EVALUATION OF RIVERBANK FILTRATION CAPACITY BASED ON HYDROGEOLOGICAL CHARACTERISTICS IN DEASAN-MYEON, KOREA

Source water quality in the lower reach of the Nakdong River is distinctly lower than in its upper section. Changwon City, the capital of Gyeong-Nam Province, located in the lower reach of the Nakdong River operates two riverbank filtration sites to provide municipal water to residents. On one of the two sites, water balance analysis and groundwater flow modeling are used to estimate riverbank filtration capacity. Pumping tests and grain size experiments are conducted to estimate hydrogeological properties required for numerical modeling. The riverbank geology is predominately near-surface layered sand overtopping a highly conductive sand/gravel unit and moderately conductive weathered andesitic tuff base layer. Based on grain size analyses, the sand/gravel layer (11.2m in thickness) acts as the main aquifer with an average hydraulic conductivity of 1.2°¿10^-3 ms^-1, and the remaining sand has a hydraulic conductivity of 10^-4ms^-1. Pump tests reveal that transmissivity is on the order of 10^-3-10^-2m²s^-1, and storativity ranges from 10^-3 to 10^-4. Since the total thickness of saturated layer is less than 40m, estimates of hydraulic conductivity are in the order of 10^-4ms^-1, in agreement to those estimated using an empirical relation to grain size. Direct runoff was estimated using the SCS-CN method, baseflow was computed from data of the Nakdong River basin, and ET was calculated using the Thornthwaite method. Our water balance analysis estimated groundwater recharge as 245mmyr^-1, 20% of the average annual precipitation. Direct runoff was estimated as 155mmyr^-1, ET as 725mmyr^-1, and baseflow as 128mmyr^-1. Groundwater flow modeling indicates that 65% of the total inflow to the pumping wells originates from the Nakdong River, 13% originates from an aquifer rectilinear to the Nakdong River, and 22% originates from the aquifer collinear to the Nakdong River. Particle tracking simulations reveal average linear velocities of 2md^-1 moving from the Nakdong River toward the pumping wells and 1md^-1 flowing from the aquifer toward the pumping wells. These results indicate that filtration capacity in the study area will be on the order of 10⁴m³d^-1.

The authors wish to acknowledge the financial support of the Sustainable Water Resources Research Center under the program of the 21st Century Frontier R&D Program (project no: 3-4-1).