Abstract This paper reports a long-term field investigation of a fractured dolostone aquifer that was penetrated by a dense non-aqueous phase liquid. High-resolution source zone characterization shows the evolution of deep penetration to the back-diffusion conditions seen at the present day. Metolachlor, a common herbicide, was released into the overburden overlying a fractured dolostone aquifer within a short time window (1978–81). In 2000, the plume front arrived at a municipal supply well located 930 m down-gradient, increasing to a maximum concentration of 2 μg l −1 . Groundwater monitoring with high-resolution, depth-discrete multi-level sampling systems since 1992 shows a clearly delineated bedrock plume. Numerous rock core samples show metolachlor in the low-permeability rock matrix at the bottom of the aquifer. The mass distribution and bedrock hydraulic head pattern strongly suggest that metolachlor entered the bedrock as a free-phase dense non-aqueous phase liquid penetrating to the aquifer bottom, preferentially accumulating in some horizontal fractures, dissolving quickly as a result of the rapid groundwater flow and then diffusing into the rock matrix, where back-diffusion sustains a dilute, persistent and stable plume. Strong plume retardation by matrix diffusion and sorption has greatly mitigated the impact on water quality in the down-gradient supply well, allowing for its continued use, while back-diffusion and degradation maintain a persistent, dilute plume managed by appropriate monitoring.

The Spring Mill Lake watershed is located in the Mitchell Plateau, a karst area that developed on Mississippian carbonates in southern Indiana. Spring Mill Lake is a reservoir built in the late 1930s and is located in Spring Mill State Park. Within the park, groundwater from subsurface conduits issues as natural springs and then flows in surface streams to the lake. From 1998 to 2002, surface and subsurface hydrology and water quality were investigated to determine the types and sources of potential contaminants entering the lake. Water samples collected during base flow and a February 2000 storm event were analyzed for selected cations, anions, trace elements, selected U.S. Environmental Protection Agency (EPA) primary and secondary drinking-water contaminants, nitrogen isotopes, suspended solids, Escherichia coli , and pesticides. All of the water samples met the EPA drinking-water standards for inorganic constituents, except those collected at five sites in August 1999 during a drought. Nitrate nitrogen (NO 3 -N) concentrations were highest during base-flow conditions and displayed a dilutional trend during peak-flow periods. The NO 3 -N concentrations in water samples collected during the 2001 spring fertilizer applications tended to increase from early to late spring. All of the δ 15 N values were low, which is indicative of either an inorganic source or soil organic matter. Storm discharge contained increased concentrations of total suspended solids; thus, storms are responsible for most of the sediment accumulation in the lake. E. coli levels in 24% of the samples analyzed contained a most probable number (MPN) greater than 235/100 mL, which is the maximum acceptable level set for recreational waters in Indiana. E. coli does appear to be a potential health risk, particularly at Rubble spring. The sources of E. coli found at this spring may include barnyard runoff from a horse barn or wastes from a wastewater treatment facility. The pesticides atrazine, metolachlor, acetochlor, and simazine were detected during the spring of 2001. Atrazine, metolachlor, ace-tochlor, and simazine are used to suppress weeds during corn and soybean production. Additional sources of atrazine and simazine may result from application to right-of-ways, orchards, and managed forest areas.