NITRATE CONTAMINATION OF BEDROCK DRINKING WATER WELLS IN THE WAITS RIVER FORMATION: EAST MONTPELIER, VERMONT

In 2000, routine Vt Dept. of Agriculture testing of domestic wells in the vicinity of a large dairy farm in East Montpelier, Vt identified a grouping of bedrock wells with nitrate levels above the EPA MCL of 10 ppm. The nitrate problem has persisted in the same wells through two consecutive years of quarterly testing. Records indicated that the farm had been spreading nitrate–rich fertilizer onto a field to the SSW of the problem wells. University of Vermont students are currently working with the Vermont Geological Survey to determine the probable pathways through which the nitrates are reaching the bedrock wells.

The study area is located in the Siluro-Devonian, greenschist facies, metasedimentary rocks of the Waits River Formation which consist of punky, brown, granular crystalline limestone interbedded with rusty-weathering gray phyllite. Dissolution of the crystalline limestone may create significant secondary porosity. The bedrock is overlain by thin glacial till. The fertilized field is topographically higher than the contaminated wells.

Ductile and brittle bedrock structural data for the Waits River Fm bedrock were collected at 45 outcrop locations in areas on or adjacent to the farm. The dominant foliation in these rocks is a NNE-trending, moderately west dipping, spaced cleavage that is parallel to bedding; this foliation has been folded by a second generation of folds. Based on frequency-azimuth rose diagrams of the 470 fractures measured, the dominant population trends E-W whereas subordinate populations trend NW-SE and N-S.

The wells with the highest levels of nitrate contamination are NNE of the fertilized field. Since the dominant foliation/bedding in the Waits River Fm trends NNE and the topographic grain in the area as determined from topographic maps and airphotos also trends NNE, the solution-enhanced foliation/bedding may be responsible, in part, for bedrock ground water flow from the field to the wells. Alternatively, N-S trending fractures may also be an avenue of transport. E-W trending fractures; however, may feed ground water to the foliation/bedding or N-S fracture surfaces. This study is important because there are other farms in the Waits River Fm where bedrock ground water contamination from nitrates is known to be a problem.