ESTIMATING BACKGROUND AND THRESHOLD NITRATE CONCENTRATIONS IN A KARST AQUIFER USING PROBABILITY GRAPHS

Because of the ubiquitous nature of anthropogenic nitrate in many parts of the world, determining background concentrations from only natural sources is meaningless for many environments. Pre-settlement background nitrate-N concentrations in the U.S. were likely considerably less than 1 mg/L in most environments, but modern background values may be higher. Modern background must now include diffuse sources of nitrate such as disruption of soils and oxidation of organic matter, and atmospheric inputs from products of combustion and evaporation of ammonia from fertilizer and livestock waste. Anomalies can be defined as nitrate derived from the direct input of nitrogen to the environment due primarily to anthropogenic activities, including synthetic fertilizers, livestock waste, and septic effluent.

Cumulative probability graphs were used to identify threshold concentrations separating background and anomalous nitrate-N concentrations and to assist in the determination of sources of N contamination for 232 spring-water samples and 200 well-water samples from karst aquifers in Illinois. Thresholds were 0.4, 2.5, and 6.7 mg/L for spring-water samples, and 0.1, 2.1 and 17 mg/L for well-water samples. The lowest thresholds are assumed to represent thresholds for modern precipitation. Thresholds at 2.5 and 2.1 mg/L are interpreted to represent modern background concentrations of nitrate-N. The population of spring-water samples with concentrations between 2.5 and 6.7 mg/L represents an amalgam of all sources of nitrate in the groundwater basins that feed each spring, and concentrations greater than 6.7 mg/L were typically samples collected soon after springtime application of synthetic fertilizer. The 17 mg/L threshold for well-water samples is interpreted to be the level above which livestock wastes dominate the nitrogen sources, and this is confirmed by land use information and nitrate isotope data.

This investigation shows that the cumulative probability technique is a viable means of separating background from anomalous concentrations of nitrate-N in karst groundwater. This technique could be used in other areas to help identify background concentrations for nitrate-N and other ions in groundwater.