Nitrate | Movement of Nitrate into the Groundwater | Pesticides | Conservation Tillage | References
Introduction
Groundwater accounts for half of the drinking water we use in the United States today. This resource is susceptible to contamination from many sources. Some sources that contaminate groundwater are septic systems, infiltration of industrial runoff, landfills, and agriculture. Agriculture is a major source of pollution and most rural communities do not have the resources to treat water that may be pumped from an aquifer for drinking water use. Agriculture contributes many pollutants to the environment such as phosphates, herbicides, pesticides, nitrates and bacteria. Nitrates and pesticides are common contaminants of groundwater derived from agriculture.
Nitrate is used
as a fertilizer and is applied to the field to enhance the growth
of many crops. It can be applied in various nitrogen forms, two
common forms being ammonium and ammonium nitrate. Animal wastes
can also be applied to provide nitrogen as a fertilizer. The
nitrogen compounds in these wastes can be transformed into
nitrate in oxidized conditions. Additional hazards to groundwater
from the use of animal wastes also occur. Pathogenic bacteria can
migrate from wastes into groundwater. In cases such as poultry
manure, heavy metals like copper can be released into the ground
and depending on the valency state of the copper ion, it can
migrate into the groundwater. (AP, 1995) Animal wastes are often
applied directly to fields as a natural fertilizer, or are found
in collection ponds where the nitrogen compounds can then migrate
into the soil system and then the groundwater.
Nitrate is a particular concern because of the health effects related to consumption of nitrate in drinking water. Nitrate in water can potentially cause many health problems. The most familiar problem associated with elevated levels of nitrate (45mg/l as NO3-) is methemoglobinemia, or "blue baby syndrome" (Salley, 1992).This is a condition resulting from the inability of blood to deliver enough oxygen to the body as a result of the presence of nitrate. Another health problem related to nitrate is cancer. Once ingested, nitrate reacts to form nitrite which then undergoes a mechanism that, in conjunction with organic compounds, forms N-Nitroso compounds in the stomach. Many of these compounds are known to be carcinogenic and hence, increased nitrate levels in drinking water can increase cancer risks. Nitrate can also contribute to hormonal and endocrine dysfunction by disrupting the thyroid, the gland responsible for these functions (EWG 1996). The average American consumes nitrate in their drinking water at a rate of about 2 mg per day. This is only about three percent of the daily exposure, the remaining proportion is is in food products. In areas of nitrate contamination, however, consumption via drinking water was estimated to be 160 mg per day, about 69 percent of daily exposure and about triple that of average exposure (EWG, 1996).
Movement of Nitrate into the Groundwater: Nitrates,
as discussed above, can be applied to a soil to improve fertility
either with a commercial fertilizer of as animal waste. The
movement of this type of nitrogen through the soil will be used
as an example. Agricultural soils range in pH, generally from
around pH of 5 to a pH of 7. One characteristic that lends soils
to agricultural use is a fair amount of cation exchange capacity
(CEC). This CEC is a reflection of the soils surface's ability to
electrostatically bind cations in the diffuse double-layer of the
soil solution. This attraction evolves from the soil's net
negative surface charge, often found on clay minerals such as
montmorillionites, iron oxides, such as geothite, and silicate
oxides and sesquioxides. Nitrate, however, exhibits a net
negative charge. Unfortunately, many soils do not contain
significant amounts of anion exchange capacity, therefore the
nitrates remain soluble in the soil solution and move with soil
water movement (Eick, 1997). (Interview with Dr. Eick in wav. or aif.) When
there is no plant uptake, or the amount of nitrogen exceeds that
of plant uptake, nitrate leaches through the soil into any
groundwater that might be below (Table 1). Because nitrate is
soluble, it will remain in the groundwater unless otherwise
reduced.
Table 1. The higher the r-value, the more nitrate likely to be found in the groundwater.
| Correlation of Groundwater Nitrate concentration with: | r-value |
| %clay in soil | -0.49 |
| Total fertilizer used | 0.28 |
Microbial action is the main driving force in redox engines. As an electron acceptor, NO3- is used by microbes in reduced environments providing the electric potential (Eh) of the system is no higher than 1.245 volts, otherwise the microbes will use other electron acceptors that may be present (Eick, 1997). Nitrate can be reduced to form nitrogen oxides and diatomic nitrogen.
Pesticides, herbicides and insecticides,
are all widely used chemicals in the agricultural world.
Contamination of groundwater by pesticides can be traced to use
on agricultural land (VWRC, 199?). Pesticides and herbicides are
used to prevent competition from either insects or other plants
for resources needed by crops. Without the use of these
chemicals, it is often very difficult for the agriculturist to
produce sufficient yields of crop. This is especially important
when using conservation tillage and because more chemicals need
to be applied. However, "there has been extensive evaluation
of the effectiveness of conservation tillage in reducing erosion
and degradation of surface water quality, [but] fewer studies
have considered the impact on pesticide leaching" (
Heatwole, 1992).With many companies producing pesticides, it is
difficult to generalize the effects of pesticides on human health
or their movement into the groundwater system. There are 240
major chemicals used in the manufacture of pesticides, some of
which are not understood, and each company may use different
formulas to create their pesticides. A common group of chemicals
used in pesticides are s-triazines. Atrazine is an example of a
widely used s-triazine, applied to corn, sorghum, rangeland,
sugarcane and turf. Some older, familiar examples of pesticides
are 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane, or DDT,
heptachlor, and dieldrin.
Once a pesticide has been applied to a field, its fate is controlled by several processes. Though there are many chemicals to take into account when considering their movement, they will all be effected by various soil chemistry properties. Sorption and desorption processes, degradation, transport and solubility are all important factors when considering pesticide fate (Heatwole, 1992).
DDT, Heptachlor, and Dieldrin are good examples of the affects of the identity and arrangement of functional groups attached to the molecule. DDT, a non-polar molecule, is considered insoluble with solubility values somewhere between 0.001-0.008 mg/l. However, it has a planar structure with little steric interaction, allowing it to absorb to a soil surface. It is a persistent chemical, having a field half-life of 2-15.6 years (Hornsby, 1996). Heptachlor, used for small grains, is more soluble than DDT. It does sorb, though its structure and various functional groups creates steric interaction hindering its sorption. Dieldrin is a large, soluble molecule (solubility 0.19-0.25 mg/l) that has such strong steric interaction and shape that it is unable to sorb to soils. These properties affect which pesticides will be transported to the groundwater.
Atrazine, as an example of s-triazines, has been widely researched in concern with its sorption. Its sorption in the surface soil horizon is correlated to organic matter, whereas further down in the soil column, exchangeable acidity and clay content of the soil are the factors affect sorption. Because atrazine forms a weak conjugate acid, its sorption is directly related to the pH of the soil. An exception is at extremely low pHs the is a decrease in sorption due to the competition of protons on the soil surface but soils of this acidity (pH<4) are not commonly agricultural. Atrazine though sorbing at some lower pHs, has a significantly lower sorption coefficient (100ml/g) than the previous pesticides. Atrazine is also very soluble, 30-70 mg/l, in its basic form, allowing it to travel with water flow.
The movement of these pesticides through the soil matrix can also be applied to how the pesticides act once in the groundwater. Percent organic matter in soils increases the sorption of pesticides because of its high CEC, but once in an aquifer, there is little organic matter for the pesticides to sorb to. As the clay content or presence of various oxides of the aquifer increases, sorption of pesticides will increase. However, most aquifers are low in clay content, having a much larger proportion of sand or coarser material which does not promote sorption (Scalf, 1992).
Conservation Tillage is
considered any kind of tillage that leaves 30% or more of the soil covered with crop residue. It is
intended to reduce the amount of water and soil losses from
runoff in comparison with conventional tillage. Since less water
is allowed to runoff, the volume of water that infiltrates
increases. This also means that there is a possibility that the
cropland contaminants that are carried with water will infiltrate
as well, increasing the risk of groundwater contamination.
Contaminants such as nitrates and pesticides, which are ever
present on croplands, can be a risk to human health if they
infiltrate the drinking supply. In the next section we will
examine the effects of conservation tillage on the rate of
infiltration of nitrates and fertilizers in order to see what
risks or benefits may come from it.
On average, no-till systems, a form of conservation tillage, increase the infiltration rate of pesticides. This does not necessarily reflect the no-till system’s effect on leaching. In a study done in Maryland Coastal Plain soils, herbicides such as atrazine and metolachlor were found in higher concentrations in soils that had been conventionally tilled as compared to those that use the no-till system and after heavy rainfalls, herbicide leaching increased in no-till soils if the herbicides had been recently applied. There were, however, no differences found in groundwater concentrations of the herbicides under either method of tillage. (Heatwole et. al., 1992) Similar studies have been done on the effects of conservation tillage methods on nitrate infiltration with ambiguous results. A study done by Thomas and associates in Kentucky found that nitrogen (surface applied in the form of ammonium nitrate) leached more quickly under no-till systems than in conventionally tilled ones. This finding was based on the higher nitrogen concentrations found below 90 cm in no-till systems. Another study conducted in Iowa, however, found opposite results. Much less leaching occurred in no-till systems as compared to conventional systems and in a more recent study in Kentucky, no differences were found.
These ambiguous results can be explained by differences in soil properties. In the soils in the first Kentucky study and in the Iowa study, most soil water movement occured through large pores in the no-till area. In the Iowa soil, the majority of the nitrogen was already present within the soil, so when water moved through the larger pore spaces, it bypassed the majority of the nitrogen so that little leaching could occur. (Logan et. al., 1987)
In conclusion, few generalities can be made on the influence of conservation tillage in nitrate and pesticide/herbicide leaching and its ability to reach the groundwater supply. The studies that have been done show that there are many variables that influence the leaching ability of these products. Factors such as application methods, amount applied, amount of rainfall, and other specific properties of the soils themselves play important roles in rate of leaching. More studies need to be done to better understand the relationships of all these factors to the different systems of tillage and their effects on groundwater quality.
Associated Press. "Poultry Companies Agree on Environmental Protection Plan." Bay Journal. 5 (1995) : 1-2 http://www.gmu.edu/bios/Bay/journal/95-05/poultry.htm
Brady, Nyle C., and Weil, Ray R. The Nature and Properties of Soils. New Jersey: Prentice-Hall, 1996.
Eick, Matt. "Environmental Soil Chemistry" ENSC 4734 Lecture, Virginia Tech. Blacksburg, VA, 1997
Environmental Working Group. "Pouring It On: Health Effects of Nitrate Exposure."Feb. 1996
Heatwole, Conrad d., et al. Fate and Transport of Pesticides in a Virginia Coastal Plain Soil. Blacksburg: Virginia Tech, 1992
Hornsby, Arthur G., Wauchope, R. Don, Herner, Albert E. Pesticide Properties in the Environment. New York, Springer-Verlag, 1996
Logan, Terry J., et al. Effects of Conservation Tillage on Groundwater Quality. Chelsea, Lewis Publishers, 1987
Salley, Bryan W. Subsurface Transport of Fertilizer-Applied Nitrogen on the Eastern Shore of Virginia. Diss. Virginia Tech, Sept. 1992
Scalf, Marion R., et al. Fate of DDT and Nitrate in Groundwater. Diss. 1968
Sources
Pictures of Fields by Jeff Ensley
Nitrogen Reaction and Table by Jeff Ensley
Pesticide structures provided by Dr. Matt Eick
Atrazine map provided by Gail p. Thelin, USGS; to view Sources and Limitations of map: http://water.wr.usgs.gov/pnsp/use92/mapex.html
Links
http://water.wr.usgs.gov/pnsp/gw/index.html
Student authors: Ryan Reed and Jeff Ensley
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
