Interpreting Narrative Objectives for Biostimulatory Substances for California Central Coast Waters

This technical paper describes an approach for interpreting the 1994 California Central Coast Water Quality Control Plan (Basin Plan) narrative language stating that “waters shall not contain biostimulatory substances in concentrations that promote aquatic growths to the extent that such growths cause nuisance or adversely affect beneficial uses.” In this approach, Central Coast Water Board staff employed Basin Plan Objectives, U.S. Environmental Protection Agency (U.S. EPA) standards, guideline values from the literature, our own monitoring data, and modeled estimates of potential algal growth and resultant oxygen deficits. The resulting numeric endpoints can be used for regional water quality assessments and to support assessment decisions for the California Integrated Report for addressing Clean Water Act Sections 303(d) and 305(b). To conduct this analysis, we have relied heavily upon data collected by the Central Coast Ambient Monitoring Program (CCAMP). CCAMP conducts monitoring for the Central Coast Water Board and is the Central Coast regional component of the California Surface Water Ambient Monitoring Program. CCAMP data can be viewed at www.ccamp.org. We identified a pool of long-term monitoring locations, or “sites”, from the extensive CCAMP dataset that have always met either warm or cold water oxygen objectives based on both monthly grab samples and 24-hour continuous monitoring. From this dataset, we identified an upper range for dissolved oxygen concentration of 13 milligrams per liter (mg/L), over which site oxygen concentrations rarely or never fell. We established 13 mg/L as an upper limit for oxygen, to address the U.S. EPA ‘Gold Book’ (1986) water quality standard for excessive gas saturation. We identified a reference subset of the initial set of sites that showed no other signs of eutrophication, such as oxygen levels over 13 mg/L, water column chlorophyll a exceeding 15 micrograms per liter (ug/L) or observed floating algal cover exceeding 50%. We examined nutrient characteristics of data from this reference set to identify a proposed screening criterion of 1.0 mg/L nitrate as nitrogen (mg/L NO3-N) to protect aquatic life. This number represents the 95