ALBERT

Description: We evaluated sources and pathways of groundwater recharge for a heterogeneous alluvial aquifer beneath an agricultural field, based on multi-level monitoring of hydrochemistry and environmental isotopes of a riverside groundwater system at Buyeo, Korea. Two distinct groundwater zones were identified with depth: (1) a shallow oxic groundwater zone, characterized by elevated concentrations of NO3- and (2) a deeper (〉10-14 m from the ground surface) sub-oxic groundwater zone with high concentrations of dissolved Fe, silica, and HCO3-, but little nitrate. The change of redox zones occurred at a depth where the aquifer sediments change from an upper sandy stratum to a silty stratum with mud caps. The δ18O and δ2H values of groundwater were also different between the two zones. Hydrochemical and δ18O-δ2H data of oxic groundwater are similar to those of soil water. This illustrates that recharge of oxic groundwater mainly occurs through direct infiltration of rain and irrigation water in the sandy soil area where vegetable cropping with abundant fertilizer use is predominant. Oxic groundwater is therefore severely contaminated by agrochemical pollutants such as nitrate. In contrast, deeper sub-oxic groundwater contains only small amounts of dissolved oxygen (DO) and NO3-. The 3H contents and elevated silica concentrations in sub-oxic groundwater indicate a somewhat longer mean residence time of groundwater within this part of the aquifer. Sub-oxic groundwater was also characterized by higher δ18O and δ2H values and lower d-excess values, indicating significant evaporation during recharge. We suggest that recharge of sub-oxic groundwater occurs in the areas of paddy rice fields where standing irrigation and rain water are affected by strong evaporation, and that reducing conditions develop during subsequent sub-surface infiltration. This study illustrates the existence of two groundwater bodies with different recharge processes within an alluvial aquifer. © 2009 John Wiley & Sons, Ltd.

Description: To investigate the origin and behaviour of nitrate in alluvial aquifers adjacent to Nakdong River, Korea, we chose two representative sites (Wolha and Yongdang) having similar land-use characteristics but different geology. A total of 96 shallow groundwater samples were collected from irrigation and domestic wells tapping alluvial aquifers. About 63% of the samples analysed had nitrate concentrations that exceeded the Korean drinking water limit (44.3 mg 1-1 NO3-), and about 35% of the samples had nitrate concentrations that exceeded the Korean groundwater quality standard for agricultural use (88.6 mg 1-1 NO3-). Based on nitrogen isotope analysis, two major nitrate sources were identified: synthetic fertilizer (about 4‰ δ15N) applied to farmland, and animal manure and sewage (15-20‰ δ15N) originating from upstream residential areas. Shallow groundwater in the farmland generally had higher nitrate concentrations than those in residential areas, due to the influence of synthetic fertilizer. Nitrate concentrations at both study sites were highest near the water table and then progressively decreased with depth. Nitrate concentrations are also closely related to the geologic characteristics of the aquifer. In Yongdang, denitrification is important in regulating nitrate chemistry because of the availability of organic carbon from a silt layer (about 20 m thick) below a thin, sandy surface aquifer. In Wolha, however, conservative mixing between farmland-recharged water and water coming from a village is suggested as the dominant process. Mixing ratios estimated based on the nitrate concentrations and the δ15N values indicate that water originating from the village affects the nitrate chemistry of the shallow groundwater underneath the farmland to a large extent. Copyright © 2003 John Wiley and Sons, Ltd.

Description: A geochemical study was carried out in a small spa area (Onyang Spa, Korea) where intensive pumping of deep thermal groundwater (1 300000 m 3 year-1) is taking place. This has caused the deep fractures to lose their artesian pressure and the upper shallow fractures have been encroached by shallow, cold waters. To quantify the influence of long-term heavy pumping on the quality of the geothermal water, groundwater sampling and chemical analysis, water-level measurement, and well loggings were performed for the selected deep thermal wells and shallow cold wells. Chemical analysis results indicate a big contrast in water chemistry and origins between the two water types. Shallow groundwater shows a wider concentration ranges in solutes that are closely related to human activity, illustrating the water's vulnerability to contamination near the land surface. Plots of water chemistry as a function of fluoride reveal that the quality of the thermal water was greatly influenced by the shallow, cold groundwater and that intensive pumping of the deep thermal groundwater has caused the introduction of shallow groundwater into the deeper fractures. Although the deep and the shallow fractures were piezometrically separated to some extent, a mixing model based on fluoride and nitrate indicated that the cold-water fractions in the thermal wells are up to 50%. This suggests that the thermal water is faced with water quality degradation by the downward flow of the shallow, cold water. Restriction on the total of all the pumpage permits per unit area is suggested to restore the artesian pressure of the deep thermal aquifer and to prevent cold-water intrusion in the study area. Copyright © 2004 John Wiley & Sons, Ltd.

Description: The weathering rate of plagioclase was estimated in the groundwater system of a sandy, silicate aquifer formed after the Wisconsin Glacial Stage. The study area is an isthmus lying between Crystal and Big Muskellunge Lakes in northern Wisconsin, USA. Plagioclase occupies 3% of the quartz and K-feldspar dominated sediments. Groundwater in the study area is recharged in part by precipitation through the isthmus soils and in part by seepage from Crystal Lake, which is of low ionic strength and chemically in steady state. Water analysis revealed that the chemistry of groundwater recharged from Crystal Lake is regulated by mineral dissolution reactions. The rate constant for plagioclase was estimated using mass balances for sodium concentrations along a groundwater flowline from Crystal Lake. For this calculation, various kinds of hydrological/mineralogical information were used: groundwater flow path from oxygen isotope analysis, groundwater travel times from flow modelling, mineral composition from microprobe analysis and surface area of minerals from BET (Brunauer-Emmett-Teller) analysis. The overall range of the estimation was less than an order of magnitude (3.5 × 10-16 to 3.4 × 10-15 mol/m2/s). The result is up to three orders of magnitude slower than the previous field estimates, which applied geometric methods in measuring mineral surface areas. However, this result is somewhat higher than the estimates reported by other BET area-based studies, which were undertaken on soil profiles having different hydrological conditions. This rate difference is interpreted as a result of higher mineral reactivity owing to younger sediment age. The rate difference is smaller when this result is compared with the estimates from the soils of similar age, indicating that the differences in hydrological condition are not sufficient to explain the weathering rate discrepancy between the laboratory and field studies, which is up to five orders of magnitude. Copyright © 2002 John Wiley and Sons, Ltd.