Agricultural Contaminants Fertilizers

5.3.6 Agricultural Contaminants Fertilizers

As a result of population growth, the available arable land per capita has decreased and the demand for agricultural production has increased. Although the contribution of agricultural production to the gross national product (GNP) in some countries has fallen, production in absolute terms has increased greatly. This can be attributed to technological developments such as increased mechanization, intensive agriculture, irrigation, and the use of fertilizers and pesticides. Most forms of agricultural land use constitute an impor- tant diffuse or nonpoint source of contamination of soils, surface water, and groundwater (UNESCO, 1998).

Increases in soil fertility have been attained specially since 1960 by massive applica- tion of inorganic fertilizers. Organic fertilizers have been used since the early history of development, but the increased use of inorganic fertilizers began in the 1940s, reaching its peak during the “Green Revolution” (1960 to 1970). For example, Foster (2000) shows how the use of artificial fertilizers affected British agriculture between 1940 and 1980.

A threefold increase in food production was accomplished by a 20-fold increase in the use of fertilizers. The unused nitrogen has, therefore, been lost to the atmosphere by denitrification and leached to surface and groundwater as nitrate, or remains stored in the unsaturated zone (Buss et al., 2005). This nitrate in the unsaturated zone will con- tinue to serve as a long-term source of groundwater contamination even if application of fertilizers were to discontinue today.

Since the 1980s, the application of fertilizers remained at relatively the same level or has been decreasing in most European countries, South and North America (including the United States), and Australia. During the same time period, total fertilizer consumption in Asia almost doubled and is projected to more than double again by 2030 (UNESCO, 1998). Nevertheless, consumption in Asia represented only 19 percent of European con- sumption in 1990. The largest users of fertilizers are Europe and the United States (100 to 150 kg/ha, with a 50 to 55 percent mineral components). In comparison, most Latin American and African countries use less than 10 kg/ha (UNESCO, 1998).

Altogether, 16 mineral elements are known to be necessary for plant growth, but only three are needed in large quantities—nitrogen, phosphorus, and potassium. The others, called secondary elements and microelements, are generally required for cell metabolism and enzymes and are required in very small amounts. Nitrogen is the most critical element in the fertilizer program. It is lacking in nearly all agricultural soils because it leaches readily and therefore has to be applied on a regular basis (UNESCO, 1998). Potassium also leaches readily and has to be applied at the same rate as nitrogen, whereas phosphorous accumulates in the soil and does not leach readily to the subsurface. Phosphorous is, therefore, the main nonpoint source of contamination by surface runoff, which results in eutrophication of surface water bodies.

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Organic fertilizers contain essential nutrients (nitrogen, phosphorous, and potas- sium) and also stimulators and a considerable quantity of microbes that support bio- logical activity and are needed for mineralizing nutrients. Worldwide, they commonly include animal manure, crop residues, municipal sewage sludge and wastewater, and a wide variety of industrial and organic wastes (UNESCO, 1998).

The first nitrogen fertilizer used commercially, Peruvian guano, formed by deposition of excreta by seawolf, was organic in nature. It is very likely that the low levels of perchlo- rate found in groundwater throughout agricultural areas in California and elsewhere can

be attributed to widespread use of guano during the first half of the twentieth century. Perchlorate, a mineral salt of chlorine, also associated with manufacturing of rocket fuel, ammunitions, and firework, is one of the most notorious emerging contaminants.

Pesticides If treatment is not used to protect plants, insects and fungus can destroy crops. Unfor-

tunately, so far, the only proven efficient method for plant protection on a large scale is through the application of chemicals. Plant extracts have been used as pesticides since Roman times, nicotine since the seventeenth century and synthetic pesticides since the 1930s (Paul Muller discovered the insecticide properties of DDT in 1939). Today, new active compounds are registered in different countries every year and usually have to be handled with care because of their toxic properties (Fig. 5.13).

The word pesticide refers to any chemical that kills pests and includes insecticides, fungicides, and nematocides; it also generally includes herbicides. Extensive use of pes- ticides is not confined to rural agricultural areas only. They are commonly used in both urban and suburban settings on lawns, parks, and golf courses. According to UNESCO (1998), data on pesticide use are not available for most countries. It is, however, known that the use of pesticides is high in developed countries where the total amount of chem- icals used per hectare varies from 1 to 3 L/year (insecticides), and from 3 to 10 kg/yr (fungicides). In developing countries, pesticides are often unavailable and beyond the financial capabilities of farmers, and their use is limited to a few crops.

The use of herbicides is increasing worldwide. Normally, herbicides are applied at a rate varying from 5 to 12 kg/ha. Preemergence herbicides are applied at lower rates (1 to 4 kg/ha).

Pesticides used in the 1950s and 1960s were generally characterized by low aqueous solubility, strong sorption by soil components, and broad-spectrum toxic effects. These pesticide properties are now known to accumulate in the environment and cause ad- verse impacts on aquatic ecosystems via persistence and biomagnifications. Examples of such pesticides are chlorinated hydrocarbon insecticides, including DDT and dieldrin. However, only small amounts of these types of pesticides are likely to reach groundwater systems (UNESCO, 1998). In contrast, newer pesticides are more soluble, less sorbed, and readily degradable and have more selective toxicological effects. As a result, pesticide application rates in developed countries have generally declined, but the solubility and mobility characteristics of these compounds may lead to considerable groundwater con- tamination. While the presence of nitrate in many aquifers in the world has been widely reported, fewer cases of contamination by pesticides have been reported so far. Reasons for this could be the potential time lag in the response of groundwater systems to this contaminant input, the high costs involved in their chemical analysis, and, in some cases, the disregard of the degradation products is the main reason (UNESCO, 1998).

GroundwaterQuality

F IGURE 5.13 Wearing gloves, mask, and other protection is part of handling farm chemicals safely. (Photograph courtesy of Tim McCabe, National Resources Conservation Service.)

In 1992, the USEPA issued the Pesticides in Ground Water Database (1971–1991), which showed that nearly 10,000 of 68,824 tested wells contained pesticides at levels that exceeded drinking water standards or health advisory levels. Almost all the data were from drinking water wells. The USEPA has placed restrictions on 54 pesticides found in groundwater, 28 of which are no longer registered for use in the United States but may still be present in soils and groundwater due to this widespread historic use (Tiemann, 1996).

Figure 5.14 shows the results of a nationwide study conducted during 1992 to 2002 and published by the USGS in 2006. One or more pesticides or their degradation products were detected in water more than 90 percent of the time during the year in streams draining watersheds with agricultural, urban, and mixed land uses. In addition, some

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(a)

Pesticides in water

(b) Organochlorine compounds in fish and sediment

Agricultural areas SW

Agricultural areas

Urban areas SW

Urban areas

Undeveloped areas SW

Undeveloped areas

Mixed land uses SW

Mixed land uses

Percentage of samples with (groundwater) with one or more detections

Percentage of time (streams) or samples

one or more detections F IGURE 5.14 (a) Pesticide occurrence in stream water (SW), shallow groundwater (SGW), and

major aquifers (MA) in the United States; most pesticides in this group are in use. (b) Organochlorine pesticides in fish tissue (FT) and streambed sediment (BS); most pesticides in this group are no longer used. (Modified from Gilliom et al., 2006.)

organochlorine pesticides that have not been used in the United States for years were detected along with their degradation products and by-products in most samples of whole fish or bed sediment from streams sampled in these land use settings. Pesticides were less common in groundwater but were detected in more than 50 percent of wells sampled to assess shallow groundwater in agricultural and urban areas.

As mentioned earlier, in addition to natural geologic sources, there are many anthro- pogenic sources of arsenic. The most important are derived from agricultural practices, such as the application of pesticides and herbicides. Inorganic arsenic was widely applied

before it was banned for pesticide use in the 1980s and 1990s. Lead arsenate (PbHAsO 4 ) was the primary insecticide used in fruit orchards prior to the introduction of DDT in 1947. Inorganic arsenicals have also been applied to citrus, grapes, cotton, tobacco, and potato fields. For example, historic annual arsenic loading rates up to approximately 490 kg/ha (approximately 440 lb/acre) on apple orchards in eastern Washington led to ar- senic concentrations in soil in excess of 100 mg/kg (Benson, 1976; Davenport and Peryea, 1991; from Welch et al., 2000). Agricultural soils in other parts of the United States also have high arsenic concentrations exceeding 100 mg/kg due to long-term application (20–40 years or more) of calcium and lead arsenate (Woolsen et al., 1971, 1973). Early studies suggested that arsenic in eastern Washington orchards was largely confined to the topsoil, although evidence for movement into the subsoil has been cited (Peryea, 1991). This apparent movement of arsenic suggests a potential for contamination of shal- low groundwater. Application of phosphate fertilizers creates the potential for releasing arsenic into groundwater. Laboratory studies suggest that phosphate applied to soils con- taminated with lead arsenate can release arsenic to soil water. Increased use of phosphate

GroundwaterQuality

at relatively high application rates has been adopted to decrease the toxicity of arsenic to trees in replanted orchards. Laboratory results suggest that this practice may increase arsenic concentrations in subsoil and shallow groundwater. Application of phosphate onto uncontaminated soil may also increase arsenic concentrations in groundwater by releasing adsorbed natural arsenic (Woolsen et al., 1973; Davenport and Peryea, 1991; Peryea and Kammereck, 1997; Welch et al., 2000).

In some irrigated regions, automatic fertilizer feeders are attached to irrigation sprin- kler systems. When the pump is shut off, water flows back through the pipe into the well, creating a partial vacuum that may cause fertilizer to flow from the feeder into the well. It is possible that some individuals even dump fertilizers (and perhaps pesticides) directly into the well to be picked up by the pump and distributed to the sprinkler system (USEPA, 1990).

Aurelius (1989; from USEPA, 1990) described an investigation in Texas where 188 wells were sampled for nitrate and pesticides in 10 counties where aquifer vulnerabil- ity studies and field characteristics indicated the potential for groundwater contami- nation from the normal use of agricultural chemicals. Nine pesticides (2,4,5-T, 2,4-DB, metolachlor, dicamba, atrazine, prometon, bromacil, picloram, and triclopyr) were found present in 10 wells, 9 of which were used for domestic supply. Also, 182 wells were tested for nitrate, and, of these, 101 contained more than the regulatory limit. Of the high nitrate wells, 87 percent were used for household purposes. In addition, 28 wells, of which 23 were domestic, contained arsenic at or above of 0.05 mg/L, which was the MCL at the time (current MCL for arsenic is 0.01 mg/L).

Concentrated Animal Feeding Operations CAFOs result from the consolidation of small farms with animals into larger operations, leading to a higher density of animals per unit of land on CAFOs than on small farms. For example, in 2005, the United States produced over 103 million pigs at 67,000 pro- duction facilities (USDA, 2006a, 2006b; from Sapkota et al., 2007). Facilities housing over 55,000 pigs accounted for more than half of the total U.S. swine inventory, reflecting the increasing consolidation and concentration of U.S. swine production (USDA, 2006a). This trend in swine production has resulted in the concentration of large volumes of manure in relatively small geographical areas. Manure is typically stored in deep pits or outdoor lagoons and then applied to agricultural fields as a source of fertilizer. However, as a result of runoff and percolation events, components of manure, including human pathogens and chemical contaminants, can impact surface water and groundwater prox- imal to swine CAFOs, posing risks to human health (Anderson and Sobsey, 2006; Sayah et al., 2005). Specific swine production practices, including the use of nontherapeutic levels of antibiotics in swine feed, can exacerbate the risks associated with exposures to manure-contaminated water sources (Sapkota et al., 2007).

Elevated concentrations of nutrients, metals, bacteria, and a number of other chemi- cals and pathogens are observed in surface and groundwater in many agricultural areas throughout the United States. Excess nutrients may be an important contributing factor for the growth and increase in dinoflagellates such as Pfiesteria. Many of the infectious organisms that cause illness in animals can also cause disease in humans and can survive in water. The most common pathogens that pose a human-health risk include Salmonella spp., Escherichia coli O157:H7 (E. coli), Campylobacter spp., Listeria monocytogenes, as well as viruses and protozoa such as Cryptosporidium parvum and Giardia. These organisms have been found in groundwater in a number of communities (Rice et al., 2005).

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In a study of surface water and groundwater situated up- and downgradient from a swine facility, Sapkota et al. (2007) found antibiotic-resistant enterococci and other fecal indicators. Collected samples were tested for susceptibility to erythromycin, tetracycline, clindamycin, virginiamycin, and vancomycin. The results of the study show that the me- dian concentrations of enterococci, fecal coliforms, and E. coli were 4- to 33-fold higher in down- versus upgradient surface water and groundwater. Higher minimal inhibitory concentrations for four antibiotics were observed in enterococci isolated from down- ver- sus upgradient surface water and groundwater. Elevated percentages of erythromycin- and tetracycline-resistant enterococci were detected in downgradient surface waters, and higher percentages of tetracycline- and clindamycin-resistant enterococci were detected in downgradient groundwater. The authors concluded that these findings provide addi- tional evidence that water impacted by swine manure could contribute to the spread of antibiotic resistance.