Sinking Particle Flux in the Subtropical Oligotrophic Northwestern Pacific from a Short-term Sediment Trap Experiment

Time-series sediment traps were deployed in the subtropical oligotrophic northwestern Pacific (SONP) at three depths from August to September 2015 to better understand vertical flux of sinking particles. Sinking particles were collected at 5-day intervals over the sediment trap deployment period. The average total mass flux at water depths of 400 m, 690 m, and 1,710 m was 9.1, 4.4, and 4.1 mg m^-2day^-1, respectively. CaCO₃ materials constituted 50 to 70% of sinking particles while in comparison particulate organic carbon (POC) constituted up to 20%. A synchronous variation of total mass flux was observed at the three depths, indicating that calcite-dominated particles sank from 400 to 1,710 m within a 5-day period. POC flux at these water depths was 2.4, 0.38, and 0.31 mg m^-2day^-1, respectively. Our results indicate low transfer efficiencies of 16% from 400 to 690 m and 13% for the 400 to 1,710 m depth range. The estimated transfer efficiencies were significantly lower than those observed at the K2 station in the northwest Pacific subarctic gyre, presumably because of a prevalence of pico-cyanobacteria in the SONP. Because cyanobacteria have a semi-permeable proteinaceous shell, they are more readily remineralized by bacteria than are siliceous phytoplankton in the northwest Pacific subarctic gyre. Continued surface water warming and expansion of the SONP will likely have a profound impact on ocean acidification in the northwest Pacific, possibly affecting the transfer efficiency of sinking POC to the deep-sea.     

The Interactive Effects of Elevated CO<Subscript>2</Subscript> and Ammonium Enrichment on the Physiological Performances of <Emphasis Type="Italic">Saccharina japonica</Emphasis> (Laminariales,Phaeophyta)

Environmental challenges such as ocean acidification and eutrophication influence the physiology of kelp species. We investigated their interactive effects on Saccharina japonica (Laminariales, Phaeophyta) under two pH conditions [Low, 7.50; High (control), 8.10] and three NH ₄ ⁺ concentrations (Low, 4; Medium, 60; High, 120 μM). The degree of variation of pH values in the culture medium and inhibition rate of photosynthetic oxygen evolution by acetazolamide were affected by pH treatments. Relative growth rates, carbon, nitrogen, and the C:N ratio in tissue samples were influenced by higher concentrations of NH ₄ ⁺ . Rates of photosynthetic oxygen evolution were enhanced under elevated CO₂ or NH ₄ ⁺ conditions, independently, but these two factors did not show an interactive effect. However, rates of NH ₄ ⁺ uptake were influenced by the interactive effect of increased CO₂ under elevated NH ₄ ⁺ treatment. Although ocean acidification and eutrophication states had an impact on physiological performance, chlorophyll fluorescence was not affected by those conditions. Our results indicated that the physiological reactions by this alga were influenced to some extent by a rise in the levels of CO₂ and NH ₄ ⁺ . Therefore, we expect that the biomass accumulation of S. japonica may well increase under future scenarios of ocean acidification and eutrophication.     

Characteristics and Ecosystem Changes of Marine Algal Communities in Wangdol-cho on the East Coast of Korea

Ocean Science Journal - This study sought to examine the characteristics and patterns of marine algae community changes in Wangdol-cho in order to establish a theoretical basis for the development...     

ARD generation and corrosion potential of exposed roadside rockmass at Boeun and Mujoo,South Korea

Acid rock drainage (ARD) is a longstanding problem often associated with the resulting corrosion due to the acidity generated from sulfidic oxidation. To evaluate characteristics of ARD and corrosion, samples from the road side rock mass of Boeun and Mujoo were analysed using X-ray diffraction, acid/base accounting and Leaching tests. The results indicated that many samples had a pyritic origin and can be regarded as acid-generating rocks. The Leaching test showed that the average pH of the leachates of samples from both Boeun and Mujoo were moderately acidic, ranging from 3 to 4. Interestingly, as acidity increases from pH 4, the SO₄₋, Fe, Al and Mg concentrations increase abnormally. Samples from roadside slope of Mujoo showed high corrosive potential. Maximum sulfide oxidation rate of a sample taken from Mujoo was as high as 5,166 mg/kg/week.     

Seasonal characteristics of the Tsushima Current in the Tsushima/Korea Strait obtained by a fine-resolution numerical model

The horizontal distribution of the Tsushima Current in the Tsushima/Korea Strait is assessed by a fine-resolution numerical experiment. The comparison of the model results with the observations along a section crossing the strait shows that the model represents relatively well, the general tendency of what was observed, such as the separation of the Tsushima Current into the western and eastern streams by the Tsushima Island. In summer, strong and relatively uniformly distributed surface currents enhance the formation of the wake downstream of the Tsushima Island. The axis of the countercurrent, embedded in the wake, is closer to the western stream. Anti-cyclonic eddies are shed near the downstream tip of the Tsushima Island and propagate along the boundary between the western stream and the wake. The exchange of water between the western stream and the wake takes place through the intermediation of these eddies. There is a net water supply from the western stream to the wake, which is then carried to the eastern stream by the countercurrent via the eastern coast of the Tsushima Island. In winter, currents, strongly barotropic, tend to have banded structures, especially in the region downstream of the western channel where isobaths converge in the downstream direction. The eddies found in this region in winter appear to be fundamentally different from those associated with the Tsushima Island wake. The necessary condition for barotropic instability is satisfied for the monthly mean currents in this region, suggesting that the currents are barotropically unstable in this region in winter.     

A study on the treatment of antifouling paint waste from shipyard

A study on the treatment of antifouling paint waste from shipyards, including sandblast waste and ship hull washing wastewater, was performed. The sandblast waste could be effectively detoxified by heat treatment, and the efficiency was affected by the temperature of the heating vessel and treatment time. The removal efficiency of total organotin compounds from the sandblast waste was over 99% at 1000 °C and treatment for 1 h. For the treatment of ship hull washing wastewater by the solvent extraction, ship diesel was a good solvent for the tributyltin (TBT) extraction, and the proper amount of solvent was about 10 mL for TBT extraction from 1 L of wastewater. The extraction efficiency of TBT was significantly affected by the agitation intensity. The TBT in the wash wastewater was rapidly extracted within 1 h. The level of the TBT residual in the wastewater extracted for 1 h was 2.8 μg L⁻¹, and this was further decreased to 0.8 after 5 h extraction.     

Comparison of groundwater age models for assessing nitrate loading,transport pathways,and management options in a complex aquifer system

In an aquifer system with complex hydrogeology, mixing of groundwater with different ages could occur associated with various flow pathways. In this study, we applied different groundwater age‐estimation techniques (lumped parameter model and numerical model) to characterize groundwater age distributions and the major pathways of nitrate contamination in the Gosan agricultural field, Jeju Island. According to the lumped parameter model, groundwater age in the study area could be explained by the binary mixing of the young groundwater (4–33 years) and the old water component (>60 years). The complex hydrogeologic regimes and local heterogeneity observed in the study area (multilayered aquifer, well leakage hydraulics) were particularly well reflected in the numerical model. The numerical model predicted that the regional aquifer of Gosan responded to the fertilizer applications more rapidly (mean age: 9.7–22.3 years) than as estimated by other models. Our study results demonstrated that application and comparison of multiple age‐estimation methods can be useful to understand better the flow regimes and the mixing characteristics of groundwater with different ages (pathways), and accordingly, to reduce the risk of improper groundwater management plans arising from the aquifer heterogeneity.     

Unexpected nationwide nitrate declines in groundwater of Korea

Elevated levels of nitrate in groundwater are an important concern for health and the environment. The overapplication of nitrogen fertilizer to croplands is one of the major sources of high nitrate content in groundwater. In this study, we analyse the nitrate concentrations in Korean groundwater based on data from groundwater quality monitoring wells (n = 1,022–2,072), which were sampled twice annually over a recent 13‐year analysis period (2001–2013). We report that groundwater nitrate levels are decreasing, despite steadily increasing groundwater use. The maximum nitrate concentration decreased from 168.91 to 48.11 mg/L, whereas the mean values also show a gradual decreasing trend. Non‐parametric Mann–Kendall tests on nitrate concentrations also confirm the decreasing trend. The nitrate decrease is more clearly evident in agricultural groundwater as compared to domestic and drinking groundwaters. This decrease of nitrate in groundwater coincides with a large decline in nitrogen fertilizer application due to reduced cropland areas, more sustainable agricultural practices, and progressive improvement of sewage disposal services. This study proposes that the long‐term adoption of best practices in agriculture has had a positive impact on groundwater nitrate control.     

Effect of NAPL exposure on the wettability and two‐phase flow in a single rock fracture

Surface‐wetting properties are an important cause of changing the groundwater and two‐phase fluid flows. Various factors affecting the surface wettability were investigated in a parallel‐walled glass fracture with non‐aqueous phase liquid (NAPL) (gasoline, diesel, trichloroethylene, and creosote) wetted surfaces. First, the effect of the duration of NAPL exposure on wettability change was considered at pre‐wet fracture surfaces using the various NAPL species, and the result showed that the surface became hydrophobic after the exposure time of NAPL exceeded 2000 min. Second, the initial wetting state of the surface affected the timing when the wettability change begins as well as the extent of the wettability change in an NAPL‐wetted rock fractures. Under the dry condition, the wettability change was completed within a very short time of exposure to NAPL (~5 min), and then it finally reached the intermediate and weakly NAPL wetting (contact angle of 118°). Under the pre‐wet condition, a relatively long time of exposure (~5000 min) was needed to observe the obvious change of the surface wettability, which was changed up to strongly NAPL wetting (contact angle of 142°). Third, the wettability changed by NAPL exposure was stable and maintained for a long time, regardless of water flushing rate and temperature. Finally, the wettability change by the exposure of NAPL on parallel fracture surfaces was evaluated at various groundwater flow velocities. Result showed that groundwater flow velocity has an important impact upon measured contact angle. Although fracture surfaces were exposed to NAPL at the low groundwater flow velocity, the wettability was not changed from hydrophilic to hydrophobic when the contact time between NAPL and mineral surfaces was not sufficient owing to the pulse‐type movement of NAPL. This implies that the variation of exposure pattern due to groundwater flow on the wettability change can be an important factor affecting the wettability change of fracture surface and migration behaviour at natural fractured rock aquifers in case of NAPL spill. Copyright © 2015 John Wiley & Sons, Ltd.     

Solute dynamics across the stream‐to‐riparian continuum under different flood waves

To enhance the understanding of solute dynamics within the stream‐to‐riparian continuum during flood event‐driven water fluctuation (i.e., flood wave), a variable saturated groundwater flow and solute transport model were developed and calibrated against in situ measurements of the Inbuk stream, Korea, where seasonal flooding prevails. The solute dynamics were further investigated for flood waves (varying by amplitude [A], duration [T], roundness [r], and skewness [t_p]) that were parameterised by real‐time stream stage fluctuations. We found that the solute transferred faster and farther in the riparian zone, especially within the phreatic zone, above which in the variable saturated zone the concentration required a significantly longer time, particularly at higher altitudes, to return to the initial state. By comparison, solute transferred shallowly in the streambed where the solute plume exhibited an exponential growth trend from the centre to the bank. The dynamic changes of solute flux and mass along the stream–aquifer interface and stream concentration were linked to the shape of flood wave. As the flood wave became higher (A↗), wider (T↗), rounder (r↘), and less skewed (t_p↗), the maximum solute storage in aquifer increased. Maximum stream concentration (C_str?max) not only presented a positive linear relationship with A or t_p but also showed a negative logarithmic trend with increasing T or r. The sensitivity of C_{str_max} to A was approximately two times that of t_p, and between these values, the r was slightly more sensitive than T. C_str?max linearly increased as hydraulic conductivity increased and logarithmically increased as longitudinal dispersivity increased. The former relationship was more sensitive than the latter.