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WORK PLAN 0 • FOR GROUND WATER WELL INSTALLATION AND MONITORING PORT OF STOCKTON-ROUGH AND READY ISLAND,CALIFORNIA 2.3.4 Quality of Shallow Groundwater Background ground water quality at RRI was evaluated in a study performed by the Navy (TtEMI, 2001). Based on the study results, a representative background ground water concentration was selected for RRI using the following selection scheme: • The 95 percent upper confidence level of the mean (UCL) of background concentrations in ground water at Rough and Ready Island was selected if calculated (TtEMI, 2001). • If the 95 percent UCL was not calculated (typically because there were too few detections), then the average of the minimum and maximum values measured in the background ground water study at RRI was selected. Comparing the background ground water quality with the lowest water quality objective listed in A Compilation of Water Quality Goals (Regional Board, 2008) reveals that aluminum, antimony, arsenic, cadmium, iron, lead, manganese, molybdenum, nickel, thallium, bromide, chloride, fluoride, sodium, TDS, and turbidity to be above the water quality objective. These constituents are likely present at relatively elevated concentrations as a consequence of the natural hydrogeology, geochemistry, and historical land reclamation activities in the Delta. Based on these data, background shallow ground water is not suitable as a domestic water supply without extensive pretreatment. As cited above, this observation is consistent with the Basin Plan, which anticipates water quality in the Delta to be less than the designated beneficial uses. 2.3.5 Calculating Attenuation Capacity Soil has the capacity to absorb, or attenuate, chemicals that are dissolved in water. The capacity of soil to attenuate these chemicals is a function of several complex parameters and processes. The dredge material placed at Rough and Ready Island would be placed on Merritt Silty Clay/Loam foundation soil consisting of organic rich silts with CEC values of 5meq/100g for this soil type. This is lower than measured values typical of RRI and other results from Delta Islands, but this lowest measured CEC value will be used to calculate attenuation capacity. Assessing the potential for dredge sediments to impact water resources as a consequence of reuse at the MO-1 site requires two important data. First, a reasonable estimate of the potential release of chemicals from the dredge sediment is required. September 2014 E Environmental Risk Services F�=4 Corporation .<