TRACKING AGRICHEMICAL TRANSPORT TO SURFACE WATER/GROUNDWATER WITH ISOTOPIC AND CHEMICAL PARAMETERS

Geochemical and isotopic data were utilized to help understand the transport of nutrients and herbicides in a study of a private farm within the somewhat poorly drained Nottawa Creek Watershed, south central Michigan. Geology of the 4.5 km² study area is comprised of a thin glacial drift cover overlying shale bedrock. A sampling network provided water quality data from 9 glacial drift observation wells, 2 streambed wells, 4 domestic drift wells, Nottawa Creek surface water, and 8 tile drains. Depth to groundwater varied from about 5 to 25 feet. The 18-month study, from December 1999-July 2001, afforded 11 sampling events to characterize relationships between groundwater flow systems and Nottawa Creek.

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.

Tile drains are a major contributor of nitrogen and phosphorus to surface water. Despite this, Nottawa Creek nutrient concentrations were consistently low. However, there is a correlation between herbicides in tile drains and herbicides in Nottawa Creek. A seasonal herbicide detection peak occurs in June-July and tapers off dramatically in early fall.