Effects of water-table changes following rainfall events on arsenic fate and transport in groundwater-surface water mixing zones.

This study investigated the effects of groundwater-surface water (GW-SW) interactions on the fate and transport of arsenic (As) following rainfall events and subsequent water-table changes in GW-SW mixing zones, comprising the riparian and hyporheic zones, near an abandoned gold mine. During the dry and wet periods, stream conditions changed from flow-through to gaining, respectively. Water-table changes caused by rainfall events controlled flow paths between riparian zones and the stream, affecting spatiotemporal variation in the redox and pH conditions of the aquatic environment. Subsequently, the fate and transport of As in GW-SW mixing zones was responsive to variations in redox and pH conditions. Through the oxidative dissolution of As-bearing sulfide minerals and the reductive dissolution of iron (Fe) oxides with adsorbed As, As was released into the groundwater in the riparian zones and transported to the stream and streambed along the baseflow discharge. However, As was also immobilized in the sediment through adsorption onto Fe-oxides and coprecipitation with calcium (Ca) and zinc (Zn), suggesting that the sediment acts as a sink-and-source of As in aquatic environments. Therefore, water-table changes and GW-SW interactions could play an important role in the fate and transport of As in aquatic environments, specifically groundwater-riparian-streambed-stream systems. The findings of this study will provide scientific insights into the mechanisms of As in aquatic environments, aiding in improved decision-making to ensure safe and sustainable water management in response to future climate change.

Nitrate vulnerability of groundwater in Jeju Volcanic Island, Korea.

Groundwater is the sole source of water for about 670,000 residents of Jeju Island, which is a volcanic Korean island. Since the 1990s, nitrate contamination of groundwater has emerged as a major environmental issue. To ensure the sustainability of water resources, this study aimed to develop a vulnerability model for nitrate contamination as a preventive measure. Based on intrinsic vulnerability determined using the DRASTIC model, the effects of anthropogenic parameters related to NO3 sources and groundwater use (land use and the hydraulic gradient, respectively) on contamination were tested using a geographic information system (GIS). The correlation between groundwater nitrate distribution and vulnerability was considerably stronger compared to the DRASTIC method, with the correlation coefficients (r) increasing from -0.048 to 0.562 and -0.069 to 0.481 in the western and eastern regions, respectively. However, in the southern and northern regions, nitrate concentrations in groundwater are low, likely due to the heavily paved land surface that resulted from urbanisation, such that groundwater vulnerability appeared negligible. To prevent further nitrate contamination in coastal groundwater, management policies for land use and groundwater exploitation should be enacted along with continuous groundwater monitoring at the regional scale.

Nitrate contamination of coastal groundwater: Sources and transport mechanisms along a volcanic aquifer.

Groundwater is the sole water supply source on Jeju volcanic island in Korea and increasing levels of nitrate contamination have raised serious public concerns. This study was objected to understand the nitrate contamination mechanisms of the coastal groundwater overlain by a low-permeability layer in the western part of the island. The 2-yrs of quarterly groundwater monitoring from Sep. 2016 to Oct. 2018 revealed that NO3-N concentrations of 60.7% of sampled wells exceeded 10 mg/L, and those of 17.8% in the range of 3-10 mg/L, indicating those wells are already contaminated or under influence of potential sources. The spatio-temporal variation of groundwater chemistry and stable isotopic signatures suggested that nitrate originated mainly from chemical fertilizers due to agricultural activities and in part from liquefied manure affected by local livestock industries. Two transport processes appeared to be responsible for the nitrate in the confined coastal aquifer: 1) the direct penetration from the land surface through the wellbore leakage at incompletely grouted wells, and 2) the addition of nitrate-contaminated groundwater from upgradient areas where the confining layer is pinched out. Presently the potential nitrate sources are distributed up to 600 m above mean sea level(amsl), and the isotopic signatures implies that the provenance of coastal groundwater could be upland areas of up to 1280 m amsl. Subsequently, nitrate sources at upland areas could be picked up and carried down to the coastal groundwater under the confined condition of the basaltic aquifer that the regional groundwater recharge occurs. Consequently, groundwater contamination in the coastal aquifer should be considered in the big picture of groundwater system accommodating both regional and local flows.

Hydrochemical variations in selected geothermal groundwater and carbonated springs in Korea: a baseline study for early detection of CO2 leakage.

A baseline hydrochemistry of the above zone aquifer was examined for the potential of CO2 early detection monitoring. Among the major ionic components and stable isotope ratios of oxygen, hydrogen, and carbon, components with a relative standard deviation (RSD) of <10 % for the seasonal variation were selected as relatively stable. These components were tested for sensitivity to the introduction of 0.1 mol/L CO2 (g) using the PHREEQC simulation results. If the relatively stable components were sensitive to the introduction of CO2, then they could be used as indicators of CO2 leakage into the above zone. As an analog to the zone above CO2 storage formation, we sampled deep groundwater, including geothermal groundwater from well depths of 400-700 m below the ground surface (bgs) and carbonated springs with a high CO2 content in Korea. Under the natural conditions of inland geothermal groundwater, pH, electrical conductivity (EC), bicarbonate (HCO3), δ18O, δ2H, and δ13C were relatively stable as well as sensitive to the introduction of CO2 (g), thus showing good potential as monitoring parameters for early detection of CO2 leakage. In carbonated springs, the parameters identified were pH, δ18O, and δ2H. Baseline hydrochemistry monitoring could provide information on parameters useful for detecting anomalies caused by CO2 leakage as measures for early warning.

Kinetics of Dimethylated Thioarsenicals and the Formation of Highly Toxic Dimethylmonothioarsinic Acid in Environment.

Dimethylmonothioarsinic acid (DMMTAV) is a highly toxic, thiolated analogue of dimethylarsinic acid (DMAV). In comparison, a further thiolated analogue, dimethyldithioarsinic acid (DMDTAV), and DMAV both exhibit lower toxicity. To understand the environmental conditions responsible for forming DMMTAV, the kinetics of DMAV thiolation are examined. The thiolation of DMAV is pH-dependent and consists of two consecutive first-order reactions under excess sulfide conditions. The first thiolation of DMAV to form DMMTAV is faster than the second one to DMDTAV. DMMTAV is therefore an intermediate. The first reaction is first-order in H2S at pH 6.0 and 20 °C; therefore, the overall reaction is second-order and the rate coefficient in this condition is 0.0780 M-1 s-1. The rate coefficient significantly decreases at pH 8.0, indicating that H2S(aq) triggers the thiolation of DMAV. The second reaction rate is significantly decreased at pH 2.5; therefore, reaction under strongly acidic conditions leads to accumulation of highly toxic DMMTAV in the early stages of thiolation. The transformation of DMDTAV to DMMTAV is catalyzed in the presence of ferric iron. Formation of DMMTAV should be considered when assessing risk posed by arsenic under sulfidic or sulfate reducing conditions.