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Section 5 <br />Both in the lead up to and then after the onset MBI-1 injection operations, <br />groundwater quality data has been collected from down gradient monitoring wells <br />SAR-10 and SAR-11 1 to track the fate and transport of recycled water, as well to <br />monitor post-injection water quality. In 2015, monthly sampling of monitoring well <br />SAR-10 occurred in the lead up to MBI-1 injection operations, with biweekly <br />sampling commencing thereafter, Quarterly sampling occurred at SAR-1 1 until <br />late May 2015, after which bi-weekly sampling began. <br />Chloride is a conservative tracer, and thus the arrival of GWRS water at a <br />monitoring well can be inferred from the progressive change in quality with each <br />well zone by comparing the pre-injection concentration (-13-18 mg /L, depending <br />on the specific zone) to the GWRS injection supply (-10 mg/L currently). Sulfate <br />is typically considered less conservative in the subsurface, but features a greater <br />difference between the background concentration (-39-44 mg/L) as compared to <br />the current GWRS injection supply (-1 mg/L), Both chloride and sulfate show the <br />clear arrival of GWRS water at all zones, first at SAR-1 0 (ranging between -1 -4 <br />weeks, depending an the zone) and then at SAR-1 1(-6-12+ weeks). <br />The mobilization of arsenic from aquifer sediments has been observed at some <br />locations down gradient of GWRS water injection at the Talbert Seawater <br />Intrusion Barrier and surface recharge in the Anaheim Forebay area. Arsenic <br />and chloride trends from monitoring wells SAR -10 and SAR-1 I were evaluated to <br />assess the potential for arsenic mobilization due to injection at MBI-1. At SAR- <br />10, the pre-injection background arsenic concentration ranged from non-detect <br />(ND; less than 1 ug/L) to 2.5 ug/L', with the arrival of GWRS water, <br />concentrations have increased to as high as 6.5 uglL. SAR-1 0 zones 2 and 4 <br />appear to have reached peak arsenic concentrations of 3-0 ug/L and 3,4 ug/L <br />before beginning a downward trend, The Maximum contaminant level (MCL) <br />drinking water limit for arsenic is 10 ug/L. <br />Vanadium is regulated as a notification level (NL) and response level (RL) <br />contaminant in drinking water by the California State Water Resources Control <br />Board's Division of Drinking Water (DDW). The NL is 50 ug/I and the RL is 500 <br />ug/L. Public water system well owners are required to notify the local governing <br />boards (e.g., city council) when a well exceeds the NIL, but the well may remain <br />In service. If the RL is exceeded, then DDW recommends the well be taken out <br />of service. <br />Minor mobilization of vanadium has been previously observed with the arrival of <br />GWRS water in the subsurface at small number of monitoring wells, but with all <br />increases being temporary and remaining well below the 50 ug/L NL Vanadium <br />trends from monitoring well SAR-1 0 were evaluated. The SAR-1 0/1 zone <br />increased from a background concentration of 4-5 ug/L to a maximum of 27,3 <br />LJg/L with the arrival of GWRS water, before subsequently declining to 16,2 ug/L. <br />qW -24;Project Final EIR 5-112 <br />Mid Basin Centennial Park gp6n ( <br />