IDENTIFYING UNKNOWN SOURCES OF NITRATE IN TWO CENTRAL ILLINOIS WATERSHEDS

Nitrate pollution due to fertilizer input is of major concern to water quality in the US Midwestern corn belt. Six Mile Creek and Money Creek in Central Illinois share near-identical agricultural land-use, yet the respective municipal supply reservoirs, Lake Evergreen and Lake Bloomington, present significantly different water chemistries. Lake Bloomington is listed as impaired water by Clean Water Act § 303(d) due to periodic exceedance of the EPA’s 10 mg/L safe drinking water limit for nitrogen. The lake’s shoreline has several dozen homes operating on septic systems, which may contribute to the nitrate issues. In contrast, Lake Evergreen is surrounded by forested parkland and is unimpaired by excess nitrate in the drinking water. The objective of this study is to explore potential sources of nitrate in each watershed and lake, using nitrate and oxygen isotopes d15N and d18O to develop longitudinal nitrate concentration profiles. Stream and lake water samples were collected during late summer, when streamflow was low and early summer when stream was high. Samples were analyzed for major ions and N and O isotopes. Nitrate concentrations were relatively low (0.40 mg/L to 0.55 mg/L N) under low flow conditions in the late summer/early fall but the maximum values were much higher (0.53 mg/L to 15.10 mg/L) under high flow conditions in the late spring/early summer. Preliminary analysis indicates there are significant isotopic differences in nitrate between stream and lake samples from the same watershed, however, it does appear that there is an isotopic difference in nitrate between the two watersheds. The results suggest that while the isotopic concentration varies between the basins, the results did not allow a differentiation between septic and agricultural sources of nitrate.