All surveyed lakes partially support swimming use. Low levels of dissolved oxygen, often naturally occurring, are the major cause of impairment of designated uses in estuaries. Bacterial contamination, primarily from agricultural runoff, onsite wastewater disposal, and municipal wastewater treatment plants also causes impairment in estuaries. Major causes of impairment in lakes include nutrients and noxious aquatic plants.
Agricultural production is the predominant source of impairment in lakes.
Other sources include urban runoff, municipal point sources, land disposal, construction runoff, and natural sources. In rivers and streams, agriculture is the major source of water quality degradation, followed by hydro-habitat modification, natural sources, and municipal point sources. Causes of water quality impairment from these sources include thermal modification, pathogen indicators, pH, and low dissolved oxygen. Ground Water Quality Washington reports ground water contamination by metals, trace elements, nitrates, pesticides, petroleum, and synthetic organic chemicals. Sources include industrial activities, agriculture, municipal wastewaters, mining, and onsite sewage systems.