The Importance of Hydrologic Factors on the Relative Eutrophic Impacts of Point and Non-Point Pollution in a Reservoir

Abstract

The Occoquan Reservoir, located downstream of a rapidly urbanizing Northern Virginia (USA) area, is a highly eutrophic water supply, which exhibits all the symptoms of excessive enrichment. Data collected during an intensive monitoring program since 1972 have vividly demonstrated the relative impacts of stormwater runoff and point-source sewage dis-charges on water quality.

From 1969 through 1976, reservoir quality steadily worsened despite a reduction of 72 percent in point-source phosphorus loadings. The worst conditions occurred in 1975 during an exceptionally wet summer; and in that year, most of the nitrogen and phosphorus (85 and 89.5%, respectively) entered the reservoir via stormwater runoff. By contrast, a marked improvement in reservoir quality was observed at most places in the reservoir during 1976 and 1977 when the worst drought on record occurred. Only in the headwaters where municipal sewage entered via one tributary were there serious problems.

Conclusions derived from 7 years of monitoring data were:

• 1) point-source discharges affect reservoir quality worst in dry periods during the algal growing season;

• 2) nonpoint contributions of nutrients during winter and early spring wet periods are largely responsible for spring algal blooms; and

• 3) already poor reservoir quality can be degraded further by nonpoint nutrient contributions during extremely wet summers.