USGS1995
Development in the Chesapeake Bay region has adversely affected the water quality of the Bay. The general degradation in the Bay has resulted in the decline of commercial fishing industries and has reduced the area of aquatic vegetation that provides food and habitat for fish and shellfish. In order to assess the effectiveness of programs aimed at reducing the effects of excess nutrients and suspended solids on Chesapeake Bay, it is necessary to quantify the loads of these constituents into the Bay, and to evaluate the trends in water quality. This report presents the results of a study funded by the Virginia Department of Environmental Quality Chesapeake Bay and Coastal Programs and the U.S. Geological Survey, to monitor and estimate loads of selected nutrients and suspended solids discharged to Chesapeake Bay from five major tributaries in Virginia. The water-quality data and load estimates provided in this report also will be used to calibrate computer models of Chesapeake Bay. Water-quality constituents were monitored in the James and Rappahannock Rivers over a 5-year period, and in the Pamunkey, Appomattox, and Mattaponi Rivers over a 4-year period. Water-quality samples were collected from July 1, 1988 through June 30, 1993, for the James and Rappahannock Rivers; from July 1, 1989 through June 30, 1993, for the Pamunkey and Appomattox Rivers; and from September 1,1989 through June 30, 1993, for the Mattaponi River. Water-quality samples were collected on a scheduled basis and during stormflow to cover a range in discharge conditions. Monitore water-quality constituents, for which loads were estimated include total suspended solids (residue, total at 105° Celsius), dissolved nitrite-plus-nitrate nitrogen, dissolved ammonia nitrogen, total Kjeldahl nitrogen, total nitrogen, total phosphorus, dissolved orthophosphorus, total organic carbon, and dissolved silica. Organic nitrogen concentrations were calculated from measurements of ammonia and total Kjeldahl nitrogen, and organic nitrogen loads were estimated using these calculations. Other selected water-quality constituents were monitored for which loads were not calculated. Daily mean load estimates of each constituent were computed by use of a seven-parameter log-linear regression model that uses variables of time, discharge, and seasonality. Concentration of total nitrogen ranged from less than 0.14 to 3.41 mg/L (milligrams per liter), with both extreme values occurring at the Rappahannock River. Concentration of total Kjeldahl nitrogen ranged from less than 0.1 mg/L in the James, Rappahannock, and Appomattox Rivers to 3.0 mg/L in the James River. Organic nitrogen was the predominant form of nitrogen at all stations except the Rappahannock River, where nitrite plus-nitrate nitrogen was predominant, and organic nitrogen comprised the majority of the measured total Kjeldahl nitrogen at all stations, ranging from 0.01 mg/L in the Appomattox River to 2.86 mg/L in the James River. Concentration of dissolved ammonia nitrogen ranged from 0.01 mg/L in the Pamunkey River to 0.54 mg/L at the James River. Concentration of nitrite-plusnitrate nitrogen ranged from 0.02 to 1.05 mg/L in the James River. Concentrations of total phosphorus ranged from less than 0.01 mg/L in the Rappahannock and the Mattaponi Rivers to 1.4 mg/L in the James River. Dissolved orthophosphorus ranged from less than 0.01 mg/L in all five rivers to 0.51 mg/L in the James River. Total suspended solids ranged from a concentration of less than 1 mg/L in all five rivers to 844 mg/L in the Rappahannock River. Total organic carbon ranged from 1.1 mg/L in the Appomattox River to 110 mg/L in the Rappahannock River. Dissolved silica ranged from 2.4 mg/L in the James River to 18 mg/L in the Appomattox River. The James and Rappahannock Rivers had high median concentrations and large ranges in concentrations for most constituents, probably because of a greater number of point and nonpoint sources of nutrients and suspended solids, and differences in land use when compared with the other basins. A significantly higher median concentration and greater range of dissolved orthophosphorus generally occurred at the James River than in all other rivers, which primarily is due to the greater number of point sources, such as municipal waste-water treatment plants. The Rappahannock River had significantly higher median concentrations and greater ranges of dissolved nitrite-plus-nitrate nitrogen and total nitrogen than other rivers, probably derived from agricultural sources. Total organic carbon was highest in the Mattaponi and Pamunkey River Basins that contain expanses of wetlands. The Appomattox River had the highest concentration of dissolved silica. The median monthly load of total nitrogen ranged from 16,500 kg (kilogram) in the Mattaponi River to 371,000 kg in the James River. Total Kjeldahl nitrogen ranged from a median monthly load of 12,500 kg in the Mattaponi River to 205,500 kg, also in the James River. Organic nitrogen comprised the majority of the total Kjeldahl nitrogen load in all five rivers, ranging from a median monthly load of 11,251 kg in the Mattaponi River to 3,299,500 kg in the James River. The median monthly load of dissolved ammonia nitrogen was 1,130 kg in the Mattaponi River and was as much as 21,050 kg in the James River, whereas nitrite-plus-nitrate nitrogen ranged from a median monthly load of 4,065 kg in the Mattaponi River to 156,500 kg in the James River. The median monthly load of total phosphorus ranged from 1,670 kg in the Mattaponi River to 61,600 kg in the James River, whereas the median monthly load of dissolved orthophosphorus ranged from 350 kg in the Mattaponi River to 25,900 kg in the James River. Total suspended solids ranged from a median monthly load of 241,500 kg in the Mattaponi River to 20,050,000 kg in the James River. Total organic carbon ranged from a median monthly load of 167,000 kg in the Mattaponi River to 2,100,000 kg in the James River. The median monthly load of dissolved silica ranged from 209,500 kg in the Mattaponi River to 3,625,000 kg in the James River. In general, annual loads for complete years of data collection were greatest at the James River for all constituents, probably because of the much higher discharge, greater basin size, and higher rates of runoff. Yields, or computations of loads per square mile of basin area, were generally highest at the Rappahannock River for total suspended solids, total Kjeldahl nitrogen, nitrite-plusnitrate nitrogen, and total nitrogen. Dissolved orthophosphorus was the only constituent with a yield consistently greater at the James River. Yields of total phosphorus were highest for the James and Rappahannock River basins, whereas yields of dissolved ammonia nitrogen, total organic carbon, and dissolved silica were similar for all five river basins. Quality-assurance analyses that compare the results of the Virginia Division of Consolidated Laboratory Services and the U.S. Geological Survey Laboratory indicate that there are statistically significant differences between the laboratories for several constituents. Differences between laboratories were found to be caused by differences in analytical reporting limits, differences in analytical technique, or a slight bias at both laboratories. Quality-assurance data were used to address analytical technique problems, and to qualify final concentrations and loads.