MONITORING TEMPORAL VARIATIONS OF AGRICULTURAL CONTAMINANTS IN NEBRASKA'S RAINWATER BASIN WETLANDS

The wetlands of south-central Nebraska’s Rainwater Basin are considered of international importance as a habitat for millions of migratory birds. They may also provide recharge to groundwater used for drinking and irrigation water. These wetlands could also be used to remediate agricultural runoff if they are found to remove contaminants before water percolates to the groundwater. The Rainwater Basin extends across 17 counties and covers approximately 4,000 square miles. These wetlands are disappearing quickly due to agricultural drainage and filling for construction of roads and concrete pits. Siltation and pollution from fertilizers and pesticide runoff may also be endangering the basins. Eight federal and state-owned wetlands were chosen across the Rainwater Basin to monitor surface and ground water chemistry. At each site, a shallow well, approximately 30 feet below the water table, and a deep well, at least 50 feet below the shallow well, were installed and sampled once in the summer of 2009. Wetland surface water was sampled monthly throughout the summer beginning in April, 2009. The concentration of nitrates and atrazine levels are of concern as the wetlands interspersed in this region are surrounded by fields of corn and soybeans. An analytical scan of 24 known pesticides, including atrazine, was performed using GC/MS and nitrates were measured along with major anions by ion chromatography at the Water Sciences Laboratory at the University of Nebraska-Lincoln. Preliminary results show that pesticides are present in the wetlands prior to the growing season. Acetochlor (0.07 to 0.19 ppb), alachor (0.53 ppb), atrazine (0.12 to 0.41 ppb), DEA (0.05 to 0.08 ppb) and metolachlor (0.06 to 0.1 ppb) were detected in the surface waters in April before pesticides were applied for the year. Ammonia (up to 3.33 mg/L) and nitrates (up to 3.65 mg/L) were found in some, but not all of the wetlands. It will be important to determine whether these levels increase throughout the growing season as a result of agricultural runoff. Identifying the levels of these contaminants in the local wells will help determine if nitrates and atrazine are reaching the groundwater. Further chemical and isotopic analysis will show how the contaminants persist throughout the year and how evaporation affects the wetlands.