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, pesticides, siltation, flow alteration, and low dissolved oxygen.
Agricultural production is the predominant source of impairment in lakes.
Other sources include urban runoff, land disposal, septic tanks, and natural sources. In rivers and streams, agriculture is the major source of water quality degradation, followed by industrial point sources and hydrohabitat modification. Causes of water quality impairment from these sources include thermal modification, pathogen indicators, and ammonia. The highest priority ground water issues in Washington are nitrates, pesticides, and other agricultural chemicals from fertilizer applications, pesticide applications, and septic tanks. Washington provides financial incentives to encourage compliance with permit requirements, the principal vehicle for regulating point source discharges.