AUTOGENIC VS. ALLOGENIC RECHARGE: SEARCHING FOR THE SOURCE OF THE STREAM in CAVE WITHOUT A NAME, BOERNE, TX

It has been previously hypothesized that flow in the main stream in Cave Without a Name (CWAN), in Boerne, TX, is allogenically sourced, at least in part, from the Guadalupe River (Veni, 1994). Using newly available geochemical, isotopic, and hydrologic data, this hypothesis has been reassessed. USGS real-time hydrograph data from the Spring Branch, TX gauging station on the Guadalupe River were compared with stage data from the Main Stream in CWAN to quantify storm response behaviors in each stream from September 2009 to October 2011. In particular, we investigated correlations between the presence, magnitude, and timing of storm pulses at both sites. During this time period, the Guadalupe River data contain records of several small storm pulses which have no corresponding signal in the CWAN data. These likely represent responses to isolated precipitation in distant and upstream portions of the Guadalupe drainage basin that did not occur near CWAN. Isotopic data also reveal distinct differences between the two sampling locations. Samples collected from the river contain a distinct evaporitic trend, while the cave stream and drip sites do not. Also, the mean isotopic values of the cave stream are significantly different than those of the river (df = 81, p = 0.003), but not significantly different than those of epikarst drip waters collected at two sites in the cave (df = 63, p = 0.74). After a re-survey of the cave with very accurate vertical control, the most upstream accessible point in the cave stream is 1.35 km away from and 6 m higher than the most reasonable potential connection with the Guadalupe River. Combined, these data suggest that flow in the main cave stream in CWAN is primarily derived from local, autogenic, diffuse and direct recharge sources, and that a direct connection with the Guadalupe River is unlikely.