Abstract
The Orange County Sanitation District (OCSD) is a public agency that provides treatment of wastewater for residents in Orange County, California. The final effluent characteristics at OCSD have been altered in the last 18 years due to the ~20-fold increase of sodium hypochlorite usage for disinfecting waste streams from 2002 to 2010, implementation of the Ground Water Replenishment System (GWRS) in 2008, and conversion to full secondary treatment, together with a major reduction of sodium hypochlorite usage in 2011. This study analyzed infauna data gathered from 1994 to 2016 at the zone of initial dilution (ZID) and reference stations located on the San Pedro Shelf to assess the effects of these treatment process changes on biota in the receiving water. Impacts to the infauna community within the ZID were observed during the period of high chlorine usage, including the increased proportion of the pollution tolerant polychaete Capitella capitata complex from 6.2% in 2003 to nearly 60% in 2010, the lowest Infaunal Trophic Index score in 2009, and the highest Benthic Response Index score in 2006. The degradation of the infauna community in the ZID was correlated with chlorination and was coincidental with the initiation of the GWRS. The infauna community at the ZID recovered rapidly after the implementation of full secondary treatment as indicated by the significant reduction of the relative abundance of C. capitata complex from 59.8% in 2010 to less than 0.1% after 2012, and by the markedly improved community health index scores. This study demonstrated the composition and biointegrity of the infauna community at the ZID varied in response to changes in the wastewater treatment process. Caution should be exercised at wastewater treatment plants when relatively higher dosages of sodium hypochlorite are used over a multi-year period, as this may negatively impact aquatic biota in the receiving water.
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