Shinya Nakashita

Variation in properties of the sediment following electrokinetic treatments

ABSTRACT Many studies have reported variation in properties of the sediment within electrokinetic treatments (EKTs). However, we aim to reveal the variation in properties of the sediment following EKTs through laboratory experiments. We collected sewage-derived sediment from a littoral region, and passed it through a 2-mm sieve. We used a potentiostat to cause electrical current in EKT. We measured the sediment properties such as pH, redox potential (ORP), and hydrogen sulphide (H2S) concentration at the end of EKT and at 30 days following EKT. Results showed decreases in pH, increases in ORP, and decreases in H2S concentration at the end of EKT. Compared with the sediment without EKT, the decrease in ORP for the sediment within EKT was higher at 30 days following EKT. These suggest that anaerobic digestion of organic compounds occurs in the sediment following EKT, of which the oxidants produced by EKT serve as electron acceptors and organic compounds serve as electron donors. Furthermore, we found that EKT can remove H2S from the sediment and reduce H2S production in the sediment within EKT when compared to the case without EKT. These ensure that EKT can be used to remove H2S and control H2S production in the sediment.

INFLUENCE OF COASTAL GROUNDWATER ON BRACKISH WATER ENVIRONMENT IN A TIDAL ESTUARY

INTRODUCTION Understanding the hydrological dynamics of brackish water bodies is considered important for the effective conservation and management of the natural environment of tidal estuaries. Especially, fresh water inflows and residence times have a marked effect on the salinity within an estuary. Residence time, which refers to the mean time required to transport dissolved or suspended matter out of a water body, can be one of the limiting factors affecting phytoplankton abundance within a system (Monbet, 1992). Howarth et al. (2000) reported that a decrease in river inflows results in increased residence times and primary production in estuaries. Moreover, since the settlement and deposition of suspended matter is dependent on residence time, short residence times are considered to improve the water quality of estuarine water. A box model using salt as a conservative tracer is often applied to estimate both fresh water inflows and residence time within a system, both of which are considered important indicators of the physical condition of the estuarine environment. However, while the box model is simple in principle, the cases to which it can be applied are relatively limited, particularly in tidal estuaries because of its non-steady flow. Tidal flats along the periphery of tidal estuaries are important habitats for a variety of coastal organisms, particularly as nurseries for fish. The tidal flats of Ota River Estuary support numerous benthic fauna (16 Eucrustacea spp., 14 Pelecypoda spp. and Gastropoda spp., and 6 Polychaeta spp.) (Hibino et al., 2006). The salinity and dissolved oxygen concentration of the pore water in these areas and the particle size of the bed material has been shown to restrict the habitat preference of these species (Lalli and Persons, 1993). Submarine groundwater discharge is also important for material circulation in coastal region (Simmons, 1992; Moore, 1996; Burnett, 2003). Nonetheless, the groundwater environment of tidal flats is relatively poorly understood.

A pilot study on remediation of muddy tidal flat using porous pile

In order to prove that porous piles are effective in remediating muddy tidal flat sediments and increasing the biomass, field experiments were carried out at the tidal flat of a brackish river located in Hiroshima City, Japan. Porous piles with a diameter of 16cm and height of 50cm were installed in the muddy sediment that covers the sand layer of the tidal flat. After installation, concentrations of dissolved oxygen in interstitial water in and around the porous piles increased to a maximum concentration of 6mg/l due to enhancement of the groundwater flow. The increase of dissolved oxygen in the interstitial water produced a decrease in the concentration of ammonia and an increase in the individual number of benthos at the porous pile site. From these results, we concluded that the porous pile is an effective technology for remediation of muddy tidal flats.

Evaluation of Granulated Coal Ash as Artificial Seabed for Eelgrass

ABSTRACT Nakashita, S.; Nakamoto, K.; Koshikawa, Y.; Kim, K.-H., and Hibino, T., 2017. Evaluation of granulated coal ash as artificial seabed for eelgrass. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 40–44. Coconut Creek (Florida), ISSN 0749-0208. Granulated coal ash (GCA) is a by-product of coal-fired thermoelectric power stations. In this study, GCA was used to create a functional artificial seabed for eelgrass. Moreover, the capability of GCA for the growth of eelgrass in an indoor experiment was examined to elucidate the ability of eelgrass to establish itself and grow in a GCA artificial seabed created in an actual marine environment. The eelgrass in the GCA artificial seabed branched and adapted similar to that on sandy ground. A stable eelgrass community was established in one year in an actual marine environment. Furthermore, five years after transplantation, the eelgrass community was maintained. For the GCA artificial seabed, the oxidation–reduction potential of the bottom sediments was approximately 100 mV higher than that of the surrounding ground, and the amount of sulfide decreased to approximately one third of that of the surrounding ground. Even five years after construction, a stable environment was preserved. Therefore, it is concluded that GCA is an effective material for an eelgrass seabed.

MEASUREMENT OF MUD FLOC–SETTLING VELOCITY USING A LASER DIFFRACTION PARTICLE SIZE DISTRIBUTION ANALYZER

In-situ and laboratory measurements of particle setting velocity through different techniques, such as settling tubes and video systems, suffer from human and random sampling errors and lack of accuracy. The development of a method to improve the accuracy of the settling velocity measurements has been a challenge for researchers. We propose a method using a laser diffraction particle size distribution analyzer (Shimadzu, SALD-2000J) to estimate the particle settling velocity with increased accuracy. SALD-2000J measures continuously the average intensity of light scattered and the particle size distribution of mud flocs, which were used to estimate the particle settling velocities. The SALD-2000J method produced high accurate results saving both labor and time. The predicted settling velocity presented the same order of magnitude of those reported in previous studies. The settling velocity was used in the simulation of particle size distribution variation, producing a relative error of particle size distributions (obtained from SALD-2000J and the simulation) interior to 5%. The results suggested that the proposed method can be applied to the prediction of settling velocities of mud flocs and inorganic particles both in fresh and saline water environments.

Study of Upwelling of Suspended Substance with Density Inversion

2010年6月から現地で実証実験が行われている.実証実 験では,200 m×300 mの海底に石炭灰造粒物を5 cm, 10 cm,20 cm(それぞれ,St.1,St.2,St.3とする)の厚 さで散布し,底質改善の効果を検証している(井上ら, 2011).さらに,石炭灰造粒物施工の効果を検討するた めに,石炭灰造粒物未施工区であるSt.6を対照区として 設定した.また,本実証実験では石炭灰造粒物の散布に より造粒物層内に50 %程度の間隙が形成されたことを確 認している(日比野ら,2011). (2)海底近傍で形成される水温の鉛直分布 海田湾海底では2011年2月~11月にSt.2,St.3,St.6に おいて海底上10 cm,0 cm,石炭灰造粒物層もしくは泥 層下5 cm,15 cm,25 cmに水温計が設置され,水温の連 続測定が行われた.図-2には海田湾海底の(a)St.2と St.6で測定された水温,(b)St.2における海底上10 cmか らの水温差,(c)St.6における海底上10 cmからの水温差 の経時変化が示されている.St.2,St.6の海底上10 cmの 水温は最大でも0.2°C程度の差しかないため(図-2(a)), St.2,St.6においては海底上10 cmの水温を基準にして水 温差を算出した.また,8月には設置機器の回収,交換 のため 2週間程度の欠測がある.本解析においては, 2010年の現地調査結果によって海底面上5 cmと20 cmに おける調査期間中(2週間)の塩分差の平均値が0.02 psu 程度であったことから(上野ら,2011),水温のみに着 目した解析を行った. 11月から5月にかけて海底上10 cmと造粒物層,泥層 密度逆転に伴う濁質粒子の巻き上がりに関する研究 Study of Upwelling of Suspended Substance with Density Inversion