Journal Journalof ofCoastal CoastalResearch Research

SI 64

pg -- pg 1633 1637

ICS2011 ICS2011 (Proceedings)

Poland

ISSN 0749-0208

Estimation Method of Oxygen Penetration Depth into the Bottom Sediment at Strongly-Enclosed Sea Area T. Shigematsu† and T. Endo† †School of Engineering, Osaka City University, Osaka, 558-8585,Japan [email protected]

‡School of Engineering, Osaka City University, Osaka, 558-8585,Japan [email protected]

ABSTRACT Shigematsu, T. and Endo, T., 2011. Estimation method of oxygen penetration depth into the bottom sediment at strongly-enclosed sea area. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), 1633 – 1637. Szczecin, Poland, ISSN 0749-0208 Oxygen consumption by bottom sediment is modeled based on field investigation in a eutrophied port and harbor through a whole year. Based on measured sediment oxygen consumption flux using a chamber and a dark bottle with fluorescence type oxygen sensors, biological and physical-chemical oxygen consumption processes are modeled with water temperature and dissolved oxygen concentration near the sea bottom. It is presented that these factors can be estimated by water temperature and dissolved oxygen above the sea bottom. Through modeling sediment oxygen penetration depth is a useful parameter to represent sea bottom environment as a remediation. Further, estimation method of oxygen penetration depth is improved to take into the change of the dissolved oxygen above sea bottom consideration. ADDITIONAL INDEX WORDS: Sediment Oxygen Consumption, Field Investigation, Chamber method,

INTRODUCTION It is well known that anoxic and/or hypoxic water has been spreading in enclosed sea over the world by during occurrence of density stratification caused by water temperature and salinity. Naturally, stratification is easy to be generated and developed in enclosed sea since sea water doesn’t move so much as geographical features. Human activities such as construction of breakwater, dike, port, harbor, and so on generate stronger stratification. As a result, oxygen transfer from into the sea bottom is interrupted by the stratification. On the other hand, excessive nutrient from land into enclosed sea causes eutrophication. Eutrophication stimulates algal growth and much organic matter settles and accumulates, which enhances a sediment oxygen demand. There is little knowledge of generation source of hypoxia and anoxia. According to the investigation of water quality conducted over the whole area of the Osaka bay at every year since 2004 to 2010, the bottom port and harbor area in summer seasons have been under low oxygen concentration. It implies the possibility which port and harbor area may be a generation source of hypoxia and/or anoxia. Authors have developed a breakwater promoting vertical circulation flow as one of remediation methods of port and harbor area(Endo at al.2005, 2006, 2007). To examine effect of the breakwater in strongly-stratified and bottom hypoxic port and harbor, oxygen consumption in water and by sea bottom sediment should be estimated. Dissolved oxygen concentration above sea bottom, nutrients, and biomass and/or diversity of benthos is sometimes used as a remediation index, to represent status of environment and/or effect of a technology and technique for remediation. It is difficult, however, to say that the indexes as mentioned above are not appropriate. Because variances of them are quite large in time and space, it is difficult to obtain useful

information by time and space resolution of field investigation such as many researcher and engineers carry out. In this study, field investigation through a whole year was conducted to evaluate the oxygen consumption of the bottom sediment. A chamber method with fluorescent dissolved oxygen concentration meter was used to measure the obtained time series of oxygen concentration in a chamber. Using the obtained time series of oxygen concentration in a chamber, the oxygen consumption flux into the sediment was estimated. Further, the oxygen consumption flux was modeled as a function of biological and chemical-physical oxygen consumption.

STUDY SITE AND INVESTIGATION METHOD Field investigation was carried out in a port at the Yamato river mouth in Osaka Bay, JAPAN for a whole year (see Fig. 1). The maximum tidal range of closed-off of Osaka Bay is approximately 1.8m. Because 30m3/s of fresh water is supplied from the Yamato River, salinity stratification always appears in the port. Although maximum water depth in the port (the length and width are approximately 1,000 m and 510 m respectively) is approximately 18 m, the water depth of the port entrance is shallower 2~12m due to the accumulation of sand from the Yamato river. Field investigation was carried out at the port entrance and the averaged water depth was approximately 10~11m through the whole investigation period. Vertical profiles of salinity, water temperature and DO were measured at almost every neap tide. Measurements for salinity, water temperature and water density profiles were conducted using a CTD (Compact CTD: Conductivity, Temperature, and Depth, Alec Electronics Co. Ltd.) with depth at 0.1 m vertical sampling intervals. The DO data were measured by using a galvanic-type DO sensor (Model-58: YSI co. Ltd) or A fluorescent dissolved oxygen concentration meter

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Estimation Method of Oxygen Penetration Depth into Sea Bottom Sediment

the chamber. DO concentration in the chamber had been monitored by a fluorescent dissolved oxygen concentration meter. After confirming that the DO concentration became higher enough, the chamber was sealed up by a rubber stopper. Dissolved oxygen in the chamber is consumed by sediment and sea water. In order to separate sediment oxygen demand, a lightresistant bottle was used. After pouring the bottom sea water into the dark bottle and sealing it up without water exchange, the dark bottle was set on the sea bed. Time variation of dissolved oxygen concentration, Cd, in the light-resistant bottle was measured by another fluorescent dissolved oxygen concentration meter (LDOHQ30d: Hach Company). Sediment oxygen consumption flux across the sediment-water interface was evaluated by time variation of Cc and Cd as follows: ∂ (Cc − Cd ) V (1) FDO = − ∂t A Cc and Cd were recorded at every 5 minutes in the memories. Figure 1. Location of field investigation. (LDO-HQ30d: Hach Company) with depth at 1 m intervals. At the same time, sediment oxygen consumption was measured by a chamber method with a fluorescent oxygen concentration meter. The volume of plastic chamber used in this study is V=10-2 m3 and the chamber covered A=0.71 m2 sea bottom. In order to yield uniform concentration of dissolved oxygen in the chamber, the sea water in the chamber was slowly stirred up by a submersible motor pump. A fluorescent dissolved oxygen concentration meter (LDO-HQ30d: Hach Company) was equipped to measure oxygen concentration, Cc, in the chamber. Strong stratification due to salinity and temperature and excessive nutrient inputs in the investigation field result in low dissolved oxygen concentrations in the bottom water during summer season. When DO concentration of the bottom water, Cb, is extremely low, the time variation of Cc is not observed. Hence, measurements of Cc were started after exchanging the sea water in the chamber. As shown in Fig. 2, the plastic chamber used in this study has a weight and rubber tube at the bottom of it to prevent generation of the gap between the chamber and sea bottom. With keeping the vent opening, the chamber was put on the sea bottom so that the bottom sediment might be not disturbed as possible. As shown in Fig. 2, the plastic chamber used in this study has a weight and rubber tube at the bottom of it to prevent generation of the gap between the chamber and sea bottom. With keeping the vent opening, the chamber was put on the sea bottom as preventing bottom sediment from disturbing as calm as possible. Hypoxia and anoxia water 0.5 m above the sea bottom was collected into a tank by using a Van-Dorn water sampler. After aerating, the collected water was supplied into the chamber on the sea bottom through a 4 mm tube connected between the tank and

RESULTS OF FIELD INVESTIGATION Measurement results of water temperature, salinity and dissolved oxygen concentration through investigation period is shown in Fig. 3. Although the temperature of water a few meters below water surface was slightly higher in summer season, water temperature was vertically almost constant. Salinity were constant, approximately 30 spur, except a few meters below the water surface. It can be interpreted that the Yamato River strongly influences the environmental structure of the investigation site. Fig. 3 (c) shows the measurement results on the DO profiles. According to Fig. 3 (c), it is found that an anoxic state had been continued in the range of the bottom to middle of depth from July to November in 2008. As above mentioned, the investigation site investigated in this study, which is an example of a typical case of port and harbor located in an enclosed bay and affected by river water with rich eutrophication, has been in inferior environment during half of a year.

MODELING SEDIMENT OXYGEN CONSUMPTION It is well known that DO concentration in a chamber has a tendency to decrease linearly with time by benthos activities under rich dissolved oxygen. This means that biological DO consumption is constant. On the other hand, DO consumption has a tendency to be proportional to the spatial gradient of DO with time by some chemical reactions. Considering interface between the bottom water and the sea bottom sediment, the spatial gradient of DO can be represented by the bottom DO concentration Cb and the distance from the bottom surface to reduced layer, namely the

Tank

Tank

Tank

Chamber Motor

Light-resistant Bottle

DO Meter Rubber tube Aerated Water

Rubber stopper Sediment

Sediment

Figure 2. Measurement method. Journal of Coastal Research, Special Issue 64, 2011 1634

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Oxygen Consumption

distance from bottom surface [m]

Shigematsu and Endo

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2008. May

Jun.

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DO consumption and that the later term depends on chemical DO consumption.

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 ∂C   Cc  (2) FDO = α + β   = α + β   ∂z   L  where α is a coefficient with unit of concentration flux, β is a coefficient with unit of velocity, and L represents oxygen penetration depth. α and β'=β / L for every measured data during investigation period were estimated by fitting to measured DO flux FDO.

10 8

FORMULATION OF CONSTANT TERM

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(b) distance from bottom surface [m]

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(c) DO Figure 3. Vertical profiles of (a) water temperature, (b) salinity, and (c) desolved oxygen concentration. Open and black circle represent beginning and end data of each monthrespectively.

As mentioned above, the constant term α of eq. (1) is considered as biological oxygen consumption in this study. Although the sediment oxygen consumption is considered to depend on accumulated organic nutrients intrinsically, it is independent on the organic nutrients in this study because too much organic nutrients accumulate on the bottom in eutrophied waters. Therefore, it is reasonable presumption that α must depend on water temperature and DO of the bottom water, Cb. Fig. 5 shows the measured relationships among α, water temperature, and Cb. Fig. 5 shows that α has a tendency to be larger over 20 degree although α varies widely. Generally, activities of creatures become higher with increase in water temperature. Because creatures can’t live without enough DO, however, even if water temperature becomes higher. This is the reason why α varies widely in the region of water temperature 20 degrees or more and low DO concentration Cb. Hence, we propose to model the

Water

Seabottom

DO concentration Penetration depth

Sediment

(a) Figure 4. Illustration of oxygen penetration depth in sediment. oxygen penetration depth, OPD (see Fig. 4). Oxygen penetration depth depends on the diffusivity in water-sediment interface and oxygen supply to bottom layer from surface layer. Therefore, oxygen penetration depth can be considered to be an index for environmental status of the bottom sediment. Some researchers (e.g. Hanne,1992) tried to measure OPD by using a microelectrode. The microelectrode is very expensive and difficult to be used in a field investigation especially deeper sea area. In this study, we tried to estimate the oxygen penetration depth based on the time series of oxygen consumption flux obtained by measured DO concentration, Cc, in a chamber. DO fluxes were modeled as a summation of a constant flux term and a proportional term to gradient of DO at the bottom surface in this study. We infer that the former term depends on biological

(b) Figure 5. Relationship between constant term and (a) water temperature, (b) DO concentration of the bottom water.

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Estimation Method of Oxygen Penetration Depth into Sea Bottom Sediment

Figure 6. Illustration of oxygen penetration depth. in sediment.

Figure 8. The relationship between coefficient �and oxygen penetration depth.

constant term, biological oxygen consumption, by nondimensional unsaturation degree, Cs-Cb (3) C* = Cs where Cs is saturate DO concentration calculated by the Weiss formula and depends on water temperature and salinity. Fig. 6 shows the relationship between C* and α. As shown in Fig. 6, the constant DO flux, α, obtained from field investigation can be estimated by using non-dimensional unsaturation degree C* with enough accuracy.

FORMULATION OF TERM PROPORTIONAL TO DO CONCENTRATION GRADIENT In order to formulate the other term of DO consumption flux proportional to the spatial gradient of DO, the OPD has to be estimated. In this study, the OPD was estimated from void ration, θ, and sediment oxygen consumption, FCb, when the DO concentration in the chamber, Cc, equals to the bottom water DO concentration, Cb. DO concentration o which obtained from a time series of the DO flux by the field investigation, by the same method by Cai (1996), 2θ ⋅ D ⋅ Cb L= (4) FCb where D is the O2 diffusivity in sediment pore water. The relationship between the estimated OPD by eq. (4) and DO concentration of the bottom water is shown in Fig. 7. The line in Fig. 7 is the estimated regression equation as follows. L = (7.29Cb + 1.89) × 10 −5

(5)

Figure 7. Relationship between oxygen penetration depth and DO concentration of the bottom water.

Figure 9. Comparison between calculated and measured result of oxygen consumption process in a chamber. The reason why the OPD is not naught even if Cb equals zero can be considered that the sea bed had been oxidized before starting measurement by supplying water into the chamber. Fig. 7 implies that the OPD can be estimated by only DO concentration of the bottom water. Further the relationship between the coefficient β estimated by the OPD is shown in Fig. 8. Although there are some deviations, the coefficient β can be estimated with reasonable accuracy. The line in Fig. 8 shows the regression curve represented by 2.53 × 10 −9 (6) L and correlation coefficient was 0.82. Considering that oxygen diffusivity in the water is 0.78x10-9 m2/s, it can be shown that estimation result using the penetration depth is quite reasonable. By using estimation of DO flux, time variance of DO concentration in a chamber at the strong anoxic status is calculated. Fig. 9 shows the comparison between calculated and measured result. It is found that estimation method above mentioned overestimates sediment oxygen consumption. As shown in Fig. 8, the OPD can be estimated by DO concentration of the bottom water, Cb. The reason of overestimation can be considered that estimation method calculate the OPD using a constant Cb during calculation although the DO concentration of the bottom water in the chamber, Cc, keeps changing during calculation. Therefore, the sediment oxygen consumption was calculated by using DO concentration in the chamber, Cc, instead of Cb at every renewal time, τ, which is decided with employing the surface renewal theory.

β=

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function of non-dimensional oxygen unsaturation degree obtained by water temperature and DO concentration of the bottom layer. Chemical oxygen consumption was formulated as a function of DO concentration of the bottom layer and oxygen penetration depth into the sea bottom sediment. These formulations enable the estimation of oxygen concentration in the surface of the sediment without using a complex ecosystem simulation model or expensive measurement instruments. Through the modeling of the sediment oxygen consumption and prediction of the change of the oxygen concentration in the bottom water using the model, it was shown that the oxygen penetration depth was a useful index to express the bottom sediment environment.

RFFERENCES

Figure 10. Estimation of oxygen concentration consumption by using a variable OPD

τ=

4D

(7)

πβ 2

Fig. 10 shows the comparison of an oxygen consumption process estimated by the both methods. The estimation by a variable oxygen concentration, i.e. a variable OPD, agrees very well to measurement result rather than that by a constant OPD. This implies that the oxygen penetration depth, OPD, is significant indicator for environmental remediation. This study presents a conventional estimation method of the OPD and calculation method of a sediment oxygen consumption process. In the conference, prediction of environmental remediation in anoxic and/or hypoxic state in a port and harbor will be also presented using the method by this study.

Cai, Wei-Jun; Frederick L. Sayles, 1996. Oxygen penetration depths and fluxes in marine sediment, Marine Chemistry, 52, 123-131. Endo, T. and Shigematsu, T. and Oda, K., 2005. Large-scale Experiments on Effectiveness of a Breakwater Promoting Vertical Circulation Flow, Proceedings of Asian and Pacific Coasts, pp.531-542. Endo, T. and Shigematsu, T., 2006. Numerical and experimental study of new breakwater promoting vertical circulation flow, Proceedings of the 30th International Conference on Coastal Engineering, ASCE, pp. 4,779-4,791. Endo, T. and Shigematsu, T., 2007. Experimental and numerical study on rearation by a breakwater for generating vertical circulation flow, International Conference on Coastal Structures 2007, pp. 1089-1100. Rasmussen, H. and Jorgensen, B.B., 1992. Microelectrode studies of seasonal oxygen uptake in a coastal sediment: role of molecular diffusion, Marin Ecology Progress Series, Vol. 81, 289-303.

CONCLUSIONS A measurement method of sediment oxygen consumption in an enclosed sea area, which is eutrophied without enough dissolved oxygen for aquatic creatures, was proposed in this paper. Based on the measurement results, sediment oxygen consumption flux was formulated as a summation of biological and chemical oxygen consumption. Biological oxygen consumption was formulated as a

ACKNOWLEDGEMENT This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (B), 20360263.

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