2024 Volume 46 Issue 2 Pages 85-93
Open-loop ground-source heat pump systems are available at many places in Japan based on enormous groundwater resources. However, the installation has yet to be stocked due to the unpredictable risk of biological clogging in pumping and injection wells. Some field evidence based on measurements, especially in injection wells, would contribute to system designing and planning but could be more extensive in previous studies. This study reported monitoring and analysis of the clogging influence on two injection wells during four years, from 2018 to 2022, in the system installed for an official building in Sapporo City, Japan. First, this report shows annual variations in iron concentrations in the groundwater sampled from the injection wells, indicating the concentrations increased by two-order in magnitude half a year after the system started to work. Although the well screens were blushed each year, the values remained still higher by one order in magnitude. This report also performed the microbial community analysis in the well screen to reveal biological clogging by ion-oxidation bacteria. The annual clogging influence was evaluated by multiple-injection testing. The hydraulic conductivity related to an area within and nearby the well radius was determined yearly by history matching water levels in the injection wells between the observations and theoretical solutions. As a result, the logarithm of hydraulic conductivity decreased with time linearly in both two injection wells, and the gradient was steeper in the injection well with the larger intercept than that in another well, indicating an injection well in highly permeable aquifers might be influenced rapidly by biological clogging.
