USING GROUNDWATER TRACERS TO ASSESS THE FATE AND TRANSPORT OF NITRATES IN AQUIFERS NEAR LARGE DAIRY FARMS IN CENTRAL VERMONT

Nitrate contamination of bedrock and surficial aquifers is a problem at some large dairy farms in central Vermont. The Vermont Agency of Agriculture (VAA) routinely monitors wells around these farms for nitrates and herbicides and requests hydrogeological expertise from the Vermont Geological Survey (VGS) when nitrate levels > 10 ppm (MCL) persist. The consumption of elevated nitrate concentrations in water can cause “blue-baby syndrome” in infants and may lead to the formation of nitrosamine in the digestive tract, a possible long-term cancer risk. Since 2001, the VGS and VAA have collaborated to understand the fate and transport of nitrates and propose solutions.

By plotting nitrate vs. time for all wells in an area where groundwater is contaminated, different patterns often emerge for spatially distinct well groups, which may reflect nitrate source areas and groundwater flow directions. These well groups can then be further evaluated using other tracers such as corn herbicides, major and trace elements, nitrogen and oxygen isotopes, and recharge-ages.

Corn herbicides are compounds sprayed on fields to control nuisance plant species that may affect the growth of corn, an important feed crop for cows. Although the herbicide levels measured in groundwater are well below health standards, the absolute and relative herbicide levels can give important clues for nitrate fate and transport. We are currently plotting all parent and daughter levels vs. time and all daughter parent/daughter ratios for all wells within each farm area vs. time.

We use major and trace elements for each well in order to geochemically fingerprint groundwater from specific bedrock formations and anions to document anthropogenic activity. Nitrogen and oxygen isotopes can determine whether the nitrate in groundwater is derived from industrial fertilizer or manure and septic waste and whether denitrification or dilution processes are active in the aquifer. Average recharge ages of groundwater are ascertained through the analysis of chlorofluorocarbons, sulfur hexafluoride, and tritium.

To fully understand the fate and transport of nitrate in an aquifer system, the chemical tracer data must be integrated with a 3-D conceptual model that is constructed from geologic maps, the spatial analysis of well driller reports, and geophysical logging.