Assessment of surface-water quality and water-quality control alternatives, Johnson Creek Basin, Oregon
Johnson Creek flows through a basin of approximately 51 square miles with mixed land uses over a reach of approximately 24 river miles from southeast of Gresham, Oregon, to its confluence with the Willamette River in Milwaukie, Oregon. Land uses within the basin include forested and agricultural lands, suburban residential, urban, and light industrial. Surface runoff and ground-water flow from the basin's areas of various land-use contain concentrations of some nutrients, trace elements, and organic compounds at levels exceeding U.S. Environmental Protection Agency (USEPA) criteria. Concentrations of dissolved cadmium, copper, lead, mercury, and silver, total recoverable chlordane, dieldrin, and dichlorodiphenyltrichloroethane (DDT) plus metabolites indicate that sources of at least one or more of these constituents exist in virtually every reach of Johnson Creek. Crystal Springs Creek is a major source of nutrients in lower Johnson Creek. Concentrations of dissolved nitrate and orthophosphorus in Johnson Creek are elevated at low flow, and are reduced by dilution when urban runoff flows into the creek during storms. Total-phosphorus concentrations exceed USEPA criteria at several sites in Johnson Creek during low flow, and at all sites during periods of storm runoff. The low-flow concentration of dissolved silver exceeded the USEPA Fresh Water Chronic Toxicity (FWCT) criterion only in Crystal Springs Creek. Concentrations of dissolved cadmium, copper, lead, and mercury exceeded FWCT criteria at selected sites in Johnson creek basin during storm runoff.