water chemistry of shoal creek and … chemistry of shoal creek and waller creek, austin, texas, and...

WATER CHEMISTRY OF SHOAL CREEK AND WALLER CREEK, AUSTIN, TEXAS, AND POTENTIAL SOURCES OF NITRATE

By Patricia B. Ging, Roger W. Lee, and Steven R. Silva

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 96-4167

Prepared in cooperation with the CITY OF AUSTIN

Austin, Texas 1996

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

For additional information write to: Copies of this report can be purchased from:

District Chief U.S. Geological SurveyU.S. Geological Survey Branch of Information Services8011 Cameron Rd. Box 25286Austin, TX 78754-3898 Denver, CO 80225-0286

CONTENTS

Abstract ............................................................................................................................^^ 1Introduction .......................................................................................................................................................................... 1

Purpose and Scope .................................................................................................................................................... 2Description of the Study Area and Historical Water-Chemistry Data ...................................................................... 2Nitrogen and Oxygen Isotopes in the Nitrate Ion ..................................................................................................... 2Acknowledgments..................................................................................................................................................... 7

Water Chemistry of Shoal Creek and Waller Creek ............................................................................................................. 7Collection of Samples ............................................................................................................................................... 7Chemical Characteristics .......................................................................................................................................... 7

Potential Sources of Nitrate ................................................................................................................................................. 12Conclusions .......................................................................................................................................................................... 17References ............................................................................................................................................................................ 20

FIGURES

1 3. Maps showing:1. Location of study area and sampling sites, Austin, Texas ......................................................................... 32. Land use for Shoal Creek and Waller Creek watersheds, Austin, Texas ................................................... 43. Soils for Shoal Creek and Waller Creek watersheds, Austin, Texas ......................................................... 5

4-8. Graphs showing:4. Approximate compositional ranges of 5 15N and 6 18O data for sources of nitrate .................................... 65. pH and alkalinity values grouped by sampling site for Shoal Creek and Waller Creek, Austin,

Texas .......................................................................................................................................................... 86. Dissolved calcium, magnesium, and sodium concentrations grouped by sampling site for

Shoal Creek and Waller Creek, Austin, Texas ........................................................................................... 97. Dissolved chloride, sulfate, and nitrogen concentrations grouped by sampling site for Shoal

Creek and Waller Creek, Austin, Texas ..................................................................................................... 108. Relations between sodium and chloride concentrations in stormflow and base flow for Shoal

Creek and Waller Creek, Austin, Texas ..................................................................................................... 119 10. Trilinear diagrams of water from:

9. Shoal Creek, Austin, Texas ........................................................................................................................ 1310. WallerCreek, Austin, Texas ...................................................................................................................... 14

11 13. Graphs showing:11. 5 15N and 5 18O data for stormflow and base-flow samples from Shoal Creek and Waller Creek,

Austin, Texas, relative to compositional ranges for sources of nitrate ...................................................... 1512. 5 15N and 5 18O averages by sampling date for all sampling sites in Shoal Creek and Waller

Creek, Austin, Texas .................................................................................................................................. 1813. 5 15N and 5 18O data for all sampling sites in Shoal Creek and Waller Creek, Austin, Texas,

and ellipses having radii of 2 standard deviations from averages of stormflow and base-flow samples ...................................................................................................................................................... 19

TABLES

1. Chemical data and isotope ratios for stormflow samples from Shoal Creek and Waller Creek, Austin,Texas ..............................................................................................................................................................^ 21

2. Chemical data and isotope ratios for base-flow samples from Shoal Creek and Waller Creek, Austin,Texas ..............................................................................................................................................................^ 23

3. Selected chemical data for rainfall samples taken near Shoal Creek and Waller Creek watersheds,Austin, Texas ........................................................................................................................................................ 16

4. Average nitrate concentrations, oxygen-18 values, and land-use information for stormflow and base- flow samples from Waller Creek, Austin, Texas .................................................................................................. 16

CONTENTS iii

Abbreviations and Acronyms

cols./ 1 00 mL, colonies per 100 millilitersft, footmi 2 , square milemg/L, milligram per literMCL, maximum contaminant levelMCC, Microelectronics and Computer Technology Corp.USGS, U.S. Geological Survey

Per mil: A unit expressing the ratio of stable-isotopic abundances of an element in a sample to those of a standard material. Per mil units are equivalent to parts per thousand. Stable-isotopic ratios are computed as follows:

R (standard)-x^QQ,

whereX is the heavier stable isotope, andR is the ratio of the heavier, less abundant stable isotope to the lighter stable isotope in a sample or standard.

The 8 values for stable-isotopic ratios discussed in this report are referenced to the following standard materials:

Element ______________ R _______________________ Standard identity and reference _________

oxygen oxygen-!8/oxygen-16(6 18O) Vienna-Standard Mean Ocean Water (Fritz and Fontes, 1980, p. 11)

nitrogen nitrogen- 1 5/nitrogen- 1 4 (8 ' 5N) Standard atmospheric nitrogen, referenced to National Bureau ofStandards, NBS-14 nitrogen gas (Fritz and Fontes, 1980, p. 16)

IV

Water Chemistry of Shoal Creek and Waller Creek, Austin, Texas, and Potential Sources of Nitrate

By Patricia B. Ging, Roger W. Lee, and Steven R. Silva

Abstract

Steep limestone slopes, thin soils, sparse vegetation, and impervious cover within the Shoal Creek and Waller Creek watersheds, Austin, Texas, contribute to rapid runoff that can quickly carry contaminants such as nitrate, into the creeks. Land use within the watershed is predominantly residen tial (single-family and multifamily dwellings). Impervious cover within both watersheds was about 55 percent during 1994 95.

Water samples were collected for chemical analysis at seven sites in the Shoal Creek and Waller Creek watersheds from September 1994 to April 1995. Samples were collected during 4 stormflow events and 3 base-flow periods. Water samples were analyzed for major ions and nutrients as well as for nitrogen and oxygen isotopes in the nitrate anion. Concentrations of dissolved constitu ents, including nitrate, are smaller in stormflow samples than in base-flow samples. Calculated dis solved solids range from 16 to 187 milligrams per liter for stormflow samples and from 213 to 499 milligrams per liter for base-flow samples.

Nitrogen and oxygen isotopes in dissolved nitrate in conjunction with water chemistry were used to indicate sources of nitrate in surface water. A combination of atmospheric nitrate, and soil nitrate and ammonium fertilizer is the most likely source of nitrate in stormflow samples (assuming that there is little or no use of synthetic nitrate fer tilizers in the watersheds). Nitrogen and oxygen isotopic data in nitrate for stormflow samples are in or near the isotopic composition ranges for atmo spheric nitrate, and soil nitrate and ammonium fer tilizer sources. Nitrogen and oxygen isotopic data in nitrate for base-flow samples are in or near the

isotopic composition ranges for soil nitrate and ammonium fertilizer, and sewage and animal waste sources of nitrate. Sewage is the most likely source of nitrate in base flow because of the potential for older sewer lines to leak, the proximity of sewer lines to creek beds, and an excess of chloride relative to sodium at some sampling sites (an indi cator of the presence of sewage) under base-flow conditions. Nitrate in the creeks at any given time is a mixture that results predominantly from sur face sources (atmospheric nitrate, soil nitrate and ammonium fertilizer) during stormflow and pre dominantly from subsurface sources (sewage) dur ing base flow.

INTRODUCTION

Urban development can have an appreciable influence on water-quality conditions. Impervious cover in developed areas can result in increased runoff, conveying contaminants from point and nonpoint sources to local streams (City of Austin, 1990b). Any contaminant on land surface has the potential to quickly enter Shoal Creek and Waller Creek during storms owing to a large amount of impervious cover. Data pre viously collected by the U.S. Geological Survey (USGS) and the City of Austin have indicated the pres ence of nitrate, as nitrogen, in Shoal Creek and Waller Creek (Welborn and Veenhuis, 1987; City of Austin, 1990a); however, the data are inadequate to determine the sources of nitrate in the watersheds. Shoal Creek and Waller Creek drain into Town Lake, which is one of the water supplies for Austin. Increased nitrate concen trations in Town Lake could affect the quality of Aus tin's public water supply. Application of 5 15N and5 18O isotopic data from dissolved nitrate can indicate sources of nitrate in the Shoal Creek and Waller Creek water sheds and could contribute information useful for the improvement of water quality in these creeks.

Abstract

Purpose and Scope

The purpose of this report is to characterize the water chemistry in the Shoal Creek and Waller Creek watersheds and to indicate potential sources of nitrate in these watersheds. Twenty-five water samples were collected during stormflow conditions, and 14 water samples were collected during base-flow conditions at sites on the creeks or adjacent storm sewers from Sep tember 1994 to April 1995. Concentrations of selected constituents and isotopes of nitrogen and oxygen in the nitrate ion are used to show the water chemistry and potential sources of nitrate in the watersheds.

Description of the Study Area and Historical Water-Chemistry Data

The Shoal Creek and Waller Creek watersheds in the City of Austin (fig. 1) contain steep limestone slopes, thin soils, sparse vegetation, and impervious cover. The Shoal Creek watershed has a drainage area of approximately 13 mi. There is a 320-ft decrease in altitude from the uppermost (highest elevations) part of the watershed to its outlet at Town Lake. The Shoal Creek watershed contains primarily residential areas with single-family dwellings (fig. 2). Soil type is pre dominantly "urban land" in the watershed (fig. 3) with about 55-percent impervious cover during 1994-95. The "urban land" soils classification represents soils altered and obscured by development that do not resemble soils described in the various series (U.S. Department of Agriculture, Soil Conservation Service, 1974).

The Waller Creek watershed has a drainage area of approximately 5.6 mi. As in the Shoal Creek watershed, altitude in the Waller Creek watershed decreases about 320 ft from the uppermost part of the watershed to its outlet at Town Lake. The Waller Creek watershed consists primarily of residential areas with single-family and multifamily dwellings. Soil types were predominantly urban land in the Waller Creek watershed with about 55-percent impervious cover during 1994^-95.

Both the Shoal Creek and Waller Creek water sheds contain wastewater lines for transport of sewage, but some septic tanks might exist in both watersheds (George Chang, City of Austin, oral commun., 1995). The exact location and condition of the septic tanks, if any, were not be determined during this investigation. One landfill located in the Waller Creek watershed con

tains drums filled with chemicals (George Chang, City of Austin, oral commun., 1995).

Historical water-chemistry data available for the Shoal Creek and Waller Creek watersheds were exam ined to determine variability of nitrate concentrations in the creeks. Although chemical data were collected as early as 1942, no consistent dataset of long-term water-quality monitoring was extant to determine tem poral trends in water chemistry in the watersheds. The USGS collected water-chemistry data at six locations along Shoal Creek from 1942 to 1946 and from Shoal Creek at 12th Street (station 08156800) from January 1975 to present (1995). The USGS collected water- chemistry data from Waller Creek at 38th Street (sta tion 08157000) from August 1954 to December 1962 and from Waller Creek at 23d Street (station 08157500) from October 1970 to August 1971. Dissolved nitrate concentrations (as nitrogen) generally ranged from 0 to 2 mg/L for both creeks (Raymond Slade, Jr., U.S. Geological Survey, oral commun., 1995) and were below the MCL of 10 mg/L for drinking water recom mended by the U.S. Environmental Protection Agency (1996). City of Austin staff also have collected water- chemistry data at Shoal Creek at Woodhollow and at various locations along Waller Creek from 1992 to present. Dissolved nitrite plus nitrate concentrations (as N) ranged from 0 to 2 mg/L and total nitrogen con centrations (as N) ranged from 0 to 7.5 mg/L, also below the recommended drinking-water standard (George Chang, City of Austin, oral commun., 1994).

Nitrogen and Oxygen Isotopes in the Nitrate Ion

The combined stable isotopic ratios I5N/ I4N (8 I5N in per mil) and I8O/ I6O (8 I8O in per mil) in the nitrate ion, NO3 ", can be useful in surface- and ground-water investigations to determine sources of nitrate. Nitrate in surface and ground water can be derived from one or more of the following sources: (1) microbial decay of organic matter in soils, (2) ani mal waste and sewage, (3) synthetic fertilizers, and (4) rainfall. The usefulness of 8 I5N alone in identifying these sources is limited by overlapping ranges of 8 I5N values and by changes in the 8 I5N values of the sources caused by microbial denitrification and other processes. However, by combining 8 I5N and 8 I8O analyses, identifiable separations for all sources of nitrate except ammonium fertilizers and bacterially produced soil nitrate are possible (Carol Kendall,

Water Chemistry of Shoal Creek and Waller Creek, Austin, Texas, and Potential Sources of Nitrate

97°44'/J

97-46

EXPLANATION

Watershed boundaries

Location and number of sampling site. Sites not on streams are on storm sewers or tributaries. Site numbers refer to tables land 2

Waller Creek watershed

Shoal Creek watershed

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Figure 1. Location of study area and sampling sites, Austin, Texas.

INTRODUCTION

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EXPLANATION

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Figure 2. Land use for Shoal Creek and Waller Creek watersheds, Austin, Texas (Fatima Paiva, City of Austin, written commun., 1995).

4 Water Chemistry of Shoal Creek and Waller Creek, Austin, Texas, and Potential Sources of Nitrate

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U.S. Geological Survey, written commun., 1995). In addition, recent studies indicate that 8 15N and 8 18O data in approximately a 2:1 ratio (8 I5N:8 I8O) could indicate denitrification (Amberger and Schmidt, 1987; Bottcher and others, 1990).

Nitrate formed from reduced nitrogen species obtains its oxygen in two parts from local water (atmo spheric, surface, or ground water) and one part from dissolved oxygen (8 18O equal to about +23 per mil) (Amberger and Schmidt, 1987). Therefore, nitrate formed from decay products in the soil zone, from reduced nitrogen species in animal waste and sewage, and from synthetic ammonium fertilizers all tend to have similar 8 I8O values. Actual 8 18O values of bacterially produced soil nitrate from recent studies aver aged a few per mil heavier (ratios more positive) than predicted from two parts local ground water and one part dissolved oxygen because (1) local water in the soil zone underwent some evaporation and became iso- topically heavier than predicted for the saturated zone, and (2) ground-water nitrate contains heavier 8 18O

values from nitrate fertilizers or atmospheric sources (Aravenaand others, 1993; Wassenaar, 1995). The approximate compositional ranges depicted in figure 4 are drawn from these observations and from published and unpublished data (Heaton, 1986; Aravena and others, 1993; Durka and others, 1994; Wassenaar, 1995; Kendall and others, in press).

Conversion of nitrite to nitrogen (denitrification) can occur in slightly reducing geochemical environ ments where dissolved oxygen is low. Denitrification causes the remaining nitrate reservoir to become slightly heavier in the isotopic ratios of nitrogen and oxygen in the NO3" ion (Kendall and others, in press). Denitrification can occur in ground water, stagnant surface water, and some reducing soils, and could substantially alter nitrogen and oxygen isotopic ratios in these systems.

Solute and ion-pair ratio data can supplement nitrogen isotope data to identify sources of nitrate (Collins, 1975; Whittemore, 1988). Solutes that are conservative such as chloride, bromide, and iodide


serve as tracers when transported with nitrate, resulting in ratios that might identify a nitrate source. Bromide and iodide tracers can identify components of fertilizer entering surface water. Dissolved chloride in conjunc tion with 8 I5N data has been used to identify nitrate sources for municipal sewage (Avimelech and Raveh, 1976).

Acknowledgments

The project was done in cooperation with the City of Austin. Members of the Environmental and Conser vation Services Department who participated in the project are George Chang, Project Manager; Jim Hubka and David Johns, scientific staff; Tom Brown, Steve Raymond, John Watkins, and Rene Avila, data collec tion. USGS staff who assisted with data collection are Mike Dorsey, Searcy Jacobs, Sonya Jones, and Joy Lizarraga.

WATER CHEMISTRY OF SHOAL CREEK AND WALLER CREEK

Collection of Samples

Samples were collected for chemical analysis at 7 sites in the 2 watersheds, 3 in the Shoal Creek water shed and 4 in the Waller Creek watershed, from Septem ber 1994 to April 1995 (fig. 1; tables 1, 2 at end of report). Samples were collected during 4 stormflow events and 3 base-flow periods for a total of 39 samples. Some sites did not sustain base flow and therefore base- flow samples were not collected at those sites. One sam pling site. Shoal Creek at MCC (site I), was used as the best available background site. The other sampling sites were Shoal Creek at Woodhollow Drive (site 2), Shoal Creek at 12th Street (site 3), Waller Creek at 45th Street (site 4), Waller Creek at 38th Street (site 5), Waller Creek at Hancock golf course (site 6), and Waller Creek at 5th Street (site 7). Waller Creek at 45th Street and Waller Creek at 5th Street sampling sites are storm sew ers draining into Waller Creek and are not on the main stem of the creek. Waller Creek at Hancock golf course sampling site is on a small tributary to Waller Creek. All other sampling sites are located on the main channels.

Sample collection was done primarily by the USGS. Staff of the City of Austin collected storm samples from sites 5, 6, and 7. For quality assurance, replicate samples and one equipment blank also were collected. An equipment blank is a sample of the deion- ized water used to decontaminate field equipment.

Three replicate samples from three locations at different times indicated that chemical constituent concentra tions were within acceptable limits. The equipment blank showed no significant amounts of chemical con stituents or fecal coliform. However, fecal streptococci measured 340 cols./100 mL in the equipment blank, indicating that some contamination of equipment was possible (table 2) and these data should be interpreted with caution.

Temperature, specific conductance, pH, and alkalinity were measured in the field when possible. Water samples were analyzed at the USGS National Water Quality Laboratory in Denver, Colo., for major cations, major anions, and nutrients. The stable isotopes of nitrate, 8 I5N and 8 I8O, were analyzed by the USGS in Menlo Park, Calif. Fecal coliform and fecal strepto cocci bacteria were measured at the USGS laboratory in Austin, Tex.

Chemical Characteristics

Graphs of selected constituent concentrations grouped by sampling sites for Shoal Creek and Waller Creek watersheds illustrate variations of chemical con centrations for stormflow and base flow in the streams (figs. 5 7). With increased streamflow during storms, constituent concentrations, except for nitrate, phos phate, and bacteria, were consistently smaller in sam ples from both creeks. The smaller concentrations of most constituents in stormflow samples compared to base-flow samples probably were the result of dilution.

Solute concentrations were variable for both watersheds during stormflow and base flow. Magne sium concentrations are smaller toward watershed out lets during stormflow and during base flow on Shoal Creek (fig. 6). Sodium, chloride, and sulfate concentra tions are larger toward watershed outlets during base flow (figs. 6, 7). These differences in concentrations toward the watershed outlets might not be typical. More sampling sites within each watershed are needed to define chemical changes toward the outlets. Solute con centrations of base flow at Shoal Creek at 12th Street probably were affected by wastewater discharge from local businesses rather than seepage from shallow ground water, based on field observation of discharge outfalls from nearby commercial areas. Solute concen trations during base-flow conditions from Waller Creek at 45th Street might have been affected by leaks from a city water main.

WATER CHEMISTRY OF SHOAL CREEK AND WALLER CREEK 7

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Bromide concentrations are near method detec tion limits for all samples taken in both watersheds (tables 1,2); therefore any relation between bromide and nitrate sources could not be determined. Some sites indicate excess chloride concentrations with respect to sodium concentrations (greater than I: I mole ratio of chloride to sodium) (fig. 8), but no correlation between

nitrate and chloride concentrations could be deter mined. Samples with excess chloride with respect to sodium could indicate sewage contamination (Avimelech and Raveh, 1976).

Nitrate (as N) concentrations in samples col lected from the Shoal Creek watershed range from 0.19 to 0.75 mg/L in stormflow and from 0.02 to 1.9 mg/L

8 Water Chemistry of Shoai Creek and Waller Creek, Austin, Texas, and Potential Sources of Nitrate

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EXPLANATION

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EXPLANATION

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Figure 8. Relations between sodium and chloride concentrations in stormflow and base flow for Shoal Creek and Waller Creek, Austin, Texas.


in base flow (below the MCL of 10 mg/L) (tables 1,2). In samples collected from the Waller Creek watershed, nitrate (as N) concentrations range from 0.02 to 1.2 mg/L in stormflow and from 0.06 to 0.86 mg/L in base flow. Nitrate concentrations are larger in stormflow samples in Waller Creek at Hancock golf course (where no base flow occurred) than in stormflow at the other sites.

In the combined Shoal Creek and Waller Creek watersheds, fecal coliform counts range from less than 1 to 84,000 cols./100 mL, and fecal streptococci counts range from less than 1 to 310,000 cols./100 mL in stormflow, indicating relatively high levels of fecal bacteria in runoff (table 1). Ratios of fecal coliform to fecal streptococci below 0.7 indicate animal sources of fecal bacteria (American Public Health Association, 1981). Ratios above 4.1 indicate human sources. Ratios in the range of 0.7 to 4.1 indicate mixes from both sources. Most ratios of fecal coliform to fecal strepto cocci are less than 0.7 in stormflow samples, indicating that the source of bacteria is predominantly from ani mal wastes. In base flow, fecal coliform counts range from less than 1 to 1,600 cols./lOO mL, and fecal strep tococci counts range from less than 1 to 960 cols./100 mL for both watersheds, indicating much lower levels of fecal bacteria in base flow than stormflow in the watersheds (table 2). Less than 1 col./100 mL recorded for bacteria at various times at the Waller Creek at 45th Street site might be a result of chlorinated water leak ing from a water line into the creek, killing most or all fecal bacteria. pH values exceeding 9.0 with low alka linity at this site, similar to values for city of Austin tap water (P.C. Bennett, University of Texas at Austin, oral commun., 1995), also indicate a leaking water line.

Trilinear diagrams indicate general water type using the major cation and major anion data from the water samples (figs. 9, 10). The water type in both Shoal Creek and Waller Creek is calcium bicarbonate, which is expected in a limestone terrane. Calculated dissolved solids in water samples collected from the Shoal Creek and Waller Creek watersheds range from 16 to 187 mg/L for stormflow samples and from 213 to 499 mg/L for base-flow samples. Dissolved solids con centrations generally are 2 to 4 times greater during base flow than during stormflow (tables 1, 2). Constit uent concentrations in the samples collected for this project are typical for water from urbanized areas; and they are within ranges of recommended drinking-water standards (U.S. Environmental Protection Agency, 1996).

POTENTIAL SOURCES OF NITRATE

The potential sources of nitrate in the Shoal Creek and Waller Creek watersheds include fertilizers, sewage, animal wastes, atmospheric, and soil sources. Each source contributes a distinct nitrogen and oxygen isotopic signature in the nitrate ion (fig. 4).

Graphs of 6 I5N and 6 I8O data relative to compo sitional ranges for sources of nitrate for the most part show distinct separation between stormflow and base- flow samples for the Shoal Creek and Waller Creek watersheds (fig. 11). Isotopic data for stormflow sam ples (6 I5N from +1.7 to +10.6 per mil and 6 I8O from +8.9 to +39.5 per mil) are in or near the isotopic com position range of synthetic nitrate fertilizer. However, synthetic nitrate fertilizer is an unlikely source of nitrate because most fertilizers applied to land surface in the watersheds are ammonium fertilizers (George Chang, City of Austin, oral commun., 1996). Because synthetic nitrate fertilizer is not a likely source of nitrate, the isotopic data for stormflow probably repre sents combinations of atmospheric nitrate, and soil nitrate and ammonium fertilizer sources. Although concentrations of nitrate in rainfall generally are small (table 3), atmospheric nitrate could contribute to the total nitrate load and affect the range of isotopic data. Isotopic data for base-flow samples (5 I5N from +8.4 to +13.1 per mil and 6 I8O from-0.08 to+19.0 per mil)are in or near the isotopic composition ranges for soil nitrate and ammonium fertilizer, and sewage and ani mal waste sources of nitrate.

Livestock or animals other than domestic pets and urban/suburban-dwelling mammals and birds are not known to be in the watersheds. However, sewage contamination of ground water from sewer lines and (or) septic tanks could be substantial in older residen tial areas (more than 20 years old). In time, sewer lines and (or) septic tanks degrade, possibly leaking sewage containing nitrate into the ground water. In addition, the sewer lines commonly are in or near the creek beds; therefore, leakage from these lines can discharge directly into the stream. Shoal Creek and Waller Creek pass through older parts of Austin, about one-half of which are residential areas older than 20 years (George Chang, City of Austin, oral commun., 1995).

Geographic information system (G1S) coverage of land use was available for the Waller Creek water shed to allow some interpretation of the isotopic data with respect to land use represented by the sample sites. Mean concentrations of dissolved nitrate increase


Num

ber

Sam

plin

g Lo

catio

n an

d D

ate

80

o St

orm

flow

Ba

se fl

ow

60\ Ca 40

Mg 20

1 2 6 9 11 13 16 17 18 22 24 27 33 34 35 36 37 38

20

40

60

8080

60

40

20

Shoa

l Cre

ek a

t MC

C; 0

9-07

-94

Shoa

l Cre

ek a

t Woo

dhol

low

Driv

e; 0

9-07

-94

Shoa

l Cre

ek a

t Woo

dhol

low

Driv

e; 0

9-09

-94

Shoa

l Cre

ek a

t 12

th S

treet

; 09-

09-9

4Sh

oal C

reek

at M

CC

; 11

-05-

94Sh

oal C

reek

at W

oodh

ollo

w D

rive;

11-

05-9

4Sh

oal C

reek

at

12th

Stre

et;

11-0

5-94

Shoa

l Cre

ek a

t MC

C; 0

1-11

-95

Shoa

l Cre

ek a

t Woo

dhol

low

Driv

e; 0

1-11

-95

Shoa

l Cre

ek a

t MC

C; 0

3-13

-95

Shoa

l Cre

ek a

t Woo

dhol

low

Driv

e; 0

3-13

-95

Shoa

l Cre

ek a

t 12

th S

treet

; 03-

13-9

5Sh

oal C

reek

at

12th

Stre

et; 0

4-20

-95

Shoa

l Cre

ek a

t Woo

dhol

low

Driv

e; 0

4-20

-95

Shoa

l Cre

ek a

t M

CC

; 04-

20-9

5Sh

oal C

reek

at M

CC

; 04-

26-9

5Sh

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reek

at W

oodh

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w D

rive;

04-

26-9

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Trili

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as.

Num

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Stor

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Base

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Sam

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Wal

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Wal

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09-9

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W

alle

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94

Wal

ler C

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ours

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1-05

-94

Wal

ler C

reek

at 4

5th

Stre

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11-0

5-94

W

alle

r Cre

ek a

t 38t

h St

reet

; 01-

11-9

5 W

alle

r Cre

ek a

t 38t

h St

reet

(dup

licat

e); 0

1-11

-95

Wal

ler C

reek

at 3

8th

Stre

et; 0

3-13

-95

Wal

ler C

reek

at 5

th S

treet

; 03-

13-9

5 W

alle

r Cre

ek a

t 45t

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; 03-

13-9

5 W

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r Cre

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t 5th

Stre

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4-20

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Wal

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4-20

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Wal

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at 4

5th

Stre

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4-20

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Wal

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Wal

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80

60

40

20

20

40

60

80

CA

TIO

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ustin

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as.

Shoal Creek

trLU Q.

«u

70

60

50

40

on

?o

10

0

in

0

O

Atmospheric ^ sources

T

------_---_.O.--...l-, Synthetic nitrate Q O o

fertilizers ^D; ^

. ..... . ..... .... ....................................^ _.. P

[ Soil nitrate and \ I = ammonium fertilizers ii «N

i i i

ShoalCreekat MCC, stormflow

Shoal Creek at Woodhollow, stormflow

Shoal Creek at 12th Street, stormflow

Shoal Creek at MCC, baseflow

Shoal Creek at Woodhollow, baseflow

Shoal Creek at 12th Street, base flow

iSewage and animal wastes ;

i

! ,

-5 0

Waller Creek10 15 20 25

00

70

60

50

40

30

20

10

0

in

Atmospheric sources

*0

O

Synthetic nitrate j

D O.......................................JUJ..............................,, .

Soil nitrate and | ammonium fertilizers

i

i i i

O Waller Creek at 45th Street, stormflow

O WallerCreek at 38th Street, stormflow

D WallerCreek at Hancock, stormflow

* WallerCreek at 5th Street, stormflow

WallerCreek at 45th Street, baseflow

T Waller Creek at 38th Street, base flow

T

1*i

Sewage and animal wastes ! i

, ,

-5 0 10

515N, IN PER MIL

15 20 25

Figure 11. 515N and 518O data for stormflow and base-flow samples from Shoal Creek and Waller Creek, Austin, Texas, relative to compositional ranges for sources of nitrate.

POTENTIAL SOURCES OF NITRATE 15

Table 3. Selected chemical data for rainfall samples taken near Shoal Creek and Waller Creek watersheds, Austin, Texas

[mg/L, milligrams per liter; cols./lOO mL, colonies per 100 milliliters; , no data; <, less than]

Sample date

10-20-93

02-28-94

03-10-94

03-13-94

04-05-94

04-1 1-9405-03-94

Dissolved nitrate, asN

(mg/L)

3.8

.11

.05

.24

.11

.12

Phosphate, (asPCV3) dissolved

(mg/L)

0.08

.10

.05

<.05

.18

<.05

<.005

Fecal coliform

(cols./100 mL)

0

0-

-

0

~

-

Fecal streptococci

(cols./100 mL)

0

50

2,000--

5,300

-

Joseph Malina, University of Texas at Austin, written commun., 1995.

Table 4. Average nitrate concentrations, oxygen-18 values, and land-use information for stormflow and base- flow samples from Waller Creek, Austin, Texas

[mg/L, milligrams per liter; per mil, parts per thousand]

Samplesite no.(fig- D

6

4

5

7

Station name

Waller Creek at Hancock golf course

Waller Creek at 45th Street



Average

dissolvednitrate as N

(mg/L)

0.76

.39

.37

.14

Average8 180

(per mil)

15.6

19.8

28.9

31.7

Imperviouscover

(percent)

10.0

43.2

46.8

93.0

Land use

Golf course

Suburban

Urban/suburban

Urban

and isotopes of oxygen become lighter (ratios more negative) with decreasing impervious cover for the land uses represented by the samples (table 4) (Jim Hubka, City of Austin, written commun., 1996).

Concentrations of nitrate from Waller Creek at 5th Street were lowest and the 6 I8O values heaviest among all samples for the watershed. Low concentra tions of nitrate and heavy 6 I8O values are consistent with a predominantly atmospheric source of nitrate (fig. 11, table 3). The stormflow samples from Waller Creek at 5th Street represent drainage from a subwatershed with more than 90 percent impervious cover (table 4), which would tend to minimize nitrate from fertilizer sources, soil sources, and sewage and animal

wastes; thus nitrate concentrations and 6 I8O data from a site associated with an essentially impervious drain age area are consistent with a predominantly atmo spheric source of nitrate.

Greater mean nitrate concentrations and lighter 6 I8O values in samples from the three sites in the Waller Creek watershed associated with less impervi ous cover (table 4) indicate sources of nitrate in addi tion to atmospheric nitrate (fig. 11). Specifically, the nitrate and isotopic data from stormflow samples at 45th Street, 38th Street, and Hancock golf course reflect combinations of atmospheric nitrate, and soil nitrate and ammonium fertilizer sources (assuming, as previously stated, that there is little or no use of


synthetic nitrate fertilizers in the watershed); whereas the nitrate and isotopic data from base-flow samples from Waller Creek at 45th and 38th Streets are most consistent with a sewage and animal waste source of nitrate.

Excess chloride concentrations relative to sodium have been used in previous investigations at other loca tions as tracers of sewage contamination (Avimelech and Ravch, 1976). Graphs of relations between sodium and chloride concentrations in stormflow and base flow indicate a source of excess chloride relative to sodium at some sampling sites, particularly under base-flow conditions (fig. 8). These sites are Shoal Creek at Wood- hollow, Shoal Creek at 12th Street, Waller Creek at 45th Street, and Waller Creek at 38th Street. Excess chloride concentrations relative to sodium at Waller Creek at 45th Street could be the result of chlorinated drinking water leaking into the creek. Chloride-to-sodium molar ratios greater than 1 at the other sites indicate that base flows in Shoal Creek and Waller Creek could have con tained sewage effluent, which would be consistent with the6 l5Nand6 l8Odata.

Figure 12 shows 8 I5 N and 8 I8O averages by sam pling date. The correspondence in trends between 8 I5N and 5 I8O from one sampling date to the next indicates that the measured values are responding to varying pro portions of two general endmember compositions: one of relatively low 8 I5N and high 8 I8O and the other of relatively high 8 I5N and low 8 I8O. Figure 13 shows all of the isotopic data for the stormflow and base-flow samples and ellipses having radii of 2 standard devia tions from average superimposed to highlight the dis tinction between sources of nitrate in stormflow and base-flow samples. The data of figures 12 and 13 are consistent with the conclusion that nitrate concentra tions in the creeks at any given time are mixtures result ing from surface sources (atmospheric nitrate, and soil nitrate and ammonium fertilizer) during stormflow and predominantly from subsurface sources (sewage and soil nitrate and ammonium fertilizer) during base flow.

Denitrification is not a likely cause of the heavy 8 I5N values because the samples were collected from oxygenated surface waters. Furthermore, there is no apparent tendency among the base-flow samples toward heavier isotopic ratios of 8 18O in the nitrate ion (fig. 11), a likely characteristic if dcnitrification were occurring (Kendall and others, in press).

CONCLUSIONS

In general, stormflow in the Shoal Creek and Waller Creek watersheds contains smaller amounts of dissolved constituents than base flow. Magnesium con centrations decrease toward basin outlets during base- flow conditions; whereas, other constituent concentra tions (sodium, chloride, and sulfate) increase toward basin outlets during base-flow conditions. Nitrate (as N) concentrations are below the MCL of 10 mg/L. Fecal bacteria counts are greater in stormflow samples than in base-flow samples.

The general water type in the Shoal Creek and Waller Creek watersheds is calcium bicarbonate, which is common in limestone terranes.

8 I5N and 8 I8O isotopic data from dissolved nitrate indicate that the sources of nitrate in stormflow of both watersheds probably arc not the same as the sources of nitrate in base flow.

8 I5N and 8 I8O isotopic data in nitrate for stormflow samples are in or near the isotopic composition ranges for atmospheric nitrate, and soil nitrate and ammonium fertilizer sources. A combination of atmo spheric nitrate, and soil nitrate and ammonium fertilizer sources is the most likely cause of nitrate in stormflow samples (assuming that there is little or no use of syn thetic nitrate fertilizers in the watersheds).

8 I5N and 8 I8O isotopic data in nitrate for base- flow samples are in or near the isotopic composition ranges for soil nitrate and ammonium fertilizer, and sewage and animal waste sources of nitrate. A combina tion of sewage nitrate and soil nitrate and ammonium fertilizer sources is the most likely source of nitrate in base-flow samples. Sewage is considered the predomi nant source because of the potential for older sewer lines to degrade, the proximity of sewer lines to creek beds, and an excess of chloride relative to sodium at some sampling sites (an indicator of the presence of sewage) under base-flow conditions.

Impervious cover could restrict the primary nitrate source to atmospheric. In the Waller Creek watershed, nitrate concentrations, and 8 I5N and 8 I8O isotopic data in nitrate for stormflow samples from one site that represents drainage from an essentially imper vious subwatershed indicate an atmospheric source is most likely; whereas nitrate and isotopic data from stormflow samples representing three subwatersheds with less than 50-percent impervious cover indicate possible soil nitrate and ammonium fertilizer sources in addition to atmospheric sources.

CONCLUSIONS 17

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REFERENCES

Amberger, A., and Schmidt, H., 1987, Naturliche iso- topengehalte von nitrat als indikatoren fur dessen herkunft: Geochimica et Cosmochimica Acta, v. 51, p. 2,699-2,705.

American Public Health Association, 1981, Standard meth ods for the examination of water and wastewater: 15thed.

Aravena, R., Evans, M., and Cherry, J., 1993, Stable isotopes of oxygen and nitrogen in source identification of nitrate from septic systems: Groundwater, v. 31, p. 180-186.

Avimelech, Y., and Raveh, J., 1976, Nitrate leakage from soils differing in texture and nitrogen load: Journal of Environmental Quality, v. 5, p. 79 81.

Bottcher, J., Strebel, O., Voerkelius, S., and Schmidt, H., 1990, Using isotope fractionation of nitrate-nitrogen and nitrate-oxygen for evaluation of microbial denitrification in a sandy aquifer: Journal of Hydrology, v. 114, p. 413^24.

City of Austin, Environmental and Conservation Services Department, 1990a, Removal efficiencies of storm- water control structures: 42 p.

____1990b, Stonnwater pollutant loading characteristics for various land uses in the Austin area: 77 p.

Collins, A., 1975, Geochemistry of oilfield waters, in Devel opments in petroleum science: New York, Elsevier Publishing Co.. p. 496.

Durka, W., Schulze, E., Gebauer, G., and Voerkelius, S., 1994, Effects of forest decline on uptake and leaching of deposited nitrate determined from I5N and I8O mea surements: Nature, v. 372, p. 765 769.

Fritz, Peter, and Fontes, J.C., 1980, Introduction, in Fritz, Peter, and Fontes, J.C., eds., Handbook of environmen tal isotope geochemistry, v. 1, The terrestrial environ ment, A: Amsterdam, The Netherlands, Elsevier Publishing Co., p. 1-19.

Heaton, T, 1986, Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere A review: Chemical Geology, v. 59, p. 87-102.

Kendall, C., Silva, S., Chang, C., Burns, D., Campbell, D., and Shanley, J., in press. Use of the 8 I5N and 8 |8O of nitrate to determine sources of nitrate in early spring runoff in forested catchments, in Conference and Inter national Symposium on Isotopes in Water Resources Management, Vienna, Austria, March 20-14, 1995, Proceedings: International Atomic Energy Agency.

U.S. Department of Agriculture, Soil Conservation Service, 1974, Soil survey of Travis County, Texas: Washington, D.C., 123 p.

U.S. Environmental Protection Agency, 1996, Drinking water regulations and health advisories: Washington, D.C., Office of Water, 16 p.

Wassenaar, L., 1995, Evaluation of the origin and fate of nitrate in the Abbotsford aquifer using the isotopes of I5N and I8O in NO3": Applied Geochemistry, v. 10, p. 391^105.

Welborn, C.T., and Veenhuis, J.E., 1987, Effects of runoff controls on the quantity and quality of urban runoff at two locations in Austin, Texas: U.S. Geological Survey Water-Resources Investigations Report 87-4004, 101 p.

Whittemore, D., 1988, Bromide as a tracer in ground-water studies Geochemistry and analytical determination, in Conference on Ground Water Geochemistry, Denver, Colo., February 16-18, 1988, Proceedings: p. 339-354.


w *

Tabl

e 1.

Che

mic

al d

ata

and

isot

ope

ratio

s fo

r st

orm

flow

sam

ples

from

Sho

al C

reek

and

Wal

ler C

reek

, A

ustin

, Te

xas

[ft, f

eet;

fVVs

, cub

ic f

eet p

er s

econ

d; (

^S/c

m, m

icro

siem

ens

per c

entim

eter

at 2

5 de

gree

s C

elsi

us; c

ols./

lOO

mL,

col

onie

s pe

r 10

0 m

illili

ters

; mg/

L, m

illig

ram

s pe

r lit

er; C

aCO

3, ca

lciu

m c

arbo

nate

; H, h

ydro

gen;

,

no d

ata;

<, l

ess

than

; M

CC

, M

icro

elec

troni

cs a

nd C

ompu

ter T

echn

olog

y C

orp.

; gc

, gol

f cou

rse;

>, g

reat

er th

an;

fet,

fixed

end

poin

t titr

atio

n; N

O2+

NC>3

, nitr

ite p

lus

nitra

te; 8

l5N

, rat

io o

f nitr

ogen

-15

to n

itrog

en-1

4; 8

l8O

, rat

io o

f oxy

gen-

18 to

oxy

gen-

16]

Sam

ple

site

no.

St

atio

n na

me

(fig-

1)

2 3 1 2 3 1 2 3 3 2 1 5 6 7 5 6 4 5 6 7 4 7 6 4 5

Shoa

l Cre

ek a

t Woo

dhol

low

Dr.

Shoa

l Cre

ek a

t 12

th S

t.Sh

oal C

reek

at M

CC

Shoa

l Cre

ek a

t Woo

dhol

low

Dr.

Shoa

l Cre

ek a

t 12

th S

t.Sh

oal C

reek

at M

CC

Shoa

l Cre

ek a

t Woo

dhol

low

Dr.

Shoa

l Cre

ek a

t 12

th S

t.

Shoa

l Cre

ek a

t 12

th S

t.Sh

oal C

reek

at W

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w D

r.Sh

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reek

at M

CC

Wal

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at 3

8th

St.

Wal

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at H

anco

ck g

cW

alle

r Cre

ek a

t 5th

St.

Wal

ler C

reek

at 3

8th

St.

Wal

ler C

reek

at H

anco

ck g

cW

alle

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ek a

t 45t

h St

.W

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h St

.W

alle

r Cre

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t Han

cock

gc

Wal

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reek

at 5

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t.W

alle

r Cre

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h St

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St.

Wal

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at H

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c

Wal

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5th

St.

Wal

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8th

St.

Dat

a

09-0

9-94

09-0

9-94

1 1-0

5-94

11-0

5-94

1 1-0

5-94

03-1

3-95

03-1

3-95

03-1

3-95

04-2

0-95

04-2

0-95

04-2

0-95

09-0

9-94

09-0

9-94

1 1-0

5-94

1 1-0

5-94

1 1-0

5-94

1 1-0

5-94

03-1

3-95

03-1

3-95

03-1

3-95

03-1

3-95

04-2

0-95

04-2

0-95

04-2

0-95

04-2

0-95

Tim

e

0030

0130

0145

0210

0305

0226

0245

0312

0150

0215

0235

0035

0043

0130

0210

0230

0235

0240

0246

0301

0342

0043

0124

0125

0127

Gag

e he

ight

(ft

)

1.05

4.50 .6

0.9

0

3.26 1.46

1.60

9.05

4.37 .9

6.4

7

4.35 .0

8.2

54.

10 .20

.08

5.03 .5

0

.50

.35

.32

.40

.14

4.34

Spec

if-

Spec

if-

P '

_.

ic c

on-

ic c

on-

Dis-

.

. w

hole

, du

e-

due-

ch

arge

fie

ld

(nS/

cm)

(nS/

cm)

Shoa

l Cre

ek25

445

9 -

282

134

155

268

280

191

148

202

125

137

1.94

0 -

158

480

-

293

306

256

Wal

ler

Cre

ek

100

204

32

3268

69

370

368

317

320

119

120 48 138 87 278

187

142

7.7

7.8

7.7

7.5

7.9

7.7

7.5

7.8

- - 7.9

7.6

7.2

7.8

7.6

9.1 7.6

- 6.9

7.5

- - -

pH,

wat

ar

who

le,

lab

(sta

n

dard

un

its)

7.9

7.6

78 7.6

7.7

7.8

7.7

7.9

7.4

7.8

7.5

7.8

7.9

7.0

7.0

7.2

8.5

7.8

7.6

7.3

7.4

7.7

7.5

7.5

7.6

Feca

i co

n

form

(c

ois.

/ 10

0 m

L)

37.0

0042

.000

8.00

034

.000

64.0

002,

000

21,0

0018

,000

60,0

0019

,000

2,00

0

84.0

0036

.000

> 10

.000

84,0

00

58.0

00 <l

21,0

00 -

24.0

00 <1

62,0

0048

.000

4,40

060

.000

Feca

i H

afd"

stre

pto-

ne

SS>

Aci

dity

COCc

i (m

g°L

(mg/

L (c

ols.

/ l

9 as

H)

100

mL)

C

aC03

)

30.0

0071

,000

6,80

025

.000

90.0

0038

.000

99.0

0011

0,00

0

160.

000

84,0

0029

.000

130,

000

64.0

00>8

6,00

010

0,00

0

63,0

00 <l

74,0

00 -

170.

000

9,20

021

0,00

031

0.00

0

49.0

0019

0.00

0

100

<0.i

i20

<.l

58

<.l

100

<.i

60

<.l

39 48 47 94 120

110 44

<.i

75

<.l

12

<.l

26

<.i

i30

<.l

85

<.l

34 42 17 44 18 83 82 36

Cal

cium

, di

s

solv

ed(m

g/L)

34 43 i9 34 22 13 16 17 33 38 35 16 23 4.2

8.5

44 16 12 14 6.2

12 6.4

29 21 13

Mag

ne

si

um,

dis

so

lved

(m

g/L)

4.1 3.0

2.6

4.7 1.3 i.5 1.9 1.0

2.8

5.1 5.7 1.1 4.2 .3 l.i

5.1

11.9 1.7 .3 3.3 .5 2.5 7.1 .8

Sodi

um

, di

s

solv

ed

(mg/

L)

8.1

6.9

3.3

10 4.8 1.4

3.3

2.1

8.6

11 4.7

2.3

5.2 .8 2.6

15 241.

82.

0 i.4 5.8 1.7

10 15 2.2

Pota

s

sium

, di

s

solv

ed(m

g/L)

2.8

3.4 1.9

2.4

2.1

2.7

2.3

2.5

3.4

2.9

3.3

2.0

13.7 2.0

10 3.9

2.1

6.0 .9 3.4 1.3

13 4.4

2.1

I'll'>O -C7 C

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Tabl

e 2.

Che

mic

al d

ata

and

isot

ope

ratio

s fo

r ba

se-fl

ow s

ampl

es fr

om S

hoal

Cre

ek a

nd W

alle

r C

reek

, Au

stin

, Tex

as

[ft,

feet

; (iS

/cm

, mic

rosi

emen

s pe

r cen

timet

er a

t 25

degr

ees C

elsi

us; c

ols./

lOO

mL,

col

onie

s pe

r 10

0 m

illili

ters

; mg/

L, m

illig

ram

s pe

r lite

r; C

aCO

i, ca

lciu

m c

arbo

nate

; H, h

ydro

gen;

M

CC

, Mic

roel

ectro

nics

and

Com

pute

r Tec

hnol

ogy

Cor

p.; <

, les

s th

an; , n

o da

ta; f

et, f

ixed

end

poin

t titr

atio

n; N

Oj+

NC

^, n

itrite

plu

s ni

trate

; 5

N, r

atio

of n

itrog

en-1

5 to

nitr

ogen

- 14

; 518

O, r

atio

of o

xyge

n-18

to o

xyge

n-16

]

Sam

ple

site

no.

Stat

ion

nam

e (fi

g.1)

Table

2

1 2 1 2 3 1 2 2 4 4 5 5 5 5

Shoa

l Cre

ek a

t MC

C

Shoa

l Cre

ek a

t Woo

dhol

low

Dr.

Shoa

l Cre

ek a

t MC

C

Shoa

l Cre

ek a

t Woo

dhol

low

Dr.2

Shoa

l Cre

ek a

t 12

th S

t.

Shoa

l Cre

ek a

t MC

C

Shoa

l Cre

ek a

t Woo

dhol

low

Dr.

Shoa

l Cre

ek a

t Woo

dhol

low

Dr.2

Wal

ler C

reek

at 4

5th

St.

Wal

ler C

reek

at 4

5th

St.2

Wal

ler C

reek

at 3

8th

St.

Wal

ler C

reek

at 3

8th

St.

Wal

ler C

reek

at 3

8th

St.2

Wal

ler C

reek

at 3

8th

St.

Equi

pmen

t bla

nk

Dat

a

09-0

7-94

09-0

7-94

01-1

1-95

01-1

1-95

01-1

1-95

04-2

6-95

04-2

6-95

04-2

6-95

09-0

7-94

09-0

7-94

09-0

7-94

01-1

1-95

01-1

1-95

04-2

6-95

04-2

6-95

Tim

e

0745

0945

0840

1000

1105

0940

1040

1050

1045

1047

1354

1245

1250

1235

0845

Gag

e he

ight

(ft

)

0.10 .0

1

.30

.01

'1.7

7

.30

.05

.05

.01 .01 2.00

2.10

2.10

2.10

Spec

ific

cond

uc

tanc

e,

field

(n

S/cm

)

935

582

934

842

873

913

671

671

417

417

555

705

705

712

Spec

ific

cond

uc

tanc

e,

ieb

(nS/

cm)

795

567

914

826

891

890

666

667

410

410

553

706

704

702 3

H. «

Shoa

l Cre

ek

7.6

7.4

7.4

8.0

7.4

7.7

7.7

7.7

Wili

er C

reek

9.7

9.7

8.3

8.5

8.5

8.2

Feca

l Fe

cal

£ £

aun

iisj

.,_-

, .

.,

. .

loo

mL)

10

0 m

L)

7.9

130

900

7.9

860

780

7.6

1,60

0 70

0

7.8

16

112

7.6

72,0

00

700

7.6

110

340

7.5

140

760

7.5

190

960

9.6

<1

<1

9.6

<1

<1

8.3

- 76

0 52

0

8.1

120

92

8.0

120

92

7.9

740

860

7.7

<l

340

Har

d

ness

, A

cidi

ty

tota

l (m

g/L

(mg/

L as

as

H)

CaC

O3)

460

<0.1

220

<.l

480

.3

350

.1

370

.1

450

280

280

110

<.l

110

<.l

200

<.l

270

<.l

260

.1

260 1

Cal

cium

, di

s

solv

ed

(mg/

L)

130 67 140

110

130

130 91 89 13 13 54 88 84 84

.3

Mag

ne

sium

, di

s

solv

ed

(mg/

L)

33 12 32 19 11 31 14 14 18 19 15 12 12 12 .0

4

Sodi

um,

dis

so

lved

(m

g/L)

21 30 20 29 44 18 27 27 34 35 36 40 33 40 <.02

Pota

s

sium

, di

s

solv

ed

(mg/

L)

0.9

4.7 .8 2.9

4.5 1.1 2.6

2.6

3.8

4.0

3.4

3.3

3.2

2.8

w

Foot

note

s at

end

of t

able

.

£ £ a I I I § Q > | H S3

I § 0 1 o st .

laoi

ez.

unem

icai

aai

a an

a is

oiop

e

Sam

ple

site

no.

St

atl0

" na

me

1 Sh

oal C

reek

at M

CC

2 Sh

oal C

reek

at W

oodh

ollo

w D

r.

1 Sh

oal C

reek

at M

CC

2 Sh

oal C

reek

at W

oodh

ollo

w D

r.

3 Sh

oal C

reek

at

12th

St.

1 Sh

oal C

reek

at M

CC

2 Sh

oal C

reek

at W

oodh

ollo

w D

r.

2 Sh

oal C

reek

at W

oodh

ollo

w D

r.2

4 W

alle

r Cre

ek a

t 45t

h St

.

4 W

alle

r Cre

ek a

t 45t

h St

.2

5 W

alle

r Cre

ek a

t 38t

h St

.

5 W

alle

r Cre

ek a

t 38t

h St

.

5 W

alle

r Cre

ek a

t 38t

h St

.2

5 W

alle

r Cre

ek a

t 38t

h St

.

Equi

pmen

t bla

nk

ratio

s io

r

Dat

e

09-0

7-94

09-0

7-94

01-1

1-95

01-1

1-95

01-1

1-95

04-2

6-95

04-2

6-95

04-2

6-95

09-0

7-94

09-0

7-94

09-0

7-94

01-1

1-95

01-1

1-95

04-2

6-95

04-2

6-95

oase

-Tio

w s

amp

Alk

alin

ity,

wat

er,

_

diss

olve

d,

Tim

e to

tal,

fet

(mg/

L as

C

aC03

)

0745

40

0

0945

14

0

0840

41

0

1000

28

0

1105

0940

41

0

1040

23

0

1050

23

0

1045

69

1047

69

1354

13

0

1245

19

0

1250

19

0

1235

18

0

0845

2

lies i

rom

bno

ai u

reeK

ana

v

Sulfa

te,

Chl

orid

e,

Fluo

ride,

di

s-

dis-

di

s

solv

ed

solv

ed

solv

ed

(mg/

L)

(mg/

L)

(mg/

L)

Shoa

l Cre

ek

34

38

0.1

44

61

.2

35

33

.1

60

69

<.l

110

81

.3

34

30

.1

35

49

.1

35

48

.1

Wal

ler C

reek

42

64

.3

41

64

.3

41

65

.3

53

78

.5

53

77

.5

52

75

.5

.04

.04

<.l

vane

r ur

eeK

, au

sun,

lex

a

Solid

s N

itro-

B

rom

ide,

;°

"a*

j ge

n,dl

s-

cSQUnm

° ni

trate

, so

lved

c^

'1-

dis-

(mfl/L)

w

n

"5-

0.25

44

9 1.

9

.10

307

.02

.25

471

.96

.15

438

.11

.39

499

.71

.23

495

.83

.07

357

.08

.06

358

.07

.20

213

.06

.21

214

.09

.27

296

.55

.39

391

.86

.39

378

.86

.40

381

.80

<.01

--

.01

is uo

nui

Nitr

o

gen,

ni

trite

, di

s

solv

ed

(mg/

L)

0.01 <.01

<.01

<.01 .0

2

.02

.05

.06

.01

.01 .02

.03

.03

.04

.01

nuea

Nitr

o

gen,

N

02+

N03

, di

s

solv

ed

(mg/

L)

1.9 .0

2

.96

.11 .72

.85

.13

.13

.07

.10

.57

.89

.89

.84

.02

Nitr

o

gen,

am

mo

ni

a,

dis

so

lved

(m

g/L)

0.02 .0

2

.13

.09

.20

.11 .27

.29

.35

.34

.01

.06

.06

.06

.01

Phos

ph

orus

, 81

5N

8180

dl

s-

(per

(p

er

solv

ed

mil)

m

il)

(mg/

L)

0.18

+8

.4

-0.0

8

.02

.12

+9.6

+1

0.6

.00

+13.

1

.01

+11.

9 +1

2.7

.14

+9.2

+1

1.6

.21

- +1

6.2

<.01

+9

.0

+16.

9

.08

+9.1

+1

7.7

.08

- +1

9.0

.18

+11.

2 +1

8.9

.04

+11.

2 +1

2.7

.13

+11.

7 +1

3.4

.21

+12.

0 +1

6.1

<.01

--

+16.

2

' Sta

ff h

eigh

t for

zer

o flo

w a

t wire

wei

ght.

2Rep

licat

e.

water chemistry of shoal creek and … chemistry of shoal creek and waller creek, austin, texas, and...

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