EXH0644024680

Stormwater becomes water of the state when it enters natural or modified natural water bodies. Ecology, as of 1995, had not established an approved design storm for determining stormwater mixing zones. Whether the design storm approach is still adhered to is unknown. If adhered to, the recommended design storms/flows would be: a) For the acute zone, the peak flow generated by the 2-year, 72-hour event or the average flow generated by the 2-year, 2-hour storm. b) For the chronic zone, the average flow generated by the 2-year, 72-hour event (or the .9.9???.9flow generated by the 3-year, 96-hour storm). Although the 7Q10 (7-day/10-year) receiving water flow has traditionally been considered critical condition for continuous discharges, stormwater discharges are not continuous. Other possible critical receiving water conditions could be defined as: a) For the acute zone, the 1Q3 flow (since metal standards are one-hour concentrations not to be exceeded more than once every three years), which can be approximated by interpolating between the 1Q5 and 1Q2 flows. b) For the chronic zone, the 5-year mean low flow for the lowest month from August through May. - 4 Receiving water background concentration must be considered. The 90thpercentile concentration based on al/available receiving water data should be used when possible. Mixing zones might not be allowed where background concentrations already exceed water quality standards. Mixing zone size criteria exceedance may be allowed under certain circumstances. No formal process is currently available for determining whether a mixing zone would meet the prerequisites for a size…

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PSCAA Advisory Council Meeting Agenda
Add to CalendarMWednesday Mar 11, 9:00am - 11:35am
1904 3rd Avenue, Suite 105, Seattle, WA 98101
Call to order – Roll Call – Quorum Established – Land Acknowledgement We would like to acknowledge that our Agency operates in four present-day counties, which are the traditional lands of the Coast Salish people, past and present. This acknowledgement does not take the place of authentic relationships with indigenous communities in our jurisdiction and [...]