The first step in reducing agricultural NPS pollution is to focus on the primary water quality problem within the watershed: the water quality use impairment must be identified and the type and source of the pollutant(s) must be defined. Once the problem has been clearly defined and documented, the critical area (the area that contributes the majority of the pollutant to the water resource) can be identified. Land treatment should then be implemented on these critical areas.
Land treatment consists of the installation or utilization of best management practices (BMPs). Best management practices are used to control the generation or delivery of pollutants from agricultural activities to water resources and to prevent impacts to the physical and biological integrity of surface and ground water. BMPs can be either structural (for example, waste lagoons, terraces, sediment basins, or fencing) or they can be managerial (for example, rotational grazing, fertilizer or pesticide management, or conservation tillage). Both types of BMPs require good management to be effective in reducing agricultural nonpoint source pollution.
The installation or use of one structural or management BMP is rarely sufficient to control the pollutant of concern. Combinations of BMPs that control the same pollutant are generally most effective. These combinations, or systems, of BMPs can be specifically tailored for particular agricultural and environmental conditions as well as for a particular pollutant.
The Rural Clean Water Program (RCWP), a federally sponsored experiment in controlling agricultural NPS pollution conducted during the 1980s, demonstrated the importance of using BMP systems to control agricultural NPS pollution (Gale et al., 1993). In the RCWP projects, systems of BMPs were used to control a range of pollutants, such as sediment, nutrients, and bacteria. The following discussion of BMP systems and their effectiveness is based on the knowledge gained through the efforts of the 21 RCWP project teams.
In the Nebraska RCWP project, sediment in Long Pine Creek was impairing fish production. Excess sediment originated from streambank erosion, irrigation return flows, and intensive grazing in the riparian zone. Although the pollutant (sediment) was the same, the pollutant sources were different. Three BMP systems had to be devised: a BMP system to control streambank sediment, a BMP system to control sediment from irrigated farmland, and a BMP system to decrease sediment from riparian zone grazing lands. Several individual BMPs were combined into each of the three BMP systems for controlling sediment. Nine individual BMPs were combined for the streambank system, twenty-four BMPs were used for the irrigated croplands system, and ten practices were integrated for the grazing lands system. A total of 37 individual BMPs were used during the project. Some BMPs were specific for controlling sediment from a single source (such as diversions), while other practices controlled sediment from two or more sources (such as fencing).
A system of BMPs designed to address a specific pollutant from a particular source must comprehensively address the pollution problem. In coastal Oregon, where rainfall often exceeds 120 inches per year, dairy farmers installed animal waste management BMPs to reduce fecal coliform runoff into the nearby estuary. Although 12 individual BMPs comprised the animal waste management system (see Table 2), these installations did not qualify as an effective BMP system because they dealt only with manure storage and were not comprehensive in controlling both the pollutant source and delivery of the pollutant to the water resource of concern. Complete and effective control of the bacterial and nutrient contamination reaching Tillamook Bay required that waste application management be employed: land application of manure had to be conducted at the appropriate season, time, and rate. It was also necessary to include waste utilization as part of the BMP system.
There is no single "best" BMP system to control a particular pollutant. Rather the BMP system should be determined by the type of pollutant; the source of the pollutant; the agricultural, climatic, and environmental conditions; the economic situation of the farm operator; and the experiences of the system designers. For example, a similar water quality problem existed in both the Massachusetts and Oregon RCWP projects. In both projects, shellfish production was impaired in an estuary because of fecal coliform contamination caused by runoff from surrounding dairy farms. Animal waste management BMPs were installed, along with other types of BMPs. In the Oregon project, 12 individual BMPs were needed to control the animal manure in the barnyard area, whereas in Massachusetts only four were needed. However, both projects implemented the most appropriate set of BMPs for their environment. The regions where these two projects are located have different climatic and ecological characteristics, requiring different approaches to mitigate the animal waste problem. Some of the animal waste management BMPs installed in Oregon were designed to keep the rain off the manure (guttering and roofing). Because rainfall in Massachusetts is much lower, these types of structures were not needed. In addition, Oregon farmers had to install extremely large waste storage structures to contain the manure during continuous rain events. Because of the drier climatic conditions in Massachusetts, these large waste storage structures were not necessary. Even though the water quality problem was similar for the Oregon and Massachusetts RCWP projects, each project had to design a system of BMPs that was appropriate to the specific conditions encountered.
Transport of agricultural chemicals to surface and ground water can be controlled by reducing the pollutant load reaching the water resource; retarding the transport of pollutant (either by reducing water transported, and thus pollutant transport, or through chemical or biological transformation); or remediating the pollutant in the water system. An individual BMP can only control the pollutant at its source, during transport, or in the water. Systems of BMPs are generally more effective in controlling the pollutant since they can be used at two or more points in the pollutant delivery system. For example, the objective of the Iowa RCWP project was to reduce the loss of soil from cropland, which was resulting in sedimentation and turbidity in Prairie Rose Lake. A system of BMPs was designed not only to reduce soil detachment, thus reducing the potential for soil to erode, but also to retard off-site transport of eroded soil. Critical area planting (NRCS Code 342) and conservation tillage system (NRCS Code 329) were used to reduce the amount of on-site soil loss. Terraces (NRCS Code 600), underground outlets (NRCS Code 620), diversions (NRCS code 362), grassed waterways (NRCS code 412), and sediment retention basins (NRCS Code 350) were installed to slow sediment transport to the lake. Systems of BMPs can be measured for effectiveness. In the Taylor Creek - Nubbin Slough RCWP project in Florida, greater than 50% reductions in total phosphorus concentrations were documented by water quality monitoring at the project outlet and in subwatersheds where many BMPs had been implemented. In contrast, water quality in subwatersheds with little BMP implementation or increased cattle densities showed increases in total phosphorus concentrations. These monitoring results supported the conclusion that the system of BMPs implemented was effective in reducing phosphorus delivery to Lake Okeechobee.
Because financial resources are generally limited, BMP system implementation should be prioritized. Systems of BMPs should first be implemented at those locations in the critical area that contribute the largest proportion of pollutant(s). The remaining critical area locations can then be treated with BMP systems as feasible given availability of funds.
NRCS. 1994. National Handbook of Conservation Practices. Natural Resources Conservation Service (formerly Soil Conservation Service), U.S. Department of Agriculture, Washington, DC.
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