ENVIRONMENTAL BENEFITS OF AN AGING WASTEWATER TREATMENT SYSTEM UPON WETLANDS AND INSTREAM FLOW

Discharge from wastewater treatment systems may beneficially support wetland habitat and stream baseflow. The City of Warden in arid central Washington (annual precipitation=10 inches per year) operates a 1960’s-era food processing wastewater treatment system consisting of lagoon treatment and sprayfield irrigation. Process water derives from 400-foot deep municipal supply wells completed in the underlying basalt aquifer. The unlined lagoons overlie unconsolidated and lithified (caliche) sediment and basalt aquifers that contain potable groundwater for domestic and municipal supply. Nitrate and TDS concentrations in the influent decline along the wastewater treatment pathway. Additional makeup water blended with the effluent reduces nitrate and TDS concentrations before spray discharge.

The Washington State Department of Ecology requires a demonstration that lagoon leakage does not impact groundwater quality; otherwise the City of Warden would have to line the treatment system lagoons and construct additional sprayfields, an option that the City could not afford and could result in plant closure. Site evaluation included characterizing the local water balance, the rates of nutrient loading and reduction, and the vertical leakage rates from the lagoons. Low permeability caliche adequately protects the water supply basalt aquifer. Excess effluent water not discharged to the spray system discharges vertically to the shallow subsurface and laterally through seeps to a local stream. The net result of the treatment system operation transfers deep, isolated groundwater from basalt aquifers to beneficially support wetlands downgradient of the wastewater system and supplements approximately 0.5 cfs of baseflow to the stream. Lining the lagoons would result in transferring much of the process water to evapotranspiration, a loss of the wetland, and reduction of instream flow.