MAJOR- AND TRACE-ELEMENT LITHOCHEMISTRY OF EDWARDS (BALCONES FAULT ZONE) AQUIFER HYDROSTRATIGRAPHIC UNITS, SOUTH-CENTRAL TEXAS, USA

The Edwards (Balcones Fault Zone) Aquifer is a major karstic limestone aquifer system in south-central Texas, USA. The aquifer serves as the primary water supply for approximately 2 million people and is integral to the ecosystems of major springs that are home to several threatened and endangered species. Because of its importance to the region, the aquifer’s flow system and water quality have been studied extensively over several decades. Many of these studies have included detailed lithologic and hydrologic analyses of the aquifer matrix, but there have been few studies done to characterize the lithochemistry of the hydrostratigraphic units comprising the aquifer. Recent investigations into the trace element chemistry of groundwater in the aquifer system have highlighted the need to understand the potential trace element contributions from the rock matrix.

Pre-existing cores from both the freshwater and brackish water zones of the Edwards Aquifer were identified and sampled. All seven of the hydrostratigraphic units of the aquifer and the overlying Georgetown Limestone were present in each core. The rock samples were collected at regular intervals and analyzed for major- and trace-element compositions using x-ray fluorescence (XRF) spectrometry and inductively coupled plasma mass spectrometry (ICP-MS). Major-element concentration differences between the brackish and freshwater samples are consistent with freshwater flushing and diagenetic processes including dedolomitization, calcite recrystallization, and replacement by SiO₂. These processes have also affected the trace-element distribution in the cores, with some element concentrations, like Sr and Ba, significantly reduced in the freshwater samples, while others, like Cr and Ni, increased in the freshwater samples. Other elements of interest, such as Zn and Pb, have similar concentrations in both the freshwater and brackish water samples. Variations in chemical composition effectively highlight the transitions between hydrostratigraphic units. Results also suggest the presence of clays and other non-carbonate minerals in hydrostratigraphic units that have lower hydraulic conductivity.