5.2 Bad-water Line

The Edwards Aquifer freshwater system interfaces with down-dip anoxic, sulfide-rich brackish (TDS > 10³ mg/L) to saline (> 10⁴ mg/L) “bad water.” The interface between these two waters has sulfur-reducing bacteria that form the base of a chemolithoautotrophic ecosystem at great depths (100 to 600 m) in which organisms can obtain the necessary carbon for metabolic processes from carbon dioxide in the environment (Engel, 2007). This autochthonous system (energy derived from within) is unlike normal cave systems and supports a greater species richness than adjacent sites, which are allocthonous (where nutrients enter the aquifer from the surface). It is inferred that small-scale dissolution is enhanced along the bad-water interface. To date, these findings are restricted to the San Antonio area, but the nutrients generated along the brackish-water line may support ecosystems over a broader range of the aquifer.

The Edwards Aquifer ecosystem is complex and highly varied. Understanding of the system has evolved with the examination of microbial processes (e.g., along the brackish-water line) and DNA analyses of aquatic organisms (e.g., Devitt, 2019). It should be expected that new organisms that are dependent on the aquifer will be discovered. Loss of spring flows during droughts is a major threat to the ecosystem. This threat led to new water laws and management policies. Lowering water tables and increased water pollution are threats to species that have limited habitat ranges.