A geophysical study of an arsenic contaminated area in the Ron Phibun District, Southern Thailand

Sammanfattning: Geophysical surveys has been carried out in an arsenic contaminated area, the Ron Phibun District, southern Thailand, where tin and associated minerals, like arsenopyrite and pyrite, have been extracted from granites. The geophysical methods used are resistivity measurements, seismic refraction, GPR, self-potential, and magnetic susceptibility. Mining activities were started in this area for almost 10 decades ago and this together with natural processes led to contamination of arsenic in the environment. The geophysical studies have been conducted in this project with the aims to (1) create an image of underground structures and subsurface water flow, (2) to determine the distribution of arsenic contamination from the geophysical data, if a relationship between the arsenic content and the physical properties can be demonstrated. The Pole-Pole array used for resistivity pseudosection gives very good information of the subsurface geological structures. It provides information of moist and dry areas. Resistivity mapping techniques indicate a low resistivity thin layer of ca 6m to 20m depth in the western part of the study area. In the eastern part, a similar low resistivity but thicker layer is found at ca 8m to 30m depths. A high resistivity layer, interpreted as basement rocks, is found at shallow depth (10m) in the western part, while it is detected at a depth deeper than 20m in the eastern part. The basement rocks is estimated to strike in NE-SW at about 39°E, which is the boundary between the low resistivity part in the east and the high resistivity part in the west. It is interpreted as a possible fault or ridge. The eastern part is probably the main mining area that has been mined by dredging. The thick-low resistivity layer can be a track of a dredger, while the western part may have been mined at a shallower depth. When comparing the distribution of apparent resistivity and arsenic content in soil elution at 0.3 and 1m depths, there is no correlation between them. A fairly good correlation between high arsenic contents in auger water (0.5 - 5.0 mg/l) and low resistivity (25 - 100 ohm. m) is found at the depth 3.5m and 5m, which is below the groundwater table. Such a correlation is supported by the fact that the conductivity of water in which arsenic is dissolved is higher than that of pure water. However, the resistivity may only roughly reflect the distribution of arsenic content in the auger water (