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Prepared by: Lee Haller, Patrick McCarthy, Terrence O'Brien, Joe
Riehle, and Thomas Stuhldreher
The two most important compounds that result from the reaction of these gases and rainwater are nitrate (NO3-, an anion) and ammonium (NH4+, a cation). In the atmosphere major sources of nitrate include reactions caused by lightning, photochemical oxidation in the stratosphere, chemical oxidation of ammonia, soil production of NO by microbial processes, and fossil fuel combustion (Gaillard, 1995). Ammonia in the air comes from fertilizer manufacturing, anaerobic decay of organic matter, bacterial decomposition of excreta, and the burning of coal (Gaillard, 1995). Anthropogenic activities have a major impact on the levels of these compounds that are found in both rain water and the atmosphere. Many of the major sources of nitrate and ammonium come from the use and production of fertilizers and the burning of fuels, as listed above.
Nitrate that leaves the atmosphere can be converted back into elemental nitrogen, through the process of denitrification. This often takes place in the soil through the activity of bacteria that reduce the nitrate. Ammonium can undergo the process of nitrification, which is an oxidation reaction, that converts it to nitrate. Through this mechanism, the nitrogen in the ammonium ion is released back into the atmosphere (Berner and Berner, 1987). After the conversion from elemental into nitrogenous ions in solutions of rainwater, the nitrogen in these compounds can be exhausted back to the atmosphere by the pathways previously described, thus completing the cycle.
When natural sources contribute a high concentration of nitrate to the groundwater it is usually as a result of anthropogenic disturbance. One example of this is the effect of forested areas on the leaching of nitrate to the groundwater. Natural, mature forests conserve nitrogen but human disturbances can lead to nitrate pollution of the groundwater. However, while this is a potential problem for groundwater, forests represent a very small source of nitrogen compared to agriculture (Hallberg and Keeney, 1993).
1) taken up by plants
2) stored in soil
3) lost to atmosphere
4) lost to groundwater
5) lost to runoff (Bocher, 1995)
Many studies have shown that most of the nitrogen, about 30 to 50 percent is taken up by the plant. According to an United States Golfing Association study only one to two percent of the nitrogen is leached beyond the root zone (Bocher, 1995). This finding may be slightly biased because this is the result that the USGA desires. Also, this result may occur only when the nitrogen fertilizer is applied carefully and properly. Certain circumstances could lead to more of the nitrogen leaching to the groundwater. Six main factors affect nitrogen leaching:
1) nitrogen rate - One study showed that at one pound of nitrogen per
1,000 square feet, no leaching occurred.
2) nitrogen source - Slow-release fertilizers are a nitrogen source
that can reduce the chance of leaching.
3) application timing - In late fall, plants take up less nitrogen
and there is a greater chance for leaching to occur.
4) irrigation practices - The more irrigation that takes place the
greater the chances for nitrate leaching.
5) soil texture - The sandier the soil the more chance for nitrate
leaching.
6) age of site - Younger sites usually have less organic matter and
need to be fertilized more therefore increasing the chance of leaching.
(Bocher, 1995)
One example of proof that farming is a major cause of groundwater pollution is that nitrate problems are most common in the spring, which is the time that farmers apply nitrogen fertilizer to their fields. Also, in a study done by Burkart and Kolpin (1993) it is found that samples of water from wells surrounded by more than 25% land in corn and soybean have a dramatically larger frequency of excess nitrate (30%) than wells with approximately 25% of the surrounding land in corn or soybean (11%) . Also many of the same factors that effect nitrogen leaching in turfgrass affect it in crop fields. For example, the use of irrigation increases the chance of nitrate pollution. "The frequency of excess nitrate was also larger where irrigation was used within 3.2 km of a well (41%) than where no irrigation was used (24%)" (Burkart and Kolpin, 1993, p. 654). In areas where "the soils over the aquifer are predominantly sand, sorption of herbicides is limited and the rate of recharge is rapid, resulting in a relatively large potential for contamination of aquifers with ... nitrates" (Burkart and Kolpin, 1993, p. 654).
One problem caused by farms results from the grazed grasslands and feedlots. In grazing pastures animal wastes are concentrated in small pastures, this leads to inefficient use of nitrogen and causes the potential for groundwater contamination by nitrate. This problem is even worse in Europe where grazing pastures are usually more intensively fertilized than in the U.S., therefore there is more nitrate available to be leached to the groundwater (Hallberg and Keeney, 1993). Even small farms can contribute to the problem of excess nitrates because of the high concentrations of manure that they may have in the barnyard or feedlot areas (Hallberg and Keeney, 1993).
One of the better ways to get rid of manure is to use it to fertilize cropland. "Such organic material is often considered a desirable nitrogen source because the nitrogen is in the mineralization-immobilization cycle longer and thus is more slowly available" (Hallberg and Keeney, 1993, p. 303). For this reason, it is a safer fertilizer than chemical fertilizer. However manure use does have many drawbacks such as variable composition and quality and the extra time for nitrogen to be mineralized may not coincide with the high rate of nitrogen needed by the crop. The main problem is the fact that an accurate estimation of net nitrogen availability is very difficult to determine (Hallberg and Keeney, 1993). Therefore farmers usually apply an excess of manure to the crop to insure that enough nitrogen will be available for the growing process.
Obviously the more nitrogen fertilizer a farmer uses the greater the chance of nitrate pollution of groundwater. "Farmers still consider nitrogen fertilizer 'cheap insurance' against crop failure" (Looker, 1991). Approximately one dollar's worth of fertilizer could bring in ten dollars of corn if the soil has a lack of nitrogen. So the farmer would, financially speaking, much rather add too much nitrogen than too little. To add to this problem, it is very difficult to determine exactly how much nitrogen a crop will need before harvest time due to yearly change in yields and weather conditions. Even if farmers cut down on nitrogen fertilizer, there will still be some nitrate leaching. As Dennis Keeney, the director of the Leopold Center for Sustainable Agriculture at Iowa State University, states, "Even if farmers add no fertilizer to fields, tilling the earth with machinery makes land more susceptible to leaking nitrogen" (Looker, 1991). Although sustainable practices may not eliminate nitrates, it might lower them to a safe level. Obviously, if there is a chance of nitrogen pollution when no fertilizer is applied, the chance of pollution is greatly increased when a large amount of fertilizer is applied.
Methemoglobinemia is the condition in the blood which causes infant cyanosis, or blue-baby syndrome. Methemoglobin is probably formed in the intestinal tract of an infant when bacteria converts the nitrate ion to nitrite ion (Comly, 1987). One nitrite molecule then reacts with two molecules of hemoglobin to form methemoglobin. In acid mediums, such as the stomach, the reaction occurs quite rapidly (Comly, 1987). This altered form of blood protein prevents the blood cells from absorbing oxygen which leads to slow suffocation of the infant which may lead to death (Gustafson, 1993; Finley, 1990). Because of the oxygen deprivation, the infant will often take on a blue or purple tinge in the lips and extremities, hence the name, blue baby syndrome (Comly, 1987). Other signs of infant methemoglobinemia are gastrointestinal disturbances, such as vomiting and diarrhea, relative absence of distress when severely cyanotic but irritable when mildly cyanotic, and chocolate-brown colored blood (Johnson et al., 1987; Comly, 1987).
Treatment of infant cyanosis is simple once the condition has been recognized. If the patient is mildly affected, then he/she must simply refrain from drinking from the contaminated well for a few days and the body will replenish the hemoglobin by itself in a few days (Johnson et al., 1987). However, if the patient is severely cyanotic, methylene blue must be administered intravenously in a dosage of 1-2 mg/kg of body weight for a ten-minute period and improvement should be prompt (Johnson et al., 1987).
Methemoglobinemia most often affects infants of less than six months in age. Comly cites several factors that make infants more susceptible to nitrate compounds that adults. The primary reason is that infants possess much less oxidizable hemoglobin than adults, so a greater percentage of their hemoglobin is converted to methemoglobin which greatly decreases the blood's ability to carry oxygen. Other possible reasons are that nitrite ions may be more strongly bound by infantile hemoglobin due to immaturity of certain enzymes, and that the kidneys of infants have inferior excretory power which may favor retention of nitrite for longer periods of time (1987).
Steps can be taken to prevent the child from becoming a victim of methemoglobinemia. Residents of rural areas should have their wells tested, especially if pregnant women or infants are consumers of the well water. If the well is contaminated, other water source alternatives are other safe wells, bottled water, a new, deeper well, or a water purification system which is capable of removing the nitrates (Johnson et al., 1987). Comly suggests that because cyanotic babies usually contract methemoglobinemia from the water used to prepare their formulas, formulas which use diluted whole milk are less risky than those prepared from powdered or evaporated milk which require large amounts of water in preparation (Lukens, 1987). Breast feeding or the use of bottled water in formula preparation offer the safest solution, especially if the groundwater quality is unknown (Johnson et al., 1987).
Since 1945, there have been over 2000 cases of infant methemoglobinemia reported in Europe and North America with 7 to 8 percent of the afflicted infants dying (Rail, 1989). However, problems can be severe as shown in a specific 1950 report, there were 144 cases of infant methemoglobinemia with 14 deaths in a 30 day period in Minnesota (Johnson et al., 1987). This of course was an isolated case. However, it shows that nitrate concentrations in well water can increase to deadly levels rapidly and the issue of nitrate contamination should not be ignored.
1) Raw water source substitution: In this case an entirely new sources
of drinking water is used to replace the heavily polluted water.
2) Blending with low nitrate waters: As a simple example, if the current
well water supply contains 15 mg/L of nitrates, then this could be combined
with an equal amount of water with a concentration of 5 mg/L to achieve
a safe concentration of 10 mg/L.
3) Connection to an existing regional system: This involves using a
system that is already set up to service the area, instead of drawing water
from the contaminated well.
4) Organizing a regional system: This is similar to the use of an existing
regional system. One can "...form a new regional utility by joining with
other nearby systems which may be having similar water quality problems..."
(Guter, 1981, p. 19).
The advantages of these methods, especially combining existing resources, is the spread of the costs of monitoring water quality amongst many different areas. This greatly reduces expenses and helps to provide safer drinking water to larger numbers of people. However, these applications can only be utilized if the contamination of nitrate is confined to a specific area, otherwise tapping into other local or regional sources to dilute the water would only result in perpetuating the problem.
Besides these methods of providing safer waters with lower nitrate concentrations, there are treatment methods. The most important idea to note about these processes, however, is that none of them are completely effective in removing all nitrate from well water, or any other subsurface water. Each one of these method's success rates depends on the conditions of plant operation and the other contaminants found in the water. The main sources of research for nitrate removal consist of ion exchange, bio-chemical denitrification, and reverse osmosis. Today the primary system in use is ion exchange.
The first part of the process is the selection of an appropriate resin for the removal of the specific problematic ion, which in this case is nitrate. However, current resins are not completely nitrate selective, and often remove other anions before removing the nitrogenous compound. "Resin beds are made up of millions of tiny spherical beads, which usually are about the size of medium sand grains" (Guter,1981, p. 21). As the solution passes through these beds, the chloride anions are released into the water, removing first the sulfate ion, then the nitrate radical. The entire process is composed of four major steps to remove the selected ions from solution:
1) Resin recharge
2) Anion exchange
3) Resin becomes "exhausted"
4) Resin regeneration
In the first step of the process, the bed is recharged, reaching its maximum exchange capacity. The resin at this time has enough chloride ions to carry out the exchange as the solution passes through the complex. The ion exchange is the next part of the process. The resin bed begins to remove the sulfate radicals first, then when the majority of S042- has been removed from the water the exchange of nitrate and chloride begins. The completion of this phase is the third step as the resin becomes "exhausted" of the ion used for exchange. At this point no more anions leave the solution. Finally, in the fourth component of the process, the bed is regenerated by passing a strong solution over the resin displacing the removed ions with the chloride (Cl-) ion (Guter, 1981).
This method of nitrate removal does not completely eliminate the contaminant from solution. However, "one such facility [of ion exchange] in the San Joaquin Valley resulted in a nitrate reduction from 16 to 2.6mg/L" (Moore, 1991,p. 238). The cost of the removal amounted to 24.2 cents/1000 gal (Moore, 1991). So far this has proven to be the most effective and efficient treatment process.
6H+ + 6NO3- + 5CH3OH -> 3N2 + 5CO2 + 13H2O (Zajic, 328).
By using a chemical such as ethanol, the removal of nitrate is possible. Sometimes it is necessary to convert the nitrogen from the ammonium ion into nitrite with the use of nitrosomas (specialized bacteria) to facilitate the removal of all nitrogen from the solution (Shuval, 1977). The nitrite compound is then oxidized to nitrate, which can then be eliminated by the reaction shown above.
Besides the use of special bacteria, photosynthetic algae can remove nitrates from water. Using the stoichiometric relationship of (Zajic, 329):
aCO2 + cNO3- + ePO43- + (c+3e)H+ + 1/2(b-c-3e)H2O ->
CaHbNcOdPe + (a+b/4+c/5-d/2-5e/4)O2
Both of these processes can be somewhat effective in removing nitrate, however, biological organism are influenced by other toxic chemicals or compounds that may be found in the water. These toxins can reduce greatly the effectiveness and efficiency with which the organisms eliminate the nitrate solution (Organization for Economic Co-Operation and Development, 1974). Another important note about these processes is that "... the practice of prechlorination greatly reduces the effectiveness of such techniques. Nitrates are, in most cases, rapidly oxidized by chlorine..." (Moore, 1991, p. 238). However, the greatest benefit of the bio-chemical denitrification is the fact that the nitrogen is completely removed in its gaseous elemental form (Organization for Economic Co-Operation and Development, 1974). There is no residue or problems with disposal.
In 1990, according to the U.S. Department of Agriculture, the rate of nitrogen fertilizer use in Iowa (a state whose farmers lead the nation in cutting back on nitrogen) was 127 pounds per acre (Looker, 1991). However, the director of the Leopold Center for Sustainable Agriculture at Iowa State University, Dennis Keeney, believes that farmers could eventually use only 75 pounds per acre and still have no drop off in yields. Mr. Dan Stadtmueller is an example of an Iowan farmer who greatly reduced his fertilization practices. According to an article in the Des Moines Register, Mr. Stadtmueller "is a miser with nitrogen fertilizer". Some of Stadtmueller's fields get as little as 60 pounds of fertilizer per acre, without displaying a decreasing yield (Looker, 1991).
There have been some steps taken to try and lessen the amount of nitrogen fertilizer used by farmers. One such measure is a law written by then member of the Iowa House of Representatives, Paul Johnson. This law taxed fertilizer and pesticides and used the money raised from this tax to research and show farmers how to use fewer chemicals without losing money (Looker, 1991). Also, Alfred Blackmer, an Iowa State University agronomist devised a test that enables farmers to measure nitrogen already in the soil more accurately. Dan Stadtmueller, the "miser" of nitrogen fertilizer, switched to a method of farming called ridge tillage in 1975. This method enables him to put small amounts of fertilizer in permanent seedbeds instead of covering the entire field. Stadtmueller switched to this method in 1975 and insists that it is more profitable. However in 1991 only about two percent of farmers in Iowa used the method (Looker, 1993). Stadtmueller figures that this is because the majority of the farmers are afraid of change (Looker, 1993). This also represents the problem with the tests and laws that have recently been formed, it might take some time to convince farmers that they can switch to new techniques without losing money in the process.
To determine the safety of private wells, state environmental agencies have surveyed and tested wells. In Iowa, where anthropogenic inputs of nitrates due to intensive agriculture are high, a state-wide rural well-water survey was conducted. The survey was performed between April 1988 and June 1989, taking 686 samples from across the state. While the study was limited to Iowa, the Iowa Department of Natural Resources claims that the results can be extrapolated to other rural areas with intensive agricultural production. The natural background concentration of nitrate-nitrogen in Iowa is less than 2 mg/L. Higher concentrations indicate a loading from anthropogenic sources (Kross et al. 1993).
The study revealed that many private wells suffer from nitrate contamination; approximately 18.3% of Iowa's private, rural wells have NO -N concentrations exceeding the EPA health advisory level. Results also show that the contamination of shallow wells (less than 15m in depth) is much more prevalent than contamination of deep wells. Thirty-five percent of wells less than 15m deep exceed the 10 mg/L threshold. The mean concentration for these shallow wells was even over the health advisory limit (Kross et al. 1993). However, in Iowa contamination of deep wells has grown more common in recent years, indicating a more pervasive problem.
Doctors at the State University of Iowa Medical Center have encountered many babies suffering from diarrhea and other symptoms consistent with methemoglobinemia. After a battery of tests to determine the cause, it was found that all of these infants were being fed water from private wells in Iowa. The NO -N level of the water from these wells was found to range from 64 to 140ppm and the severity of the symptoms appears to roughly correspond to the nitrate levels in the water. Doctors from Cedar Rapids, Fort Dodge and hospitals across the state have documented many additional cases of apparent nitrate-induced methemoglobinemia (Comly, 1945).
With regard to the nitrate problem in groundwaters the best suggestion to avoid health risks is to have wells checked frequently and to reduce the fertilization of fields. The overload of nitrogenous fertilizers to the soils actually kills the biota that help to provide nitrogen to the soil, which the crop plants can use. By using much lower amounts of fertilizers these crops may still be as productive as those produced under heavily fertilized soils, due to the healthier environment for the microbes. If the farmer adds large amounts of fertilizer in the beginning then he is forced to use more and more each year. Using only moderate to low amounts at the outset allows the farmer to avoid the entrapment into this vicious cycle. Furthermore, many of the aforementioned prevention methods can be incorporated to help reduce nitrate leaching from the soil into the groundwater. Slurrystores and concrete lagoon pits can greatly reduce the concentration of nitrate. By avoiding over-irrigation of a field both turfgrass managers and farmers can help to control the leaching of nitrate to the groundwater.
The clean-up of nitrate from the contaminated waters is not an easy job. So far, the most effective and widely used technique for removal is ion exchange model FGA-60N 30,000 grain whole house nitrate unit. Other processes are either in an experimental stage or not as universally employed. The nitrate can most effectively be removed in a plant and is not treated while still in the aquifer. While nitrate cannot be completely removed from groundwater, the use of treatment methods such as ion exchange and the adoption of preventative measures, will help to reduce nitrates to biologically safe levels.
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