SEDIMENT AND BACTERIA TRACING IN MANTLED KARST OF NORTHWEST ARKANSAS

Springs flowing from mantled-karst in northwest Arkansas have significant concentrations of fecal coliform (dominated by E.coli) from a variety of potential sources. The subsurface fate of these bacteria is poorly understood. To understand the transport and storage mechanism of E.coli in a mantled-karst environment, four tracers were injected during spring 2001 in Savoy Experimental Watershed (SEW), Arkansas, including rhodamine dye, chloride, E.coli and lanthanum-labeled clay. The labeled-clay tracer is considered representative of finer aquifer sediments and it was proposed that E.coli would be associated with the clays during transport and storage. Primary discharge from the basin is two springs (located 500 meters from injection point) that were instrumented for continuous discharge allowing determination of tracer load and calculation of percent capture. In April, dye and chloride were injected to assess travel times and required tracer concentrations. These tracers arrived approximately twelve hours after injection and peak concentrations occurred approximately nineteen hours after injection. Percent recovery of dye and chloride during a three-day period was 30% and 60%, respectively. A second injection in May, included dye, E.coli, and lanthanum-labeled clay. These tracers were injected during extremely low-flow conditions allowing the E.coli and clay to settle out of suspension within the aquifer, while the more conservative dye moved through as a continuous plume. The dye arrived approximately nineteen hours after injection and peak concentration occurred approximately at twenty-seven hours. Twenty percent of the dye was captured during the first three days of sampling, while only 1% of the clay was captured. Dye captured during the second tracer test was lower because spring discharge was 12% lower. Six days after injection a storm pulse mobilized the bacteria and clay tracer, resulting in an additional 60% capture for the labeled-clay. During the storm pulse, peak concentrations of the clay tracer were coincident with peak concentrations of E.coli, suggesting that ground-water storage and transport mechanisms are similar for bacteria and clay. These results indicate that sediment tracers can be used for studying the transport of bacteria in mantled-karst environments.