Water quality in Barton Springs and is associated outlets,
Eliza, Old Mill and Upper Barton Springs, declines as flows
decline. In addition, storm water impacts as indicated by
specific conductance increase as spring flows decrease.
During droughts, declining water quality is due to greater
percentages of saline water mixing with "normal" Edwards
Aquifer waters. Regression formulas allow for predictions of
concentrations of specific conductance, nitrate, chloride,
sulfate, and sodium that might be expected during severe
SR-06-09 Page 1 of 15 November 2006 EFFECTS OF LOW SPRING DISCHARGE ON WATER QUALITY AT BARTON, ELIZA, AND OLD MILL SPRINGS, AUSTIN, TEXAS By David A. Johns P.G., Hydrogeologist, Environm ental Resource Management Division, Watershed Protection & Development Review Department, City of Austin. Abstract Water quality in Barton Springs and is associated outlets Eliza, Old Mill and Upper Barton Springs, declines as flows decline. In addition, storm water impacts as indi cated by specific conductance increase as spring flows decrease. During droughts, declining water quality is due to greater percentages of saline water mixing with Â“normalÂ” Edwards Aquifer waters. Regression formulas allow for predictions of concentrations of specific conductance, nitrate, chloride, sulfate, and sodium that might be expected dur ing severe droughts. Introduction The quality of water discharging from Barton Springs, including main Barton, Eliza, Old Mill and Upper Barton Springs, is effected by many factors, including the quality of water in creeks recharging the Edwards Aquifer feeding Barton Springs, the amount of urbanization in upland areas over the recharge zone of the aquifer, rain events in contributing drainages, and the amount of water discharging from the springs which affects leakage from adjacent aquifers. As urbanization expands in the southern areas of the BSEA, demands for water also increase. Water pumped from the Barton Springs Segment of Edwards Aquifer (BSEA) has increased from approximately 0.5 cfs in 1956 to approximately 10.5 cfs in 2003 (BSEACD Management Plan, 2003). Modeling by the Bureau of Economic Geol ogy (Scanlon and others, 2000) and the Barton Springs/Edwards Aquifer Conservation District (Smith and Hunt, 2004) indicates a direct relationship between pumping and spring flows; pumping 10 cfs causes a 10 cfs decline in spring flows. This has raised concerns that pumping during droughts may contribute to drying of Barton Springs when cumulative discharge of the springs dropped as low as 9.6 cfs during the drought of records in the 1950Â’s (USGS, 1990; Smith and Hunt, 2004). As drought progresses and spring flows drop, there is associated degradation in baseflow water quality of the springs contributed to influx from the saline water zone (Senger and Krietler, 1984; Slade and others, 1986; COA, 1997). In addition, storm water impacts to spring water quality are greater during drought because there is less dilution. Declining water quality during drought may threaten the existence of endangered species dependent on Barton Springs. Understanding how chemical constituents change as spring flows drop is important to protection of the endangered species dependent on spring flow from the aquifer and management of aquifer
SR-06-09 Page 2 of 15 November 2006 pumping. During the drought of 1996 cumulative spring discharge dropped to a low of 17 cfs in August and discharge from Eliza and Old Mill outlets was perilously low. This raised concerns that these outlets may dry up completely during a re-occurrence of the drought of record, especially with current levels of pumping from the aquifer. Methods Discharge data for Barton Springs is available from the USGS since the late 1880Â’s to present, daily data is available since 1978. The long-term average cumulative discharge from the springs is 53 cfs. Historical chemical data is sparse in the early part of the 1900Â’s but there is a wealth of data beginning in 1978 as the USGS and the City of Austin began a cooperative monitoring program at Barton Springs. Years of data coll ection over wide ranging flow conditions allows analysis of the impacts of spring discharge on water quality and an estimate the impacts of drought on water quality. A number of common chemical constituents for Barton Springs were examined to quantify their behavior related to flows and predict behavior during severe droughts. Constituents examined include nitrate-nitrogen, specific conductance, turbidity, dissolved oxygen, temperature, and the common ions (calcium, magnesium, sodium, chloride, sulfate, fluoride, and alkalinity). Toxic chemicals and metals were not examined. The data spans from 1922 to 2003. The range in flows with corresponding chemical data is from 12.4 cfs to 130 cfs. Multi-probe data loggers in Barton Springs since 1993 provide a nearly continuous record of temperature, specific conductance, dissolved oxygen, pH, depth, and, since 1994, turbidity. Total dissolved gas was added in 2002. Continuous data provides a much more detailed picture of the gradual and acute impacts from rain events and flows on water quality, a critical component of understanding a karst spring system, missed by traditional grab sampling. Chemical data from the associated springs of Eliza and Old Mill are only available from 1994 to present. Upper Barton Springs is an ephemeral spring, drying up entirely when cumulative spring flows drop below approximately 40 cfs, and so the effects of drought conditions were not examined. For these analyses, the term Â“droughtÂ” is used to describe periods of low discharge from Barton Springs. Previous analyses of Barton Springs chemistry, (COA, 1997) use arbitrary flow boundaries of 35-40 cfs, 41-64 cfs, and 65-70 cfs to differentiate between low, average, and high flow conditions at the springs. Based on data an alyzed for this report, low flow periods and droughts impacting water quality in Barton Springs will include flows at or below 40 cfs. Linear regression equations derived from drought data were used to predict constituent concentrations below existing data ranges. Water from the saline zone is characterized by chemical data from two saline zone wells (YD 58-50-301, 58-50-902). Barton Springs baseline chemistry is characterized by water discharging from the main spring during average and high flow conditions (greater than 40 cfs). Continuous multiprobe data loggers installed in Main Barton Springs since 1993 are used to examine acute short-term impacts of rain events. Specific conductance in storm water is collected from continuous multiprobe data loggers at Upper Barton Springs during times that they are pe riodically over whelmed by flooding Barton Creek. Rainfall totals are from COA Flood Early Warning System rain gage records.
SR-06-09 Page 3 of 15 November 2006 Results The relationship between spring discharge and ion chemistry has been previously documented (Senger and Kreitler, 1984; Slade and others, 1986; COA; 1997). Senger and Krietler and Slade and others illustrated a relationship between spring discharge and sodium and chloride. COA further refined this relationship with other ions, specific conductance and nitrate. Also, a previously unrecognized relationship of nitrate to flow has also been demonstrated (COA 1997; Turner, 2000; Turner and Johns, 2005; Herrington, 2005) although it is less pronounced than with the ions. Most of the major constituents increase as flow decreases, except dissolved oxygen and total suspended solids that decrease as flow decreases. Data graphs indicate that nutrient and ion concentr ations are similar in Barton and Eliza where as most constituent concentrations are greater in Old Mill under all flow conditions. The data indicate that in Barton, Eliza, and Old Mill Springs, changes in water chemistry begin to occur when cumulative spring flows drop below approximately 40 cfs. As illustrated in Figures 1, 2, and 3, there is a pronounced steepening of concentration trends in the springs as flow decreases, especially notable in Old Mill Spring. Regression equations derived from flow and concentration data were used to estimate theoretical concentrations in the springs when spring discharge drops to 5 and 1 cfs. Table 1 shows the equations where Â“xÂ” is cumulative spring discharge and Â“yÂ” is the constituent concentration. Data for nitrate included all flow conditions because trends at individual springs are not expressed usi ng only data during low flow conditions.
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SR-06-09 Page 5 of 15 November 2006 Table 1. Linear regression equations for selected constituents where flow < 40 cfs, except nitrate where all flows are used. Old Mill Spring Barton Spring Eliza Spring Sulfate y = 1.4032x + 111.19 R2 = 0.4503 y = 0.4987x + 53.277 R2 = 0.1618 y = 0.1723x + 45.126 R2 = 0.0381 Chloride y = 2.2138x + 151.63 R2 = 0.4604 y = 1.4693x + 89.822 R2 = 0.426 y = 0.5382x + 58.02 R2 = 0.1497 Sodium y = 1.021x + 84.059 R2 = 0.265 y = 0.5541x + 43.302 R2 = 0.2052 y = 0.3528x + 35.431 R2 = 0.3017 Nitrate y = 0.0046x + 1.6 R2 = 0.3507 y = 0.0031x + 1.5935 R2 = 0.2494 y = 0.0019x + 1.3475 R2 = 0.0679 Specific Conductance y = 7.4961x + 1153.6 R2 = 0.1792 y = 3.508x + 781.33 R2 = 0.1978 y = 3.0777x + 816.16 R2 = 0.3595 Magnesium y = 0.0634x + 27.645 R2 = 0.0495 y = 0.3893x + 35.317 R2 = 0.3908 y = 0.0865x + 25.88 R2 = 0.1483 Calcium y = 0.0524x + 84.601 R2 = 0.0033 y = 0.2525x + 73.967 R2 = 0.198 y = 0.0567x + 79.558 R2 = 0.0037 Fluoride y = 0.0069x + 0.5111 R2 = 0.3924 y = 0.0072x + 0.5039 R2 = 0.4983 y = 0.0037x + 0.3877 R2 = 0.2104 Table 2 summarizes concentration increases and predicted concentrations at 5 and 1 cfs of combined discharge for several constituents based on the regression equations in Table 1. The greatest changes in major ion concentrations are in chloride, sulfate, sodium, and fluoride. Calcium, magnesium, and alkalinity concentrations change the least. The ratios of calcium to magnesium decrease (slightly lower calcium and higher magnesium) with discharge and is indicative of older water that has been in contact longer with the carbonate host rocks. TSS does not show a well-defined trend with discharge a lthough observations and secchi disk data indicate increasing water clarity with decreasing flows (COA/WPDRD Unpublished data).
SR-06-09 Page 6 of 15 November 2006 Table 2. Average concentrations of selected constituents during normal/high flow and low flow (drought) and estimated concentrations at 5 and 1 cfs combined discharge from Barton Springs. Nitrate is a nutrient of particular interest, b ecause it commonly increases in concentration with urbanization and nitrate has recently been documented to increase over time in Barton Springs during low and high flow conditions (Turner, 2000; Herrington, 2005). No trend in nitrate concentration is seen at Barton Springs if onl y low flow nitrate concentrations are used. Extrapolation of existing trends shows a relatively small increase in nitrate concentrations with decreasing discharge. Predicted concentrations of all constituents at spring discharge rates of 5 and 1 cfs can be misleading, since it assumes there are no additional inputs in the system from over time. Ten years of continuous data demonstrate the effects of low flow on dissolved oxygen and specific conductance in main Barton Springs. Figure 4 indicates that in 1996 and 2000 when springs flows dropped into the 20-30 cfs range, DO concentrations dropped from Â“normalÂ” range of approximately 6 mg/L to around 4 mg/L or lower. Low DO during low flows is a critical concern to survival of the species during drought. Similarly, specific conductance during these two periods of low flow increased from around 600-650 uS/cm to around 700-750 uS/cm in the main spring (Figure 5). Normal/High Flow Avg. Low Flow Avg.5 cfs Est.1 cfs Est Normal/High Flow Avg. Low Flow Avg.5 cfs Est.1 cfs Est mg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/L Barton Springs 14.1127.1240.5342.7526.0149.0582.4888.35 Eliza Springs 17.525.0333.6735.082942.1455.3357.48 Old Mill Springs 26.7554.3578.9583.0444.0587.21140.56149.42 Normal/High Flow Avg. Low Flow Avg.5 cfs Est.1 cfs Est Normal/High Flow Avg. Low Flow Avg.5 cfs Est.1 cfs Est mg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/L Barton Springs 28.3438.7150.7852.781.341.511.591.59 Eliza Springs 36.1540.0444.2744.951.181.331.341.35 Old Mill Springs 44.7470.36104.17109.791.211.491.581.60 Normal/High Flow Avg. Low Flow Avg.5 cfs Est.1 cfs Est Normal/High Flow Avg. Low Flow Avg.5 cfs Est.1 cfs Est mg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/L Barton Springs 60868076477126.96.36.1990.50 Eliza Springs 6357238018130.140.270.470.38 Old Mill Springs 7239411116114188.8.131.520.50 Averages based on data from 1922 -2002 except nitrate which is based on data from 1978-2002 Normal and high flows defined as greater than 40 cfs combined discharges from Barton Springs Low flow defined as less than or equal to 40 cfs combined discharges from Barton Springs 5 cfs average is a linear extrapolation from low flow data except nitrogen 5 cfs average is a linear extrapolation from all data for nitrogen Specific ConductanceFluoride SodiumChloride SulfateNitrite+Nitrate N
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SR-06-09 Page 8 of 15 November 2006 Saline Zone Contributions Encroachment of water from the Edwards saline water zone is most likely responsible for the sharp changes in ion chemistry in Barton Springs when discharge drops, although there may also be an unrecognized Trinity water component as well. The saline water zone is defined by concentrations of total dissolved solids greater than 1000 mg/L with equivalent specific conductance values in the range of 14,000 uS/cm. Chloride, sulfate, and sodium concentrations are particularly high in the saline water zone and these three constituents increase the most in spring water during low flows. Figure 6 illustrates the increase in contribution of water from the saline zone to flow for Barton Springs by increasing strontium and sodium content based on relationships established by Senger and Kreitler (1984). Based on a relationship of sulfate to fluoride and fluoride concentrations in Glen Rose wells (COA, 1997), the waters discharging from all the spring outlets do not have a large component of Glen Rose water, they more appear to be Edwards. Any Glen Rose waters entering the Edwards far upgradient may be mixed or equilibrated with the Edwards and therefore indistinguishable. Additional analysis is necessary to determine if the Glen Rose makes significant contributions of water to the Edwards Aquifer. Movement of saline water into the freshwater zone has been documented in the San Antonio area (Groschen,1998). This occurrence is not documented in the BSEA but increases in specific conductance and ion chemistry suggests it is occurring. The spring water chemistry indicates that saline water flows into the freshwater zone during periods of when the water table is low. Drawdowns of Barton Springs Pool (when gates in the lower pool dam are opened to partly drain
SR-06-09 Page 9 of 15 November 2006 and lower water levels in the pool) cause a localized drop in the water table (Senger and Kreitler, 1984). A spike in specific conductance of the spring water shortly after drawdown and increasing concentrations of chloride, sulfate, and sodium indicate influx of more saline water (Figure 7). A small drop in specific conductance when the pool is refilled suggests that there is constant contribution of more mineralized wate r during drawdowns. It is likely that drawdowns mimic lower water table conditions that could be present during drought, especially when considering influx of water from the saline zone. The dropping water levels in Barton Springs Pool locally lowers the water table (Senger and Kreitler, 1984) and the resulting reduction in hydrologic head allows an influx of saline water into the conduits feeding the springs, raising specific conductance and changes ionic composition of spring water. Higher concentrations and higher ratios of calcium to magnesium also indicate that Â“olderÂ” water is discharged during pool drawdowns and droughts as expected if saline water was discharging (water is more saline and has higher calcium/magnesium ratio in the aquifer due to longer periods of contact towards equilibrium with hosting aquifer rocks). Estimates of the amount of contribution of wate r from the saline zone during droughts can assist in determining potential impacts to endangered species. Calculations based on simple mixing of water characteristic of the saline zone and Barton Springs average/high flow water were used to match the average and estimated concentrations of constituents from Table 3. This produced an estimate of water discharging from the springs from the saline zone. During low flows (< 40 cfs), saline water making up approximately 0.5% of the water in Barton Springs and approximately
SR-06-09 Page 10 of 15 November 2006 1.75% of the water in Old Mill Spring matches well with average concentrations of sodium, sulfate, chloride, and specific conductance. When flow drops below historical lows, for example to 5 cfs, it is predicted that water from the saline zone will make up approximately 1-to-1.25% and 3-to-3.25% of water in Barton and Old Mill Springs respectively. Although this amounts to only 0.4-to-1.3 gallons per second of saline water, toxicity testing indicates that a solution of approximately 6.25% saline water with a conductivity of approximately 1,100 uS/cm can be fatal to salamanders (COA, 1999). Table 3. Estimated Water from the Saline Zone in Barton and Old Mill Springs. SodiumSulfateChlorideSpecific Conductance mg/Lmg/Lmg/LuS/cm Calculated Saline Average* 25402425373514100 Calculated BS/normal and high flow avg14.1 28.326.0608 Calculated BS/low flow avg** 184.108.40.20680 Calculated Old Mill Low flow avg** 54.470.487.2941 Estimated Barton Springs BS/5 cfs*** 40.550.882.5764 Estimated Old Mill Springs BS/5 cfs*** 79.0140.6104.21113 Estimated 3.25% saline 102.5112.2155.81080 Estimated 3% saline 89.9100.2137.31013 Estimated 2% saline 64.676.3100.2878 Estimated 1.75% Saline 58.370.390.9844 Estimated 1.25% saline 45.758.372.4777 Estimated 1% saline 39.452.363.1743 Estimated 0.75% saline 33.146.353.8709 Estimated 0.5% saline 26.740.344.6675 Average of analyses from wells YD 58-50-301 and YD 58-50-902 ** Average from 1922 to 2003 below 40 cfs *** Estimated based on linear regression from data below 40 cfs Storm Water Impacts Multiprobe data loggers measuring physical water quality parameters every 15 minutes in main Barton Springs and other outlets provide a detailed picture of large and small impacts of rain events. Magnitude of these impacts, as measured by changes in specific conductance, vary according to rain volumes and spring flows. Ot her contributing factors include location and intensity of rain, antecedent moisture, and creek flow conditions although these are not considered here. Specific conductance of storm water in Barton Creek was used to present recharge water. This data was collected when floods periodically inundated a multiprobe in Upper Barton Springs
SR-06-09 Page 11 of 15 November 2006 upstream of Barton Springs Pool. As expected, the data clearly illustrates greater acute impacts from large rain events (Figure 8). However, the data also shows that during low spring flows all rain events have greater impacts on water quality than during average to high spring flows resulting in a greater storm water runoff fraction of spring discharge (Figure 9). For example, at main Barton Springs a one inch rain during av erage or high spring flows might drop specific conductance around 20 uS/cm whereas the same rain might drop specific conductance by 50 uS/cm during low flows. This indicates that stor m water runoff comprises a greater percentage of spring flows. In fact, during low flows storm water can make up over 30% of spring discharge following large rain events (Table 4). Of course, any urban contaminants in the storm water would be in correspondingly higher concentrations as well. Higher concentrations of storm water runoff pollutants on salamanders already potentia lly stressed from depressed dissolved oxygen and higher salinity could result in significant behavioral and/or physical changes.
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SR-06-09 Page 13 of 15 November 2006 Table 4. Percentage of storm water in Barton Springs and changes in specific conductance. Regression equations derived from these data wher e Â“yÂ” is the percentage of stormwater and Â“xÂ” is rainfall are: Barton Springs low flow (<40 cfs): y = 0.1504x Â– 0.0439 Barton Springs average and high flows (>40 cfs): y = 0.0345x Â– 0.0025 BARTON SPRINGS LOW FLOWS (<40cfs) Change in Specific Conductance StormwaterRain at Loop 360 uS/cm Percentinches 122.0%0.5 356.0%0.7 305.0%0.7 457.0%0.75 559.5%0.8 508.5%1 12320.5%1.4 22035.0%2.7 BARTON SPRINGS AVG/HIGH FLOWS (>40cfs) Change in Specific Conductance StormwaterRain at Loop 360 uS/cm Percentinches 40.7%0.28 20.5%0.44 71.5%0.6 71.5%0.75 153.0%0.8 204.0%0.9 30.5%1.1 102.0%1.25 213.5%1.26 529.5%1.4 183.5%1.4 285.0%1.5 529.5%1.5 245.0%1.5 193.5%1.6 112.0%1.8 6713.0%2.2 346.0%2.44 479.0%2.8 5812.0%3 7414.0%4.6
SR-06-09 Page 14 of 15 November 2006 Recommendations There are a number of potential projects to could further investigate water quality impacts associated with low flows at Barton Springs. They include: Data indicates that there is a lag between lowering water tables and water quality impacts to the springs. Identifying potential locations of saline water injections into conduits leading to the springs could be explored. Since it appears that high water tables impede the influx of saline water to main Barton Springs, staff could experiment with weirs in BS Pool spillways as tool for artificially creating high local water tables and thus reducing impacts of drought on water quality. This technique might also be useful in sl owing release of water form the aquifer and therefore serve as a water conservation measure. Examine feasibility of other engineered means to augment water quantity and quality during severe droughts to protect salamander species. Conduct toxicity testing on salamanders or a surrogate species to determine if predicted concentrations of ions and or saline water during drought threatens endangered species. Conduct toxicity testing on salamanders or a surrogate species to determine if predicted percentages of stormwater in springs threatens endangered species. Continue analysis of spring water chemistry, including isotopes and trace elements, to identify any Trinity water inflow duri ng droughts or during periodic drawdowns. Continue deployment of multiprobe data loggers to collect detailed data from as many spring outlets as feasible. Collect additional data, including from multiprobes, salamander population surveys and dissolved gas, from Upper Barton Springs to determine if high concentrations of stormwater in springs flows is related to gas bubble trauma. Conclusions Decreasing flows in Barton Springs results in increasing concentrations of most major constituents in spring water accompanied with decreasing levels of dissolved oxygen. Data indicates concentration increases are likely due to increasing contribution of saline zone water to spring flow, making up several percent of spring flow during severe drought. Low spring flow concentrations of constituents generally double over average/high flow concentrations. Regression equations indicate concentrations of many dissolved constituents will triple over average/high flow concentrations at 5 cfs of sp ring discharge. In addition, stormwater runoff has greater effects on the main Barton Springs duri ng low flows due to less dilution, temporarily making up 30% of spring flow during low spring flows and following large rain events. It is currently unknown if these conditions w ill threaten the survival of the endangered salamanders. Predicted concentrations are less than concentrations that caused total mortality in a toxicity test of related salamander species. However, there is currently insufficient data from those toxicity tests to determine if these predicted concentrations pose a threat to the species.
SR-06-09 Page 15 of 15 November 2006 References Barton Springs/Edwards Aquifer Conservation District, 2003; District Management Plan, Adopted October 30, 2003, 62p. City of Austin. 1997. The Barton Creek report. City of Austin, Drainage Utility Department, Environmental Resources Management Division. Water Quality Report Series COA-ERM/ 1997. April 22, 1997. Austin, Texas. City of Austin, 1999, Jollyville Plateau Water Quality and Salamander Assessment. City of Austin, Watershed Protection Department, Environmental Resources Management Division. Water Quality Report Series COA-ERM 1999-01. June 22, 2001. Austin, Texas. Groschen, G.E., and Buszka, P.M., 1997, Hydroge ologic framework and geochemistry of the Edwards aquifer saline-water zone, south-centr al, Texas: U.S. Geological Survey WaterResources Investigations Report 97Â–4133, 47 p. Senger, R. K. and Kreitler, C. W. 1984. Hydrogeology of the Edwards Aquifer, Austin Area, Central Texas: The University of Texas at Austin Bureau of Economic Geology Report of Investigations 141 35p. Scanlon, B. R., Mace, R. E., Dutton, A. R., and Reedy, A., 2000, Predictions of Groundwater Levels and Spring Flow in Response to Future Pumping and Potential Future Droughts in the Barton Springs Segment of the Edwards Aquifer; Bureau of Economic Geology, The University of Texas at Austin, Prepared for the Lower Colorado River Authority. Slade, R. M., Jr., Dorsey, M. E., and Stewart, S. L. 1986. Hydrology and Water Quality of the Edwards Aquifer Associated with Barton Springs in the Austin area, Texas: U.S. Geological Survey Water-Resources Investigations Report 86-4036 117 p. Smith, B.A, and Hunt, B,B, 2004. Evaluation of Sustainable Yield of the Barton Springs Segment of the Edwards Aquifer, Hays and Travis Counties, Central Texas: Barton Springs/Edwards Aquifer Conservation District, 36 p. Turner, M. A., 2000. Update of Barton Springs Water Quality Data Analysis Â– Austin, Texas. City of Austin, Watershed rotection Department, Environmental Resources Management Division. Water Quality Report Series COA-ERM 2000-03. Turner, M. A. and Johns, D. A, 2005. Long-Term Trends in the Water Quality of Barton Springs, Austin, Texas, in Norwine, Jim, Giardino, J. R., and Krishnamurthy, Sushma, eds. Water for Texas. Texas A&M University PressÂ…Â… U. S. Geological Survey. 1990. Water Resources Data. Texas, Water Year 1990: U. S. Geological Survey Water-Data Report TX 90.