a b s t r a c tThrough field and numerical studies, this paper describes the importance of thermal dispersivity in designing groundwater heat pump (GWHP) systems. A pushepull test using heat as a tracer was performed to estimate the thermal dispersivity of the aquifer of this study at the Han River Environment Research Center in Korea. Measured data during the test were compared to the results of threedimensional (3-D) groundwater flow and heat transport simulations. From the best fit between the measured data and the simulated results, thermal dispersivity was estimated to be 0.4 m. To evaluate the effects of the thermal properties on subsurface heat transport associated with GWHP systems, sensitivity analysis was also performed. The analysis confirmed that, despite small changes based on the estimated values, thermal dispersivity of the aquifer had a great influence on the subsurface temperature distribution as well as the extent of the thermal plume. Because groundwater pumping and injection can cause flow velocity around wells to be faster than natural groundwater velocity, thermal dispersion in this elevated velocity condition will have a considerable impact on the heat transport process with operation of the GWHP system.

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The extent of denitrification using multiple isotopes, groundwater age, and physicochemical data for groundwater was compared in a small agricultural area near a river in Yangpyeong, South Korea. In 2014, the shallow groundwater at one site had high concentrations of NO3-N (74-83 mg L -1 ). The δ 15 N-NO3 values for groundwater in the study area ranged between +9.1 and +24.6‰ in June and +12.2 to +21.6‰ in October. High δ 15 N-NO3 values (+10.7 to +12.5‰) in both sampling periods indicated that the high concentrations of nitrate in the groundwater originated from application of organic fertilizers and manure. In the northern part of the study area, some groundwater samples showed elevated δ 15 N-NO3 and δ 18 O-NO3 values, which suggest that nitrate was removed from the groundwater via denitrification, with N isotope enrichment factors ranging between −4.8 and −7.9‰ and O isotope enrichment factors varying between -3.8 and -4.9‰. Similar δD and δ 18 O values of the surface water and groundwater in the south appear to indicate that groundwater in that area was affected by surface water infiltration. The mean residence times (MRTs) of groundwater showed younger ages in the south of the study area (10-20 years) than in the north (20-30 years). Hence we conclude that denitrification processes under anaerobic conditions with longer MRT of groundwater in the northern part of the study area removed considerable amounts of nitrate.This study demonstrates that multi-isotope data combined with physicochemical data and age-dating information can be effectively applied to characterize nitrate contaminant sources and attenuation processes.

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