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| Albemarle-Pamlico NAWQA |
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ABSTRACT
Agricultural practices, such as how the land is tilled and how much and in what manner pesticides and herbicides are used, are major sources of sediment, nutrients, and synthetic organics in surface-water runoff and of nutrients and organics in ground water. The extent, however, to which agricultural practices serve as a nonpoint source of pollution is largely a function of how the agricultural land is managed.
Farmers can use land-management practices that control erosion, increase soil moisture, and reduce the transport of farm chemicals and fertilizer in runoff from the fields. These methods, which are generally referred to as best-management practices, include development of grassed waterways and field borders, strip cropping, contour farming, and crop rotation. In contrast, when traditional or standard land-management practices are used, waterways are poorly maintained, crop production is continuous and without rotation, and the rows are plowed straight without regard to slope or topography.
To better define how agricultural land-management practices affect water quality, the USGS in cooperation with the Guilford Soil and Water Conservation District and the U.S. Soil Conservation Service began a 6-year study in 1984 of four small basins in the Piedmont of North Carolina. The Piedmont, a physiographic province extending from Virginia through Alabama, is characterized by clayey soils, rolling topography, and abundant rainfall. This area was chosen because of the highly erosive nature of the soils and the ongoing local effort to convert existing farmland to best-management practices. Results of this study should be transferable to similar agricultural lands throughout the Piedmont physiographic region of the Southeastern United States.
The study is designed to monitor chemicals applied to the land through farming practices as well as nutrients resulting naturally from atmospheric deposition. It also monitors water quantity and quality of overland runoff, concentrations of chemical constituents percolating through the clay soils in the unsaturated zone, and constituents reaching the ground water. Farmers cooperating in the study are helping keep detailed records of the chemicals applied to their fields and of their farming activities such as plowing. Data collection is scheduled to end September 1990.
Four areas including two row-crop fields, a mixed land-use basin, and a forested basin were selected for study. The row-crop fields are adjacent-one having best-management practices (7.4 acres) and the other having standard-management practices (4.8 acres). The amount of sediment, nutrients, and selected organics in runoff and the volume of runoff were monitored for the two fields as were the nitrate plus nitrite and pesticide content of soil water. In the mixed land-use basin (665 acres), changes over time in water-quality constituents in runoff are being monitored at a streamflow gage, as standard-management practices are converted to best-management practices over the duration of the project. In the forested basin (44 acres), background hydrologic and chemical-quality conditions are monitored. These areas are within a 4-mile radius, and the effects of atmospheric deposition, which is monitored at one of the agricultural field sites, are assumed to be equal among all four areas.
Analysis of surface-water-runoff data through May 1989 indicates that for the two row-crop fields, in general, concentrations of sediment, nutrients, and selected organics in runoff from the field having best-management practices are dramatically lower than the concentrations found in the field having standard practices. A general relation appears to exist for nutrients and sediment concentrations for the four sites and for precipitation. With the exception of nitrogen, the lowest constituent concentrations are measured in precipitation, followed by increasing concentrations in runoff from the forested area, the field site having best land-management practices, and the mixed land-use site, with the standard-practices field site having the highest concentrations. Interestingly, nitrate plus nitrite concentrations were found to be higher in precipitation than those measured in runoff at the forested site, probably because the nutrient was bound up by the forest litter and also used by the plants.
The difference in sediment concentrations between the two agricultural fields is striking. The standard-management practices field had a mean concentration of 11,200 milligrams per liter (mg/L), compared with the best-management practices field mean concentration of 3,230 mg/L. Sediment concentrations for the mixed land-use basin were generally lower than those observed for the agricultural field sites because of the presence of 14 small farm ponds in the basin that act as sediment traps.
The erosion process, which creates suspended sediment and the resulting sediment yield, tends to sort the soil particles by carrying away the fine siks and clays associated with most of the soil fertility. Sediment yields-the amount of suspended sediment moving past the runoff gage-are consistently higher in the field having standard land-management practices than yields in the field having best land-management practices. During the 1987 water year, the sediment yield from the basin having standard practices was almost 36 tons per acre, compared with 5.4 tons per acre that came off the field having best-management practices.
A seasonal comparison of runoff differences between the two agricultural fields shows that the greatest amount of runoff occurred in the standard-management practices field during the growing season (May-September) and in the best-management practices field in the barren season (October-April). This is probably because during the growing season there is more bare, hardened ground, which promotes runoff, in the standard-management practices field. In contrast, this same field has more plowed ground in the barren season. Plowing the ground increases the surface runoff and promotes infiltration by causing water to be trapped. In spite of the fact that the runoff is greater on the average in the best-management practices field during the barren season, sediment yields during this period are still lower than those observed for the standard-management practices field. This difference indicates that the effect of best-management practices in reducing sediment losses is significant enough to compensate for the additional runoff.
Click to see larger version of Figure 1.
Figure 1.--Comparison of total phosphorus concentrations, in milligrams per liter, in samples of precipitation, and of surface-water runoff from the forested and mixed land-use basins and the best land-management and standard land-management field sites.
The effects of land-management practices on ground-water quality are not as apparent as the effects on surface water. Mean concentrations of total nitrate plus nitrite in the upper unsaturated zone (3 feet below land surface) are higher in the field having standard-management practices where concentrations averaged 1.7 mg/L compared with 1.0 mg/L in the field having best-management practices. Water samples collected in the unsaturated zone 6 feet below land surface and from ground water from 10 to 15 feet below land surface, however, have concentrations slightly higher in the best-management practices field than those measured in the standard-practices field. Levels of pesticide residues measured in soil samples tend to be higher in the best-management practices field compared with the standard-management practices field.
USGS hydrologists are continuing their sampling of surface and ground water in the four study areas. The project results thus far support the need for further study of the effects of land-management practices in clay-type soils on ground-water quality. In particular, the hydrologists will be interested to see what changes over time occur in water quality in the mixed land-use basin as it is converted to best-management practices.
Click to see larger version of Figure 2.
Figure 2.-- Comparison of mean concentrations of total nitrate plus nitrite, in milligrams per liter, in samples of surface-water runoff, of water in the unsaturated zone, and of ground water from the two field sites having best land management and standard land management practices.
Hill, C.L., and Harned, D.A., 1990, The effects of agricultural land-management practices on surface and ground water in the Piedmont of North Carolina, in United States Geological Survey Yearbook, fiscal year 1989, p. 36-38.
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