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 (<5m depth), because there are other properties that may also cause a decrease in the resistivity. From GPR measurements the depth to the groundwater table is obtained. It is as expected rather flat and ranges in depth from about 0.4m to1.3m. The GPR wave velocity above the groundwater table is 106.78m/ms and the average relative dielectric permittivity (er) is about 7.89. The low depth of penetration of the signal indicates a shallow (< 4m) high conductivity layer along the measurement profiles, which agrees well with resistivity data. The seismic refraction survey gives a clear picture of subsurface geological structures along the main profile. Three layers in the subsurface are suggested. The first layer is soil or weathered surface material; the thickness ranges from 1.4m to 5m and the P-wave velocities are between 374m/s and 572m/s. The second layer is clay or mud, sand (wet), floodplain alluvium, and the thickness ranges between 6.4 and 29.6m and the P-waves velocities range between 1,227m/s and 2,244m/s. The third layer is the basement rock. The P-wave velocity ranges from 2,541m/s to 3,636m/s. The depth to the third layer ranges between 9.1m and 34.2m. The variation of the P-wave velocity of the third layer along the profile indicates different types of basement rocks and the positions of contact zones. The SP data indicate moist and dry areas, which agree with the resistivity data. This information can also be used to estimate the possible occurrence of high arsenic content in groundwater. The direction of surface water or groundwater flow in the study area is determined by a streaming potential, which may be caused by water flowing from the mountain outside to the foothill within the study area. Other sources of SP anomalies are probably caused by moisture content, electrolytic content, chemical activity, diffusion potential and some minerals left over from previous mining activity. The susceptibility (median k <1,874x10-6 SI) of the surface soil in the study area represents the distribution of ferromagnetic or/and paramagnetic minerals. The large variation of the soil susceptibility is possibly caused by the mining turning the soil up side down, burning of housing wastes, covering of the sediments by flooding, and covering the original soils for plantation. No correlation between the susceptibility and arsenic content was found. This study resulted in a new and continuous image of the subsurface structures. On the basis of this an area that is suitable for digging a well free from arsenic contamination, has also be suggested.
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