Jeong-Yong Cheon

Factors affecting the distribution of hydrocarbon contaminants and hydrogeochemical parameters in a shallow sand aquifer

The distributions of hydrocarbon contaminants and hydrogeochemical parameters were investigated in a shallow sand aquifer highly contaminated with petroleum hydrocarbons leaked from solvent storage tanks. For these purposes, a variety of field investigations and studies were performed, which included installation of over 100 groundwater monitoring wells and piezometers at various depths, soil logging and analyses during well and piezometer installation, chemical analysis of groundwater, pump tests, and slug tests. Continuous water level monitoring at three selected wells using automatic data-logger and manual measuring at other wells were also conducted. Based on analyses of the various investigations and tests, a number of factors were identified to explain the distribution of the hydrocarbon contaminants and hydrogeochemical parameters. These factors include indigenous biodegradation, hydrostratigraphy, preliminary pump-and-treat remedy, recharge by rainfall, and subsequent water level fluctuation. The permeable sandy layer, in which the mean water table elevation is maintained, provided a dominant pathway for contaminant transport. The preliminary pump-and-treat action accelerated the movement of the hydrocarbon contaminants and affected the redox evolution pattern. Seasonal recharge by rain, together with indigenous biodegradation, played an important role in the natural attenuation of the petroleum hydrocarbons via mixing/dilution and biodegradation. The water level fluctuations redistributed the hydrocarbon contaminants by partitioning them into the soil and groundwater. The identified factors are not independent but closely inter-correlated.

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 groundwater chemistry affected by an abandoned metal mine within a dam construction site, South Korea

Groundwater chemistry in and around an abandoned metal mine within a large dam construction site was evaluated. For this purpose, a total of 50 groundwater samples were collected and chemically analysed for major ions and toxic metals. The pHs of the groundwaters were near neutral to slightly basic, with an apparent increase along the flow direction away from the mine waste dump. Higher values of electrical conductivity with a mean of 532 μS cm−1 were observed in the mine area. Dissolved oxygen concentrations over 5 mg l−1 clearly indicated an oxygenated groundwater environment. The high concentration of sulphate was derived from weathering and oxidation of sulphide minerals. Hydrochemistry of the groundwater samples is characterized by the relatively significant enrichment of Ca2+ and SO42−. Results of factor analysis also indicated that the dominant mechanisms or continuing processes affecting the distribution of the chemical parameters in the study area are various geochemical reactions including dissolution of sulphide and carbonate minerals, and removal of calcium and sulphate by dilution through mixing with Ca2+− and SO42−-poor groundwaters. In the mine area, substantial groundwater contamination by toxic metals including Zn, Al, Fe and Mn was observed. Most of the toxic metals decreased gradually or dramatically with distance from the mine, although some metals were also found in high concentrations in the downstream areas. Compared with the contamination levels of the surrounding farmland soils and stream sediments, the levels of contamination of groundwater by toxic metals were relatively low. The low concentrations are ascribed to the high pH and highly oxygenated conditions, which reduce toxic metal mobility. Significant groundwater contamination with toxic metals was strictly limited within the immediate vicinity of the mine waste dump but the ubiquitous distribution of the toxic metals with slightly elevated levels all over the study area may be attributed to the same geology and mineralogy as in the mine area.

Evaluation of Contaminant Concentrations in Wet and Dry Seasons during Pump-and-Treat Pilot Tests

This study was performed to examine use of the pump-and-treat method for remediation of TCE, CF and CT in groundwater contaminated by DNAPL. The Woosan industrial complex is located in Wonju, about 120 km east of Seoul, Korea. Two pumping wells (KDPW7 and KDPW8) and five monitoring wells (KDMW7, KDMW8, KDMW9, KDMW10, and SKW2) were installed for the test. An asphalt laboratory is a main source of the extensive subsurface contamination at this site. To evaluate change in the concentrations of TCE, CF, and CT in groundwater in the study area, three rounds of pump-and-treat pilot tests were performed (6 July to 6 August, 22 August to 6 September, and 19 September to 2 December in 2011). The groundwater levels and the concentrations of TCE, CF, and CT exhibited negative correlations in the wet season but positive correlations in the dry season, which suggests that the TCE concentrations were mainly controlled by dilution through rainfall during the wet season and by residual TCE, CF, and CT in the unsaturated zone during the dry season. These possibilities should be considered in the full-scale remediation plan.

Effects of Dissolved Compounds in Groundwater on TCE Degradations Reaction by Nanoscale Zero-Valent Iron

Nanoscale zero-valent iron (NZVI) particles were tested as remediation media for groundwater contaminated by organic pollutants (e.g., TCE, trichloroethylene). The contaminated groundwater contained anions (NO3, Cl, SO4, and HCO3) and natural organic matter (NOM). Treatability of commercial NZVI particles (NANOFER 25, Nanoiron, Czech) was tested by using a synthetic groundwater and the field groundwater samples. More than 95% of 1.8 mM TCE was removed within 20 hours with a NZVI dosage of 25 g/L (k = 0.15 hr). Repetitive degradation experiments revealed that the removal capacity of NANOFER 25 was 0.19 mmole TCE/g NZVI. TCE degradation reactions were not substantially affected by the presence of each anion with concentrations as high as 100 times the average field concentrations. However, when the four anions (NO3, Cl, SO4, HCO3) were present simultaneously. the degradation reactivity and removal capacity were decreased by 60% (k = 0.069 hr) and 10%, respectively. The k value of TCE degradation in the presence of NZVI (25 g/L) with dissovled organic carbon of 2.5 mg/L was also decreased by 84% (k = 0.025 hr). In the experiments with the field groundwater, more than 90% of 1.8 μM TCE, which is the concentration of TCE at the source zone, was removed within 10 hours with a NANOFER 25 dosage of 25 g/L. The results imply that the contaminated groundwater can effectively be treated by NANOFER 25 with more information on the hydrogeology of the site.

Relationship between Water Levels and TCE Concentrations during Pump-and-Treat Tests for a TCE-Contaminated Aquifer

Pilot tests of a pump and treat system were performed three times in the wet season and two times in the dry season over 139 days in a trichloroethylene (TCE) contaminated aquifer. Both groundwater levels and TCE concentrations were measured at two pumping and five monitoring wells. The measured data were used to examine the relationships between groundwater levels, TCE concentrations, and pumping rates. In the wet season, the groundwater levels and TCE concentrations showed negative correlations at both pumping wells and some of the monitoring wells due to heavy rainfall by dilution. At the other monitoring wells, they showed positive correlations because of influence of a residual TCE around the wells. In the dry season, they had positive correlations at most wells. TCE concentrations were also influenced by the pumping rates in a zone including residual TCE. However, TCE concentrations did not show any correlation with the pumping rates out of the zone.

A review on clogging mechanisms and managements in aquifer storage and recovery (ASR) applications

Clogging, the major obstacle in aquifer storage and recovery (ASR) applications, causes declines in recharge rates and ultimately the failure of artificial recharge systems. Clogging is generally triggered by physical processes, which involve the accumulation of suspended solids from recharge water, the release of fine particles from soils and aquifers and their migration, and the formation of gas barriers from the entrapped air in recharge water and biogenic gases. Physical clogging is then followed by biological processes (e.g., the growth of microorganisms and the accumulation of biomass). It can be further aggravated by chemical reactions leading to mineral precipitation. Prior to artificial recharge, source water can be pretreated via sedimentation, coagulation, filtration, advanced oxidation, and disinfection to lessen the clogging potential. Although such preventive measures are useful, clogging will eventually occur in recharge systems, thus necessitating the rehabilitation. The rehabilitation methods include backwashing, scrubbing, surging, jetting, under-reaming, acidification, and biociding. Notably, the methods applied vary with the type of recharge systems: surface infiltration systems, vadose zone infiltration systems, and aquifer injection wells. Among them, surface infiltration systems are most cost-effective and easiest to install and maintain. Even if field conditions are not adequate for their implementation, surface infiltration systems can be used in conjunction with the other recharge systems.