Se-Woong Chung

Occurrence of micropollutants in four major rivers in Korea

There has been an increasing concern in recent years over the presence of numerous micropollutants in river water. Simultaneously, risk assessment of micropollutants has become increasingly important due to their high toxicity. Therefore, the aims of this study were to characterize the levels of micropollutants in rivers, to identify the sources of these pollutants, and to evaluate their risk quotients (RQ) to the ecological system. Monitoring data from 159 sites distributed across four major rivers in Korea were collected from the Ministry of the Environment (KME) reports. Tests were performed to check for the presence of 35 pollutants in river waters, from which 29 pollutants were detected. Organochlorine pesticides (OCPs) occurred most frequently, at low concentrations, whereas volatile organic compounds (VOCs) and phthalates were detected at higher concentrations. Based on this study, it was suggested that the factories near the sampling sites where pollutants were found represent one of the main sources of chemicals. After comparing the industrial activities with the measured pollutants, although the released amounts were not reported, representative intermediate chemicals were found in the river water. The RQs of most VOCs were below one, despite their concentration range being higher, compared with other pollutants. Methyl bromide and di-n-octyl phthalate occurred frequently in the Han River Basin and the Nakdong River Basin, respectively, and their RQs were consistently high, so further studies should focus on their exact emission source in order to reduce ecological risk. The results suggest that it will be necessary to develop methods of risk assessment that are more tailored to the various micropollutants present in river water, in addition to the implementation of water treatment systems to reduce ecological risk.

Estimating time-variable transformation rate of atrazine in a reservoir

Abstract The persistence of atrazine, one of the most applied herbicides in corn cropping areas, in an aquatic environment is dependent upon environmental conditions, i.e. temperature, sunlight, and presence of microorganisms. As these conditions vary seasonally, accurate determination of a time-variable degradation rate is important for the prediction of its fate and transport in surface water. A mass balance was performed to estimate the time-variable transformation rate (or half-life) of atrazine in the Saylorville Reservoir, Iowa. Calculated atrazine concentrations were compared with field data to verify the estimated half-life, which agreed reasonably well with the trends of observed values. A significant inverse relationship between the half-life and the hours of sunlight was obtained, showing the effectiveness of photodegradation. Estimated annual atrazine budget showed that 60% of the atrazine transported into the reservoir exited unchanged via outflow releases, while 40% was by kinetic processes within the reservoir. The half-life obtained in this study can be used as a preliminary value of degradation rate in the fate modeling of atrazine in reservoirs under similar environmental conditions.

Modeling metal-sediment interaction processes

Sensitivity and uncertainty analysis of contaminant fate and transport modeling have received considerable attention in the literature. In this study, our objective is to elucidate the uncertainty pertaining to micropollutant modeling in the sediment-water column interface. Our sensitivity analysis suggests that not only partitioning coefficients of metals but also critical stress values for cohesive sediment affect greatly the predictions of suspended sediment and metal concentrations. Bayesian Monte Carlo is used to quantify the propagation of parameter uncertainty through the model and obtain the posterior parameter probabilities. The delineation of periods related to different river flow regimes allowed optimizing the characterization of cohesive sediment parameters and effectively reducing the overall model uncertainty. We conclude by offering prescriptive guidelines about how Bayesian inference techniques can be integrated with contaminant modeling and improve the methodological foundation of uncertainty analysis. Sensitivity and uncertainty analysis was performed for sediment-metal modeling.Suspended sediment predictions are sensitive to critical erosion stress.Sediment bed-water partitioning coefficient is critical for metal predictions.River flow dynamics affect contaminant fate and model parameter sensitivity.Strategies to improve uncertainty analysis of sediment-metal modeling are discussed.

The effect of hydrodynamic flow regimes on the algal bloom in a monomictic reservoir

The effectiveness of a proposed curtain weir to be installed in the transitional zone of a eutrophic reservoir located in monsoon areas on the control of algal blooms in the lacustrine zone where drinking water withdrawals occur was assessed with various hydrodynamic flow regimes. A two-dimensional hydrodynamic and eutrophication model that can accommodate vertical displacement of the weir following the water surface changes was developed and validated using field data obtained from two distinctive hydrological years; drought (2001) and wet (2004). The model adequately reproduced the temporal and spatial variations of temperature, nutrients and phytoplankton concentrations in the reservoir. The efficacy of the curtain weir method found to be diverse for different hydrological conditions and dependent on the inflow densimetric Froude number (Fr(i)). Algal blooming was considerably mitigated by curtailing the transport of nutrients and algae from riverine zone to lacustrine epilimnion zone during the drought year as long as Fr(i) < 1.0. However, some flood events with Fr(i) > 1.0 transported nutrients and algae built upstream of the weir into the downstream euphotic zone by strong entrainments in 2004. Numerical experiments revealed that the efficiency of the weir on the control of algal blooming becomes marginal if the Fr(i) > 3.0.

Application of SWAT for the Estimation of Soil Loss in the Daecheong Dam Basin

미국농무성에서 토양과 토지이용 특성을 고려한 대규모 유역의 유출해석과 토양침식량 및 비점오염원 부하를 해석하기 위해 개발한 SWAT 모델을 대청댐 유역에 적용하여 토지이용 특성별 토양침식량을 산정하였다. 연구결과는 저수지관리자와 정책입안자들에게 저수지 탁수문제를 완화하기 위한 유역관리 대안의 효율성을 평가하는데 중요한 정보를 제공한다. 유출과 토양 유실량 산정에 영향을 미치는 주요한 매개변수들을 보정한 후, 모델은 실측 연간 유출성분과 월별 유황변화를 비교적 잘 예측하였다. 모의결과, 토지 이용별 단위면적당 토양침식량은 밭이 33.1 ton/ha/yr로 가장 많았으며, 임야는 수목의 종류에 따라

Characterization and modeling of turbidity density plume induced into stratified reservoir by flood runoffs.

2.3{\sim}5.4ton/ha/yr

Variability of pCO 2 in surface waters and development of prediction model

, 도시지역 1.0 ton/ha/yr, 논 0.1 ton/ha/yr의 순으로 나타났다. 토지이용면적 가중 연간 토양침식량 산정결과, 밭은 유역전체 면적에서 차지하는 비율이 단지 10% 정도에 해당하지만 연간 총 토양침식량의 55.3%를 차지하는 것으로 나타났다. 비록 본 연구에서 적용한 토양침식량 산정 방법에 농작물의 종류와 경작형태에 대한 구체적인 정보를 포함하지 못하는 불확실성이 내재되어 있으나, 연구결과는 최소한 저수지의 탁수문제를 완화하기 위해서는 상류 유역의 토양침식 조절대책이 필요함을 시사하며, 유역의 점유율에 비해 토양침식에 기여도가 가장 큰 밭농사 지역에 대한 토양침식 억제 대책을 우선 실시하는 것이 가장 효율적일 것으로 판단된다. 【The Soil and Water Assessment Tool (SWAT) developed by the USDA-Agricultural Research Service for the prediction of land management impact on water, sediment, and agricultural chemical yields in a large-scale basin was applied to Daecheong Reservoir basin to estimate the amount of soil losses from different land uses. The research outcomes provide important indications for reservoir managers and policy makers to search alternative watershed management practices for the mitigation of reservoir turbidity flow problems. After calibrations of key model parameters, SWAT showed fairly good performance by adequately simulating observed annual runoff components and replicating the monthly flow regimes in the basin. The specific soil losses from agricultural farm field, forest, urban area, and paddy field were 33.1,

Uncertainty of Discharge-SS Relationship Used for Turbid Flow Modeling

2.3{\sim}5.4

Modelling the propagation of turbid density inflows into a stratified lake: Daecheong Reservoir, Korea

depending on the tree types, 1.0, and 0.1 tons/ha/yr, respectively in 2004. It was noticed that about 55.3% of the total annual soil loss is caused by agricultural activities although agricultural land occupies only 10% in the basin. Although the soil erosion assessment approach adopted in this study has some extent of uncertainties due to the lack of detailed information on crop types and management activities, the results at least imply that soil erosion control practices for the vulnerable agricultural farm lands can be one of the most effective alternatives to reduce the impact of turbidity flow in the river basin system.】

Calibration of CE-QUAL-W2 for a monomictic reservoir in a monsoon climate area

In monsoon climate area, turbidity flows typically induced by flood runoffs cause numerous environmental impacts such as impairment of fish habitat and river attraction, and degradation of water supply efficiency. This study was aimed to characterize the physical dynamics of turbidity plume induced into a stratified reservoir using field monitoring and numerical simulations, and to assess the effect of different withdrawal scenarios on the control of downstream water quality. Three different turbidity models (RUN1, RUN2, RUN3) were developed based on a two-dimensional laterally averaged hydrodynamic and transport model, and validated against field data. RUN1 assumed constant settling velocity of suspended sediment, while RUN2 estimated the settling velocity as a function of particle size, density, and water temperature to consider vertical stratification. RUN3 included a lumped first-order turbidity attenuation rate taking into account the effects of particles aggregation and degradable organic particles. RUN3 showed best performance in replicating the observed variations of in-reservoir and release turbidity. Numerical experiments implemented to assess the effectiveness of different withdrawal depths showed that the alterations of withdrawal depth can modify the pathway and flow regimes of the turbidity plume, but its effect on the control of release water quality could be trivial.