Dynamics of Nitrate, Phosphorus, and Suspended Sediment Transport in Two Agricultural Streams in Central Illinois

Abstract

Nutrients such as nitrate and phosphorus are necessary for life, but excessive amounts can be detrimental. Large amounts of nutrients entering bodies of water can lead to hypoxic zones such as the one in the Gulf of Mexico. Nutrients are also problematic in drinking water reservoirs, as high concentrations of nitrate in drinking water can cause health conditions such as blue baby syndrome and high phosphorus concentrations can lead to algal blooms. Suspended sediment leads to reservoir sedimentation, habitat degradation, and is able to transport particulate nutrients. High nutrient and sediment concentrations are a recurring problem in the drinking water reservoirs for the City of Bloomington, Illinois where water is drawn from two reservoirs -Evergreen Lake and Lake Bloomington. The primary source of these nutrients is from agriculture, which dominates the land use in the area. To better understand the dynamics of nitrate, phosphorus, and suspended sediment transported into these reservoirs, water samples were collected at the major tributary for each reservoir -Six Mile Creek for Evergreen Lake and Money Creek for Lake Bloomington. SedEvent, an autosampler system that uses a turbidity threshold sampling method to determine when a rain event is occurring, was used to collect water samples at both tributaries. Water samples were analyzed for nitrate, total phosphorus, and dissolved reactive phosphorus using flow injection analysis (FIA). Samples were analyzed for suspended sediment by filtration and drying of samples. Results showed high nutrient and suspended sediment concentrations and loads in both creeks during or just after rain events, when discharge was high. Nitrate concentrations ranged from 1.58 to 13.3 ppm, total phosphorus concentrations ranged from 11.9 to 1250 ppb, and total suspended sediment concentrations ranged from 2.5 to 4100 ppm. Seasonal patterns in nutrient dynamics were present and, in general, water quality tended to be lower during the spring and higher during the summer. In both Six Mile and Money Creek, the majority (>70%) of phosphorus and total suspended sediment cumulative load occurred during stormflow conditions which accounted for less than 25% of flow time. The majority of nitrate cumulative load at Six Mile Creek occurred during baseflow conditions and at Money Creek, slightly more nitrate was transported during stormflow. Overall, seasonal changes in water quality coincide with agricultural activities, which suggests that alternative management practices may help improve water quality.