Contribution of nitrate-nitrogen concentration in groundwater to stream water in an agricultural head watershed.

This study characterized nitrate-nitrogen (NO3-N) concentrations in groundwater and stream water in an agricultural head watershed in South Korea and identified the pollution load of NO3-N as a result of the groundwater entering streams using field surveys, analyses of chemical constituents, and numerical modeling. The mean NO3-N concentration in groundwater was 7.373 mg/L, which is approximately 1.9 times higher than concentrations found in stream water. The groundwater and stream water samples belonged to the Ca-HCO3 type. The concentration of NO3-N in groundwater tended to increase in the lowland areas downstream. There was seasonal variations of NO3-N in both the groundwater and stream water samples, with increases in concentration during the dry season (January-April) and decreases during the wet season (June-October). The NO3-N load in stream water to that in groundwater (R) was higher during the wet season (September) than the dry season (March), with R distinctly increasing in upstream areas relative to downstream areas, indicating that during the wet season, a large amount of NO3-N is introduced into stream water from groundwater. By analyzing the relationship between groundwater and stream water and through NO3-N transport modeling, it was revealed that in the watershed, the nitrate-N load in stream water is greatly augmented by inputs from groundwater, particularly in the middle and downstream areas.

Groundwater Impacts from the M5.8 Earthquake in Korea as Determined by Integrated Monitoring Systems.

This paper describes the impacts of the M5.8(5.1) Gyeongju earthquakes on groundwater levels using data obtained from a unique coastal monitoring well. The monitoring strategy integrates conventional water level monitoring with periodic, continuous measurements of temperature and electrical conductivity (EC) within the water column of the well. Another important component of the monitoring system is a new instrument, the InterfacEGG, which is capable of dynamically tracking the freshwater-saltwater interface. Although the system was set up to monitor seawater intrusion related to over-pumping, as well as rainfall and tidal effects, it recorded impacts associated with a large earthquake and aftershocks approximately 241 km away. Seismic energies associated with the M5.8(5.1) Gyeongju earthquakes induced groundwater flows to the monitoring well through fractures and joints in the crystalline basement rocks. Temperature and EC logging data showed that the EC vertical profile declined from an average of approximately 5300 to 4800 µS/cm following the earthquakes. The temperature profile showed a trend toward lower temperatures as the depth increased, a feature not commonly observed in previous studies. Data from the InterfacEGG suggested that the rise in EC was not due to the saltwater intrusion, but from the tendency for brackish water entering the borehole to induce convective mixing at deeper depths as the seismic waves travel through the well-aquifer system. The increase in groundwater levels was caused by pulse of colder, less brackish water flowing into the well because of the earthquake. This behavior reflects an enhancement in rock permeability by removing precipitates and colloidal particles from clogged fractures, which improve the hydraulic connection with a nearby unit with a higher hydraulic head. This study suggests there is value added with a more aggressive monitoring strategy.

Hydraulic experiments for determination of in-situ hydraulic conductivity of submerged sediments.

A new type of in-situ hydraulic permeameter was developed to determine vertical hydraulic conductivity (VHC) of saturated sediments from hydraulic experiments using Darcy's law. The system allows water to move upward through the porous media filled in the permeameter chamber driven into sediments at water-sediment interface. Darcy flux and hydraulic gradient can be measured using the system, and the VHC can be determined from the relationship between them using Darcy's law. Evaluations in laboratory and in field conditions were performed to see if the proposed permeameter give reliable and valid measures of the VHC even where the vertical flow at water-sediment interface and fluctuation of water stage exist without reducing the accuracy of the derived VHC. Results from the evaluation tests indicate that the permeameter proposed in this study can be used to measure VHC of saturated sandy sediments at water-sediment interface in stream and marine environment with high accuracy.