The Case of the Underground Passage: Putting the Clues Together to Understand Karst Processes

Contaminants in surface water entering karst aquifers in focused recharge can be transported rapidly through the system to discharge at springs. Such contaminants act as anthropogenic tracers of ground-water transport; analysis of their breakthrough curves as they discharge from springs allows identification and apportionment of contaminant sources, and can provide insight into aquifer structure and function. At Barton Springs, the principal outlet for the Barton Springs segment of the Edwards aquifer, near Austin, Texas, breakthrough curves for several anthropogenic and natural tracers have been analyzed. The proportion of discharge composed of recent recharge is determined with a mixing model for oxygen-18: following rainfall, an initial increase in discharge related to pressure transfer is clearly seen, followed by a further increase consisting of an increasing proportion of recently recharged water, which reaches a maximum about 48 hours after rainfall. First appearance of contaminants varies from less than 20 to more than 40 hours after rainfall, depending on the contaminant, aquifer conditions, and location of contaminant source. Decomposition of breakthrough curves of sediment, nutrients, pesticides, and volatile organic compounds at the different spring orifices indicates the existence and approximate location of multiple conduit flow routes, contaminant times of travel, and probable contaminant sources. Differences in breakthrough curve shape and magnitude indicate urban runoff, contaminant spills, and infiltration through the soil zone as sources of different contaminants. Comparison of contaminant loads in recharging surface water to those estimated for the decomposed breakthrough curves allows apportionment of contaminants to the five watersheds contributing recharge to the aquifer.