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.
Rock Creek Sewage Treatment Plant (STP): Final NPDES Wastewater Permit Related Documents
Permitted Mixing Zone Size for Toxicity:
Waterbody: Tualatin River at about River Mile (RM) 37.7 about 200 feet downstream of the confluence with Rock Creek.
The NPDES Fact Sheet (p. 14) describes the primary Rock Creek Outfall Diffuser as follows:
Acute Mixing Zone Radius: 10 feet in all directions from the diffuser.
Acute Mixing Zone Surface Area: Estimated SA is 874 square feet based on a diffuser length of about 28 feet.
Chronic Mixing Zone Radius: 100 feet in all directions from diffuser (at least this is what the permit language allows despite the channel being only 75 feet wide).
Chronic Mixing Zone Surface Area: Estimated SA is 2,800 square feet based on a diffuser length of about 28 feet (i.e., SA is about 28 by 100 feet if only the downstream direction is considered).
Chronic Mixing Zone Volume: DEQ does not provide Tualatin River MZ depth in the fact sheet so MZ volume is not yet determinable.
"3.4. Rock Creek AWTF
The Rock Creek AWTF (Outfall R001) is located at river mile 37.7 and is approximately 200 downstream of the confluence of Rock Creek. The diffuser pipe is located on the bottom of the river in the center of the channel. The Tualatin River is about 75 feet wide at this location. The diffuser takes up about 1/3 of the channel and consists of 12 ports spaced at 2.5 foot intervals along to outfall pipe. The new wet weather outfall pipe (Outfall R003) is a single port outfall from the 96-inch pipe on the north river bank about 50-feet downstream of Outfall R001."
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