HISTORY OF NITRATE CONTAMINATION IN GROUNDWATER AT A CENTRAL VERMONT DAIRY FARM

The Vermont Geological Survey (VGS) and the Vermont Agency of Agriculture, Food, & Markets (VAAFM) have collaborated for ~20 years to understand the fate and transport of nitrates in aquifers underlying large dairy farms in Vermont. The VAAFM routinely monitors private wells around these farms for nitrates and herbicides and often has data that span decades. We are currently working together on farms in Bristol, East Montpelier, Hardwick, and Irasburg. This presentation will focus on the Hardwick farm.

By plotting nitrate vs. time data for all wells in a farm area where groundwater is contaminated, different patterns often emerge for spatially distinct well groups, which may reflect nitrate source areas and groundwater flow directions. Nitrate patterns can show little, moderate, and large changes over time. Well groups can be further evaluated using other tracers such as corn herbicides, major and trace elements, nitrogen and oxygen isotopes, and recharge ages.

At a dairy farm in Hardwick, nitrate and herbicide data are available for bedrock wells and springs since ~2000, and for surface water (streams and a pond) since ~2007. Additional spring locations were sampled from 2018-2020. By examining these data sets spatially, the nitrate contamination can be divided into northern (#1), central (#2), and southern (#3) areas. Area #1 consists of bedrock springs that were recently determined to have elevated (3<x<10 ppm) nitrate levels; Area #2 contains bedrock wells that have had high (>10 ppm) nitrate levels since ~2004; and Area #3 has springs that have had low (<3 ppm), elevated, and high nitrate levels since ~2004.

Although the source areas for nitrates in areas #1, #2, and #3 are generally the nearby farm fields, groundwater-surface water interaction plays a role in Area #3, where overflowing springs with elevated nitrate levels form the headwaters for streams flowing southward toward a wetland containing one contaminated spring; this spring has an irregular nitrate pattern suggesting surface water influence. Streams emerging from the wetland have nitrate and herbicide patterns nearly identical to those of this spring, suggesting a southward transition from groundwater - surface water- groundwater - surface water. Our goal is to propose nutrient management practices that will reduce nitrate levels over time.