Assessing the thermal environmental impacts of an groundwater heat pump in southeastern Washington State

A thermal analysis of a large-scale groundwater heat pump (GHP) installed in southeastern Washington State has been performed using a steady-state numerical modeling approach. Water temperature increases at the upgradient extraction wells in the system and at the downgradient Columbia River are potential concerns, especially since heat rejection to the subsurface will occur year-round. Hence, potential thermal impacts of the GHP were investigated to identify operational scenarios that minimized environmental impacts at the river, and temperature drift at the production wells. Simulations examined the sensitivity of the system to variations in pumping rates and injected water temperatures, as well as to hydraulic conductivity estimates of the aquifer. Results demonstrated that both downgradient and upgradient thermal impacts were more sensitive to injection flow rates than estimates of hydraulic conductivity. Higher injection rates at lower temperatures resulted in higher temperature increases at the extraction wells but lower increases at the river. Conversely, lower pumping rates and higher injected water temperatures resulted in a smaller temperature increase at the extraction wells, but a higher increase at the river. The scenario with lower pumping rates is operationally more efficient, but does increase the likelihood of a thermal plume discharging into the Columbia River. However, this impact would be mitigated by river water mixing with groundwater near the shoreline. Even though transient conditions were not considered, the approach demonstrated the potential for conflict between optimizing GHP operations and minimizing downgradient thermal impacts. The impact under current operational conditions is negligible, but future increases in heat rejection could require a compromise between maximizing operational efficiency and minimizing temperature increases at the shoreline. (C) 2011 Elsevier Ltd. All rights reserved.