The study unit is also characterized by five dominant land-resource provinces that are derived from general soils associations (fig. 2).This classification scheme was chosen as having the best potential for relating water quality in the study unit to a hydrogeomorphic setting. The land-resource provinces and land use were factors considered in the design of a sampling strategy to examine water-quality conditions in the study unit at different spatial and temporal scales. Nine surface-water monitoring sites were selected within the study unit to examine the effects of various land uses within specific land-resource provinces. Three monitoring site designations were used for these surface-water sites: (a) integrator sites—sampling large basins with varied land use in multiple land-resource provinces; (b) indicator sites—sampling small basins with relatively uniform land use in a singular land-resource province; and (c) intensive-fixed sites—indicator or smaller integrator sites that were selected for high frequency sampling to characterize nutrient and pesticide concentrations in urban and agricultural settings.
The 9,950 square mile Suwannee River basin, which has more than 30 percent of its area in agricultural land use, provides an ideal setting to examine the effects of fertilizer and pesticide use on water quality. The Suwannee River basin is particularly interesting because much of this river recharges directly to and receives discharge directly from the Upper Floridan aquifer, the primary source of public and private water supply in the area. Urban land use accounts for less than 2 percent of the basin, and the total population is just over 400,000 people.
Three sampling sites on the Suwannee, Withlacoochee, and Little Rivers, have a spatial relation conducive to studying the distribution and persistence of agricultural chemicals within a large part of the Suwannee River basin (fig.3). The Little River and Withlacoochee River monitoring sites are intensive-fixed sites where water samples were collected as frequently as weekly from March until July 1993, and thereafter monthly, to assess changes in the concentrations of nutrients and a wide variety of pesticides. The 1993 water year was exceptionally dry in the Southeast and the Little River basin had no flow from late July until early November 1993. As a result, no samples were collected at the Little River site during that period. Water samples were collected at the Suwannee River site, which is an integrator site, quarterly and more frequently when hydrologic conditions warranted. The factors that affect constituent concentrations in surface waters at these sites include the size of the drainage area, the soils and topography, the degree of chemical usage, the scheduling of chemical usage, precipitation intensity and duration, and the degree of surface-water/ground-water interaction.
Data collected at the Little River site in the headwaters of the Suwannee basin indicated the presence of three herbicides prometon, metolachlor, and atrazine. These herbicides generally were detected in low concentrations, but concentrations were somewhat elevated during the spring of 1993 (fig. 4). These pesticides are typically applied to croplands in the months of February through June. The highest concentrations observed for metolachlor and atrazine were 0.091 and 0.071 micrograms per liter, respectively. Concentrations of prometon, the herbicide detected in the highest concentrations, were as high as 0.14 micrograms per liter in samples from the Little River site (fig. 4). Prometon concentrations also were elevated in samples from the Withlacoochee River site, but were higher in the fall than in the spring (fig. 4). Had the Little River not been dry during the fall of 1993, prometon concentrations at the Little River site also might have exceeded concentrations measured in the spring, because herbicide application practices are similar in the two drainage basins. Preliminary examination of the water-quality data (fig. 4) and discharge hydrographs (not shown in fig. 4) for sites on the Little and Withlacoochee Rivers indicate that the highest concentrations of prometon generally were in samples collected following rainfall-runoff events. Samples for analysis of prometon were not collected at the Suwannee River site during 1993.
Nitrite plus nitrate as nitrogen concentrations in water samples collected at sites on the Suwannee, Withlacoochee, and Little Rivers in 1993 are shown in figure 5. Concentrations of nitrite plus nitrate as nitrogen at the Little River site were elevated during the spring of 1993 when application of fertilizer on crops in the basin was most intensive. After mid-April, concentrations of nitrite plus nitrate as nitrogen in surface water were notably higher at the Withlacoochee River site than at the Little River site. Concentrations of nitrite plus nitrate as nitrogen in samples from the Withlacoochee River site had large peaks during spring and fall, which probably were related to application of fertilizer on winter and summer crops. Smaller peaks in nitrite plus nitrate as nitrogen concentrations during spring and fall also were observed at the Suwannee River site. Nitrite plus nitrate as nitrogen concentrations at this site exceeded 1 milligram per liter in late spring and 0.9 milligram per liter in late summer and early fall, but were less than half the highest concentrations in samples collected at the Withlacoochee River site.
Seasonal peaks in the concentrations of the herbicide prometon and nitrite plus nitrate as nitrogen observed in the surface-water samples collected at three monitoring sites in the Suwannee basin reflect the seasonal application of agricultural chemicals to crops in the basin. Concentrations of nitrite plus nitrate as nitrogen exceeded 2.0 milligrams per liter at the Withlacoochee River site site but were well below concentrations considered a health risk in drinking-water supplies by the U.S. Environmental Protection Agency (maximum contaminant level is 10 milligrams per liter nitrate as nitrogen). Concentrations of prometon, the herbicide having the highest concentrations measured at the three monitoring sites exceeded 0.10 micrograms per liter at the Little River site site but were also well below the concentration considered to represent a health risk (105 micrograms per liter).
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