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Does Treatment Plant Effluent Constitute a Municipal and Domestic Supply (MUN)? A UAA for a MUN Use in California

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Like many states, California began the process of designating beneficial uses as directed by the Clean Water Act (CWA) and Code of Federal Regulations (CFR) over 30 years ago. In an attempt to meet the CWA goals, waterbodies were often assigned designated uses without consideration for the existence or attainability of the uses. 40CFR131.10(g) provides guidance on how to revise or remove designated uses that are not existing and not attainable through a Use Attainability Analysis (UAA). However, UAAs often pose significant challenges to states and local agencies that attempt to address issues raised by designations of uses for waterbodies as demonstrated by the application of the UAA process to San Luis Obispo (SLO) Creek in California.

SLO Creek, located along the central coast of California, is designated as having an existing Municipal and Domestic Supply (MUN) use. The City of San Luis Obispo's Water Reclamation Facility (WRF) has been discharging to the creek since the 1940s and currently discharges approximately 5 million gallons per day (MGD) of highly treated effluent to SLO Creek During the dry summer months of August and September, up to 95% of the flow in lower SLO Creek is comprised of treated wastewater. As such, the predominate source of water to SLO Creek during much of the year is WRF effluent. Additionally, the WRF has a mandatory dedication of a minimum discharge to SLO Creek of 2.5 cubic feet per second (cfs) for in-stream beneficial uses, in-stream habitat uses in particular. The interest in maintaining a 2.5 cfs discharge was in recognition that SLO Creek is effluent-dependant and effluent water quality is suitable for in-stream beneficial uses including (but not limited to) habitat for federally protected steelhead.

In 2005, a Nitrate TMDL was adopted for SLO Creek to protect the MUN use. The WRF was assigned a Waste Load Allocation (WLA) equal to the MUN objective of 10 mg/L nitrate as N.Meeting the WLA would require the WRF to upgrade treatment to include denitrification as the WRF currently nitrifies to address unionized ammonia. However, even with an upgrade to the WRF, estimated to be approximately 30 million dollars, the California Department of Public Health (CDPH) would not allow lower SLO Creek to be used as a municipal source. CDPH is concerned that even if nitrate concentrations meet the WLA, the predominate source of water to lower SLO Creek is effluent and contains other constituents, detectable or not, that could be detrimental to public health. As such, the question was raised: Why expend limited public resources to treat effluent to protect a use of water that a State agency implementing State and federal drinking water laws would not allow to occur? So began the UAA process for de-designating the MUN use of lower SLO Creek.

As required by 40CFR131.10(g), the City embarked on an evaluation of 1) whether the MUN use is existing and 2) whether the MUN use is attainable. In terms of determining whether the use was existing, the City evaluated available land use records, conducted field surveys, distributed questionnaires, and evaluated water quality data. To determine attainability, the City considered the six factors outlined in 40CFR131.10(g) as they relate to lower SLO Creek and the interplay between the City's commitment to protect steelhead habitat and meet the requirements of the MUN designation.

The focus of the paper and presentation will be to detail the approach utilized by the City, the process of evaluating alternatives to de-designation including consideration for developing sitespecific objectives, as well as the outcome of the efforts.
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Keywords: De-designation; Designated Uses; Use Attainability Analysis

Document Type: Research Article

Publication date: 2011-01-01

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