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EXPLORING CAUSES FOR INCREASING NITRATE CONCENTRATIONS IN THE LOWER ST. CROIX RIVER
Brenda Moraska Lafrancois, National Park Service, Midwest Region and St. Croix Watershed Research Station
Nitrate loads from the upper Mississippi River Basin contribute to the Gulf Hypoxia problem, and nitrate management is becoming an important concern in the Midwest and Central U.S. Monitoring data from the Metropolitan Council indicate that nitrates in the St. Croix have increased since the mid-1970s, particularly at downstream monitoring stations in Lake St. Croix. Various factors may contribute to these trends, including increases in fertilizer application and runoff, changes in wastewater treatment practices, increases in regional nitrogen oxide emissions and atmospheric deposition, and changes in soil denitrification processes.
Stable isotope analysis can help identify sources of nitrogen and clarify the relative importance of wastewater versus fertilizer inputs. δ15N signatures are measurable in water and biota. We measured nitrate concentrations and investigated patterns in δ15N signatures in water (nitrate), phytoplankton, and zebra mussel tissue. We hypothesized that nitrate concentrations would increase from upstream to downstream in Lake St. Croix, that the δ15N signal in water and biota would reflect wastewater influence at upstream sites and fertilizer influence at downstream sites, and that the δ15N signal would show less seasonal variability in zebra mussels than in phytoplankton or water.
We collected water samples for nitrate concentration and stable isotope analysis from seven Lake St. Croix sites (from near Arcola to near Prescott), from five tributaries (Brown’s Creek, Valley Creek, Trout Brook, and the Willow and Kinnickinnic Rivers), and from two wastewater treatment plants (Hudson and St. Croix Valley, near Bayport). We sampled zebra mussels from the five lowermost St. Croix sites and phytoplankton from all mainstem St. Croix sites. We repeated the sampling five times from May to October 2008, capturing a range of hydrologic conditions.
Results from the first four sampling events indicate that mean nitrate-N concentrations for Lake St. Croix sites ranged from 0.12 mg/l near Stillwater and Bayport to 0.34 mg/l near Prescott. Mean tributary nitrate-N concentrations ranged from 0.73 mg/l in Brown’s Creek to 5.49 mg/l in Valley Creek, and mean wastewater treatment plant concentrations were 4.98 mg/l and 13.98 mg/l for Hudson and Stillwater, respectively. Concentration results indicate that the pattern of increasing nitrate levels from upstream to downstream is caused mainly by inputs from lower tributaries to Lake St. Croix. Stable isotope analyses are underway, and preliminary results may be available by Rendezvous-tim