CHEMICAL, ISOTOPIC, AND HYDROGEOLOGIC INVESTIGATONS OF AN AGRICULTURALLY IMPACTED AREA, SOUTH CENTRAL MICHIGAN

A 4.5 km² agricultural area within the somewhat poorly drained Nottawa Creek Watershed, south central Michigan, was investigated to help understand transport and transformation processes of nutrients and herbicides to surface water. Geology of the study area is comprised of a thin glacial drift cover overlying shale bedrock. A numerical triangular finite element one layer two-dimensional groundwater flow model was developed to evaluate regional groundwater flow. Selected horizontal hydraulic conductivities ranged from 10^-3 ft/day to 350 ft/day. Field-scale infiltration modeling with the computer program GLEAMS, substantiated with water level measurements, suggest significant recharge events occur during summer, as well as fall and spring. Depth to groundwater varied from about 5 to 25 feet.

Geochemical data was collected for 12 sampling events over 22 months, from December 1999-September 2001, for Nottawa Creek surface water, 9 water table observation wells, 4 domestic drift wells, 2 streambed wells, and 8 tile drains. Tile drains are a major contributor of nitrogen and phosphorus to surface water. Despite this, Nottawa Creek nutrient concentrations were consistently low.

Tritium results from the two streambed wells, which are relatively uncontaminated, indicate groundwater discharging into Nottawa Creek is post-bomb water. Values of d¹⁵N and d¹⁸O from nitrate from most wells and tile drains suggest field applied synthetic fertilizer, composted manure, and liquid manure is transformed or not present. Conversely, wells adjacent to an animal waste lagoon and a compost pile exhibit an animal waste isotopic signature.

Concentrations detected by immunoassay are reported for triazine and chloroacetanilide residues. Triazines are leached vertically via preferential flow and transported to surface water by tile drains. Low or non-detect triazine concentrations in observation wells suggest significant degradation or retardation. Comparatively, chloroacetanilide test results indicate a much greater detection of residue in wells while largely undetected or at low concentrations in tile drains.