Application of the Ecosystem Model to the Mathematical Simulation of Water Environment Dynamic under Anaerobic State in the Organically Polluted Agricultural Reservoir

Trong hồ chứa bị ô nhiễm hữu cơ, tình trạng phân tầng do nhiệt diễn ra mạnh mẽ hơn dẫn tới sự gia tăng của quá trình yếm khí trong hồ chứa. Quá trình này kết hợp với ô nhiễm hữu cơ trong hồ làm gia tăng quá trình phát thải các chất gây hại từ lớp bùn sình dưới đáy hồ trong thời gian bị yếm khí. Trong nghiên cứu này, chúng tôi tiến hành ứng dụng mô hình toán nhằm đánh giá môi trường nước dưới tác động của tình trạng yếm khí trong hồ bị ô nhiễm hữu cơ.

Master’s thesis

Application of the Ecosystem Model to the Mathematical Simulation of Water Environment Dynamic under Anaero- bic State in the Organically Polluted Agricultural Reservoir

Hoang Quang Duong

Laboratory of Water Environment Engineering Department of Bioproduction Environmental Sciences Graduate School of Bioresource and Bioenvironmental Sciences

Kyushu University

2.2.2 Water quality analysis by laboratory experiment 0 4

3 Dynamic characteristics of water quality under anaerobic state 25

3.1 Annual characteristics of water quality under anaerobic state in the reservoir 25

4 One-dimensional vertical ecosystem model and scenario analysis 43

4.1.1 Construction of one-dimensional vertical ecosystem model 43

1 Introduction

There is growing public concern about environmental issues, and such topics as global warming and sustainable growth are frequency the subject of comment in the press and other media Public pressure continues to grow for greater efforts to be made up the environment and remedy the pollution and degradation that has arisen over the last hundred years

Water makes up a major part of the environment, together with land and air, and is vital to the maintenance of life (J.C.CURRIE and A.T.PEPPER 1993) Water, like energy, clean air and solid organic and soil organic matter makes an essential contribution to the maintenance of economic productivity, social well-being and lifestyle and the maintenance of nature and ecosystem services and various aspects should be treated from that perspective Water resources are under stress world- wide and one of the elements of this stress-water quality has increasingly become a crucial issue for researchers and scientists in their studies (Tasuku Kato 2005) With the developing industrialization and increasing populations, the range of requirements for water has increased together with greater demands for higher quality water It is estimated that 8% of worldwide water use is for household purposes (drinking water, bathing, cooking, sanitation and gardening), 22% for industrial uses (mainly hydropower or nuclear power), and 70% for crop irrigation (Sterling and Vintinner, 2008) Regarding the demand for quality water for agricultural irrigation, there are no national standards or regulations for irrigation water quality Irrigation of food crops presents a possible health risk to food consumers if the quality of the irrigation is inadequate, particularly with respect to pathogens and toxic compounds In term of agricultural use of water, agricultural ponds are dominant a considerable amount of water for irrigation The properties closed water body leads to accumulating the pollution sources in the lake It depends on the characteristics of a lake that has its own pollutants sources One

of the most common pollutants is organic contaminant which is the reason of many serious water issues in the water bodies The annual temperature cycle is one of the most significant determinants

of the physical, chemical, and biological interactions of a lake or reservoir The significance of mal stratification to the eutrophication process and to reservoir restoration lies in the separation of the upper and lower lake zones during the summer through differences in water density The separa- tion of layer prevents the transport oxygen has caused the loss of oxygen in the deep part of the reservoir which is called anaerobic state As summer progress, the process increase phosphorus, ni- trogen in the hypolimnion and frequency are transported vertically in winter season into the water surface area where to subsidize algal growth This biological process response to the excess nutrient input to the lake is known as eutrophication Eutrophication is responsible for many water supply problems (WHO, 1981) Common lake and reservoir problem are excessive algal growth, deficient fishery Water quality in terms of transparency, color, and the odor is often related to a number of

ther-issues Although many methods have been developed to rectify the adverse impacts of cultural trophication It consists of evaluation and controlling activities of the water quality in field, simulation model in the laboratory or mathematical modeling on the computer etc Among them, using mathe- matical modeling is a high effective method to assess the relative importance of processes affecting eutrophication and the potential success of different lake treatment strategies Moreover, the lake manager must acquire a sufficient database to determinate what dominant lake progresses to cause a lake problem, decide which management techniques affect lake processes sufficiently to reduce the problem, and evaluate cost and benefits of management techniques Modeling is a tool that can pro- vide the information for the decision-making process In addition, the mathematical model has a ca- pacity for utilizing in multi-objective Ideally, it offers the lake manager a sophisticated evaluation at

eu-a compeu-areu-atively low cost to eu-aneu-alyze expensive leu-ake meu-aneu-agement techniceu-al eu-and to optimize the money spent on a lake management program Because of the strength of its, the mathematic model has ap- plied for worldwide to simulation the variability of dissolved elements such as nitrogen (NH4+, NO3-

) or major ions Carbonate (C), Calcium (Ca)…etc as well as the seasonal variation of Chl.a and DO

in eutrophic water bodies However, the evaluation and simulation the variability nutrient salts centration are considerable differences between aerobic lake and anaerobic lake It is explained that the concentration of nutrient varies dramatically depending on the erosion mechanism in the bottom anaerobic condition lake Hence, the internal nutrient loading of phosphorus and nitrogen can’t dis- play sufficiently in these modeling Moreover, there is no research on modeling the variation of sul- fide under anaerobic state in the organically polluted reservoir To overcome these limitations, the ecosystem model meets the requirement two criteria (i) simulation the characteristic of dynamic char- acteristics of water quality (ii) reflect the biological-chemical process simulation such as nitrification- denitrification, iron reduction and sulfate reduction under anaerobic conditions

con-The purpose of this study is to develop a modified ecosystem model based on a general dynamic model in order to apply for water quality simulation under anaerobic conditions To achieve this objective, the operation is followed by these steps (1) Evaluation and analysis the dynamic char- acteristic of water quality throughout by data observation collection Data includes of the collection from the surface to bottom of the lake at the central of study areas and laboratory activities consist of concentration measurements, biological determination (2) The results are utilized to correct and develop by applying Fortran programming computer software

hydro-2 Study area and observation data

2.1 Study area

The target of the study is a regulating reservoir (No 5 reservoir) (Figure 2.1) that is located in

Ito campus of Kyushu University, Itoshima Peninsula, Fukuoka Prefecture, Japan No 5 reservoir is

a deep water body (water surface area of ca 13,800 m3, pondage of ca 63,000 m3, and maximum water depth of 8m), was created to store rainfall and supply water for cultivation activities at the downstream crop area There are two box culverts that include box culvert 1 (BC1), located at the

Southwest of the reservoir, and box culvert 2 (BC2), located at the North of the reservoir (Figure

2.1) The inflow from BC1 is treated by a pollution treatment facility to reduce the concentration of

organic material before going to the No.5 reservoir Most of the organic pollution comes from BC2 where runoff directly flows into the reservoir without any treatment The watershed of the reservoir

is deforestation area and agricultural regions Therefore, runoff usually carries a lot of humic acids from these regions to the reservoir, making heavy organic pollution in the water body Every summer, the high concentration of organic matter is the main reason to exacerbate the thermal stratification, leading to the drop of DO at the deep parts of the reservoir At the benthic zone, due to lack of DO, many biochemical reduction reactions such as iron reduction, sulfate reduction occur, release nutrient salts, iron, and sulfide, causing considerable deterioration of water environment

2.2 Observation data

In 2015, the monitoring periods began on April 1 and completed on December 9 The water quality analysis was conducted once a week In the 6-month survey, there are two kinds of water quality analysis activity were implemented, including outdoor analysis activities and indoor analysis activities

2.2.1 Outdoor analysis activities

The outdoor analysis activities that consist of DO, ORP, EC, pH, water temperature and water samples collection were conducted on the fixing point at the center of the reservoir Water samples were collected 1-m intervals from 0m to 8m Nine water samples in total were contained in the two liters plastic bottles and save on the boat before were analyzed in the laboratory by many measure- ments to get the water quality data DO, ORP, EC, pH and water temperature were directly recorded

in the reservoir 0.5-m intervals by using a water quality probe A Secchi-disk, circular disk 30 cm in diameter, was used to determine transparency The meteorological data that consist of air temperature, relative humidity, wind speed, solar radiation, and rainfall were measured automatically every ten

minutes The detailed data of outdoor analysis are shown in Figure 2.2 and from Table 2.1 to Table 2.6

2.2.2 Indoor analysis activities

Water samples were analyzed in the laboratory in order to determine the concentration of water quality indicators including Chlorophyll-a, TN, TP, TOC, DOC, NH4-N, NO3-N, PO4-P, Cl, E254,

SO42-, silica, total iron ion, sulfide The concentration of Chlorophyll-a was determined by using a submersible fluorescence probe (FluoroProbe, bb-Moldaenke, Germany) The concentration of NH4-

N was measured by Quick Ammonia AT-2000 The analysis of NO3-N, SO42- and Cl were analyzed

by an ion chromatography method using AS50 auto-sampler (was manufactured by DIONEX ration) The concentration of sulfide was determined by using methylene blue method (DR5000, HACH) The TOC, DOC were analyzed by using wet-ultraviolet oxidation method (Sievers 900, GEAI) The TN, TP, PO4-P, E254, total iron ion, silica were measured using a portable spectropho-

Corpo-tometer (DR5000, HACH) The detailed data of indoor analysis are shown from Table 2.7 to Table

5 10 15

4 5 6 7 8 9 10 11 12 0

500 1000

Table 2.1 Water temperature observation in 2015 (oC)

Table 2.4 ORP observation in 2015 (mV)

Table 2.6 Transparency observation in 2015

Days

Extinction Coefficient

ency

Table 2.18 Total iron ion in 2015 (mg/l)

3 Dynamic characteristics of water quality under anaerobic state based on field tions

observa-3.1 Annual characteristics of water quality under anaerobic state in the reservoir

No 5 reservoir is a turbidity reservoir due to organic materials that be carried by runoff that the

average transparency is approximate 2.0 m (Figure 3.1) The turbidity of the reservoir prevented light

from reaching the deep part of the reservoir, leading to the quick drop of light intensity in vertical in

the water body (Figure 3.2) Therefore, thermal stratification occurs usually in this reservoir every summer As can be seen from the Figure 3.3, the thermal stratification in the reservoir began in April

and became stronger over time The highest thermal stratification occurred in July and August when the difference in temperature was up to 20oC Due to thermal stratification, the water body is divided into two main layers including epilimnion and hypolimnion and causes large differences in density between two layers These vertical density variations have important impacts on wind mixing and

DO (Jonas and Dake 1969) The division is very resistant to wind mixing and prevents diffusion of

DO In addition, reduction of photosynthesis at depth and organic matter decay near the bottom

sed-iment leads to drop down DO concentration at depth (Koretsky et al 2011) As shown in Figure 3.4,

DO decreased gradually over time in 2015 and was exhausted at 7 m and 8 m for approximately a six month period There was a significant difference between DO at surface and bottom At the surface,

DO usually remained approximately 10 mg/l In contrast, DO was 0 mg/l at the bottom sediment

This condition extended until December when thermal stratification disappeared (Figure 3.3) and

DO increased again (Figure 3.4) The state that DO is less than 0.5 mg/l could be considered as an

anaerobic state (Chen and Liu 2003) According to observation data, it could conclude that anaerobic

state occurred above the benthic zone from the end of May to December (Figure 3.4)

During anaerobic state, lack of DO leads to falling quickly the oxidation-reduction potential

(ORP) and increase the electrical conductivity (EC) (Figure 3.5 and 3.6) It is well known that ORP

is used to estimate the ability of a lake or river to cleanse itself and break down waste products ORP depends on the amount of dissolved oxygen as well as others elements that function similarly to oxygen The negative ORP indicates that DO exhaust and terminal electron acceptors are NO3, FeO(OH), and SO4 Therefore, reduction reactions such as denitrification, ammonification, iron re- duction and sulfate reduction will occur in the water body They are reasons of releasing nutrients from sediment and generation of sulfide in the closed water body through reduction processes (Gor- don 2007) In the No.5 reservoir, under anaero-

bic state, ORP usually reach high negative

value (up to -200 mV) near the bottom sediment

(Figure 3.5) The high negative value of ORP 2

the bottom As can be seen from Figure 3.5 and Figure 3.6, when ORP reduced to -144 mV at 8 m

on May 27th, EC also began to increase The high value of EC at 7 m and 8 m that maintained during

the condition where DO was 0 mg/l and ORP was lower than -150 mV showed the significant release

of reduction reaction products from bottom sediment

Figures 3.7 to 3.14 represent the seasonal change and vertical variation of water quality

indica-tors from April to December in 2015 It shows that in 2015, under anaerobic state, the water ment dynamic had significant change over time, especially at 7 m and 8 m As described in the pre- vious part, DO deficiency causes NO3-N, FeO(OH) and SO42- to become electron acceptors instead

environ-of oxygen, and be used by anaerobic microorganisms Therefore, the concentration environ-of these water quality indicators decreased rapidly On the other hand, opposition to the loss of NO3-N and SO42-, the concentration of nutrient salts (NH4-N and PO4-P), total iron, and sulfide increased quickly at 7

m and 8 m They are products of reduction reaction including ammonification, iron reduction, and sulfate reduction The release of NH4-N near the bottom began to rise dramatically at the end of July corresponding to the low concentration of NO3-N (Figure 3.7 and 3.8) It could be interpreted by the

release of ammonia from organics that are being decomposed (Gordon and Higgins 2007) and the inhibition of nitrification and ammonia assimilation under anaerobic state (Beutel 2006) Due to the

resulting built up of ammonia, TN was very high concentration during anoxic condition (Figure 3.9)

PO4-P and total iron ion, as well as sulfide, started to increase when NO3-N almost was exhausted They are products of two reduction processes consist of iron reduction and sulfate reduction It is well known that iron (III) bind phosphate in the oxidized sediment (Mortimer 1971) Therefore, when DO appears in the water body, the concentration of PO4-P and the total iron ion is usually very low Under anaerobic condition, the iron reduction is maintained by nitrate-reducing bacteria and phosphate and iron increase concomitantly as a result of reduction process of co-precipitate iron-phosphate and or- ganic carbon forms (Skoog and Arias-Esquivel 2009) Thus, it is clear to see PO4-P and total iron ion increased quickly after exhaustion of NO3-N (Figure 3.8, 3.10 and 3.12), and led to the dramatic rise

of TP (Figure 3.11) Figures 3.14 showed the seasonal change of sulfide in 2015 Before anaerobic

state occurred, sulfide was almost zero and only begun to build up near the bottom from July 29th This increase is due to generation of sulfide from sulfate reduction that occurs strongly under extreme anoxic condition Moreover, the occurrence of sulfate reduction could be verified by a significant decrease of SO42- in the same time with the increase of sulfide (Figure 3.13) The observation data

of sulfide showed that some time sulfide suddenly dropped in concentration and then increased again

It was assumed that sulfide decreased under anaerobic state because of the formation of insoluble compounds such as FeS (Uwabara and Lexander 1999; Haaning Nielsen et al 2005)

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