Impact of Midwest Farming Practices on Surface and Groundwater

Chapter 22

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Impact of M i d w e s t F a r m i n g Practices on Surface and Groundwater Equally

J. L. Hatfield and D. B. Jaynes 1

National Soil Tilth Laboratory, Agricultural Research Service, USDA 2150 Pammel Drive, Ames, IA 50011-4420 Nonpoint source pollution has been linked with agricultural practices across the United States. Farming practices can be modified to improve ground and surface water quality and have a positive impact on sustainability and adaptability by the farmer. Farming practices can alter the hydrologic balance of a landscape and ground and surface water quality problems can be related to the flow of water through the landscape. Surface runoff, which is a major nonpoint source problem, can be reduced by adoption of conservation tillage practices. To fully understand the impact of farming practices on ground and surface water quality requires that each farming practice be examined from a mechanistic view of water management.

Farming is intense in the Midwest with over 80% of the agricultural land area in some aspect of grain crop production. Coupled with the intense crop production is the use of herbicides and nitrogen fertilizer as part of the production system. Herbicide use in the Midwest represents nearly 60% of all the herbicides applied in the United States (i). These herbicide use patterns have remained fairly stable over the past 10 years with atrazine being used on nearly 65% of the com area. Nitrogen fertilizer is used on nearly 97% of the grain crop area of the Midwest. With these intensive levels of inputs there have been detections of herbicides and nitrate-nitrogen in surface and ground water of the Midwest. In the last 10 years several reconnaissance studies of the ground and surface water resources of the Midwest have been undertaken by a number of different groups (2-6). These studies have shown that pesticides, including atrazine (2chloro-4-ethylamino-6-isopropylamino-.y-triazine) were detected in almost half of the groundwater samples. The differences among studies were related to analytical reporting limits, well selection criteria, andtimeof sample collection (4). Kolpin et ai (5) found that in 837 samplesfrom303 wells across the Midwest, five of the 266

U.S. Government work. Published 1998 American Chemical Society Ballantine et al.; Triazine Herbicides: Risk Assessment ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


267 six most frequently detected compounds were herbicide metabolites. They suggested that levels of metabolites may be more frequently detected in groundwater and that degradation pathways of herbicides in the soil need to be better understood. Kolpin et al. (7) found detectable levels of herbicides or atrazine metabolites in 28.4% of the 303 midwestern wells sampled in 1991. None of the wells sampled had herbicide concentrations that exceeded standards for safe drinking water. Nitrate also has been found in shallow ground water samples in the Midwest. Burkart and Kolpin (2) reported nitrate-nitrogen concentrations above the Federal EPA standard for safe drinking water (10 μg L" ) in 6% of their samples. Madison and Burnett (8) found 6.4% of the 123,656 wells sampled throughout the Midwest had nitrate-nitrogen concentrations above 10 μ g L" and 13% had concentrations between 3 and 10 μ g L" . Hallberg (3) suggested that movement of herbicides and nitrate into ground water wells would be dependent upon the intensity of the farming practices and the hydrologie and geologic conditions. Goolsby and Battaglin (9) analyzed data from surface water samples from the Midwest and found that concentrations and mass transport of herbicides follow an annual cycle. In their reconnaissance study, several drainage basins in the Mississippi River watershed were sampled beginning in 1989. They found that less than 3% of the herbicide mass applied to crop land was transported into streams; however, this mass was sufficient to cause atrazine concentrations to exceed 3 pg/L" portions of the Mississippi River for short periods of time. Peak herbicide concentrations were found in storm runoff during May, June, and July with other detections occurring throughout the year. Concentrations were related to the amount applied. Nitrate-nitrogen concentrations throughout the year exhibited a different pattern than herbicides with the highest concentrations in the winter and spring and the lowest during the summer. There is little information about the magnitude of agrichemical loads to surface and shallow ground water from agricultural watersheds. There has also been little mechanistic investigation of the attributes of farming practices that could be managed to influence ground and surface water quality. Hatfield (10,11) described how different attributes of farming practices could be modified to improve environmental quality. This report will show where the potential lies in modifying farming practices for further improvements in ground and surface water quality. 1

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Impact of Farming Practices on Hydrology Movement of herbicides and nitrate-nitrogen are determined by movement of water. Therefore, to develop a mechanistic model of farming practices and water quality, it is important to understand the patterns of water movement associated with different farming practices. The distribution and amount of precipitation occurring throughout the year are determined by movement of fronts and air masses across a region. The amount of precipitation that a specific area receives is dependent upon its position on the earth and size of the land area, and nearness to mountains. These are basic concepts that underlie the input side of the hydrologie

Ballantine et al.; Triazine Herbicides: Risk Assessment ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


268 balance. Another important factor is the application of irrigation water to a field from either a ground or surface water source. Precipitation that falls on the earth may infiltrate the soil, or be intercepted by the leaves and stems of the vegetation canopy and subsequendy evaporated, or run off from the field into nearby streams or rivers. Of the precipitation that infiltrates the soil, a portion may be returned to the atmosphere by evaporation from both the soil and crop canopy. Any excess may move through the soil profile beyond the reach of the root system and ultimately become part of the ground water. One can imagine the soil as a leaky container that can lose water from the surface if water is applied more quickly than the soil can absorb the water; and from the bottom the amount applied is beyond the ability of the soil to hold water. Water may be removed from the soil volume through evaporation from the soil surface. Evaporation occurs when the soil is wet and rapidly decreases as the soil dries. Evaporation is a small portion of the water lost from agricultural systems and may represent less than 15% of the annual precipitation (Hatfield and Prueger, unpublished data). Growing crops explore the soil volume with a root system that extracts water from various soil layers depending upon the stage of crop growth and available soil water. Water taken up by the root system is moved to the leaves where evaporation occurs as part of the process of cooling the leaves. This process is referred to as transpiration and is a critical part of plant growth. Without an adequate soil water supply, agricultural plants undergo stress resulting in reduced yield potential. In agriculture, the soil water evaporation process and transpiration are combined into a term, évapotranspiration. Evapotranspiration removes water from the soil profile; the amount being determined by atmospheric conditions. Hatfield (12) described this process and the various techniques available to either estimate or directly measure évapotranspiration. These methods have been used to determine the rate of water use by different cropping systems and to compare cropping systems for their efficiency in using water for crop growth. The critical aspect of évapotranspiration for assessment of nonpoint source pollution and farming practices is the seasonal relationship between precipitation and évapotranspiration. This can be understood by examining the records from three locations shown in Figure 1. Precipitation throughout the year is dependent upon the general climate of the area as shown in this example from Ames, Iowa; Peoria, Illinois; and West Lafayette, Indiana. The precipitation and évapotranspiration distribution patterns throughout the year are similar among locations with the highest rainfall during the summer months. While the distribution patterns are similar, the monthly amounts are different, which leads to a greater excess in the soil profile at Peoria and West Lafayette than at Ames in the early spring. This additional water could lead to either increased leaching or greater potential surface runoff in the eastern part of the Corn Belt. There would be similar soil water use patterns with latitude across the Midwest due to temperature and length of the growing season. There are differences in évapotranspiration caused by different tillage practices. Hatfield et al. (13) showed that no-tillage practices that increased the crop residue on the soil surface decreased the soil water evaporation rates in the


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early season and increased évapotranspiration later in the growing season because of the increased soil water in the soil profile. Typically, in central Iowa, the yearly total of évapotranspiration is approximately 650 mm. The largest portion of this amount occurs in the summer, and less than 150 mm occurs from October through April. These seasonal patterns show that the greatest chance of surface runoff from fields would occur in the early spring in the Midwest. Surface Runoff Resulting from Farming Practices. Baker and Laflen (14), after studying the effects of tillage practices on herbicide washoff, concluded that washoff water from crop residue would either run off or infiltrate into the soil based on a combination of factors. These factors were the timing and intensity of rainfall and the infiltration capacity of the soil. Baker and Shiers (75) found that the largest concentration of cyanazine [2-[[4-chloro-6-(ethylamino)-l,3,5-triazin2-yl]amino]-2-methylpropionitrile], alachlor [2-chloro-2'-6'diethyl-N-(methoxymethyl)-acetanilide], and propachlor [2-chloro-N-isopropylacetanilide] occurred after the first rainfall event following their application. Rainfall events wiUmi 12 hours of herbicide application are responsible for considerable loss from washoff from the crop residue. Whether there is movement from the edge of the field will depend upon the amount of rainfall and the infiltration rate into the soil. Surface runoff of herbicides from the edge of fields is a short-lived event as depicted in Figure 2. In this event, within the Walnut Creek watershed in Iowa, atrazine was transported in the surface runoff which occurred shortly after application. Concentrations were measured between 25 and 30 pg L for atrazine while metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-l-methylethyl)acetamide) was measured at 75 to 80 μ g L" (76). Fawcett et al. (17) summarized the results of different natural rainfall studies across the United States and concluded that conservation tillage practices reduced herbicide runoff by 70% compared to moldboard plowing. The no-till practice has been shown to increase infiltration and reduce surface runoff and sediment transport because of the effect of the residue on protecting the soil surface. Conversely, Myers et al. (18) found that no-till practices increased surface runoff. There is a difference in the studies reported in the literature on herbicide runoff when natural rainfall is compared to simulated rainfall. Fawcett et al. (17) found that rainfall simulation studies that applied very heavy rainfall amounts after herbicide application produced the highest concentrations in the runoff. They also stated that this type of event would rarely occur. Under normal conditions though there would still be some infiltration of water into the soil and subsequent movement of herbicide into the upper soil layer. Surface runoff will be more likely to occur when the soil is saturated and there is an excess of precipitation compared to water use. The central and eastern part of the Corn Belt, as illustrated by Peoria and West Lafayette, have a greater chance of soil profile saturation in the spring as compared to Ames. Moving from east to west across the Corn Belt, the chances of surface runoff of herbicides to decrease would be expected. However, surface runoff is a result of sloping land and as the slope increases, there is increased chance of a runoff event after rainfall. Hills in western Iowa will experience a greater chance of surface runoff in the 1

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spring than will relatively level central Iowa even with less rainfall. Even though there is an excess of precipitation above evaporation in the late fall and early winter, there is little chance of surface runoff because the soil profile has been depleted of soil water by the crop water use over the summer and there is little herbicide available at the surface to be moved by surface runoff events. All of these factors must be integrated to assess the effect of changing farming practices on water quality. Leaching Resulting from Farming Practices. The quantity of water that infiltrates the soil may increase the amount of water moved through the root zone and toward the ground water. This movement is likely to occur in the portion of the year immediately after application when there is little water use by the crop and the soil profile is saturated. Soluble herbicides and nitrate-nitrogen are easily transported with moving water, and when water moves below the bottom of the root zone, chemicals are moved out of the root zone. This would be considered offsite movement and subject to the overall water flow patterns within the landscape. Leaching through the soil profile would be dependent upon the amount of crop residue, the presence or absence of cracks and macropores in the soil profile, and the distribution of water within the soil profile. Green et al. (19) found that increasing the amount of corn residue on the soil surface decreased the time for the peak atrazine concentrations to occur in the leachate from more permeable soil columns. They suggested that crop residue intercepts the herbicide and permits a more steady pattern of infiltration into the soil. Under crop residue, the herbicide was moving more slowly through the soil profile. Gish et al. (20) evaluated no-till compared to tilled fields and found in the no-till field that 28% of the water samples collected below the root zone had detectable amounts of atrazine ranging from 0 to 10 pg L" . In the tilled fields, 53% of the samples had detectable levels of atrazine ranging from 3 to 35 pg L" . In contrast, an earlier study by Isensee et al. (21) and Gish et al. (22) reported that increased infiltration under no-tillage conditions enhanced the chemical transport because of increased preferential flow through the root zone. The effects of management practices on preferential flow and chemical movement through the soil profile need to be more completely evaluated across a wider range of soils and soil water management practices. Herbicide movement through no-till soils is complicated because of the addition of organic matter to the soil with the adoption of this farming practice. Blevins et al. (23) found that organic matter increased in the surface layer of longterm no-till plots. Dick (24) also reported that reducing the intensity of tillage increased the organic matter content. Organic matter has been correlated with changes in herbicide activity, and Upchurch et al. (25) reported that the soil organic fraction accounted for 66% of the variation in herbicide activity. Increases in soil organic matter have been related to changes in herbicide activity and degradation. Hudson (26) reported that increases in soil organic matter content led to an increase in the soil water holding capacity. Changes in the soil organic matter content will lead to changes in water movement patterns through increased storage within the soil profile but will also change herbicide interaction with the 1

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273 soil. The recent work by Gish et al. (20) suggests an interaction between the movement of herbicides and biological activity in the soil profile caused by changes in soil organic matter content. Movement of herbicides through the soil profile results in groundwater and subsurface drainage concentrations that are often below the 3.0 μ g L" Health Advisory Level (HAL). The H A L is a standard for the annual average concentration in drinking water and is set using wide safety margins assuming daily intake over lifetime. Studies conducted within Walnut Creek in Iowa on the quality of subsurface drainage water show a responsiveness to changes in the soil water content as evidenced by the increase in atrazine concentration during periods with increased flow. These data, collected during July and August of 1992, show that peak atrazine concentrations are less than 1 pg L" and are typically less than 0.5 pg L ' (Figure 3). Nitrate-nitrogen concentrations in contrast, decrease with increased subsurface flow and range from 10 to 15 pg L" under normal flow. These changes in concentrations with increased flow would suggest that there is considerable preferential movement through the soil profile. Owens et al. (27) measured nitrate-nitrogen concentrations in suction lysimeters below a continuous corn and corn-soybean rotation at Coshocton, Ohio and found concentrations from 10 to 14 μ g L ' . They also found a seasonal variation from 1 to 20 pg L" caused by the crop rotation. This led them to conclude that shallow ground water could be protected by adopting a crop rotation pattern over continuous corn production. Kanwar et al. (28) found in north central Iowa that continuous corn with no-tillage had the largest water loss through the soil profile but the lowest nitrate-nitrogen concentrations in subsurface drainage compared to moldboard plow and chisel plow. In contrast, the results of Hatfield et al. (29) from Walnut Creek watershed in central Iowa showed no seasonal trend in the nitrate-nitrogen concentrations in the subsurface drainage waterflow. This difference may be due to the scale of the project and the observations of field-sized areas rather than small plots or due to the differences among the soils at the study sites. Leaching of herbicides and nitrate-nitrogen is a process of offsite movement that can affect either surface water or ground water quality. In areas with subsurface drainage, the discharge is moved across the landscape and is transported to open channels where the water becomes part of the surface water. In areas without subsurface drainage, the water that leaches through the soil profile can become part of the ground water recharge. There is a balance at all locations regarding the partitioning among surface runoff, subsurface drainage, and ground water recharge. The balance will be impacted by the type of farming practice and the patterns of crop water use throughout the year.


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Studies in the Midwest on Water Quality There are many unknowns about the effect of farming practices on water quality. To address these questions and begin to develop a research program that would address the issues associated with farming practices, a research and education effort began in 1990 in the Midwest. The Management Systems Evaluation Areas (MSEA) Program was established in 1990 by the United States Department of


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275 Agriculture (USDA)-Agricultural Research Service (ARS). As described by Onstad et al. (30), this program was established to evaluate the effects of agricultural chemicals on ground water quality in areas representing a variety of soil, geologic, and climatic conditions. An additional goal was to develop best management practices (BMPs) that protect ground water from agricultural chemical contamination while addressing the economic, environmental, and social needs of midwest agricultural production. Administration of the program is a cooperative effort between the USDA-ARS, USDA-Cooperative State Research Education and Extension Service (CSREES), State Agricultural Experiment Stations, Natural Resources Conservation Service (NRCS), United States Geologic Survey (USGS), and United States Environmental Protection Agency (USEPA). Research is conducted at five primary sites in Iowa, Minnesota, Missouri, Nebraska, and Ohio and at several additional sites within these and adjacent states. Current research activities at these sites focus on the protection of ground water in specific aquifers or geographical areas. Research objectives at these MSEA research sites are to: a) measure impacts of prevailing and modified farming systems on chemical constituents of ground water and surface waters; b) identify and increase the understanding of factors and processes that control fate and transport of agricultural chemicals; c) assess impacts of agricultural chemicals and practices on ecosystems; d) assess benefits of using modified farming systems in the Midwest; e) evaluate social and economic impacts of adopting modified farm management systems; and f) transfer appropriate technology to other agricultural areas (31). The Iowa MSEA project goal is to evaluate the effects of agricultural management systems on ground water and surface water quality in three regions of the state (32). These sites are located in the northeast part of the state on the Iowan surface landform; in southwest Iowa, near Treynor, on the deep loess deposits; and on the Des Moines Lobe landform region near Ames. Walnut Creek watershed near Ames, Iowa was the site selected for an intensive watershed scale study. Spécifie research objectives for the Iowa MSEA project are: a) quantify physical, chemical, and biological factors that affect transport and fate of agricultural chemicals; b) determine effects of crop, tillage, and chemical management practices on quality of surface runoff, subsurface drainage, and groundwater recharge; c) integrate information from the first two objectives with data on soil, atmospheric, geologic, and hydrologie processes to assess impacts of these factors on water quality; d) evaluate socioeconomic effects of current and newly developed management practices; and e) understand the ecological effects of agricultural chemicals, distinguishing them from impacts of other agricultural practices (32). Observations from Walnut Creek Watershed The Walnut Creek watershed has been described in a detailed report on research protocols and descriptions by Sauer and Hatfield (33). In this report, observations on surface discharge and shallow ground water will be reported. Farming


276 practices within Walnut Creek watershed are typical of the Midwest with the percent of land area treated with atrazine very similar to the average for the Midwest. Nitrogen fertilizer rates for the watershed are slighdy below the statewide average of 150 kg ha" , however, they are typical of the Des Moines Lobe landform region. Observations have been made with shallow ground water piezometers installed around a series of fields typical of the Midwest with chisel plow, ridge tillage, and no-till and in a corn-soybean rotation.


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Shallow Ground Water Observations. Observations through 1993 in the shallow wells showed only one atrazine detection above the 3 pg L H A L and this was in the 1.5-3 m depth below the soil surface. These wells were placed at the edge of the fields having a history of atrazine use within the watershed since the mid-1960's. Therefore, the observations should reflect the history of field application. Similar results were found for alachlor, metribuzin (4-amino-6-(l,ldimethylethyl)-3-(methylthio)-l,2,4-triazin-5(4H)-one), and metolachlor in these piezometers (27). The mean atrazine concentration for the 1.5-3 m depth was 0.065 pg L" and would support the general conclusion of Burkart and Kolpin (2) that there are a few detections above the H A L . Observations made in the deep ground water wells (>150 m) within Walnut Creek watershed revealed no detections of any herbicide, and nitrate-nitrogen concentrations less than 1 pg L" . Edge of field samples show that there is little historical movement of atrazine under a range of climatic conditions and farming practices. 1

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Surface Water Observations. Surface water discharge from the 5600 ha Walnut Creek watershed has been monitored since 1991 and shows only a small amount of herbicide loss (29). Stream discharge from the watershed would represent a composite of subsurface drainage and surface runoff from fields. There is a continual movement of water from the watershed, and nitrate-nitrogen concentrations typically average between 15 and 20 pg L" in the spring and 5-10 pg L" in the late summer. Herbicide concentrations are more dependent the precipitation events within the watershed. Loads in 1991 and 1993 were the largest for all herbicides and can be attributed to the spring rains in 1991 and the large summer rains in 1993. These large rain events caused an increase in the surface runoff and subsurface drainage. In 1992 and 1994, the loads were quite low due to the reduced number of large rain events early in the growing season. Atrazine loss ranged from 0.2 to 7.0% of the amount applied during the growing season. Concentrations of atrazine at the discharge point of Walnut Creek have averaged less than 0.5 pg L" since 1990 (Figure 4). These samples are collected as weekly grab samples from automated samplers located at the discharge point. There is considerable scatter throughout the year in response to the variation in the distribution of rainfall events. Surface water quality within Walnut Creek is influenced by the subsurface drainage system and the hydrology of the watershed. There is little evidence of an effect of crop rotation on nitrate-nitrogen or herbicide movement into the subsurface drainage systems. The concentrations within selected subbasins of the watershed reflect the overall water movement within the watershed. 1

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278 Concentrations of atrazine detected in the subsurface drain lines may be residual atrazine from previous growing seasons since atrazine was introduced in the mid1960's. The amounts that are lost, however, are quite small. Runoff concentrations from the edge of single fields in excess of 25 pg L" in 1991 are not reflected in the concentrations leaving the watershed because only a small portion of the stream flow consists of surface runoff in Walnut Creek watershed (Figure 4). Atrazine application across the watershed is evenly distributed and field records show that about 50% of the land area receives some atrazine. Metolachlor is more widely used as a herbicide in crop production within Walnut Creek. Both of these herbicides are soluble in water and easily moved with water flow. Atrazine losses with current farming practices in Walnut Creek are less than the amounts reported by Goolsby and Battaglin (9). Nitrate-nitrogen losses are higher than reported by Goolsby and Battaglin (9) since there is a greater proportion of subsurface drainage and grain crop production in Walnut Creek compared to the Midwest. Nitrate-nitrogen concentrations are higher in subsurface drainage discharge than in surface runoff.


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Modifications of Farming Practices to Improve Water Quality Water movement is the primary transport mechanism for herbicides and nitratenitrogen, and efforts to modify farming practices to reduce the loss of herbicides below current levels must start with this mechanism. Surface runoff can be reduced through the adoption of conservation tillage practices as demonstrated by Fawcett et al. (77). Surface runoff of herbicides occurs when the soil is saturated and there is heavy rainfall shortly on fields after broadcast applications. Total surface water discharge from a watershed does not exhibit the concentrations or loads from an individual field since herbicides are not uniformly applied across a watershed. This can be greatly reduced by incorporation of herbicides at the time of planting as described by Baker and Mickelson (34). Incorporation of a herbicide into the upper soil layer allows for more rapid infiltration and soil-bind, thus reducing the potential for movement with surface runoff. In conservation tillage systems, incorporation is not always possible due to the need to leave as much residue on the surface as possible. In these cases, delaying application when the soil is saturated and there is heavy rain predicted could potentially reduce surface transport of herbicides. This concept needs to be evaluated for adoption into herbicide management practices. Leaching through the soil profile is considered to be the opposite effect of decreasing surface runoff. There are few studies that have examined the interrelationships between these two processes at the field and watershed scale for herbicide and nitrate-nitrogen transport. Leaching may increase with conservation tillage as demonstrated by Kanwar et al. (28); however, they found a decrease in the herbicide and nitrate-nitrogen concentrations in the leachate so that there is a net decrease in total load. Nitrogen management could include a change in the time of application to more closely link the application with peak crop needs. This would reduce the potential for leaching since the nitrogen would be placed in the soil at a time when there is both rapid uptake of water and nitrate and little



279 potential for leaching. There are many offsetting processes that occur within the soil profile as changes in farming practices are made, and these are related to the organic matter content and preferential flow pathways through the soil. There is litde information to compare these mechanisms across soils and management practices, although we can assume that increasing the organic matter content in the surface not only increases infiltration, but also increases the degradation rate of herbicides due to increased biological activity. Much of the Corn Belt has subsurface drainage; a situation which offers the potential for management of the tile system to reduce offsite movement. Management of subsurface drainage to reduce movement in the spring could impact both nitrate-nitrogen and herbicide lossesfromfields and improve water management for the crop later in the growing season. Modification of farming practices to achieve environmental goals is not a new research area. Stewart et al. (35,36) developed the first report on the interrelationships of nonpoint source pollution and farming practices. The principles detailed in those reports are valid today, as they were 20 years ago. The concepts of modifying farming practices have the potential to reduce both the onsite and offsite impacts. However, as the studies from Walnut Creek show, herbicide losses are small and modifications in farming practices will provide only slight reductions in herbicide loads. Field management practices coupled with landscape changes around fields, e.g., vegetative filter or buffer strips, can provide environmental benefits. Surface runoff appears to be the largest contributor to herbicide loss across the Midwest and some reductions will be possible with the adoption of conservation tillage. Baker et al. (37) evaluated the effect of filter strips on herbicide removal and showed that removal efficiency is based on antecedent soil water in the vegetative filter strip, runoff volume, and herbicide concentration. Summary Farming practices can be modified to achieve water quality goals. These modifications need to be developed for each particular region of the United States and evaluated locally in cooperation between local research communities and growers. The research community will gain insights into the newest farming practices and the modifications can be quickly transferred to and adopted by local growers. These changes can be integrated into current practices by farmers and landowners without major changes in their farming operations. This partnership can yield positive results for soil management, profit of farming operations, and environmental quality. Literature Cited 1. Gianessi, L. P.; Puffer, C. M. Herbicide use in the United States. Resources for the Future, Qual. of the Environ. Div., 1991, Washington, D.C. 2. Burkart, M. R.; Kolpin, D. W. J. Environ. Qual. 1993, 22, pp. 646-656. 3. Hallberg, G. R. Agric. Ecosys. Environ. 1989, 26, pp. 399-367.


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10. 11. 12. 13.

14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

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