Myeong-Jae Yi

Dependency of hydrologic responses and recharge estimates on water-level monitoring locations within a small catchment

Estimation of groundwater recharge is essential for planning sustainable groundwater development. In this study, recharge ratios were estimated using the groundwater hydrographs obtained from monitoring wells located in a small catchment (0.256 km2). The catchment area is a triangular alluvial plain bounded by gentle hills and a large stream. For the recharge estimation, the modified water-table fluctuation (WTF) method was used. Although the study area is very small, the groundwater hydrographs were very different depending on the thickness of the unsaturated zone and the distance of the monitoring well from the main stream. Where the unsaturated zone is thick, the groundwater level changes smoothly in response to rainfall and resulting amplitude is small. In addition, the hydrographs closer to the stream showed quicker responses and larger fluctuations. Resultant recharge estimates showed a very wide variation between 2.5 and 20.1%. Compared with 12.7% estimated by other study mainly based on the water budget analyses, the recharge ratios were underestimated in upgradient area (hills) while they were overestimated in downgradient area (stream). Therefore, for representative and appropriate recharge estimation based on the groundwater hydrographs, it appears that the monitoring location is most important.

Combined performance of pumping and tracer tests: A case study

A combined pumping and tracer test was conducted at a highly fractured aquifer system. The hydrogeologic units underlying the test site are reclamation soil, weathered rock layer, and fractured layer. The fractured layer is the main aquifer for this site. Prior to pumping and tracer tests, slug tests were conducted at four test wells. The test data revealed existence of a low permeability zone near well OB-1. Generally the estimated hydraulic conductivities are in the order of 10−4 cm/sec. A pumping test with a discharge rate of 57 m3/d was performed for 1,230 min. The pumping test data analysis yielded coherent hydraulic conductivity values with those of the slug tests. However, the separate analysis for each monitoring well based on conventional analytical solutions with highly strict boundary conditions and homogeneity assumption cannot efficiently show the potential existence of the low permeability zone. During the pumping test, when the water levels of the pumping and monitoring wells are stabilized, a convergent radial tracer test was conducted. From the observed tracer concentration, a longitudinal dispersivity of 0.3 m was obtained, which is well consistent with the values in the prominent literature considering the test scale. This study excellently demonstrated a method completing a combined pumping and tracer test at one time.

Evaluation of seawater intrusion on the groundwater data obtained from the monitoring network in Korea

Over-exploitation of groundwater induced seawater intrusion throughout coastal plains of Korea. To protect groundwater resources from the seawater intrusion hazards, a monitoring programme was undertaken. Groundwater data obtained from 45 seawater intrusion monitoring wells were evaluated in this study. The groundwater levels were mainly affected by pumping for agricultural irrigation and tidal fluctuations. Decreases in water levels accompanied increases in electrical conductivities. Vertical profile of electrical conductivity indicated progressive seawater encroachment. A detailed investigation and mitigation measure should be undertaken for the areas where electrical conductivities are continuously increasing.

Evaluation of Long-term Data Obtained from Seawater Intrusion Monitoring Network using Variation Type Analysis

With groundwater data of seawater intrusion monitoring network in coastal areas of Korea`s main land, we analyzed types of seawater intrusion through the coastal aquifer. The data including groundwater level, temperature and electrical conductivity obtained from 45 monitoring wells at 25 watershed regions were evaluated. Based on statistical analysis, correlation analysis and variation type analysis, groundwater levels were mainly affected by rainfall and artificial pumping. About 78% of the monitoring wells showed average temperature higher than and about 58% of them showed minimum variations less than . Electrical conductivities showed a large magnitude of variation and irregular characteristics compared with groundwater levels and temperatures. Average electrical conductivities lower than were observed at 28 monitoring wells while those of higher than were done at 9 monitoring wells. From the cross-correlation analysis, groundwater levels were mostly affected by precipitation while temperature and electrical conductivity showed very low correlation. Meanwhile tidal variations strongly affected the groundwater levels comparing to precipitation. We classified the long-term monitoring data according to variation types such as constant process, linear trend, cyclic variation, impulse, step function and ramp. Impulse type was dominant for variations of groundwater level, which was largely affected by rainfall or artificial pumping, the constant process was dominant for temperature. Compared with groundwater level and temperature, electrical conductivities showed various types like linear trend, step function and ramp. According to the discrepancy of variation characteristics for monitoring data at each well in the same region, periodical analysis of monitoring data is essentially required.