Water quality in south-central Texas, Texas, 1996–98

2000 CONTENTS

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM .............................................................. IV SUMMARY OF MAJOR FINDINGS ................................................................................................... 1 INTRODUCTION TO THE UPPER PART OF THE SOUTH-CENTRAL TEXAS STUDY UNIT ......... 3 MAJOR FINDINGS ............................................................................................................................ 6 Stream Quality Is Affected Most by Urban Activities .................................................................... 6

NATIONAL PERSPECTIVE—Trends in Water Quality Associated With Urban Development, as Reflected in Bottom Sediment of Lakes and Reservoirs, Are Not Unique to San Antonio ... 12

NATIONAL PERSPECTIVE—Nationally and Locally, Biological Community Status Is Related to Watershed Development ......................................................................................... 13

Edwards Aquifer Water Quality Remains Excellent ..................................................................... 14

NATIONAL PERSPECTIVE—The Frequencies of Pesticide Detection in Ground Water Vary in Comparison With Those in Ground Water Nationally—Compounds Detected Were the Same ......................................................................................................................... 16

Trinity Aquifer Water Quality Is Mostly Unaffected by Human Activities ....................................... 20

STUDY UNIT DESIGN ...................................................................................................................... 22 GLOSSARY ....................................................................................................................................... 24 REFERENCES .................................................................................................................................. 25 APPENDIX A—WATER-QUALITY DATA FROM SOUTH-CENTRAL TEXAS IN A NATIONAL CONTEXT .......................................................................................................................................... 27

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

THIS REPORT summarizes major findings about water quality in south-central Texas that emerged from an assessment conducted between 1996 and 1998 by the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program. Water quality is discussed in terms of local and regional issues and compared to conditions found in all 36 NAWQA study areas, called Study Units, assessed to date. Findings also are explained in the context of selected national benchmarks, such as those for drinking-water quality and the protection of aquatic organisms. The NAWQA Program was not intended to assess the quality of the Nation’s drinking water, such as by monitoring water from household taps. Rather, the assessments focus on the quality of the resource itself, thereby complementing many ongoing Federal, State, and local drinking-water monitoring programs. The comparisons made in this report to drinking-water standards and guidelines are only in the context of the available untreated resource. Finally, this report includes information about the status of aquatic communities and the condition of instream habitats as elements of a complete water-quality assessment.

Many topics covered in this report reflect the concerns of officials of State and Federal agencies, water-resource managers, and members of stakeholder groups who provided advice and input during the assessment. Residents who wish to know more about water quality in the areas where they live will find this report informative as well.

THE NAWQA PROGRAM seeks to improve scientific and public understanding of water quality in the Nation’s major river basins and ground-water systems. Better understanding facilitates effective resource management, accurate identification of water-quality priorities, and successful development of strategies that protect and restore water quality. Guided by a nationally consistent study design and shaped by ongoing communication with local, State, and Federal agencies, NAWQA assessments support the investigation of local issues and trends while providing a firm foundation for understanding water quality at regional and national scales. The ability to integrate local and national scales of data collection and analysis is a unique feature of the USGS NAWQA Program.

South-Central Texas is one of 51 water-quality assessments initiated since 1991, when the U.S. Congress appropriated funds for the USGS to begin the NAWQA Program. As indicated on the map, 36 assessments have been completed, and 15 more assessments will conclude in 2001. Collectively, these assessments cover about one-half of the land area of the United States and include water resources that are available to more than 60 percent of the U.S. population.

IV National Water-Quality Assessment Program

Stream and River Highlights

The South-Central Texas Study Unit encompasses the Nueces, San Antonio, and Guadalupe River Basins. The 1996–98 assessment involved only the upper part of the Study Unit. Streams and rivers that originate in the rugged hills of the Edwards Plateau generally gain water as they flow southeastward toward the Edwards aquifer outcrop (recharge zone). As they flow across the highly permeable, faulted, and fractured rocks of the recharge zone, most lose substantial amounts of flow directly into the aquifer.

Summary of Major Findings Water Quality in South-Central Texas

INTRODUCTION TO THE UPPER PART OF THE SOUTH-CENTRAL TEXAS STUDY UNIT

The upper part of the South-Central Texas Study Unit (hereinafter, Study Unit) encompasses parts of the topographically rugged and picturesque Edwards Plateau and the comparatively flat, gently coastward-sloping Gulf Coastal Plain physiographic regions. An abrupt topographic break, the Balcones escarpment, separates the two landforms (fig. 1).

San Antonio is the principal urban area in the Study Unit. The agricultural areas of primary interest are west of San Antonio. Rangeland predominates in the Edwards Plateau.

Introduction to the Upper Part of the South-Central Texas Study Unit The Study Unit contains the Edwards aquifer in the Balcones fault zone, a zone of northeastward-trending parallel faults that straddles the Balcones escarpment. The Study Unit also contains the Trinity aquifer (fig. 2) in the “Hill Country,” the local name for the eastern part of the Edwards Plateau in the region.

An Unusual Physical System

Surface water and ground water in the Study Unit are uniquely interrelated. Rainfall reaches streams deeply incised into the marl, shale, and limestone of the Edwards Plateau as springflow and surface runoff. The streams generally gain water as they flow southsoutheastward from headwaters in the higher elevations of the plateau. As the major streams flow across the faulted and fractured carbonate rocks of the Edwards aquifer outcrop (recharge zone), they lose substantial amounts of flow directly into the highly permeable aquifer.

Ground Water Predominates

Ground water accounts for nearly all of the water supply in the Study Unit (fig. 3), and the Edwards aquifer, one of the most productive aquifers in the world, is the principal source. Withdrawals from the Edwards aquifer meet the water-supply needs of more than 1.5 million people in the greater San Antonio region and support farming and ranching west of San Antonio. The aquifer sustains the flows of Comal and San Marcos Springs, which attract tourists to the region, yield base flow to the Guadalupe River to meet downstream water requirements, and provide habitat for several threatened and endangered species. The Edwards aquifer harbors diverse aquatic communities above and below land surface; at least 90 described species, one-half of which are subterranean, are unique to the region.

Figure 2. Areal extent of the Edwards and Trinity aquifers sampled during 1996–98 (modified from [4]). The outcrop of the Edwards aquifer essentially is the recharge zone; the subcrop essentially is the confined zone. The outcrop of the Trinity aquifer is approximately coincident with the Hill Country. The subcrop of the Trinity aquifer (not shown), which extends beneath the Edwards aquifer, is not a source of water supply in the region and was not sampled.

The Trinity aquifer, although much less productive than the Edwards aquifer, is the most commonly used and reliable source of water supply in the Hill Country.

Development is Increasing

The watersheds of major streams in the Edwards Plateau that recharge the Edwards aquifer were largely undeveloped rangeland in the late 1990s—but that is changing. A common sight in the area is the construction of new

Water Quality in South-Central Texas subdivisions on land that for generations has been devoted to ranching. Edwards, Bandera, and Kendall Counties were among the top 10 statewide and the top 50 nationally in percentage of population increase during 1990–98. Urban development in the Edwards aquifer recharge zone, particularly in Bexar County (San Antonio), also is increasing. Bexar County ranked fourth among counties statewide and 15th nationally in numeric population increase during 1990–98. Continued development on a scale suggested by those U.S. Census Bureau statistics has the potential to affect surface- and ground-water quality in the Study Unit.

Figure 3. Nearly all the withdrawals from the Study Unit were ground water. About 70 percent of the ground-water withdrawals were from the Edwards aquifer.

The Edwards Aquifer Is Particularly Vulnerable to Contamination

The highly permeable rocks that compose the Edwards aquifer recharge zone, and development in the watersheds of major streams in the Edwards Plateau and in the recharge zone, make the Edwards aquifer particularly vulnerable to contamination. That vulnerability and the dependence of so many people on the aquifer combine to make the water quality of the Edwards aquifer and the streams that recharge it a critical issue for the future of the region.

Climate and Hydrologic Conditions Can Affect Water Quality

Hydrologic conditions in the Study Unit often are extreme. Months-long droughts that strain water supplies and produce widespread crop failure commonly are followed by wet periods that include torrential rains and flash floods. Such was the pattern during the 1996–98 period of intensive sampling (figs. 4, 5).

Whether hydrologic conditions are wet or dry can affect water quality. For example, during wet conditions, proportionately more of the streamflow is surface runoff. Runoff that contains excessive nutrients, pesticides, or other contaminants washed off land sur-

Figure 5. The water level in Edwards aquifer index well J–17 in Bexar County rose substantially during wet periods following the regional droughts of 1996 and 1998; and reflects the effects of pumpage at San Antonio as well as natural fluctuations in recharge and discharge. Although pumpage at San Antonio has increased fivefold since the 1930s, Edwards aquifer water levels show no long-term declines because the aquifer readily accepts recharge when rainfall is plentiful.

Introduction to the Upper Part of the South-Central Texas Study Unit faces can degrade stream quality. During dry conditions, stream quality is more strongly influenced by the quality of the base flow (and wastewater-treatment plant discharge, if present).

Figure 4. Daily mean streamflow of the Frio River at Concan, a basic site for sampling, reflects the regional droughts of 1996 and 1998 and the subsequent wet periods. Streamflow at the site can increase almost instantly by a factor of 100 or more for short periods in response to intense rainfall. The region near the Balcones escarpment is more prone to flash floods than anywhere else in the Nation [5, p. 63].

The Study is a Benchmark for Changes to Come

This study is more of a benchmark from which to measure future water-quality changes than a documentation of problems. Population growth and development bring changes in land use, and land use can have a strong influence on water quality. Accordingly, part of the study design focuses on land use. Surface-water sampling was done to assess the effects of urban, agricultural, and rangeland use on stream quality during stormflows and during normal and low flows. Ground-water sampling was done to assess the overall quality of ground water in the Edwards and Trinity aquifers and to assess the relation between ground-water quality and urban land use in the recharge zone in San Antonio. Synoptic samples were collected primarily from domestic and monitor wells in the Edwards aquifer recharge zone, public-supply wells in the Edwards aquifer confined zone, and domestic wells in the upper and middle zones of the Trinity aquifer. (See “Study Unit Design,” p. 22.)

Stream Quality Is Affected Most by Urban Activities

Chemical and biological indicators in monitored streams in key land-use settings in the Study Unit (fig. 6) show that stream-quality degradation tends to be associated with urban land to a greater degree than with agricultural land or rangeland. With a few exceptions, pesticides, VOCs, and trace elements were either not detected or detected at concentrations below levels of concern for human health or aquatic life in all monitored streams.

Discharges from wastewater treatment plants had the greatest effect on nutrient concentrations of any identifiable source in the Study Unit [6]. Nitrate concentrations were appreciably higher in an urban stream downstream from

Figure 6. Stream-water sampling in key land-use settings in the Study Unit was done at 9 sites, bed-sediment sampling at 15 sites, and fish-tissue sampling at 14 sites.

Water Quality in South-Central Texas four wastewater treatment plants (San Antonio River at Elmendorf) than in streams in other land-use settings (fig. 7). Nitrate concentrations in the San Antonio River at Elmendorf generally increased as the percentage of wastewater-treatment plant discharge (effluent) in streamflow increased (fig. 8). Although the San Antonio River historically has not been a source of drinking water, the median nitrate concentration at Elmendorf (11 milligrams per liter [mg/L]) exceeded the U.S. Environmental Protection Agency (USEPA) maximum contaminant level (MCL) for drinking water (10 mg/L). No aquatic-life guideline for nitrate has been established. The flow-weighted mean nitrate concentration at Elmendorf, 5.8 mg/L, was about the 90th percentile among 372 NAWQA stream sites nationwide.

Ammonia at sufficient concentrations, which vary with water

Nutrients in Water

temperature and acidity [7], can be toxic to fish. Median ammonia concentration at Elmendorf was 0.1 mg/L, the same as the estimated national background concentration in streams essentially unaffected by human activities [8]. Modern treatment plants effectively remove ammonia from wastewater by converting it to nitrate.

Land use had less effect on nutrient concentrations than proximity to wastewater treatment plants. The median nitrate concentration in an urban stream not affected by wastewater-treatment plant discharge (Salado Creek at San Antonio) was 0.56 mg/L, about the same as the estimated national background nitrate concentration (0.6 mg/L) in streams essentially unaffected by human activities [8]. The median nitrate concentration in an agricultural (cropland) stream (Medina River at La Coste) was 2.0 mg/L, higher than that of Salado Creek and showing the effect of runoff containing nitrate from fertilizers. The median is typical of median nitrate concentrations in surface water draining agricultural areas in selected NAWQA Study Units nationwide [8]. Median nitrate concentrations in four rangeland streams (Frio, Sabinal, Guadalupe, and Blanco Rivers) were near background levels (0.34 to 0.77 mg/L).

Nitrate concentrations vary over time in response to hydrologic conditions, wastewater-treatment plant discharges, plant and algal uptake, and timing of fertilizer application. However, no seasonal pattern in concentrations associated with any land-use setting was evident (fig. 7).

Total phosphorus concentrations also were highest in the San Antonio River at Elmendorf. The median total phosphorus concentration was 1.6 mg/L, and all concentrations were greater than the USEPA goal of 0.1 mg/L to control nuisance algae and aquatic plant growth. The flow-weighted mean total phosphorus concentration at Elmendorf, 1.2 mg/L, was about the 98th percentile among 372 NAWQA stream sites nationwide. The median total phosphorus concentration in Salado Creek at San Antonio, the urban stream not affected by wastewater-treatment plant discharge, was 0.066 mg/L, and individual concentrations commonly exceeded the USEPA goal of 0.1 mg/L. The median total phosphorus concentration for the Medina River at

Figure 7. Nutrient concentrations were consistently higher in streams influenced by urban wastewater-treatment plant discharge (San Antonio River) and agricultural runoff (Medina River) than in an urban stream (Salado Creek) or a rangeland stream (Frio River). Greater rainfall and streamflow during 1997 and early 1998 diluted nitrate concentrations associated with wastewater-treatment plant discharge in the San Antonio River at Elmendorf.

Figure 8. Differences in rainfall-runoff relations with flow in the San Antonio River at Elmendorf account for variability in the dilution of wastewater-treatment plant discharge. Concentrations of nitrate in the river increased appreciably with the percentage of wastewater-treatment plant discharge in the streamflow.

Major Findings La Coste, the agricultural stream, was 0.030 mg/L; and the median concentration for each of the four rangeland streams was less than 0.010 mg/L, the minimum reporting level. Total phosphorus concentrations in all streams exceeded the USEPA goal during stormflows. Higher phosphorus concentrations during stormflows is consistent with the adherence of phosphorus to sediment particles and the increase in suspended sediment concentrations during stormflows.

More pesticides and their breakdown products (hereinafter, pesticides) were detected in Salado Creek at San Antonio (25 of 83 analyzed) than in the San Antonio River at Elmendorf downstream from the wastewater treatment plants (18 of 83) and the Medina River at La Coste (15 of 83) [6]. Samples from rangeland streams were not analyzed for pesticides. The maximum concentrations of 7 of the 10 pesticides detected in all three streams occurred in Salado Creek at San Antonio. In all three streams, some pesticide concentrations increased in the spring, which is consistent with the time of year for application.

Table 1. Pesticides and VOCs in stream water for which at least one sample exceeded an aquatic-life guideline

No concentration of a pesticide for which a drinking-water standard has been established (25 of 31 pesticides detected) exceeded that standard in any of the three streams. However, aquatic-life guidelines (established for 17 of 31 pesticides detected) were exceeded in at least one sample for 5 pesticides in Salado Creek at San Antonio, 3 insecticides in the

Water Quality in South-Central Texas

Pesticides in Streams—How Toxic to Aquatic Life?

San Antonio River at Elmendorf, and 1 insecticide in the Medina River at La Coste (table 1).

The most frequently detected pesticides in all three streams were atrazine, deethylatrazine, and prometon (fig. 9). Atrazine was detected in all of the samples from each of the three streams.

The number of pesticides and the most frequently detected pesticides in each stream change when frequency of detection is based on a common concentration of 0.05 microgram per liter (μg/L). (See box, p. 9.) In Salado Creek at San Antonio, 13 pesticides were detected on the basis of the common concentration (compared with 25 detected without regard to a common concentration). The five most frequently detected pesticides and their detection frequencies based on the common concentration were

Figure 9. Pesticides and VOCs were detected more frequently in urban streams than agricultural streams. The five pesticides and VOCs most frequently detected in stream water are shown. More herbicides than insecticides (shaded brown) were detected.

In the San Antonio River at Elmendorf, only 5 pesticides were detected on the basis of the common concentration (compared with 18). The five detected and their detection frequencies were In the Medina River at La Coste, 5 pesticides also were detected on the basis of the common concentration (compared with 15). Detection frequencies of all 5 pesticides based on the common concentration were less than 10 percent.

About Frequencies of Detection

The number of pesticides detected and, in general, the frequencies of detection based on a common concentration of 0.05 μg/L are substantially less than those not based on a common concentration. This result underscores the fact that the majority of pesticide detections in stream water represent extremely low concentrations. Less than 0.05 μg/L means less than 1 part contaminant per 20 billion parts water, which is about the same concentration as an aspirin tablet dissolved in an olympic-size swimming pool.

All of the surface-water samples contained more than one pesticide, which is consistent with national NAWQA findings that show pesticides commonly occur in mixtures of several compounds [13]. Drinking-water standards for combinations of pesticides have not been established, and very little is known about the effects of mixtures of pesticides on aquatic life.

The most VOCs (33 of 86 analyzed) were detected in the

Major Findings San Antonio River at Elmendorf. Twenty-one VOCs were detected in Salado Creek at San Antonio. As was the case with pesticide detections, the fewest VOC detections (15) were in the Medina River at La Coste [6]. The maximum concentrations of 7 of the 10 VOCs detected in all three streams were from the San Antonio River at Elmendorf. No samples from rangeland streams were analyzed for VOCs. Of the 20 VOCs detected for which drinking-water standards and (or) aquatic-life guidelines have been established, one VOC—trichloromethane— exceeded the Canadian water-quality guideline for the protection of aquatic life [14] in 8 of 13 samples in the San Antonio River at Elmendorf (table 1). Trichloromethane is moderately toxic to aquatic life [15], but it is not persistent in surface water; one-half of it will have evaporated after several days. No other VOC concentrations in any of the three streams approached the levels of concern for the protection of human health or aquatic life.

Four of the 5 VOCs most frequently detected in the San Antonio River at Elmendorf (bromodichloromethane, chlorodibromomethane, trichloromethane, and tribromomethane) (fig. 9), are by-products of water chlorination. The presence of these trihalomethane compounds is consistent with the fact that a major part of the flow of the river was wastewater-treatment plant discharge. On the basis of a common concentration of 0.1 μg/L, only 5 VOCs were detected (compared with 33 detected without regard to a common concentration). The same four trihalomethanes were detected at essentially

Water Quality in South-Central Texas the same frequencies as those detected without regard to a common concentration (fig. 9). The fifth, methylbenzene, was detected in 31 percent of the samples.

Four of the 5 VOCs most frequently detected in Salado Creek at San Antonio (cis-1,2-dichloroethene, methylbenzene, acetone, and carbon disulfide) (fig. 9) are industrial chemicals with a variety of uses. They probably entered the stream in urban runoff. The fifth VOC most frequently detected (methyl tert-butyl ether [MTBE]) was 1 of 3 VOCs detected on the basis of a common concentration of 0.1 μg/L. MTBE was the only one of the three with a detection frequency (50 percent) greater than 10 percent. MTBE is a gasoline additive used primarily to reduce air pollution and has received recent publicity because of its potential to contaminate ground water. Although the use of MTBE in gasoline in the area has not been mandatory, it likely has been in some of the gasoline supplied to service stations and subsequently entered the environment during refueling at service stations or from engine exhaust, leaking storage tanks, or spills.

The five VOCs most frequently detected in the Medina River at La Coste (fig. 9) are hard to associate with a specific agricultural use. None of the five were detected on the basis of a common concentration of 0.1 μg/L. Two other VOCs, benzene and methylbenzene, were detected on the basis of the common concentration (compared with 15). The frequency of detection for both was 14 percent.

As with pesticides in stream water, the substantially smaller number of VOCs detected and, in general, the reduced frequencies of detection based on a common concentration show that the majority of VOC concentrations were extremely low—less than 1 part contaminant per 10 billion parts water.

The occurrence of environmentally persistent organochlorine compounds (organochlorine insecticides and polychlorinated biphenols [PCB]), semivolatile organic compounds (SVOC) including polycyclic aromatic hydrocarbons (PAH), and trace elements (primarily metals) was assessed by measuring their concentrations in stream sediment and fish and clam tissue in key land-use settings. (Urban and urban/effluent dominated samples were grouped together as urban.)

Table 2. Concentrations of contami- nants in urban, agricultural, and

Sediment samples from 15 sites (fig. 6) were analyzed for 32 organochlorine pesticides, total PCBs, and 63 SVOCs. The greatest number of contaminants detected, and generally the highest concentrations (table 2), occurred in urban stream sediment. An average of 26 contaminants per site were found at 6 urban sites. An average of 10 contaminants per site were found at 3 agricultural sites; and an average of 8 contaminants per site were found at 6 rangeland sites.

The total organochlorine pesticide concentrations (sum of concentrations of all pesticides detected at each site) in some urban sediment samples were among the highest of 836 NAWQA stream sites nationwide. Total concentrations at 3 of the 6 urban sites ranked in the top 12 percent. In contrast, no pesticides were detected in sediment from 2 of the 3 agricultural streams, and none were detected in sediment from any of the 6 rangeland sites. PCBs were not detected in any stream-sediment samples.

The stream-sediment samples also were analyzed for 44 trace elements. Although the frequencies of detection were similar among urban, agricultural, and rangeland sites (23 to 28 per site), the concentrations generally were highest in urban sediment (table 2).

For the 27 organochlorine pesticides and total PCBs analyzed in wholebody and carcass (wholebody without liver) fish tissue from nine sites, the greatest number of contaminants, and generally the highest concentrations (table 2), also were found in urban samples. An average of 5 contaminants per sample were found in 7 urban fish-tissue samples; an average of 2 contaminants per sample were found in 10 agricultural fish-tissue samples; and an average of 2 contaminants per sample were found in 4 rangeland fish-tissue samples. The fish-tissue samples were composites from 3 to 9 fish of the same species, typically common carp (Cyprinus carpio).

The total organochlorine pesticide concentrations in urban wholebody and carcass fish-tissue samples ranked in the middle-tohigh range among concentrations in wholebody tissue from 505 NAWQA sites nationwide. Three of the 7 urban sample concentrations were in the top 25 percent. As with total organochlorine pesticide concentrations in sediment, concentrations in agricultural and rangeland fish-tissue samples generally were lower than those in urban samples. Only 2 of the 10 agricultural samples and none of the 4 rangeland samples contained total organochlorine pesticide concentrations in the upper 50 percent of concentrations nationwide.

Total PCB concentrations in wholebody and carcass fish-tissue samples generally followed the same pattern. Concentrations were in the top 15 percent of wholebody PCB concentrations nationwide in 4 of the 7 urban samples, 1 of the 10 agricultural samples, and none of the 4 rangeland samples.

Trends in Water Quality Associated With Urban Development, as Reflected in Bottom Sediment of Lakes and Reservoirs, Are Not Unique to San Antonio

Water Quality in South-Central Texas

Nationally and Locally, Biological Community Status Is Related to Watershed Development

In clam (Corbicula) tissue from 12 sites, the concentrations of trace elements generally were highest in the samples from urban sites (table 2). In fish-liver tissue from nine sites, however, no clear association between trace element concentration and watershed land use was evident.

At some sites, concentrations of some sediment contaminants exceeded the Canadian sediment guidelines for the protection of aquatic life [17]. The guidelines are termed “probable effects levels” (PEL). The PEL is the concentration above which adverse effects on aquatic life are predicted to occur frequently. PELs have been established for 18 of the 43 organochlorine compounds and SVOCs and 8 of the 28 trace elements detected in sediment. In one each of 4 urban sediment samples, DDT, DDE, chlordane, and lead concentrations exceeded the respective PEL; in 1 rangeland sediment sample, the mercury concentration exceeded the PEL.

Guidelines to protect fish-eating wildlife (for the State of New York) [18] have been established for 8 of the 10 organochlorine compounds detected in wholebody and carcass fish tissue but not for the 19 trace elements detected in clam and fish-liver tissue. Total PCB concentrations in 6 of the 7 urban fish-tissue samples and 2 of the 10 agricultural

Major Findings fish-tissue samples exceeded the guideline for PCBs. The guideline for total DDT was exceeded in 1 of the 7 urban fish-tissue samples.

Intense rainstorms on terrain conducive to rapid runoff result in frequent floodflows in the Study Unit. In June 1997, about a month before biological samples were collected, severe flooding on the Frio and Sabinal Rivers affected the biological communities, particularly the invertebrates. Estimated recurrence intervals for the peak floodflows at the sampling sites on the Frio and Sabinal Rivers were 15 and 90 years, respectively [19]. For both rivers, the invertebrate communities were more degraded in 1997 than in 1996 and 1998

Figure 10. Fifty-eight of the Edwards aquifer wells sampled were in the recharge zone, and 30 were in the confined zone. Most of the recharge-zone wells were for domestic supply or monitoring, and nearly all of the confined-zone wells were for public supply. Almost all of the urban recharge-zone samples contained both pesticides and VOCs. Most of the 31 sampled wells from the upper and middle zones of the Trinity aquifer were for domestic supply. About one-third of the Trinity aquifer samples contained no pesticides or VOCs, and only three samples contained both.

Water Quality in South-Central Texas during considerably drier conditions. Changes in the percentages of certain algae reflected less siltation and reduced nutrient concentrations after scouring floodflows. The fish communities were not noticeably affected by the flooding.

More than 30 non-native aquatic species, many of tropical origin, are known to exist in the Study Unit [20, table 5]. Non-native aquatic species are considered a threat to native species [21]. However, the findings of this study show that native fish species remain dominant at the locations sampled. The redbreast sunfish (Lepomis auritus), originally introduced in Texas as a game fish, was the most common non-native fish species, particularly in the Blanco River where clear-flowing water favors this species.

Edwards Aquifer Water Quality Remains Excellent

The quality of water in the Edwards aquifer is “excellent” according to the Edwards Aquifer Authority (EAA), the State agency charged with managing, conserving, preserving, and protecting the aquifer [22]. Comprehensive analyses of water samples from 88 wells (fig. 10) (one sample per well) in the Edwards aquifer in urban, agricultural, and rangeland areas of the recharge and confined zones support that characterization. However, the fact that water samples contained detectable concentrations of pesticides and VOCs, even though the levels were well below allowable maximums in drinking water, shows that human activities can affect the aquifer.

Hydrogeology and Land-Use Distribution—Good for Edwards Aquifer Water Quality

The occurrence of contaminants in the Edwards aquifer is influenced by hydrogeology and land use. The faulted and fractured limestone of the Edwards aquifer recharge zone allows unrestricted downward movement of water containing contaminants into the ground-water-flow system, whereas the confined zone has a buffer (confining unit) between land surface and the aquifer that restricts the downward movement of water and contaminants. (See box above.) Thus, within the recharge zone, land use noticeably influences water quality; but in the confined zone, land use has much less effect.

Nitrate, which dissolves readily in water, is widespread in the Edwards aquifer. It was detected in all but 1 of 88 samples. The median nitrate concentration was 1.4 mg/L in the recharge zone and 1.7 mg/L in the confined zone.

Primarily public-supply wells were sampled in the confined zone. The median concentration of 1.7 mg/L in that zone, although well below the USEPA MCL for drinking water (10 mg/L), was in the top 10 percent of median nitrate concentrations of major aquifers sampled by NAWQA nationwide. The highly permeable, faulted and fractured rocks of the recharge zone readily allow infiltration of water that contains nitrate and dissolved oxygen. Nitrate is more stable under aerobic conditions. Nitrate commonly migrates large distances from recharge areas in fractured-rock aquifers that contain considerable dissolved oxygen [26]. The median dissolved oxygen concentration in the confined zone was 6.0 mg/L.

Orthophosphate, which accounts for nearly all the dissolved phosphorus, was less prevalent than nitrate. Orthophosphate was detected in 49 of 88 well-water samples. Concentrations throughout the aquifer were low; the median concentration was 0.015 mg/L.

Seventeen of the 83 pesticides analyzed in recharge-zone samples and 18 of the 47 pesticides analyzed in confined-zone samples were detected (fig. 10); 17 of 18 were the same in both zones. At least one-half of the water samples with a pesticide detection contained two or more pesticides. The concentration of each of the

The Frequencies of Pesticide Detection in Ground Water Vary in Comparison With Those in Ground Water Nationally— Compounds Detected Were the Same

Water Quality in South-Central Texas 13 pesticides detected for which drinking-water standards or guidelines have been established was substantially less than the respective allowable maximum [27]. However, standards for combinations of pesticides have not been established, and very little is known about the effects of mixtures of pesticides on human health.

Figure 11. Pesticides and VOCs in the Edwards aquifer were most frequently detected in urban recharge-zone wells. Shown are those pesticides and VOCs detected in more than 10 percent of samples. Only one insecticide, p,p'-DDE (shaded brown), a breakdown product of DDT, was detected in more than 10 percent of samples; the other pesticides shown here are herbicides.

More pesticides were detected and frequencies of detection were greater in urban recharge-zone samples than in nonurban recharge-zone or confined-zone samples (fig. 11). Pesticide usage in urban areas likely is higher than in nonurban areas in the recharge zone. Nonurban areas in the recharge zone primarily are rangeland where usage is nonexistent or very low. Little if any direct (downward) recharge occurs in the confined zone compared with the recharge zone. Although pesticide usage in urban and agricultural areas overlying the confined zone could be greater, fewer pesticides will reach the aquifer because of the lack of direct recharge.

As was the case for stream water, the pesticide detection picture changes considerably when detection frequency is based on a common concentration of 0.05 μg/L. Among urban recharge-zone samples, only five pesticides were detected (compared with 17 without regard to a common concentration). Atrazine was the most frequently detected but only in 5.6 percent of the samples. On the basis of the common concentration, no pesticides were detected in any nonurban recharge-zone or confined-zone samples (compared with 5 and 6, respectively, without regard to a common concentration). As in stream water, the majority of pesticide detections thus represent concentrations of less than 1 part contaminant per 20 billion parts water.

Four of the 5 pesticides most frequently detected in ground water from urban recharge-zone wells— the herbicides deethylatrazine, atrazine, simazine, and prometon (fig. 11)—were the same as 4 of the 5 pesticides most frequently detected in urban streams (Salado Creek at San Antonio and San Antonio River at Elmendorf) (fig. 9). Although both sites are

The Edwards Aquifer Harbors Diverse Subterranean and Unique Spring-Dependent Aquatic Species

downstream from the recharge zone, the contaminants detected at the sites likely are typical of contaminants in urban runoff in northern San Antonio, which is in the recharge zone.

Atrazine and deethylatrazine were the most frequently detected pesticides in Edwards aquifer water and were among the top three most frequently detected pesticides in stream water. Although atrazine was detected in more than three-fourths of urban recharge-zone wells, the maximum measured concentration was about 23 times less than the drinking-water MCL, 3 μg/L. No drinking-water standard or guideline has been established for deethylatrazine.

Thirty-four of 86 VOCs analyzed were detected in samples

Water Quality in South-Central Texas from Edwards aquifer wells. Unlike pesticides, the fewest VOCs (12) were detected in urban recharge-zone samples. Sixteen were detected in nonurban recharge-zone samples, and 27 were detected in confined-zone samples. In general, however, frequencies of detection were greatest in urban recharge-zone samples.

Trichloromethane, the most frequently detected VOC, was detected in three-fourths of the urban recharge-zone and confined-zone samples (fig. 11). Measured concentrations of trichloromethane were very low; the largest was about 80 times less than the drinking-water MCL, 100 μg/L.

Tetrachloroethene also was frequently detected. It was detected in three-fourths of the urban recharge-zone samples. The largest measured concentration of tetrachloroethene was about 12 times less than the drinking-water MCL, 5 μg/L.

When detection frequency is based on a common concentration of 0.1 μg/L, only 7 VOCs were detected—5 in urban recharge-zone samples, 3 in nonurban recharge-zone samples, and 5 in confined-zone samples. Trichloromethane remained the most frequently detected VOC, but it was detected only in about 20 percent of urban and nonurban recharge-zone samples. No other VOC was detected in more than 10 percent of recharge-zone or confined-zone samples.

MTBE, a gasoline additive of recent concern because of its potential to contaminate ground water, was detected in 2 samples, 1 from the urban recharge zone and 1 from the confined zone. Concentrations were more than 200 times less than the lifetime health advisory, 20 μg/L.

Arsenic, ranked first on the Agency for Toxic Substances and Disease Registry (ATSDR) and USEPA 1999 list of priority hazardous substances [29], was detected at concentrations many times less than the current (2000) drinking-water MCL of 50 μg/L (fig. 12). The median concentration of this naturally occurring element was about 1 μg/L, still less than a proposed new standard of 5 μg/L that is being considered for adoption by the USEPA in 2001.

Lead, ranked second on the ATSDR and USEPA 1999 list of priority hazardous substances, also was detected in Edwards aquifer samples. Lead concentrations ranged from 1 to 9 μg/L with a median of about 2 μg/L (fig. 12), well below the drinking-water action level for lead, 15 μg/L. Lead was detected only in previously existing domestic and public-supply wells and not in any of 30 PVC monitor wells. The monitor wells were constructed in the urban recharge zone in cooperation with the Edwards Aquifer Authority as a part of the 1996–98 assessment. This finding indicates that detections of lead could be related to metal parts of the wells or pumps.

Figure 12. A comparison of concentrations in samples from wells completed in the Edwards and Trinity aquifers shows that, in general, the Trinity aquifer contains higher concentrations of dissolved constituents than the Edwards aquifer.

Radon is a colorless, odorless, radioactive gas that forms naturally from uranium in rocks. Ground water in contact with some rock types—for example, light-colored volcanic rocks, granites, and dark-colored shales—can contain elevated concentrations of radon [30].

Radon was detected in 41 of 58 wells in the Edwards aquifer. (Radon was not analyzed in samples from the 30 monitor wells constructed in the urban recharge zone.) Concentrations ranged from 80 to 780 picocuries per liter (pCi/L), with a median concentration of 150 pCi/L. The 75thpercentile radon concentration in samples from wells completed in the Edwards and Trinity aquifers ranked 32 among 35 NAWQA Study Units nationwide.

Radon dissolved in water generally poses a smaller health risk than radon in indoor air, which has been linked to lung cancer in humans [31]. The USEPA has proposed an MCL for radon in drinking water of 300 pCi/L and an alternative MCL of 4,000 pCi/L, the higher level applicable when accompanied by a

Major Findings mitigation program to address radon risks in indoor air. About 25 percent of the sample concentrations from the Edwards aquifer exceeded the proposed MCL of 300 pCi/L.

Trinity Aquifer Water Quality Is Mostly Unaffected by Human Activities

As in the Edwards aquifer, the presence of pesticides and VOCs in ground water of the upper and middle zones of the Trinity aquifer is evidence that human activities can affect the aquifer; but as of the late 1990s, the effects of human activities were minimal. The concentrations of these contaminants were well below drinking-water standards and guidelines, and the water quality of the aquifer remains influenced primarily by the natural processes of water interacting with surrounding rock. Concentrations of some of the products of these natural processes—dissolved solids, sulfate, and iron—exceeded nonenforceable guidelines related to esthetic effects in drinking water in some samples; some concentrations of strontium and radon exceeded a lifetime health advisory (strontium) and a proposed drinking-water standard (radon).

In the largely undeveloped Hill Country, 28 of 31 mostly domestic wells sampled were in rangeland settings, 2 were in urban settings, and 1 was in an agricultural setting. Because of the predominance of rangeland, the hydrogeologic characteristics of the aquifer, and the depth to water—the median depth to water in sampled wells was 209 feet—land use probably has not been a major influence regionally on aquifer water quality.

Water Quality in South-Central Texas

Although Both the Trinity and Edwards Aquifers Predominantly are Limestone, Hydrogeologic Differences Contribute to Differences in Water Quality

The 31 Trinity aquifer wells yielded hard water that generally was high (greater than 500 mg/L) in dissolved solids and rich in calcium, bicarbonate, magnesium, and sometimes sulfate. The concentrations of some common constituents (fig. 12) illustrate the chemical variability of Trinity aquifer water and, in large part, reflect the mineral composition of the rocks that compose the aquifer. Dissolved solids concentrations in 19 of 31 samples were greater than the USEPA nonenforceable drinking-water guideline of 500 mg/L. Five of 31 sulfate sample concentrations exceeded a similar nonenforceable guideline of 250 mg/L. Iron was detected in 23 of 31 samples, and concentrations exceeded the nonenforceable guideline of 0.3 mg/L in 7 of the samples. Strontium, which was detected in each of 29 samples, exceeded the USEPA lifetime health advisory level of 17 mg/L in 2 of the 29 samples.

Nitrate was detected in 27 of the 31 well-water samples but generally at very low concentrations. The median concentration was 0.12 mg/L. Although the water that recharges the Trinity aquifer and much of the water that recharges the Edwards aquifer originate in the same region—the Edwards Plateau—the median nitrate concentration in Trinity aquifer samples was about 14 times less than that in Edwards aquifer recharge-zone samples. The difference largely is attributable to the ease with which water flows vertically to the subsurface in the Edwards aquifer recharge zone relative to that in the Trinity aquifer.

Orthophosphate was detected in 16 of the 31 well-water samples. As with nitrate, concentrations were very low. The median concentration was 0.015 mg/L.

Only 4 of 83 pesticides analyzed were detected in 7 of the 31 Trinity aquifer well-water samples (fig. 10). Atrazine was detected in 3 samples, prometon in 2 samples, and chlorpyrifos and diazinon in 1 sample each. Unlike Edwards aquifer and national NAWQA findings that show that pesticides commonly occur in mixtures of several compounds [13], none of the samples contained more than one pesticide.

The pesticide concentration in each sample was very low—at or near the minimum reporting level—and tens of times less than the applicable drinking-water standard or guideline. When detection frequency is based on a common concentration of 0.05 μg/L, no pesticides were detected in any Trinity aquifer water sample.

Analyses of water samples from 31 Trinity aquifer wells detected 16 VOCs of the 86 analyzed. Carbon disulfide was the VOC most frequently detected (fig. 11); it was detected in 11 of 31 samples.

None of the eight VOCs detected for which drinking-water standards or guidelines have been established had concentrations near those standards or guidelines. MTBE was not detected in any sample. On the basis of a common concentration of 0.1 μg/L, the number of VOCs detected dropped from 16 to 3, which reiterates the fact that VOC concentrations were very low. The three VOCs were detected in only one sample each.

As in the Edwards aquifer, arsenic and lead were detected in the Trinity aquifer at low concentrations relative to current (2000) or proposed drinking-water standards. All arsenic concentrations were less than 2 μg/L (fig. 12); the median lead concentration was 2.1 μg/L. Whether lead is actually in the aquifer or was introduced by metal parts associated with the wells is unknown.

Radon was detected in 30 of 31 Trinity aquifer water samples in concentrations throughout a range similar to that in Edwards aquifer samples (fig. 12). The median concentration, 295 pCi/L, was about twice that in Edwards aquifer samples and about the same as the USEPA-proposed MCL of 300 pCi/L. The median radon concentration of Trinity aquifer samples is greater than that of Edwards aquifer samples probably because granitic rocks north of the Hill Country, the likely source of the radon, are closer to the Trinity aquifer than to the Edwards aquifer.

STUDY UNIT DESIGN

Ground-Water Chemistry—Aquifer surveys were done to provide a broad assessment of water quality in the Edwards aquifer (recharge and confined zones) and the Trinity aquifer (upper and middle zones). The aquifer surveys involved sampling primarily existing domestic and public-supply wells. The primary objective of the land-use study was to characterize the effects of urban land use on the quality of recently recharged ground water in the Edwards aquifer. A second objective was to learn more about the human and natural factors that affect ground-water quality.

Water Quality in South-Central Texas Study Unit Design

GLOSSARY

Water Quality South-Central Texas

REFERENCES

Glossary and References Water Quality South-Central Texas

APPENDIX—WATER-QUALITY DATA FROM SOUTH-CENTRAL TEXAS IN A NATIONAL CONTEXT

For a complete view of South-Central Texas data and for additional information about specific benchmarks used, visit our Web site at http://water.usgs.gov/nawqa/. Also visit the NAWQA Data Warehouse for access to NAWQA data sets at http://water.usgs.gov/nawqa/data.

Water-Quality Data in a National Context Water Quality in South-Central Texas Water-Quality Data in a National Context Water Quality in South-Central Texas Water-Quality Data in a National Context Water Quality in South-Central Texas

A COORDINATED EFFORT National Water-Quality Assessment (NAWQA) Program South-Central Texas

U.S. Geological Survey Circular 1212

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