Ecological Modelling 152(2002)65–75A water quality modeling study of the Nakdong River,KoreaSeok Soon Park a,*,Yong Seok Lee baDepartment of En 6ironmental Science and Engineering ,College of Engineering ,Ewha Womans Uni 6ersity ,Daehyun -Dong ,Seoul 120-750,Republic of KoreabDepartment of En 6ironmental Engineering and Management ,Hallym College of Information and Industry ,Chuncheon ,Republic of KoreaReceived 22May 2001;received in revised form 9November 2001;accepted 26November 2001AbstractA water quality modeling study was performed for water quality management of large river systems where autochthonous sources and denitrification play an important role in biochemical oxygen demand (BOD)and nitrogen dynamics.The model was based on the USEPA’s QUAL2E and several modifications were made in the computer code to overcome limitations of QUAL2E.This paper outlines changes which were made in the model in addressing some of water quality problems and demonstrates the model application to the Nakdong River,Korea.The changes include the modification of computational structure and the addition of new constituent interactions,such as conversion of algal death to BOD,denitrification,and dissolved oxygen (DO)change caused by fixed plant.To validate the program modifications,both QUAL2K and QUAL2E were applied to the same reaches of river and the results were compared.The water quality parameters included in the model were DO,BOD,nitrogen and phosphorus series,and chlorophyll-a .Both QUAL2K and QUAL2E model represent the field data quite well except for some parameters of QUAL2E.In BOD,DO,and total nitrogen,there are significant discrepancies between the results of two models,where QUAL2K displayed better agreement with the field measurements than QUAL2E due to QUAL2K’s ability to simulate the conversion of algal death to BOD,fixed plant DO,and the denitrification.©2002Elsevier Science B.V.All rights reserved.Keywords :Water quality model;River;QUAL2E;Denitrification;Eutrophication /locate /ecolmodel1.IntroductionThe Nakdong River,which is one of four major river systems in Korea,serves as an importantwater resource for the south eastern area (Fig.1).The river drains an area of 23,817km 2and length of the main stream is over 525km.Currently,about 7million people reside within the basin and more than 13million people intake a drinking water from the river.During the last several decades,the combination of rapid population growth coupled with industrial and urban devel-*Corresponding author.Tel.:+82-2-3277-3546;fax:+82-2-3277-3275.E -mail address :ssp@ewha.ac.kr (S.S.Park).0304-3800/02/$-see front matter ©2002Elsevier Science B.V.All rights reserved.PII:S 0304-3800(01)00489-6S.S.Park,Y.S.Lee/Ecological Modelling152(2002)65–75 66opment had resulted in a serious deterioration of water quality in downstream area.Major pollu-tion sources include domestic sewage,industrial wastewater,livestock discharge,and urban and agricultural runoff(Kim et al.,1998).In addition to such high anthropogenic pres-sures,the barrage was built in estuary to protect fresh water resources from saltwater intrusion, such that the riverflow become very slow and almost impounded in downstream area.This im-Fig.1.The Nakdong River and watershed.S.S.Park,Y.S.Lee/Ecological Modelling152(2002)65–7567Fig.2.Observed profiles of water qualities in the Nakdong River.are generally good in the upstream area but be-come seriously deteriorated in midstream,espe-cially after the confluence of the Kumho River, where the Daegu metropolitan area,the third largest city,is located.The construction and upgrading of several mu-nicipal wastewater facilities and the enforcement of effluent requirements on some of the industrial sources has resulted in noticeable improvements in the water quality of some of the tributaries.In spite of such improvements in the tributaries and the wastewater discharges,however,the down-stream water quality remains in the same serious condition as before(Fig.3).The locations of water quality measurements in Fig.3,are shown in Fig.1.During lowflow period,the downstream water quality was even worse than before the construction of wastewater facilities.This unrea-sonable cost-benefit effect can be attributed to the algal bloom in the impounded area.The algal bloom was often observed in thefield measure-ment during last several years.In order to examine the relationship between the pollution control strategies and the receiving water quality,the water quality modeling study was performed in the Nakdong River and the results are presented in this paper.The modeling study includes development and application of a computer model,which is suitable for the large river with a heavy algal growth.2.Model developmentThe USEPA’s QUAL2E model(Brown and Barnwell,1987)was selected as a best available one for use in the Nakdong River based on system nature which was described in Section1, after a review of several water quality models which can be adopted on personal computer,such as WASP5(Ambrose et al.,1987),CE-QUAL-W2,STREAM(Park and Lee,1996),DYRESM (Hamilton and Schladow,1997),SIREM1(Park and Uchrin,1997).The model was originally de-veloped in1970and has been followed by several modifications,as shown in Fig.4.Currently,the QUAL2E model has been the most widely used stream model(Drolc and Koncan,1999;Somly-pounded downstream supplies a drinking water to the Pusan metropolitan area,the second largest city in Korea.Fig.2presents observed profiles of 5-day biochemical oxygen demand(BOD5),dis-solved oxygen(DO),total nitrogen(TN),and total phosphorus(TP)in main stream and major tributaries.As shown in thisfigure,water qualitiesS.S.Park,Y.S.Lee/Ecological Modelling152(2002)65–75 68ody et al.,1998;Yang et al.,2000).The model is numerically accurate and includes an updated ki-netic structure for most conventional pollutants (Chapra,1997;McCutcheon and French,1981; Roesner et al.,1977).Several limitations of the QUAL2E model, however,have been reported(Park and Uchrin, 1990;Park and Lee,1996).One of the major inadequacies of the QUAL2E model is the lack of provision for conversion of algal death to BOD, which is autochtonous source of organic matter (Ambrose et al.,1987;Park and Uchrin,1996, 1997).Since the algal bloom is an important phenomenon found in downstream area,this in-adequacy of the model should be repaired to simulate the water quality dynamics in the Nakdong River.This river has shown the signifi-cant denitrification in bottom layer in water column,and DO changes caused by periphyton and macrophyte in riparian zone(Kim et al., 1998).In QUAL2E model,however,these phe-nomena are not included.In addition,the maxi-mum numbers of reaches,computational elements,and junctions are limited in currently available version of the QUAL2E model,such that the model cannot simulate the large river system with high accuracy.In this study,therefore,modifications were made in the computer code to overcome such limitations and the modified version was named as QUAL2K,which stands for2000Year Version of USEPA’s QUAL2E.The major enhancements of the QUAL2K model include the expansion of computational structure and the addition of new constituent interactions,such as algal BOD,deni-trification,and DO change caused byfixed plant. The evolution of QUAL2K model was displayed in Fig.4and the differences between two models were summarized in Table1.Most of the model equations included in QUAL2K are same as in QUAL2E,except for DO,BOD,and nitrate.The equations modified in QUAL2K were compared with those in QUAL2E in Table2and all notations were described in Appendix A.As shown in this table,sink of DO by algae in QUAL2E was separated into two components in QUAL2K,which are respiration and death of algae.Algal death was converted toFig.3.Time series of water qualities in major tributary and main stream.S.S.Park,Y.S.Lee/Ecological Modelling152(2002)65–7569 Fig.4.Evolution of water quality models for a river system.BOD in QUAL2K by the expression of h4z2A, similar as that of algal respiration.h4is oxygen uptake per unit of algal respiration/death(mg-O2/ mg-A),z2is algal death rate(1/day),and A is algal biomass(mg/l).QUAL2K also accounts for the resuspension of BOD from bottom area by including the term of K4/D and DO change caused byfixed plant was added in QUAL2K,by the expression of(u1−u2)/D.K4is the bottom resuspension rate of BOD(g O2/m2per day),D is the average depth(m),and u1and u2are oxygen consumption and production rate by attached plant respiration(g O2/m2per day),respectively. Denitrification process is assumed to occur in anaerobic sediment layer or only in water column under extremely low DO conditions(Park and Jaffe,1999).This process includes reduction of Table1Differences between QUAL2E and QUAL2KQUAL2E QUAL2K Model parameterYesNoAutochthonous sourceYesNoDenitrificationYesNoDO change byfixed plant156Number of maximum junctions25100 Number of maximum reaches1000250Number of maximum elementsS .S .Park ,Y .S .Lee /Ecological Modelling 152(2002)65–7570Table 2Modi fied equations in QUAL2K,compared with those in QUAL2E Parameter Model EquationsQUAL2EDOdOd t=K 2(O s −O)+(h 3v −h 4z )A −K 1L −K 5D−h 5i 1N 1−h 6i 2N 2QUAL2KdOd t=K 2(O s −O)+(h 3v −h 4z 1)A −K 1L −K 5D −h 5i 1N 1−h 6i 2N 2+u 2−u 1DQUAL2E BOD d Ld t=−K 1L −K 3L QUAL2Kd L d t =−K 1L −K 3L +K 4D +h 4z 2A QUAL2ENitrate dN 3d t=i 2N 2−(1−F )h 1v AQUAL2KdN 3d t =i 2N 2−(1−F )h 1v A −i 3N 3in water column even if denitri fication occurs near around sediment water interface.In QUAL2K,therefore,water column oxygen level was not included in denitri fication equation.As shown in the table,only reduction of nitrate is assumed as a first order reaction by the expression i 3N 3,where i 3is the rate constant of denitri fication and N 3is nitrate nitrogen.The reduction of BOD by denitri fication can be treated in the model as sedimentation rate (K 3)which can decrease BOD without consuming oxygen.3.Model applicationTo validate the program modi fications,both QUAL2K and QUAL2E were applied to same reaches of the river and the results were com-pared.Based on the river characteristics as well as the model structure,the study area was divided into 54reaches.The main stream and the tribu-taries were divided into 26and 28reaches,respec-tively.Since the model structure of QUAL2K is same as that of QUAL2E,the input data are almost identical in both models,except for some kinetics of the modi fied water quality interactions.Reaches and pollution sources in each tributary were given in Table 3.Each reach was then subdivided into uniform computational elements,which form the basis for model ’s finite difference numerical solution.The size of these elements was chosen to be 1.0km.Schematics of the system segmentation is dis-played in Fig.5,along with locations of tributaryBOD as well as nitrate,and DO level is an important parameter to control this process.In certain model,such as WASP,where sediment and vertical DO can be computed,these parame-ters are considered in model equation.Since QUAL2K assumes complete vertical mixing same as QUAL2E,such low DO level cannot be foundTable 3Reaches and pollution sources in the Nakdong River model Distances (km)Branches No.reaches No.non-point No.point 34112Main stem 926312Naesungchun 12Younggang 10101684Wichun 21292Kanchun 11816Kumhogang 27113Hoichun 44313Hwanggang 1070524Namgang 3622MilyanggangS.S.Park,Y.S.Lee/Ecological Modelling152(2002)65–7571 Fig.5.System segmentation and location of pollution sources.flow inputs as well as type of pollution sources.P, S,and NS in Fig.5,indicates point source,tribu-tary stream,and non-point source,respectively. The measured river geometry was used to deter-mine the hydraulic characteristics in each reach using the trapezoidal method,which is given as an option in QUAL2E model(Brown and Barnwell, 1987).The velocity,cross-sectional area,and depth are computed from the riverflow by the trial and error solution of Manning’s equation and the method is fully described in Chapra (1997).Both QUAL2K and QUAL2E were calibrated and verified in steady-state mode using average spring(March to May)and fall(September to November)conditions during1996–2000,respec-tively.Since very light rainfall is usually observed during spring and fall in Korean peninsula,al-S.S.Park,Y.S.Lee/Ecological Modelling152(2002)65–75 72most steady-state conditions are obtained in river flow and pollution loads.Thefield data were collected by the national water qualities monitor-ing program of the Korea Ministry of Environ-ment.The water quality parameters included in the model were DO,BOD,nitrogen and phospho-rus series,and chlorophyll-a.All parameters were modeled in a steady-state ing the input conditions,the model was run and the results compared to thefield measurements.System co-efficients were appropriately adjusted until rea-sonable agreement between model results and field measurements was achieved.All values of system coefficients used in QUAL2E were same as those in QUAL2K except for the algal respiration,death,and denitrifica-tion.The values of system coefficients were based on the typical values cited in the model documen-tation(Bowie et al.,1985;Brown and Barnwell, 1987)and the calibrated values of selected coeffi-cients were summarized in Table4.As shown in this table,algal respiration rate(z)in QUAL2E was separated into respiration rate(z1)of living algae and death rate(z2),which is converted to BOD,in QUAL2K.In order to examine the contribution of BOD by algal death,benthic source of BOD was set to be zero in QUAL2K. Benthic source of BOD was calibrated with set-tling rate in this study to set same input data in QUAL2E and QUAL2K.The denitrification rate was calibrated and ranged from0.0to0.35(1/ day)in this system.The calibrated values of all other coefficients were located within the general ranges recommended by Brown and Barnwell (1987).In order to test the ability of the models to predict water quality conditions under different ambient weather andflow condition,model verifi-cation studies were performed using average fall conditions.The system coefficients in QUAL2K as well as QUAL2E were kept identical to those values determined during the model calibration. Both QUAL2K and QUAL2E model results were compared withfield measurements infigures (Figs.6and7).Field measurements are displayed as mean and95%confidence intervals in allTable4Values of major system coefficients used in Nakdong River QUAL2E and QUAL2KModel aUnitSymbolDescription ValueRange bAlgae1/day0.25zRespiration rate(dead+living)0.05–0.5E&Kz11/dayDeath rate K–0.080.17–K1/dayz2Respiration(living)h3mg-O2/mg-A E&K 1.4–1.8 1.6 Oxygen production1.6mg-O2/mg-AOxygen uptake E&Kh4 1.6–2.3BOD and DO0.2–0.40.02–3.4E&KDeoxygenation rate1/dayK1K31/day E&K−0.36–0.360.05–0.98 Settling rateK4g O2/m2per dayBenthos source K–0.0Kg O2/m2per day 2.0–5.0u2–O2production byfixed plantNitrogen and phosphorusi0Organic-N decay1/day E&K0.02–0.40.1–0.30.01–0.1 Organic-N settling rate|41/day E&K0.001–0.1i11/dayOxidation of NH3–N E&K0.10–1.000.4–0.8 Oxidation of NO2–N i21/day E&K0.20–2.0 1.0–2.01/dayDenitrification0.0–0.35–Ki3Organic-P decay1/dayi4E&K0.01–0.70.1–0.5|51/dayOrganic-P settling E&K0.001–0.10.01–0.1a E&K,QUAL2E and QUAL2K;K,QUAL2K only.b Values given in Brown and Barnwell(1987).S .S .Park ,Y .S .Lee /Ecological Modelling 152(2002)65–7573Fig.6.Model calibration results.signi ficant discrepancies between the results of two models.From these figures,it is to note that QUAL2K displayed better agreement with the field measurements than QUAL2E.This is due toFig.7.Model veri fication results.figures.The figure shows that both models repre-sent the field data quite well in some parameters,where the model prediction lines coincide in figures.In BOD,DO,and TN,however,there areS.S.Park,Y.S.Lee/Ecological Modelling152(2002)65–75 74QUAL2K’s ability to simulate the conversion of algal death to BOD,fixed plant DO,and the denitrification,which are not available in QUAL2E.4.Summary and conclusionA water quality modeling study was performed in the Nakdong River,where autochthonous sources and denitrification play an important role in BOD and nitrogen dynamics.The Nakdong River,one of four major river systems in Korea, serves as an important water resource for the south eastern area.Currently,about7million people reside within the basin and more than13 million people intake a drinking water from the river.In order to examine the relationship be-tween the pollution control strategies and the receiving water quality,the water quality model-ing study was performed in this river and the results are presented in this paper.The modeling study includes development and application of a computer model,which is appropriate for the large river with a heavy algal growth.The model was based on the USEPA’s QUAL2E and several modifications were made in the computer code to overcome limitations of QUAL2E.These modifications include the addi-tion of new water quality interactions,such as conversion of algal death to BOD,dentrification, and DO change caused byfixed plants.In addi-tion,the maximum number of reaches,computa-tional elements,and junctions are extended to be applicable for the large river system.To validate the program modifications,both QUAL2K and QUAL2E were applied to the same reaches of river and the results were compared.The water quality parameters included in the model were DO,BOD,nitrogen and phosphorus series,and chlorophyll-a.Both QUAL2K and QUAL2E model represent thefield data quite well except for some parameters of QUAL2E.In BOD,DO, and TN,there are significant discrepancies be-tween the results of two models,where QUAL2K displayed better agreement with thefield measure-ments than QUAL2E due to QUAL2K’s ability to simulate the conversion of algal death to BOD,fixed plant DO,and the denitrification.AcknowledgementsThis study was partially supported by the Ko-rea Environment Institute(KEI).The authors gratefully acknowledge the support and the water quality data provided by KEI.Appendix A.Nomenclaturedissolved oxygen(mg/l)OO s saturated dissolved oxygen(mg/l)biochemical oxygen demand(mg/l)Lnitrite nitrogen(mg/l)N2nitrate nitrogen(mg/l)N3A algal biomass(mg/l)averaged water depth(m)DK1deoxygenation rate coefficient(1/day) reaeration rate coefficient(1/day)K2K3settling rate coefficient(1/day)K4benthos source rate of BOD(g O2/m2per day)sediment oxygen demand(g O2/m2perK5day)v specific growth rate of algae,(1/day)algal respiration plus death rate(1/day)zz1algal respiration rate(1/day)algal death rate(1/day)z2i1rate constant for the oxidation of ammo-nia nitrogen(1/day)rate constant for the oxidation of nitritei2nitrogen(1/day)specific denitrification rate(1/day)i3fraction of algal nitrogen taken from am-Fmonia pool(0–1)fraction of algal biomass that is nitrogenh1(mg-N/mg-A)oxygen production per unit algal growthh3(mg-O2/mg-A)oxygen uptake per unit of algal respiration h4(mg-O2/mg-A)oxygen uptake per unit of NH3oxidationh5(mg-O2/mg-N)h6oxygen uptake per unit of NO2oxidation (mg-O2/mg-N)u1O2consumption byfixed plant respiration(g O2/m2per day)S.S.Park,Y.S.Lee/Ecological Modelling152(2002)65–7575O2production 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