PA-013:

Effect of temperature change on microbial community structure in groundwater during long-term thermal loading experiments

Ground source heat pump system utilizes groundwater with relatively constant temperature as heat source and heat sink. In addition to such technologies, widespread subsurface utilization and urban heat-island phenomena as well as the global warming are expected to cause the change of subsurface temperature (up to 100 m in depth). Although it is well expected that such temperature changes would affect geological structure, groundwater quality, and subsurface microorganisms, little has been understood as to what would actually happen. In this study, we investigated the archaeal and bacterial community structures in groundwater during long-term thermal loading experiment.
For the experiment, warm water (40_C, 40 L/min) was circulated in a 50 m-deep U-tube, which was installed in the campus of Saitama University, for 13 months. Subsequently, the site was kept under natural cooling condition for 14 months. Groundwater was collected every 1_2 weeks from two confined aquifers (GL -17 m, -39 m) in monitoring wells installed at 1 (W1), 2 (W2), 5 (W5) and 10 m (W10) distances from the U-tube. The 16S rRNA genes based quantifications and community structure analyses for Bacteria and Archaea were carried out.
As the result of thermal loading, groundwater temperature was elevated by up to 7_8_C in W1, while there was almost no change in W10. 16S rRNA gene copy numbers of Bacteria and Archaea fluctuated between 5.3×10³_3.2×10⁷ and 2.7×10_1.4×10⁵ copies/mL, respectively, during both heating and cooling period, but no effect of temperature change was observed. PCR-DGGE and MiSeq analyses showed that species compositions of Bacteria and Archaea gradually changed in all wells. Among them, some anaerobic bacteria increased in accordance to groundwater temperature increase, while some aerobic bacteria inversely decreased. These results suggest that subsurface temperature elevation caused anaerobic environment in groundwater at the experimental site.

keywords:Microbial community in groundwater,Subsurface temperature change,Ground source heat pump,16S rDNA amplicon analysis