NPDES Permits Potentially Affecting Water Quality
Final NPDES Wastewater Permits - Issued by Oregon Department of Environmental Quality
This list consists of permit documents* (see end) made available by Oregon DEQ and is not all inclusive. All documents are PDF except where indicated.
Forest Grove Sewage Treatment Plant (STP): Final NPDES Wastewater Permit Related Documents
Permitted Mixing Zone Size for Toxicity:
Waterbody: Tualatin River at about River Mile (RM) 53.8. Wastewater discharges are not allowed during specified low flow conditions in the Tualatin River.
The NPDES Fact Sheet (p. 13) describes the primary Forest Grove Outfall as follows:
Acute Mixing Zone Radius: 10 feet in all directions from outfall pipe (a single pipe discharge is assumed because no multi-port diffuser is called out in the fact sheet).
Acute Mixing Zone Surface Area: Estimated SA is 314 square feet.
Chronic Mixing Zone Radius: 100 feet in all directions from diffuser (at least this is what the permit language allows despite the channel width being unquantified).
Chronic Mixing Zone Surface Area: Estimated SA is 2,500 square feet based on an assumed MZ width of 25 feet (i.e., SA is about 25 by 100 feet if only the downstream direction is considered and the MZ width is assumed in lieu of DEQ providing it).
Chronic Mixing Zone Volume: DEQ does not provide Tualatin River MZ depth in the fact sheet so MZ volume is not yet determinable.
"3.2. Forest Grove WTF
The Forest Grove WTF discharges treated wastewater to Tualatin River at river mile 53.8 from Outfall FOOl. This location is near the intersection of Fernhill Road and Geiger Road about 3/i of a mile south of the treatment facility. This facility is not allowed to discharge during low flow periods in the Tualatin River. Historically the permit has limited discharge based on a calendar date. This renewal proposes to incorporate actual stream flow monitoring to determine the seasonal discharge time. Using a continuous stream gauge, it is required that the permittee discontinue discharging to the river when the seven day median value of daily mean flow is less than 250 cfs at the Tualatin River at Farmington (River Mile 33.3) or July 1st at the latest. Discharge to the stream may resume anytime after September 30th provided the seven day median of daily mean flow exceeds 350 cfs. This change is proposed because it has long been recognized that the wet weather season (which greatly influences the available stream flow) is not controlled by calendar dates and varies from one year to the next. The Department is confident that with adequate streamflow available the discharge from this facility will not contribute to or cause water quality standards violations."
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