ISOTOPIC IDENTIFICATION OF THE SOURCES OF NITRATE IN WELLS AND SPRINGS OF A KARST PLAIN IN SOUTHWESTERN ILLINOIS

The Illinois sinkhole plain, located on the western margin of the Illinois Basin, is the most karstified region of the state. Over 10,000 sinkholes, numerous large springs and the largest caves in the state characterize the area. The majority of caves and sinkholes occur within or overly the calcite-rich Mississippian-age Ste. Genevieve and St. Louis Limestones. A thin layer of Illinoian glacial till and residuum, overlain by about 10 m of wind-blown loess cover most of the bedrock. Nearly half of the residents obtain their water from the shallow karst aquifer.

Water samples were collected from 10 springs and 17 wells during different seasons of the year. Several nitrate end-member samples were also collected including: N-fertilizers, livestock waste, septic effluent, field runoff, and atmospheric NO₃^- from precipitation. Five wells had NO₃^- concentrations in excess of the USEPA¢s drinking water standard of 10 mg-N/L. Relative to background levels (£1.4 mg-N/L), the NO₃^- concentrations were anomalously high in most the wells and all the springs, suggesting that sources other than naturally occurring soil-organic matter have contributed NO₃^- to the groundwater. Agriculture is the dominant land use in the area, and the number of private septic systems is increasing due to urban development.

Chemical and isotope results of some dissolved constituents varied seasonally in the springs, attesting to rapid infiltration of surface and soil water to the karst aquifer. Isotopic data were the most definitive for distinguishing NO₃^- sources. d¹⁵N and d¹⁸O of the NO₃^- ranged from 3.2 to 25 ^o/oo and 7.2 to 18.7 ^o/oo respectively. Isotope results suggest the sources of NO₃^- in the springs are dominated by N-fertilizer with some possible influence of atmospheric NO₃^- and, to a lesser extent, animal/septic waste. The trend of the d¹⁵N and d¹⁸O data also suggests denitrification occurs in the shallow karst hydrologic system prior to discharging in the springs.

Nitrate concentrations in the well samples varied greatly. Isotope results indicate the source of NO₃^- in wells with high concentrations (13 to 80 mg-N/L) originated primarily from septic/livestock waste, while the dominant source of NO₃^- in wells with lower concentrations (2 to 12 mg-N/L) was N-fertilizer.