Agricultural Nitrogen in Groundwater

by Paul Schmidt and Erik Nieman

Fall, 1997

The purpose of this site is to give a brief overview of the nitrogen found in groundwater from agricultural sources, and answer questions like: How much nitrogen is in groundwater? Where does it come from? What are the health risks? and What can be done to remediate the problem?

To What Extent is Nitrogen Found in Groundwater?

Nitrogen in groundwater in the form of nitrates is a large problem in the U.S.. Most of this nitrogen comes from agricultural sources. The EPA federal standard for maximum concentration of nitrate in drinking water is 10 mg/L as nitrogen. Much research has been conducted in order to determine the amounts of nitrates found in drinking water wells. "USGS data show that the 20 states with the largest agricultural marketings in 1989 had a notably higher percentage of wells with nitrate concentrations above 10 mg/L than the remaining 30 states; 7.1% compared to 3.0% respectively." (Fedkiw 2) Research conducted by private firms also links high nitrogen content in wells with agricultural activity. "Monsanto reported that the frequency of wells with nitrate exceeding 10 mg/L doubled for wells located on farm property." (Fedkiw 5) "A survey of 1,430 randomly selected and sampled drinking water wells in agricultural areas of 26 states, conducted in 1988 and 1989 by the Monsanto Agricultural Products Company, found nitrate above 10 mg/L in 4.9% of the wells. For wells on farmsteads only, however, the proportion was 10%." (Fedkiw 3)

above pictures from: wwwrvares.er.usgs.gov/nawqa/fs-092-96/fig1.html

What are the sources of nitrogen in groundwater?

Agriculture is the primary source of the elevated nitrate levels although in some rare cases certain geologic units can be the source of the nitrate."In some cases, the detailed descriptions of the aquifers suggest agricultural use of nitrogen fertilizers as the source of high nitrate levels. Nitrate sources other than applied fertilizers may be involved such as wastes from livestock, dairy, or poultry operations, and accidents or inadequate precaution in handling fertilizers near well sites." (Fedkiw 4) "There are locations in at least 14 states with high agricultural production where nitrate contamination has been associated with the application of nitrogen fertilizers. In some cases it has been demonstrated that the leaching of nitrate is accelerated by irrigation." (Fedkiw 19)

What are the Health Risks to Humans?

Now that we know how much nitrogen is in groundwater, and where it comes from, lets take a look at how many people are actually exposed to it and what can happen if one is exposed to a high concentration (above 10mg/L). "Out of an estimated 219 million people using public drinking water supplies, approximately 1.7 million are exposed to nitrate levels above 10 mg/L. About two-thirds of those exposed , 1.1 million are served by public water systems using groundwater supply sources. Almost 27,000 infants a year are exposed to tap water with nitrate levels exceeding 10 mg/L." (Fedkiw 7)

Infants are the primary concern, because they are the most vulnerable. "The EPA standard for nitrate in drinking water is set at 10 mg/L to protect babies under about 3 months of age, the most nitrate sensitive segment of the U.S. population. Such infants are much more sensitive to nitrate toxicity than the rest of the population for a variety of reasons. For example, bacteria that live in the digestive tracts of such infants convert nitrate into toxic nitrite. Nitrite transforms hemoglobin to methemoglobin, preventing transport of oxygen and producing symptoms of asphyxation (blue baby syndrome). After babies reach an age 3 to 6 months, acid in their stomachs increases, thereby creating an unfavorable environment for the bacteria causing the problem." (Fedkiw 29)
Infants aren't the only ones at risk, it is possible that high nitrate concentrations can cause cancer in adults. "Nitrate itself is not directly carcinogenic. However, there is recognition that nitrate could be converted to nitrite in the human body and react with secondary and tertiary amines to form nitrosamines - which have been identified as potent carcinogens." (Fedkiw 30)

What Can be Done About the Problem?

Luckily, the most effective method of removing nitrates from subsurface water is naturally occuring, through a process called denitrification. "Denitrification reduces nitrate levels in soils and has been identified as basic factor contributing to the generally low levels of nitrate found in the groundwaters of the Southeastern United States." (Fedkiw 19) "Denitrification is a process carried out by micro-organisms in soil or soil water that converts nitrate into nitrogen in its gaseous form and returns it back to the atmospere. Denitrification takes place under saturated or wet conditions when the soil is deficient in oxygen -or anaerobic- and sufficient carbon for energy is available from organic materials." (Fedkiw 28)

"Denitrification has traditionally been viewed as an undesirable process among agriculturalists due to the loss of nitrate from the soil. In recent years, it has received attention due to its potential beneficial role in reducing the nitrate content of soil water below the crop rooting zone that is leaching into groundwater." (Fedkiw 28)

above picture by Paul Schmidt

Denitrification can be partially controlled through a Best Manage Practice called Water Table Management, if sufficient carbon is present in the soil. "Water table management to increase denitrification is now being considered and studied in several states as a Best Management Practice (BMP) to preclude or significantly reduce the leaching of nitrogen, particularly in the post harvest period before spring planting." (Fedkiw 19)

So what exactly is a Best Management Practice? "A Best Management Practice or BMP is a practice or combination of practices that are determined by a state or designated areawide planning agency to be the most effective and practicable means of controlling point and nonpoint pollutants at levels compatible with environmental quality goals. Such as Water Table Management or Conservation Tillage." (Heatwole 1)

Water table management seems to be an effective way to remediate the problem as recent research shows. "In recent years water table management through controlled subsurface drainage and subirrigation has taken advantage of the denitrification process to reduce the movement of nitrogen to groundwater. The idea is to maintain the water table at varying depths above the drain tile during the growing season but particularly in the period between harvest and spring tillage. This practice manages the saturated soil depth between the tile and the soil surface to promote denitrification and subsequently reduce nitrate loading in the groundwater." (Fedkiw 28) "Various studies completed in Iowa since 1985 indicate that optimum water table management practices can result in lower nitrate concentrations in subsurface and groundwater quality. Studies in North Carolina indicate controlled subsurface drainage may reduce nitrate concentrations in drainage discharges by up to 20% compared to no control drainage systems." (Fedkiw 28) picture below from: www.epa.gov/ow

Although water table management can help reduce the problem, other BMPs may actually make the problem worse. BMPs such as Conservation Tillage which increase infiltration may accelerate the flow of nitrates into the groundwater. "Conservation tillage leaves at least 30% of the soil surface covered with crop residue after planting." (Heatwole 5) "Conservation tillage systems increase infiltration and reduce average annual runoff by about 35%." (Heatwole 5) "Tillage destroys the continuity of macropores such as earthworm tunnels and decayed root channels." (Heatwole 5) If no or little tillage is taking place then, "dissolved chemicals move 2-20 times faster than predicted by Darcy flow, passing the root zone where most biological and chemical transformations take place, and hence reach the groundwater." (Heatwole 5)

"Emerging research indicates that on some soils, available BMPs that reduce the amount of nitrogen fertilizer application for optimum production are not sufficient to reduce the soil solution concentrate of nitrogen in the root zone after harvest to a level below 10 mg/L. This research indicates a need for improved practices in such situations and for new technology that can reduce the postharvest solution concentrate to an acceptable level." (Fedkiw 19)

References:

Fedkiw, John. Nitrate Occurance in Waters. USDA, Washington D.C., September 1991.

Heatwole, Conrad. Agricultural BMP's Applicable to Virginia. VPI&SU, Blacksburg, 1991.

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