IDENTIFYING UNKNOWN SOURCES OF NITRATE IN TWO CENTRAL ILLINOIS WATERSHEDS

Nitrate pollution is of major concern to water quality in agricultural watersheds due to fertilizer inputs, a problem that is quite significant in the Midwestern corn belt, USA. Six Mile Creek and Money Creek in Central Illinois share near-identical agricultural land-use yet their 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 10mg/L safe drinking water limit for nitrogen; whereas, Lake Evergreen does not have that issue. Lake Evergreen is surrounded by forested parkland, while Lake Bloomington’s shoreline has several dozen homes that operate on septic systems, which may be contributing to Lake Bloomington’s nitrate issues. The objective of this study is to explore potential sources of nitrate in each watershed and lake using nitrate isotopes d15N and d18O and to develop longitudinal nitrate concentration profiles. We collected stream and lake water samples during late summer when streamflow was low. 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). Preliminary analysis indicates there is not much of an isotopic difference 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. We intend to conduct a second sampling event in the upcoming spring season to attempt capturing high stream flow and higher nitrate concentrations than were seen in the late fall sampling. In this study, we will provide current and future planners a better understanding of nitrate sourcing and will help with remediation and prevention of unsafe levels of nitrate in our natural and municipal water supplies.