In Southern California, the majority of waterbodies are comprised primarily of wastewater treatment plant effluent and urban runoff and many of the waterbodies are concrete-lined channels. As a result, wastewater treatment plants usually do not receive dilution in the calculation of
their effluent limits and are often discharging to waterbodies with high pH and temperatures. Because of these conditions, some agencies found that compliance with ammonia objectives was going to be challenging, even with the installation of nitrification and denitrification. Consequently,
in 1999, the City of Los Angeles, County Sanitation Districts of Los Angeles County (CSDLAC), and the City of Burbank began the development of a sitespecific freshwater objective for ammonia. The chosen approach was to develop a Water Effects Ratio (WER) downstream of ten wastewater treatment
plant discharges in the Los Angeles River, San Gabriel River, and Santa Clara River watersheds. Samples were collected at ten stations, each downstream of a wastewater treatment plant. At all but one station, four acute Hyalella azteca toxicity tests (primary test) and one chronic Pimephales
promelas (fathead minnow) test (secondary test) were collected. Additionally, at five stations, a chronic Hyalella azteca test was conducted to confirm that the use of acute tests to establish WER values was appropriately conservative for the purposes of this study. The acute H.
azteca tests were conducted during both dry and wet weather to assess the impacts of different seasons on the WER. The acute H. azteca tests resulted in WERs ranging from 1.395 to 2.303. The chronic H. azteca tests demonstrated much higher WERs, ranging from 7.025 to 44.59.
Therefore, it was determined that it was conservative to use the acute tests to calculate the final WERs. The fathead minnow tests all had WERs around 1, ranging from 0.937 to 1.714. The usual application of a WER is to multiply the geometric mean of all of the WERs by the objective to obtain
the site-specific objective. As a result of the lower WERs for the fish tests, an alternative approach was developed that only adjusted the invertebrate data used to calculate the ammonia criteria and the fish data used in the criteria equation calculation were not adjusted. To calculate the
SSOs for a waterbody, a new criteria equation was developed based on EPA guidance for determining aquatic life criteria. The SSOs are all equal to the pH relationship multiplied by the lower of 1) the H. azteca value adjusted by the WER or 2) the lowest fish value. This ensures that
the SSOs are protective of both fish and invertebrates. The site-specific objectives for ammonia were adopted by the Los Angeles Regional Water Quality Control Board as an amendment to the Los Angeles Region Water Quality Control Plan on June 7, 2007.
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