• Journal of Environmental Science International
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• v.13 no.12
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• pp.1089-1102
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• 2004

Seasonal variation of biogeochemical characteristics was determined in Lake Shihwa from October 2002 to August 2003. When the lake was artificially constructed for the freshwater reservoir in 1988, the development of the strong haline density stratification resulted in two-layered system in water column and hypoxic/anoxic environment prevailed in the bottom layer due to oxidation of accumulated organic matters in the lake. Recently, seawater flux to the lake through the sluice has been increased to improve water quality in the lake since 2000, but seasonal stratification and hypoxic bottom layer of the lake still developed in the summer due to the nature of artificially enclosed lake system. As the lake is still receiving tremendous amount of organic matters and other pollutants from neighboring streams during the rainy summer season, limited seawater flux sluicing into the lake may not be enough for the physical and biogeochemical mass balance especially in the summer. The excess of accumulated organic matters in the bottom layer apparently exhausted dissolved oxygen and affected biogeochemical distributions and processes of organic and inorganic compounds in the stratified two-layered environment in the summer. During the summer, ammonia and dissolved organic carbon remarkably increased in the bottom layer due to the hypoxic/anoxic condition in the bottom layer. Phosphate also increased as the result of benthic flux from the bottom sediment. Meanwhile, dissolved organic carbon showed the highest value at the upstream area and decreased along the salinity gradient in the lake. In addition to the sources from the upstream, autochthonous origin of particulate organic carbon from algal bloom in the lake might be more important for sustaining aggravated water quality and development of deteriorated bottom environment in the summer. The removal of trace metals could be attributed to scavenging by strong insoluble metal-sulfide compounds in the hypoxic/anoxic bottom layer in the summer.

• Journal of Marine Life Science
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• v.5 no.1
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• pp.25-33
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• 2020

The Asan Bay, which has semi-diurnal tide with macro-tidal range, is affected by both freshwater discharge from the sluice gates in the sea dikes and tidal seawater inputs from the Yellow sea. Understanding water quality change in response to tides is important since tides can impact the short-term variations in physical and chemical water properties as well as the response of biological properties. The diel variations in water quality were seasonally investigated at 2 hour intervals from a fixed station in the Asan Bay. In the results, water temperature and salinity consistently fluctuated in phase or out of phase with tidal height. Especially salinity was positively correlated with tidal height. The concentrations of total suspended solids were higher in the bottom water than in the surface and fluctuated greatly over the tidal cycle recording higher values at low tide than at high tide. Nitrite+nitrate levels also fluctuated out of phase with tidal height and correlated negatively with tidal height. Other nutrients also showed a similar pattern. The pattern was distinct in July when freshwater was discharged before the field sampling. The concentrations of organic materials, total nitrogen and total phosphorus greatly fluctuated over the tidal cycle and were generally out of phase with tidal height. Most materials except particulate organic forms were correlated with salinity indicating that freshwater inputs were sources for the materials similarly to the dissolved inorganic nutrients. The results suggest that water quality (except dissolved oxygen and pH) and nutrients including organic materials was largely affected by tides in the Asan Bay.

• Journal of Marine Life Science
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• v.5 no.2
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• pp.72-80
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• 2020

A sea dike in the Yeongsan River estuary was constructed in 1981 to supply water and reclaim tidal flats for agriculture, separating the estuary into the freshwater and seawater zones. However, the sluice gates are frequently opened and freshwater is discharged in summer when more rainfall is recorded than other seasons, then converting the estuary to brackish water system. In this study, the direct effect of freshwater discharge was investigated by monitoring daily variation in water properties and phytoplankton size structure before and after the freshwater discharge events from 2013 to 2015. Freshwater discharge resulted in a sharp decrease in salinity and dissolved oxygen (DO) at surface water whereas it increased the turbidity of water column. However, salinity did not decrease sharply in 2014 when freshwater was discharged one day before the monitoring and salinity remained low prior to the monitoring. Levels of nutrients especially dissolved inorganic nitrogen (DIN) increased after the discharge and this contributed to potential limitation of nutrients such as P or Si rather than N in the estuary. Freshwater discharge also caused the changes in phytoplankton biomass (chlorophyll a) and size structure although their responses were different between years. The changes may affect growth of grazers and thus structure of marine food web by alternating food availability in the Yeongsan River estuary.

• Journal of the Korean Society of Marine Environment & Safety
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• v.25 no.1
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• pp.89-95
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• 2019

Mooring observations of water temperature and salinity were conducted to investigate the effects of freshwater discharge patterns on the mouth of Nakdong River from April 2017 to March 2018. More than $500-1000m^3\;s^{-1}\;d^{-1}$ of freshwater was frequently discharged into the estuary throughout the rainy season, but less than $200m^3\;s^{-1}\;d^{-1}$ was discharged through the normal season. Sluice gates of the estuarine barrage operated depending on the tide level during spring tide, but they were constantly open during neap tide. Water temperature and salinity fluctuated regularly with intermittent discharges of freshwater, whereas they were stable while freshwater discharge was continuous. Mean salinity was 29 during the study period. Salinity exceeded the mean value in the normal season and rapidly recovered after a temporary reduction. In contrast, water with salinity below the mean value prevailed in the estuary for three months over the rainy season. These results indicate that water temperature and salinity were affected by the amount of freshwater discharge, as well as the frequency of discharge on a large scale and the time over which the freshwater discharge continued.

• Journal of Wetlands Research
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• v.23 no.2
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• pp.189-200
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• 2021

To improve the water quality of agricultural reservoirs, constructed wetlands are applied in many places. These are technologies that establish ecosystems and important design factors include water depth distribution, inflow and outflow, water flow distribution, hydraulic residence time, water quality treatment efficiency, aspect ratio, and the distribution of open water and covered water surfaces. For high efficiency during the operation of a constructed wetland, the design needs to be optimized and this requires consideration of the different types and length of the intake dam as well as the type and connection of wetland cells. Therefore, this study was conducted to investigate and suggest factors that needs to be considered during the design and for efficient operation measures through field surveys of 23 constructed wetlands that have been established and operated in agricultural reservoirs. Results of the field investigation shows that several sites were being operated improperly due to the malfunctioning or failure of the water level sensors, sedimentation in the intake dam, and clogging of the mechanical sluice frames. In addition, it was found that as the length of the inlet channel increases, the ecological disconnection between the intake dam upstream and the wetland outlet downstream also increases and was identified as a problem. Most of the wetlands are composed of 2 to 5 cells which can result to poor hydraulic efficiency and difficulty in management if they are too large. Moreover, it was found that the flow through a small wetland can be inadequate when there are too many cells due to excessive amounts of headloss.

• Journal of Soil and Groundwater Environment
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• v.26 no.4
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• pp.27-43
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• 2021

Since climate factors, such as precipitation, temperature, etc., show repeated patterns every year, it can be said that future changes can be predicted by analyzing past climate data. As with groundwater, seasonal variations predominate. Therefore, when a drought occurs, the groundwater level is also lowered. Thus, a change in the groundwater level can represent a drought. Like precipitation, groundwater level changes also have a high correlation with drought, so many researchers use Standard Groundwater Level Index (SGI) to which the Standard Precipitation Index (SPI) method is applied to evaluate the severity of droughts and predict drought trends. However, due to the strong interferences caused by the recent increase in groundwater use, it is difficult to represent the droughts of regions or entire watersheds by only using groundwater level change data using the SPI or SGI methods, which analyze data from one representative observation station. Therefore, if the long-term groundwater level changes of all the provinces of a watershed are analyzed, the overall trend can be shown even if there is use interference. Thus, future groundwater level changes and droughts can be more accurately predicted. Therefore, in this study, it was confirmed that the groundwater level changes in the last 5 years compared with the monthly average groundwater level changes of the monitoring wells installed before 2015 appeared similar to the drought occurrence pattern. As a result of analyzing the correlation with the water storage yields of 3,423 agricultural reservoirs that do not immediately open their sluice gates in the cases of droughts or floods, it was confirmed that the correlation was higher than 56% in the natural state. Therefore, it was concluded that it is possible to re-evaluate agricultural droughts through long-term groundwater level change analyses.

• Journal of Korean Society of Disaster and Security
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• v.16 no.4
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• pp.157-164
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• 2023

The estuary dam is a structure installed and operated in a closed state except when flood event occurs to prevent inland saltwater intrusion and secure freshwater supply. However, the closed state of dam leads to issues such as eutrophication, so it is necessary to examine the extent of saltwater intrusion resulting from the opening of sluice gates. Groundwater, due to its subsurface conditions and slow flow characteristics, is widely analyzed using numerical models. OpenGeoSys, an open-source software capable of simulating Thermal- Hydraulic- Mechanical- Chemical phenomena, was adopted for this study. Simulations were conducted assuming natural flow conditions without dam and operating considering busy farming season, mostly from March to September. Verification of the model through analytical solutions showed error of 3.7%, confirming that OpenGeoSys is capable of simulating saltwater intrusion for these cases. From results simulated for 10 years, considering for the busy farming season, resulted in about 46% reduction in saltwater intrusion length compared to natural flow conditions, approximately 74.36 m. It may be helpful to make choices to use groundwater as a water resource.

• Journal of Wetlands Research
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• v.17 no.1
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• pp.62-74
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• 2015

Urban areas are variously under threat including deterioration of ecological functions. Many pond wetland types have been created as part of an effort to improve and restore this urban environment. This study was arranged to examine improvement plans of wetlands in urban areas by analyzing semi-natural wetlands in farm areas. As for environment for water quality, it suggested the inflow of natural water neighboring rivers or the direct inflow of rain as the improvement plans. The result which analyzed soil pH, OM, and T-N content of the soil environment mentioned that urban areas supplied artificial sluices, removed apoptotic bodies, and used artificial soil and waterproofing materials and use of natural materials in design and construction, the sluice state of the natural form, and negligence of autumn plants were suggested as the improvement plans. Florae appeared in the subject sites of the study have found that there are 35 families 69 species in urban areas and 53 families 142 species in rural areas. As the average has found that there are 18.5 families 29.3 species in 4 urban areas and 26.3 families 53.5 species in 4 rural areas, the big difference between them was analyzed. As the cause has found that there are differences in yearly plants in farming areas when compared to urban areas, creation of various basic environments including soil and water quality was suggested to make yearly plants settle down widely. Naturalized plants have found that there are no big differences between urban areas and rural areas. However, the average of the naturalized ratio in urban areas is 17.4% as the naturalized plants are about 1/4 of the appeared plants. As it was analyzed to be higher than 7.7%, the average of the naturalized ratio in farming areas as the big difference, creation of various inhabiting environments was suggested to make more yearly plants appear like the analyzed result of the life type. Consideration of placement, materials, and inhabiting environments was suggested to make creation of wetlands well appreciated to improve functions of wetlands in urban areas. It is expected that the above results of the study will be utilized in creation and improvement of the pond wetlands which can play a huge role in increase and improvement of biological diversity in urban areas.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.22 no.4
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• pp.187-198
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• 2017

Dissolved and particulate organic carbon concentrations and fluxes were measured and estimated for the Yeongsan River during 2006~2015. The dissolved organic carbon (DOC) concentrations ranged from 2.49 to $4.39mg{\cdot}C/L$ with a variance of 30.1% (${\sigma}_x/\bar{x}$), and showed a simple correlation to algal bloom and precipitation. The particulate organic carbon (POC) concentrations had gradually decreased from 6.68 to $0.19mg{\cdot}C/L$ for 10 years, and changed definitely with weir construction in 2011. Based on the relationships between POC and suspended particulate matters and between POC and chlorophyll-a, we found out that the distinct variation of the origin and composition of POC was caused by stagnation and screening effect of the dammed river. The total organic carbon (TOC) concentrations dropped to 52.3% (from 8.26 to $3.94mg{\cdot}C/L$) as the POC concentrations diminished to more than 94.8% after weir construction, in which the DOC forms up to 90.9%. The fluxes of TOC, based on the relationship between the annual TOC concentration and the discharge of Yeongsan dike sluice, were $2.56{\sim}19.41{\times}10^9g{\cdot}C/yr$, and showed a great deal of variability in 2011. Since then the TOC flux dropped to $5.40{\times}10^9$ (2011~2015) from $14.54{\times}10^9g{\cdot}C/yr$ (2006~2010). These results suggest that the weirs trapped annually $1.83{\times}10^9g{\cdot}C$ on a river bed, but released in great levels of dissolved organic form at their exits.

• Korean Journal of Ecology and Environment
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• v.47 no.2
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• pp.71-81
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• 2014

A survey was carried out to understand the influence of hydrology on the composition, abundance and adaptive strategies of phytoplankton in artificial Lake Hwaseong, an estuarine reservoir with seawater exchange through a sluice. Samples were collected seven times from May to October 2012. Hydrological events (seawater exchange, rainfall) resulted in a wide variation in salinity along with nutrients and turbidity. Shifts in the dominant phytoplankton composition occurred on every survey. Chlorophyll-a ranged from 9.7 to $104.1{\mu}g\;L^{-1}$. Multivariate analysis allowed us to identify the four phases on phytoplankton community change. Phase I (May~June) was characterized by small-sized Gymnodinium sp. and Heterosigma akashiwo dominated in warm temperature and high salinity derived from seawater exchange, and followed by Cylindrotheca closterium blooms due to rainfall and winds during phase II (July and September). During phase III (August), the dominance of Oscillatoria spp. was correlated with high temperature and low salinity. Abundant cryptomonads were associated with lower temperature during phase IV (October). Adaptive strategies were identified in the phytoplankton as morphological and physiological characteristics. These strategies identified small-sized flagellates as CR-strategists, fast-growing opportunistic species, which might favor the weak stratification of lake due to the seawater exchange during phase I and IV. Dominant species during phase II and III were characterized with R-strategists, medium-sized stress-tolerant species, which might favor turbulence by river flow. The results indicate that stronger stratification following the termination of seawater exchange for the freshening might intensify the predominance of smaller flagellates. In conclusion, this study suggests that hydrology may drive phytoplankton community change and blooms through the controls of salinity, turbulence and nutrients.