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5. BASIS OF PLANNING The mercury in the Willamette River system is believed to originate from natural volcanic and mineral sources and legacy mining wastes and atmospheric deposition in the fiver headwaters. The MWMC has monitored its influent and effluent for mercury for many years. The facility does not use, store, treat or discharge mercury in significant amounts and should not impact the water quality status for that pollutant. Arsenic Based on data from the regional wastewater program's ambient water quality monitoring program, DEQ has included the Willamette River in Eugene-Springfield as water quality limited for arsenic, based on exceedances of the current human health criterion of 0.002 pg/L. As mentioned above, DEQ is expected to adopt new criteria for toxic substances in the near future, and this criterion is expected to be revised upwards to 0.014 gg/L. The ambient monitoring data2 indicate that the river would continue to be listed if the criterion is revised upwards. This arsenic in the river is believed to be from natural sources. Arsenic is present in rock formations in Lane County, and in some areas arsenic is commonly fotmd in groundwater. A TMDL for arsenic will be developed but is not on DEQ's current TMDL schedule. Other Parameters Data from the Eugene-Springfield ambient monitoring program indicate no exceedances of other water quality criteria at any of the Willamette River monitoring sites, including e. coIi and metals2. Nutrient concentrations do not approach levels of concern. Ammonia-nitrogen is typically less than 0.1 mg/L. Nitrate+nitrite nitrogen and total phosphorus concentrations do show a measurable increase downstream of the WPCF discharge. For example, average nitrate+nitrite - N concentrations increase from 0.02 mg/L to 0.11 mg/L, and total phosphorus from 0.04 mg/L to 0.09 mg/L2. Dissolved oxygen (DO) concentrations in the Eugene-Springfield area and downstream of the WPCF discharge meet applicable criteria for cold water uses. If the desibmation of the Willamette River in Eugene-Springfield as a salmon and steelhead spawning area from October 15 to June 15 remains in effect, the corresponding DO criterion would be 11.0 mg/L. Monitoring data indicate that the DO concentrations are frequently below this criterion. Summary Willamette River water quality in the Eugene-Springfield area generally exceeds applicable water quality criteria, with the principle exception of temperature. The background water quality does not create any significant issues related to the treated wastewater discharge, both at current and future planned flows. 2 City of Eugene, 2003. City of Eugene Storrnwater Annual Report. May 2003. MWMC_5.0_REVS_VKS.DOC 5-25