phosphorus fluxes to the environment from mains water ......41 • seasonality + scenarios in p...
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Phosphorusfluxestotheenvironmentfrommainswaterleakage:Seasonalityandfuturescenarios1
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Ascott,M.J.a,Gooddy,D.C.a,Lapworth,D.J.a,Davidson,P.b,Bowes,M.J.c, Jarvie,H.P.candSurridge,3
B.W.J.d4
5
aBritishGeologicalSurvey,MacleanBuilding,Crowmarsh,Oxfordshire,OX108BB,UnitedKingdom.6
bEnvironment Agency, KingsMeadowHouse, KingsMeadow Road, Reading, Berkshire, RG1 8DQ,7
UnitedKingdom.8
c Centre for Ecology & Hydrology, Maclean Building, Crowmarsh, Oxfordshire, OX10 8BB, United9
Kingdom.10
dLancasterEnvironmentCentre,LancasterUniversity,Lancaster,LA14YQ,UnitedKingdom.11
12
CorrespondingAuthor:Ascott,M.J.*13
a British Geological Survey, Maclean Building, Crowmarsh Gifford, Oxfordshire OX10 8BB, UK.14
matta@bgs.ac.uk,+44(0)149169240815
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Abstract21
Accuratequantificationof sourcesofphosphorus (P)entering theenvironment isessential for the22
managementofaquaticecosystems.Pfluxesfrommainswaterleakage(MWL-P)haverecentlybeen23
identified as a potentially significant source of P in urbanised catchments. However, both the24
temporal dynamics of this flux and the potential future significance relative to P fluxes from25
wastewatertreatmentworks(WWT-P)remainpoorlyconstrained.UsingtheRiverThamescatchment26
inEnglandasanexemplar,wepresentthefirstquantificationofboththeseasonaldynamicsofcurrent27
MWL-Pfluxesandfuturefluxscenariosto2040,relativetoWWT-PloadsandtoPloadsexportedfrom28
thecatchment.ThemagnitudeoftheMWL-Pfluxshowsastrongseasonalsignal,withpipeburstand29
leakageeventsresultinginpeakPfluxesinwinter(December,January,February)thatare>150%of30
spring(March,April,May)/autumn(September,October,November)fluxes.WeestimatethatMWL-31
P isequivalenttoupto20%ofWWT-Pduringpeak leakageevents. Winterrainfalleventscontrol32
temporal variation in bothWWT-P and riverine P fluxes which consequently masks any signal in33
riverinePfluxesassociatedwithMWL-P.TheannualaverageratioofMWL-PfluxtoWWT-Pflux is34
predictedtoincreasefrom15to38%between2015and2040,associatedwithlargeincreasesinP35
removal at wastewater treatment works by 2040 relative to modest reductions in mains water36
leakage.However,furtherresearchisrequiredtounderstandthefateofMWL-Pintheenvironment.37
FuturePresearchandmanagementprogrammesshouldmorefullyconsiderMWL-Panditsseasonal38
dynamics,alongsidethelikelyimpactsofthissourceofPonwaterquality.39
Highlights40
• Seasonality+scenariosinPfluxesfrommainswaterleakage(MWL-P)quantified41
• MWL-PcomparedtowastewaterPflux(WWT-P)andcatchmentPexport42
• WinterburstMWL-Pflux>150%ofspring/autumnflux,equaltoupto20%ofWWT-P43
• MWL-P/WWT-Pratiopredictedtoincreaseto38%by2040duetoWWTPremoval44
• MWL-PfluxandseasonalityshouldbeconsideredinfuturePresearchandmanagement45
3
Keywords46
Phosphorus,sourceapportionment,eutrophication,mainswater,leakage47
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GraphicalAbstract49
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1500
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Was
tew
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2005 2006 2007 2008 2009 2010 20110
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P flu
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Wastewater Treatment WorksMains Leakage
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1 Introduction59
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Eutrophication associatedwith phosphorus (P) export fromagricultural land andwithwastewater61
treatmenteffluentisawell-knowncauseofwaterqualityimpairment(Elseretal.2007,Hiltonetal.62
2006,Vollenweider1968),thatcanresultinharmfulalgalblooms,fishkillsandadverseimpactson63
animalandhumanhealth(FunariandTestai2008).Effluentfromwastewatertreatmentworkshas64
beenrecognisedasamajorsourceofPattheglobalscale(Moréeetal.2013)whichhasbeenshown65
toimpairwaterqualityinriversintheEurope(Jarvieetal.,2006),Asia(Lietal.2015),andtheUSA66
(Dubrovskyetal.2010).IntheUK,Pfluxesfromwastewatereffluenthasbeenestimatedtoaccount67
for 78% of annual riverP loads (WhiteandHammond2009). The implementationof theUrban68
WastewaterTreatmentDirective(EuropeanUnion1991)resultedinsubstantialdecreasesinPloads69
fromUKwastewater treatmentworks (WWT-P), leading to largedecreases in fluvialP fluxes. For70
example,in2015,fluvialfluxesofPinEnglandandWaleswereestimatedtohavedeclinedtolessthan71
50%of1974fluxes(Worralletal.2015).72
Recentstudieshaveshownthatincatchmentswithalargeproportionofurbanlandcover,leakageof73
P added to mains water (MWL-P) as phosphate is a potentially significant but previously largely74
overlookedsourceofPwithintheenvironment(Gooddyetal.2017).Phosphateisaddedtomains75
watertoreduceplumbosolvency(Hayes2010),withtherisksassociatedwithleadindrinkingwater76
derived from pipework being a significant public health concern (Edwards et al. 2009). Lead77
consumption in humans has been associated with cognitive development problems in children78
(Bellinger et al. 1987) as well as increased risk of heart disease and stroke (Pocock et al. 1988).79
Phosphateisaddedtomainswatertoconvertleadcarbonatetoleadphosphate.Leadphosphateis80
anorderofmagnitudelesssolublethanleadcarbonateandresultsintheformationofleadphosphate81
precipitates on internal pipe surfaces (Hayes 2010). In theUK it is estimated that >95% ofwater82
suppliesaredosedwithphosphate(CIWEM2011).IntheUSA,overhalfofsuppliesaredosed(Dodrill83
5
andEdwards1995). Dosingachieves finalP concentrations in tapwater thatareestimated tobe84
between700–1900µgP/L(UKWater IndustryResearchLtd2012),withphosphatedosinghaving85
beenahighly successfulengineering solution to reduce leadconcentrations indrinkingwater. For86
example,in2011,99.0%ofrandomtapwatersamplesinEnglandandWalesmetthedrinkingwater87
limitof10µgPb/L(CIWEM2011).88
Leakageofmainswaterisagloballysignificantchallenge,withthecostofnon-revenuewater(leakage89
andunbilledconsumption)estimated tobe$14billionperyear (WorldBank2006).National-scale90
leakage rates in England andWales are reported to be up to 25% of the water that enters the91
distributionnetworkandwatercompaniesplanfutureleakageratesovera25-yearplanninghorizon92
(2015-2040).Leakageofphosphate-dosedmainswaterasapotentiallyimportantsourceofPinthe93
environmentwas firsthypothesisedbyHolmanet al. (2008). Gooddyet al. (2015)made the first94
quantificationofthisfluxonanannualbasisforEnglandandWales,whichwassubsequentlyrefined95
byAscottetal.(2016a).96
97
However, understanding the temporal variability of MWL-P is also important, because biological98
impacts of P in aquatic ecosystems also show significant temporal variability, with autotrophic99
biomass growth occurring inmany systems primarily during spring (March, April,May) and early100
summer(June,July,August)(Bowesetal.2012a).Anumberofstudies(Bireketal.2014,Cocksand101
Oakes 2011, UKWater Industry Research Ltd 2007, 2013) have highlighted seasonal dynamics in102
leakageofwaterfrommainssupplies.Higherratesofbothburstsandlow-levelcontinuous“invisible”103
leakageoccurduringwinter(December,January,February),duetopipecontractionandexpansion.104
However,workonPtodatehasonlyquantifiedtheannualfluxofPfrommainswaterleakageandthe105
temporaldynamicsof this sourcearepoorlyconstrained. Whilst substantial reductions inWWT-P106
loadstorivershavebeenachieved(Worralletal.2015),elevatedPconcentrationsremainasignificant107
concern, with 40% of the water bodies in England not achieving “good status” under theWater108
6
Framework Directive (European Union 2000) due to high reactive P concentrations (Environment109
Agency 2015). Consequently, in England andWales, substantial further investment inwastewater110
treatmentisplannedto2020withanestimatedtotalexpenditureof£2.1billion(GlobalWaterIntel111
2014).Similarprogrammesexist internationally, forexample in theUSA(SewageWorld2016)and112
across Europe (Water Technology 2016). In parallel,water companies continue to reduce leakage113
basedonwatersavingdrivers,whichwillinturnreduceMWL-Pfluxes.Despitetheseprogrammes,114
noestimatesofhowtherelativecontributionsofMWL-PandWWT-Ptotheenvironmentmaychange115
inthefuturehavebeenmade.IfPsourcestoaquaticecosystemsaretobemanagedeffectively,and116
the impactsofMWL-Preduced, it isessential thatthetemporaldynamicsofMWL-Pandpotential117
futurechangesinfluxesfromthissourceofParebetterconstrained.Theresearchreportedherewas118
undertakentoexaminethefollowinghypotheses:119
1. MWL-Pfluxesshowseasonaltrends,withincreasedfluxesinwinterassociatedwithincreased120
burstsandinvisibleleakage121
2. SeasonalityinMWL-PfluxescanbedistinguishedfromtemporalvariabilityinotherPsources122
suchaswastewatertreatmentworkseffluent123
3. TherelativeimportanceofMWL-Pfluxeswillincreaseinthefuture,asWWT-Pfluxesdecrease124
duetoimprovementsintertiarytreatment.125
126
Using the River Thames, a large lowland catchment in the UK, as an exemplar, we tested these127
hypotheses by deriving historic, daily MWL-P, WWT-P and riverine P fluxes. Subsequently, we128
validatedourapproachbycomparingourderivedannualfluxesofPfromMWLandWWTsourcesto129
fluxes reported in previous studies.We then undertook time series analysis usingmultiple linear130
regression modelling to develop an improved understanding of the temporal dynamics and the131
processescontrollingMWL-P,WWT-PandriverinePfluxes.Further,wederivedfuturescenariosfor132
MWL-PandWWT-P,basedon25yearplansforleakagereductionandWWT-Premovalrespectively133
7
aspublishedbytheUKenvironmentalregulatorandwatercompanies.Finally,weprovideasummary134
ofkeyresearchprioritiesinthecontextofMWL-P,andconsiderhowtomanagethissourceofPacross135
arangeofdifferenthydro-socioeconomicsettings (countrieswhichcurrentlyundertakephosphate136
dosing,countriesconsideringthisstrategy,countriesactivelyavoidingit).137
138
2 MaterialsandMethods139
2.1 Studyarea140
141
WequantifieddailyMWL-P,WWT-PandriverinePfluxesforthenon-tidalThamescatchment(Figure142
1). The Thames is a relatively large (9948 km2) lowland catchment in southern England. The143
catchmentispredominantlyunderlainbytheCretaceousChalk,withOoliticlimestonesattheheadof144
thecatchmentandareasoflowpermeabilityclaysaroundOxfordandLondon.Meanannualrainfall145
in the catchment at Oxford is 745mm (Marsh andHannaford 2008). Although the catchment is146
relativelyrural,withc.45%oflandclassifiedasarable(Fulleretal.2013),therearealsoanumberof147
majorurbanareas(London,Oxford,Reading)resultinginahighcatchmentpopulationdensity(c.960148
people/km2(Merrett2007).149
150
Watersupplyinthecatchmentisprovidedbyfourdifferentwatercompanies(ThamesWater,Affinity151
Water,SuttonandEastSurreyWaterandSouthEastWater)whichoperateacrossatotalof10largely152
self-contained water resource zones (WRZs, Environment Agency (2012)). Each individual water153
companyreportsannualleakagerates.Leakageratesarereportedtobehigh,reachingupto26%of154
water input to thedistributionnetwork (CommitteeonClimateChange2015).Thecatchmenthas155
beensubjecttopreviousresearchtoestimatetheannualcontributionofMWL-PandWWT-Ptothe156
8
RiverThames(Gooddyetal.2017).Buildingonthis initialresearch,Ascottetal.(2016a)estimated157
thatc.30%oftheMWL-PfluxinEnglandandWaleswasderivedfromtheriverbasindistrictinwhich158
thenon-tidalThamescatchmentislocated.Wastewatertreatmentoccursthroughoutthecatchment159
and137watercompanywastewatertreatmentworksserving>2000populationequivalents(p.e)are160
present.Sincethelate1990s,PfluxesfromWWTshavebeenreducedinthecatchmentthroughthe161
introductionoftertiarytreatment,primarilydrivenbytheEUUrbanWasteWaterTreatmentDirective162
(Kinniburgh and Barnett 2010, Powers et al. 2016). Consequently, P concentrations in the River163
Thamesfellbyc.90%between1992and2005(KinniburghandBarnett2010).Despitethis,thereis164
scantevidencethateutrophicationriskintheThameshasreduced(Bowesetal.2014),withanumber165
ofrivers inthecatchmentexhibitingexcessivephytoplanktonbiomassandPconcentrationsabove166
thoseatwhichthegrowthofautotrophscanbeexpected(>c.80µgSRP/l,Bowesetal.(2012b)).167
168
(b)(a)
Rivers
Water Resource Zone Boundaries
Thames Catchment
Urban Areas
WWTsCatchment Export Monitoring PointOxford Weather Station
169
Figure 1 Location of the non-tidal Thames catchment within England (a) and the water resource zones, wastewater170treatmentworks serving > 2000 p.e., catchment exportmonitoring point and theOxfordweather stationwithin the171catchment(b).ContainsOrdnanceSurveydata©Crowncopyrightanddatabaseright(2017).172
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2.2 Pfluxtimeseriesderivationandanalysis175
2.2.1 Datasources176177
Table1summarisesthedatasourcesusedtoderivedailytimeseriesofMWL-P,WWT-Pandriverine178
Pfluxesinthisstudyfortheperiod2001-2011.MWL-Pwasestimatedbasedonpublishedleakage179
ratesandestimatesofPconcentrationsindrinkingwater.WaterqualitysamplingofWWTdischarges180
andtheRiverThamesoccursatamuchlowerfrequency(c.every10daystomonthly)incomparison181
tothedailyflowdatathatareavailable.Consequently,toderivedailyWWT-PandriverinePfluxes,182
missing concentration data were estimated using concentration-flow relationships, as detailed in183
section2.2.3.184
185
Table1DatasourcesusedtoderivedailytimeseriesofMWL-P,WWT-PandriverinePfluxesfor2001-2011.186
Flux DataTimeperiod Frequency Source
MWL-P
WRZlevelleakageratesforWRZsinnon-tidalThamescatchment
2011-2013 Annual WRMPtables
LeakageratesforThamesWater
2001-2011 Daily
CocksandOakes(2011)
PO4-Pdosingconcentration
2001-2011 Annual Comberetal.(2011)
PO4-Pdosingextent2001-2011 Annual
Hayesetal.(2008),CIWEM(2011)
RiverineP
PO4-PconcentrationforThamesatTeddington
2000-2015 Monthly
EnvironmentAgencyFlowsfortheThamesatTeddington
2000-2015 Daily
WWT-P PO4-PWWTdischargeconcentrations
2005-2011
Variable(meanfrequency=10days,medianfrequency=27days)
EnvironmentAgencyWWTdischargeflows2005-2011 Daily
187
10
188
189
2.2.2 MWL-P190
191
Annualwaterresourcezonelevelleakagerates(inML/day)for2011-2013wereextractedfromwater192
resourcesplanningtablesavailableonwatercompanywebsites(AffinityWater2014,SoutheastWater193
2014,SuttonandEastSurreyWater2014,ThamesWater2014).Dailyleakagedatafor2001to2011194
(asreportedbyCocksandOakes(2011))forthewholeThamesWatersupplyareawereusedasthe195
basis forestimatingtheMWL-Pflux.This is theonlypublicly-availablesub-annual leakagedata for196
boththecatchmentandtheUK.TheThamesWatersupplyareacovers76%ofthenon-tidalThames197
catchment that is the focus for the research reported here, but also includes London in the198
downstreamtidalThamescatchment.Toaccountforthisdiscrepancy,wecalculatedtheratioofthe199
annualleakageratesinthewaterresourcesplanningtablesforthewholeThamesWatersupplyarea200
versus the10WRZs in thenon-tidalThamescatchment.Wethenusedthis ratio toscale thedaily201
ThamesWaterleakagetimeseries(CocksandOakes2011)toprovideadailyleakagetimeseriesfor202
thenon-tidalThamescatchmenttoTeddingtonforthesameperiod.Overtheperiod2001to2011,203
theextentofPdosingofmainswaterandtheconcentrationsusedhaveincreased.Basedonpublished204
reportsofdosingconcentrations(Comberetal.2011)andthespatialextentofdosing(CIWEM2011,205
Hayesetal.2008,UKWaterIndustryResearchLtd2012),weusedtheconservativeapproachadopted206
byGooddyetal.(2017)toestimatethehistoricchangesinPdosingconcentrationsandextentswithin207
thenon-tidalThamescatchment.208
209
2.2.3 RiverinePandWWT-Pfluxderivation,fluxcomparisonandtimeseriesanalysis210
211
RiverinePfluxderivation212
11
213
The daily net export of P (2001-2016) from the catchment was derived using observed214
orthophosphate-P (PO4-P) concentration and daily flow data for the River Thames at Teddington215
(Figure1). Inorderto infillmissingconcentrationdatatoderiveadailyPfluxtimeseries,wefirst216
undertooksinglechangepointdetectionusinganasymptoticpenaltyvalueof0.05(ChenandGupta217
2011,KillickandEckley2014)todeterminewhethertherewasastatisticallysignificantchangeinthe218
meanandvarianceoftheconcentrationtimeseriesassociatedwithhistoricreductionsinPloadsfrom219
WWTs(KinniburghandBarnett2010).FollowingtheapproachofBowesetal.(2014),wethenused220
non-linear least-squares regression toderive separateconcentration-flow relationshipsbeforeand221
afteranychangepointsthatwereidentified:222
𝐶𝐶 = 𝐴𝐴𝑄𝑄% (1)223
WhereCisconcentration(mgP/L),Qisflow(m3/s)andAandBareempiricallyderivedconstants.The224
derivedrelationshipswereusedtoestimatePconcentrationswheredataweremissingbeforeand225
afterthechangepoint.Thecompleteconcentrationandflowtimeserieswasthenusedtoderivea226
dailyPfluxtimeseries.227
228
WastewaterTreatmentWorksPFluxEstimation229
230
WastewatertreatmentworksintheThamescatchmentfallintotwocategories:(1)monitoredsites231
whereflowsare>50m3/d;and(2)unmonitored,smallsiteswithflows<50m3/d.Outputsfromthe232
ThamesSourceApportionmentGeographicalInformationSystems(SAGIS,Comberetal.(2013))tool233
usingpopulationequivalentdataandconcentrationestimateshaveshownthatsmallunmonitored234
sitescontribute<1%ofthetotalWWT-PloadtotheRiverThames.Consequently,thesesiteshavenot235
beenconsideredfurtherintheresearchreportedhere.236
237
12
ForthemonitoredWWTsites,dailyflowmonitoringdatafor2005–2011wereusedinconjunction238
with PO4-P concentration data to derive daily P fluxes for each site. A similar approach to that239
describedabovefortheriverinePfluxestimateswasusedforinfillingofmissingconcentrationdata240
usingconcentration-flowrelationshipsoftheformofequation1.ThedatesforimplementationofP241
removalschemesatWWTsintheThamescatchmentwereprovidedbytheenvironmentalregulator242
for England. Where P removal schemes were implemented at a WWT during the study period,243
separate concentration-flow relationships were estimated for before and after implementation.244
WherenochangesintreatmentprocessesforPremovalatWWTsoccurredduringthestudyperiod,245
asingleconcentration-flowrelationshipfortheWWTwasused.ThederiveddailyPfluxesforeach246
individualWWTwerethensummedtoderiveatotal,dailycatchmentfluxofWWT-P.247
248
Fluxcomparisonandtimeseriesanalysis249
250
TovalidateourdailyestimatesofMWL-PandWWT-P,wederivedannualmean fluxes foreachof251
thesePsourcesandthencomparedthesetopreviouslypublishedestimatesofthesamefluxesforthe252
Thamescatchment(Comberetal.2013,Gooddyetal.2017,Haygarthetal.2014).Wealsocalculated253
adailytimeseriesoftheratioofMWL-PtoWWT-P.ThederivedMWL-P,WWT-PandriverinePtime254
serieswere initiallycomparedbyvisual inspection toassess the timingandmagnitudeofpeaks in255
fluxes.Wethenassessedwhetherthereweresignificantlongtermtrendsandchangesinthevariance256
inMWL-PandMWL-P/WWT-P ratiousingaMannKendall test (Pohlert 2018) anda changepoint257
analysisusinganasymptoticpenaltyvalue=0.01(KillickandEckley2014).258
259
InordertoassessthecontrolofcoldweathereventsontemporalvariabilityinMWL-P,weundertook260
correlation analyses using historic dailyminimum temperature data for the Oxford station in the261
13
Thamescatchment(Figure1).WealsoquantifiedcorrelationsbetweendailyrainfallanddailyWWT-262
P anddaily riverineP loads to assess the roleof short term rainfall events in controllingP fluxes.263
Finally,todeterminehowmuchofthevariationindailyriverinePloadcouldbeexplainedbydaily264
MWL-PanddailyWWT-P,wedevelopedsimplemultiplelinearregressionmodelsusing(1)WWT-P265
onlyand(2)WWT-P+MWL-Pasexplanatoryvariables.Wecomparedthecoefficientofdetermination266
betweenthetwomodelsandusedapartialF-test(RDevelopmentCoreTeam2016)todeterminethe267
impactofaddingtheMWL-Pvariableonmodelpredictivepower.Thereissubstantialuncertaintyin268
thefateofMWL-PintheenvironmentassociatedwiththerelativeimportanceofdifferentPretention269
(claysorption,soilandsedimentaccumulation)andtransportprocesses(groundwater,surfacewater270
andthesewernetwork)(Gooddyetal.2017).Inthiscontext,ourresearchdoesnotaimtomakea271
directcausallinkbetweentimeseriesvariabilityinWWT-P,MWL-PandriverinePfluxes.Thepurpose272
the comparison of fluxes described here is threefold: (1) to provide indications of the relative273
contributionsof thesesources to theenvironment; (2) to improveunderstandingof theprocesses274
controllingtemporalvariabilityinthesesources;and(3)toimproveunderstandingoftherelationships275
betweenthesources.276
277
278
2.3 DerivationoffuturescenariosofMWL-PandWWT-P279280
Scenarios for future reductions in both MWL-P andWWT-P were constructed. Planned leakage281
reductions foreachof the10WRZs in theThamescatchment for2015-2040wereextracted from282
watercompanywaterresourceplanningtables(AffinityWater2014,SoutheastWater2014,Sutton283
andEastSurreyWater2014,ThamesWater2014). EstimatesoffuturereductionsinWWT-Pwere284
providedbytheenvironmentalregulatorasnewdischargepermitlimitsforP(inmgP/L).InEngland285
and Wales investments in wastewater treatment are planned on a five-year Asset Management286
14
Programme(AMP)cycle.Investmentsover2015–2020(AssetManagementProgramme6(AMP6))287
arefullyplannedandcosted. Furtherintothefuture,aprovisional“longlist”of improvementsin288
WWTsthatarepotentiallyrequiredinordertomeetgoodecologicalstatusinthecatchmenthasbeen289
derivedbasedonsourceapportionmentmodelling(Comberetal.2013).Thislisthasbeensubjectto290
acostbenefitanalysiswhichhasdividedsitesintothoseatwhichenhancedPremovaliscostbeneficial291
andthoseatwhichthisisnot.Intotal,fourWWTimprovementscenariosweredeveloped,asdetailed292
below:293
1. Baseline(2015)294
2. AMP6(2020)–plannedandcostedWWTimprovementsandleakagereductionprogrammes295
for2015–2020296
3. Costbeneficialplanningperiod(2040a)–plannedleakagereductionsandcostbeneficialWWT297
improvementsfor2015–2040298
4. Full planning period (2040b) – planned leakage reductions and all potential WWT299
improvementsfor2015–2040300
ScenarioswereimplementedbyapplyingthenewdischargepermitlimitsforWWTstothebaseline301
observed concentration-flow data in 2015. Given the uncertainty in future WWT and leakage302
reductionbeyond2020,anannualaverageapproachwasusedtoderiveannualfluxesfromMWL-P303
andWWT-Pin2015,2020andthetwo2040scenarios.304
305
306
307
308
15
3 Results309310
3.1 MWL-Pfluxestimation311
312
Figure 2 shows the evolution of the MWL-P flux over the period 2001 to 2011 for the Thames313
catchment. Figure2(c)showsthetotaldaily leakage(CocksandOakes2011) for2001–2011for314
ThamesWater. Over the period 2001 – 2011, the volumeof leakagehas declineddue towater315
companies actively locating and fixing leaks in the distribution network. After accounting for this316
reduction,theleakagetimeseriesshowsarelativelystrongcorrelationatlag=0withdailyminimum317
temperature(Pearsoncorrelation,r=0.63,p=1.392x10-14).Thetimeseriesshowsaclearseasonal318
signalassociatedwithincreasedburstsandinvisibleleakageinwinter.Thewintersof2009,2010and319
2011wereparticularlycold,withmeanDecembertemperatures1.1,2.0and4.8degreesbelowlong320
termaverage(1971-2000)respectively(MetOffice2016).Thisresultedinlargeincreasesinleakage321
of up to 50% compared to the preceding autumn months (September, October, November). In322
summer(June,July,August),smallincreasesofupto6%comparedtospring(March,April,May)values323
arealsoobserved,whichhasbeenattributedtosoilexpansionandcontractionresultinginpipefailure324
(CocksandOakes2011).Figure2(d)showstheestimateddailyfluxofMWL-Pfor2001-2011forthe325
Thamescatchment.Whilstthevolumeofleakagehasreducedsubstantially,increasesinthespatial326
extentofdosingand inphosphatedosingconcentrations counteract these reductions, resulting in327
significant(MannKendalltrendtest,p<2.2x10-16)netincreasesinMWL-Pfluxesofc.300%overthe328
period 2001 to 2011. Seasonality increases through time with a significant change in variance329
identifiedat08/09/2008(singlechangepointdetectionusinganasymptoticpenaltyvalue=0.01)and330
theratioofdailyannualmaximum/minimumMWL-Pratesincreasingfromc.130%in2002–2008to331
150%in2008–2011.332
333
16
334
Figure2:EstimatesofPdosingconcentrations(a)andextents(b)for2001–2011,dailyleakageratesreportedbyCocks335andOakes(2011)(c),andthederivedMWL-Pflux(d)336
337
338
339
3.2 WWT-Pfluxestimation340
341
Figure3reportsdatarelatedtotheWWT-PfluxfortheThamescatchment.Substantialdecreasesin342
PconcentrationinWWTeffluentsoccurredbetween2005and2007,associatedwiththeintroduction343
ofPremovaltechnology.FromMarch2008to2012,nosignificantfurtherPremovalwasimplemented344
in the catchment. Consequently, effluent P concentrations have been relatively stable and the345
frequency of P monitoring reduced. The slight increase (c. 5.4 to 5.6 mg P/L) in effluent P346
concentrationsover this periodmay reflect anoverall decrease inP removal efficiency in existing347
WWTsduetochangesinplantoperationandinfluentPloads(TchobanoglousandBurton2013).There348
700
800
900
1000
Dosi
ng (
g P/
L)μ
0.5
0.6
0.7
0.8
0.9
Dosi
ng e
xten
t (-)
2002 2004 2006 2008 2010
600
700
800
900
1000
Leak
age
(Ml/d
)
2002 2004 2006 2008 2010
200
300
400
500
600
700
800
900
MW
L-P
(kg
P/d)
(a) (b)
(c) (d)
17
isconsiderablevariationintheWWTflows(Figure3(a))particularlyduringhighflowperiodsinwinter349
associatedwithrainfalleventswhenflowscanbeuptotwiceaslargeasdryweatherflows(DWF).350
ThederivedWWT-Pflux(Figure3(c))reflectsbothlongtermchangesinPconcentrationsfollowing351
investmentinPremovaltechnologiesandshorttermvariabilityinWWTflowsassociatedwithrainfall352
events.353
354
355
Figure3TotalWWTflows(a),meandailyPconcentrationsandnumberofsamples(b)andthederivedWWT-Pfluxand356rainfalltimeseriesforOxford(c)357
358
3.3 Fluxcomparison359
360
Table2reportstheannualmeanMWL-PandWWT-Pfluxesderivedintheresearchreportedhereand361
inpreviousstudiesoftheThamescatchment.TheannualestimatesofMWL-Pinthisstudyarewithin362
therangeofvaluesreportedbyGooddyetal.(2017)whoconsideredarangeofdifferentplausible363
1000
000
2000
000
Tota
l WW
T F
low
(m
3 /d) (a)
05
1525
Num
ber o
f sam
ples
/day
5.2
5.6
6.0
6.4
Mea
n W
WT
conc
(mg
P/L
)
(b)
WWT conc.No. of samples
010
2030
40Ra
infa
ll (m
m)
2006 2008 2010 2012
1500
2500
WW
T flu
x (k
g P
/d) (c)
WWT fluxRainfall
1030
20
18
historicmainswaterdosingextents.WWT-Pfluxesbroadlycorroboratepreviousfindingsreportedby364
Haygarthetal.(2014)andComberetal.(2013).Fluxesestimatedinthecurrentstudyfor2006-2008365
aresomewhathigher(c.29%)thanestimatesmadeusingobservedsolublereactivePconcentrations366
inWWTdischargesinconjunctionwithsourceapportionmenttools(Comberetal.2013).Fluxesfrom367
thesourceapportionmentmodellingwereadjustedbytheUKenvironmentalregulator(Environment368
Agency 2016) to take into account new and enhanced P removalwhich is the likely cause of this369
discrepancy.Ourestimatesarealsolarger(c.50%)thanthosecalculatedbyHaygarthetal.(2014),370
whousedasimpleapproachbasedonpopulationequivalentsandassumedtotalPconcentrationsto371
estimateWWT-Pfluxforthecatchment.372
Table2AnnualmeanMWL-PandWWT-Pfluxesderivedinthisstudyandpreviousstudies373
374
PeriodMWL-P(ktP/yr) WWT-P(ktP/yr)
Gooddyetal.2017 Thisstudy
Haygarth et al2014
Comber et al2013 Thisstudy
2001 0.019-0.029 0.025 1.18 - -2006-2008 - - 0.52 0.672011 0.068-0.089 0.082 0.38 - 0.58
375
3.4 FluxcomparisonbetweenMWL-PandWWT-PwithRiverinePExport376
377
3.4.1 RiverinePfluxestimation378379
Figure4showstheresultsofthechangepointdetectionanalysisandthederivedconcentration-flow380
relationshipsbeforeandafter thechangepoint for theRiverThamesatTeddington.A substantial381
decrease in the mean and variance of riverine phosphate concentrations was observed in 2006,382
associatedwithreductionsinPloadsfromWWTs.Fortheperiod2000–2006,theconcentration-flow383
relationshipshowsdecreasesinconcentrationswithincreasingflow,associatedwithdilutionofWWT384
inputs. This corroborates concentration flow relationships derived by Bowes et al. (2014) in the385
19
Thamesbasin.From2006to2015,thevariabilityinconcentrationwithflowissubstantiallyreduced.386
Between0and100m3/ddecreases inconcentrationwith increasingflowareobserved,associated387
withdilutionofpoint sourceP inputs.However, from100 to500m3/d, there is limitedchange in388
concentration with increasing flow which reflects a balance of both point source dilution and389
mobilisationofdiffusePsourcesacrossthisrangeofdischarges.390
391
392
Figure4Observedphosphateconcentrations(mgP/L)intheRiverThamesatTeddingtonandmeanconcentrations393(dashedline)beforeandafterthechangepoint(a)andconcentration-flowrelationshipsforeachoftheseperiods(b).394
395
396
3.4.2 ComparisonofMWL-P,WWT-PandriverinePfluxes397398
Figure5showstheratioofMWL-PandWWT-Pfluxes(“MWL-P/WWT-P”)andtheirrelationshipwith399
Pflux intheRiverThamesatTeddington. From2005to2011there isasignificanttrend inMWL-400
P/WWT-P(Mann-Kendalltrendtest,p<2.2x10-16).TheratioofMWL-PtoWWT-Pincreasesfrom401
fromc.7-10in2005to15%in2011.Thisisduetotwofactors:(1)anincreaseintheextentofmains402
waterPdosingthroughtheperiod,asshowninFigure2;and(2)adecreaseinWWT-Ploadsdueto403
investmentinPremovaltechnology.404
2000 2005 2010 2015
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Con
cent
ratio
n (m
g P/
L)
(a)
0 50 100 150 200 250 300 350
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Flow (m3 /d)
Con
cent
ratio
n (m
g P
/L)
2000-2006, C = 1.036X-0.197
2006-2015, C = 0.343X-0.143
(b)
20
SeasonalityintheratioofMWL-PtoWWT-Pincreasesthroughtheperiod,duetotheincreasedextent405
ofdosing.ThereisasignificantincreaseinthevarianceofMWL-P/WWT-Pfrom22/11/2008onwards406
(singlechangepointdetectionusinganasymptoticpenaltyvalue=0.01(ChenandGupta2011,Killick407
andEckley2014)).PeaksinMWL-P/WWT-Pofupto20%occurinthewintersof2009and2011in408
comparisontovaluesaround10%in2005-2008.Thisseasonalityalsogivessomeinitialinsightinto409
theprocessescontrollingMWL-Pfluxes.Inthewinterof2009/10arapidincreaseinMWL-Poccurs410
followedbyarapidincreaseinWWT-P.ThiscausesMWL-P/WWT-Ptoriseandfallagainrapidly.Itis411
likelythesetrendsaretheresultofaparticularlycoldweatherperiodresultinginanincreaseinleaks412
andbursts.Thisislikelytobefollowedbyaperiodofactiveleakagecontroltoreduceleakagerates413
backtolevelspriortothecoldweatherperiod,ashasbeenreportedbyUKWaterIndustryResearch414
Ltd(2007). Atthesametimeaperiodofwetweatheroccurswhichresults in increasedinflowsto415
WWTs.ThisresultsinincreasesinWWToutflows(Figure3(a))butflowincreasesaresignificantlyless416
thantypicaldesignflowsforfulltreatmentofinfluentduringstorms(upto3timesDWF,with4–6417
timesDWFretained instormtanks(Sauletal.2007)). Any influent inexcessofstormtankdesign418
criteria as well as combined sewer overflows are likely to be discharged directly to the nearest419
watercourse.ThesewillallcombinetoreducetheeffectivenessoftheWWTstoremoveP.420
421
Figure5(a)and(d)showthedailyWWT-PfluxandcatchmentPexport.Thereareweakbutsignificant422
correlations betweenWWT-P anddaily rainfall (Pearson correlation, r = 0.23, p < 2.2 x 10-16) and423
catchment export anddaily rainfall (Pearson correlation, r = 0.18, p < 2.2 x 10-16). Figure6 shows424
observedandmodelledriverinePfluxesusingMLRmodelsconsidering:(1)WWT-Ponly;and(2)WWT-425
PandMWL-Ptogether.Themultiplelinearregressionmodelusedtodeterminewhethertheriverine426
P load can be explained by MWL-P and WWT-P resulted in a relatively modest coefficient of427
determination (adjusted R2 = 0.41). TheMLRmodel suggested thatWWT-P andMWL-P are both428
significantexplanatoryvariables(P<2x10-16). RemovingMWL-Pfromtheregressionsignificantly429
21
reduced thepredictivepowerof themodel (F=242,P<2x10-16, forpartial F-testbetweenMLR430
models considering WWT-P only and WWT-P + MWL-P) resulting in a lower coefficient of431
determination(adjustedR2=0.33).432
010
2030
40R
ainf
all (
mm
)
1000
2000
3000
WW
T flu
x (k
g P/
d)
(a)
WWT flux Rainfall
-50
510
1520
Tem
pera
ture
(°C
)
MWL-P flux Temperature
150
200
250
300
MW
L-P
Flux
(kg
P/d) (b)
0.05
0.15
0.25
MW
L-P/
STW
-P ra
tio
(c)
010
2030
40R
ainf
all (
mm
)
2005 2006 2007 2008 2009 2010 2011
020
0040
0060
00
Riv
er P
Flu
x (k
g P/
d)
(d) River flux Rainfall
433
Figure5DailyfluxesofWWT-Pandrainfall(a),MWL-Pandtemperature(b)fortheThamescatchment,theratioofMWL-434PtoWWT-P(c)andtheriverinefluxofPoutofthecatchmentandrainfall(d)435
22
436
Figure6:Observed(black)andmodelledriverinePfluxesfortheexportoftheThamescatchmentusingtheMLRmodel437consideringWWT-Ponly(blue)andWWT-PandMWL-P(red)438
439
440
3.5 FutureloadsofMWL-PandWWT-P441442
Table3reportsthederivedfutureMWL-PandWWT-Ploadsforannualaverageconditions.WWT-P443
fluxes are predicted to decrease to 2020 associated with the AMP6 programme, with further444
decreasespredictedto2040.WhenconsideringallWWTimprovementsto2040,atotaldecreasein445
WWT-Pfluxof0.41ktP/yrisestimated.SmallreductionsinMWL-Pof0.01ktP/yeararepredictedto446
occur to 2040. In total, the reduction in MWL-P loading is c. 2% of the reduction in WWT-P.447
Consequently, the ratio ofMWL-P toWWT-P is predicted to increase from c. 15% to 38% under448
averageconditions.449
450
451
2005 2006 2007 2008 2009 2010 2011
010
0020
0030
0040
0050
00
Riv
erin
e P
flux
(kg
P/d
)ObservedLinear model (WWT-P only)Linear model (WWT-P and MWL-P)
23
452
Table3FutureMWL-PandWWT-Ploadings453
454
ScenarioInformation FluxesMWL-P/WWT-P%
ScenarioName TimeSlice
WWTImprovements
LeakageImprovements
WWT-PFlux (ktP/yr)
MWL-PFlux (ktP/yr)
AnnualMean
Baseline 2015 - - 0.62 0.09 14.52
AMP6Programme 2020 AMP6
ProgrammeAMP6 LeakageProgramme 0.61 0.08 13.11
2040Plans(CostBeneficial)
2040
CBA"LongList"2040 Plan LeakageProgramme
0.57 0.08 14.04
2040 Plans (Allsites) Full"LongList" 0.21 0.08 38.10
TotalPfluximprovement(ktP/yr) 0.41 0.01
455
456
457
458
459
460
461
462
24
4 Discussion463
4.1 Seasonaldynamicsandrelative importanceofMWL-PandWWT-P to464
riverPloads465
466
EstimatesofMWL-PandWWT-PfluxesfortheThamescatchmentmadeintheresearchreportedhere467
illustratethatMWL-PisapotentiallysignificantsourceofPintheenvironmentwhichhassofarbeen468
largelyoverlookedinPsourceapportionmentandmanagementstrategies.Inparticular,thedistinct469
seasonalsignal(Figure2)inMWL-PresultsinhigherratiosofMWL-PtoWWT-Pthroughthewinter470
periods.TheseasonalityinMWL-Pthatwashypothesizedinsection1hasnotbeenreportedtodate471
andrepresentsadevelopmentinourunderstandingofthisPsource.472
473
TheprocessescontrollingseasonalityinMWL-Parecomplex.Whilstcorrelationsbetweenminimum474
air temperatures and MWL-P are reported in this paper, it should be noted that other non-475
meteorologicalprocessesalsohaveapotentiallysignificantimpactonMWL-Pvariability.Coldweather476
isthepredominantcontrolonthetimingandmagnitudeoftheinitialoutbreakofwatermainsbursts477
duringawinterleakageevent.Astemperaturesincrease,thereislikelytobesomenaturalreduction478
ininvisibleleakageassociatedwithpipeexpansion.However,ratherthananymeteorologicalfactor,479
themajorityofthereductioninleakageafteraneventistheresultofactiverepairstoburstwater480
mains(UKWaterIndustryResearchLtd2007).Thiscombinationofmeteorologicalandengineering481
processes is likely to be broadly applicable in developed countries with significant temperature482
fluctuationsandconsequentlyasimilarseasonalityinMWL-Pwouldbeexpected.Phosphatedosing483
isstartingtobedevelopedinothertemperatecountries(e.g.Ireland(IrishWater2015,Mockleretal.484
2017)).Inthesecountries,asdosingextentsandconcentrationsincrease,anincreaseintheboththe485
absolutemagnitudeandseasonalityofMWL-Pfluxes,asreportedinFigure2,wouldbeanticipated.486
25
487
RainfalleventshaveanimpactontheobservedcorrelationsbetweenWWT-P,MWL-PandriverineP488
fluxes. Winter rainfalleventscauseshort term increases inbothWWT-PandriverineP fluxesbut489
throughdifferentprocesses.Highrainfallresultsinincreasedinflowstowastewatertreatmentworks.490
Theremaybesomedilutionof influentPduringtheseeventsdueto increasedcontributionsfrom491
runoff and groundwater infiltration (De Bénédittis and Bertrand-Krajewski 2005), but these are492
unlikely to be captured in the flux estimates due to the paucity of concentrationmeasurements.493
Influentflowsbelowthepeakstormflowtreatmentdesigncriteria(typically3xDWF(Sauletal.2007))494
willbetreatedanddischargedtowatercourses. Excessflowswillbetemporarlydivertedtostorm495
tanks and flows > 6 xDWFwill be directly discharged to rivers. Winter rainfall events can cause496
increasesinriverinePfluxesthroughanumberofprocesses.Increasedagriculturalrunoff,in-stream497
mobilisation of P associated with sediments, combined sewer overflows (CSOs) and wastewater498
treatmentworksdischargescanallcontributetoincreasingPloadsduringwinterstormevents(Bowes499
et al. 2008, Jarvie et al. 2006). These processes controlling short term temporal changes in both500
riverinePfluxesandWWT-PfluxesmaskanycorrelationbetweenMWL-PandriverineP.501
502
InthecontextofMWL-P,thereissomeuncertaintyintheextentofPdosingandtheconcentrations503
used. Just aswater companiespublically releasewater resourceplanningdata,makinghistoric P504
concentrationsanddosingextentsavailablewouldbebeneficialtorefiningMWL-Pestimates.Given505
the potential significance of this flux, assessment of MWL-P using observed leakage and P506
concentrationdataatthelocalscaleusingdistrictmeteredarea(DMA)datacouldprovideimportant507
newinsights.Further,theuncertaintysurroundingtheultimatefateofMWL-Pfollowingleakageis508
significant. As discussed previously, comparisonswithWWT-P and riverine P fluxes such as those509
presentedinFigure5arebeneficialastheyprovideindicationsoftherelativecontributionofdifferent510
sourcestotheenvironment.ItislikelythatMWL-PmakessomedirectcontributiontoriverinePfluxes,511
26
buttherelativeimportanceofgroundwater,surfacewaterandthesewernetworkaspathwaysfor512
MWLP-Parelargelyunknownatthistime.Moreover,thespringandsummergrowingperiodisoften513
perceived to be themost biologically-critical period inmany receivingwaters (Jarvie et al. 2006),514
meaning thatwinterpeaks inMWL-Pwhichdodirectly contribute to riverineP loadsmayhavea515
limitedimmediateimpactonriverineecosystems.However,therearelikelytobetimelagsbetween516
the flux of P fromawatermains leak and this flux reaching a river,with a rangeof transit times517
dependingoncatchmentsize,transportpathwayandchangeinPconcentrationalongthepathway.518
Consequently, it isplausiblethatwinterpeaks in leakageofP fromwatermainsonlyreachariver519
networkinmorebiologically-criticalperiodsoftheyear.Longtransittimesarelikelyincatchments520
withasignificantlythickunsaturatedzoneandlonggroundwaterflowpaths.Inthesecatchmentsit521
isplausiblethatMWL-Pcontributesto legacyPstores ingroundwater (Holmanetal.2008)ashas522
been observed for nitrate (Ascott et al. 2017, Ascott et al. 2016b). River sediments can also523
accumulate P (Sharpley et al. 2013), with time lags of < 1 year associated with in-stream P524
remobilisationduringhighriverflowevents.Thesein-streamaccumulation-remobilisationprocesses525
will result in further lags betweenMWL-P that reaches a river and P riverine exports.Whilst not526
considered in thisstudy,agricultureremainsasignificantPsourceto theenvironment (Whiteand527
Hammond2009)andshouldclearlybeaccountedforinfuturePmanagementstrategies.Muchofthe528
likelytemporaldynamicsofthefateofMWL-P(higherwinterfluxeswithpotentialcatchmentlegacy529
storesandlags)areanalogoustothefateofnonpointPsourcessuchasagriculture(Sharpley2016).530
TheanticipatedimpactofmeasurestoreducePfluxesfrombothmainsleakageandagricultureshould531
betemperedwithknowledgethatcatchmentlagsandreleaseofexistingnonpointlegacyPsources532
maysignificantlydelayanywaterqualityimprovements.533
534
535
27
536
4.2 MWL-P and WWT-P Scenarios: Implications for drinking water and537
wastewatermanagement538
539
Table 3 illustrates that futureWWT-P reductions are likely to be substantially larger thanMWL-P540
reductions,andconsequentlythiswillresult inagreaterrelativecontributionofMWL-PtoP loads541
delivered into the environment in the future. Whilst policy responses to minimise MWL-P were542
previously discussed by Gooddy et al. (2017), the implications of future increases in the relative543
contribution of MWL-P have not been considered to date. Such increases are likely to occur in544
countrieswherePdosingiscurrentlybeingconsideredinfutureleadreductionstrategies(e.g.Ireland545
(IrishWater2015)andSouthKorea(Lee,pers.comm.)).Inthesecountries,thepotentialimpactsof546
MWL-P shouldbe considered inenvironmental impact assessments. Moreover, in lessdeveloped547
countries,currentwastewaterPremovalandmainswaterPdosingmaybelessextensive.Asboth548
environmentalandpublichealthstandardsimprove,itisplausiblethatinthesecountriesmainswater549
P dosing and WWT P removal may increase substantially, resulting in increases in the relative550
importanceofMWL-P.551
552
A number of European countries have adopted policies that actively avoid phosphate dosing on553
environmentalgrounds(CIWEM2011).CountriessuchastheNetherlandsundertakepHcorrection554
andcentralizedwatersofteningtoensureleadconcentrationsarebelowtherequiredstandard(Hayes555
2012).Phasbeenshowntobealimitingnutrienttobiofilmgrowthinanumberofdifferentdrinking556
watersystems(Liuetal.(2016)andreferencestherein)andconcernhasbeenraisedhistoricallythat557
thepresenceofPinwatermains(throughdosingorotherwise)wouldresult inincreasedbacterial558
counts in watermains (Miettinen et al. 1996). The current and future significance ofMWL-P in559
28
comparisontootherPsourcesidentifiedinthisstudyaddstothebodyofevidencetosupportpolicies560
that avoid phosphate dosing,where the extent of leadpiping in the distribution network is small561
enoughtosafeguardhumanhealthprotectedwithoutdosing.562
563
Acrossthesedifferenthydro-socioeconomicsettings,futuremanagementofMWL-PandWWT-Pis564
likely to be a significant challenge. Bothwater utilities and environmental regulators have often565
historicallybeendividedbetweencleanwaterandwastewater(Ofwat2017).Withinthecleanwater566
sector,theindustryhasoftenbeenfurtherdividedbetweendrinkingwaterquality(responsibleforP567
dosing) and water resources management (responsible for leakage) (Deloitte 2014). Moreover,568
regulatory drivers for changes in water management have typically addressed single issues (e.g.569
drinking water quality via the EU Drinking Water Directive (European Commision 1998),570
environmental quality via the EU Water Framework Directive (European Union 2000)), whereas571
addressingMWL-Prequirespolicyinterventionsacrossmultiplefields.WherePdosingandWWTP572
removalare in their infancy,waterandenvironmentalmanagerswillneedtoengagestakeholders573
fromacrossthesedisciplinesatanearlystageofdevelopment. Thiswillensurethatstrategiesfor574
managingPsourcestotheenvironmentconsiderbothWWT-PandMWL-P,andthatreductions in575
WWT-Parenotoffsetby increases inMWL-P. Wherethesepracticesarealreadywellestablished,576
integration of MWL-P into leakage targets and catchment P permits may be an effective policy577
intervention (Gooddy et al. 2017). Adopting these strategies will ensure that P sources to the578
environmentaremanagedeffectively,whilstsafeguardhumanhealth.579
580
4.3 ResearchprioritiesforMWL-P581
582
29
Basedontheuncertaintieshighlightedabove, thereareanumberofkeyresearchprioritieswhich583
needtobeaddressedifMWL-PanditsassociatedseasonalityaretobeeffectivelyintegratedintoP584
managementstrategies.Thefateofwaterfollowingaleakiscurrentlypoorlyunderstood.Itcanbe585
hypothesisedthatthefatewillvarydependingonthetypeofleakasfollows:(1)Visiblewinterbursts586
causingrapidtransporttorivers,groundwaterandsewers,(2)Invisiblewinterleakagewithagreater587
rechargetogroundwater,(3)Summerleakageassociatedwithchangesinsoilmoisturedeficit(SMD)588
causingpossiblelossbyevapotranspirationandtogroundwater,(4)Backgroundleakagewithaslower589
transport and greater recharge to groundwater. Background leakage is likely to have the largest590
impactinthelongterm,becausetheseleaksarerelativelydifficulttolocateandstoprelativetobursts591
(Edie2016,Lambert1994).592
593
ThefateofPwithinthesubsurfacefollowingaleakaddsfurtheruncertainty.ThepHofmainswater594
isoftenincreasedcomparedtorawuntreatedwaters(Flemetal.2015)whichwillinhibitformation595
of iron and aluminium phosphate precipitates, resulting in increased P mobility. However, after596
releaseintotheenvironmentMWL-Phasawiderangeofpotentialfates(claysorption,soil/sediment597
accumulation,flowtorivers,groundwaterorthesewernetwork).Dependingonthetimeofyearand598
thetypeofleakidentifiedabove,thefateofMWL-Pmayvarysubstantially.Visiblewinterburstsand599
invisiblewinterleakagenotcapturedbythesewernetworkmayresultinrapidPtransporttorivers600
and groundwater, potentially accelerated by high rainfall events following coldweather. Summer601
leakageassociatedwithchanges insoilmoistureandbackground leakage is likely toresult inslow602
transport and potentially sorption and accumulation of P in soils. Understanding the relative603
significanceofthesedifferentfatesandlagsinthehydrologicalsystemisessentialifMWL-Pistobe604
managedinthefuture.605
606
30
Long term changes inMWL-P seasonality are also likely to occur due to climate change. Climate607
projections(UKCP092080s,Mediumemissionsscenario)showincreasesindailymean,maximumand608
minimumtemperaturesacrosstheUK,withslightlygreaterincreasesinsummerthanwinter.Whilst609
changestoannualprecipitationtotalarepredictedtobesmall,winterandsummerprecipitationare610
predictedtoincreasebyupto33%anddecreasebyupto40%respectively(MetOffice2010,Murphy611
etal.2009).Warmerwintersmayreducewinterbursts,andwarmersummersmayincreaseleakage612
relatedtosoilmovement.ConsequentlyitisplausiblethattheseasonalcomponentofMWL-Pmay613
becomemorebimodal,withpeaksinbothwinterandsummer.Differencesinthetemporalrainfall614
distributionarealsolikelytoimpactthefateofPwithinthewidercatchment.Increasedwinterrainfall615
mayresultinmorepronouncedwinterPflushing(Johnsonetal.2009).Interactionsbetweenclimate616
change induced temperature and rainfall effects on P fluxes are likely to be complex and require617
furtherinvestigation.618
619
620
621
5 Conclusions622
623
Thisstudyhasquantifiedseasonalityandfuturescenariosof fluxesofP frommainswater leakage624
(MWL-P)totheenvironmentforthefirsttime.MWL-Pshowsastrongseasonality,withpeakfluxes625
duringburstandleakeventsinwinter>150%offluxesduringotherseasons. Duringpeakevents,626
MWL-Pisequivalenttoc.20%ofPfluxesfromwastewatertreatmentworks(WWT-P).Amoderate627
cross correlation between WWT-P and riverine P fluxes is observed as the short term temporal628
variations inbothofthesefluxesaretheresultofwinterrainfallevents. Thismasksanypotential629
31
correlationbetweenMWL-PandriverinePfluxes. Asubstantial increase intheratioofMWL-Pto630
WWT-Pispredictedto2040associatedwithimplementationofsubstantialwastewaterPremovaland631
minimalmainswaterleakagereduction.FurtherresearchisrequiredtounderstandthefateofMWL-632
Pintheenvironment,futurechangesinMWL-Ploadings,andpotentialapproachestointegratethisP633
sourceintowatermanagementstrategies.634
6 Acknowledgements635
636
TheresearchwasfundedbytheUKsNaturalEnvironmentResearchCouncil(NERC)NationalCapability637
resourcesdevolvedtotheBritishGeologicalSurvey.Thispaper ispublishedwithpermissionofthe638
ExecutiveDirector,BritishGeologicalSurvey(NERC).639
640
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642
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