Assessment of hydrogeological and water quality parameters, using MRS and VES in the Vientiane Basin, Laos

Sammanfattning: Water is essential for all life on the planet, sustaining and ensuring the earth's ecosystem. Groundwater from a global perspective provides about 50% of the potable water supplies, 40% of the industrial water and 20% of the irrigated agriculture. For drinking water, deep groundwater has many advantages compared to surface water and shallow groundwater, since it demands little or no treatment and the access is secured against temporary droughts. However, salinity in deep groundwater is common in coastal areas and in areas where rock salt is occurring naturally in the soil and bedrock. Drilling wells is expensive and unprofitable if it is made without knowing the groundwater potential and the location of salt affected groundwater. Vertical Electrical Soundings (VES) has been widely used to characterize aquifers and to identify salt affected water since there may be a relation between the conductivity of the aquifer and the salinity of the water. However, it is not possible to distinguish high conductive groundwater from e.g. increased clay content. Magnetic Resonance Sounding (MRS) gives a direct image of the water content of the ground and hence the vertical distribution of an aquifer. By combining MRS with VES, salt affected groundwater could be distinguished from high conductive sediments and freshwater aquifers. The aim of this study is to define and characterize water bearing geological formation and to test the possibility of using geophysical techniques to determine hydrogeological parameters and water quality parameters relating to salinity in the Vientiane basin, Laos. The investigation area is part of the Khorat Plateau where halite is naturally occurring as shallow as 50 m in depth in the Thangon Formation. In total, 32 sites and 28 sampled wells, located in three different areas were investigated. MRS and VES recognized the stratigraphic unit N2Q1-3, consisting of alluvial unconsolidated sediments, as the main water bearing unit. The aquifer thickness varies between 10 to 40 m and the depth to the main aquifer range from 5 to 15 m. The water content is here relatively high, up to 16 % and decay times varying between 100 and 400 ms, suggest a mean pore size equivalent to medium sand to gravel. The resistivity is highly variable, but usually around 10-100 ohm-m, which suggests fresh water. Hydraulic and storage related parameters such as transmissivity, hydraulic column, specific yield and specific storage have been estimated from MRS and transverse resistance has been estimated from VES. MRS together with VES has also shown to be a useful and important tool in identifying the salt related clay layer of the Thangon Formation. This layer is characterized by low water content and a resistivity lower than 5 ohm-m. This formation can be found in all 3 areas on depth of 20 to 50 m. Several approaches have been used to look for a relation between the water conductivity collected from wells and the aquifer conductivity determined from VES, including Archie's law. The best correlation is achieved using a linear fit between VES conductivity and water conductivity for deep wells (EC[μS/cm ]= 0.3821VES[μS/cm], R2=0.81). The conductivity of water from shallow wells does not show any relation to VES conductivity (R2=0.09). This is probably because deep wells usually have higher conductivity than shallow wells and hence contributing more to the aquifer conductivity. Interpolated maps of TDS in area 3 from deep wells together with maps of the conductivity of the bottom layer and aquifer layer, MRS transmissivity and hydrostatic column indicates that the salt originate from the underlying layer situated beneath the main aquifer. The most important water quality parameters influencing the water conductivity are TDS, hardness and chloride, which all have a high correlation to the EC of water and thus VES conductivity. This makes MRS and VES a very promising tool for guidance of future drillings and water quality estimation.