Modelling Acidity Propagation in a Coastal Zone with Acid Sulphate Soils

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1 Modellg Acdty Propagato a Coastal Zoe wth Acd Sulphate Sols N.D. Phog 1,, C.T. Hoah, T.P. Tuog 3, H. Malao ad T.Weaver 5 1 Uversty of Nog Lam, Ho Ch Mh Cty, Vetam. Iteratoal Water Maagemet Isttute (IWMI), Southeast Asa Regoal Offce, Peag, Malaysa. 3 Iteratoal Rce Research Isttute (IRRI), Los Baños, Phlppes. Uversty of Melboure, Vctora, Australa. 5 URS Australa Pty Ltd Emal: godagphog@pgrad.umelb.edu.au Keywords: Water qualty, salty, acdty, polluto, water maagemet, ttrato, ruoff. EXTENDED ABSTRACT Acdc polluto caused by ruoff from acd sulphate sols (ASS) ca be detrmetal to agrculture, aquaculture ad the evromet. It s dffcult to quatfy the amout of acd loadg, how t trasports ad pollutes the surroudg caal etworks. I a coastal zoe, these pheomea are further complcated by the tdal effect o the water dyamcs ad the chemcal reactos betwee sale ad acd water. Ths paper presets a model that ca smulate the propagato of acd polluto a caal etwork a coastal zoe wth ASS. The model was developed ad coupled wth a prevously valdated hydraulc ad salty model VRSAP (Vetam Rver System Ad Plas). Ra water dssolves the acd materal exposed o caal embakmet durg excavato ad dredgg ad dras t to caal as ruoff ad bypass flow. The amout of acd loadg from each lear meter of caal embakmet was quatfed by a multple ste expermet carred out 5 ad used as put of the model. It was calculated as fucto of sol types, the age (years after last dredgg) of caal embakmet ad rafall. The teracto betwee the acdc water ad bracksh water the recevg caal s smulated by emprcal fuctos. These fuctos, establshed by laboratory ttrato expermets, descrbe the chage of the recevg water at dfferet salty levels as t receved creasg amout of ttrat at dfferet. The salty ad acdty at odes the caal etwork were calculated at each tme step by solvg the solute trasport equato system the VRSAP. The effect of sale water o acdty reducto s corporated to calculatos for water acdty segmets ad at odes. Evaluato of model performace calbrato ad valdato steps clude both statstcal crtera ad graphcal dsplays for comparso. The statstcal dcators of model performace are the square of lear regresso coeffcet (r ) ad the model effcecy (EF) (Aderse et al., 1). For graphcal comparso, the graphs showg smulated versus measured mothly averages of all motored stes ad mothly at 3 specfc stes the study area are used. The model was calbrated wth ad salty measured mothly from December to Aprl or b-mothly from May to July at 3 motored stes 3. Durg calbrato, the dsperso (DISP), legth of ruoff path o caal embakmet (ROC) ad seepage across embakmet (SP) were parameterzed to obta the best ft betwee the smulated ad measured values 3. Subsequetly, the model was valdated wth the measured 5. Geerally, the VRSAP wth ew acdty module ca smulate the average reasoably well as reflected through the comparso of the model outputs wth measured data 3 for calbrato ad 5 for valdato. creased the dry seaso, reached a maxmum o 15 March, ad gradually decreased later moths whe the ray seaso started. The model also descrbes the effect of sale water reducg acdty the caal etwork. The smulato of the acd propagato the study area could result evaluatg the relatve effectveess of dfferet strateges for acd load reducto the coastal zoes wth ASS. 17

2 1. INTRODUCTION Besdes salt water truso, acd water from acd sulphate sols s a commo problem the coastal zoes ad lmts agrculture ad aquaculture. Studyg the tdal bufferg to eutralze the acdty a sgle caal, Idrarata et al. () showed that some alkalty forms sea water such as bcarboate (HCO 3 - ) ad carboate CO 3 - could have the acd eutralzg capacty. It s possble to determe the result due to ths capacty f mxg wth acdc water occurs. May models were developed to smulate water qualty parameters, for example the ISIS Qualty (Halcrow ad HR Wallgford, 1999), a fully featured water qualty smulator that cludes advecto, dffuso of coservatve, decayg pollutats, water temperature, sedmet trasport, teracto wth qualty determats wth sedmet, phytoplakto ad. However, these models do ot work or apply for acd propagato a complex caal etwork uder tdal effect ad salty truso the coastal zoes. Truog et al. (1995) preseted a model for acd water trasport caal etwork the Pla of Reeds of the Mekog delta. The model was based o emprcal regresso betwee ad other toxc elemets that assumed jurbate equlbrum exstg the o sale, acd water evromet. But the smulated dd ot agree wth the measured values. The authors attrbuted the falure of the model to the correct reflecto of chemcal processes of acdty the feld drag to the caals. I summary, t s dffcult to quatfy the amout of acd loadg ad how t trasports ad pollutes the surroudg caal etworks. Furthermore, the coastal delta the hydrodyamc model s a mportat platform for modellg of water qualty. Ths study troduces a ew acdty module coupled wth hydrodyamc ad salty modules of a exstg model to smulate the propagato of acdty the caal etwork of the Camau pesula, Mekog Delta, Vetam.. ACID POLLUTION IN THE STUDY AREA The study area s the Camau pesula located the souther part of the Mekog delta, Vetam wth a total area of 99,599 ha (Fg. 1). Acd sulphate sol (ASS) cocetrated the wester part occupes 5% of ts total area (SNIAPP, ). Ma producto systems the area are rce the easter part, shrmp the wester part ad shrmp-rce the termedate zoe (Hoah et al., 1). There are two dstctve seasos: the ray seaso from May to November wth roughly 9% of the aual rafall of 1, mm ad the dry seaso from md-november to Aprl. The hydrologcal regme s govered by the flow of the Mekog rver ad the tde from the East ad West Seas. A dese mamade caal etwork the area acts as a trasportato route, rrgato ad draage codut. Sce the caals are heavly slted, dredgg s mplemeted every 5 to 1 years. Tde West Sea J Boudary of Ca Mau pesula Og Doc rver m3/s Ca Lo rver 1 m3/s Nh quo Phuoc Log Pho Sh Ho phog Fgure 1: The Camau pesula, Mekog Delta ad ts ma rver systems. Durg excavato ad dredgg of caals the ASS areas, the sulphurc ad pyretc materals from subsols were put ad exposed o the caal embakmets resultg the oxdato of the pyrte (Cook et al., ) ad formg acdty whch s brought up to the sol surface by capllary rse durg the dry seaso. Ra water dssolves the acd materal from caal embakmet ad dras t to caal as ruoff ad bypass flow. Every year the acdc load propagatg the caal etwork results acd polluto over a large area. Durg the early part of the ray seaso, water the caals could drop from about 7 to below a large area (the estmated polluto area s, ha). 3. METHODOLOGY Fresh water zoe Ma caals from Bassac rver m3/s Sale water zoe Tde East Sea My Thah rver 1 m3/s Gah Hao LEGEND rver Shallow acd sols Deep acd sols m3/sec No-acd sols Water maagemet ut 3.1 Icorporato of acdty module to hydraulc-salty model VRSAP The VRSAP (Vetam Rver Systems ad Plas) wth ts hydraulcs ad salty modules was calbrated ad valdated wth data 199 ad 199 the Camau pesula (Hoah et al., 1). VRSAP s a umercal model usg Sat-Veat equatos for solvg complex flow ad mass trasport problems a complex etwork of tercoectg ope chaels. It s based o the cotuty ad the mometum equatos for each caal segmet: 171

3 Q z + Bc = q (1) 1 R 1/ K = AC R C = z α ( Q / A) α Q ( Q / A) / Q / + + = g g A K Q () where; Q s the dscharge (L 3 T -1 ), x s the dstace alog the segmet, B c s the caal wdth, cludg storage area, averaged over the segmet (L), z s the water level (L), t s tme (T), q s the lateral flow per ut legth (L 3 T -1 L -1 ) to the segmet, α o ad α are adjustmet coeffcets, varyg from 1. to 1.1, g s the gravty accelerato (LT - ), A s the cross-secto area (L ), C s the Chezy coeffcet for bottom roughess (L - T -1 ), R s the hydraulc radus (L), s the Mag's coeffcet. The model apples the cocept of advecto ad dsperso (Harlema, 1971) to smulate salty truso: ( Ac S) ( QS) S + ( Dx A ) = qsv (3) where; A c s the cross secto, cludg storage area evely spread alog the segmet (L ), S s the salt cocetrato (ML -3 ), D x s the logtudal dsperso coeffcet (L T -1 ), S v s the salt cocetrato of lateral flow (ML -3 ). The ew acd module s corporated to the VRSAP model after the hydraulc ad salty modules as show Fgure. Start The geeral equatos of acdty trasport cludg the effect of salty o decreasg acdty are (): C ( QS) = ( AD A A x C ql ) + λ ( CL C) () A where; A s the cross secto of caal segmet (L ), C s the acd cocetrato of caal water (ML -3 ), D x s the logtudal dsperso coeffcet (L T - 1 ), C L s the acd cocetrato of lateral flow (ML - 3 ), λ s the rate of decreasg acdty by effect of salty [1/T] ad q L s lateral flow rate [L 3 /T-L]. Fgure 3 explas detals of the computato acdty module. The advecto ad dsperso (frst ad secod compoets o the rght of equato ()) are calculated block c, the last compoet o the rght of equato () dealg wth the acd load to the caal etwork caused by ruoff, bypass or seepage s calculated blocks b ad c. By usg the mplct fte dfferece scheme to solve the acd equato system as for salty, the acdty at all odes s computed block e. The last step block f s the recalculato of acdty at odes by takg to accout the effect of salty o acdty reducto. For = 1 to segmet a. Embakmet type ad wdth, sol type, dredgg year b. Lateral acd load from embakmet of segmet t = Topographc data of 7 segmets, 15 odes, felds Iput tal codto for Z, Q, S at t = No t = t + 1 Hydraulc module Iput Z, Q, S at boudares Establshg the equato system of Z t = Ed T Output Z, S of caals, felds, Q segmets Store Q, Z, S for ext tme step New S at odes ad felds Yes Ed Acdty module c. Iteracto betwee lateral acdc flows ad water segmet d. Calculatg advecto ad dsperso compoets of acdc flow segmet e. Establshg ad solvg equato system for acdty at odes f. Recalculatg acdty at odes by takg to accout of the effect of salty o acdty reducto. g. Ttrato curves expressg chage of of recevg sale water whe ttrated wth acd water. Solvg the equato system of Z New Z at odes ad felds & ew Q segmets Solvg the equato system of S Establshg the equato system of S Salty module Notes: Z, S are water level ad salty at odes ad felds, ad Q s dscharge segmets. t s curret tme step. Ed T s last tme step. Fgure : Icorporato of acdty module to VRSAP model Fgure 3: Flowchart of calculato acdty module Iput data for acdty module cludes parameters of caal segmet that fluece the acd load to the caal such as embakmet type (road or o road), caal level (prmary, secodary or tertary level for determg the embakmet wdth), sol type (severe, medum or o ASS), year of the last 17

4 dredgg/excavato ad the felds coected to the segmet. The put acd loadg from the caal baks were calculated by the regresso model establshed from results of a feld expermet (Secto 3.). I addto, to calculate the chage of the sale recevg water caused by acdc flow, the ttrato curves establshed from a laboratory expermet are also used the acdty module (Secto 3.3). 3. Quatfyg acdty flows from caal embakmet to caal etwork A feld expermet was carred out the study area from 1 Aprl 5 to 15 July 5 to verfy that acdty released from the caal embakmet deped o ts sol type ad ts age. The expermet cluded 7 stes alog dfferet secodary caals the study area. The ste selecto was based o two factors: ASS type (dstgushed by the exstg ad depth of the sulphurc horzo alog sol profle,.e. at -5 cm depth: severe ASS, 5-1 cm depth: medum ASS, ad wthout sulphurc horzo: o ASS) ad age of the embakmet (umber of years after the last dredgg: ether <, to 3 or >3 years). Data from the feld expermet were used to geerate the regresso equatos that smulate ruoff ad ts acdty for every combato of sol type ad age of the embakmet. Computato of acd loadg Acdty s expressed term of [H + ] per volume ut or = 1/log (H + ). The daly total acdty of ruoff per ut of caal legth (TA) s defed as: TA = [ H + ] x RO (mmol H +.m -1.day -1 ) where: RO s ruoff (l.m -1.day -1 ). I the acdty module, TA at day s calculated from the cumulatve total acdty at day (Ts ) ad prevous day -1 (Ts -1 ): TA = Ts Ts -1 Ts ad Ts -1 are looked up from the emprcal regresso model betwee the cumulatve acdty ad daly rafall establshed for each combato of sol type ad age of the embakmet. The expermet results that the bypass acd load was very low compared wth ruoff ad seepage compoets, therefore t was eglgble the acd module. The motored acdty of seepage across caal embakmet dd ot vary much durg the expermet, hece average values obtaed from expermetal data for each sol type were used ad assumed uchaged durg dry seaso ad begg of ray seaso. Expermet setup Daly rafall, ruoff ad bypass volume were measured by usg a collecto system at each expermetal ste that comprsed of a stadard ra gauge ( dameter ad tall), a ruoff collector box ad a bypass collector box (Fg. ). Rawater, ruoff ad bypass water were stored a ce box ad trasported rght after samplg to a moble laboratory located at a farmer s house earby these expermetal stes for measuremet of ad acdty. Seepage water across caal embakmet was also sampled oce a week by usg a vacuum samplg techque wth sucto cups (Phog, 1). Acdty of all ruoff, bypass ad seepage water samples was aalysed usg the ttrato method (APHA, 199). The Darcy s equato was appled for calculatg seepage flow where the hydraulc gradet s the dfferet betwee water levels felds ad caals. The saturated hydraulc coductvty Ks were measured at 3 locatos each of caals wth dfferet sol types. However, the varato of Ks does ot cause a sgfcat varato seepage flow, therefore we use a average value of.1m.s -1 for all combatos of sol type ad age of the embakmet. Bypass collector box Ruoff collector box Fgure : Ruoff ad bypass water collecto o caal embakmet 3.3 Iteracto betwee acdc flow ad recevg water body Whe acdc water from the baks dras to a caal, the of caal water wll be lower because of the addto of H + to the water body. The chage of the recevg water body depeds o () the rato betwee the recevg water volume ad the acdc flow ad () the salty cocetrato of the recevg water, because sea water cotas bcarboate that has a eutralzato capacty. A ttrato expermet was carred out to smulate the teracto betwee the acdc flow ad the caal water. The ttrats represeted the acdc flow wth dfferet acdty levels ( 3 to 7); ad the recevg water (recpet) samples represeted 173

5 the caal water wth dfferet salty levels (, 1,, 3 or 55 dsm -1 ). The ttrats were prepared by dlutg extracted sol soluto of a severe ASS sample ( =3.1 at sol water rato 1:5) take from the study area. The recevg water samples were prepared by usg the sea water (salty 7 dsm -1, 7.) take at Gah Hao ear the East Sea sde ad the fresh water (salty dsm -1 ad =.9) take at upstream of the Qua Lo Phug Hep caal (QLPH). To establsh a ttrato curve, the ttrat was dropped cotuously to 5 ml recpet sample. The ttrat volume ad of the sample are recorded durg ttrato. The put was stopped whe the of the water sample was almost uchaged or the volume of ttrat was approxmately more tha 1 tmes that of the recpet sample. The ad volume of ttrat were plotted to draw the ttrato curves (or curves) for the recpet samples wth dfferet salty levels. 3. Large scale motorg of ad salty The avalable data 3 ad 5 clude salty ad measured durg the dry ad the begg of ray seaso o 1 days (15/1 of prevous year, 15/1, 15/, 15/3, 15/, 3/, 15/5, 3/5, 15/ ad 3/ of 3 ad 5) at 3 motored stes the study area. The acdty stuato 3 was more serous tha 5 because more caals were dredged -3 tha -5. The 3 ad 5 measured data were used for calbrato ad valdato of the acd module, respectvely. 3.5 Model calbrato ad valdato The calbrato was carred out by adjustg the coeffcets used the acdty module that could ot be measured easly or accurately the felds: the dsperso coeffcet DISP, the legth of ruoff path o caal embakmet ROC ad the seepage across embakmet SP. The adjusted coeffcets calbrato stage were remaed uchaged valdato stage. To evaluate the model performace, two statstcal dcators ad two llustrators were used: a. Two statstcal dcators: the square of lear regresso coeffcet (r ) ad the model effcecy (EF). EF was calculated as: EF = ( M M ) ( S M ) ( M M ) = 1 = 1 = 1 where s umber of pars of smulated ad measured data; S s smulated value ; M s average value of measured data; ad M s measured value. These dcators were calculated from 1 pars of smulated ad measured mothly averages of 3 motored stes. Rakg of model performace s gve Table 1 (Aderse et al., ). Table 1: Rakg of model performace by r, EF Performace level r EF 1. Very good >. >.95. Good Far Poor <.5 <.7 b. Two llustrators: the smulated versus measured mothly averages of all 3 motored stes (Fg. ) ad mothly at 3 specfc locatos Nh Quo (NQ), Pho Sh (PS) ad Phuoc Log (PL) (locatos Fg.1 ad results Fg.7) alog the QLPH caal where salty was referred for decso of the operato of sluce gates.. RESULTS AND DISCUSSIONS.1 Acd load from caal embakmet All the regresso models of ruoff versus daly rafall ad cumulatve total acdty versus cumulatve rafall at 7 expermetal stes (Tables 3 ad ) are valuable (r >.9) ad sgfcat, except at ste 7 (o-ass). These models are appled for other caals wth same combatos of sol type ad age of the embakmet as those stes. However, the released acdty s assumed to be zero f embakmet s road or o-ass type or alog a prmary caal. Table : Regresso models of ruoff (RO - m3.s -1.m -1 ) versus daly rafall (R - mm) Ste ASS type Age Regresso equato r 1 Medum > 3yrs RO =.5 R.5.77** Severe > 3yrs RO= 1.3 R 1.9.9** Medum -3 yrs RO=. R 1..3** 5 Severe -3 yrs RO= 1.1 R 3..91** 3 Severe 1 yrs RO=.9 R ** Medum 1 yrs RO=.11 R -.3.1** 7 No ASS > 3yrs RO=.5 R 3.7. s Note: ** : the regresso equato s very sgfcat at.1 probablty. s : the equato has o sgfcace. Table 3: Regresso models of cumulatve total acdty (Ts - mmol H + ) versus cumulatve rafall (Rs - mm) Ste ASS type Age Regresso equato r 1 Medum > 3yrs Ts = 1. Rs ** Severe > 3yrs Ts = 1. Rs ** Medum -3 yrs Ts = 1. Rs 3.99** 5 Severe -3 yrs Ts = 9.9 Rs ** 3 Severe 1 yrs l Ts =.5l Rs ** Medum 1 yrs l Ts = 3. lrs ** 7 No ASS > 3yrs 17

6 . Ttrato curves of sale water Ttrato curves are gve Fgures 5a, 5b ad 5c for recpet wth dfferet salty (S =, 1,, 3, 55 dsm -1 ) ttrated by ttrats wth = 3, 5 ad 7, respectvely. These curves are used to determe the reducto of acdty correspodg to salty a caal segmet. The tal of recpet rages from 7.5 to 7. depedg o ts alkalty. Whe ttrat wth low (= 3) s added to recpet sample, the of recpet wll decrease sharply because a lot of base (alkalty) was cosumed to eutralze the acd cocetrato. Near the equvalece pot whch s at.3 to.9 depedg o the alkalty of recpet (APHA, 199), the chage occurs very quckly, therefore the graphs Fgure 5a s extremely steep aroud ths pot. dry seaso, reached a maxmum o 15 March, ad gradually decreased later moths whe the ray seaso started. From Jauary to March wthout rafall, the caal etwork ofte remaed stable (7.5 to 7.7) but lower tha of sea water (7.) possbly due to the permaet acdty load by seepage alog caals. I 5, although the amout of rafall was smlar to that 3, both smulated ad measured values at most stes were remaed hgher tha 7 utl 3 Jue. These hgh values were explaed by less dredgg actvtes -5 that caused less acdty release 5 compared to 3. 1 a. Calbrato Measured Smulated b. Valdato of recpet 7 5 a. ttra te d b y ttra t p H = 3 S = d S. m -1 S=1 ds.m -1 S= ds.m -1 S=3 ds.m -1 S=55 ds.m -1 15/1 15/ 15/3 15/ 3/ 15/5 3/5 15/ 3/ Year 3 15/1 15/ 15/3 15/ 3/ 15/5 3/5 15/ 3/ Year 5 of recpet of recpet V (ttra t) / V (re c p e t) Fgure 5: Ttrato curves showg the chage of the recpet due to addto of ttrat wth = 3 (a), 5 (b) ad 7 (c). Note: Values x-axes ad y-axes are ot same three cases. O the other had, whe the ttrat wth hgh (= 7) s added, a large volume of ttrat s eeded to decrease sgfcatly the of recpet as show Fgure 5c. Fgure 5b presets the trastoal case betwee two cases 5a ad 5c whe ttrato s doe wth ttrat of = 5..3 Calbrato ad valdato b. t t r a t e d b y t t r a t p H = V (ttra t)/ V (re c p e t) c. t t r a t e d b y t t r a t p H = V (ttra t)/ V (re c p e t) Geerally, the acdty module ca smulate the average reasoably well as show by the comparso of the model outputs wth measured data 3 for calbrato ad 5 for valdato (Fgures a ad b). creased the Fgure : The smulated ad measured mothly averages of 3 stes from 15 December to 3 Jue 3 ad 5. The measured ad smulated 3 ad 5 at three specfc locatos, NQ, PL ad PS are show Fgures 7a to 7f. At PL ad PS located the sale area, a qute good ft s foud comparso betwee smulated ad measured 3 ad 5 (Fgures 7c, 7d, 7e ad 7f), especally the dry seaso from 15 December to 3 Aprl. I the ray seaso (after 3 Aprl), the smulated was over estmated, ether slghtly as at PL (Fgures 7c ad 7d) ad hghly as at NQ (Fgure 7a ad 7b). The reasos could be the ouform rafall dstrbuto (but oly data from some meteorologcal statos the study area were used the model) ad the operato of small sluce gates (that was ot take to cosderato the model). As NQ s located the trastoal area betwee fresh water ad sale water, larger varato of salty at ths locato causes hgher smulated tha the measured (Fgure 7a ad 7b). Table shows that the model performace dcators of calbrato wth 3 data are very good ad good (levels 1 ad ). O the other had, those of valdato are oly good ad far (levels ad 3), possbly because acdty dd ot vary much 5. However, these results show that wth the methods appled estmato of acdty from embakmets ad calculato of acdty reducto wth sale 175

7 water, the acdty module ca smulate the acd propagato the study area a. Nh Quo Year 3 c. Phuoc Log Year 3 e. Pho Sh Year 3 Measured Smulated 15/1 15/ 15/3 15/ 3/ 15/5 3/5 15/ 3/ Fgure 7: Smulated ad measured at Nh Quo (a & b), Phuoc Log (c & d) ad Pho Sh (e & f) for 3 (left graphs) & for 5 (rght graphs). Table : Model performace results by usg the average mothly Model ru Data set r EF Calbrato 3.7** (1) +.7 () + Valdato 5.73** () +. (3) + Note: ** sgfcat at.1 probablty. + value sde bracket s the performace level as classfed Table CONCLUSIONS b. Nh Quo d. Phuoc Log f. Pho Sh Year 5 Year 5 Year 5 15/1 15/ 15/3 15/ 3/ 15/5 3/5 15/ 3/ The VRSAP model wth ew acdty module could be appled for smulato of acd propagato the study area. The locatos ad the umber of dredged caals each year ASS areas are the ma factors that determe the acd load by ruoff draed from caal embakmets ad propagatg the caal etwork. I addto, the model also smulates the effect of salty o reducg acdty. The smulato of the acd propagato dcates that wth sutable caal dredgg pla ASS areas ad salty cotrol such as solatg ewly dredged caals ad usg sale water for reducg acdty, areas flueced by acd water ca be elmated the coastal zoes wth acd sulphate sols. REFERENCES APHA, 199. Stadard Methods for the Examato of Water ad Wastewater (17th ed.). Amerca Publc Health Assocato, Washgto DC. 155pp. Aderse, J., Dybkjaer D., Jese K.H., Refsgaard J.C. ad Rasmusse K.. Use of remotely sesed precptato ad leaf area dex a dstrbuted hydrologcal model. Joural of Hydrology () 3-5. Cook F. J., Hcks W., Garder E. A., Carl G. D. ad Froggatt D. W... Export of Acdty Draage Water from Acd Sulphate Sols. Mare Polluto Bullet Vol. 1, No. 7-1, pp Hoah C.T., Tuog T.P., Kam S.P., Phog N.D., Ngoc N.V., Lehma E. 1. Usg GISlked hydraulc model for maagg water qualty coflct for shrmp rce producto the Mekog Rver Delta, Vetam. Proceedgs of MODSIM 1, Volume 1: Natural Systems (part oe). pp. 1-. Idrarata, B., Glamore W. C. ad Tularam G. A.. The effects of tdal bufferg o acd sulphate sol evromets coastal areas of NSW. Geo. & Geol. Eg., : Halcrow ad HR Wallgford, ISIS Qualty user maual. Halcrow Group Ltd ad HR Wallgford Ltd, Oxfordshre, UK. Harlema, D. R. F Oe-Dmesoal Model. Estuare Modelg: A Assessmet. pp TRACOR, Ic., New York, NY, U.S.A. Phog, N.D, 1. Dyamcs of salty ad salty effects o rce producto of rce_shrmp system the Mekog Delta, Vet Nam. Thess (M.Sc.) - Uversty of the Phlppes at Los Baos. Bblog. Icludes bblographcal refs. (p ). SNIAPP,. Evaluato of sol ad water resources for lad use plag Camau pesula. Techcal report submtted to Vetamese Govermet. Sub-Natoal Isttute for Agrcultural Plag ad Projecto Vetam. 7pp ( Vetamese). Truog, T.V., Dac N.T. ad Phe H.T., 199. Smulato of acd water movemet caals. Joural of Hydrology, 1 (199) pp