แบบจำลองชั้นหินสโทแคสติคและการประเมินความไม่แน่นอนแบบจำลองน้ำบาดาลของแอ่งหาดใหญ่

Abstract

Groundwater flow modeling of the Hat Yai Basin (HYB) conducted using deterministic method based on single conceptual model may not be sufficient to obtain realistic results. While, aquifer characterization based on geostatistical method has yet to be assessed. This study presents the analysis of aquifer characteristic by using geostatistical method and development the stochastic hydrofacies model. Assessment of conceptual model uncertainties by using model averaging approach is conducted in order to select the best conceptual model for assessing groundwater safe yield. Lithologic data from borehole logs were used to analyze and reclassified into 14 lithoclasses. For the purpose on geostatistical modeling, 14 lithoclasses were simplified into three hydrofacies unit (HFU), corresponding to three permeability classes, namely; high, moderate, and low permeability. Spatial statistical analysis of three HFUS was conducted and hydrofacies models were developed based on indicator geostatistical method. Analytical result shows that the horizontal correlation length of aquifer units ranges 1,770 to 2,335m and that of aquitard unit is approximately 1,188m. The vertical correlation lengths which can be regarded as the average thickness for aquifer and aquitard units are 25-30m and 23m, respectively. The spatial dependency class results show moderate and strong dependency in horizontal and vertical directions, respectively. Stochastic hydrofacies model generated using sequential indicator simulation (SISIM) and post-processed using the categorical transformation (TRASCAT) algorithms clearly correspond to the geological conceptual model, such that three main aquifer layers, namely the Hat Yai (HY), the Khu Tao (KT), and the Kho Hong (KH) aquifers, are well preserved. The results of hydraulic conductivity (K) estimation from lithologic logs using thickness weight average method in order to describe the statistical distribution of hydraulic conductivity shows that the values hydraulic conductivity for the Hat Yai, the Khu Tao, and the Kho Hong aquifers well correspond with hydraulic conductivity values from field pumping test. The median values of the effective hydraulic conductivities are 1.70x10, 1.40x10, and 1.95x105 m/sec for the Hat Yai, the Khu Tao, and the Kho Hong aquifers, respectively. A right-skewed distribution (Positive skewed) of hydraulic conductivity are observed for the the Hat Yai, the Khu Tao, and the Kho Hong aquifers. Additionally, the median values anisotropy ratio of the Hat Yai, the Khu Tao, and the Kho Hong aquifers are 11, 10, and 12, respectively. Assessment of conceptual model uncertainty for the Hat Yai Basin by using model averaging method shows that the best conceptual model, having highest model probabilities of 67%, is model Mic. This model consists of homogeneous and anisotropic of hydraulic conductivity, general head boundary at eastern and western flank basin, no flow boundary at southern part, constant head boundary at Songkhla Lake and Gulf of Thailand, river boundary at the U-Tapoa and Rattaphum rivers, and groundwater recharge boundary on top model. An assessment of groundwater balance uncertainty result reveals that highest of model uncertainty about of about 65% is from lateral outflow. Oppositely, constant head boundary has the lowest uncertainty of 6%. These results clearly depicted an importance of conceptual model uncertainty assessment. Groundwater potential of the Hat Yai Basin has been assessed based on future increase of groundwater usage in term of a ratio of annual groundwater recharge. The result shows that groundwater safe yield of the HYB could be not over 3.5 times of present pumping rate or about 75 Mm3 per year. This safe yield can be used with confidence with no harm on both qualitative and quantitative on groundwater from the Hat Yai, the Khu Tao, and the Kho Hong aquifers over 30 years (2007-2037). For groundwater vulnerable areas which consist of zone 1 (Sing Ha Nakorn district area), zone 6A (Hat Yai municipality) and zone 7 (Vicinity of Songkhla Lake), groundwater pumping rate should be controlled within 1.5 times of present pumping rate.