Geochemical evolution of karst vadose water and brush clearing impacts on recharge in central Texas

Abstract

Groundwater geochemistry is used to investigate flow paths, water residence times, and rock-water interaction processes, which is especially important in central Texas where groundwater flow is unpredictable and difficult to study due to the anisotropic nature of karst terrains. The first part of this study is a multiyear monitoring study that identifies and quantifies the processes controlling vadose drip water evolution in a cave, Natural Bridge Caverns, central Texas. Three different types of drip water are identified. Group 1 drip sites (n=3) are characterized by strong seasonal variations in Mg/Ca and Sr/Ca that are driven by seasonal fluctuations in calcite precipitation related to winter cave ventilation. Group 2 drip sites (n=4) exhibit correlations between drip water composition (Mg/Ca, Sr/Ca, and ⁸⁷Sr/⁸⁶Sr) and measures of water flux (rainfall and drip rate). Mass balance modeling demonstrates rock-water interactions (i.e., dissolution-reprecipitation processes involving carbonate minerals comprising host Cretaceous carbonate rocks) can account for drip water compositions. Group 3 sites (n=2) exhibit limited geochemical, physical or temporal correlations. Group 3 sites likely reflect a combination of Groups 1 and 2 processes, as drip water composition suggests both varying extents of rock-water interaction and calcite precipitation. The results of this study provide insight on the processes controlling the geochemical evolution of vadose karst waters and can be applied toward uncoding the paleoclimate signals recorded in speleothems. More specifically, in areas where cave-air CO₂ fluctuates seasonally, speleothem Mg/Ca and Sr/Ca variations may serve as chemical indicators of annual laminae, and speleothem growth may be biased. The second part of the study uses changes in drip rate and drip water geochemistry to evaluate the affects of brush clearing on recharge. Brush clearing is commonly used to increase stream and spring flow in central Texas even though it is not clear whether or not brush clearing enhances recharge. Drip rate and drip water composition were monitored every four to six weeks from May 2004 to April 2008. Brush clearing above the cave was conducted from April 2007 to July 2007. Drip rate and drip water compositions were compared at nine drip water sites, five of which are directly beneath an area cleared during this study. There were no changes in drip rate, Mg/Ca, Sr/Ca, or ⁸⁷Sr/⁸⁶Sr at drip sites beneath the cleared area that could be attributed to the brush clearing. The lack of change in drip water compositions and drip rates indicate that the brush clearing did not have a discernible impact on recharge to the cave, and suggests brush clearing does not have an impact on vadose recharge.