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SPX CORPORATION ORDER R5-2019-0018 SPX MARLEY COOLING TECHNOLOGIES GROUNDWATER CLEANUP NPDES NO. CA0081787 d. Hardness-Dependent CTR Metals Criteria. The CTR and the NTR contain water quality criteria for seven metals that vary as a function of hardness. The lower the hardness the lower the water quality criteria. The metals with hardness-dependent criteria include cadmium, copper, chromium III, lead, nickel, silver, and zinc. This Order has established the criteria for hardness-dependent metals based on the hardness of the receiving water (actual ambient hardness) as required by the SIP' and the CTR 2. The SIP and the CTR require the use of"receiving water" or"actual ambient" hardness, respectively, to determine effluent limitations for these metals. The CTR requires that the hardness values used shall be consistent with the design discharge conditions for design flows and mixing zones'. Where design flows for aquatic life criteria include the lowest one-day flow with an average reoccurrence frequency of once in ten years (1Q10) and the lowest average seven consecutive day flow with an average reoccurrence frequency of once in ten years (7Q10).4 This section of the CTR also indicates that the design conditions should be established such that the appropriate criteria are not exceeded more than once in a three year period on average.' The CTR requires that when mixing zones are allowed the CTR criteria apply at the edge of the mixing zone, otherwise the criteria apply throughout the water body including at the point of discharge. 6 The CTR does not define the term "ambient," as applied in the regulations. Therefore, the Central Valley Water Board has considerable discretion to consider upstream and downstream ambient conditions when establishing the appropriate water quality criteria that fully complies with the CTR and SIP. Summary findings The ambient hardness for the Stockton Diverting Canal is represented by the data in Figure F-1, below, which shows ambient hardness ranging from 56 mg/L to 89 mg/L based on collected ambient data from March 2014 through September 2017. Given the high variability in ambient hardness values, there is no single hardness value that describes the ambient receiving water for all possible scenarios (e.g., minimum, maximum). Because of this variability, staff has determined that based on the ambient hardness concentrations measured in the receiving water, the Central Valley Water Board has discretion to select ambient hardness values within the range of 56 mg/L (minimum) up to 89 mg/L (maximum). Staff recommends that the Board use the ambient hardness values shown in Table F-4 for the following reasons. i. Using the ambient receiving water hardness values shown in Table F-4 will result in criteria and effluent limitations that ensure protection of beneficial uses under all ambient receiving water conditions. ii. The Water Code mandates that the Central Valley Water Board establish permit terms that will ensure the reasonable protection of beneficial uses. In this case, using the lowest measured ambient hardness to calculate effluent ' The SIP does not address how to determine the hardness for application to the equations for the protection of aquatic life when using hardness-dependent metals criteria. It simply states, in Section 1.2, that the criteria shall be properly adjusted for hardness using the hardness of the receiving water. 2 The CTR requires that, for waters with a hardness of 400 mg/L (as CaCO3), or less, the actual ambient hardness of the surface water must be used (40 C.F.R. § 131.38(c)(4)). 3 40 C.F.R. §131.3(c)(4)(ii) 4 40 C.F.R. §131.38(c)(2)(iii) Table 4 5 40 C.F.R. §131.38(c)(2)(iii) Table 4, notes 1 and 2 6 40 C.F.R. §131.38(c)(2)(i) ATTACHMENT F— FACT SHEET F-14