FLUID LOGGING AND DISCRETE SAMPLING OF THE FRESHWATER/ SALINE-WATER INTERFACE OF THE EDWARDS AQUIFER, SOUTH CENTRAL TEXAS: IMPLICATIONS FOR GROUNDWATER FLOW AND ORIGIN OF SALINITY (PART 2-GEOCHEMISTRY)

To fully understand the interaction between the freshwater zone (FWZ) and the saline-water zone (SWZ) of the Edwards Aquifer, discrete samples were taken from monitor wells with screened intervals (open bore hole) >150m.  Sample depths were selected based on vertical profiles of temperature and specific conductance obtained from fluid logging. Discrete intervals then were sampled using a 6-liter Kemmerer sample flask. Samples were analyzed for major ions, trace metals, stable isotopes (dD, d¹⁸O, d¹³C), dissolved gases (noble and major), and tritium.  Total dissolved solids (TDS) ranged from 360 mg/L to 19,000 mg/L with Cl^- concentrations ranging from 10 mg/L to 9,000 mg/L.  The FWZ water is dominated by HCO₃^-, whereas the SWZ water is dominated by Cl^-.  ⁴He values (ranging from 62 to 357556 mcc/kg) strongly correlate with Cl^- concentrations and have a distinctive R/RA of 0.22 +/- 0.007.  Occasionally, noble gas abundances are fractionated with respect to air saturated water because of hydrocarbons trapped in the SWZ.   Stable isotopes (dD and d¹⁸O) indicate that the sampled waters are meteorically derived and deviate slightly from the meteoric water line with increasing TDS.  Relative age constraints suggest that the FWZ waters are dominantly modern (post 1950), whereas the saline waters are considerably older.  Standard mixing relations indicate that the SWZ is simply a zone of mixing between fresh water and brine, however the gradational change observed in the SWZ indicates that the mixing process is complex.  Logging and sampling results suggest that waters in the FWZ move rapidly through high conductivity strata (fractured/conduit flow), while waters in the SWZ migrate more slowly either because of lower hydraulic conductivity or lower recharge rates.  Within the SWZ, trapped connate water has mixed with fresh, meteoric water over time to produce the saline water.  In the FWZ, connate water has been removed over time because large volumes of fresh water have flushed the aquifer system.