CHARACTERIZING FRACTURED ROCK AQUIFERS CONTAMINATED WITH NITRATE AT LARGE DAIRY FARMS IN CENTRAL VERMONT

The Vermont Geological Survey (VGS) and Agency of Agriculture (VAA) have collaborated for 17 years to understand the fate and transport of nitrates at large dairy farms in Vermont, where groundwater from farm and domestic wells (and springs) exceeds the maximum contaminant level (MCL) of 10 ppm. The VAA routinely monitors wells around these farms for nitrates and herbicides and requests VGS expertise when elevated nitrate levels are persistent. Aquifer characterization studies were completed at farms in Sheldon and East Montpelier, are ongoing in Hardwick and Sutton, and are planned in Bristol and Irasburg. The goal of these studies is to build a geologic framework that can be used to guide nutrient management practices.

The aquifer characterization process is multi-disciplinary and incorporates components of physical (geologic mapping, spatial analysis of well driller logs) and chemical (major and trace elements, nitrate, herbicides, nitrogen and oxygen isotopes, and groundwater tracer-ages) hydrogeology. Once a physical 3-D geologic framework is constructed, the chemical tracers give a 4^th dimension. Aquifer characterization is in an intermediate stage in both Hardwick and Sutton.

The Hardwick and Sutton farms are situated in interbedded gray phyllites and sandy marbles of the Waits River Formation, where secondary porosity can develop parallel to bedding. The bedrock is overlain by porous and permeable sandy till of variable thickness. Steeply-dipping fractures that strike N-S and E-W are common.

Nitrate records for wells and springs surrounding the Hardwick farm span ~20 years and indicate elevated contamination levels for extended periods. Herbicide levels, which are well below drinking water standards, are an important groundwater tracer for farm activity. Peaks in herbicide abundances are out-of-phase with those of nitrates. Significant changes in nutrient management practices have yet to reduce nitrate levels.

The Sutton farm area was chosen for study because of elevated nitrate levels in a bedrock well serving the town school. Although nitrate levels have decreased below the MCL in this well, elevated levels persist in other wells. Our preliminary analysis suggests that different nitrate sources (point and non-point) affect spatially separate groupings of contaminated wells and springs.