Megan Andring

12/4/2008

GIS Final Report

Chemical Hydrogeology of the Hill Country, Texas

Introduction

In conjunction with the Groundwater Management Area-9 (GMA-9) in central Texas, the goal of this work was to support a larger effort to evaluate the chemical hydrogeology of ten counties, specifically the variability and distribution of sulfate in groundwater within these ten counties (Figure 1).  Adopted in 2002, the Texas Groundwater Management Area delineations were intended to provide for the conservation, protection, recharging, and prevention of waste of groundwater.  Increased growth and development in the Austin area has spurred development in the Hill Country surrounding the city, in turn creating additional stresses on the local groundwater system.

The Hill Country surrounding Austin relies heavily on groundwater to readily supply potable water to citizens.  Current pumping rates have already begun to affect local spring flows.  Groundwater Management Areas are charged with optimizing groundwater usage for both residential and environmental purposes.  Increased development in the Hill Country has amplified the number of active wells and will certainly continue to do so in the future.  Because of a linear (east-west) zone of groundwater with an unusually high concentration of sulfate, the ability to drill potable wells within parts of Blanco and Hays Counties has been compromised (Figure 2).

Sulfate concentrations in wells within the GMA-9, especially in parts of Blanco and Hays Counties, are up to ten times greater than the concentration recommended by the Texas Commission on Environmental Quality for drinking water standards.  The goal of this project was to analyze sulfate variability in space in order to determine possible sulfate sources and explanations for its linear spatial distribution.  Understanding the variability of sulfate within groundwater in the Hill Country is imperative in developing a sound groundwater management plan capable of providing water of appropriate quantity and quality to the residents and springs of the Hill Country.

During the Cretaceous (65 MYA) central Texas was covered by shallow seas.  As a result, the current geology is dominated by limestone and other evaporites including gypsum and halite (Table 1).  Springs and caves associated with a karstic landscape are prevalent in the Hill Country surrounding Austin.  It is likely that the sources of sulfate in the Hill Country are interbedded layers of gypsum within local geologic formations.

Figure 1.  Map of study area.

Figure 2.  Study area.  Red oval indicates zone of high sulfate.

Mineral

Chemical Formula

Limestone

CaCO3

Halite

NaCl

Gypsum

CaSO4 * 2H2O

Table 1.  Chemical formulas of evaporite minerals commonly found in central Texas.

Data

Five data sources were used to complete this study.  Links to the website for each source can be found at the conclusion of this paper.

The National Hydrography Dataset (NHDPlus) provided extensive stream data as well as elevation data in the form of DEMs for the study area (Figure 3).  The Texas Commission on Environmental Quality (TCEQ) website provided information relating to standards for chemical constituents in groundwater.  The Texas Natural Resources Information System (TNRIS) website provided GIS data for geologic outcrops and faults within the study area.  Chemical sampling data for over 8,000 well samples in the study area were acquired from the website of the Texas Water Development Board (TWDB).  The TWDB provides chemical data for wells by county and typically includes wells that have been repeatedly sampled over the last thirty years.  Latitude and longitude for each sampled well were provided as a separate shape file from the TWDB (Figure 4).  The USGS National Map Seamless Server was also used to acquire elevation data for the study area.  A geochemical spreadsheet previously developed by the author was used in calculating various chemical parameters, including but not limited to various expressions of concentration, ionic strength, and mineral saturation indices.

Figure 3.  NHDPlus data portal.

Figure 4.  Locations of wells sampled within study area.

Methods

Chemical Data

After well chemistry data was downloaded for each county it was entered into an existing spreadsheet created by the author capable of calculating multiple chemical attributes including charge balance, ionic strength, and mineral saturation indices.  Well samples that were not charge balanced were discarded.  Nearly 8,500 charge-balanced samples exist over the study area.  In addition to being sorted by county, samples were sorted by the geologic unit from which they were taken.

Cross-plots of chemical constituents (Figure 5) were made in order to look for relationships that may be useful in analyzing the chemical processes occurring at depth in the region.  Stoichiometric relationships among chemical constituents may indicate precipitation or dissolution of various minerals.  For example, a one to one relationship between sodium and chloride may indicate (1) dissolution of halite, (2) precipitation of halite, or (3) evaporation and concentration of rain water.  Twelve different cross-plots showing the relationships of various chemical constituents were made for well samples in each county, totaling 120 plots.  Constituents are plotted in units of millimoles per liter (millimoles chemical constituent per liter of sampled water).  Chemical concentrations initially reported in units of milligrams per liter were divided by the gram formula weight of the respective constituent, giving a concentration in millimoles per liter.  The standard for sulfate in drinking water as given by the Texas Commission on Environmental Quality is 300 mg/L or 3.1 mmol/L.

Figure 5.  Cross-plot of sodium concentration versus chloride concentration in millimoles per liter for wells in Hays County.  Samples are separated by geologic unit from which they were sampled.

Chemical Data in GIS

In order to display chemical data spatially, a spreadsheet containing all well samples was saved as an Access database file.  Spatial data for each well was combined with chemical data by joining the two databases in ArcMap.  Once the join was completed, chemical concentrations could be displayed spatially (Figure 6).  Maps of concentrations of standard cations and anions along with total dissolved solids were made for the study area.

Figure 6.  Sulfate concentration in mg/L for all wells in study area.

Inverse Distance Weighting Interpolation

Inverse distance weighting (IDW) was used as a method of interpolation to represent concentrations of chemical constituents in space using available well samples.  Because sulfate variability is the main focus of this study, the geologic units containing the highest concentrations of sulfate were identified and include the Glen Rose, Edwards, Georgetown, Cow Creek, and Trinity formations.  IDW was performed to interpolate sulfate variability for all wells in the study site (Figure 7) as well as for samples from individual geologic units.

Figure 7.  Sulfate variability for all wells in study area using inverse distance weighting as method of interpolation.

Stream Buffer Creation

After comparing locations of samples with sulfate concentrations greater than the TCEQ standard to the location of streams, it appeared early on that the highest concentrations of sulfate were located within interfluves, the area between streams.  In order to analyze this further, a one mile buffer was created around streams in the study area.  Samples in the study area with sulfate concentrations higher than 300 mg/L were separated from those whose concentrations were below 300 mg/L.  Of the samples with sulfate concentrations greater than 300 mg/L, those that lie outside of the one mile stream buffer were excluded from those that lie within the buffered zone (Figure 8).

Figure 8.  All samples with sulfate concentrations greater than 300 mg/L and one mile stream buffer.

Results & Interpretations

Chemical Data

The relationship between calcium and sulfate as shown on cross-plots for Blanco and Hays Counties are consistent with dissolution of gypsum; after a certain point calcium and sulfate both increase together (Figure 9).  Plots of sulfate concentration with depth show a general increase in concentration with depth (Figure 10).  Cross-plots also indicate a strong relationship between sulfate concentration and total dissolved solids.  The highest sulfate values are found in samples from the Edwards, Georgetown, and Glen Rose Formations.

Figure 9.  Calcium versus sulfate in mmol/L for samples in Hays County.

Figure 10.  Sulfate in mg/L versus depth for Hays County.

Chemical Data in GIS

As mentioned previously, the highest concentrations of sulfate are distributed linearly between Blanco and Hays Counties (Figure 11).  In agreement with the chemical cross-plot data, the highest concentrations of calcium are generally found in the areas that contain the highest sulfate concentrations (Figure 12).  The highest concentrations of total dissolved solids are also found in areas with high sulfate concentrations (Figure 13).  This data also supports the theory that dissolution of gypsum is responsible for the high concentrations of sulfate in the region.

Figure 11.  Sulfate in mg/L for all samples in study area.

Figure 12.  Calcium in mg/L for all samples in study area.

Figure 13.  Total dissolved solids in mg/L for all samples in study area.

The results of the inverse distance weighting interpolations were not as useful as originally planned, mostly because of the dataset used.  Samples were separated by the geologic unit from which they were sampled so that interpolated surfaces of sulfate concentration could be made for each unit (Figure 14).  Because samples from each geologic formation were not taken uniformly across the study area, an implicit bias exists when trying to interpolate sulfate concentrations for each unit across the study area.  The interpolations are useful for regions where many samples exist, but lack validity where the samples from individual units are rare.  Interpolations confirm that highest sulfate concentrations are found within the Glen Rose, Edwards, and Georgetown Formations.

Figure 14.  Variability in sulfate concentration for Glen Rose Formation samples using inverse distance weighting as method of interpolation.

Elevation

Sulfate concentration variability was compared with elevation over the study area (Figure 15).  Upon first inspection it appeared that most samples with high sulfate concentration existed on the interfluves between streams.  The presence of high sulfate zones in interfluves supports the theory that a layer of gypsum within the Glen Rose formation may be responsible for elevated sulfate concentrations.  As groundwater flows toward streams and springs in the region, one would expect that a gypsum layer would be preferentially flushed in these zones, with the remnants of the gypsum layer remaining in the interfluves.

Figure 15.  Sulfate concentration for all wells displayed over elevation.

Stream Buffer

Blanco, Hays, and Travis Counties are affected the most by wells that produce water with unsatisfactory sulfate concentrations (Figure 18).  Surprisingly, eighty percent of Blanco County wells containing high sulfate concentrations lie within interfluves (lie outside the one mile stream buffer) while only thirty percent of high sulfate wells in Hays County lie outside of the buffer.  Only twenty-five percent of high-sulfate Travis County wells lie outside of the buffer.   Groundwater in the region generally flows east-southeast.  It is possible that as groundwater flows from Blanco County toward the springs in Hays and Travis Counties, sulfate is being flushed toward the zone within the stream buffer.

County

Total Samples

% of Samples with SO4 >300 mg/L

% of Samples with SO4 >300 mg/L Outside of 1 Mile Stream Buffer

Bandera

401

7.98

71.88

Bexar

1364

3.45

53.19

Blanco

848

10.14

80.23

Comal

560

3.39

31.58

Gillespie

748

0.80

83.33

Hays

603

8.96

31.48

Kendall

559

4.29

62.50

Kerr

517

2.13

18.18

Medina

1474

2.65

76.92

Travis

1347

15.74

25.94

Table 2.  Summary of samples with sulfate values greater than 300 mg/L by county and their relationship to the one mile stream buffer.

Summary

(1)  Cross-plots of chemical constituents suggest that dissolution of gypsum is occurring within the system.

(2)  Cross-plots of sulfate versus depth show that sulfate generally increases with depth and the Glen Rose, Edwards, and Georgetown Formations are characteristically high in sulfate.

(3)  Areas of high sulfate concentration correspond to areas of high calcium and TDS concentrations, supporting the theory that dissolution of gypsum layers within the Cretaceous units of central Texas is responsible for high sulfate concentrations observed in wells.

(4)  Most high-sulfate wells in Blanco County are located on interfluves while most high-sulfate wells in Hays and Travis Counties are located within one mile of streams.

(5)  Data supports the theory that interbedded gypsum layers within the Glen Rose and other Cretaceous formations are being flushed as groundwater flows toward springs and streams in an east-southeast direction.

Future Work

In addition to the work completed for this project, well logs from the area will be analyzed for the presence of any gypsum-bearing strata that may represent possible sources of sulfate.  Water samples have been collected for isotope analyses in the hopes that isotope data may give insights into both the age of groundwater as well as potential flowpaths.

Data Sources

National Hydrography Dataset Plus

http://www.horizon-systems.com/nhdplus/data.php

Texas Commission on Environmental Quality

http://www.tceq.state.tx.us/

Texas Natural Resources Information System

http://www.tnris.state.tx.us/

Texas Water Development Board

http://www.twdb.state.tx.us/home/index.asp

USGS National Map Seamless Server

http://seamless.usgs.gov/

Acknowledgements

I would like to thank Ron Fieseler, Blanco County groundwater manager, for access to data and for his suggestions throughout this research.  I would also like to thank Dr. Phil Bennett of the Jackson School of Geosciences at the University of Texas at Austin for his guidance and assistance in developing the geochemical spreadsheet that was used in analyzing water chemistry.