Modelling salt accumulation in an oval irrigated with recycled water

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1 2th Intenational Congess on Modelling and Simulation, Adelaide, Austalia, 1 6 Decembe Modelling salt accumulation in an oval iigated with ecycled wate M.M. Rahman a, D. Hagae a, B. Maheshwai b, P. Dillon c School of Computing, Engineeing and Mathematics, Univesity of Westen Sydney, Locked Bag 1797, Penith, NSW 1797, Austalia School of Science and Health, Univesity of Westen Sydney, Locked Bag 1797, Penith, NSW 2751, Austalia CSIRO Land and Wate, PMB2 Glen Osmond, SA 564, Austalia muhit.ahman@uws.edu.au Abstact: Recycling of wastewate is impotant fo achieving sustainable use of wate in both uban and ual aeas. Cuently, in Austalia, thee ae stict egulations on disposal and euse of wastewate. Iigation of spoting fields is one of the most common applications of wastewate ecycling. Howeve, thee is an inceased isk of soil salinity in the vadose zone when ecycled wate is used fo iigation. This is due to the geneally inceased levels of salt in the ecycled wate compaed to that of town wate supply. This pape pesents salt tanspot modelling fo a spoting oval (Geygums Oval) in Westen Sydney which has been iigated with ecycled wate fo moe than fou yeas. The modelling was caied out fo 1277 days using HYDRUS 1D, which is a one dimensional salt tanspot model. Meteoological data was used to specify atmospheic bounday conditions, which was collected fom neaest weathe station of Penith Lakes, 4 km fom the Geygums Oval. Aveage total dissolved solids (TDS) of ecycled wate vaied in the ange of mg/l. Hydaulic loading ate and salt loading of applied iigation wate wee 16 mm/yea and 713 kg/ha/yea, espectively. The esults of salt tanspot modelling of ecycled wate fo iigation showed a 77% incease (fom.38 g/l to.67 g/l) in the oot zone soil wate TDS concentation ove the peiod of 1277 days. Howeve, duing the simulation peiod, pedicted TDS concentation showed significant fluctuations, which can be attibuted to the influence of ainfall on soil wate content and the movement of salt within the ootzone. The inceasing tend was suppoted by a limited numbe of field data points. The field data collected geneally suppoted the findings of the model. Howeve, moe field data ae equied to validate the model. The esults also indicate that weathe conditions, paticulaly ainfall, appea to have significant impact on the accumulation of salt in the soil. That is, accumulation of salt in the soil due to ecycled wate applications can be moe citical in the aid than in the topical ainfoest climatic conditions. Fo aid egions, due to limited availability of wate, thee is an inceased pessue fo using ecycled wate fo iigation. The esults of this study appea to indicate that, if the ecycled wate is applied indisciminately, the soils in this egion may be unde sevee isk of salinization. Howeve, futhe studies ae equied to quantify the isk of salinisation. Keywods: Salt tanspot modelling, Uban iigation, Vadose zone 273

2 1. INTRODUCTION Recycled wate is the teated wastewate afte emoving solids and cetain impuities. Chaacteistics of ecycled wate depend on its souce, teatment level and geogaphic location. Recycled wate chaacteistics can be classified accoding to its physical, chemical and biological aspects. Despite significant benefits of ecycled wate, thee ae seveal concens elated to envionmental and health isks. One such concen elates to the incease of salinity including sodicity and bicabonate hazads in iigated fields. Salinity is the concentation of soluble salts in wate that ae measued as total dissolved solids o electical conductivity (EC) of soil solution. As wate evapoates fom soils o is used by the plants, salts ae left behind. This phenomenon inceases the concentation of salts in the soil with time. This incease in salt concentation in the soil can advesely influence the amount of wate a plant can take up fom the soil due to the osmotic effect it ceates. Seveal studies have epoted inceased salinity due to the polonged use of ecycled wate fo iigation. Distinct long-tem effects of ecycled wate use in tems of salinity have been obseved in agicultual field by Dikinya & Aeola (21). Afte thee yeas of iigation with ecycled wate, the electical conductivity (EC) in soil inceased fom 15 to 235 µs/cm and Na + concentation inceased fom 2.95 to 5.75 meq/1g of soil. Jahantigh (28) epoted 95% incease of salinity levels fo a field which used ecycled wate fo iigation ove five yeas. Incease of salinity in tems of EC, Na + and Cl - ae also epoted in a numbe of othe studies (Xu et al., 21; Yang et al., 26; Heidapou et al., 27; Gloaguen et al., 27; Leal et al., 29; Alacón and Pedeo, 29; Wang et al., 23; Klay et al., 21 and Adove et al., 212). In the case of public open space including spoting oval iigation, Candela et al. (27) investigated the effect of ecycled wate iigation in a golf couse in Spain fo two yeas. They obseved that Na + inceased in the top.6 m of soil pofile due to wate application and evapotanspiation. Afte fou yeas of investigation of nine golf couses in Southen Nevada, which wee iigated using ecycled wate, Devitt et al. (27) epoted that the soil salinity levels followed a sinusoidal seasonal cuve, whee 7% of all the peaks occued in summe. This indicated that the type of season has an effect on the salinisation to some extent. Salt tanspot modelling is an efficient way to quantify salinity isk in iigated fields (Ragab 22, Bachceci and Suat 27). The salinity issue can be undestood by modelling the inteaction among the soil, wate and salt within the vadose zone. The main objective of the pesent study is to develop a methodology to pedict soil salinity levels of a spoting oval that is iigated with ecycled wate fo a numbe of yeas. The modelling was caied out using HYDRUS 1D, which is a one dimensional salt tanspot model, to pedict the soil salinity levels. 2. STUDY AREA The Geygums oval ( ʹS, ʹE), is situated in Canebook, NSW, Austalia (Figue 1). The oval is used fo athletics duing summe season and Austalian Rules football duing the winte season. This oval is iigated by spinkle method using the ecycled wate fom the Penith Sewage Teatment Plant (STP) since Januay 28. The ecycled wate used fo iigation is stoed in a 25, L concete tank and up to 553 kl/day of ecycled wate is used fo iigation of the oval. Aveage TDS of ecycled wate vaied in the ange of mg/l. Hydaulic loading ate and salt loading of applied iigation wate was 16 mm/yea and 713 kg/ha/yea, espectively. Mean annual ainfall in the study aea is mm, whee wettest months ae fom Octobe though to Mach and diest fom Apil to Septembe. Figue 1. Map showing locations of Geygums oval and Penith Sewage Teatment Plant (STP). 3. HYDRUS-1D SIMULATION MODEL Hydus 1D (Simunek et al., 29) simulates one dimensional wate flow and solute tanspot in incompessible, poous, vaiably satuated media. The model can be used fo diffeent egimes but in this study tansient system is used. Fo wate flow modelling Hydus 1D uses Richads equation (Celia et al., 199; Xu and Shao, 22): 2731

3 θ t z K zz h K zz z z S = (1) Whee, K zz = satuated hydaulic conductivity [LT -1 ], θ = volumetic moistue content [L 3 L -3 ], h = pessue head [L], z = depth in the vetical diection (positive upwad) [L], t = Time [T], S = sink tem epesenting wate uptake by plant oots [L 3 L -3 T -1 ]. Fo Hydaulic popeties, Van Genuchten s equation (198) was used, which povides elationships between the volumetic moistue content, effective satuation, hydaulic conductivity and specific moistue stoage: θ s θ h < θ + p θ(h) = ( 1+ ( a h ) ) m θ s h (2) K s S l e 1 1 S 1/m e m 2 h < K(h) = K s h (3) Whee, m=1-(1/n), n>1 S e θ θ = θ θ s Whee, S e is effective satuation [Unitless], a is the soil wate etention function [L -1 ], m and n ae empiical paametes [unitless], θ is esidual moistue content [L 3 L -3 ], θ s is satuated moistue content [L 3 L -3 ], l is poe connectivity paamete. The value of l in the hydaulic conductivity function was estimated to be about.5 as an aveage fo geneal soil (Simunek et al., 29). Fo solute tanspot, it was assumed that the solutes wee non-eactive and thee was no solubilization o dissolution of soil mineals. The assumption pemitted the salinity in soil to be modeled based on the convection-dispesion equation fo noneactive solution (Robets et al., 29). The patial diffeential equation govening one dimensional advective-dispesive tanspot fo tansient flow in a vaiable satuated soil can be witten as (Xu and Shao, 22; Simunek et al., 29; Bunsi et al., 28): c c c θ R θd + q = (5) z z t z z z Whee, ρ d R = 1+ bk θ ρ b is bulk density [ML -3 ], K d is distibution coefficient [L 3 M -1 ], c is concentation of chemical in liquid phase [ML -1 ], q z is the volumetic flux density [LT -1 ], D z is dispesion coefficient [L 2 T -1 ] that accounts fo both diffusion and hydodynamic dispesion. D z is calculated using Simunek et al. (25): D z = α 1 q z θ + D dτ (6) Whee, α 1 is longitudinal dispesivity [L], D d is the ionic o molecula diffusion coefficient in fee wate [L 2 T -1 ], τ is a totuosity facto. The totuosity facto was evaluated by using Millington and Quik equation (Millington and Quik, 1961): τ = θ 7 3 θ s 2 (7) The plant solute uptake was assumed to be negligible in the pesent study. The govening wate flow and solute tanspot equations wee solved using upsteam weighting finite element method (Simunek et al., 29) Soil pofile and initial conditions The salt tanspot modelling was caied out fo a soil pofile of 1m. The oot zone was assumed as to.4 m. Initial soil wate content was set to a unifom value of.2 (m 3 m -3 ) thoughout the soil pofile. Initial (4) 2732

4 solute concentation was calculated fom electical conductivity of soil wate (EC sw ), which was detemined using the following elationship (Ayes and Westcot, 1985; Stevens et al., 28): EC sw = 2 EC e (8) Whee, EC e is the satuated paste electical conductivity, whose value was taken fom Maheshwai (211) as.296 ds/m. Textual distibution of soil was caied out of samples collected fom the oval and the textue was detemined as sandy loam compising of 74.6% sand, 9.5% silt and 15.9% clay. The bulk density of the soil fom the study aea vaied fom 763 kg/m 3 at depth.25 m to 1355 kg/m 3 at depth m. Bulk density of top soil (.25 m) is less than the usual ange of 11 to 16 kg/m 3 because ecycled oganics wee mixed with the soil in this depth to incease its poosity Time vaiable bounday conditions Atmospheic and fee dainage conditions wee defined as bounday condition at the suface and bottom of the soil pofile, espectively. Fee dainage condition at the bottom of soil pofile is assumed to designate the soil pofile as patially satuated. Atmospheic bounday conditions wee specified using meteoological data. Fom these data, daily values of the efeence evapotanspiation ate (ET ) wee calculated using Penman- Monteith method (Simunek et al., 29). Daily values of ET wee geneated by HYDRUS 1D, which was in the ange of mm/day. Meteoological data wee collected fom neaest weathe station of Penith Lakes (station numbe 67113), 4 km fom the Geygums Oval (Bueau of Meteoology, 212). Annual ainfall duing yea 28 was mm which is aound 2% moe of mean annual ainfall of mm in the study aea. Annual ainfall in the study aea duing yea 29, 21 and 211 wee mm, mm and 74.6 mm, espectively. Vaiation of ainfall, iigation ate and ET ove the study peiod is shown in Figue 2. Rainfall and iigation (mm/d) Rainfall (mm) Iigation wate application ate (mm/d) ETo (mm) ETo (mm) Figue 2. Vaiation of ainfall amount, ET and amount of iigation wate applied Iigation system and scheduling Geygums Oval has an above gound automatic wateing system consisting of Rainbid Eagle E9 spinkle heads. A submesible pump opeates the iigation system fom a 22, L tank. Geygums oval was iigated with ecycled wate since Januay 28. The data elated to monthly use of ecycled wate fo iigation wee collected fom Penith City Council Gounds-man s logbook fo the peiod between Januay 28 and June 211(Sheen, 212). Using the monthly data appopiate application ates wee calculated by taking into account the iigation fequency. Fom Januay 28 to Febuay 29 the iigation fequency was thee days pe week but the iigation scheduling was deceased to 2 days pe week fom Mach 29 to June 211. Figue 3 epesents measued TDS value in iigation wate and the salt load applied duing the iigation. Salt Load of iigation wate (Kg/ha/month) Salt load (kg/ha) TDS (mg/l) Figue 3. Vaiation of monthly salt load (indicated by columns) applied on the land and TDS concentation (indicated by the line) of ecycled wate used fo iigation. 1 5 TDS of iigation wate (mg/l) 2733

5 4. RESULTS AND DISCUSSION 4.1. Soil hydaulic and solute tanspot paametes The hydaulic paametes including esidual wate content (θ ), satuated wate content (θ s ), shape paametes (α and n) and satuated hydaulic conductivity (K s ) wee detemined fom the paticle size distibution and bulk density of the soil with Rosetta model, which is implemented in HYDRUS 1D (Simunek et al., 29) (Table 1). Dispesivity values at diffeent depths wee taken fom Vandeboght and Veeecken (27) and Kanzai et al. (212). Root wate uptake paametes wee taken as equal to default values suggested in the built-in libay of HYDRUS fo tuf gass. Table 1. Soil hydaulic and solute tanspot paametes Depth θ θ s α n K s l Dispesivity (m) [-] [-] [1/cm] [-] [cm/d] [-] [cm -1 ] Salt accumulation in soil ove time The HYDRUS 1D simulation peiod fo this study was set fo 1277 days fom Januay 28 to June 211. This peiod was selected mainly due to the availability of ecycled wate (RW) data. The aveage TDS concentation in the soil wate in the oot zone is shown in Figue 4 fo the above peiod. As shown in this Figue, thee appeas to be significant fluctuations in the TDS concentation of the soil wate ove the study peiod. Root zone TDS concentation deceased duing ainfall events, which can be due to the flushing of accumulated salt by the ain wate towads the lowe layes of the soil. Simila obsevations wee epoted by Ramos et al. (211). Howeve, thee is geneally an inceasing tend in TDS concentations. To demonstate the impact of ecycled wate TDS concentation on salt accumulation in oot zone, a scenaio analysis was caied out fo diffeent iigation wate conditions as shown in Figue 4. As expected the soil wate TDS concentation coesponding to iigation with twice the ecycled wate TDS concentation (1.8 g/l) was found to be significantly highe than the concentations obtained fo the iigation wate with town wate TDS concentation (.14 g/l). It is evident fom this figue that using town wate as iigation wate educes the salt accumulation. In fact, as shown in Figue 4, use of town wate fo iigation esults in insignificant accumulation of salt in the oot zone. Daily aveage ainfall fo the month (mm/d) Rainfall (mm) Root zone ( -.4 m) TDS conc. (g/l) fo RW TDS conc of.54 g/l Root zone ( -.4 m) TDS conc. (g/l) fo RW TDS conc of 1.8 g/l Root zone ( -.4 m) TDS conc. (g/l) fo town wate TDS conc of.14 g/l Root zone TDS concentation (g/l) Figue 4. Vaiation of oot zone TDS concentation. The pedicted TDS concentations at depths -.2 m and.7-1 m wee compaed with the measued TDS concentations on two occasions (June 28 and June 21) as shown in Figue 5. The measued TDS concentations at both the depths confim inceasing tend of the TDS in soil wate ove two yeas, which is in ageement with Figue 4. The TDS concentation measued in June 28 at depth.7-1 m is close to the pedicted value. Similaly, TDS measued in June 21 at depth -.2 m is close to the pedicted value. Since only two obseved data points fo the soil salinity available, it is, at this stage, difficult to genealize the findings. Moe obseved salinity data ae equied to conclusively establish the eliability of the model to 2734

6 pedict the salt accumulation in spoting oval. Although the simulation pedicts an inceasing patten of soil wate TDS concentation at ootzone (Figue 4), yet the simulated TDS is not high enough to affect the gowth of tufgass (Kikuyu) existing in Geygums oval. Accoding to NRMMC 26, salt toleance limit of Kikuyu is 1.8 to 3.6 g/l, which is well above the maximum simulated soil wate TDS concentation of 1.37 g/l. 1.5 TDS (g/l) Pedicted TDS concentation at depth -.2 m Pedicted TDS concentation at depth.7-1 m Measued TDS at -.2 m Measued TDS at depth.7-1 m Figue 5. Obseved and pedicted TDS at diffeent depth 5. Conclusions The esults of salt tanspot modelling of using ecycled wate fo iigation show 77% incease fo oot zone TDS concentation ove 1277 days. Howeve, duing the simulation peiod, pedicted TDS concentation showed significant fluctuations, which can be attibuted to the influence of ainfall on soil wate content and the movement of salt within the oot zone. Scenaio analysis showed that highe is the TDS concentation in the ecycled wate, highe is the accumulation in the soil oot zone. Fo the peiod consideed in this study, that is 1277 days (3.5 yeas), the simulated TDS concentation is below the salt toleance limit of Kikuyu gass existing in Geygums oval. Howeve, the tends in the cuent data indicate that the salt accumulation may each o even exceed salt toleance limits in the long un. As evidenced by the esults obtained, weathe conditions, paticulaly ainfall, appeas to have significant impact on the accumulation of salt in the soil. This means that the long-tem accumulation of salt in the soil depends on, besides soil type, the ainfall and evapoation pattens. That is accumulation of salt in the soil due to ecycled wate applications can be moe citical in the aid than in the topical ainfoest climatic conditions. Fo aid egions, due to limited availability of wate, thee is a pessue fo using ecycled wate fo iigation. The esults of this study appea to indicate that, if the ecycled wate is applied inappopiately, the soils in this egion may be unde sevee isk of salinization. Howeve, futhe studies ae equied to quantify the isk of salinisation. Acknowledgements The authos acknowledge the school of computing, engineeing and mathematics, Univesity of Westen Sydney, fo poviding suppot fo the eseach epoted in this manuscipt. The authos also wish to acknowledge M. Mal Sheen of Penith city council fo poviding data elated to Geygums oval iigation. The lead autho also acknowledges the suppot of CSIRO Wate fo a Healthy County Pogam though a postgaduate eseach top-up awad. The authos ae gateful to the eviewes fo thei extemely useful comments. REFERENCES Adove, M., Faús, E., Moyà, G. & Vadell, J Chemical popeties and biological activity in soils of malloca following twenty yeas of teated wastewate iigation. Jounal of envionmental management, 95, S188-S192. Alacón, J. & Pedeo, F. 29. Effects of teated wastewate iigation on lemon tees. Desalination, 246, Ayes, R. S. & Westcot, D. W Wate quality fo agicultue, Food and Agicultue Oganization of the United Nations Rome, Italy. Bunsi, T., Sivakuma, M. & Hagae, D. 28. Numeical modeling of tace tanspot in unsatuated poous media. Jounal of Applied Fluid Mechanics, 1,

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