The objective of the project is to establish a conceptual groundwater model over the lower Nam Kam Basin in order to apply a numerical technique for the prediction of the impact of saline water transport due to the proposed weir across the Nam Kam River. Hydrogeological investigations including mapping, drilling, piezometer installations and monitoring were systematically conducted during 1997 to 1998. Brackish groundwater is saturated under the area with a depth of 30-60 m. Groundwater regionally flows from the south (the Phu Phan Range) to the north and discharges to the Nam Kam River. Another direction is from the northern region to the southern region, discharging to the central region. A two-dimensional model was constructed along the principal gradient in the NW-SE direction. There are several local recharge and discharge areas across the Nam Kam floodplain. A local groundwater flow is active within the depth of 2 m to 30 m below the ground surface within the sand and gravel unit. Simulations were calibrated with hydraulic heads and salinity of groundwater in the piezometers. It is found that the recharge and evapotranspiration rates are 1% to 40% of the rainfall and 10% to 15% of a pan evaporation, respectively. The ranges of horizontal hydraulic conductivity to vertical hydraulic conductivity are 0.1 to 0.01. The possible longitudinal dispersivity values of the hydrostratigraphic units are 20 m to 500 m, but the transverse dispersivity is less than the longitude by one order of magnitude. The comparison of calculated heads and measured heads give a root mean square error of less than 1 m. The different salinity concentrations are still in a range of 2000-5000 mg/l. Ten year simulation of saline water transport indicates that the reservoir ponding with water level at +140.5 m above mean sea level may divert groundwater flow and discharging to the northern boundary of the reservoir at Ban Don Kao.
Fractured rock aquifers provide the most extensive groundwater resources in Northeast Thailand. The hydraulic conductivity (K) of these aquifers controls the flow of water and is therefore, an essential parameter for groundwater modeling and management. K values may be directly determined by conducting pumping tests at the aquifer or by performing fracture analysis using data acquired from outcrops. The K value at outcrop should be greater than that at aquifer of deeper position due to the effect of aperture reduction by overburden compression. The goal of this study was therefor to find the correlation between K values determined from outcrop with those directly measured at an aquifer. This study was conducted on the Phu Phan sandstone aquifer at the Huay Luang watershed, Udon Thani Province of Northeast Thailand. The orientation, aperture, and spacing of fractures were measured at four outcrops and used for determining the K values by fracture analysis. Single well pump test data from 17 wells in the Phu Phan aquifer (30-120 m depth) were analyzed to obtain transmissivity (T) and K. Both sets of the K values, from outcrop and aquifer, were correlated using the plotting position of Weibull. Both plots were linear on a semi-log scale but the outcrop curve was steeper. Lognormal distributions fitted both plotting positions quite well. These results show that for the same probability value, K values found from the pumping test to be about an order of magnitude smaller than K values determined from outcrop data.
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