Multivariate analysis of groundwater quality and modeling impact of ground heat pump system

The ground source heat pump system (GSHP) has recently become a popular building heating or cooling method, especially in North America, Western Europe, and Asia, due to advantages in reducing energy consumption and greenhouse gas emission. Because of the stability of the ground temperature, GSHP can effectively exchange the excess or demand heat of the building to the ground during the building air conditioning in the different seasons. The extensive use of GSHP can potentially disturb subsurface soil temperature and thus the groundwater quality. Therefore the assessment of subsurface thermal and environmental impacts from the GSHP operations is necessary to ensure sustainable use of GSHP system as well as the safe use of groundwater resources. This study aims to monitor groundwater quality during GSHP operation and to develop a numerical model to assess changes in subsurface soil temperature and in groundwater quality as affected by GSHP operation. A GSHP system was installed in Fuchu city, Tokyo, and consists of two closed double U-tubes (50-m length) buried vertically in the ground with a distance of 7.3 m from each U-tube located outside a building. An anti-freezing solution was circulated inside the U-tube for exchanging the heat between the building and the ground. The temperature at every 5-m depth and the groundwater quality including concentrations of 16 trace elements, pH, EC, Eh and DO in the shallow aquifer (32-m depth) and the deep aquifer (44-m depth) were monitored monthly since 2012, in an observation well installed 3 m from the center of the two U-tubes.Temporal variations of each element were evaluated using multivariate analysis and geostatistics. A three-dimensional heat exchange model was developed in COMSOL Multiphysics4.3b to simulate the heat exchange processes in subsurface soils. Results showed the difference in groundwater quality between the shallow and deep aquifers to be significant for some element concentrations and DO, but insignificant for pH, EC and Eh. The differences may be attributed to difference in retention periods between the two aquifers. High correlations were observed among Li, B, As, Se and Si. The multivariate analysis classified the trace elements into major groups representing different hydrochemical characteristics in the groundwater. After six-month operation of the GHSP, the temperature in the soil profile in the observation well increased about 0.2 - 0.5 oC. Although the change in temperature is fairly small, the numerical model still well reproduced the observed temperature patterns and the temperature changes in the soil profile. The change in groundwater quality was non-significant before and after operating the GSHP for six months, but further investigations are needed to assess impact of longer-term GSHP operation periods.