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Ground Water Injection FS -3- 10 December 1991 <br /> Marley Cooling Tower Company <br /> electrochemical treatment effluent combined with ground water; and a <br /> 25/25/50 mix of the ion exchange and electrochemical treatment effluents <br /> combined with ground water. <br /> However, these analytical methods were not used to evaluate whether the <br /> "recipient" ground water has the potential for the formation of precipitate. <br /> It is unknown whether the "recipient" ground water is undersaturated, <br /> saturated or oversaturated for the inorganic constituents of concern (i .e. , <br /> carbonates and sulfates) . Therefore, because the formation of precipitate <br /> was generally not expected with most of the mixing scenarios, it is unclear <br /> how much potential the "recipient" ground water may have for the formation <br /> of the precipitate. <br /> Page 17. A sensitivity analysis was performed for the ion concentration and for pH. <br /> This analysis was performed using the worst-case mixing scenario consisting <br /> of the highest concentration of each ion and the highest pH. As part of the <br /> sensitivity analysis, the worst-case scenario should have been performed <br /> using concentrations representative of the ground water in the area of the <br /> North Yard versus concentrations representative near the Stockton Diverting <br /> Canal . <br /> Page 18. The FS Report discusses the use of scale inhibitors, such as hydrochloric <br /> acid, as part of periodic maintenance. The use of any chemicals which <br /> potentially could impact the water quality, such as scale inhibitors or <br /> bactericides, would need to be further evaluated to define the zone of <br /> treatment around the injection wells and to determine if any supply wells <br /> could be affected by the addition of the chemicals. Potential impacts from <br /> the addition of chemicals to nearby surrounding supply wells was a concern <br /> with the Department of Health Services, as discussed in my conversation with <br /> Mr. Joe Spano on 25 October 1991. The potential impacts and the overall <br /> benefits of using these chemicals for the operation and maintenance needs to <br /> be identified. <br /> Page 22. The Injection FS Report states that two injection wells were evaluated. The <br /> Report does not state why more injection wells were not used in the FS. I <br /> concur with the use of two wells, as operation of these wells may be <br /> switched during periods of down time for maintenance. However, more <br /> injection wells properly spaced may ultimately be needed to gain the desired <br /> hydraulic affects on the plume. The Report should have indicated why two <br /> injection wells were considered. <br /> Page 22. An injection rate of 500 gpm, combined for the two wells, was selected for <br /> evaluation in the FS. It is unclear why 500 gpm was used for the <br /> evaluation. The Report acknowledges that more injection wells may be needed <br /> if the combined injection rate is less than 250 gpm each. However, Marley <br /> has requested that the NPDES permit be revised to increase flows to 700 gpm. <br /> Therefore, if flows from the treatment plants are increased to this amount, <br /> then the injection wells would be inadequate. <br /> In addition, a difference in the injection flow rate, other than at 500 gpm, <br /> could potentially affect the gradients. If the flow rate is reduced, the <br /> potential detrimental hydraulic affects on the plume at the North Yard <br /> location may be minimized, whereas if the flow rates are increased, then <br />