hydrogeology of south-central st. croix, u.s. virgin … · hydrogeology of south-central st....
TRANSCRIPT
Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
By Robert P. Graves
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 93-4162
Prepared in cooperation with the
U.S. VIRGIN ISLANDS WATER AND POWER AUTHORITY
San Juan, Puerto Rico
1995
U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary
U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director
For additional information write to:
District ChiefU.S. Geological SurveyGSA Center651 Federal Drive, Suite 400-15Guaynabo, Puerto Rico 00965
Copies of this report can be purchased from:
U.S. Geological Survey Earth Science Information Center Open-File Reports Section, MS 517 Box 25286, Denver Federal Center Denver, CO 80225
CONTENTS
Abstract................................................................................................................................................^ 1
Introduction..............................................................................................^^ 1
Approach..........................................................................................................._^
Description of the area.........................................................................................................................................4
Geology ..............................................................................................................................................................^
Ground-water hydrology.................................................................................................................................... 10
Ground water occurrence and movement............................................................................................ 10
Hydraulic characteristics of the aquifer............................................................................................... 15
Water Quality ..................................................................................................................................................^
Summary ............................................................................................................................................................32
References.......................................................................................................................................................... 33
ILLUSTRATIONS
1. Map showing location of study area, wells, and well fields in south-centralSt. Croix, U.S. Virgin Islands ..............................................................................................................2
2. Bar graph showing mean monthly rainfall for Beth Upper New WorksStation, St. Croix, U.S. Virgin Islands 1982-91...................................................................................4
3. Bar graphs showing monthly rainfall at Beth Upper New Works Station, St. Croix, U.S. Virgin Islands, 1989 through September 1992, and mean-monthly rainfall 1982-91 ...........................................................................................................9
4-6. Graphs showing:
4. Daily mean ground-water levels in selected wells at the Adventure well field, and monthly rainfall at Beth Upper New Works Station, St. Croix, U.S. Virgin Islands, March 1990 through September 1992............................................................ 11
5. Daily mean ground-water levels in selected wells at the Golden Grove well field, and monthly rainfall at Beth Upper New Works Station, St. Croix, U.S. Virgin Islands, 1989 through September 1992........................................................................ 12
6. Daily mean ground-water levels in wells VIWAPA 02, Fairplains, and VIWAPA 23A, Barren Spot, and monthly rainfall at Beth Upper New Works Station, St. Croix, U.S. Virgin Islands, 1989 through September 1992.............................. 13
Contents
7. Map showing configuration of the water table in south-central St. Croix,U.S. Virgin Islands, July 1987........................................................................................................... 14
8. Diagrams showing major constituents in ground water from wells in theAdventure and Golden Grove well fields, St. Croix, U.S. Virgin Islands......................................... 17
9. Diagrams showing major constituents in ground water from wells in theFairplains and Barren Spot well fields, St. Croix, U.S. Virgin Islands.............................................. 17
TABLES
1. Description of test holes drilled during 1989 in south-central St. Croix,U.S. Virgin Islands...............................................................................................................................5
2. Well name and site identification number of wells drilled in south-centralSt. Croix, U.S. Virgin Islands ..............................................................................................................8
3. Estimated transmissivity from specific capacity for selected wells at theAdventure well field, St. Croix, U.S. Virgin Islands......................................................................... 15
4. Chemical analyses of ground-water samples from wells in south-centralSt. Croix, U.S. Virgin Islands ............................................................................................................ 18
5. Chloride concentration, specific conductance, and water levels for theU.S. Virgin Islands Water and Power Authority's municipal wells in theAdventure well field, St. Croix..........................................................................................................22
6. Chloride concentration, specific conductance, and water levels for theU.S. Virgin Islands Water and Power Authority's municipal wells in theGolden Grove well field, St. Croix....................................................................................................24
7. Chloride concentration, specific conductance, and water levels for theU.S. Virgin Islands Water and Power Authority's municipal wells in theFairplains well field, St. Croix...........................................................................................................27
8. Chloride concentration, specific conductance, and water levels for theU.S. Virgin Islands Water and Power Authority's municipal wells in theBarren Spot well field, St. Croix........................................................................................................ 28
IV Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
CONVERSION FACTORS, ABBREVIATED WATER-QUALITY UNITS, AND ACRONYMS
Multiply By To obtain
inch 25.4 millimeter
foot 0.3048 meter
foot squared per day ' 0.09290 meter squared per day
gallon per minute 0.06308 liter per second
gallon per minute per foot 0.2070 liter per second
Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C = 5/9 (°F - 32)
Abbreviated water-quality units used in this report:
microgram per liter (jig/L)
milligram per liter (mg/L)
microsiemen per centimeter at 25 °C (|iS/cm)
Acronyms used in this report:
National Oceanic and Atmospheric Administration (NOAA)
U.S. Geological Survey (USGS)U.S. Virgin Islands (USVI)U.S. Virgin Islands Water and Power Authority (VIWAPA)
1 The standard unit for transmissivity is cubic foot per day per square foot times foot of an aquifer thickness [(ft3/d)/ft2]ft.
In this report, the mathematically reduced form, foot squared per day (ft2/d), is used for convenience.
Conversion Factors, Abbreviated Water-Quality Units, and Acronyms
Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
By Robert P. Graves
Abstract
The subsurface geology of south-central St. Croix consists of alluvium and underlying carbonate rocks. Ground water occurs under water-table conditions in the alluvial deposits, the post-Kingshill Carbonates, and the Kingshill Limestone deposits. These deposits are hydraulically connected and are considered to be a single hydrologic unit. Depending on the location, depth to the top of the water-table aquifer can range from 5 to 68 feet below land surface. The top of the Jealousy Formation is considered to be the bottom of the water-table aquifer and lies from 85 to more than 120 feet below land surface. Well yields in south- central St. Croix ranged from less than 5 gallons per minute to 80 gallons per minute. Aquifer specific capacity at selected wells ranged from 1 to 14 gallons per minute per foot of drawdown. Aquifer transmissivity estimated from specific capacity ranged from 180 to 3,300 feet squared per day.
Ground water in south-central St. Croix is of the sodium-chloride type. Ground-water samples collected from selected wells had chloride concentrations ranging from 67 to 4,400 milligrams per liter, and dissolved solids concentrations ranging from 619 to 7,540 milligrams per liter. Depending on location, connate water or saltwater intrusion are suspected of being the source of sodium chloride in the ground water of the study area.
INTRODUCTION
Freshwater is a scarce and expensive commodity on St. Croix, U.S. Virgin Islands (USVI). To meet the demand for drinking water, all available sources of water are used including ground water, water collected in rooftop-rainfall catchments, and desalinated seawater.
Freshwater supply on St. Croix is provided to residents by the Virgin Islands Water and Power Authority (VIWAPA) from a combination of sources, primarily ground water and desalinated seawater. The high cost of desalination ($16.00 per one thousand gallons of water produced, 1991) has prompted VIWAPA to consider increasing their water supplies through additional ground- water development, primarily in the south-central part of the island. The extent to which ground-water resources in this area can be further developed will depend, in large part, on the hydrogeology of the area, about which relatively little is known. Recognizing the need for a better understanding of the hydrogeology of the area, the U.S. Geological Survey, in cooperation with VIWAPA, initiated a study of the ground-water resources of south- central St. Croix in 1989. This study, which is described herein, focused on four areas where wells existed and some geologic, lithologic, and hydrologic data were available. These areas, the Adventure, Golden Grove. Faiiplains, and Barren Spot well fields (fig. 1). were studied in detail but the knowledge of the hydrogeologic system in these areas was used to provide insight into the hydrogeology and ground-water resources throughout south-central St. Croix.
Introduction 1
64°49'15" 64°48'
17°43'
17°41'45"
\ / \ ' "/rV.A/"^ -\ ' \ /. v - jLwe V !>,H :^^ J-*v- '/'" ' V^kVi \ *' % V *~ - r> ' Is ^RS* v^ ' \ s . " > 1 v ' ".-""^ 't^1 * v ^ -f. / K ','--v-- MoA - V - ' ADVENTURE \ IV I VS *
BETHLEHEM
ADVENTURE v 4 » WELL FIELD V V\ \
,."v '«* ",. f4 IS -' BETH UPPER .. NEW WORKS M STATION
CARIBBEAN SEA
BASE FROM USGS TOPOGRAPHIC MAPCHRISTIANSTED. V.I.. ANDFREDERIKSTED, V.I. QUADRANGLES, 1982.CONTOUR INTERVAL 20 FEET,DOTTED LINES REPRESENT 10-FOOT CONTOURS.
EXPLANATION
29© WELL SITE AND SITE DESIGNATION
Figure 1. Location of study area, wells, and well fields in south-central St. Croix, U.S. Virgin Islands.
2 Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
64 46'45" 64'45'30"
17°43'
17'41'45"
CARIBBEAN SEA
BASE FROM USGS TOPOGRAPHIC MAPCHRISTIANSTED, V.I , ANDFREDERIKSTED, V.I. QUADRANGLES, 1982.CONTOUR INTERVAL 20 FEET,DOTTED LINES REPRESENT 10-FOOT CONTOURS
EXPLANATION
29© WELL SITE AND SITE DESIGNATION
Figure 1. Location of study area, wells, and well fields in south-central St. Croix, U.S. Virgin lslands~(Continued
Introduction 3
This report summarizes the results of a study conducted between January 1989 and September 1992, to define the hydrogeology of selected lands in south-central St. Croix. The objectives of this study were to determine:
1. the occurrence and movement of ground water,
2. aquifer characteristics, and
3. ground-water quality in the area.
APPROACH
The objectives of this investigation were addressed primarily by analyses of data collected during test-hole drilling by VTWAPA. From April to August 1989, VIWAPA drilled 28 test holes (table 1). Of these test holes, 10 were completed as observation wells, 15 were completed as municipal water wells, and 3 were back filled and abandoned. Of the 10 observation wells completed, 5 were instrumented by the USGS with continuous water-level recorders. These water-level monitoring sites were in operation from March 1990 until September 1992. Of the 15 municipal water wells completed, specific capacity tests to determine aquifer transmissivity were conducted on 4 wells and all wells were sampled for water-quality analysis. The test-hole drilling for this program was completed using the reverse- air circulation drilling method with an open-center reverse-air drill bit (Graves, 1992). Data used in this report include data from these test holes and wells and data from 13 VIWAPA wells completed prior to this investigation (table 2). Two of the existing VIWAPA wells were instrumented with continuous water-level recorders during January 1989, and 11 of the existing wells were sampled for water-quality analysis.
The locations of all wells used in this study are shown in figure 1. Well numbers, names, and site identification numbers for these wells are given in table 2. The well numbers in table 2 apply only to this report. However, the site-identification numbers conform with the established USGS Ground-Water Site Inventory numbering system (Mathey, 1990). The site-identification number does not change and can be used to reference a specific well in other USGS publications. Information regarding well- construction data and lithologic information for the 28 test-holes drilled by VIWAPA in 1989 is given in table 1.
DESCRIPTION OF THE AREA
South-central St. Croix has been characterized as undulating lowlands with rounded hills (Cederstrom, 1950). Surface drainage out of the area is by way of Bethlehem and River Guts, two ephemeral streams that flow only in response to heavy rainfall (fig. 1).
The mean annual air temperature recorded by the National Oceanic and Atmospheric Administration (NOAA) from 1982 to 1991 for St. Croix was 79.4 degrees Fahrenheit (U.S. Department of Commerce, 1982 to 1991). Mean-annual rainfall recorded by NOAA from 1982 to 1991 at the Beth Upper New Works Station (fig. 1) was 40.66 inches. Mean-monthly rainfall between 1982 and 1991 ranged from 1.48 inches in February to 6.48 inches in November (fig. 2). Monthly rainfall during this study (January 1989 to September 1992) ranged from less than 1 inch to more than 14 inches and differed substantially from the mean-monthly rainfall in some months (fig. 3).
^ 5
< 1LJJ
M ON
Figure 2. Mean-monthly rainfall for Beth Upper New Works Station, St. Croix, U.S. Virgin Islands, 1982-91. (Data from the U.S. Department of Commerce, National Oceanic and Atmospheric Administration, 1982 to 1991)
Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
Tab
le 1
. D
escr
iptio
n of
tes
t ho
les
drill
ed d
urin
g 19
89 in
sou
th-c
entr
al S
t. C
roix
, U
.S.
Virg
in I
slan
ds
[fig
, fig
ure;
mm
-dd-
yy, m
onth
-day
-yea
r; i
n, i
nche
s; f
t-bl
sd,
feet
bel
ow l
and
surf
ace
datu
m;
gal/m
in,
gallo
ns p
er m
inut
e; V
IWA
PA,
Vir
gin
Isla
nds
Wat
er a
nd P
ower
Aut
hori
ty;
<, l
ess
than
; ?,
ind
icat
es t
hat
cont
act
is u
ncer
tain
; N
A,
data
not
ava
ilabl
e]
o a (D
Well
number
Date
Diameter
(see
drilled
of
fig
Well name
(mm-dd-
casing
1)
(see table 2)
yy)
(in)
1 VIWAPA 1,
Adventure
04-19-89
4 (Observation Well)
2 VIWAPA 2,
Adventure
04-21-89
4 (Observation Well)
3 VIWAPA 2A,
Adventure
04-25-89
6 (Municipal Water Well)
4 VIWAPA 3, Adventure
04-28-89
4 (Observation Well)
5 VIWAPA 4, Adventure
05-01-89
4 (Observation Well)
6 VIWAPA 5,
Adventure
05-05-89
6 (Municipal Water Well)
7 VIWAPA
6, Adventure
05-06-89
6 (Municipal Water Well)
8 VIWAPA 7, Adventure
05-11-89
6 (Municipal Water Well)
9 VIWAPA 8, Adventure
05-12-89
NA
Depth
of
Depth
test
of
Screen
Estimated
hole
casing
interval
yield
(ft-
(ft-
(ft-
(gal/
Subsurface material
blsd)
blsd)
blsd)
min)
(ft-blsd)
127
60
60-80
5 Topsoil
Kingshill Limestone
Jealousy Formation
100
20
20 -
40
25
Topsoil
Alluvial deposits
Kingshill Limestone
60
20
20 -
60
20
Topsoil
Alluvial deposits
Kingshill Limestone
110
50
50-90
< 5
Topsoil
Alluvial deposits
Kingshill Limestone
Jealousy Formation
100
40
40-80
< 5
Topsoil
Alluvial deposits
Kingshill Limestone
100
20
20 -
80
50
Topsoil
Alluvial deposits
Kingshill Limestone
100
20
20 -
80
25
Topsoil
Alluvial deposits
Kingshill Limestone
100
20
20 -
80
30
Topsoil
Alluvial deposits
Kingshill Limestone
100
TEST HOLE
NA
Topsoil
BACK FILLED
Alluvial deposits
Kingshill Limestone
0 -
08 -
108 -
0 -
06 -
43 -
0 -
06 -
43 -
0 -
06 -
24
-
107
-
0 -
08 -
35 -
0 -
08 -
43 -
0 -
07 -
45 -
0 -
05 -
13 -
0 -
05 -
10 -
08
108
127 06
43
100 06
43
60 06
24
107
110 08
35
100 08
43
100 07?
45
100 05?
13
100 05?
10
100
><
Q.
O
(Q
(D
O_
O
(Q O
(D Si. o>
p O ( 5"
Tab
le 1
. D
escr
iptio
n of
tes
t ho
les
drill
ed d
urin
g 19
89 in
so
uth-
cent
ral
St.
Cro
ix,
U.S
. V
irgin
Isl
ands
-Con
tinue
d
[fig
, fig
ure;
mm
-dd-
yy,
mon
th-d
ay-y
ear;
in,
inc
hes;
ft-
blsd
, fee
t bel
ow l
and
surf
ace
datu
m;
gal/m
in,
gallo
ns p
er m
inut
e; V
IWA
PA,
Vir
gin
Isla
nds
Wat
er a
nd P
ower
Aut
hori
ty;
<, l
ess
than
; ?,
ind
icat
es t
hat c
onta
ct is
unc
erta
in;
NA
, dat
a no
t av
aila
ble]
Well
number
(see
fig
Well name
1)
(see table 2)
Date
Diameter
drilled
of(m
m-dd
- casing
yy)
(in)
Depth
oftest
hole
(ft-
blsd)
Depth
ofcasing
(ft-
blsd
)
Screen
interval
(ft-
bl
sd)
Estimated
yield
(gal/
min)
Subsurface material
(ft-blsd)
10
VIWAPA 9, Adventure
05-15-89
NA
11
VIWAPA 10,
Adventure
05-16-89
NA
12
VIWAPA 11,
Adventure
05-17-89
4 (Observation We
ll)
13
VIWAPA 12,
Adventure
05-18-89
(Observation Well)
15
VIWAPA 17
, Adventure
06-09-89
(Observation We
ll)
16
VIWAPA 18,
Adventure
06-13-?
(Municipal Water Well)
17
VIWAPA 19
, Adventure
06-15-89
6 (M
unic
ipal
Water Well)
18
VIWAPA 20
, Adventure
06-19-89
6 (Municipal Water Well)
21
VIWAPA 13,
Golden
05-22-!
Grove
(Mun
icip
al Water Well)
22
VIWAPA 14,
Golden
05-25-!
Grove
(Observation We
ll)
90 70
110
20
TEST HOLE
BACK FILLED
TEST HOLE
BACK FILLED
20 -
i
90
20
20 -
120
30
100
2040 -
55
NA
NA
100
20
20 -
80
55
95
10
10-40
< 5
25
100
20
20 -
80
50
100
20
20 -
95
60
30 -
115
30
20-40
< 5
55 -
75
Topsoil
0-05
Kingshill Limestone
05 -
90
Topsoil
0 -
10?
Kingshill Limestone
10 -
70
Topsoil
0 -
10?
Kingshill Limestone
10 -
96Jealousy Formation
96 -
100
Topsoil
0-05
Kingshill Limestone
05 -
100
Topsoil
0-05
Alluvial deposits
05 -
15Kingshill Limestone
15 -
95
Topsoil
0 -
05?
Alluvial deposits
05 -
17Kingshill Limestone
17 -
85Jealousy Formation
85 -
90
Topsoil
0 -
07?
Alluvial deposits
07 -
25Kingshill Limestone
25 -
98Jealousy Formation
98 -
100
Topsoil
0 -
05?
Alluvial deposits
05 -
17Kingshill Limestone
17 -
98Jealousy Formation
98 -
100
Topsoil
0 -
06?
Alluvial deposits
06 -
57Kingshill Limestone
57 -
120
Topsoil
0 -
06?
Alluvial deposits
06 -
50Kingshill Limestone
50 -
100
Tabl
e 1.
D
escr
iptio
n of
test
hol
es d
rille
d du
ring
1989
in s
outh
-cen
tral S
t. C
roix
, U
.S.
Virg
in I
slan
ds-C
ontin
ued
[fig
, fig
ure;
mm
-dd-
yy, m
onth
-day
-yea
r; i
n, i
nche
s; f
t-bl
sd, f
eet b
elow
lan
d su
rfac
e da
tum
; ga
l/min
, gal
lons
per
min
ute;
VIW
APA
, V
irgi
n Is
land
s W
ater
and
Pow
er A
utho
rity
; <,
les
s th
an;
?, i
ndic
ates
tha
t co
ntac
t is
unc
erta
in;
NA
, da
ta n
ot a
vaila
ble]
Well
number
Date
Diameter
(see
drilled
of
fig
Well name
(mm-dd-
casing
1)
23 24
i (see table 2)
yy
) (in)
VIWAPA 15,
Golden
05-30-89
6Grove
(Municipal Water Well)
VIWAPA 16
, Golden
06-06-89
6Grove
(Municipal Water Well)
Depth
of
Depth
test
of
Screen
Estimated
hole
casing
interval
yield
(ft-
(ft-
(ft-
(gal
/ Subsurface material
blsd)
blsd)
blsd)
min)
(ft-blsd)
100
31
31 -
92
20
Topsoil
Alluvial deposits
Kingshill Limestone
110
30
30 -
100
25
Topsoil
Alluvial deposits
Kingshill Limestone
005 50
005 55
- 05
- 50
- 100
- 05
- 55
- 110
25
VIWAPA 21,
Golden
Grove
(Municipal Water Well)
06-21-89
Topsoil
0Alluvial deposits
05Kingshill Limestone
45
o (D
V> O 5' 5' D 2. (D 3 D)
26
VIWAPA 21A, Golden
06-23-89
4Grove
(Observation We
ll)
36
VIWAPA 23
, Barren
08-12-89
6Spot
(Municipal Water Well)
37
VIWAPA 23A, Barren
08-08-89
4Spot
(Observation Well)
38
VIWAPA 24,
Barren
06-30-89
6Spot
(Municipal Water Well)
39
VIWAPA 24A, Barren
07-05-89
6 Spot
(Municipal Water Well)
40
VIWAPA 25,
Barren
06-27-89
6Spot
(Municipal Water Well)
100
15
15 -
70
60
Topsoil
Alluvial deposits
Kingshill Limestone
120
80
80 -
120
50
Topsoil
Post-Kings hi 11
Carbonates
110
70
70 -
110
80
Topsoil
Post-Kingshill
Carbonates
110
70
70 -
110
80
Topsoil
Post-Kingshill
Carbonates
120
80
80 -
120
35
Topsoil
Post-Kingshill
Carbonates
110
70
70 -
110
70
Topsoil
Alluvial deposits
Kingshill Limestone
0 -
05 -
55 -
0 -
02 -
0 -
01 -
0 -
01 -
0 -
03 -
0 -
05 -
20 -
05 55100 02
120 01
110 01
110 03
120 05 20
110
Table 2. Well name and site identification number of wells drilled in south-central St. Croix, U.S. Virgin Islands
[*, indicates well drilled by the Virgin Islands Water and Power Authority (VIWAPA) during 1989, wells without this designation completed prior to 1989; DPW, Department of Public Works]
Well number
(see figure
I*2*3*4*5*6*
7*8*9*
10*11*12*
13*
1415*16*17*18*
192021*22*23*24*25*26*
2728293031
3233343536*
37*38*39*40*
41
Well 1)
VIWAPA 1VIWAPA 2VIWAPA 2AVIWAPA 3VIWAPA 4VIWAPA 5
VIWAPA 6VIWAPA 7VIWAPA 8VIWAPA 9VIWAPA 10VIWAPA 11
VIWAPA 12VIWAPA 14VIWAPA 17VIWAPA 18VIWAPA 19VIWAPA 20
VIWAPA 1/DPW 1VIWAPA 6/DPW 6
VIWAPA 13VIWAPA 14VIWAPA 15VIWAPA 16VIWAPA 21VIWAPA 21A
VIWAPA 1VIWAPA 2VIWAPA 6VIWAPA 7VIWAPA 9
VIWAPA 3AVIWAPA 5VIWAPA 6VIWAPA 7VIWAPA 23
VIWAPA 23AVIWAPA 24VIWAPA 24AVIWAPA 25VIWAPA 31
name
, Adventure, Adventure, Adventure, Adventure, Adventure, Adventure
, Adventure, Adventure, Adventure, Adventure, Adventure, Adventure
, Adventure, Adventure, Adventure, Adventure, Adventure, Adventure
, Golden Grove, Golden Grove, Golden Grove, Golden Grove, Golden Grove, Golden Grove, Golden Grove, Golden Grove
, Fairplains, Fairplains, Fairplains, Fairplains, Fairplains
, Barren Spot, Barren Spot, Barren Spot, Barren Spot, Barren Spot
, Barren Spot, Barren Spot, Barren Spot, Barren Spot, Barren Spot
Site identification
number
174304064484600174303064481100174303064481101174308064482800174311064482601174304064481900
174304064482000174311064481100174306064484701174321064485400174318064490300174329064483000
174328064482000174311064482900174316064480800174313064481100174307064481000174309064481100
174247064475800174243064475100174240064475600174237064474900174247064475700174244064475500174247064475700174247064475701
174225064471700174225064472000174225064471900174229064471600174231064471900
174323064454600174329064454700174331064453800174316064453800174327064454600
174319064454401174319064454400174332064453400174316064454300174315064454000
8 Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
JFMAMJJASOND
1989 1990 1991JFMAMJJAS
1992
Figure 3. Monthly rainfall at Beth Upper New Works Station, St. Croix, U.S. Virgin Islands, 1989 through September 1992, and mean-monthly rainfall 1982-91. (Data from the U.S. Department of Commerce, National Oceanic and Atmospheric Administration, 1982 through September 1992)
GEOLOGY
The subsurface geology of south-central St. Croix consists of alluvium and underlying carbonate rocks. The carbonate deposits consist of the Jealousy Formation of Eocene to middle Miocene age, the Kingshill Limestone of Miocene to Pliocene age (Gerhard, Frost, and Curth, 1978), and post-Kingshill Carbonates of Pliocene and younger age (Gill and Hubbard 1986). Using this stratigraphic nomenclature the results of the test-hole drilling in south-central St. Croix revealed the following:
ALLUVIAL DEPOSITS-sand and gravel with cobbles and boulders, locally with thin lenses of silt and clay. The alluvial deposits are localized and not laterally extensive. A 5- to 8-foot thick topsoil overlies the alluvial deposits. The alluvial deposits range in thickness from 8 to 51 feet. Depth to the top of the alluvial deposits ranges from 5 to 8 feet below land surface. Depth to the bottom of the
alluvial deposits ranges from 10 to 57 feet below land surface.
POST-KINGSHILL CARBONATES a shallow water carbonate deposit which is dominantly a benthic foraminiferal packstone, with significant quantities of reef and skeletal clasts (Gill and Hubbard, 1986). Locally, these deposits were interbedded with clay, crushed coarse grained coral fragments, and conglomerates of washed out limestone boulders and volcanic gravel, cobbles, and boulders. The top of the post-Kingshill Carbonates was near land surface and this unit commonly was overlain by only a 1- to 3-foot thick topsoil. Deposits of the post- Kingshill Carbonates were localized and were penetrated only in test holes drilled in the Barren Spot well field (fig. 1, table 1). Test-hole drilling indicated that the depth to the bottom of these deposits exceeded 120 feet below land surface, at some sites.
Geology
KINGSHILL LIMESTONE-a deep-water carbonate
deposit of relatively soft, white to buff marl, which is
locally fractured (Gill and Hubbard, 1986) with iron and
manganese oxide stains common in the fracture zones.
The Kingshill Limestone generally was overlain by a 5- to
8-foot thick layer of topsoil or alluvium. Where the
bottom of the Kingshill Limestone was penetrated by test
holes, this unit had a thickness that ranged from 68 to 100
feet. Depth to the top of the Kingshill Limestone ranged
from 5 to 57 feet below land surface. Depth to the bottom
of the Kingshill Limestone ranged from 107 to more than
120 feet below land surface.
JEALOUSY FORMATION-a deep-water deposit of bluish-gray clay which contains a rich planktonic foraminiferal assemblage (Gill and Hubbard, 1986). The
Jealousy Formation underlies the Kingshill Limestone.
The top of the Jealousy Formation was from 85 to 108
feet below land surface in the test holes drilled during this
study. However, data from other wells in the area indicate
that the depth to the top of the Jealousy Formation
exceeds 120 feet below land surface in places. The
thickness of the Jealousy Formation has been estimated to exceed 1,400 feet (Gill and Hubbard, 1986).
GROUND-WATER HYDROLOGY
Fresh ground water is not plentiful in St. Croix, but it exists in sufficient quantities in some areas to be a source of water for public supply. Most of the fresh ground water exists at relatively shallow depths in unconsolidated surficial deposits or in relatively shallow limestone deposits.
Ground Water Occurrence and Movement
Ground water occurs under water-table conditions in the alluvial, post-Kingshill Carbonates, and Kingshill Limestone deposits. Where the alluvial deposits are saturated, they are considered to be hydraulically connected to the post-Kingshill Carbonates and Kingshill Limestone deposits and thus these thin units are considered a single hydrologic unit. The top of the
Jealousy Formation, which underlies all of south-central
St. Croix, is considered to be the bottom of this
hydrologic unit.
The water table fluctuates seasonally in response to
rainfall variation and can fluctuate daily in wells located
near pumping wells. Depth to the water table ranges from
approximately 5 to 36 feet below land surface in the
Adventure well field (fig. 4); 13 to 43 feet below land
surface in the Golden Grove well field (fig. 5); 20 to 26
feet below land surface in the Fairplains well field (fig. 6);
and 57 to 68 feet below land surface in the Barren Spot
well field (fig. 6).
The general direction of ground-water flow in south-
central St. Croix is south-southeast toward the coast
(Torres-Gonzalez and Rodriguez del Rio, 1990) (fig. 7).
The elevation of the water table can range from
approximately 140 feet above mean sea level in the
northern part of the study area to less than 5 feet above
mean sea level near the coast.
The quantity of water recharging the water-table
aquifer in south-central St. Croix has been estimated to be
approximately 3 percent of annual rainfall (Robison,
1972). Jordan (1973) reported that aquifer recharge in the
USVI is infrequent and probably occurs only after periods
of heavy rainfall or a series of lesser rains. He believed
that because of relatively low annual rainfall and high
average annual temperatures, most recharge occurred
after major rainstorms which produced more than 2 inches
of rain. Evidence of the occurrence of recharge following
a major rainstorm and the infrequency of aquifer recharge
is evident when water-table fluctuations are compared to
rainfall records (figs. 4, 5, and 6). During this study,
substantial increases in ground-water levels were
recorded only after three heavy rainfall events in
September 1989 (14.24 inches), October 1990 (13.16
inches), and May 1992 (10.90 inches).
10 Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
opQ
ao:
10
20
30
10
20
30
40
VIWAPA 17, ADVENTURE USGS OBSERVATION WELL 174316064480800
MAMJJ ASOND
1990
JFMAMJJ ASOND
1991
JFMAMJJAS
1992
i i i I i i i i I I i i I I I I I I I I I I I I I r
VIWAPA 03, ADVENTURE USGS OBSERVATION WELL 174308064482800
MAMJJ ASOND
1990
JFMAMJJASOND
1991
JFMAMJJAS
1992
10
20
30
15
10
VIWAPA 02, ADVENTURE USGS OBSERVATION WELL 174303064481100
MAMJJ ASOND
1990
JFMAMJJ ASOND
1991
JFMAMJJAS
1992
§ g
BETH UPPER NEW WORKS STATION ST. CROIX, U.S. VIRGIN ISLANDS
MAMJJ ASOND 1990
1,1.1.1.1.1.1.1.1.1.JFMAMJJ ASOND
1991
JFMAMJJAS
1992
Figure 4. Daily mean ground-water levels in selected wells at the Adventure well field and monthly rainfall at Beth Upper New Works Station, St. Croix, U.S. Virgin Islands, March 1990 through September 1992.
Ground-Water Hydrology 11
LL OCD
LLJ CO
10 I I I I I
20
gQ 30
LLJ
40
2 50
VIWAPA 21A, GOLDEN GROVE USGS OBSERVATION WELL 174247064475701
I V
JFMAMJJ ASONDJFMAMJJ ASONDJFMAMJJ ASONDJFMAMJJ AS
1989 1990 1991 1992
VIWAPA 06 / DPW 06, GOLDEN GROVE USGS OBSERVATION WELL 174243064475100
JFMAMJJ ASOND JFMAMJJ ASON OIJ FMAMJJ ASOND35
15
10
1989 1990 1991JFMAMJJAS
1992
.M.I.1.1,1.1.1.1.1.
BETH UPPER NEW WORKS STATION ST. CROIX, U.S. VIRGIN ISLANDS
JFMAMJJ ASONDJFMAMJJ ASOND1989 1990
JFMAMJJASOND 1991
JFMAMJJAS 1992
Figure 5. Daily mean ground-water levels in selected wells at the Golden Grove well field and monthly rainfall at Beth Upper New Works Station, St. Croix, U.S. Virgin Islands, 1989 through September 1992.
12 Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
cr=jGO 6
58
60
62
63
VIWAPA 23A, BARREN SPOT USGS OBSERVATION WELL 174319064454401
JFMAMJ JASON D J FMAMJJ ASOND
1989 1990JFMAMJ JASOND
1991
JFMAMJJ AS
1992
24
28JFMAMJ JASOND
1989
J FMAMJJ ASOND JFMAMJ JASOND
1991
JFMAMJJAS
1992
Z 00< I= O
JFMAMJ JASOND
1989
BETH UPPER NEW WORKS STATION ST. CROIX, U.S. VIRGIN ISLANDS
J FMAMJJ ASOND
1990
JFMAMJ JASOND
1991
JFMAMJJAS
1992
Figure 6. Daily mean ground-water levels in wells VIWAPA 02, Fairplains, and VIWAPA 23A, Barren Spot, and monthly rainfall at Beth Upper New Works Station, St. Croix, U.S. Virgin Islands, 1989 through September 1992.
Ground-Water Hydrology 13
64'48' 64'45'30"
44'17"
17'43' -r
/Srv -^
v /i; s . ^'
^Ww*' V«r U -*,- **&%* 2r -~J &$* -**<& ''*&?, *.$**»# . .., .' /- ^ /:- . ' *,*^ek.»-^t*i :> ^ >. t"")ji>* f ' \~- - '^^ " ' *''-'Sx-^'M-*- «i;̂ '-''vfi''' \ < ?> <F<\ £ \>' S \ ^tj i ' ,-««.- ' ' "^<fe^^-:%''V/ **/ S >>. -.^:,^^,,X5jS:-->5..> " -^- -/?, '/ ^X- ^. , . .-' Vt ' »» -.» ,'^W^'^^^\'^^\^ J* V\ L . ^ ' ,
CARIBBEAN SEA
BASE FROM USGS TOPOGRAPHIC MAPCHRISTIANSTED, V.I.. ANDFREDERIKSTED, V.I. QUADRANGLES, 1982.CONTOUR INTERVAL 20 FEET.DOTTED LINES REPRESENT 10-FOOT CONTOURS.
EXPLANATION
LINE OF EQUAL WATER-TABLE ALTITUDE, IN FEET ABOVE MEAN SEA-LEVEL Interval is variable. All contours are approximately located.
DIRECTION OF GROUND-WATER FLOW
Figure 7. Configuration of the water table in south-central St. Croix, U.S. Virgin Islands, July 1987. (Adapted from Torres-Gonzalez and Rodriguez del Rio, 1990)
14 Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
Hydraulic Characteristics of the Aquifer
The ground-water development potential of an aquifer is dependent upon hydraulic characteristics of transmissivity and storage coefficient. Transmissivity is a measure of the ability of the aquifer to transmit water and is defined as the product of the hydraulic conductivity and the aquifer thickness. The storage coefficient is the volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head.
During this study, aquifer transmissivities in south- central St. Croix were estimated to range from 180 to 3,300 feet squared per day. Transmissivity values for wells located in the Adventure well field were determined from four specific capacity tests. The specific capacity values determined from these tests ranged from 1 to 14 gallons per minute per foot of drawdown (table 3). The specific capacity values were converted to transmissivity using a method described by Meyer (1963), that relates well diameter, specific capacity, and the aquifer storage coefficient to aquifer transmissivity. Aquifer storage coefficients were not determined from field tests for this report, but rather are assumed to be typical water-table storage coefficients, which range from 0.1 to 0.3 (Lohman, 1979). The specific capacity/aquifer transmissivity relation developed by Meyer (1963) assumes a water-table storage coefficient of 0.1.
The variation in aquifer transmissivity values in the Adventure well field can be explained by the presence or absence of intersecting fracture zones within the Kingshill Limestone. Where the density of fracture zones increase, the aquifer transmissivity will increase.
Field tests to determine specific capacity were conducted only in the Adventure well field. However, well yield was estimated for all 28 test holes drilled in 1989 (table 1). The test to estimate well yield was completed by forcing, at a constant rate, compressed air through the drill pipe and measuring continuous flow out of the test hole. Results of this testing indicated somewhat higher potential aquifer yields in the Barren Spot well field, where yields of 35 to 80 gallons per minute were measured, than in the other well fields. Yields of less than 5 to 60 gallons per minute and less than 5 to 80 gallons per minute were measured in the Adventure and Golden Grove well fields, respectively. No wells were drilled or specific capacity tests conducted in the Fairplains well field for this study. However, Torres-Gonzalez (1991) indicated that aquifer transmissivity in the Fairplains area ranged from 3,000 to 4,000 feet squared per day.
The relatively wide range of yields in the study area probably is due to the nonhomogeneity of the aquifer material. High yields in the Post Kingshill are due to the primary depositional porosity of the shallow-water limestone deposits and interlayered conglomerates and
Table 3. Estimated transmissivity from specific capacity for selected wells at the Adventure well field, St. Croix, U.S. Virgin Islands (Estimates based on method described by Meyer, 1963)
[(gal/min)/ft, gallons per minute per foot of drawdown; ft2/d, feet squared per day]
Well name
VIWAPA 05, Adventure
VIWAPA 14, Adventure
VIWAPA 18, Adventure
VIWAPA 19, Adventure
Specific capacity [(gal/min)/ft]
2
1
1
14
Transmissivity (ft2/d)
420
180
180
3,300
Length of test (hours)
19
19
20
20
Ground-Water Hydrology 15
volcanics, as well as to subsequent diagenetic changes within the limestone. Generally, where the Post Kingshill is tapped by wells, the yields will be consistently high. In the Kingshill Limestone high yields are common only in areas where there is a high density of intersecting fracture zones. The range of yields in the alluvial deposits are limited by the thin (10- to 30-foot), saturated thickness of the deposits. In areas where the thickness of the saturated alluvial deposits is greater, the yields probably are greater.
WATER QUALITY
In order to describe the quality of ground water in south-central St. Croix, 33 samples were collected from 25 wells and analyzed for common cations and anions and trace metals (table 4). Results of these analyses indicated that the ground water in the Golden Grove, Fairplains, and Barren Spot well fields was predominantly a sodium- chloride type water (Piper, 1953; figs. 8 and 9). In the Adventure well field, the ground-water type was mixed; no one cation or anion exceeded 50 percent of the total milliequivalents of cations or anions.
In an island setting, ground water that is a sodium chloride type water commonly is considered to contain a mixture of freshwater and seawater (Gomez-Gomez, 1984). The source of the seawater could be from the lateral migration of seawater from the ocean through the aquifer, from the upconing of saline water as a result of excessive pumping, or from the presence of connate seawater which has not been completely flushed out of the aquifer. To help identify the source of the sodium chloride in ground water in south-central St. Croix, ; ground-water samples were collected monthly from selected wells in each well field and analyzed for chloride concentration and specific conductance (tables 5, 6,7, and 8). If the seasonal variations in the chloride concentrations are small, connate water is the most likely source of the sodium chloride; assuming that pumping at the well fields is relatively constant. If the variations are large, sodium chloride is most likely the source of upconing or lateral migration (intrusion) of seawater which changes with the seasonal water-level fluctuations in the aquifer, and the related movement of the saltwater- freshwater interface.
In the Adventure and Golden Grove well fields, it is suspected that connate water is the source of saltwater, whereas in the Fairplains and Barren Spot well fields it is suspected that intrusion is the source of saltwater. In the Adventure and Golden Grove well fields, relatively small seasonal variations in chloride concentration indicate that connate water might be the principle source of the chloride. In the Adventure well field, the ranges of seasonal variation in chloride concentrations in samples collected from June 1990 to September 1992, from wells 6, 7, 16, 18, and 19 were 66, 292, 14, 22, and 14 milligrams per liter (mg/L), respectively (table 5). In the Golden Grove well field, the variations in chloride concentrations in samples collected from August 1990 to September 1992, from wells 1, 13, 15, 16, and 21 was 70, 80, 84, 68, and 56 mg/L,VesrJectively (table 6). With the exception of the significant change in chloride concentration in water from well 7 in the Adventure well field (292 mg/L), the relatively small seasonal variation in chloride concentrations and the relatively large water- level fluctuations of as much as 20 feet in the two well fields indicate that connate ground water probably is the predominant source of the sodium chloride. The seasonal variation in chloride concentration of 292 mg/L in well 7 of the Adventure well field might indicate possible upconing of saltwater into the Adventure well field at that site.
In the Fairplains and Barren Spot well fields the greater seasonal variations in chloride concentrations indicate that seawater intrusion into the aquifer might be the principal source of the chloride. In the Fairplains well field, the variation in chloride concentrations in samples collected from June 1991 to September 1992, from wells 7 and 8 were 290 and 710 mg/L, respectively (table 7). In the Barren Spot well field, the variations in chloride concentrations in samples collected from August 1990 to September 1992, from wells 3A, 5,6,7, 25, 24,23, and 31 was 94, 220, 112, 94, 162, 250, 82, and 164 mg/L, respectively (table 8). The greater seasonal variations in chloride concentrations, and relatively small seasonal water-level fluctuations (approximately 6 feet) indicates that seawater intrusion could be the principal source of chloride in water from wells in these two well fields.
16 Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
CONSTITUENTS ADVENTURE WELL FIELD GOLDEN GROVE WELL FIELD
Ca
K
Mg
Na
Cl
C0 3
HC0 3
S0 4
Calcium
Potassium
Magnesium
Sodium
Chloride
Carbonate
Bicarbonate
Sulfate
Figure 8. Diagrams showing major constituents in ground water from wells in the Adventure and Golden Grove well fields, St. Croix, U.S. Virgin Islands. Values shown are percent of total milliequivalents per liter. Numbers refer to wells in table 2.
CONSTITUENTS FAIRPLAINS WELL FIELD BARREN SPOT WELL FIELD
Ca
K
Mg
Na
Cl
C03
HC03
S04
Calcium
Potassium
Magnesium
Sodium
Chloride
Carbonate
Bicarbonate
Sulfate
Figure 9. Diagrams showing major constituents in ground water from wells in the Fairplains and Barren Spot well fields, St. Croix, U.S. Virgin Islands. Values shown are percent of total milliequivalents per liter. Numbers refer to wells in table 2.
Water Quality 17
< Q.
Tab
le 4
. C
hem
ical
ana
lyse
s of
gro
und-
wat
er s
ampl
es f
rom
wel
ls in
sou
th-c
entr
al S
t. C
roix
, U
.S.
Virg
in I
slan
ds
[fig
, fig
ure;
mm
-dd-
yy, m
onth
-day
-yea
r; (
iS/c
m, m
icro
siem
ens
per
cent
imet
er a
t 25
degr
ees
Cel
sius
; de
g °C
, deg
rees
Cel
sius
; m
g/L
, mill
igra
ms
per
liter
; V
IWA
PA
, Vir
gin
Isla
nds
Wat
er a
nd P
ower
Aut
hori
ty;
DP
W, D
epar
tmen
t of P
ublic
Wor
ks]
igeology of
South-Cer
Well
number
(see
Well
fig
1 name
and
:?
table 1)
O 3. p. c J° <3 5 5T Q) 3 Q.
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
DPW
1VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
2A
05 06 07 14 18 19 20
Adventure
Adventure
Adventure
Adventure
Adventure
Adventure
Adventure
Adventure
3 6 7 814 16 17 18
Date
(mm-dd-yy)
03-11-91
03-12-91
03-12-91
03-11-91
04-17-91
03-11-91
07-31-91
03-11-91
Spe
cific
con
duct
ance
(labor
atory
value)
(U,S/cm)
1, 2, 2, 1, 1, 1, 1, 1,
870
020
030
600
700
430
810
660
PH
(field
value)
( stand
ard
units)
7.3
7.1
7.3
7.3
7.3
7.2
7.3
7.3
Temp
er
ature
(deg 26 27 26 27 25 26 29 26
°C)
.5 .0 .5 .0 .5 .5 .0 .5
Hard
ness
(mg/L
as
CaC0
3)
330
360
310
190
270
320
230
230
Alka
linity
(labor
atory
value)
(mg/L
as
CaC0
3)
597
648
634
553
528
494
577
566
Calcium,
dis
solved
(mg/L
as Ca)
56 57 45 35 42 60 39 44
Magne
sium,
dis
solved
(mg/L
as Mg
)
45 52 47 24 40 42 32 28
Sodium
dis
solved
(mg/L
as Na
)
290
330
350
290
260
190
290
290
Golden Grove
13 13 15 15 16 16 21 21 01 06 07 09 3A 05 06 07 23 23 24 24 25 25 31 31
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Fairplains
Fairplains
Fairplains
Fairplains
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
19 21 21 23 23
24 24 25 25 27 29 30 31 32 33 34 35 36 36 38 38 40 40 41 41
02-12-91
04-17-90
01-22-91
04-17-90
01-22-91
04-17-90
01-22-91
04-17-90
01-22-91
01-10-91
01-10-91
01-10-91
01-10-91
01-25-91
01-28-91
01-28-91
01-25-91
04-16-90
01-28-91
04-16-90
01-25-91
04-16-90
01-25-91
01-28-91
07-24-91
1, 1, 1, 2, 1, 2, 1, 1, 1,
12, 5, 3, 2, 3, 3, 2, 2, 3, 3, 4, 3, 3, 2, 3,
930
990
630
480
360
220
440
950
820
600
970
830
320
860
010
440
760
380
670
260
690
680
760
997
400
8.3
7.1
7.1
7.1
7.1
7.1
7 .1
7.2
7.1
7.0
7 .2
7.2
7.4
7.5
7.7
7.7
7 .7
7.5
7.6
7.6
7.6
7.5
7.5
7.7
7.3
28 27 27 28 27 28 27 28 27 27 27 27 28 27 27 27 26 28 26 27 27 27 25 25 27
.0 .0 .0 .0 .5 .5 .5 .0 .0 .5 .5 .5 .0 .5 .0 .5 .5 .0 .0 .5 .0 .5 .5 .0 .5
390
400
280
410
340
400
290
390
420
3,400
1,200
550
380
170
110
110
130
130
150
280
210
260
140
160
320
537
510
449
587
488
557
480
509
527
278
504
560
539
541
596
561
559
546
566
504
549
531
559
426
508
75 70 48 75 66 71 52 71 77
660
220
110 68 43 25 28 34 32 37 69 54 60 35 25 79
50 55 38 53 42 54 40 52 55
420
150 67 50 16 11 10 12 13 15 25 19 27 12 24 31
280
270
250
410
180
350
200
270
230
1,400
800
620
370
780
620
510
570
720
750
870
740
740
550
170
610
Tabl
e 4.
C
hem
ical
ana
lyse
s of
gro
und-
wat
er s
ampl
es fr
om w
ells
in s
outh
-cen
tral
St.
Cro
ix,
U.S
. V
irgin
Isl
ands
-Con
tinue
d[f
ig, f
igur
e; m
m-d
d-yy
, mon
th-d
ay-y
ear;
mg/
L, m
illig
ram
s pe
r lit
er;
VIW
AP
A, V
irgi
n Is
land
s W
ater
and
Pow
er A
utho
rity
; D
PW, D
epar
tmen
t of P
ublic
Wor
ks]
Well
number
(see
Well
fig
1 name
and
Date
table 1)
(mm-
dd-y
y)
Water
Quality
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
DPW 1
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
2A
0506
0714
1819
20
Adventure
Adventure
Adventure
Adventure
Adventure
Adventure
Adventure
Adventure
3 6 7 814 16 17 18
03-11-91
03-12-91
03-12-91
03-11-91
04-17-91
03-11-91
07-31-91
03-11-91
Potas
sium,
dis
solved
(mg/L
as K)
0 1
.4 .4 .6 .5 .6 .2 .4 .7
Sulfate,
dis
solved
(mg/L
as S0
4)
94 86 84 94 53 71 84 92
Chlo
ride,
dis
solved
(mg/L
as Cl
)
230
260
260
150
200
150
220
170
Fluo-
ride
, dis
solved
(mg/
L as
F)
0.80 .80
.80
.90
.60
.50
1.00 .70
Sili
ca,
dis
solved
(mg/L
as
Si0
2)
43 44 41 44 41 48 41 45
Dis
solved
solids
(mg/L)
1,12
01,
220
1,210
970
954
859
1,050
1,010
Golden Grove
13 13 15 15 16 16 21 21 01 06 07 09 3A 05 06 07 23 23 24
24
25
25
31
31
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Fairplains
Fairplains
Fairplains
Fairplains
Barren Sp
otBarren Sp
otBarren Sp
otBarren Sp
otBarren Sp
otBarren Sp
otBarren Sp
ot
Barren Sp
ot
Barren Sp
ot
Barren Sp
ot
Barren Sp
ot
Barren Sp
ot
19 21 21 23 23 24 24 25 25 27 29 30 31 32 33 34 35 36 36 38
38
40
40
41
41
02-12-91
04-17-90
01-22-91
04-17-90
01-22-91
04-17-90
01-22-91
04-17-90
01-22-91
01-10-91
01-10-91
01-10-91
01-l'0-91
01-25-91
01-28-91
01-28-91
01-25-91
04-16-90
01-28-91
04-16-90
01-25-91
04-16-90
01-25-91
01-28-91
07-24-91
1 1 1 1 1 1 6 3 2 1 9 8 5 6 8 9 8 6 12
5 12 8
.9 .1 .7 .1 .9 .1 .7 .2 .2 .0 .5 .5 .4 .0 .8 .5 .8 .6 .5 .0
.5
.0
.7
.0
.6
100 86 75
140 51
110 65 91 87
450
290
240
190
250
200
130
130
200
220
290
240
230
130 39
230
270
300
210
400
140
350
160
290
240
4400
1600 860
350
780
510
380
480
670
700
880
720
760
470 67
650
.60
.20
.50
.30
.40
.20
.50
.20
.30
.10
.30
.90
.10
.70
.80
1.00
1.00 .20
.70
.30
.80
.10
.90
.80
.90
44 47 39 58 39 54 39 48 40 36 38 38 38 27 26 23 26 32 27 33
27
34
26
26
27
1, 140
1, 140 930
1,49
0813
1,320
845
1,13
01,050
7,540
3, 400
2,270
1,390
2,230
1,760
1,420
1,59
02, 000
2,100
2, 480
2,140
2,180
1,56
0 619
1, 940
Tabl
e 4.
C
hem
ical
ana
lyse
s of
gro
und-
wat
er s
ampl
es fr
om w
ells
in s
outh
-cen
tral S
t. C
roix
, U
.S.
Virg
in I
slan
ds-C
ontin
ued
[fig
, fig
ure;
mm
-dd-
yy, m
onth
-day
-yea
r; m
g/L
, mill
igra
ms
per
liter
; |J.
g/L,
mic
rogr
ams
per
liter
; V
IWA
PA,
Vir
gin
Isla
nds
Wat
er a
nd P
ower
Aut
hori
ty;
DPW
, Dep
artm
ent
of P
ublic
Wor
ks; , i
ndic
ates
dat
a no
t av
aila
ble;
<, l
ess
than
]0 geolog
y of
South-
§ sl to o 3 y c to '< <3 5' ii[ Q. (0
Well
number
(see
Well
fig
I name
and
table 1)
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
DPW
1
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
2A Adventure
05 Adventure
06 Adventure
07 Adventure
14 Adventure
18 Adventure
19 Adventure
20 Adventure
Golden Grove
13 Golden Grove
13 Golden Grove
15 Golden Grove
15 Golden Grove
16 Golden Grove
16 Golden Grove
21 Golden Grove
21 Golden Grove
01 Fairplains
06 Fairplains
07 Fairplains
09 Fairplains
3A Barren Spot
05 Barren Spot
06 Barren Spot
07 Barren Spot
23 Barren Spot
23 Barren Spot
24 Barren Spot
24 Barren Spot
25 Barren Spot
25 Barren Spot
31 Barren Spot
31 Barren Spot
3 6 7 814 16 17 18 19 21 21 23 23 24 24 25 25 27 29 30 31 32 33 34 35 36 36 38 38 40 40 41 41
Date
(mm-dd-yy)
03-11-91
03-12-91
03-12-91
03-11-91
04-17-91
03-11-91
07-31-91
03-11-91
02-12-91
04-17-90
01-22-91
04-17-90
01-22-91
04-17-90
01-22-91
04-17-90
01-22-91
01-10-91
01-10-91
01-10-91
01-10-91
01-25-91
01-28-91
01-28-91
01-25-91
04-16-90
01-28-91
04-16-90
01-25-91
04-16-90
01-25-91
01-28-91
07-24-91
Nitro
gen,
N02+N0
3 dis
solved
(mg/L
as N) __ -- __ -- --
1.2 --
2.7
2.8
3.2
3.3 -- _- -- --
5.2
6.1
5.2
6.3
Man-
Iron,
ganese
dis-
dis
solved
solved
(|lg/L
(|lg
/Las Fe)
310 103 6 5 9 5
123 3
103
103 9 3
20 10 10 10 10 10 10 10 20 1020
0 10 40 10 14 20
as Mn
)
210 101 2 6 1 1 1 1 1
101
101 1 1
100 10 30
340 10 10 10 10 10 10 10 10 10 10 23 10
Barium
dis
solved
(ug/L
as Ba
)
2710
0100 31 21 40 24 39 48 46 32
100 20
100 21 47 42
400
100
100
100
100
100
100
100
100
100
100
100
100
100
110
100
Alumi
nium
dis
solved
(ug/
Las
Al
)
<10 15 12 10
<10 20
<10 10
<10 10 12 10
<10 10 14
<10
<10
<10
<10 11
<10 10
<10 10
<10 10 11
<10 20
Arsenic
dis
solved
(ug/L
as As
)
1 1 1 1 1 1 1 1 1
1
1
1
1 1 1 1 1 1 2 3 1--
1--
1
1 1 1
Cadmium
dis
solved
(ug/
Las
Cd
)
1 1 1 2 1 1 1 1 1
1
1
1
1 2 1 1 1 1 1 1 1--
1--
1
1 1 1
Chro
mium
dis
solved
(ug/L
as Cr)
1 1 1 1 1 1 1 1 1
1
1
1
1 6 1 1 1 1 1 2 2--
2--
1
1 1 1
Tab
le 4
. C
hem
ical
ana
lyse
s of
gro
und-
wat
er s
ampl
es f
rom
wel
ls i
n so
uth-
cent
ral
St.
Cro
ix,
U.S
. V
irgin
Isl
ands
-Con
tinu
ed
[fig
, fig
ure;
mm
-dd-
yy, m
onth
-day
-yea
r; |lg
/L, m
icro
gram
s pe
r lit
er; V
IWA
PA, V
irgin
Isla
nds
Wat
er a
nd P
ower
Aut
horit
y; D
PW,
Dep
artm
ent o
f Pub
lic W
orks
; ,
indi
cate
s da
ta n
ot a
vaila
ble]
o
Well
number
(see
Well
fig
1 name
and
Date
table 1)
(mm-dd-yy)
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
DPW
1VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
VIWAPA
2A
0506
07 14 18 19
20
Adventure
Adventure
Adventure
Adventure
Adventure
Adventure
Adventure
Adventure
3 6 7 814 16 17 18
03-11-91
03-12-91
03-12-91
03-11-91
04-17-91
03-11-91
07-31-91
03-11-91
Mer
cury
dis
solved
(jig/L
as Hg)
0.2 .9 .3 .1 .1 .2 .1 .2
Sel-
nium
dis
solv
ed
(jig/L
as Se
)
1 1 2 2 2 2 1 3
Silver
dis
solved
(u.g/L
as Ag
)
1 1 1 1 1 1 1 1
Lead
dis
solved
(u.g/L
as Pb)
1 1 1 1 1 1 1 1
Golden Grove
13 13 15 15 16 16 21 21 01 06 07 09 3A
05 06 07 23 23 24 24 25 25 31 31
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Golden Grove
Fairplains
Fairplains
Fairplains
Fairplains
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
Barren Spot
19 21 21 23 23 24 24 25 25 27 29 30 31 32 33 34 35 36 36 38 38 40 40 41 41
02-12-91
04-17-90
01-22-91
04-17-90
01-22-91
04-17-90
01-22-91
04-17-90
01-22-91
01-10-91
01-10-91
01-10-91
01-10-91
01-25-91
01-28-91
01-28-91
01-25-91
04-16-90
01-28-91
04-16-90
01-25-91
04-16-90
01-25-91
01-28-91
07-24-91
.1 .3 .1 .8 .1
1.6 .2 .1 .1 .1 .1 .1 .1 .2 .3 .1 .2 .1
3
4
4
9
6 4 4 3 3 6 5 3 2
5
5
3 1 4
1
1
1
1
1 2 1 1 1 1 1 1 1
1
1
1 1 1
1
1
1
1
1 2 1 1 1 1 1 1 1
1
1
2 1 1
Tabl
e 5.
C
hlor
ide
conc
entra
tion,
spe
cific
con
duct
ance
, an
d w
ater
leve
ls fo
r th
e U
.S.
Virg
in I
slan
ds W
ater
and
Pow
er A
utho
rity'
s (V
IWA
PA
) m
unic
ipal
wel
ls
in t
he A
dven
ture
wel
l fie
ld,
St.
Cro
ix
Hydrogeology of
Soul 1 sL 0) o o. c 0) 5: 5' 5T £U Q.
(A
[mm
-dd-
yy, m
onth
-day
-yea
r; m
g/L
, mill
igra
ms
per
liter
; |iS
/cm
, mic
rosi
emen
s pe
r ce
ntim
eter
at 2
5 de
gree
s C
elsi
us;
NA
, SW
L, s
tatic
wat
er le
vel;
WP-
WL
NA
, wel
l pum
ping
-wat
er le
vel n
ot a
vaila
ble]
WE
LL
V
IWA
PA
6,
Adventu
re
Date
(mm
-dd-y
y)
12
-13
-89
12
-13
-89
01
-11
-90
01
-19
-90
01
-31
-90
02
-05
-90
02
-23
-90
02
-26
-90
03
-16
-90
03
-26
-90
04
-27
-90
06
-01
-90
06
-27
-90
06
-26
-90
08-0
1-9
108-2
9-9
109-2
7-9
110-3
0-9
112-0
3-9
10
2-0
8-9
20
2-0
5-9
20
3-1
3-9
2
04
-28
-92
05
-29
-92
07
-02
-92
07
-28
-92
09
-15
-92
Ch
lori
de
concen
trati
on
(mg/
L)
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
29
2
NA
256
26
8
276
28
0
30
0
276
28
0
27
2
28
0
NA
27
4
23
4
Specif
ic
con
du
cta
nce
(fie
ldv
alu
e)l
S/c
m)
2,5
00
2,5
00
NA
NA
1,
650
NA
1,
820
1,
900
NA
1,7
90
1,
840
NA
NA
2,0
40
NA
2,0
90
2,0
30
2,0
90
2,0
80
2,
060
2,0
10
2,0
30
2,
03
02,
10
02
,07
0
1,
890
2,
06
0
Wat
erle
vel
(in
feet
bel
ow
lan
dsu
rface)
Com
men
ts
27 .
65
27.6
527
.34
26
.45
17
.60
N
A50.1
939.5
735.4
540.6
51
9.3
318.2
51
8.6
9N
A17.3
82
6.4
2
26.9
7N
AN
AN
AN
AN
A
51 .
23
50
.74
50
.46
58
.05
58
.61
PWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
PWL
PWL
SWL
SWL
SWL
WP
-WL
NA
SWL
PWL
PWL
WP
-WL
NA
WP
-WL
NA
WP
-WL
NA
WP
-WL
NA
WP
-WL
NA
PWL
PWL
PWL
PWL
PWL
data
not
ava
ilabl
e; P
WL
,pum
ping
wat
er le
vel;
WE
LL
V
IWA
PA
7
, A
dv
en
ture
Sp
ecif
ic
con-
Ch
lori
de
du
cta
nce
Date
(mm
-dd-y
y)
12
-13
-89
01
-11
-90
01
-19
-90
01-3
1-9
0
02-1
5-9
0
02-2
3-9
00
3-1
6-9
00
3-2
6-9
00
4-2
7-9
00
6-0
1-9
00
6-2
7-9
008-0
1-9
108-2
9-9
109-2
6-9
11
0-3
0-9
112-0
3-9
10
1-0
8-9
20
2-0
4-9
20
3-1
3-9
204-2
8-9
205-2
9-9
207-0
2-9
20
7-2
8-9
20
9-1
5-9
2
concen
trati
on
(mg
/L)
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
176
188
206
206
468
450
220
NA
216
224
214
NA
(fie
ldv
alu
e)l
S/c
m)
1,2
10
NA
NA
1,
650
1,
600
1,4
90
1,0
10
NA
2,0
50
NA
NA
NA
1,8
10
1,7
10
1,7
80
1,7
90
2,
630
2,5
60
1,8
20
NA
1,
870
1,8
60
1,7
00
NA
Wate
rle
vel
(in
feet
belo
wla
nd
surf
ace)
Co
mm
ents
11.5
312.4
51
2.4
9
12.2
513.2
7
33.0
52
9.2
32
9.2
2
12.2
812.3
012.4
212
. 8
5N
AN
A
NA
NA
NA
NA
50
.81
21 .
08
47
.1
9
47 .
69
NA
21
.06
PWL
PWL
PWL
PWL
SWL
PWL
PWL
PWL
SWL
SWL
SWL
SWL
WP
-WL
NA
WP
-WL
NA
WP
-WL
NA
WP
-WL
NA
WP
-WL
NA
WP
-WL
NA
WP
-WL
NA
SWL
PWL
PWL
WP
-WL
NA
SWL
I)
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Water Quality 23
<§
Tab
le 6
. C
hlor
ide
conc
entr
atio
n, s
peci
fic c
ondu
ctan
ce,
and
wat
er le
vels
for
the
U.S
. V
irgin
Isl
ands
Wat
er a
nd P
ower
Aut
horit
y's
(VIW
AP
A)
mun
icip
al w
ells
in
the
Gol
den
Gro
ve w
ell f
ield
, S
t. C
roix
[DPW
, Dep
artm
ent
of P
ublic
Wor
ks;
mm
-dd-
yy, m
onth
-day
-yea
r; m
g/L
, mill
igra
ms
per
liter
; |iS
/cm
, mic
rosi
emen
s pe
r ce
ntim
eter
at
25 d
egre
es C
elsi
us;
NA
, dat
a no
t av
aila
ble;
SW
L, s
tatic
wat
er le
vel;
PWL
, pum
ping
wat
er le
vel;
WP
-WL
NA
, wel
l pum
ping
-wat
er le
vel n
ot a
vaila
ble]
>logy of
South o <D 1 CO o
3. c <a 5' 5f 0) Q.
(A
WELL--VIWAPA
Date
(mm-dd-yy)
03-16-90
03-26-90
04-27-90
05-29-90
06-26-90
08-07-90
08-27-90
10-04-90
11-01-90
11-26-90
01-03-91
02-05-91
03-04-91
03-25-91
05-01-91
06-17-91
08-01-91
08-29-91
09-25-91
10-24-91
12-03-91
01-08-92
02-05-92
03-13-92
04-30-92
05-27-92
06-26-92
07-28-92
09-15-92
Chloride
concen
tration
(mg/
L) NANA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
330
270
266
260NA
274
280NANANANA
NA
NA
NA
NA
1 , Golden
Specific
con
ductance
(fie
ldvalue
(IS/
cm)
NA
NA
1,500
NANA
NA
NA
NA
NANA
NA
NA
1,840
1, 900
1,870
1, 850
2,000
1, 980
NA
1, 990
2,000
NA
NA
NANA
NA
NA
NANA
Grove,
(DPW
Water
level
(in
feet
below
land
surface)
30.40
30.28
47.86
31.83
47.73
40.35
43.38
32.99
20.54
19.05
20.33
47.36
47 .86
47.62
47.84
NA
48.03
47.80
30.78
NANA
33.20
33.72
35.38
36.30
34 .99
33.85
34.41
35.72
1) Comments
SWL
SWL
PWL
SWL
SWL
PWL
PWL
PWL
SWL
SWL
SWL
PWL
PWL
PWL
PWL
WP-WLNA
PWL
PWL
SWL
WP-WLNA
WP-WLNA
PWL
PWL
SWL
SWL
SWL
SWL
SWL
SWL
Date
(mm-dd-yy)
12-13-89
01-11-90
01-19-90
02-01-90
02-22-90
03-16-90
03-26-90
04-17-90
05-29-90
06-26-90
08-07-90
08-28-90
11-01-90
11-26-90
01-03-91
02-05-91
03-04-91
03-25-91
05-01-91
06-17-91
08-01-91
08-29-91
09-25-91
10-24-91
12-03-91
01-08-92
02-05-92
03-13-92
04-30-92
05-27-92
06-26-92
07-28-92
09-15-92
WELL--VIWAPA
Chloride
concen
tration
(mg/
L)
NANA
NA
NANA
NA
NA
NA
NANANANA
250NANA
NANA
272
290
280
294
286
292 NA
304
310
294
330
290 NA
NA
NANA
13,
Golden Grove
Specific
con
ductance
(fie
ldvalue
(IS/
cm)
1,80
0NANA
1, 650
1,850
NA
1,730
1,740
2,110
NANANA
1,340
1,330
NA
1,570
1,730
1, 810
1, 810
1, 800
1, 930
1, 980
1, 990
NA
1, 980
1, 940
1, 940
1,930
1, 960
NA
NA
NANA
Water
level
(in
feet
below
land
surface)
32.80
34.80
34.98
28.00
42.80
42. 99
42.63
40.01
34.13
34.11
44.6
443.99
NA
NA
25.75
26.10
30.13
31.75
32.58
34.20
34.24
41.33
43.56
32.73
47 .37
48.31
53.24
50.87
54.55
35.71
34.20
34.85
35.89
Comments
PWL
PWL
PWL
SWL
PWL
PWL
PWL
PWL
SWL
SWL
PWL
PWL
WP-WLNA
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
PWL
PWL
PWL
SWL
SWL
SWL
SWL
Tab
le 6
. C
hlor
ide
conc
entr
atio
n, s
peci
fic c
ondu
ctan
ce,
and
wat
er le
vels
for
the
U.S
. V
irgin
Isl
ands
Wat
er a
nd P
ower
Aut
horit
y's
(VIW
AP
A)
mun
icip
al w
ells
in
the
Gol
den
Gro
ve w
ell f
ield
, S
t. C
roix
-Con
tinue
d[m
m-d
d-yy
, m
onth
-day
-yea
r; m
g/L
, m
illig
ram
s pe
r lit
er;
(iS/
cm,
mic
rosi
emen
s pe
r ce
ntim
eter
at
25 d
egre
es C
elsi
us;
NA
, da
ta n
ot a
vaila
ble;
SW
L,
stat
ic w
ater
leve
l; PW
L, p
umpi
ng
wat
er le
vel;
WP-
WL
NA
, w
ell
pum
ping
-wat
er le
vel n
ot a
vaila
ble]
WELL VIWAPA 15,
Water
Quality ro Ol
Date
(mm-dd-yy)
12-13-89
01-11-90
01-19-90
01-30-90
02-22-90
03-16-90
03-26-90
04-17-90
05-29-90
06-26-90
08-07-90
08-28-90
10-04-90
10-30-90
11-26-90
01-03-91
02-05-91
03-04-91
03-25-91
05-01-91
06-17-91
08-01-91
08-29-91
09-25-91
10-24-91
12-03-91
01-08-92
02-05-92
03-13-92
04-30-92
05-27-92
06-26-92
07-28-92
09-15-92
Chloride
concen
tration
(mg/L)
NANANA
NANA
NANA
NA
NA
NA
NA
384NA
358NA
344NA
370
347
326
342
340
330
330
350
346
360
336
340
314
300
308
300
NA
Specific
con
ductance
(field
value
H-S/cm)
2, 400
2, 400
NA
2, 450
2,260
NA
2,210
2,300
2, 800
NA
2,280
2,440
2,370
2, 010
2, 050
NA
1,280
2, 010
2,200
2,280
2,290
2,370
2, 440
2, 400
2,370
2,390
2, 410
2,350
2,410
2,360
2, 150
2,220
2, 120
NA
Golden Grove
Water
level
(in
feet
below
land
surface)
52.06
48.66
50.80
49.53
52.72
52 .22
49.38
50.45
36.99
37.71
55.32
53.03
53.99
48.83
39.90
38.20
39.18
39.40
41.20
40.89
42.02
45. 12
49.81
53.39
NA
NANA
NA
NA
51 .16
40.43
38.53
39.15
40.46
Comments
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
WP-WLNA
WP-WLNA
WP-WLNA
WP-WLNA
WP-WLNA
PWL
PWL
PWL
PWL
SWL
WELL VIWAPA 16,
Date
(mm-dd-yy)
12-13-89
01-11-90
01-19-90
01-30-90
02-22-90
03-16-90
03-26-90
04-17-90
05-29-90
06-26-90
08-07-90
08-28-90
10-04-90
11-01-90
11-26-90
01-03-91
02-05-91
03-04-91
03-25-91
05-01-91
06-17-91
08-01-91
08-29-91
09-25-91
10-24-91
12-03-91
01-08-92
02-05-92
03-13-92
04-30-92
05-27-92
06-26-92
07-28-92
09-15-92
Chloride
concen
tration
(mg/
L)
NA
NANA
NANANA
NANA
NANA
NA
332NA
294NA
308NA
290
NA
264
314
310
306
NA
310
310
320
280
300
NA
310
322
296
306
Specific
con
ductance
(field
value
(IS/cm)
1,800
NANA
2,100
1, 900
NA
1, 970
2, 020
2,090
NA
2,000
2,350
1, 830
2,050
1,720
NA
1,380
2, 000
NA
2, 050
1,860
2, 070
2,240
NA
2,220
2,200
2,210
2,160
2, 180
NA
2, 150
2,230
2, 160
2,240
Golden Grove
Water
level
(in
feet
below
land
surface)
56.72
52.74
44.12
48.72
55.05
52.14
45.76
56.18
37.47
40.18
62 .23
58.92
60.24
43.52
39.62
41.24
42.26
41.99
31.55
44.27
45.34
51.38
55.52
40.62
57.90
65.45
NA
NA
NA
43 .77
61.48
64.53
60.28
67.48
Comments
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
WP-WLNA
WP-WLNA
WP-WLNA
SWL
PWL
PWL
PWL
PWL
M Ta
ble
6.
Chl
orid
e co
ncen
tratio
n, s
peci
fic c
ondu
ctan
ce,
and
wat
er le
vels
for
the
U.S
. V
irgin
Isl
ands
Wat
er(V
IWA
PA
) an
d P
ower
Aut
horit
y's
mun
icip
al w
ells
in t
he G
olde
n G
rove
wel
l fie
ld,
St.
Cro
ix-C
ontin
ued
I
[mm
-dd-
yy, m
onth
-day
-yea
r; m
g/L
, mill
igra
ms
per
liter
; |iS
/cm
, mic
rosi
emen
s pe
r ce
ntim
eter
at 2
5 de
gree
s C
elsi
us;
NA
, dat
a no
to.
av
aila
ble;
SW
L, s
tatic
wat
er le
vel;
PWL
, pum
ping
wat
er le
vel]
o <Q
<D
O
.
<§ >< a I I I (A
O 3 _x c (A < <3 5' £
o> Q.
(A
WELL VIWAPA 21,
Golden Grove
Date
(mm-dd-yy)
02-22-90
03-16-90
03-26-90
04-17-90
05-29-90
06-26-90
08-07-90
08-28-90
10-04-90
11-01-90
11-26-90
01-03-91
02-05-91
03-04-91
03-25-91
05-01-91
06-17-91
08-01-91
08-29-91
09-25-91
10-24-91
12-03-91
01-08-92
02-05-92
03-13-92
04-30-92
05-27-92
06-26-92
07-28-92
09-15-92
Chloride
concen
t ration
(mg/
L)
NA
NANANA
NA
NA
NA
284NA
234 NA
250NA
245
248
254
256
290
270
270
286
276
290
280
268
260
276
268
270
280
Specific
con
ductance
(field
value
)lS/cm)
1, 710
NA
1,730
1, 790
1,750
NA
1,800
2,140
1, 630
1,100
1,250
NA
1,780
1,780
1, 810
1,720
1,770
1, 910
1, 930
1, 940
1, 940
1,880
NA
NA
1, 920
1, 890
1, 900
1, 970
1,850
1,990
Water
level
(in
feet
below
land
surface)
40 39 37 38 34 35 35 42 39 30 26 27 29 30 31 32 34 32 38 37 38 35 36 37 44 44 42 40 41 42
.80
.80
.47
.24
.24
.87
.92
.13
.87
.44
.67
.39
.69
.84
.47
.61
.10
.90
.67
.91
.05
.67
.31
.07
.00
.27
.24
.44
.16
.07
Comments
PWL
PWL
PWL
PWL
SWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
Tabl
e 7.
C
hlor
ide
conc
entra
tion,
spe
cific
con
duct
ance
, an
d w
ater
leve
ls fo
r th
e U
.S.
Virg
in I
slan
ds W
ater
and
Pow
er A
utho
rity'
s (V
IWA
PA
) m
unic
ipal
wel
ls
in t
he F
airp
lain
s w
ell f
ield
, St
. C
roix
[mm
-dd-
yy,
mon
th-d
ay-y
ear;
mg/
L, m
illig
ram
s pe
r lit
er;
|0,S
/cm
, mic
rosi
emen
s pe
r ce
ntim
eter
at 2
5 de
gree
s C
elsi
us;
NA
, da
ta n
ot a
vaila
ble;
WP-
WL
NA
, w
ell
pum
ping
- w
ater
lev
el n
ot a
vaila
ble;
PW
L, p
umpi
ng w
ater
lev
el;
SWL
, st
atic
wat
er le
vel]
WELL VIWAPA 7,
Date
(mm-dd-yy)
06-27-91
07-25-91
08-29-91
09-25-91
10-23-91
01-07-92
02-04-92
03-12-92
04-30-92
05-28-92
07-01-92
07-27-92
09-14-92
Chloride
concen
tration
(mg/L)
850
780
800NANANA
NA
1,070
848
NA 870
850
NA
Specific
con
ductance
(field
value
(J,S
/cm)
3,920
3,750
3,850
NANANA
NA
4,530
4, 050
NA
4,080
4, 060
NA
Fairplains
Water
level
(in
feet
below
land
surface)
NA
48.52
48.59
25.82
28.89
48.12
48.13
48.12
48.11
3.15
48.13
48.12
4.63
Comment s
WP-WLNA
PWL
PWL
SWL
SWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
SWL
WELL --VIWAPA 8,
Date
(mm-dd-yy)
06-18-91
07-25-91
08-28-91
09-25-91
10-23-91
12-03-91
01-07-92
02-04-92
03-12-92
04-30-92
05-28-92
07-01-92
07-27-92
09-14-92
Chloride
concen
tration
(mg/L)
1,390
1,490
1,550
NA
1,530
1, 608
1, 670
1, 650
1,700
2,050
1,372
1, 698
NA
1,340
Specific
con
ductance
(field
value
US/cm)
5, 640
5,700
6,080
NA
5,830
5, 890
6,110
6, 110
6,430
7, 170
5,240
6,450
NA
5,480
Fairplains
Water
level
(in
feet
below
land
surface)
55.51
60.41
62.97
27.03
61.44
63.93
63.23
64.06
57.48
30.48
24.90
29.40
NA
24.98
Comments
PWL
PWL
PWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
No Access
PWL
0 QJ
Hydrogeology of
South-Central
CO p
o. c (O 5' 5T Q) 3 Q.
W
Tab
le 8
. C
hlor
ide
conc
entr
atio
n, s
peci
fic
cond
ucta
nce,
and
wat
er le
vels
for
the
U.S
. Virg
in I
slan
ds W
ater
and
Pow
er A
utho
rity'
s (V
IWA
PA)
mun
icip
al w
ells
in
the
Bar
ren
Spot
wel
l fie
ld,
St.
Cro
ix
[mm
-dd-
yy,
mon
th-d
ay-y
ear;
mg/
L,
mill
igra
ms
per
liter
; |j.
S/cm
, mic
rosi
emen
s pe
r ce
ntim
eter
at
25 d
egre
es C
elsi
us;
NA
, dat
a no
t av
aila
ble;
WP-
WL
NA
, w
ell p
umpi
ng-
wat
er le
vel
not
avai
labl
e; P
WL
, pum
ping
wat
er l
evel
; SW
L,
stat
ic w
ater
lev
el]
WE
LL
V
IWA
PA
3
A,
Date
(mm
-dd
-yy)
02
-26
-90
03
-26
-90
0
4-2
5-9
00
5-2
1-9
00
6-2
5-9
00
8-0
6-9
00
8-2
9-9
01
0-0
4-9
01
0-3
0-9
01
1-1
9-9
01
2-2
7-9
00
2-0
6-9
10
3-0
5-9
10
3-2
5-9
10
5-0
2-9
10
6-2
0-9
10
7-2
4-9
10
8-2
7-9
10
9-2
3-9
11
0-2
2-9
11
1-2
5-9
10
1-0
9-9
20
2-0
4-9
20
3-1
2-9
20
4-2
7-9
20
5-2
8-9
20
7-0
1-9
20
7-2
7-9
20
9-1
4-9
2
Ch
lori
de
co
ncen
trati
on
(mg
/L)
NA
N
A
NA
NA
NA
NA
78
0N
A874
NA
79
6N
AN
AN
A820
800
79
88
10
NA
78
0792
850
NA
NA
850
840
836
860
870
Specif
ic
con
ducta
nce
(fie
ld
valu
e(a
s/cm
)
3,5
20
3,
14
0
3,
37
03
,54
0N
A3,
46
03,
83
02,
690
3,
15
03,
83
0N
A3,
900
NA
NA
3,5
40
3,
80
03,
83
03,
94
0N
A3,
930
3,
940
3,
940
NA
NA
4,0
50
4,0
90
4,
03
04,
11
04,
10
0
Barr
en
S
po
t
Wate
r le
vel
(in
feet
belo
w
lan
dsu
rface)
NA
N
AN
A6
3.3
76
0.2
66
6.3
26
6.0
26
6.5
06
2.8
25
8.
9264
.44
65 .
25
47.9
06
5.3
86
5.1
36
5.8
667
. 6
566.1
56
5.3
26
5.3
06
5.2
36
6.
055
9.7
36
2.6
162.7
96
2.9
462
.72
62
.12
62
.98
Com
men
t s
WP-
WL
NA
W
P-W
LN
A
WP-
WL
NA
PWL
SWL
PWL
PWL
PWL
PWL
PW
L
PW
L
PW
L
SW
L
PW
L
PW
L
PW
L
PW
L
PW
L
SWL
PW
L
PW
L
PW
L
SWL
SWL
PW
L
PW
L
PW
L
PW
L
PW
L
WE
LL
V
IWA
PA
5
, B
arr
en
S
pot
Date
(mm
-dd
-yy
)
12-1
3-8
9
02-2
6-9
0
02
-26
-90
03-2
6-9
00
4-2
5-9
00
5-2
1-9
006-2
5-9
00
8-0
6-9
00
8-2
9-9
01
0-0
4-9
010-3
0-9
011-1
9-9
01
2-2
7-9
002-0
6-9
103-0
5-9
10
3-2
5-9
10
5-0
2-9
10
6-2
0-9
107-2
4-9
108-2
7-9
10
9-2
3-9
11
0-2
2-9
11
1-2
5-9
101-0
9-9
202-0
4-9
20
3-1
2-9
204-2
7-9
205-2
8-9
207-0
1-9
207-2
7-9
209-1
4-9
2
Chlo
ride
concen
trati
on
(mg
/L)
NA
N
A
NA
NA
NA
NA
NA
NA
NA
NA
508
NA
500
NA
NA
704
484
510
504
520
500
510
522
522
546
516
516
506
510
NA
500
Sp
ecif
ic
con
du
cta
nce
(fie
ld
valu
e(i
S/c
m)
NA
2,
620
NA
2,
800
NA
2,7
90
2,7
80
2,7
90
2,
880
2,
830
2,
840
2,8
60
NA
2,
880
NA
2,8
90
2,
810
2,
970
3,
000
3,0
60
3,0
90
3,0
50
3,
090
3,
080
3,0
90
3,0
70
3,0
80
3,
100
3,0
40
NA
3,0
30
Wat
er
level
(in
feet
bel
ow
la
nd
surf
ace)
72.4
2
70.1
4
70
.16
70
.74
77
.12
77.5
579.8
379.2
979.1
078.
977
6.8
17
7.0
47
9.2
377.1
56
7.9
57
6.6
877
.02
77.7
477
.95
78
.22
76.7
277
.4
377.6
67
8.0
877.4
97
8.3
378.8
579.0
578.6
268.1
078.5
2
Co
mm
ents
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
Tabl
e 8.
C
hlor
ide
conc
entra
tion,
spe
cific
con
duct
ance
, an
d w
ater
leve
ls fo
r th
e U
.S.
Virg
in I
slan
ds W
ater
and
Pow
er A
utho
rity'
s m
unic
ipal
wel
ls in
the
B
arre
n S
pot w
ell f
ield
, S
t. C
roix
-Con
tinue
d[m
m-d
d-yy
, mon
th-d
ay-y
ear;
mg/
L, m
illig
ram
s pe
r lit
er;
(iS/
cm, m
icro
siem
ens
per
cent
imet
er a
t 25
degr
ees
Cel
sius
; N
A, d
ata
not
avai
labl
e; P
WL
, pum
ping
wat
er le
vel;
SWL
, sta
tic w
ater
le
vel]
WELL VIWAPA 6,
Q) fl? o c £.
5'
Date
(mm-dd-yy)
07-05-90
07-05-90
08-06-90
08-29-90
10-04-90
10-30-90
11-19-90
12-27-90
02-06-91
03-05-91
03-25-91
05-02-91
06-20-91
07-24-91
08-27-91
09-23-91
10-22-91
11-25-91
01-09-92
02-04-92
03-12-92
04-27-92
05-28-92
07-01-92
07-27-92
09-14-92
Chloride
concen
tration
(mg/L)
NANANA
304NA
416NANA
NANA
384
320
354
354
342
368
340
350
340
354
330
318NA
NANA
NA
Specific
con
ductance
(field
value
(iS/
cm)
2,019
2,019
2,019
2,090
2, 115
2,300
2,325
NA
2,328
NA
2,356
2,294
2,366
2,347
2,375
2,470
2,384
2,375
2,380
2,390
2,280
2,330
NANANA
NA
Barren Spot
Water
level
(in
feet
below
land
surface)
76.63
76.63
78.81
76.55
76.39
75.07
75.24
75.30
75.28
74 .77
75.14
75.95
76.28
76.46
76.58
75.92
75.94
76.06
76.44
76.28
76.72
76.79
76.05
75.76
75.13
75.71
Comment s
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
SWL
SWL
WELL VIWAPA 7, Barren Spot
Date
(mm-dd-yy)
05-21-90
06-25-90
08-06-90
08-29-90
10-04-90
10-30-90
11-19-90
12-27-90
02-06-91
03-05-91
03-25-91
05-01-91
06-20-91
07-24-91
08-27-91
09-23-91
10-22-91
11-25-91
01-09-92
02-04-92
03-12-92
04-27-92
05-28-92
07-01-92
07-27-92
09-14-92
Chloride
concen
tration
(mg/L)
NA
NANA
458NA
480NA
436NANA
NA
490
520
530
524
478NA
460
468
450
440
436NA
448
442
454
Specific
con
ductance
(fie
ldvalue
US/c
m)
2,835
2,565
2,477
2, 470
1, 936
2,562
2, 602
NA
NANA
NA
2,716
2, 839
2,717
2, 889
2,792
NA
2, 688
2, 630
2,560
2,540
2, 610
NA
2,580
2,520
2, 620
Water
level
(in
feet
below
land
surface)
55.42
55.58
55.47
55.27
52.84
54.41
54.88
54.52
55.04
52.45
55.58
53.32
55.64
58.15
55.77
55.63
52.46
55.66
55.78
55.82
55.96
55.32
53.11
53.74
53.73
53.69
Comments
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
PWL
Tab
le 8
. C
hlor
ide
conc
entr
atio
n, s
peci
fic c
ondu
ctan
ce,
and
wat
er le
vels
for
the
U.S
. V
irgin
Isl
ands
Wat
er a
nd P
ower
Aut
horit
y's
mun
icip
al w
ells
in t
he
Bar
ren
Spo
t wel
l fie
ld,
St.
Cro
ix-C
ontin
ued
Hydrogeology of
Sout 3- 3 S_ (/) p o. c j° 5: 5' 5T 0) Q.
W
[mm
-dd-
yy,
mon
th-d
ay-y
ear;
mg/
L,
mil
ligr
ams
per
lite
r; j
iS/c
m,
mic
rosi
emen
s pe
r ce
ntim
eter
at
25 d
egre
es C
elsi
us;
NA
, av
aila
ble;
PW
L, p
umpi
ng w
ater
lev
el;
SW
L,
stat
ic w
ater
lev
el;
US
GS
, U
.S.
Geo
logi
cal
Sur
vey]
Date
(mm
-dd-y
y)
12-1
3-8
90
1-1
1-9
00
1-3
0-9
00
2-2
6-9
003-1
5-9
0
03-2
6-9
0
04
-16
-90
05
-25
-90
06
-25
-90
08
-06
-90
08
-29
-90
10-0
4-9
010-3
0-9
01
1-1
9-9
01
2-2
7-9
002-0
6-9
10
3-0
5-9
10
3-2
5-9
105-0
2-9
10
6-2
0-9
106-2
4-9
10
8-2
7-9
109-2
3-9
110-2
2-9
11
1-2
5-9
101-0
9-9
20
2-0
4-9
203-1
2-9
204-2
7-9
20
5-2
8-9
207-0
1-9
207-2
7-9
20
9-1
4-9
2
WE
LL
V
IWA
PA
25,
Specif
iccon-
Ch
lori
de
du
cta
nce
co
ncen
- (f
ield
trati
on
valu
e(m
g/L
) |lS
/cm
)
NA
1
,13
0N
A
NA
NA
N
AN
A
3,0
00
NA
N
AN
A
NA
N
A
3,3
50
NA
3
,14
0N
A
3,5
20
NA
3,4
10
672
1,9
20
NA
2
,47
0N
A
NA
NA
2
,64
0N
A
NA
NA
2,7
40
NA
N
AN
A
NA
534
2,9
40
57
0
3,0
90
58
0
3,0
60
562
3,1
60
572
3,3
80
580
3,2
50
59
0
3,2
50
57
6
3,1
70
580
3,1
60
568
3,0
70
514
3,0
60
514
3,0
60
534
3,0
20
540
2,9
60
510
3,0
20
Barr
en
S
pot,
Wate
r le
vel
(in
feet
belo
wla
nd
su
rface)
52
.06
52.1
253.3
353.4
353.6
052.3
1
53.7
557.6
258.4
16
7.0
159.7
059.2
058.7
856.2
056.5
755.3
254.9
455.1
657.5
957.9
858.1
858.3
257.4
557.5
157.7
258.1
757.9
85
8.1
657.9
358.2
757.9
657.8
357.9
9
(US
GS
-22)
Com
men
ts
PWL
PWL
PWL
PWL
SWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
PWL
SWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
data
not
ava
ilab
le;
WP
-WL
NA
WEL
L V
IWA
PA
24,
Date
(mm
-dd-y
y)
12-1
3-8
90
1-1
1-9
00
1-3
0-9
002-2
6-9
003-1
5-9
0
03-2
6-9
0
04
-16
-90
05
-21
-90
06
-25
-90
08
-06
-90
08
-29
-90
10-0
4-9
010-3
0-9
011-1
9-9
012-2
7-9
00
2-0
6-9
103-0
5-9
10
3-2
5-9
10
5-0
2-9
106-2
0-9
10
7-2
4-9
108-2
7-9
109-2
3-9
110-2
2-9
11
1-2
5-9
10
1-0
9-9
202-0
4-9
20
3-1
2-9
204-2
7-9
205-2
8-9
20
7-0
1-9
20
7-2
7-9
209-1
4-9
2
Chlo
ride
concen
trati
on
(mg
/L)
NA
NA
NA
NA
NA
NA
N
AN
AN
AN
AN
AN
A674
NA
678
NA
NA
726
NA
700
682
704
NA
900
900
738
740
700
676
650
674
690
680
Barr
en
Spot,
Specif
iccon
du
cta
nce
(fie
ldv
alu
e|lS
/cm
)
2,5
00
NA
3,8
00
NA
NA
NA
3,
85
0N
AN
AN
AN
AN
A3,2
80
3,5
60
NA
1,2
50
NA
3,5
00
NA
3,5
60
3,
490
3,
610
NA
4,
320
4,4
30
3,
690
3,
680
3,
630
3,5
30
3,5
30
NA
3,5
80
3,5
30
, wel
l pu
mpi
ng-w
ater
leve
l no
t
(US
GS
-23)
Wate
r le
vel
(in
feet
belo
wla
nd
su
rface)
56.9
956.7
056.8
356.5
157.2
156.8
9
57.3
86
4.0
06
0.5
558.7
96
7.2
06
6.8
56
4.0
057.7
06
9.7
567.4
658.4
066.
65N
A70.7
370.4
0N
A57.9
558.7
05
8.
9363.0
562.6
763.0
462.8
16
3.0
86
2.7
362.5
462.8
5
Com
men
ts
PWL
PWL
PWL
SWL
PWL
PWL
"P
WL
PWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
WP-
WL
NA
PWL
PWL
WP-
WL
NA
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
CO
<D o CO
Tab
le 8
. C
hlor
ide
conc
entr
atio
n, s
peci
fic c
ondu
ctan
ce,
and
wat
er le
vels
for
the
U.S
. V
irgin
Isl
ands
Wat
er a
nd P
ower
Aut
horit
y's
mun
icip
al w
ells
in t
he
Bar
ren
Spo
t w
ell
field
, S
t. C
roix
-Con
tinue
d[m
m-d
d-yy
, mon
th-d
ay-y
ear;
mg/
L, m
illig
ram
s pe
r lit
er;
|J,S
/cm
, mic
rosi
emen
s pe
r ce
ntim
eter
at
25 d
egre
es C
elsi
us;
NA
, dat
a no
t av
aila
ble;
PW
L, p
umpi
ng w
ater
leve
l; SW
L, s
tatic
wat
er
leve
l; U
SGS,
U.S
. G
eolo
gica
l Su
rvey
]
WELL VIWAPA
Date
(mm-dd-yy)
01-30-90
02-26-90
02-26-90
03-15-90
03-26-90
04-16-90
05-21-90
06-25-90
08-06-90
08-29-90
10-04-90
10-30-90
11-19-90
12-27-90
02-06-91
03-05-91
03-25-91
05-02-91
06-20-91
07-24-91
08-27-91
09-23-91
10-22-91
11-25-91
01-09-92
02-04-92
03-12-92
04-27-92
05-28-92
07-01-92
07-23-92
09-14-92
Chloride
concen
tration
(mg/
L) NANANA
NANA
NA
NANANA
686NA
668NA
680NA
NA
700
726
736
726
720NA
730
746NA
NANA
750
724
740
718NA
23,
Barren
Specific
con
ductance
(field
value
jlS/cm)
3,200
3, 100
NANA
3,070
3,010
3,200
3,350
3,220
3,280
2,760
2,230
3,360
NA
3, 630
NA
3,440
3,380
3, 650
3,650
3,730
NA
3,720
3,730
NA
NANA
3,750
3,750
3, 670
3,680
3, 620
Spot,
Water
level
(in
feet
below
land
(USGS-25)
surface)
Comments
65.37
66.92
NA
65.72
65.53
66.20
69.77
72.17
70.59
70.34
70.38
68.03
68.16
69.55
68.82
64.23
67.24
68.37
69.29
69.49
69.88
64.34
66.27
66.37
65.75
64.84
65.82
70.60
70.70
70.37
68.92
70.46
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
PWL
PWL
PWL
SWL
PWL
PWL
SWL
SWL
SWL
PWL
PWL
PWL
PWL
PWL
WELL- -VIWAPA
Date
(mm-dd-yy)
05-21-90
06-25-90
08-06-90
08-29-90
10-04-90
10-30-90
11-19-90
12-27-90
02-06-91
03-05-91
03-25-91
05-01-91
06-26-91
07-24-91
08-27-91
09-23-91
10-22-91
11-25-91
01-09-92
02-04-92
03-12-92
04-27-92
05-28-92
07-01-92
07-27-92
09-14-92
Chloride
concen
tration
(mg/
L) NANA
NA
630NA
NA
NANA
NA
NANA
674
672
672
700
640
630
636
614
600
620
572
550
536
554
550
31,
Barren
Specific
con
ductance
(field
value
US/cm)
3,470
NA
3,550
3,010
3,530
NA
2,460
NA
NA
NANA
3,200
3,410
3,430
3,390
3,400
3,370
3,240
3,250
3,160
3, 150
3,100
3,070
2,960
2,950
3,020
Spot Water
level
(in
feet
below
land
surface)
54.41
51.99
55.79
55.73
51.81
48.53
49.22
48.73
48.83
51.08
51.33
54.03
54.84
54.91
53.72
54.67
54.85
54.65
54.62
54.87
55.00
54.73
55.06
54.46
54.68
54.63
Comments
PWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
SWL
SWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
PWL
Chloride concentrations in ground-water samples collected during this study ranged from 67 to 4,400 mg/L (tables 4, 5, 6, 7, and 8), and the dissolved solids concentrations ranged from 619 to 7,540 mg/L (table 4). The chloride and dissolved solids concentrations in ground-water samples collected during this study exceeded the U.S. Environmental Protection Agency's (EPA) Secondary Maximum Contaminant Levels in drinking water of 250 mg/L chloride and 500 mg/L dissolved solids (U.S. Environmental Protection Agency, 1992) in about 76 and 100 percent, respectively, of the wells sampled. In terms of water supply acceptance, the recommended maximum level for chloride is based largely on taste, with the EPA recognizing that ground water having a chloride concentration as high as 500 mg/L is potable. For dissolved solids the EPA Secondary Maximum Contaminant Level is based on an increase in the mineral taste in the water and possible economic consequences, due to deterioration of plumbing and pipes, because of the high mineral content.
SUMMARY
The freshwater supply on St. Croix, U.S. Virgin Islands, is obtained from ground-water sources and desalinated seawater. Because of the high cost of desalinization, however, the Virgin Islands Water and Power Authority is considering the development of additional ground-water sources. A study of the hydrogeology of several areas in south-central St. Croix - where some geologic and hydrologic data were available - was conducted to obtain information that would be useful in the planning for any future development of ground water.
The subsurface geology of south-central St. Croix is characterized by alluvium and underlying carbonate rocks. Ground water occurs under water-table conditions in the alluvial deposits, the post-Kingshill Carbonates and the Kingshill Limestone. Depth to the top of the water- table aquifer ranges from 5 to 68 feet below land surface. The top of the Jealousy Formation is considered to be the bottom of the water-table aquifers in south-central St. Croix. The top of the Jealousy Formation at the four well
fields ranged from 85 to more than 120 feet below land surface. Well yields in the study areas ranged from less than 5 gallons per minute to 80 gallons per minute. Aquifer specific capacity at four wells in the Adventure well field ranged from 1 to 14 gallons per minute per foot of drawdown. Using Meyers model that relates specific capacity to aquifer transmissivity, aquifer transmissivities were estimated to range from 180 to 3,300 feet squared per day. The wide range of transmissivity values in the Adventure well field probably is due to the presence or absence of intersecting fracture zones within the Kingshill Limestone. Where the density of fracture zones is high, the aquifer transmissivity is large.
The wide range of yields in the study area (less than 5 to 80 gallons per minute) is due to the nonhomogeneity of the aquifer material. The relatively high yields in the
post-Kingshill Carbonates are due to the primary depositional porosity of the shallow-water limestone deposits and interlayered conglomerates and volcanics, as well as to subsequent diagenetic changes within the limestone. Generally, where the post-Kingshill Carbonates is tapped by wells, the yields are in the range of 35 to 80 gallons per minute. In the Kingshill Limestone, well yields range from 5 to 60 gallons per minute, the higher yields are primarily limited to those areas where there is a high density of intersecting fracture zones. The yields in the alluvial deposits are unknown, however, the yields will be limited by the thin, 10- to 30- foot, saturated thickness of the deposits.
Ground water in south-central St. Croix is of the sodium-chloride type. Ground-water samples collected from selected wells had chloride concentrations ranging from 67 to 4,400 milligrams per liter, and dissolved solids concentrations ranging from 619 to 7,540 milligrams per liter. Connate water probably is the source of sodium chloride in the Adventure and Golden Grove well fields. In the Fairplains and Barren Spot well fields, the greater seasonal variations in chloride concentration indicate that the sodium chloride in the ground water at these sites could be due to saltwater intrusion.
32 Hydrogeology of South-Central St. Croix, U.S. Virgin Islands
REFERENCES
Cederstrom, D.J., 1950, Geology and ground-water resources of St. Croix, U.S. Virgin Islands: U.S. Geological Survey Water-Supply Paper 1067, 117 p.
Gerhard, L.C., Frost, S.H., and Curth, P.J., 1978, Stratigraphy and depositional setting, Kingshill Limestone, Miocene, St. Croix, U.S. Virgin Islands: Amerrican Association of Petroleum Geologist Bulletin, v. 62, no. 3, p. 403-418.
Gill, I.P., and Hubbard, D.K., 1986, Subsurface Geology of the St. Croix carbonate rock system: Water Resources Research Institute, College of the Virgin Islands, Technical Report no. 26, 71 p.
Gomez-Gomez, Fernando, 1984, Water resources of the lower Rio Grande de Manati Valley, Puerto Rico: U.S. Geological Survey Water-Resources Investigations Report 83-4199, 42 p.
Graves, R.P., 1992, Geohydrology of the Aguirre and Pozo Hondo areas, southern Puerto Rico: U.S. Geological Survey Water-Resources Investigations Report 91-4124, 43 p.
Jordan, D.G., and Cosner, O.J., 1973, A survey of the water resources of St. Thomas, Virgin Islands: U.S. Geological Survey Open-File Report 72-201, 55 p.
Jordan, D.G., 1975, A survey of the water resources of St. Croix, Virgin Islands: U.S. Geological Survey Open- File Report, 73-137, 51 p.
Lohman, S.W., 1979, Ground-water hydraulics: U.S. Geological Survey Professional Paper 708, 70 p.
Mathey, S.B., 1990, National water information system user's manual, Volume 2, Chapter 4. Ground-Water Site Inventory System: U.S. Geological Survey Open- File Report, 89-587, 283 p.
Meyer, R.R., 1963, A chart relating well diameter, specific capacity, and the coefficient of transmissibility and storage, in Bentall, Ray, compiler, Methods of determining permeability, transmissivity, and drawdown: U.S. Geological Survey Water- Supply Paper 1536-1, p. 338-340.
Piper, A.M., 1953, A graphic procedure in thegeochemical interpretation of water analyses: in U.S. Geological Survey Ground Water Notes, Geochemistry, no. 12, June 1953.
Robison, T.M., 1972, Ground water in central St. Croix, U.S. Virgin Islands: U.S. Geological Survey Open- File Report (unnumbered), 18 p.
Torres-Gonzalez, Sigfredo, and Rodriguez del Rio, Felix, 1990, Potentiometric surface of the Kingshill Aquifer and hydrologic conditions, St. Croix, U.S. Virgin Islands, July 1987: U.S. Geological Survey Water- Resources Investigations Report 89-4085, 1 plate.
Torres-Gonzalez, Sigfredo, 1991, Steady-state simulation of ground-water flow conditions in the Kingshill aquifer, St. Croix, U.S. Virgin Islands, July 1987: in American Water Resources Association International Symposium on Tropical Hydrology, July 23 through 27, 1990, San Juan, Puerto Rico, AWRA Monograph Series No. 15, p. 93-108.
U.S. Department of Commerce, 1982 to 1990,Climatological data annual summary - Puerto Rico and the Virgin Islands: National Oceanic and Atmospheric Administration, v. 28-36.
U.S. Environmental Protection Agency, 1992, Secondary maximum contaminant levels (part 143, National primary drinking water regulations): U.S. Code of Federal Regulations, Title 40, Parts 100 to 149, revised as of July 1,1992, p. 772-776.
References 33