Hydrogeological characterization of baseflow to Jacob's Well spring, Hays County, Texas

Jacob's Well is a karst spring that discharges from the Trinity aquifer in central Texas. Ongoing development and increased groundwater pumping in the area around the spring are thought to be impacting the volume of discharge at the spring--however, the source of spring discharge is poorly understood. The goals of this study are to characterize the chemistry of springflow and nearby surface waters and groundwaters and to identify the source or sources of water that contribute flow to the spring under low flow conditions. The spring is located in the Middle Trinity aquifer within the Lower Cretaceous Trinity Group in central Texas. The Trinity Group is composed of alternating terrestrial and marine deposits, with most groundwater flow occurring through secondary porosity in the carbonate units. The cave leading up to the spring consists of at least two water-filled conduits that have been mapped down into the Cow Creek Limestone and several hundred meters to the north and northwest of the spring outlet. Water samples and measurements of water levels and chemical field parameters were collected at 48 study sites representing stream, spring, and well waters over a 250-km² area around the spring from May-September 2008. The area was in drought throughout this period and the results thus represent low flow conditions. Samples were analyzed for major and trace ions, organic and inorganic carbon, and isotopes of O, H, and Sr. Modeling using PHREEQC was used to calculate whether realistic phase changes in known aquifer minerals could explain the chemical differences between sampled waters along hypothesized flowpaths. Water levels indicate that groundwater flows generally to the southeast within the study area. Chemical analyses of water samples show three chemical types: (1) Ca-Mg-HCO₃ waters, which make up all spring water, all surface water, and most ground water samples; (2) high TDS Ca-Mg-SO₄ ground waters; and (3) high TDS Na-Mg-Ca-SO₄ ground waters. Measured element concentrations, strontium isotope data, and geochemical modeling results indicate that the high TDS waters most likely come from the upper member of the Glen Rose Limestone downgradient from the spring (type 2) and from the Hosston Formation in the Lower Trinity aquifer (type 3). Physical and chemical properties of the sampled waters were used to define which well and stream samples might represent water within the area contributing to baseflow at the spring. Water samples taken from seven sites north and west of the spring were found to represent possible sources of springflow during the study period. These findings suggest the possibility of two distinct flowpaths connecting the aquifer to Jacob's Well: (1) flow to the southeast within the Cypress Creek watershed along the regional groundwater flow path estimated from water level measurements, and (2) flow to the east along fracture or conduit paths that cross the surface divide between the Upper Blanco River and Cypress Creek watershedsGeological Science