They can take a long time to show up—and then treat—in groundwater.
By Jennifer Strawn
Nitrogen is one of the most important elements on Earth—about 78% of the air we breathe is composed of nitrogen gas.
It’s also essential to plant growth and survival. Nitrogen, in the form of nitrate, helps plants develop tissue and build immune systems. It also encourages seed production. That’s why most commercial fertilizers have high concentrations of nitrate.
But like a lot of things in life, there can be too much of a good thing. In surface water, too much nitrogen can lead to the overgrowth of plants and algae. This can cause unsightly scums of algae on a surface and harm aquatic life by reducing the amount of available oxygen in the water.
In groundwater used for drinking water, excess nitrates pose human health risks. When nitrates enter a person’s bloodstream, it’s converted to nitrite and competes with oxygen in hemoglobin, depriving the body’s tissue and organs of oxygen. While rare, it can result in long-term digestive and respiratory
problems, or even death.
The risks can be especially serious for expectant mothers and the very young. Infants—six months and younger—are unable to process nitrate, which leads to “blue baby” syndrome.
Long Time Coming
While fertilizers and pesticides have been widely used for decades, it sometimes takes that long for the contamination to show up in groundwater and surface water.
“It has taken us a while to appreciate that there is this connection between what we do at land surface and what shows up in groundwater,” says Paul Capel, hydrologist and research team leader of the U.S. Geological Survey National Water Quality Assessment Program’s Integrated Watersheds Studies
Team in Minneapolis, Minnesota. “There is also a connection between what shows up in groundwater and moves to the streams.”
Nitrate concentrations in groundwater have steadily increased, particularly in agricultural areas of the country. According to the USGS’ 2010 report, Nutrients in the Nation’s Streams and Groundwater, 1992–2004, the median nitrate concentration in monitoring wells increased 6% from 1993-2003. During the same time period, the proportion of wells with concentrations of nitrate greater than the 10 mg/L MCL increased from 16% to 21%.
The study also found concentrations were highest (median of 3.1 mg/L) in shallow wells less than 100 feet deep in areas with high rates of fertilizer use. It was less likely in deep wells in major aquifers.
Several factors contribute to this. For one, the longer travel times from the surface to the well allows for denitrification or attenuation during transport. The wells are also often located near urban areas where nitrate is less prevalent.
“Anytime human contamination shows up, it’s usually in the shallow groundwater system,” Capel says. “The exception to that is if there are multiple layers of aquifers and open channels between the layers of the aquifers from leaky casings or open boreholes. Then, there is a possibility of contaminating the deeper groundwater with recent chemical use.”
Cody Christensen of Christenson Well and Irrigation Inc. in Hartington, Nebraska, sees exactly what Capel describes in the northeastern part of his state, where—despite heavy agriculture—many deeper wells do not exceed 10 ppm. If nitrates exceed the MCL, it’s usually an older, shallow well
not constructed to today’s standards.
“A lot of water wells in use today are 50-plus years old in my area, and frankly, they didn’t drill them any deeper than they had to,” he adds. “If we are able to penetrate deeper into the aquifer, we can actually get away from those surface nitrates and provide somebody with a good well that way.
“Fortunately, we have so much glacial till in this area with the blue clays that mother nature has a very good way of filtering and cleaning our water as it’s soaking down and getting back into the aquifer.”
When drilling a deeper well isn’t an option, nitrates can be successfully treated with ion exchange or reverse osmosis systems.
Downey Drilling Inc. in Lexington, Nebraska, is also working on a blending plant in Burlington, Colorado. Blending takes water from wells with low nitrate and blends it with water from wells with high nitrates so they stay below the MCL.
Accomplishing this involves building a blending plant, bringing pipelines in from the existing wells and new wells to the plant and replacing the existing higher-head pumps with lower-head pumps. High service pumps are needed to be added from the plant to create enough head to get water into
elevated storage facilities.
“They’ve done tests and pinpointed (the source of the contamination),” says Tom Downey, CWD/PI, president of Downey Drilling and past president of the National Ground Water Association. “There was an old landfill in the east part of town, so they’re developing these new wells on retired irrigated
acres on the west side of town and blending it with their existing wells.”
Into the Future
Addressing future contamination could present challenges. Because of the time it takes for nitrates to reach groundwater systems, changing farming practices may do little in the short term.
“In areas where the groundwater is turned over pretty quickly, (nitrate and pesticide contamination) could be resolved relatively quickly if we can reduce the inputs to groundwater,” Capel says. “In Maryland, where it can take 30 or 40 years, it’s going to be a long process because what we do at land surface now isn’t going to show up as a change for a while.”
Conservation efforts reducing nitrates and pesticide concentrations in surface water may also have a neutral or negative effect on groundwater—and vice versa. Antierosion efforts decrease the movement of chemicals across the surface, but it also means a larger fraction infiltrates into the groundwater.
“We still are using an enormous amount of chemicals, so the ones that can dissolve in the water and move through the soil to the groundwater are very prevalent,” Capel says. “So,we’re still seeing continual inputs of nitrogen and dissolved pesticides.”
The use of cover crops, however, is proving to be one of the best solutions to preventing the transfer of nitrates to water. The plants capture the excess nitrogen and hold it in the biomass, making it less mobile to the groundwater or surface water.
In Nebraska, local natural resource districts (NRDs) are also setting up groundwater management areas where higher nitrate concentrations are found to monitor contamination. For example, the Lewis & Clark NRD found an isolated area with higher nitrates near Creighton, Nebraska, and implemented
a permitting process to prevent further contamination. The 2010 USGS study noted long-term planning is still needed as water from shallower, contaminated groundwater systems moves downward.
The Nebraska Grout Study, released in 2010, may address these concerns. This study examined the sealing performance and effectiveness of bentonite slurry grout, non-slurry bentonite, and various cement slurries. It found, while cement grout keeps its structure, it develops cracks over time and that
bentonite slurries lose their ability to seal the well properly. Dry bentonite, on the other hand, performed the best over time, both in providing a good seal and stability.
As a result of the study, Nebraska changed its grouting standards. According to the standards released by the state’s Department of Health and Human Services in 2014, the annular space must be filled from the top of the primary aquifer seal to the bottom of the surface seal. It must have at least 5 feet of grout placed 5 to 15 feet below grade or at the static water level, whichever is less.
The state is also looking into ways it can provide extra protection for older, existing wells by squeezing grout into the annular space.
“We’re trying to figure out what else we can do in areas that have nitrate issues,” Downey says. “It’s believed that it’s migrating down the annular space because it’s not sealed up. I don’t know if I’d call it a conclusion, but it’s an assumption they’re trying to study.”
Christensen feels fortunate to live in a state like Nebraska where the state Department of Health and Human Services, NRDs, and water well system professionals work hand in hand in their efforts to keep nitrate contamination from becoming a widespread problem.
“From what I’ve seen, we’re a leading state in contractors and regulators working together to solve problems and prevent future harm to groundwater,” he says. “I think that’s pretty unique.”
Jennifer Strawn was the associate editor of Water Well Journal from 2004 to 2007. She is currently in the internal communications department at Nationwide in Columbus, Ohio. She can be reached at email@example.com.