ESCHERICHIA COLI AND SEDIMENT FATE AND TRANSPORT IN A MANTLED KARST AQUIFER

Four tracers were simultaneously released into a losing stream at the Savoy Experimental Watershed, Northwest Arkansas in May 2004, with a primary goal of elucidating fate and transport of Escherichia coli (E. coli) and sediment in mantled karst aquifers. Prior investigations suggest that, unlike conservative tracers, E. coli and sediment become deposited along the flow path in the aquifer, and are resuspended with subsequent recharge events. Two dyes (Fluorescein and Rhodamine WT) previously shown to behave conservatively in this environment, 700 g (approximately 1x10¹⁵ E. coli) of Europium-tagged E. coli, and 500 g of lanthanum-tagged montmorillinitic clay were injected on the recessional limb of the hydrograph following a significant recharge event. Both dyes took about 10 days to move completely through the aquifer to two primary spring discharge points. The peak dye concentration was observed in the first day 17 hours after initial injection, whereas the first E. coli peak observed at both springs occurred about 13 hours after injection, four hours sooner than the two relatively conservative dyes. Total suspended solids (TSS) remained near typical low-flow concentrations (2-5 mg/L) over this same 10-day period. Three precipitation events large enough to increase spring discharge occurred in the 50-day period following tracer injection (7.5 cm 10 days after injection; 2.5 cm 30 days after injection; 3.6 cm 49 days after injection). During each of these recharge events, concentrations of TSS and E. coli increased sharply with rising spring stage, peaked before or just at the peak in spring stage, and dropped rapidly to background low-flow concentrations well before the springs receded to baseflow. These findings document that the fate and transport of sediment and E. coli is significantly different than conservative tracers, and that during storm-induced recharge, sediment and E. coli move through the aquifer by similar mechanisms. The study also shows that E. coli are stored and remain viable in the aquifer for extended periods. Understanding the fate and transport of E. coli and sediment in mantled karst aquifers is important, in as much as 20% of the U.S. is underlain by karst aquifers which support a largely untreated rural domestic water supply.