briefing note series note 12 urban wastewater as ...€¦ · the quality of wastewater, and its...

Urban Wastewater as Groundwater Rechargeevaluating and managing the risks and benefits

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 20thcentury.Althoughsewerageprovisionlagsbehindwatersupply,rapidgrowthinurbanwaterdemandis resulting in steadily-increasing wastewater generation by most developing cites, and this will befurther stimulated by the UN Millennium Goals for sanitation. Many sewerage systems dischargedirectlytosurfacewatercourseswithminimaltreatmentandlittledilutioninthedryseason,andthusthe‘wastewater’availableforirrigationreusecanoftenbe(ineffect)largelysewageeffluent.

● It has also become apparent that common wastewater handling and reuse practices in developingnations(whicharefrequentlyunplannedanduncontrolled)generatehighratesofinfiltrationtounder-lyingaquifers inthemorearidclimates.This incidental infiltrationisoftenvolumetricallythemostsignificant local ‘reuse’ of urbanwastewater, but onewhich is rarelyplanned andmaynot evenberecognized. It both improves wastewater quality and stores it for future use, but can also polluteaquifersusedforpotablewatersupply.Thetopichasmajorimplicationsintermsoffutureapproachestogroundwaterandwastewatermanagementinmanyrapidly-developingurbancenters.

● Thewastewaterrechargeoccursregardlessofwhethertheurbanareaisservedprimarilyby:● on-sitesanitationfacilities,withdirectsoildischargeonadiffusebasisviaseptictanksandlatrines● seweragesystems,witheffluentdischargedownstreamoftheurbancenterandreuseforirrigation.

This note deals only with the latter situation. Acknowledging the potential benefits of wastewaterreuse foragricultural irrigationandaquifer recharge, the focus isonevaluating theconsequencesofcommonpracticesofsewagehandlingandreuseindevelopingcities(Figure1A)andoncost-effectiveincrementalapproachestoreducingthegroundwaterpollutionrisk(Figure1B).

● Wastewaterinfiltrationtogroundwateroccursdirectlyfromeffluenthandlingfacilitiesandindirectlyfromexcessagriculturalirrigationindownstreamriparianareas.Researchundertheseconditionshasbeencarriedoutinanumberofareas(Table1),andthereisclearevidenceofgroundwaterrechargeatunitratesofmorethan1000-mm/a.Itcanthusbearguedthatmajorincidentalaquiferrechargeisubiquitousandshouldalwaysbeanticipatedasanintegralpartofwastewaterreuseprocesses.

Authors (GW•MATE Core Group)

StephenFoster1HéctorGarduño1AlbertTuinhof 2 KarinKemperMarcellaNanni

(1leadauthor2mainsupportingauthor)

1

world bank global water partnership associate program

Sustainable Groundwater Management:Concepts and Tools

GW•MATE Briefing Note Series

Sustainable Groundwater ManagementConcepts & Tools

GW MATEBriefing Note Series

Briefing Note SeriesBriefing Note Series

Groundwater ManagementAdvisory Team


global water partnership associate program

Briefing Note Series Note 12

2005

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● Wastewater isverypopularwithpoorer farmers,becauseof its continuousavailability, largeorganicloadandhighplantnutrientcontent.Butthereareinstancesofindiscriminatepracticesofveryhighpublichealthrisk,suchasirrigationwithrawsewageandcultivationofcropseatenuncooked.Theremayalsobe longer-termhazards if industrial effluent ispresent, suchasbuild-upof toxic elements(notablylead,chromium,boron,etc.)insoils,reductionofsoilfertilityandpossibleuptakeintothefoodchain,buttheseimportantinter-relatedtopicsareoutsidethecurrentscope.

To what extent is wastewater a groundwater pollution hazard?● Therangeofpotentialgroundwaterpollutantsinwastewaterincludespathogenicmicroorganisms,excess

nutrients anddissolvedorganic carbon, andwhere significant industrial effluent ispresent, toxicheavymetalsandorganiccompounds.However,theactualeffectongroundwaterqualitywillvarywidelywith:● thepollutionvulnerabilityoftheaquifer(Briefing Note 8)● thequalityofnaturalgroundwaterandthusitspotentialuse● theoriginofsewageeffluentandthuslikelihoodofpersistentcontaminants● thequalityofwastewater,anditsleveloftreatmentanddilution● thescaleofwastewaterinfiltrationcomparedtothatofaquiferthroughflow● themodeofwastewaterhandlingandlandapplication.

2

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






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● For common wastewater reuse practices employing unlined distribution reservoirs and with floodirrigationatfieldlevel,thereislikelytobesignificantpenetrationofpathogenicbacteriaandvirusestoaquifers inconditionsofshallowwatertableornear-surfacefracturedaquifers.Butinmostotherconditionsvadosezoneattenuationwillbeeffectiveineliminatingmostpathogens(Figure2)beforetheyreachthewatertableand(inthissense)inachievingtertiary-levelwastewatertreatment.

● However,evenunderfavorableconditionsintermsofaquifervulnerabilityandwastewaterquality,thewastewaterinfiltrationprocessalonecannotachievestrictpotablewater-qualitystandardsinphreaticaquifers.Thisismainlyaconsequenceofthefollowing:● wastewaternitrogencontentconsiderablyexceedsplantrequirementswith leachingfromirrigated

soilsandresultantnitrate(NO3)concentrationsofover45mg/lingroundwaterrecharge(Table1)● where wastewater infiltrates directly ammonical-nitrogen (NH4) is generally the stable nitrogen

speciesandlikelytoreachtroublesomelevels(Table1)● elevated dissolved organic carbon (DOC) concentrations, typically 3–5 mg/l and peaking at

6–9-mg/l(Figure3),comparedtonormalbackgroundlevelsoflessthan1–2mg/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 ‘DOCreactivity’of20–45-µg/mgandsomesamplesrecordedrelativelyhighTHMformationpotentialvaluesofover100µg/l

● possibilitythattheDOC(mainlyhumic-likeacidsandsomesterols,phthalates,phenols,detergentsandavarietyof‘notpositivelyidentifiedcompounds’)couldalsoincludetracelevelsofman-madeorganic 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) TRACELOCATION LEVEL VULNERABILITY Na Cl NO3 NH4 B DO2 DOC ELEMENTS^

LimaSuburb– 90/85 182/168 40/85 3.2/0.8 n/a n/a 5/4 n/aPeru^^

WadiDhuleil– 570* 1190* 130 1.3 1.2 2 3 Mn,ZnJordan*

Mezquital 240 220 60 <0.1 0.8 3 4 AsValley-Mexico

Leon(Gto)– 210 340 40 <0.1 0.3 2 4 Mn,Ni,Mexico** Cr,Zn

HatYai- 40 50 <1 6.2 <0.1 0 3 Mn,Fe,Thailand As

†datafromBGSet al.,1998^indicatesthosedetectedinlowconcentrations^^separatevaluesgivenforaquiferbeneathtreatmentlagoons/irrigatedfields*aquiferalsosubjecttosomesalineintrusion**wastewaterhasmajorindustrialcomponentnanotanalyzed

primaryorsecondary

variable;generally

moderatebutlocallyhigh

moderate

low

high

none,butequivalentprimarywithin

distributionsystem






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What types of measures are available for reducing risks and increasing benefits?

● Becausegroundwaterisoftenthepreferredsourceofpublicwatersupply,andisalsowidelyexploitedforprivatedomesticand sensitive industrialuse, aquiferpollutionhazard is a seriousconsideration.However,littleprogressinreducingthishazardislikelytobemadeinthedevelopingworldbysimplyadvocatingrigorousqualitystandards.Indeedtheexistenceofsuchstandardscanbecounterproductive,oftenleadingenvironmentalhealthagenciesto‘turnablindeye’tothesituationbecausetheydonothavethepersonnelcapacityandfinancialresourcestorespond.

● Thereisapressingneedtoconfronttherealityofcurrentpracticespragmatically,byidentifyingwherecost-effectiveinterventionsandincrementalinvestmentscanbestbemadetoreducetheriskstogroundwaterusers(Figure1B),ratherthanblindlyconstructingconventionalsewagetreatmentworksofquestionableoperationalsustainability.Thesepriorityactionsthenneedtobepursuedconsistently(aspartofapackagewhichincludesactionsdirectedatothercriticalissuessuchascroppingcontrols,farm-workerhealthandsoilfertility),withparticipationoftherepresentativesofalltheruralandurbanstakeholdersinvolved.

● A high priority will always be to improve wastewater characterization as an aid to the assessment ofgroundwaterpollutionhazard.Wherepotentialproblemsassociatedwithpersistentcontaminants thatposeathreattogroundwaterqualitybecomeapparent(suchashighsalinityorcertaintoxicindustrialorganicand inorganicchemicals), thebestapproachwillbe toevaluate theiroriginwithin theoverallseweragesystemandestablishthefeasibilityofcontrolatsourceorseparatecollectionanddisposal.

● Theimpactofwastewaterinfiltrationonspecificgroundwatersupplysourceswilldependnotonlyonitsimpactontheshallowaquifersystem,butalsoontheirsitingrelativetowastewaterinfiltrationarea,theirdepthofwaterintakeandtheintegrityofwellconstruction.Withcarefulcontrolofsuchfactors(andunderfavorablecircumstancesintermsofaquifervulnerabilityandwastewaterquality),compat-ibilitybetweenwastewaterreuseandgroundwatersupplyinterestscanbeachievedthrough:● increasingthedepthandimprovingthesanitarysealingofpotablewaterwells● establishingappropriatesourceprotectionareasforsuchwaterwells● increasinggroundwatermonitoringfortheindicatorsdiscussedabove● usingirrigationwellstorecovermostofthewastewaterinfiltrationandprovidea‘hydraulicbarrier’

fortheprotectionofpotablewatersupplies

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.






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● improvingirrigationwater-useefficiencyandthuswastewaterrechargetounderlyingaquifers● urgingconstraintsontheuseofshallowprivatedomesticwells.

How can wastewater and groundwater use be integrated into urban planning?

● Arelatedquestionishowcanfutureurbanwastewaterengineeringtakeadequateaccountofground-water resource interests.Currentdecisions toextendmainsseweragecoveragearenormally taken inrelationtothefollowingtechnicalandsocialfactors:● inadequate subsoil capacity todisposeof liquid effluentdue to thepresenceof 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 situsanitationanditspotentialupgradetohigherecologicalstandard.Norisadequateemphasisplacedonwaterresourceissuessuchas:● providingadditionalwatersupplyforamenityoragriculturalirrigationthroughwastewaterreusein

areasoflowaquiferpollutionvulnerability,asameansofconservingpotable-qualitygroundwater● reductionoftheingressofsalinegroundwaterintosewersinaridareas● reducingthepollutionhazardtomunicipalandprivatewellssituatedwithintheurbanarea● increasingwater-supplyneedsforwaterbornesewerage,likelytobemetinpartfromgroundwater● recognizingthataquiferstorageofreclaimedwastewaterwilloftenbethebestoveralloptionwhere

demand for irrigation water exhibits large seasonal variation, and using infiltration through thevadosezoneofaquifersfortertiarytreatmentofwastewater.

● Toachieveamoreintegratedapproachsignificantinstitutionalquestionsmustbeaddressed:● whichagencyshouldhavefinalresponsibilityforwastewatermanagement?● whatshouldbetherespectivelegalobligationsofwastewatergeneratorsandusers?● howbestcanabroaderbaseofstakeholderconsultationbeintroduced?● howshouldwastewaterdischargepermitsconsiderreusefactors?● howcantrainingonwasterwater-groundwaterrelationsbestbeimplemented?

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 .






Publication Arrangements

TheGW•MATEBriefingNotesSeriesispublishedbytheWorldBank,WashingtonD.C.,USA.ItisalsoavailableinelectronicformontheWorldBankwaterresourceswebsite(www.worldbank.org/gwmate)andtheGlobalWater

Partnershipwebsite(www.gwpforum.org).Thefindings,interpretations,andconclusionsexpressedinthisdocumentareentirelythoseoftheauthorsandshouldnotbeattributedinanymannertothe

WorldBank,toitsaffiliatedorganizations,ortomembersofitsBoardofExecutiveDirectors,orthecountriestheyrepresent.

Funding Support

GW•MATE(GroundwaterManagementAdvisoryTeam)isacomponentoftheBank-NetherlandsWaterPartnershipProgram

(BNWPP)usingtrustfundsfromtheDutchandBritishgovernments.






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● Thegroundwaterdimensionisthusstilloftenoneofthe‘missinglinks’.Majorincidentalrechargeofaquifersthroughwastewaterhandlingandreuseissowidespreadthatitshouldalwaysbecontemplatedasanintegralpartofwastewatermanagement,andthusplannedforaccordingly.Thoseresponsibleforwastewaterneedtobemadeawareofthebenefitsandhazardsofwastewaterrechargetoaquifers,andhowhydrogeologicalenvironmentsvarywithregardtopollutionvulnerability,andthustowastewatersafeloadingratesandpatterns.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: 2099–2108.

- 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. andMorris,B.L.1997.Groundwater inurbandevelopment: 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 Federation59:813–820.

- Idelovitch,E. andMichail,M.1984. Soil-AquiferTreatment—anew approach to anoldmethodof wastewaterreuse.Journal ofWater Pollution Control Federation56:936–943.

- Jimenez,B.andGarduno,H.2002.Social,politicalandscientificdilemmasformassivewastewaterreuse intheworld.InAWWAPublicationNavigating Rough Waters—Ethical Issues in the Water Industry.

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