ground water flow in aquifer systems: floridan aquifer case study
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Ground Water Flow in Aquifer Systems: Floridan Aquifer Case Study. Envi 518 September 10, 2002. Global Water Supply. Approximately 29% of the world’s fresh water resources exists in aquifers. Aquifers. Definition: A geological unit which can store and supply significant quantities of water. - PowerPoint PPT PresentationTRANSCRIPT
Ground Water Flow in Aquifer Systems: Floridan Aquifer Case Study
Envi 518September 10, 2002
Approximately 29% of the world’s fresh water resources exists in aquifers
Global Water Supply
Definition: A geological unit which can store and supply significant quantities of water.
Principal aquifers by rock type:
Unconsolidated
Sandstone
Sandstone and Carbonate
Semiconsolidated
Carbonate-rock
Volcanic
Other rocks
Aquifers
Ground water occurs when water penetrates the subsurface through cracks and pores in soil and rock
Ground Water
Natural Precipitation
melting snow
Infiltration by streams and lakes
Transpiration by plants
Artificial Recharge wells
Spread water over land in pits, furrows, ditches, or erect small dams in stream channels to detain and deflect water
Recharge
Hydrologic Cycle – Rainfall in becomes Recharge to the water table
Saturated zonebelow the water table
Water table
Soil zone
Unsaturated zone
Precipitation
Recharge to water table
Evapotranspiration
Infiltration
Runoff
Over Pumping
Cone of Depression
Drawdown
Unhealthy vs. Healthy Lake
PumpingWell
Section 21 Wells
Northern Tampa Bay (NTB)
PASCO
HILLSBOROUGH
PINELLAS
HERNANDO
POLK
N
0 10 Miles
NTB Overpumping IssueInterconnected Regional Well Fields, Southwest Florida Water Management District
0
50000
100000
150000
200000
250000
300000
350000
400000
450000
500000
1930 1940 1950 1960 1970 1980 1990
m3 /day
Cross Bar
Cypress Creek
South Pasco
Section 21
Eldridge Wilde
Cosme Odessa
CROSS BAR RANCH
COSME
SOUTH PASCOCYPRESS CREEK
ELDRIDGE WILDE SECTION 21
NTB Overpumping ImpactsExcessive Groundwater
Pumping has Caused:
Decline in aquifer water levels
Lowered water levels in lakes,wetlands & springs
Formation of sinkholes
Reduced flow in river systems
Seawater intrusion
Dock on Florida Lake in 1970’s Same Dock in 1990
Surface Water Issues
Over 50,000 homeowners in South Pasco and North Hillsborough counties have been hit with massive land subsidence, as a result of over pumping.
Negative Impacts: Sinkholes
Sinkhole Formation• dissolution of soluble carbonate rocks by weakly
acidic water• the process starts in the atmosphere, where rain
falls on the ground and percolates through the soil
• dissolves carbon dioxide gas from the air and soil, forming carbonic acid (H2CO3), a weak acid
• carbonic acid percolates through the ground cover down to the bedrock
• carbonic acid reacts with limestone and dolomite and dissolves these carbonate rocks into component ions of calcium (Ca2+), magnesium (Mg2+) and bicarbonate (HCO3
-).
CO2
H2O
CaCO3
Atmosphere
CoverSediment
CarbonateBedrock
CO2
H2O
H2CO3CaMg(CO3)3
Mg++ Ca++ HCO3-
CO2 + H20 H2CO3
Negative Impacts: Sinkholes#
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0 10 20 Miles N
Sinkholes by year# 1960-1969# 1970-1979# 1980-1984# 1985-1989# 1990-1994# 1995-2000
Wetlands and Lakes in the NTB Area
5 0 5 10 MilesN
NTB CountiesLakesWetlands
Overpumping has negative effects on surface waters as well – wetlands in the area continue to dry out
Negative Impacts: Wetlands
Radius of Influence: South Pasco Wellfield
Camp
Thomas
Lake Levels vs. Pumping
“Thirsty Tampa Bay ponders huge desalination plant” April 20, 2000
• Want to build the largest desalination plant in
the Western Hemisphere
• Projected cost of $100 million
• Could supply about 25 MGD
(about 1/10 of the region's needs)
• Critics are concerned that the high salinity wastewater pumped back into the bay will hurt the environment.
Impact of Pumping on Heads
Eldridge Wilde Mitchell Effect of Pumping on Observed Heads
0
5
10
15
20
25
1100 1600 2100 2600 3100 3600
Time (Days)
Head (Ft)
0
8
16
24
32
40
Q (Ft3/Day)
ObservedHeads
PumpingRates
Rainfall/Recharge
53 rainfall observation pointsMonthly Readings from
January 1989-January 2000
Average Rainfall Observations
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Jan-89 Jan-90 Jan-91 Jan-92 Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98
Time
Inches/Month
1
Rainfall to Recharge
Assigned rain gages to basins & used recharge equation from previous studies:
Rech (node) = Radj * ((Rechss/P(b,ss))*P(b,m)
Radj = Runoff adjustment per basin per month Rechss = Recharge of node in May 1989 Steady State Model P(b,ss) = May 1989 Rainfall per Basin P(b,m) = Rainfall per basin per month
Uses of ModelingA model is designed to represent reality in such a way that
the modeler can do one of several things:
Quickly estimate certain aspects of a system (screening models, analytical solutions, “back of the envelope” calculations)
Determine the causes of an observed condition (contamination, subsidence, flooding)
Predict the effects of changes to the system (remediation, development, waste disposal)
Types of Ground Water ModelsAnalytical Models
1-D solution, Ogata and Banks (1961)
2-D solution, Wilson and Miller (1978)
3-D solutions, Domenico & Schwartz (1990)
Numerical ModelsFlow-only models (MODFLOW)
Transport-only models (MT3D, RT3D, MODPATH, etc.)
Require a coordinated flow model, such as MODFLOW
Combined flow and transport models (BIOPLUME, FEMWATER, FLOTRAN)
NTB Model & MODFLOW Three dimensional finite difference groundwater flow model
(McDonald & Harbaugh, 1988)
Simulates horizontal flow based on following inputs: Aquifer properties - Pumpage
Recharge - Evapotranspiration
River/spring flow - General Head Boundaries
Allows for vertical interchange between layers
Surficial Aquifer
Upper Floridan (1)
Upper Floridan (2)
NTB MODFLOW Model Description
GMS
Encompasses all/or part of five counties
1500 mile2 area
Variable grid spacing (0.25 - 1.0 miles2)
62 Rows & 69 Columns
Three layers
MODFLOW Cell-centered, 3D, finite difference groundwater flow
model
Iterative solver
Initial values of heads are provided
Heads are gradually changed through “time steps” until governing equation is satisfied
Divided into a series of packages
Each package forms a specific task
Each package stored in a separate input file
MODFLOWMODFLOW based on the following partial differential equation for
three-dimensional movement of groundwater of constant density through porous earth material :
Kxx, Kyy, and Kzz = hydraulic conductivity (x, y, and z axis)
h = potentiometric head
W = volumetric flux per unit volume pumped
Ss = specific storage of the porous material
t = time
t
hSW
z
hK
zy
hK
yx
hK
x szzyyxx ∂∂
=−⎟⎠
⎞⎜⎝
⎛∂∂
∂∂
+⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂
∂∂
+⎟⎠
⎞⎜⎝
⎛∂∂
∂∂
Recharge EquationAssigned rain gages to basins & used recharge
equation from previous studies:
Rech (node) = Radj * ((Rechss/P(b,ss))*P(b,m)
Radj = Runoff adjustment per basin per month
Rechss = Recharge parameter in May 1989 Steady State Model
P(b,ss) = May 1989 Rainfall per Basin
P(b,m) = Rainfall per basin per month
Runoff Parameter
Watersheds - Runoff AdjustmentAnclote RiverBrooker CreekCoastal RiversCypress CreekDouble Branch CreekHillsborough RiverPithlachascotee RiverRocky CreekSweetwater CreekWithlacoochee River
5 0 5 10 Miles
N
Runoff AdjustmentsWatersheds
1, 3, 7, 8, 9, 10
2, 4, 5, 6
Jun 0.8 0.7
Jul 0.5 0.4
Aug 0.4 0.3
Sep 0.4 0.3
Oct 0.4 0.3
Nov 0.5 0.4
Dec 0.5 0.5
Jan 0.6 0.5
Feb 0.7 0.6
Mar 0.8 0.7
Apr 1.0 0.9
May 1.0 1.0
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
Calibration Quotient
Thiessen Polygons for May 1989 precipitation
Recharge Parameter for Calibrated Model
5 0 5 MilesN
Recharge Rate Coefficient (ft/day)Low- 0.001Moderate - 0.004High - 0.009
MODFLOW InputsRecharge/Rainfall
Variable parameter, dependent on type of rainfall used in recharge calculation
Pumping Well DataOver 1500 wells used for pumping information
Starting HeadsStarting heads interpolated from May 2000 data, Inverse Distance
Weighted Method
Observation Coverage–544 Observation PointsCreated per layer in the grid for each month; data were obtained from
District monitoring wells
MODFLOW Inputs
Qualitative Analysis
Ending Heads for NxrdRaw run for Layer 2
Qualitative Analysis – NxrdRaw Layer 1
Quantitative Analysis
∑=
−=n
iico hh
nME
1
)(1
i
n
ico hh
nMAE ∑
=
−=1
1
2
2
11
22
11
2
1112
⎟⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜⎜
⎝
⎛
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎠
⎞⎜⎝
⎛−
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎠
⎞⎜⎝
⎛−
⎟⎠
⎞⎜⎝
⎛⎟⎠
⎞⎜⎝
⎛−⎟
⎠
⎞⎜⎝
⎛
=
∑∑∑∑
∑∑∑
====
===
n
io
n
io
n
ic
n
ic
n
io
n
ic
n
ioc
hhnhhn
hhhhnr
where hc = computed head, ho = observed head
and n = number of observations
Quantitative AnalysisRainGage Run, May 01
r2 = 0.9131
-200
20406080
100120140
-20 0 20 40 60 80 100 120 140
observed head, ft
computed head, ft
NxrdRaw Run, May 01
r2 = 0.9144
-200
20406080
100120140
-20 0 20 40 60 80 100 120 140
observed head, ft
computed head, ft
NxrdAvg Run, May 01
r2 = 0.9128
-200
20406080
100120140
-20 0 20 40 60 80 100 120 140
observed head, ft
computed head, ft
NxrdGeo Run, May 01
r2 = 0.9129
-200
20406080
100120140
-20 0 20 40 60 80 100 120 140
observed head, ft
computed head, ft
NxrdTemp Run, May 01
r2 = 0.9121
-20
0
20
40
60
80
100
120
140
-20 0 20 40 60 80 100 120 140
observed head, ft
computed head, ft
The Major Aquifers of Texas
The Minor Aquifers of Texas
The Edwards Aquifer
The Edwards Aquifer
Pumpage to Date: 33,035.30 mg (million gallons)
Average Daily Pumpage: 144.26 mg
Minimum Edwards Level for 2000: 649.7’
Historic Minimum (8/17/56): 612.5’
Maximum Edwards Level for 2002: 690.5’
Historic Maximum (6/14/92): 703.3’
The Edwards Aquifer
When the limestone was exposed, it was extensively eroded creating cavities and conduits making it capable of holding and transmitting water
Then it was covered over with relatively impermeable sediments forming a confining unit
Geology of Edwards Aquifer
• Primary geologic unit is Edwards Limestone
• one of the most permeable and productive aquifers in the U.S.
• The aquifer occurs in 3 distinct segments:
-The drainage, recharge, and artesian zones
Drainage Zone of Edwards Aquifer
• located north and west of the aquifer in the region referred to as the Edwards Plateau or Texas Hill Country
• largest part of the aquifer spanning 4400 sq. miles
• water in this region travels to recharge zone
Recharge Zone of Edwards Aquifer
• Geologically known as the Balcones fault zone
• It consists of an abundance of Edwards Limestone that is exposed at the surface
-provides path for water to reach the artesian zone
Artesian Zone of Edwards Aquifer
• The artesian zone is a complex system of interconnected voids varying from microscopic pores to open caverns
• located between 2 relatively less permeable layers that confine and pressure the system
• underlies 2100 square miles of land
Artesian Wells• A well whose source of water is a confined aquifer
• The water level in artesian wells is at some height above the water table due to the pressure of the aquifer
•This level is the potentiometric surface and if it is above the land surface, it is considered a flowing artesian well
The Edwards Group
The Edwards Group
The Edwards limestone is between 300-700 ft. thick
Outcrops at the surface is tilted downward to the south and east and is overlain by younger limestone layers and thousands of feet of sediment The immense weight of this sediment layer
caused faulting in the region
Typical Dip Section
Regional Dip Section
Flowpaths of the Edwards Aquifer