Nitrate Risk Near Farms: A Hydrogeologist Explains
Farm runoff isn't just polluting Vermont lakes and streams. Nitrate from manure and fertilizer is also contaminating private drinking wells. VPR interviewed hydrogeologist Miles Waite of Waite-Heindel Environmental Management to help us understand how the nutrient gets into groundwater.
The following transcript has been lightly edited for clarity and brevity.
VPR: How does nitrate get into groundwater?
Waite: The normal nitrogen cycle is that plant matter decomposes. Bacteria and microorganisms convert that to ammonium. That, in turn, is converted to nitrite and nitrate, which can be used by the plant roots as plant food to grow for the next season.
In a farm situation you often add manure and fertilizers as nitrogen sources. In that case, all the nitrogen and nitrate in the soil may not be taken up by the plants. In that case, rainwater percolates through the soil, picks up the nitrate, and moves it away from the plant roots into groundwater, either moving through the aquifer into a well or coming out into a stream which leads to a river or a lake.
But whether nitrate leaches into groundwater can also depend on the geology of the area. If you have a thin soil layer right on bedrock, leaching is more likely. If you have a more protected situation where you have clay, on top of unconsolidated soils, above a dense layer and then rock, it could protect the nitrate from getting in there.
So high nitrate levels in groundwater could mean that there’s an excessive amount of manure being spread, or it could mean there’s just not enough soil to take up a "normal" amount of manure being spread?
If a farm’s drilled well is testing say, over 20 ppm for nitrate, and there are neighbors distributed within a one mile radius, how worried should those neighbors be?
If your farmer-neighbor’s well has exceeded the safety standard and your well is under say, 500 or 1000 feet away, you should be concerned. But it’s also very hard to predict where that nitrate is going to be. So there are studies that show that near a farm field you might have one well in a backyard of a home that tests for nitrate at 20 ppm, and the neighbor has a well 50 feet away, that tests at “non-detect.” And that’s because groundwater is very hard to predict where it is going to flow. It flows in fractures that have orientations, which means the groundwater could flow north, northwest, but it’s not going to flow where your neighbor is, so it’s hard to predict. You can’t say within a radius of “X,” everyone’s going to have 20 ppm.
And just in the same way your neighbor 50 meters away may be safe, could another neighbor a mile away not be safe?
If you lived near a farm and their well was testing high for nitrates, would you want to know about it?
Yes. Particularly if I had a shallow dug well. If I had a well that didn’t have the protection of a well casing and a deep source, I would be concerned that the manure applied on the farm field near my home could get into my water source.
How often and where do you usually find that people using shallow dug wells?
Often it is farm properties. It could be up to five percent of properties in rural Vermont have never gone through the process of drilling.