Pollution Prevention in Natural Resources Management

Transcription

1 Pollution Prevention in Natural Resources Management with a focus on Nitrates and Pesticides in Agricultural Production Systems Kenneth K. Tanji Department of Land, Air and Water Resources University of California, Davis Introduction Our agricultural production systems provide a bountiful supply of food and fiber, but they are obtained at some cost to our water, soil and air resources. For instance, agriculture is the largest single nonpoint source (NPS) of surface water pollutants, which include sediments, nutrients, pesticides, animal wastes, salinity and trace elements. According to the EPA (1989). agricultural NPS pollutants have contributed to impairing the water quality of 64% of the USA's -266,000 km (165,000 miles) of rivers and 57% of the USA's 3.3 million ha (8.1 million acres) of lakes. Furthermore, a recent national survey of drinking well water conducted by EPA (1990) reveal that at least half of the drinking wells contained detectable concentrations (0.15 mg/liter) of nitrates and about 1.2 to 2.4% contained nitrates exceeding the maximum contaminant level (MCL) of 10 mg/liter. In this same survey 4% of the rural domestic drinking wells and 10% of the community drinking wells had detectable residues of at least one pesticide though none exceeded the MCL. A significantly greater percentage of 271

2 pesticide and nitrate detections in wells were made in intensively cropped lands, where pesticide and fertilizer usages are high, areas with high groundwater vulnerability exist, or both. This paper addresses opportunities for the prevention of water pollution in agricultural crop production systems, with a focus on nitrates and pesticides. This paper contains two central themes: "everything has to go somewhere" and "an ounce of prevention is worth a pound of cure." Agricultural Crop Production Systems Agricultural crop production systems may be viewed as a "biological factory" (Hillel, 1991) utilizing solar energy, carbon dioxide from the atmosphere, water from the hydrosphere and mineral nutrients from the geosphere. Today's intensive crop production systems have evolved over millenia from food gathering of native plants and animals, to domestication of herbivorous animals, to slash-and-burn agriculture in forested lands, to extensive farming of arable land with little or no inputs of manures and fertilizers, to self sustaining farming, which involves cultivation of crops that provide food for humans and livestock, return of livestock and human wastes to the soil and cultivation of crops that fix nitrogen gas from the atmosphere. These farming systems, some of which are still being practiced, have low to moderate productivity per unit area (Frissell, Intensive agriculture in developed countries consists of continuous inputs of fertilizers and pesticides. This allows a steady export of crops and livestock products. Such agricultural systems tend to have a deleterious effect on our environment. In the past, agriculture mostly involved "on-site" 272

3 measures that enhanced the production of crops and livestock. In recent decades, we have become more aware of the "off-site" effects of farming operations, such as degradation of the quality of surface waters. And today we, including farmers, have become acutely aware of "out-of-sight" contamination of ground waters. The principal agricultural sources of NPS pollutants are nutrients. including nitrogen and phosphorus: sediments and the nutrients, pesticides, salinity and trace elements associated with them: pesticides, including herbicides, insecticides, fungicides, nematicides and miticides: dissolved mineral salts (salinity); and livestock wastes, which contain nutrients, salinity, trace elements, pathogens, and oxygen-demanding constituents. Water pollution from agricultural and other sources was initially a local problem, but has now spread to regional, national and even global levels. Factors contributing to the extensive nature of water pollution are exceedingly complex and interactive, a topic that I will be addressing later. For now, suffice it to say that factors include the mobility of the pollutant, persistence of the pollutant, and the surface soil and hydrogeological conditions which can affect an area's vulnerability to pollution. Nitrates Plants derive their essential mineral nutrients mainly from the soil, but obtain some from the atmosphere and water. Nitrogen, by far, is the nutrient that is most limiting for crop production. Under intensive crop production, the soil's reservoir of nitrogen tends to become depleted and is replenished by both natural sources, such as crop residues and animal manures, as well as by synthetic nitrogen fertilizers derived from the 273

4 atmosphere by various industrial processes. These nitrogen fertilizers include anhydrous ammonia, urea, ammonium nitrate, ammonium sulfate, calcium nitrate and sodium nitrate. An estimated one-third of all agricultural production in the USA stem from nitrogen fertilizers (Meyer and Coppock, 1980). The amount of nitrogen removed from the soil by harvested crops in the USA varies from 20 to 40% of the amount of fertilizer applied for most vegetable and fruit crops, and from 40 to 70% for most grain and forage crops (Meyer and Coppock, 1980). To assess what happens to the rest of the nitrogen fertilizer requires an understanding of the soil nitrogen cycle. uptake from atmosphere ammonia from manure run- Off I I available soil nutrients, pool A Figure 1. Flowchart agroecosystem (Frissell, of 31 transfer pathways of nutrients in an 1978). 274

5 Figure 1 (Frissell. 1978) presents the cycling of nutrients. including nitrogen, in agroecosystems. The principal nitrogen pools are plant, livestock and soil, which is further subdivided into inorganic and soil organic nitrogen pools. Arrows directed to a pool indicate inputs of nitrogen, arrows leaving a pool indicate outputs, and arrows going from one pool to another indicate transfers between pools within the agroecosystem: a total of 31 transfer pathways. More details on soil nitrogen are found in Stevenson (1982). Figure 2 (Pennsylvania State University, 1988) depicts a simplified version of the soil nitrogen cycle. Losses of nitrogen from croplands include volatilization of ammonia, microbial denitrification producing nitrogen and dinitrogen gases, surface runoff of mineral and organic forms of nitrogen, leaching of nitrates into ground waters, and removal of nitrogen in the harvested crop. Of particular interest in this paper are losses of nitrogen in surface runoff and deep percolation. including: 1. Dissolved nitrogen gases (nitrogen, dinitrogen. ammonia, etc.) 2. Soluble inorganic nitrogen (ammonium, nitrate and nitrite). 3. Soluble organic nitrogen (amino acids, sugars, etc.) 4. Particulate organic nitrogen (suspended matter consisting of plant and animal origins). 5. Sorbed inorganic nitrogen (exchangeable and fixed ammonium in sediments, etc.). The discharge of the above forms of nitrogen varies with site-specific conditions. All of the above forms may be present in surface runoffs and the first three forms may be present in percolating waters. The nitrogen species 275

6 Atmospheric nitrogen fixation Biological / fixation Volatilization Fertilizer Ammonia N Soil surface Crop uptake Leaching Water table Figure 2. The soil nitrogen cycle (Pennsylvania State University, 1988). 276

7 that are most frequently measured are nitrate or nitrate plus nitrite, while ammonium and ammonia are less frequently measured, and dissolved and suspended organic nitrogen are infrequently measured. Ammonium and ammonia in high ph waters should be more frequently analyzed. They indicate that pollution and the inefficient use of ammonia/ammonium fertilizers has occurred, and are highly toxic to aquatic organisms. Nitrate is of importance because it plays a role in eutrophication of surface waters, may cause methemoglobinemia in high concentrations, indicate that excessive leaching has occurred, and may be detrimental to certain crops during the maturation stage. Significant concentrations of nitrite may occur only under unusual anoxic (reduced) conditions. Organic nitrogen present as nitrogenous fraction of the biochemical oxygen demand (BOD), when determinations of BOD are made for more than 5 days of incubation. also indicates pollution has occurred, and may lower dissolved oxygen (DO) in the receiving waters and contribute mineralized forms of nitrogen to waters. In surface runoff from close-growing crops, such as pasture and flooded rice fields, organic nitrogen usually dominates over the mineralized forms. In widely-spaced crops, such as furrow irrigated field crops, ammonium and nitrate may be more prominent than organic nitrogen. Preventive measures to minimize nitrate accumulation in waters will be first addressed by potential management options on existing cropping systems and later by options requiring substantial changes in farming. Focusing on leaching of nitrate into ground water, the two main factors are the amount of leachable nitrate present in the crop root zone and the amount of water percolating through this root zone. The former is 277

8 influenced largely by the rate and timing of nitrogen inputs. The latter is influenced largely by the rate and timing of infiltrated precipitation or irrigation waters. These two factors are closely interrelated. Fertilizer-use efficiency. --the percentage of applied nitrogen taken up by the crop--, is a key determinant to leachable nitrate present in soils. Figure 3 presents the results of field studies of corn grown in the San Joaquin Valley of California (Meyer and Coppock, 1980). Fertilizer-use efficiency decreased and unaccounted-for losses of nitrogen (leaching and denitrification) increased with excess fertilization. Increased yield of grain per kg/ha of applied nitrogen became smaller as the rate of nitrogen application was increased (yields actually declined with excess fertilization) and the nitrate leaching potential to groundwater was increased. Some leaching of nitrate is inevitable in almost all farmlands since agriculture is practiced in an open system, but some areas are especially vulnerable to leaching. Soils most sensitive to nitrate ground water contamination are those that have high water infiltration rates, high water transmission rates throughout their profiles and low denitrification potential. Crops that create a high potential for nitrate leaching are those with low fertilizer use efficiency and those that require high nitrogen input to insure rapid vegetative growth, such as vegetable crops. Leaching of nitrate is more likely when the amount of infiltrated precipitation or irrigation exceeds the water storage capacity of the soil profile. Measures to prevent nitrate leaching losses from the use of fertilizers include proper management of the application rate, method and timing of application, and use of the proper type of fertilizer. The amount of fertilizer to be applied should be guided by soil, water, and plant tissue tests. Tests of soils and waters indicate how much nitrogen will be available to the crop. 278

9 Figure 3. Corn grain yields, crop removal of nitrogen and unaccounted-for losses of nitrogen in the San Joaquin Valley (Meyer and Coppock, 1980) Tests of plant tissues indicate whether or not the growing crop is deficient in nitrogen. Fertilizers should be incorporated into the soil by injection, disking, or plowing to minimize losses in surface runoff. Nitrogen should be applied during those periods of time when the plant most needs the nitrogen. And the pollution potential of the form of fertilizer (granular, gaseous, liquid, suspension or slurry) should be considered. Some forms are highly soluble and highly subject to leaching. Some release nitrogen over a period of time, such as slow-release nitrogen fertilizers or those containing nitrogen inhibitors. 279

10 Other management practices that help to prevent pollution include crop rotation, no-till and conservation tillage to reduce runoff losses, as well as use of legumes and animal wastes as sources of nitrogen (NRC, 1989; OTA, 1990). Pesticides In agriculture, the use of pesticides has had a history of several hundred years. Initially, the pesticides used were naturally occurring substances, such as sulfur as a fumigant, ground tobacco and its extract nicotine formulated as nicotine sulfate (Black Leaf 40), the plant pyrethrum and its crushed dried flowers or seeds, petroleum oils and various inorganic chemical, such as arsenic, lead and copper. As agriculture shifted into a more intensive crop production system the pest-predator relationship was disturbed, clearly cultivated fields were desired and crop rotation systems become limited, all of which contributed to the increasing use of herbicides, insecticides, fungicides, nematicides. and miticides. During the 1950's. new organic chemicals were discovered, and the use of synthetic pesticides escalated. The consumer began to expect attractive-looking food products without blemishes or insects. However, Rachel Carsons' Silent Spring, published in 1962, gave rise to public concern over the threat posed by pesticide to the health of people and wildlife. Pesticide contamination of our surface and ground waters continues to be a growing source of concern. The use of some pesticides, e.g., DDT, an insecticide, and DBCP and EDB, nematicides, have been banned due to their toxicity. Some of these banned pesticides and some of those currently in use are quite persistent, with half-lives of years to decades, while others have 280

11 short half'-lives of days to months. Some pesticides are acutely lethal to people and wildlife,. Others are sublethal. Still others are innocuous. Figure 4 (Sawhney and Brown. 1989) shows the reactivity and mobility of pesticides applied to croplands. The principal losses of pesticides include surface runoff and leaching, volatilization, and chemical and microbial degradation. In regard to potential ground water contamination, four major factors are involved: the properties of the pesticide, the characteristics of the soil, other characteristics of the site, and management practices. Pesticides that dissolve readily in water are likely to leach. But many moderately to highly soluble pesticides do not leach because they are adsorbed or tightly held by soil, particularly by the soil organic matter clays and fractions. Pesticides which have high vapor pressure are highly volatile and may be easily lost to the atmosphere. Some highly volatile pesticides are less soluble in water and do not contribute to the contamination of ground water. Another property of the pesticide is its degradability by chemical processes, such as photolysis or photochemical degradation, hydrolysis, oxidation, or by microbial processes. The pesticides may be degraded to innocuous products, such as carbon dioxide, water and inorganic constituents. However, some pesticides are degraded into interrmediate products which may be more toxic than the parent compound. The fate of a pesticide applied to soil depends largely on two of its properties: sorption and persistence (Rao and Hornsby. 1989). The tendency of the pesticide adsorbing to soil particles is typically evaluated by its partition coefficient (Koc) defined as the concentration ratio of the pesticide in the sorbed state to the pesticide in the soluble state. The 281

12 Figure 4. Reactions and movemenet of pesticides (Sawhney and Brown, 1989). smaller the value of Koc, the more likely it is that the pesticide will be subject to leaching. Such pesticides are referred to a "leachers". Persistence refers to the "Lasting-power" of the pesticide in question and is related to the extent it is degraded over time. This degradation time is measured in terms of half-life (t1/2). A half-life is the amount of time it takes for one-half of the original amount of the pesticide in the soil to be deactivated. The half-life of the pesticide to be completely degraded is longer than that based on deactivation. 282

13 Table 1 (Rao and Hornsby) presents partition coefficients and halflives for deactivation of pesticides used in the state of Florida. The Koc values of these pesticides in surface soils vary widely and is based on the pesticide's chemical properties. For a given pesticide, sorption is greater in soils with larger soil organic matter content and leaching is expected to be less. The t1 / 2 values also show a wide range of values. Those pesticides with half-lives of 30 days or less are classed as non-persistent, half-lives greater than 30 but less than 100 days as moderately persistent, and half-lives greater than 100 days as persistent. The second major factor influencing the potential for a pesticide to contaminate ground water is the properties of the soil. These include the soil's texture, permeability and amount of organic matter. Texture refers to the relative proportions of sand, silt and clay fractions. A coarse-textured soil, which contains mainly sand and silt, has a lower water holding capacity than finer-textured soil. Soils high in clay and organic matter offer greater opportunities for the pesticide to be adsorbed. Permeability of soils is related to pore sizes (filled with water and gases) and the distribution of pore sizes, and gives a measure of how fast water will move through the soil profile. Generally, coarser-textured soils are more permeable than finer- textured soils. These properties of the soil are important in assessing the leachability of pesticides. For instance, increasing the organic matter content of soils through incorporation of cover crops, minimum tillage and application of manures will increase the soil's ability to retain both water and pesticides and hence lessen the potential for ground water contamination. 283

14 Table 1. Pesticides Used in Florida and their Partition Coefficients and Half-Lives (Rao and Hornsby. 1989).

15 A third major factor is the other conditions of the site. In regions where the depth to the water table is shallow (several feet), the pesticide will more quickly leach into the ground water within weeks to months. In regions where the water table is deep (hundreds of feet), the leaching of pesticides to ground water would take much longer, such as decades. Furthermore, a shallow depth to the water table offers less opportunities for the pesticide to be sorbed and degraded. Soil structure refers to the manner in which soil particles are aggregated and cemented by clays and organic compounds. It affects the movement of water. The presence of macropores (channels) formed by decayed roots, earthworms and shrinking of clayey soils significantly helps pesticides to move downward. These macropores serve as preferential flow paths for water and dissolved pesticides, and thus enhances leaching. Hydrogeologic properties deeper beneath the soil profile are also important. The "underground plumbing" affects how fast and how much pesticides are likely to contaminate ground water. The presence of highly permeable materials, such as gravel, would allow greater movement of pesticide contaminated waters, while layers of clay may inhibit deep percolation. Another condition of the site to be considered is climate. Higher volumes of rainfall or irrigation applications enhances leaching of pesticides. Warmer temperatures enhance the rate of degradation and volatilization of pesticides. The fourth major factor is management practices. Pesticides sprayed on crop plants are less likely to leach, but may be more subject to runoff loss. Pesticides incorporated into the soil will have a greater tendency to be leached. Most of the pesticides detected in ground water are those that are incorporated into the soil. The rate and timing in their application have a large influence on leachability of pesticides. The larger the amount of 285

16 pesticide applied and the closer the time of application to rainfall or irrigation, the more likely it is that the pesticide will leach to ground water. In light of the above four major factors, pesticides are most likely to contaminate ground water when the pesticides used have high solubility, low adsorption to soil particles and longer persistence, when soils are coarsetextured and low in organic matter, when the site has a shallow depth to ground water and a wet climate or heavy irrigations, and when pesticides are injected or incorporated into the soil. Preventive measures for pesticide contamination of surface and ground waters are manifold. They include using pesticides only when needed: identifying the soil's vulnerability to excessive deep percolation: avoiding the use of pesticides known to be "leachers": following the label's instructions on the timing of and rate of application: applying pesticides only to the target site: delaying irrigation after the application of pesticides: to avoid irrigation runoff; and using Integrated Pest Management (IPM). IPM integrates pest control techniques that are both ecologically and economically sound. IPM involves understanding the pest in question, its host crop and its natural predators. An IPM program may include such practices as monitoring of pests: cultural controls that reduce pest problems, such as crop rotation: water management to reduce runoff and leaching and plant diseases: crop canopy management and sanitation to reduce overwintering of pests: development of varieties of crops that resist pest damage: use of the sun's heat to kill pests by placing plastic sheets on the soil surface: cover crops to maintain not only crop production, but to minimize pest problems: use of pheromone dispensers to attract males: and use of biological controls, such as predators, parasites, and pathogens (Flint, 1989). 286

17 Summary Since agriculture is practiced in an "open system", residuals of drainage waters and applied agrichemicals are inevitably produced and discharged into the environment. This open system is subject to the vagaries of climate, and consists of heterogeneous soil properties and "underground plumbing". The effects of agriculture are felt "on-site" (farmland), "off-site" (surface waters) and "out-of-sight" (ground waters). Nevertheless, protection of our soil, water and air resources could be realized by recognizing that "everything has to go somewhere" and "an ounce of prevention is worth a pound of cure". References California Department of Food and Agriculture, Nitrate and Agriculture in California, Report prepared by the Nitrate Working Group, Sacramento, California, 66 pages. Flint, M.L Annual Report, University of California Statewide IPM Project, Focus: Reducing Pesticide Use, University of California, Davis. 71 pages. Frissell, M.J., Editor. Ecosystems Cycling of Mineral Nutrients in Agricultural Developments in Agricultural and Managed-Forest Ecology, No. 3. Elsevier Scientific Publishing Company, Amsterdam, The Netherlands, 356 pages. Hillel. D.J Out of the Earth. Civilization and the Life of the Soil. The Free Press, New York, 321 pages. 287

18 Meyer, RD., and R. Coppock Nitrate Losses From Irrigated Cropland. Division of Agricultural Sciences Leaflet , University of California, 23 pages. National Research Council Alternative Agriculture, Report by the Committee on The Role of Alternative Farming Methods in Modern Production Agriculture. Board on Agriculture, National Academy of Sciences, Washington, D.C., 448 pages. Office of Technology Assessment Beneath the Bottom Line. Agricultural Approaches to Reduce Agrichemical Contamination of Groundwater. U.S. Congress of the United States, Washington, D.C. 337 pages. Rao, P.S.C. and A.G. Hornsby Behavior of Pesticides in Soils and Water. Soil Science Fact Sheet. SL40 (Revised), University of Florida, Gainesville, Florida, 4 pages. Sawhney. B.L., and K. Brown Reactions and Movement of Organic Chemicals in Soils. Soil Science Society of America Special Publication No. 22. Madison, Wisconsin. 474 pages. Stevenson, F.J Nitrogen in Agricultural Soils. Agronomy Monograph No. 22, American Society of Agronomy. Madison, Wisconsin, 940 pages. U.S. Environmental Protection Agency National Survey of Pesticides in Drinking Water Wells, Phase 1 Report, 98 pages. U.S. Environmental Protection Agency Nonpoint Source Agenda for the Future. 288