USING TRACER TESTS TO DEVELOP AN IMPROVED CONCEPTUAL MODEL FOR A LARGE KARST AQUIFER

The karstic Edwards (Balcones Fault Zone) Aquifer in south-central Texas is a critically important resource for the region, supplying drinking water to more than 2 million people and providing a dependable water source for irrigation, industrial, and recreational use. The Comal and San Marcos springs are the major natural discharge locations for the Aquifer, and the spring ecosystems are home to several threatened and endangered species. Permitted use of Edwards (BFZ) Aquifer water is regulated based on a need to maintain adequate springflows to ensure viability of the endangered species.

The complex and interconnected nature of conduits and other preferential pathways within karst aquifers can be difficult to discern, especially in a system with little regional variation in geochemistry like the Edwards (BFZ) Aquifer. Tracer testing with fluorescent dyes can provide direct evidence of hydraulic connections and flow paths within karst systems. As such, tracer testing has become an important tool in characterizing karst aquifer vulnerability to contamination, understanding the relative contribution of recharge features to aquifer flow and discharge, and identifying unexpected pathways.

Over nearly two decades, Edwards Aquifer Authority staff and consultants have conducted numerous tracer tests in the Edwards (BFZ) Aquifer. The results of some of these tests have (i) elucidated important flow paths and connections, or lack thereof, at the spring systems, (ii) provided definitive evidence to challenge some long held conceptual models of the influence of faulting on flow, and (iii) confirmed flow characteristics at the borehole-scale. We review examples of each of these tests to demonstrate how tracers can be employed at various scales to develop a better conceptual model of a large karst aquifer.