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Excerpts on Ground Waters and7lte Cleanup • Page I I from the Water Quality Control Plan concentration will be considered to comply with the objective. Consistent with Resolution No. 68-16, the Regional Water Board will impose more stringent numerical limitations (or prohibitions) which will maintain the existing quality of the receiving water, unless, pursuant to Resolution No. 68-16, some adverse change in water quality is allowed. Maintenance of the existing high quality of water means maintenance of"background" water quality conditions, i.e.,the water quality found upstream or upgradient of the discharge, unaffected by other discharges. Therefore, the water quality objectives will define the least stringent limits which will be imposed and background defines the most stringent limits which will be imposed on ambient water quality. This Basin Plan contains numerical water quality objectives for various constituents and parameters in Chapter III. Where numerical water quality objectives are listed, these are the limits necessary for the reasonable protection of beneficial uses of the water. In many instances,the Regional Water Board has not been able to adopt numerical water quality objectives for constituents or parameters, and instead has adopted narrative water quality objectives (e.g.,for bacteria,chemical constituents,taste and odor, and toxicity). Where compliance with these narrative objectives is required(i.e.,where the objectives are applicable to protect specified beneficial uses),the Regional Water Board will,on a case-by- case basis,adopt numerical limitations in orders which will implement the narrative objectives. To evaluate compliance with the narrative water quality objectives,the Regional Water Board considers,on a case-by-case basis, direct evidence of beneficial use impacts, all material and relevant information submitted by the discharger and other interested parties, and relevant numerical criteria and guidelines developed and/or published by other agencies and organizations (e.g., State Water Board,California Department of Health Services, California Office of Environmental Health Hazard Assessment, California Department of Toxic Substances Control,University of California Cooperative Extension, California Department of Fish and Game,USEPA,U.S. Food and Drug Administration,National Academy of Sciences,U.S.Fish and Wildlife Service,Food and Agricultural Organization of the United Nations). In considering such criteria, the Board evaluates whether the specific numerical criteria,which are available through these sources and through other information supplied to the Board,are relevant and appropriate to the situation at hand and, therefore, should be used in determining compliance with the narrative objective. For example, compliance with the narrative objective for taste and odor may be evaluated by comparing concentrations of pollutants in water with numerical taste and odor thresholds that have been published by other agencies. This technique provides relevant numerical limits for constituents and parameters which lack numerical water quality objectives. To assist dischargers and other interested parties,the Regional Water Board staff has compiled many of these numerical water quality criteria from other appropriate agencies and organizations in the Central Valley Regional Water Board's staff report,A Compilation of Water Quality Goals. This staff report is updated regularly to reflect changes in these numerical criteria. Where multiple toxic pollutants exist together in water,the potential for toxicologic interactions exists. On a case by case basis, the Regional Water Board will evaluate available