The California and National Toxics Rules describe water quality standards for seven metals that vary as a function of hardness: (1) cadmium, (2) copper, (3) chromium III, (4) lead, (5) nickel, (6) silver, and (7) zinc. Some of the methodologies used by California Regional Water Quality Board, Central Valley Region, for assignment of metal effluent limits may be inconsistent with the narrative toxicity objectives of the Basin Plan (i.e., “All waters shall be maintained free of toxic substances that produce detrimental physiological responses in human, plant, animal, or aquatic life”). The purpose of this paper is to describe a methodology for the assignment of metal effluent limits that will be protective under all dilution conditions when the effluent/receiving water hardness is less than 400 mg/L.
The California Toxics Rule is very clear that the hardness to be used when implementing Equation 1 is not to exceed 400 mg/L without determination of a corresponding water effect ratio [§131.38(c)(4)(i)]. The rationale provided is that at hardness in excess of 400 mg/L, the relationships between hardness and other important inorganic constituents (e.g., alkalinity, pH) may not correspond with the relationships present in the dilution waters used for development of the criteria. The preamble to the California Toxics Rule states that should the effluent cause the hardness to exceed 400 mg/L, then “the hardness used in the hardness equation is the hardness of upstream water that does not contain the effluent.”
The guidance provided by the California Toxics Rule is less clear under conditions whereby the hardness is less than 400 mg/L. The California Toxics Rule states that with waters with a hardness of 400 mg/L or less, the actual ambient hardness of the surface water shall be applied [§131.38(c)(4)(i)] and that the hardness values used shall be consistent with the design discharge conditions established for design flows and mixing zones [§131.38(c)(4)(ii)]. This regulatory guidance has been applied with considerable variability in California’s Central Valley Region (Region 5). For example, historical methods used to implement this guidance include use of the lowest effluent hardness (e.g., Order No. 5-01-122), use of the lowest receiving water hardness (e.g., Order No. 5-01-120), use of a variable limit making use of the actual hardness observed after the discharge and receiving water mix (e.g., Order No. 5-01-242), estimates of what might constitute a reasonable hardness (e.g., Order No. 5-00-171), or any methodology adopted by a state in implementation of standards (e.g., 85th percentile lowest hardness of the ambient receiving water; (USEPA, 1991)). The variability in the manner by which hardness is applied to Equation 1 is likely because the term “ambient” is not defined by statute and “ambient” is simply defined as “all around; surrounding; encompassing (Barnhart and Barnhart, 1980).”
Because of the non-linearity of Equation 1, some of the methodologies referenced above may be inconsistent with federal and state narrative toxicity objectives (i.e., “All waters shall be maintained free of toxic substances that produce detrimental physiological responses in human, plant, animal, or aquatic life”) because they result in the formation of toxicity after the mix of effluent into a receiving water when no toxicity may have been initially present in either the receiving water or whole effluent. In the sections that follow, it will be demonstrated that the methodology for assigning protective fixed (non-varying) effluent limitations must account for the non-linearity of Equation 1.
The purpose of this paper is to describe a methodologies for assigning fixed effluent limitations for hardness based metals that will be protective under all dilution conditions when the final mixed receiving water/effluent hardness is less than 400 mg/L, without being overly restrictive. Unless otherwise stated, the equations presented herein were developed for occasional effluent dominated conditions (i.e., an effluent discharge can constitute up to 100 percent of stream flow at times) and no use of environmental assimilative capacity (i.e., receiving water contaminant concentrations at water quality objectives prior to discharge of effluent). The methodologies can be easily modified to account for restricted ranges of fractional effluent to be present in a receiving water or to allow use of environmental assimilative capacity.