This manual addresses the use of a public-domain software package developed to aid engineers in the design of water treatment systems for aquifer thermal energy storage (ATES). The software, H20_TREAT, which runs in the DOS or UNIX Environment, was developed by the Pacific Northwest Laboratory and targeted to engineers possessing limited or no experience in geochemistry. To do this, the software provides guidance on geochemical phenomena that can cause problems in ATES systems (i.e., the

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... on of scale in heat exchangers, clogging of wells, corrosion in piping and heat exchangers, and degradation of aquifer materials causing a reduction in permeability). Preventing such problems frequently requires the use of water treatment systems. Because individual water treatment methods vary in cost, effectiveness, environmental impact, corrosion potential, and acceptability to regulators, proper evaluation of treatment options is required to determine the feasibility of ATES systems. The software is available for DOS-and UNIX-based computers. It uses a recently revised geochemical model, MINTEQ, to calculate the saturation indices of selected carbonate, oxide" and hydroxide minerals based on water chemistry and temperature data provided by the user. The saturation index of a specific mineral defines the point at which that mineral is oversaturated and hence may precipitate at the specified temperature. Cost calculations are not performed by the software; however, treatment capacity requirements are provided. Treatments include Na and H ion exchanger, fluidized-bed heat exchanger or pellet reactors, and CO 2 injection. The H20_TREAT software also provides the user with warning of geochemical problems that must be addressed, such as Fe and Mn oxide precipitation, SiO 2 precipitation at high temperatures, corrosion, and clay swelling and dispersion. iii ACKNOWLEDGMENTS