The City of Virginia Beach currently (1994) supplies water to about 400,000 people in southeastern Virginia. The city plans to withdraw water from the Pea Hill Arm of Lake Gaston to meet projected water needs of the population to the year 2030. The purpose of this report is to (1) describe the temporal and spatial distribution of selected water-quality constituents, (2) document current (1989) land use and land cover in the Pea Hill Arm drainage basin, and (3) discuss relations, if any, between the quality of water in the inlets within the Pea Hill Arm and land uses. The report focuses on water-quality problems in the basin, including changes in concentrations of major ions, nutrients, and algae associated with urban development adjacent to water bodies.

The Pea Hill Arm was classified as mesotrophic on the basis of the range of concentrations of total phosphorus (0.001 to 0.61 milligrams per liter); the range of concentrations of total organic-plus-ammonia nitrogen (0.2 to 1.4 milligrams per liter); and the range of concentrations of chlorophyll a (1.4 to 56 micrograms per liter). These water-quality data were collected at 3 feet below the water surface during water years 1989-90.

Thermal stratification in Pea Hill Arm generally began in April and ended in September. Water below a depth of about 25 feet generally became anoxic by June. Destratification generally began in late September and was completed by November. Lake Gaston followed the same general stratification and destratification pattern as Pea Hill Arm, except Lake Gaston was partially destratified during the summer when large amounts of water were released from John H. Kerr Reservoir and Lake Gaston Dams.

During water year 1988, streamflows were 33 percent below the long-term mean-annual streamflows at one of the major streams to Lake Gaston. Low streamflows contributed to elevated specific conductances and concentrations of sodium, calcium, magnesium, and alkalinity from October 1988 to February 1989 at sampling stations in the Pea Hill Arm and Lake Gaston.

About 75 percent of the land use in the Pea Hill Arm is forest land. The remaining 25 percent of the Pea Hill Arm drainage basin is 8 percent pasture/open land, 8 percent open water, 6 percent residential land, and 3 percent cropland. No statistical relations are present between water-quality constituents measured and developed land uses within 11 basins in the Pea Hill Arm Basin, except during periods of stormwater runoff. During a stormwater-runoff event, there was a relation between total nitrite plus nitrate and land use (Kendall's tau correlation coefficient of 0.69). The relation between the developed land use and total nitrite plus nitrate can also be related to the increased ground-water inputs during high base-flow periods.

Spatial differences in water-quality constituents as determined by Wilcoxon (matched-pairs) signed-rank tests and cluster analyses were longitudinal and primarily grouped into riverine, transition, and lacustrine zones. These zones were grouped on the basis of flow characteristics and nutrient concentrations.