Prepared in cooperation with the San Antonio Water System Origin and Characteristics of Discharge at San Marcos Springs, South-Central Texas
Discharge from San Marcos Springs, the second largest spring complex in Texas, is used as a factor for enacting drought management strategies for the Edwards aquifer, although the hydrologic connection of the springs with this regional aquifer is not well understood.
Key Findings
• Discharge at San Marcos Springs is dominated by regional recharge sources and groundwater flow paths; the contribution to recharge from local streams is relatively minor. • Different orifices of San Marcos Springs respond differently to changes in hydrologic conditions; Deep Spring is less responsive to changes in hydrologic conditions than are Diversion Spring and Weissmuller Spring. • San Marcos Springs discharge is influenced by mixing with a component of saline groundwater.
Spring Lake, created by the impounded discharge from San Marcos Springs, south-central Texas (right).
A U.S. Geological Survey employee servicing water-quality monitoring equipment at one of the orifices of San Marcos Springs in Spring Lake, south- central Texas (above).
The Edwards aquifer in south-central Texas is one of the most productive aquifers in the Nation and is the primary source of water for the rapidly growing San Antonio area. Springs issuing from the Edwards aquifer provide habitat for several threatened and endangered species, serve as locations for recreational activities, and supply downstream users. Comal Springs and San Marcos Springs are major discharge points for the Edwards aquifer (fig. 1), and their discharges are used as thresholds in groundwater management strategies. Regional flow paths originating in the western part of the aquifer are generally understood to supply discharge at Comal Springs. In contrast, the hydrologic connection of San Marcos Springs with the regional Edwards aquifer flow system is less understood. During November 2008–December 2010, the U.S. Geological Survey, in cooperation with the San Antonio Water System, collected and analyzed hydrologic and geochemical data from springs, groundwater wells, and streams to gain a better understanding of the origin and characteristics of discharge at San Marcos Springs.
U.S. Department of the Interior Fact Sheet 2013–3080 U.S. Geological Survey Printed on recycled paper November 2013 River
Marcos
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i CANYON R l 98°20'
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Streamflow-gaging station or surface-water site Groundwater well, Edwards aquifer Groundwater well, Edwards and Trinity aquifers Groundwater well, Trinity aquifer Spring Rain sample collection site National Weather Service cooperative station
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BLANCO COUNTY BLANCO Guadalupe d a PLATEAU EDWARDS # " " l l TI ON l ni o l
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Bulverde l an EXP LANA Creek S l
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COMAL COUNTY COMAL
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KENDALL COUNTY KENDALL
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l Edwards aquifer Contributing zone Recharge zone Confined zone Balcones Escarpment Municipal boundary Groundwater divide Freshwater/saline-water interface (1,000 milligrams per liter [mg/L] dissolved solids concentration) 98°40'
l Study area and locations of data- and sample-collection sites, south-central Texas. Samples were collected periodically and in response to storms. Study area and locations of data- sample-collection sites, south-central Texas. l l l l #
Boerne Base modified from U.S. Geological Survey 1:24,000 digital data Geographic Coordinate System North American Datum of 1983 Figure 1. 30°20' 30°00' 29°40' Figure 1. Study area and locations of data- sample-collection sites, south-central Texas. Samples were collected periodically and in response to storms. A wide range of climatic and hydrologic conditions during the study allowed for investigation of controls on discharge at San Marcos Springs over much of its historical range of discharge.
During the study, climatic and hydrologic conditions Previous studies have hypothesized that discharge at transitioned from exceptional drought to wetter than normal. San Marcos Springs might include notable contributions of Between November 1, 2008, and September 8, 2009, is referred recharge from nearby streams, including the Guadalupe River, to as the “dry period,” and between September 9, 2009, and Cibolo Creek, Dry Comal Creek, Sink Creek, Purgatory Creek, December 31, 2010, is referred to as the “wet period.” The York Creek, Alligator Creek, and, in particular, the Blanco wide range of hydrologic conditions that occurred during this River (Guyton and Associates, 1979; Ogden and others, 1986; 25-month study—and corresponding changes in surface-water, Johnson and Schindel, 2008). Results from this study indicate that recharge from these local streams is not a major source of groundwater and spring discharge, and in physicochemical San Marcos Springs discharge. properties and geochemistry—provides insight into the origin of Rather, discharge at San Marcos Springs is dominated the water discharging from San Marcos Springs. Three orifices by regional recharge sources and flow paths, even during wet at San Marcos Springs (Deep, Diversion, and Weissmuller hydrologic conditions when aquifer recharge is likely occurring Springs) were selected to be representative of larger springs at from local streams. Geochemical modeling results using the the spring complex (fig. 2). Potential sources of discharge at San program PHREEQC (Parkhurst and Appelo, 1999) indicate Marcos Springs include (1) regional groundwater flow that has that the proportion of local stream recharge contributing to San bypassed Comal Springs, (2) local recharge from nearby streams, Marcos Springs discharge increased from the dry period to the (3) groundwater from the saline zone of the Edwards aquifer wet period, but even under wet conditions the proportion was (downgradient of the freshwater/saline-water interface) (fig. 1), less than 30 percent, and for most hydrologic conditions it was and (4) interaquifer flow from the underlying Trinity aquifer. less than 10 percent (fig. 3).
97°56' 97°55'50"
29°53'40"
Weissmuller Spring
Diversion Spring
U D Deep Spring SpringSpring Lake U D
29°53'30"
SAN MARCOS SPRINGS FAULT SPRING LAKE DAM
East outflow structure West outflow EXPLANATION structure U Fault—U, upthrown; D, downthrown D (Guyton and Associates, 1979) Spring sample collection location 29°53'20"
Base modified from U.S. Geological Survey digital data, 1:24,000 0 0.05 0.1 MILE U.S. Department of Agriculture, 2010 Natural Agriculture Imagery Program (NAIP) 1 meter 0 0.05 0.1 KILOMETER Geographic Coordinate System, North American Datum of 1983
FigureFigure 2. 2.Location Spring Lake,of Spring San Marcos Lake, Springs San Marcos complex, Springsand location complex, of sampled and spring spring orifices. orifices sampled during this study. The Blanco River in Hays County, Texas. The Guadalupe River in Comal County, Texas.
Geochemical modeling results indicate that the proportion of local stream recharge contributing to San Marcos Springs discharge was less than 10 percent for most hydrologic conditions.
30 EXPLANATION
San Marcos Springs—Midpoint and range of stream 25 recharge proportion, by orifice (except where range is smaller than symbol size)
20 Deep Spring Diversion Spring
15 Weissmuller Spring
10
5 Modeled proportion of spring discharge composed of stream recharge, in percent
0 0 50 100 150 200 250 San Marcos Springs daily mean discharge, in cubic feet per second
FigureFigure 3. 3.Relation Relation between between San Marcos San Marcos Springs discharge Springs anddischarge the modeled and proportion the modeled of spring proportion discharge of composed spring discharge of stream recharge composed of fromstream the Blanco recharge River basedfrom theon inverse Blanco geochemical River based modeling. on inverse geochemical modeling.
Geochemical modeling results also indicate that the effect Modeling results for this and other storms indicate of storm-related recharge from local focused recharge sources that San Marcos Springs was not notably affected by was small. For example, mixing models developed by using recharge events from local streams moving rapidly through conservative tracers for the largest storm that occurred during transmissive flow paths. This hypothesis is also supported the study, a named tropical storm (Hermine, September 7, 2010), by a lack of marked changes in geochemistry from the dry indicate that recharge from the Blanco River composed less period to the wet period in time-series data from wells located than 10 percent of discharge at San Marcos Springs immediately to the north of San Marcos Springs along possible flow paths following the storm and for several months afterwards (fig. 4). from the Blanco River to the springs. 0 0
-10 Deep Spring -10 Diversion Spring
-20 90:10 mix -20
-30 70:30 mix 100 percent -30 springwater -40 -40
-50 -50
-60 50:50 mix -60 50:50 mix 100 percent -70 surface water 30:70 mix -70 Delta deuterium, in per mil -80 10:90 mix -80 -90 -90 0 5 10 15 20 25 0 Chloride, in milligrams per liter -10 Weissmuller Spring EXPLANATION -20 Modeled mixing line—In 10 percent increments between springwater -30 endmember (100 percent) and surface-water endmember (100 percent) Mixing-model springwater endmember— -40 Prestorm sample Mixing-model surface-water endmember— -50 Storm sample, Blanco River at Halifax Ranch near Kyle, Texas Sample collected in response to storm 3 -60 50:50 mix San Marcos Springs—Deep Spring San Marcos Springs—Diversion Spring
Delta deuterium, in per mil -70 San Marcos Springs—Weismuller Spring -80 Routine sample collected after storm 3, by date -90 September 29, 2010 0 5 10 15 20 25 October 29, 2010 Chloride, in milligrams per liter December 1, 2010
FigureFigure 4. 4. RelationRelation between between chloride chloride concentration concentration and deuterium and isotopesdeuterium for two-component isotopes for two-componentmixing models showing mixing proportional models mixing showing proportionalbetween surface-water mixing between (stream recharge) surface-water and springwater (stream endmembers recharge) for and San springwaterMarcos Springs endmembers samples collected for immediately San Marcos followi Springsng and several months after Tropical Storm Hermine in September 2010. samples collected immediately following and several months after Tropical Storm Hermine in September 2010.
Different spring orifices at San Marcos Springs, Comal Springs, and Hueco Springs respond differently to temporal changes in hydrologic conditions.
Differences in the geochemistry and variability of Comal discharge at Deep Spring was likely dominated by regional and Springs, Hueco Springs, and San Marcos Springs were compared less variable flow paths. The geochemical response at Diversion to better understand flow paths and sources of spring discharge. Spring was more variable than at Deep Spring. Previous studies Changes in hydrologic conditions that occurred at the beginning have also described differences between springs in the southern of the wet period resulted in large changes in discharge from the and northern parts of Spring Lake (Ogden and others, 1986). springs (fig. 5). At Hueco Springs, this change was accompanied Diversion Spring was more affected by changes in recharge by large changes in geochemistry in response to local rainfall sources, but the nature of the geochemical changes, such as the and recharge events. At Comal Springs, changes in geochemistry increase in chloride concentration (fig. 5), indicates that increased were relatively minor as discharge increased, reflecting regional discharge at Diversion Spring included an increased component flow paths and discharge sources. of saline groundwater rather than recharge from local streams. At The geochemical response at San Marcos Springs as higher discharge, the composition of Diversion Spring became discharge increased was intermediate between Hueco and more like that of Deep Spring. Weissmuller Spring, which was Comal Springs and was different for the different orifices (fig. sampled during only the wet period, had a composition and 5). Deep Spring, similar to Comal Springs, was not responsive variability similar to Diversion Spring (fig. 5), indicating that to changes in hydrologic conditions, which indicates that these two springs were likely supplied by common flow paths. Discharge at San Marcos Springs is influenced by mixing with a component of saline groundwater.
25 Geochemical modeling results consistently indicate that, 23 in addition to a dominant component of regional groundwater flow, a small contribution of saline water (more saline than 21 the freshwater Edwards aquifer) is needed to account for the 19 composition of San Marcos Springs. Both the Edwards aquifer 17 saline zone and the underlying Trinity aquifer were considered 15 as hydrologically plausible saline-water sources. Although the Trinity aquifer cannot be eliminated as a potential source, 13 modeling results indicate that mixing with a small component , in milligrams per liter per milligrams in , 11 (less than 1 percent) of saline-zone groundwater more likely 9 accounts for the composition of San Marcos Springs discharge. 7 Chloride 5 This fact sheet is based on the following report: 0 100 200 300 400 Musgrove, M., and Crow, C.L., 2012, Origin and characteristics Spring daily mean discharge, in cubic feet per second of discharge at San Marcos Springs based on hydrologic and geochemical data (2008–10), Bexar, Comal, and Hays Counties, Texas: U.S. Geological Survey Scientific EXPLANATION Investigations Report 2012–5126, 94 p., http://pubs.usgs.gov/ Spring, by orifice sir/2012/5126/. San Marcos Springs (Deep Spring) San Marcos Springs (Diversion Spring) References San Marcos Springs (Weissmuller Spring) Comal Springs (Spring 1) Guyton, W.F., and Associates, 1979, Geohydrology of Comal, Hueco Springs (Spring A) San Marcos, and Hueco Springs: Austin, Texas Department of Water Resources Report 234, 85 p. Figure 5. Relation between spring discharge for Comal, Hueco, Johnson, S.B., and Schindel, G.M., 2008, Evaluation of the Figureand 5.San Relation Marcos Springs between and springchloride discharge concentration for for Comal, samples Hueco, and optionSan Marcos to designate Springs a separate San Marcos pool for critical andcollected chloride from concetration orifices of those for samplessprings. collected from orifices of thoseperiod springs. management: San Antonio, Tex., Edwards Aquifer Authority, 109 p. Ogden, A.E., Quick, R.A., Rothermel, S.R., and Lundsford, D.L., 1986, Hydrological and hydrochemical investigation of the Edwards aquifer in the San Marcos area, Hays County, Texas: San Marcos, Tex., Edwards Aquifer Research and Data Center, 364 p. Parkhurst, D.L., and Appelo, C.A.J., 1999, User’s guide to PHREEQC (v. 2)—A computer program for speciation, reaction-path, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Water- Resources Investigations 99–4259, 312 p.
—By MaryLynn Musgrove and Cassi L. Crow
For additional information contact: Director, Texas Water Science Center U.S. Geological Survey 1505 Ferguson Lane Austin, Texas 78754–4501 http://tx.usgs.gov/
U.S. Geological Survey personnel and a diver from Texas State ISSN 2327–6916 (print) University above Weissmuller Spring, an orifice of San Marcos Publishing support provided by ISSN 2327–6932 (online) Springs in Spring Lake, south-central Texas. Lafayette Publishing Service Center http://dx.doi.org/10.3133/fs20133080