of interactive processes between water and rock in carbonate aquifers provide insight into groundwater residence time and flow paths, diffuse and conduit recharge sources, mineral- solution reactions, and mixing processes. Previous work in the Edwards aquifer provides a framework of understanding for geochemical evolution processes that affect geochemical and isotopic tracers such as Mg/Ca and Sr/Ca ratios and Sr isotopes (for example, Oetting, 1995; Sharp and Banner, 1997; Musgrove and Banner, 2004; Wong and others, 2011; Musgrove and others, 2010). As discussed in Musgrove and others (2010), Mg/Ca and Sr/Ca ratios in carbonate groundwater typically increase along flow paths as a result of water-rock interaction and progressive groundwater evolution processes (for example, calcite recrystallization, incongruent dolomite dissolution, and prior precipitation of calcite along flow paths) (Plummer, 1977; Trudgill and others, 1980; Lohmann, 1988; Fairchild and others, 2000; Musgrove and Banner, 2004). Higher Mg/Ca and Sr/Ca ratios are consistent with longer residence times and greater extents of mineral-
solution reaction. Strontium isotope ratios (87Sr/86Sr) in the
Edwards aquifer have been applied in conjunction with Mg/ Ca and Sr/Ca ratios as tracers of water-rock interaction, groundwater residence time, recharge, and the influence of soil composition on groundwater geochemistry (Oetting and others, 1996; Musgrove and Banner, 2004; Garner, 2005; Wong and others, 2011). As demonstrated in these studies, Sr
Interaction Between Surface Water and Groundwater 61
Date
0 50 100 150 200 250 300
Spring discharge, in cubic feet per second
Nov. 2008 Feb. 2009 May 2009 Aug. 2009 Nov. 2009 Feb. 2010 May 2010 Aug. 2010 Oct. 2010 Dec. 2010
580 590 600 610 620 630 640 650
Specific conductance, in microsiemens per centimeter at 25 degrees Celsius
580 590 600 610 620 630 640
Specific conductance, in microsiemens per centimeter at 25 degrees Celsius
50 100 150 200 250 300
Spring discharge, in cubic feet per second
1 2 3
B A
San Marcos Springs, by orifice (table 1) Deep Spring Diversion Spring Weissmuller Spring
EXPLANATION
1 Onset of major storm (sampled)
and identifier
Onset of major storm (unsampled) Discharge (daily mean) for San Marcos Springs (table 1) Specific conductance (daily mean), by orifice (table 1)
Deep Spring Diversion Spring Weissmuller Spring
EXPLANATION
Figure 20. Specific conductance at San Marcos Springs, south-central Texas. A, Time series (November 2008–December 2010) of
specific conductance for Deep, Diversion, and Weissmuller Springs orifices, and discharge at San Marcos Springs. B, Relation of specific conductance (Deep, Diversion, and Weissmuller Springs orifices) with discharge at San Marcos Springs.
62 Origin and Characteristics of Discharge at San Marcos Springs Based on Hydrologic and Geochemical Data (2008–10) isotope ratios in the Edwards aquifer generally decrease with
increasing water-rock interaction, approaching values similar to those of the Cretaceous-age limestone aquifer rocks, which have values ranging from 0.7074 to 0.7077 (Koepnick
and others, 1985; Oetting, 1995). Higher 87Sr/86Sr values
for groundwater relative to the aquifer rocks are indicative
of a source of more radiogenic Sr (enriched in 87Sr) to the
groundwater, which has been previously proposed to result from chemical interaction with overlying soils (Musgrove
and Banner, 2004). Lower 87Sr/86Sr values are indicative of
proportionally more interaction with limestone aquifer rocks as a result of longer groundwater residence time. At a local scale, variations in limestone composition, soil composition, flow paths, and residence time can affect individual values of these geochemical constituents.
Variations in Mg/Ca and Sr/Ca ratios and Sr isotope values for the springs were evaluated to distinguish potential differences in residence time, flow paths, and components of diffuse versus conduit flow affecting the springs. Results for Hueco Springs were consistent with marked changes in recharge sources from the dry period to the wet period. Values for these constituents at Hueco Spring A were significantly different between the dry and wet periods and covered a larger range than at Comal Spring 1 or San Marcos Springs (table 5); lower Mg/Ca and Sr/Ca ratios and higher Sr isotope values during the wet period are consistent with a large component of recently recharged, less geochemically evolved water contributing to Hueco Springs (fig. 21). In contrast with Hueco Spring A, Mg/Ca and Sr/Ca ratios and
87Sr/86Sr values at Comal Spring 1 and at Deep Spring were
not significantly different between the dry and wet periods (table 5; fig. 21). At Diversion Spring, Mg/Ca and Sr/Ca ratios were significantly different between the dry period and wet period (table 5) but covered a small range of values
relative to Hueco Spring A; 87Sr/86Sr values at Diversion
Spring were not significantly different between the dry and wet periods (table 5). Ratios of Mg/Ca and Sr/Ca at Diversion Spring during the dry period were similar to values at Deep Spring; during the wet period, Mg/Ca and Sr/Ca ratios at Diversion Spring shifted to higher values relative to the dry period (table 5; fig. 21). This shift in Mg/Ca and Sr/Ca values during the wet period is consistent with a change in the proportion of different water sources contributing to Diversion Spring: the shift is consistent with an increased contribution of more geochemically evolved, longer residence time groundwater, rather than recent recharge, such as is observed at Hueco Springs. Results for
Weissmuller Spring for Mg/Ca and Sr/Ca ratios and 87Sr/86Sr
values (collected during only the wet period) are similar to those for Diversion Spring during the wet period (table 5; fig. 21) and imply a similar origin and similar flow paths for water supplying both Diversion and Weissmuller Springs. These results indicate that discharge sources to Diversion Spring changed from the dry period to the wet period, whereas discharge sources to Deep Spring were more constant.