IDENTIFYING REGIONAL AND LOCAL CONTRIBUTIONS TO A LARGE KARST SPRING SYSTEM, COMAL SPRINGS, TEXAS, USA

Comal Springs in New Braunfels, Texas, USA is a multi-orifice karst spring of the Edwards (Balcones Fault Zone) Aquifer (Edwards BFZ) and the largest spring system in Texas with an average discharge of 289 ft³/s (8.2 m³/s). Comal Springs is home to several endangered and protected species and serves as an important public water source for local and downstream interests. Previous studies indicate discharge at Comal Springs represents a composite of sources and conduits of the large and complex Edwards BFZ Aquifer system, but tracer tests at the springs and subtle geochemical differences between some major orifices suggest that distinct local flow paths are present. Understanding the relative contribution of source areas to different orifices is an important part of assessing Comal Springs vulnerability to contamination and loss of flow from increased groundwater demand.

The Edwards Aquifer Authority (EAA) is a regional groundwater conservation district assigned to manage, enhance, and protect the aquifer system. The EAA has embarked upon an extended effort to characterize the temporal and spatial geochemistry of Comal Springs. Three main orifices at the springs are sampled bimonthly for a range of parameters including major ions, trace elements, and isotopes. The discrete sampling is supplemented by continuous monitoring of temperature and conductivity. Time-series and multivariate statistical analyses are used to evaluate changes in spring geochemistry under varying flow conditions. Results indicate that differences in water chemistry (e.g., bicarbonate and strontium concentrations) between spring orifices are persistent even as they vary with flow. Water isotope data, on the other hand, do not vary over the study period and are indicative of a large well-mixed aquifer reservoir. During periods of very low flow, some constituents (e.g., sulfate) vary in ways that suggest greater contribution to some orifices from local flow paths, which may include influx from adjacent aquifers.