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t �Y protect the beneficial use with the most stringent numeric water quality standards will protect all other uses Below, site-specific water quality standards and goals are developed 3.1.1 Water Quality Protection Standards In general, water quality protection standards focus on protecting the existing water quality whenever that water quality is better than that required to protect all present and potential beneficial uses (Resolution 68-16) Numeric water quality standards based on Resolu- tion 68-16 are associated with the background levels, which in turn are subject to the reason- able limit of detection for the residual constituent of concern As indicated in Section 2 2 1, the constituents of concern are TPH-g and BTEX compounds Parameters mentioned in the Basin Plan that are not relevant to the development of water quality protection standards for the site are pH, electric conductivity, total dissolved solids, chloride, total petroleum hydro- carbons as kerosene, total oil and grease, metal contaminants, and volatile organics These parameters are either (1) not found beneath the site (i e , total oil and grease), (2) not asso- ciated with impact identified beneath the site (i e , pH, total dissolved solids), or (3) naturally occumng (i e , metals) Reasonable limits of detection for the residual target compounds found beneath the site are shown below Concentration (. Compound arts per billion Benzene 05 Toluene 05 Eth lbenene 05 Xylenes 05 TPH-g and TPH-d 50 Where groundwater degradation has occurred, Resolution 68-16 may not strictly apply This f is because the existing water quality within the residual plume boundary does not warrant protection, m other words, the residual plume water atiality is not better than that re qutred to protect all other beneficial water uses It seems reasonable to presume that protection, the o j ive o esolution 68-16, pertains only tow her ume boundary The ramification of this interpretation is that flow (or discharge) of groundwater across the residual plume boundary with residual concentrations greater than those identified above, is not consistent with Resolution 68-16 (or site water quality protection standards) Locating the exact position of the residual plume boundary is difficult, consequently, it is proposed that a compliance boundary be established The compliance boundary will encom- pass the residual plume, and allow for a reasonable downgradient attenuation zone The water quality standards will apply to groundwater outside the compliance boundary This feature is consistent with State and Regional Board policy regarding establishing a WQLZ in `.ie3201337BI1918REV 10 August 31, 1995-