A public-domain software package is available to aid engineers in the design of water treatment systems for Aquifer Thermal Energy Storage (ATES). Geochemical phenomena that cause problems in ATES systems include formation of scale in heat exchangers, clogging of wells, corrosion in piping and heat exchangers, and degradation of aquifer materials. Preventing such problems frequently requires employing water treatment systems. Individual water treatment methods vary in cost, effectiveness, environmental impact, corrosion potential, and acceptability to regulatory bodies. Evaluating these water treatment options is generally required to determine the feasibility of ATES systems. The H2OTREAT software was developed by Pacific Northwest Laboratory for use by engineers with limited or no experience in geochemistry. At the feasibility analysis and design stages, the software utilizes 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 whether that mineral is oversaturated, hence may precipitate at the specified temperature. The saturation indices of key calcium, iron, silica, and manganese carbonates, oxides, and hydroxides (calcite, rhodochrosite, siderite, Fe(OH)3[a], bimessite, chalcedony, and SiO2) are calculated. User input is separated into “required” and “optional” data (Table 1). Currently, H2OTREAT does not perform cost calculations; however, treatment capacity requirements are provided. Treatments considered include 1) Na and H ion exchangers and pellet reactors to avoid calcite precipitation, and 2) in situ nitrate addition and cascade precipitators to prevent Fe and Mn oxide precipitation. The H2OTREAT software also provides the user with guidance on other geochemical problems that must be considered, such as SiO2 precipitation, corrosion, and environmental considerations. The sodium adsorption ratio and sodium hazard are calculated to evaluate the likelihood of clay swelling and dispersion caused by high Na concentrations (e.g., as a result of Na ion exchange water treatment used to prevent calcite precipitation). H2OTREAT is available for DOS and UNIX computers.