An integrated magnetotelluric and gamma exploration of groundwater in fractured granite for small-scale freshwater supply: a case study from the Boshan region, Shandong Province, China

Abstract

Exploration and use of groundwater in hard rocks is a difficult but yet a significant issue, especially when the local surface water source is polluted. In this study, integrated geophysical and nuclear methods are used to delineate groundwater-bearing zones in a granite terrain in the Boshan region, Shandong Province, China. Structural analysis showed that fractures extend in a NW–SE direction, which is the same as the direction of river valley. A controlled source audio-frequency magnetotelluric survey conducted along the river valley indicated several different low resistivity anomalies or potential water-bearing fracture zones. They are seated at depth ranges down to 400 m from the surface along the river valley. The locations of deep water-bearing zones were further confirmed by higher gamma radiation measurements found near the river valley. A fault with a NW–SE strike near the river was deduced as an anomalous water-bearing zone. Two tube wells were drilled targeting at the water-bearing zone that has achieved single-well water yield greater than 360 m3/day. The well water was sampled for water chemistry and stable isotope analyses. Shallow groundwater samples show similar water chemistry and isotopic feature to river water, indicating connection between the two. Deep groundwater from the water-bearing fracture zones is not hydraulically connected with shallow groundwater, ensuring safety of the deep groundwater in the granite from being affected by the polluted shallow groundwater or river water. Integrated geophysical and gamma exploration methods helped to identify two disconnected groundwater systems, offering an alternative freshwater source for the region if shallow groundwater or surface water is polluted.