er GW MATE Briefing Note Series Groundwater · world bank global water partnership associate program nt Management global water partnership associate program m Advisory Team Sustainable Groundwater Management: Sustainable Groundwater Management Tools Concepts andConcepts & Tools Briefing Note Series Note 12 30104 rev Urban Wastewater as Groundwater Recharge evaluating and managing the risks and benefits 2005 Authors (GW·MATECoreGroup) StephenFoster1HéctorGarduńo1AlbertTuinhof 2 KarinKemperMarcellaNanni ( 1leadauthor2 mainsupportingauthor) How does urban wastewater relate to groundwater? The expansion of waterborne sewerage in developing cities has gone on intermittently over many decades, with the earliest small systems having been introduced during the first half of the 20th century.Althoughsewerageprovisionlagsbehindwatersupply,rapidgrowthinurbanwaterdemand is resulting in steadily-increasing wastewater generation by most developing cites, and this will be further stimulated by the UN Millennium Goals for sanitation. Many sewerage systems discharge directlytosurfacewatercourseswithminimaltreatmentandlittledilutioninthedryseason,andthus the`wastewater'availableforirrigationreusecanoftenbe(ineffect)largelysewageeffluent. It has also become apparent that common wastewater handling and reuse practices in developing nations(whicharefrequentlyunplannedanduncontrolled)generatehighratesofinfiltrationtounder- lyingaquifersinthemorearidclimates.Thisincidentalinfiltrationisoftenvolumetricallythemost significant local `reuse' of urban wastewater, but one which is rarely planned and may not even be recognized. It both improves wastewater quality and stores it for future use, but can also pollute aquifersusedforpotablewatersupply.Thetopichasmajorimplicationsintermsoffutureapproaches togroundwaterandwastewatermanagementinmanyrapidly-developingurbancenters. Thewastewaterrechargeoccursregardlessofwhethertheurbanareaisservedprimarilyby: on-sitesanitationfacilities,withdirectsoildischargeonadiffusebasisviaseptictanksandlatrines seweragesystems,witheffluentdischargedownstreamoftheurbancenterandreuseforirrigation. This note deals only with the latter situation. Acknowledging the potential benefits of wastewater reuse for agriculturalirrigationand aquifer recharge, the focus is on evaluating the consequences of commonpracticesofsewagehandlingandreuseindevelopingcities(Figure1A)andoncost-effective incrementalapproachestoreducingthegroundwaterpollutionrisk(Figure1B). Wastewaterinfiltrationtogroundwateroccursdirectlyfromeffluenthandlingfacilitiesandindirectly fromexcessagriculturalirrigationindownstreamriparianareas.Researchundertheseconditionshas beencarriedoutinanumberofareas(Table1),andthereisclearevidenceofgroundwaterrecharge atunitratesofmorethan1000-mm/a.Itcanthusbearguedthatmajorincidentalaquiferrechargeis ubiquitousandshouldalwaysbeanticipatedasanintegralpartofwastewaterreuseprocesses. 1 r Groundwater global water partnership associate program t Management m Advisory Team Figure 1: General schemes of wastewater generation, treatment, reuse and infiltration to aquifers (A) commonly-occurring unplanned and uncontrolled situation (B) economical interventions aimed at reducing groundwater source pollution risk Wastewater is very popular with poorer farmers, because of its continuous availability, large organic loadandhighplantnutrientcontent.Butthereareinstancesofindiscriminatepracticesofveryhigh publichealthrisk,suchasirrigationwithrawsewageandcultivationofcropseatenuncooked.There may also be longer-term hazards if industrial effluent is present, such as build-up of toxic elements (notablylead,chromium,boron,etc.)insoils,reductionofsoilfertilityandpossibleuptakeintothe foodchain,buttheseimportantinter-relatedtopicsareoutsidethecurrentscope. To what extent is wastewater a groundwater pollution hazard? Therangeofpotentialgroundwaterpollutantsinwastewaterincludespathogenicmicroorganisms,excess nutrients and dissolved organic carbon, and where significant industrial effluent is present, toxic heavy metalsandorganiccompounds.However,theactualeffectongroundwaterqualitywillvarywidelywith: thepollutionvulnerabilityoftheaquifer(Briefing Note 8) thequalityofnaturalgroundwaterandthusitspotentialuse theoriginofsewageeffluentandthuslikelihoodofpersistentcontaminants thequalityofwastewater,anditsleveloftreatmentanddilution thescaleofwastewaterinfiltrationcomparedtothatofaquiferthroughflow themodeofwastewaterhandlingandlandapplication. 2 er Groundwater global water partnership associate program nt Management m Advisory Team For common wastewater reuse practices employing unlined distribution reservoirs and with flood irrigationatfieldlevel,thereislikelytobesignificantpenetrationofpathogenicbacteriaandviruses toaquifersinconditionsofshallowwatertableornear-surfacefracturedaquifers.Butinmostother conditionsvadosezoneattenuationwillbeeffectiveineliminatingmostpathogens(Figure2)before theyreachthewatertableand(inthissense)inachievingtertiary-levelwastewatertreatment. However,evenunderfavorableconditionsintermsofaquifervulnerabilityandwastewaterquality,the wastewaterinfiltrationprocessalonecannotachievestrictpotablewater-qualitystandardsinphreatic aquifers.Thisismainlyaconsequenceofthefollowing: wastewaternitrogencontentconsiderablyexceedsplantrequirementswithleachingfromirrigated soilsandresultantnitrate(NO3)concentrationsofover45mg/lingroundwaterrecharge(Table1) where wastewater infiltrates directly ammonical-nitrogen (NH ) is generally the stable nitrogen 4 speciesandlikelytoreachtroublesomelevels(Table1) elevated dissolved organic carbon (DOC) concentrations, typically 3­5 mg/l and peaking at 6­9-mg/l(Figure3),comparedtonormalbackgroundlevelsoflessthan1­2mg/l. TheseelevatedDOCconcentrationsgiverisetotwoassociatedconcerns: potential for the formation of harmful trihalomethanes (THMs) if the groundwater is disin- fected for potable supply--`affected groundwaters' from the research areas had a `DOC reactivity'of20­45-µg/mgandsomesamplesrecordedrelativelyhighTHMformationpotential valuesofover100 µg/l possibilitythattheDOC(mainlyhumic-likeacidsandsomesterols,phthalates,phenols,detergents andavarietyof`notpositivelyidentifiedcompounds')couldalsoincludetracelevelsofman-made organic chemicals potentially harmful to human health--although carcinogenic compounds, endocrinedisruptersorotherhazardouschemicalshaverarelybeenconfirmedingroundwater. Table 1: Typical composition of shallowest groundwater affected by wastewater infiltration in research areas at time of study - TREATMENT AQUIFER SELECTED DISSOLVED CONSTITUENTS (mg/l) TRACE LOCATION LEVEL VULNERABILITY Na Cl NO3 NH4 B DO2 DOC ELEMENTS^ LimaSuburb ­ moderate 90/85 182/168 40/85 3.2/0.8 n/a n/a 5/4 n/a Peru^^ primaryor secondary WadiDhuleil ­ 570* 1190* 130 1.3 1.2 2 3 Mn,Zn high Jordan* Mezquital variable; 240 220 60 <0.1 0.8 3 4 As Valley-Mexico none,but equivalent generally primary moderatebut Leon(Gto) ­ 210 340 40 <0.1 0.3 2 4 Mn,Ni, within locallyhigh Mexico** Cr,Zn distribution HatYai- system 40 50 <1 6.2 <0.1 0 3 Mn,Fe, low Thailand As datafromBGSet al.,1998^indicatesthosedetectedinlowconcentrations^^separatevaluesgivenforaquiferbeneathtreatmentlagoons/irrigatedfields *aquiferalsosubjecttosomesalineintrusion**wastewaterhasmajorindustrialcomponentnanotanalyzed 3 er Groundwater global water partnership associate program nt Management m Advisory Team Figure 2: Vadose zone attenuation of fecal pathogens from wastewater infiltration These data (from Foster etal., 1994) are for a deep highly-permeable profile of aeolian sand at a site in Lima-Peru for which acceptable levels of attenuation occur within 5 m of the land surface; this depth will vary with soil type and the most vulnerable situations will arise where groundwater table and/or fissured bedrock are at less than 3 m depth. What types of measures are available for reducing risks and increasing benefits? Becausegroundwaterisoftenthepreferredsourceofpublicwatersupply,andisalsowidelyexploited for private domestic and sensitive industrial use, aquifer pollution hazard is a serious consideration. However,littleprogressinreducingthishazardislikelytobemadeinthedevelopingworldbysimply advocatingrigorousqualitystandards.Indeedtheexistenceofsuchstandardscanbecounterproductive, oftenleadingenvironmentalhealthagenciesto`turnablindeye'tothesituationbecausetheydonot havethepersonnelcapacityandfinancialresourcestorespond. Thereisapressingneedtoconfronttherealityofcurrentpracticespragmatically,byidentifyingwherecost- effectiveinterventionsandincrementalinvestmentscanbestbemadetoreducetheriskstogroundwater users(Figure1B),ratherthanblindlyconstructingconventionalsewagetreatmentworksofquestionable operationalsustainability.Thesepriorityactionsthenneedtobepursuedconsistently(aspartofapackage whichincludesactionsdirectedatothercriticalissuessuchascroppingcontrols,farm-workerhealthand soilfertility),withparticipationoftherepresentativesofalltheruralandurbanstakeholdersinvolved. A high priority will always be to improve wastewater characterization as an aid to the assessment of groundwater pollution hazard.Where potential problems associated with persistent contaminants that poseathreattogroundwaterqualitybecomeapparent(suchashighsalinityorcertaintoxicindustrial organic and inorganic chemicals), the best approach will be to evaluate their origin within the overall seweragesystemandestablishthefeasibilityofcontrolatsourceorseparatecollectionanddisposal. Theimpactofwastewaterinfiltrationonspecificgroundwatersupplysourceswilldependnotonlyon itsimpactontheshallowaquifersystem,butalsoontheirsitingrelativetowastewaterinfiltrationarea, theirdepthofwaterintakeandtheintegrityofwellconstruction.Withcarefulcontrolofsuchfactors (andunderfavorablecircumstancesintermsofaquifervulnerabilityandwastewaterquality),compat- ibilitybetweenwastewaterreuseandgroundwatersupplyinterestscanbeachievedthrough: increasingthedepthandimprovingthesanitarysealingofpotablewaterwells establishingappropriatesourceprotectionareasforsuchwaterwells increasinggroundwatermonitoringfortheindicatorsdiscussedabove usingirrigationwellstorecovermostofthewastewaterinfiltrationandprovidea`hydraulicbarrier' fortheprotectionofpotablewatersupplies 4 r Groundwater global water partnership associate program t Management m Advisory Team Figure 3: Cl and DOC concentrations in groundwater from wastewater infiltration research areas The range of increases in shallow groundwaters above normal background values is illustrated here (data from BGS etal., 1998); some sewage effluent has elevated salinity levels as a consequence of exfiltration to sewers of brackish soil water and/or substantial flow contributions from saline industrial discharge, and can lead to groundwater recharge from wastewater infiltration containing in excess of 250 mg Cl/l . improvingirrigationwater-useefficiencyandthuswastewaterrechargetounderlyingaquifers urgingconstraintsontheuseofshallowprivatedomesticwells. How can wastewater and groundwater use be integrated into urban planning? Arelatedquestionishowcanfutureurbanwastewaterengineeringtakeadequateaccountofground- waterresourceinterests.Currentdecisionstoextendmainsseweragecoveragearenormallytakenin relationtothefollowingtechnicalandsocialfactors: inadequate subsoil capacity to dispose of liquid effluent due to the presence of low-permeability surfacestrataand/orhighwatertable,causingmalfunctionandoverflowofin situsanitationunits high-density residential development with inadequate access and/or space for removal of solid residuesfromin situsanitationunits. Sufficientconsiderationisnotgiventothenewenvironmentalproblemsthatwillbecreatedbygener- ationofsewageeffluent,comparedtothoseofexistingin situsanitationanditspotentialupgradeto higherecologicalstandard.Norisadequateemphasisplacedonwaterresourceissuessuchas: providingadditionalwatersupplyforamenityoragriculturalirrigationthroughwastewaterreusein areasoflowaquiferpollutionvulnerability,asameansofconservingpotable-qualitygroundwater reductionoftheingressofsalinegroundwaterintosewersinaridareas reducingthepollutionhazardtomunicipalandprivatewellssituatedwithintheurbanarea increasingwater-supplyneedsforwaterbornesewerage,likelytobemetinpartfromgroundwater recognizingthataquiferstorageofreclaimedwastewaterwilloftenbethebestoveralloptionwhere demand for irrigation water exhibits large seasonal variation, and using infiltration through the vadosezoneofaquifersfortertiarytreatmentofwastewater. Toachieveamoreintegratedapproachsignificantinstitutionalquestionsmustbeaddressed: whichagencyshouldhavefinalresponsibilityforwastewatermanagement? whatshouldbetherespectivelegalobligationsofwastewatergeneratorsandusers? howbestcanabroaderbaseofstakeholderconsultationbeintroduced? howshouldwastewaterdischargepermitsconsiderreusefactors? howcantrainingonwasterwater-groundwaterrelationsbestbeimplemented? 5 r Groundwater global water partnership associate program t Management m Advisory Team Thegroundwaterdimensionisthusstilloftenoneofthe`missinglinks'.Majorincidentalrechargeof aquifersthroughwastewaterhandlingandreuseissowidespreadthatitshouldalwaysbecontemplated asanintegralpartofwastewatermanagement,andthusplannedforaccordingly.Thoseresponsiblefor wastewaterneedtobemadeawareofthebenefitsandhazardsofwastewaterrechargetoaquifers,and howhydrogeologicalenvironmentsvarywithregardtopollutionvulnerability,andthustowastewater safeloadingratesandpatterns.Astrongerelementofmunicipalplanningwillbeneededfortheworst (andleastsustainable)ofpastpracticestobeavoidedinfuture. Further Reading - BGS,CNA,SAPAL,WAJ,DMRandPSU.1998.Protectinggroundwaterbeneathwastewaterrecharge sites.BGS Technical Report WC/98/39. - Bouwer,H.1991.Groundwaterrechargewithsewageeffluent.Water Science & Technology 23: 2099­2108. - Dillon,P.J.2002.Managementofaquiferrechargeforsustainability.Proc. 4th International Symposium on Artificial Recharge of Groundwater (Adelaide, September 2002). Balkema Publishers. Rotterdam, Netherlands - Foster, S. S. D., Gale, I. N. and Hespanhol, I. 1994. Impacts of wastewater reuse and disposal on groundwater.BGS Technical Report WD/94/55. - Foster, S. S. D., Lawrence, A. R. and Morris, B. L. 1997. Groundwater in urban development: assessing management needs and formulating policy strategies. World Bank Technical Paper 390. Washington, D.C.,USA. - George,D.B.,Leftwhich,D.B.,Klein,N.A.andClaborn,B.J.1987.Redesignofalandtreatment systemtoprotectgroundwater.Journal ofWater Pollution Control Federation59:813­820. - Idelovitch, E. and Michail, M. 1984. Soil-AquiferTreatment--a new approach to an old method of wastewaterreuse.Journal ofWater Pollution Control Federation56:936­943. - Jimenez,B.andGarduno,H.2002.Social,politicalandscientificdilemmasformassivewastewaterreuse intheworld.InAWWAPublicationNavigating Rough Waters--Ethical Issues in the Water Industry. Publication Arrangements TheGW·MATEBriefingNotesSeriesispublishedbytheWorldBank,WashingtonD.C.,USA.Itisalsoavailable inelectronicformontheWorldBankwaterresourceswebsite(www.worldbank.org/gwmate)andtheGlobalWater Partnershipwebsite(www.gwpforum.org). Thefindings,interpretations,andconclusionsexpressedinthisdocumentareentirelythoseoftheauthorsandshouldnotbeattributedinanymannertothe WorldBank,toitsaffiliatedorganizations,ortomembersofitsBoardofExecutiveDirectors,orthecountriestheyrepresent. Funding Support GW·MATE(GroundwaterManagementAdvisoryTeam) isacomponentoftheBank-NetherlandsWaterPartnershipProgram (BNWPP)usingtrustfundsfromthe DutchandBritishgovernments. 6