GEOLOGIC AND ANTHROPOGENIC CONTROLS on CARBON DIOXIDE IN EDWARDS AND TRINITY LIMESTONE CAVES, BEXAR COUNTY, TEXAS

Carbon dioxide is globally significant to applications of understanding climate change, determining speleogenetic controls of carbonate aquifer and reservoir diagenesis, and CO² sequestration of excess greenhouse gasses in the Earth’s atmosphere. Caves provide a unique opportunity to study several aspects of CO², including: subsurface cycling of CO² as groundwater interacts with the surrounding limestone matrix, complex temporal patterns related to the dynamic system of CO² generation in karst settings, and diurnal-seasonal-climatic ventilation patterns related to cave morphology and vadose zone permeability. Two caves at the U.S. Army Camp Bullis Training Site in northern Bexar County, Texas, found within the Edwards and Trinity aquifer systems, have been instrumented with Vaisala CARBOCAP CO² Probe GMP343 sensors and Campbell Scientific CR1000 dataloggers to continuously record CO² concentrations every 15 minutes. This high-frequency, temporal record of CO² levels in the two caves has been monitored for approximately two years. CO² levels at each cave demonstrate distinct seasonal trends; however, the two caves display reverse seasonal trends relative to each other, despite being only 3 km apart and both in the Kainer Formation. Cave A follows the trend observed by many researchers of higher summer CO² concentrations (avg 4,747 ppm) and lower winter concentrations (avg 500 ppm). Cave B displays higher concentrations of CO² in the winter (avg 12,830 ppm) and lower concentrations during the summer (avg 1,185 ppm). These reverse trends may be due to different background levels of moisture in the caves or to cave morphology and deeper sources of CO². Cave A has collapse morphology and maintains moisture year-round. Cave B has shallow maze morphology and dries completely seasonally. Understanding the processes controlling cave CO² fluctuations has wide applications in karst hydrogeology, speleothem paleoclimate reconstruction, and cave ecology.