Evaluation of Analytical and Numerical Techniques for Defining the Radius of Influence for an Open-Loop Ground Source Heat Pump System

In an open-loop groundwater heat pump (GHP) system, groundwater is extracted, run through a heat exchanger, and injected back into the ground, resulting in no mass balance changes to the flow system. Although the groundwater use is nonconsumptive, the withdrawal and injection of groundwater may cause negative hydraulic and thermal impacts to the flow system. Because GHPs are a relatively new technology and regulatory guidelines for determining environmental impacts for GHPs may not exist, consumptive-use metrics may need to be used for permit applications. For consumptive-use permits, a radius of influence is often used, which is defined as the radius beyond which hydraulic impacts to the system are considered negligible. In this paper, the hydraulic radius of influence concept was examined using analytical and numerical methods for a nonconsumptive GHP system in southeastern Washington State. At this location, the primary hydraulic concerns were impacts to nearby contaminant plumes and a water supply well field. The results reported in this paper show that distance drawdown methods for both analytical and numerical methods were generally unsuitable because they overpredicted the influence of the well system. Particle tracking yielded more reasonable results because flow paths demonstrated the probable impact on the flow system. In particular, the use of a capture zone analysis was identified as the best method for determining potential changes in current contaminant plume trajectories, which could be performed with both analytical and numerical techniques. Capture zone analysis is a more quantitative and reliable tool for determining the radius of influence with a greater accuracy and better insight for a nonconsumptive GHP assessment.