RECOVERY OF A FRACTURED BEDROCK AQUIFER FROM NITRATE CONTAMINATION IN THE CENTRAL VERMONT APPALACHIANS

In 2000, elevated nitrate levels ranging from 12-35 ppm (NO₃-N) were discovered in groundwater from numerous domestic bedrock wells that are adjacent to a large dairy farm in central Vermont. The area is underlain by interlayered gray phyllites and quartzose marbles of the Devonian Waits River Formation. Bedding planes and a ~coplanar foliation in the bedrock strike NNE-SSW and dip moderately westward whereas the dominant fracture sets dip steeply and strike E-W and NW-SE. Based on potentiometric contours, borehole camera surveys, and electromagnetic induction (EMI) surveys, these planar structures control groundwater flow. The Pleistocene sandy glacial till that overlies the bedrock is variable in thickness, ranging from 0 – 25m, reflecting the presence of localized bedrock basins. Average CFC-113 recharge ages for groundwater range from 17-22 years. 

Following detection of elevated NO₃ in bedrock wells in 2000, changes were made to field nutrient management practices and varied aquifer NO₃ trends were observed, ranging from wells that rapidly responded (i.e. NO₃ decreased rapidly) to others that exhibited slower rates of change, or did not change at all. These “fast-change”, “slow-change” and “no-change” wells were spatially distinct. When integrated with the bedrock geologic and hydrogeologic frameworks, the nitrate patterns suggest two sources: a ravine point source, emanating from a ravine where manure was stored and the non-point source provided by years of (and on-going) nutrient application on the surrounding fields. The “no-change” wells presumably reflect the long-term background levels of nitrate in the bedrock aquifer due to nutrient applications on the nearby farm fields, whereas the “fast-change” and “slow-change” patterns record the exhaustion of the ravine point source. Approximate groundwater flow rates in the bedrock aquifer were inferred from potential groundwater flow paths and range from 0.1 – 0.2 m/day. From 2005-2013, NO₃ concentrations in all wells that had been elevated in NO₃ returned to background levels of ~3-10 ppm. In terms of mitigation, the termination of the ravine point source in 1995 was the most effective nutrient management practice change. It is yet uncertain whether changes to field nutrient management practices initiated in 2000 have had any effect on nitrate levels.