Groundwater Circulation in a Karstic Geothermal Reservoir

Abstract

The deep karstic aquifer containing hot water is ideal for space heating and maybe for geothermal power generation. The hot water is characterized by high single-well yield, low salinity, gravity injection and less impact on environment when it is exploited. In order to run a karstic geothermal field sustainably, it is of high importance to identify the local groundwater circulation pattern in the field. Here we take Xiongxian geothermal field as an example to get insights into the characteristics of local groundwater circulation in karstic aquifers. Groundwater samples and surface water samples were collected, and analyzed for their hydro-chemical and isotopic contents. Results show that the water type for most of geothermal water is Na-Cl-HCO3, and their TDS is between 1.9 and 2.6 g/L. This TDS is lower than that of the overlying sandstone aquifers, exhibiting a reversal in the salinity-depth gradient. According to the data of water table, the local groundwater flow direction is from Southwest to Northeast. This is confirmed by the TDS data, which increases gradually in this direction. However, this is orthogonal to the regional groundwater flow direction that is from Northwest to Southeast. This phenomenon highlights the control of aquifer lithology and geological structures on the groundwater flow field. Recharge is from far and different sources as compared to that in the overlying sandstone aquifers. Groundwater flow speed is higher in karstic aquifers due to their higher porosity and permeability. Isotopic data illustrates weak oxygen shift, although the geothermal temperature is less than 90 ℃, which also supports the concept of high flow speed.