Estimating groundwater resources in hardrock area s - a water balance approach

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BO OLOFSSON NGU - BULL 43 9 , 2002 - PA GE15

Estimating groundwater resources in hardrock area s - a water balance approach

BO OLOFSSON

Olofsson, B.2001:Estimat ing groundwater resources in hardrock areas - a water balance approach. Norges qeoio- giskeundersekelseBulletin439,15-20.

Among decisionmakers in counties andmunicipalities,thereis a great need for simp lebut hydrogeologically cor- rectestimatio nsof thegroundwat erresources.Today'smethod s are ofte nbased onextensive field investigations andcomplicatedmathematicalflow modell ingortoo simplifiedannually based waterbalances.In thispaper,a groundw aterbalancemethodispresentedthatis especiallyadapted to smallandcomplexaquife rs,such as in hardrock,inwhich the storage capacityof the aquife rsgivesthe extraction limit s.Thebalanceis focusedon the developmentovertime of the storage, towhichgroundwa terrechargeandwit hdrawalfromwell users areadded anddrawn.A compu terisedsystem hasbeendevelop ed as an expertsystem forusers, who arenotprofessional hydro geo logists.The balancecanbeusedforcalculatingthemaximum acceptable number ofconsumers inanarea or forthe setofamaximumacceptablesanitaryst andard,based onavailable ground water resources.Thebalance method hasso farbeenused as aplanningtool wit hinseveralareasaround Stockholm, Sweden.Theresults arerea- sonable,comparedto experiencesofthereal groundwatersituations in the areas,alt ho ughatrue validation has not yet been carriedout.

BoOloisson,Dept.ofLand andWaterResourcesEngineering.Roya/ln stituteofTechnology.SE-lOO44 Stockholm, Sweden, e-mail:[email protected]

Introd uction

Many densely populated areasin the Scandinavian coun- triesare located in areas wit hlimited groundwater sources.

Some of the biggest cities,suchasStockholm,Gothenburg, Helsinkiand Bergen are located in hardrockterrain wit h a large surface area of bare outcrop and in the low er parts soils,mainly consisting of till,clay and in places sand and gravel. The outerareas,which commonlyhave a mixtureof summe r housing and perm anent housing,generallyrelyon a small-scalewater supply based on private drilled wells.

There isalso a hard exploi tati on pressure on those areas, which are usually located in attractive surround ings.The areasare slowly convertingfrom pure sum me r housing to permanent housing.In addition,the sanitary standard s in theareas are ste adily improving,whichin turn increasesthe specific demandfor fresh water and increasestheamount of sew agewater produced.A too highgroundwater ext raction sometimesleadsto salt wate r intrusionorotherground water quality problems,such as increasednit rate contentand bacteria.The municipal and county adm inist rat ions often have to take decisions wheth er an increased exploitation can be accepted,based on the availablewatersources and the possibilitiestohandl e the sewage water.Thereareacute quality problem s insome areasin municipal it ies along the Swedish eastern coastline, such as in Varmdo, Nacka, Oste raker and Norrta lj e municipal iti es in the county of Stockholm.

Basis for municipal decisions

There

is

a la rge

number of different quali tati veandquant ita-

tivemethodspreviousl yused fortheestimat io nofground- wate r quant it iesandvulnerabi lit y.The qualitativ emethods do not give absol ute valuesof discharge possibil it ies but insteadgive arelat ivemeasure. Suchmet hodsinclude local experiences and some variable methods, the DRASTIC method (AIIeretal.1987)and theLeGrand method(LeGrand 1983),whic h arevulnerability met hodsdeveloped in the USA for com parison betw eendiff erent areas.Aspecifi cvari- able method, adapte d to Swedi sh conditions, the Risk Variablemethod(the RV-me t hod),hasbeen develop ed and tested for the occurrence of salt groundwaterin Sw eden using GIS in Norrtalje municipality, Stockholm count y (Lind berg & Olofsson 1997).Althoug hthe qualitative vari- ablemethodscansomet imesgive valuable information,the results arenotalways easilyunderstand abl e, whichcan lead to inconsistent decisions. Quantit ati ve method s, on the other hand,give calculated values for the possibilities of extracti on of grou ndw ater.The methods are comm only based on directmeasurementsof physicalprope rt ies, often hydra ulic measurementsinwells and more orless com plex and sophistic ated calculat io ns using water balances or mathematicalmodelling .Thelatterhascommonlybeencar- riedoutduringbigconstructionproj ect sin someareas,eg., wit hin thenuclearwaste rep ository programme in Sw eden (fo r example,Follin1995).How ever,math ematical modellin g requiresanextensive amountof data,especiallyin hydrogeologically heterog eneou s areas,and canusuallyonlybe carried out by experts.Therefore,the costsofsuch investigations are quitehigh.The groundwater balancesthat have beencarried out, ontheotherhand, havegenerallybeentoo

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NGU-BUL L439, 2002 - PAG E 16 BOOLOFSSON

The part played by precip itation , which could infiltra te into theground, the coefficientof infiltration (C),isassumed tovary between diff erent geologicalterrains (usually a frac- tionof 0.OS-0.2). If the art ifi cial dischargefrom the area(by pumping) was equal or higherthan the recharge, watersup- ply problems could not be avoided.The method was fre- quent ly used during the 1980s (Sund & Bergman 1983, Eriksson &Tilly 1984,among others).

simple,and haveusually not taken theartificia land time- dependent discharge as well as the heterogeneous rock characteristics into consideration.

Groundwat erbalances have beenused as adecision tool in municipal planninginSwed en since thebeg in ningof the 1980s(Sund & Bergman1980).Thebalanceswere originally basedon asimpl ifi edrecharge-d ischargeapproachinwhich the annualgroundwater recharge(st rictly formed from precipitation) in an area was com pared to the annual wit h- draw alof gro und waterby householdconsumption.

The limitations of recharge and extraction

The rechargeand dischargeof groundwaterare highl yvary- ing functionsofthe spatiallyand tim e-dependentdistribu- tion of precipitation, geological conditions, topography, typ e of landuseandvegetati on, housing andsani ta ry stan- dards.Metho dsfor recharge estimationsin the most common soilsin Swed en (in till)aregivenbyJohansson (1987).

Verylittle research,how ever,has beencarried out regard ing the recharge to rock.Bergman (1972) measuredthe loss of water during art ificial sp rink ling of wateron barehardrock outcrop s,which only can give an approximate measure

under fully saturatedconditions.The flow from soil to rock is anevenmore uncertain variable.Itcan only occur at specific hydrogeologicallyconditionswhere a conductorin the rock is hydraulicallyconnected to a conductive reservoir in the soil(Olofsson 1994).

In vegetation-covered areas of southern and central Sw eden and Finland,groundwater recharge is assumed as small or negligible duringthe summerperiod,because the potent ial evapotranspiration greatly exceedsthe precip itation(Fig. 1).Therefo re,groundwater recharqe in those areas usually occursduringthe spring and autumn.

During winter, a considerable part of the precipitationis sometimesstoredin snow and added to theinfil t rati on during the snow -meltin g period. Although the potential groun dw at er recharge in the county ofStockholmis as high as 270 mm per year,the amount of groundwater that can be stored inthe hardrock is much lower in reality.In practice, maximum discharge from the hardrock is limited by the kinem at ic porosity ofthe geologicalmaterial,since ahetero- geneous fractured rock and a heterogeneous till com p rise drainable as well as non-drainablepores(Fig. 2).

Fig. 2.Fractu res inhardr ock:A=drainable,flowing.B=drainable,no flow C=notdrainable,noflow(Olofsson et al. 2000).

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C=coefficientof infiltration A=area n=numberof persons r=rest term P'C 'A=Q'n±r

P=precipitat ion Q=specific wit hdrawal

E E 160 140 120 100

80 60 40 20

o

• Prec ipitation (mm)

fZj

PoLevapotranspirat ion (mm) 0 PoLg rw.recharge (mm)

Fig.1. Monthly available amountofwater forinfilt rati on in Stockhol mcounty(based on data fromSwedishMeteorologicalHydrologicalInstitute, SMHI).

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BOOLOFSSON NGU-BUL L439,20 0 2- PAGE 17

t t

Fig. 3.A new groundwater balanceapproach;

importantfactors.

Q1-36S(withdrawal, permanent) _ Q1-36S(withdrawal , summer)

-

c=J Rock Ilffijjfijgm ... = rm

^I

_ Clay

c:=J ^Sand

iura' ^discharge

....

The balance can be mathematicallyformulated as:

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N,=Pi-ET;+R,(ifNi<O thenNi⁼0) (3)

365 m m

I

n :

^N,^'^l{J^g^•^Ag•Tg=Qi-n,±

2:

~Sg, i

1=1 9=1 9=1

h

Sg=

2:

t/Jj•d,

i=l

where:

P

=

Precipitati on (mm) N

=

^Nettinfilt rat ion(mm)

q>=Infiltrationfactor(%)

r

=

Hom ogeneit y factor(%)

n=Number ofusers

!/J

=

kinemat ic porosity(%)

ET

=

Evapot ranspirat ion(mm) R

=

Arti ficial recharge (mm) A =Area (m')

Q =Specificwithdra wal(rn') liS=change in storage (m') d =thicknessof thesoiland

rock layers(m) j

=

1,2...hare variou s soi landrock types at depth 9

=

1,2,..marevarioussoilandrocktypes at the ground

surface The kinematic porosityinhardrockisvery smal l,usually

much less than 0.05% (Carlsson & Olsson 1981, Olofsson 1991);therefore, in practice,onlyaminorpartofthe poten - tial recharge can bestored.In soil,however,the kinemat ic porosity (ofte nsimilar tothe effective porosity )is ahundr ed (in till)to a thousand(insand) timeshigh er, i.e.a 1 drn-th ick layer of well-sorted sat urated sand may contain asmuch drainablewater as a one hund red metre thick sequenceof hard crysta llinerock. In areas ofbare outcrops,suchas along agreatpart of theSwedishcoastlin e,this is afundament al limitati onforwithdrawal of groun dwater.Since much of the annualextracti onwit hin thesepart icular areasisfocusedto the summerseason, the groundwater resources areheavily st ressed duringthis period.

An extended groundwater balance model

In order to take into consideratio n the limitatio ns of recharge, withdrawal andstorage capaciti es,a wate r balance model hasbeenformulatedand tested as partof themunic- ipal planning of some areas in the archipelagoofSto ckholm.

The requirement s were that inp ut data to the balance should be given from generalised literat ure values, from exist ing hyd rom et eorological data, from existi ng topo- graphicalandgeological maps,and from off icialand unoffi- cialinfor matio nonsanita rysta ndards,housinguse,etc.The modelshouldalso take intoconsiderat ion the act ualstorage capacit ies ofthe rock and soil. Theexte ndedgroun dwate r balanceis given inFig. 3.

With drawalofgroundwateris given as:

Q-n=Qpnp+Q,'n, (5) Qp,np

=

perm anenthousing Q"n,

=

summer housing

Thegroundwaterlevelsmay somet imesvaryon a daily basis in very smallaquifers.However, in pract ice,the groundwater sit uat io nas a wholeisslowly changing dueto seasonal variation s and, hence,the groundwaterbalanceis usually madeonamonthly basis insteadofdaily,which makes thecomp ilation of data easier.

Storage

Thecent ralpart of the balanceis the storage. For eachsur- face soil(and rock),astrat igrap hyis builtup. If no informa- tiononthe stratigraphyandsoilthicknessisgiven from bor-

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NGU-BULL 439 .2002 - PAGE 18 BOOLOFSSON

Example s from eastern Sweden

The groundwaterbalancemodelhas so far been usedwithin severalareasin three municip alities in Stockholm county, Sweden(Fig.4).Somebasic informatio nandresult s fromthe calculations areshown in Table 1,compared to the realexpe- riences of thegroundwater situation in the areas.

Withdrawal of groundwater

Manyexploitati onalareas,especiallysummerhousingareas, showa sign ifi cant seasonal variationof groundwater extraction. Whereasour knowledge ofwater consumption in permanenthousing areaswitha high sanitarystandard is quite good,there is a signi ficantlackof information on consumption in summ er cottageswit hvarying sanitarystandards.A distinct ion ismadebetweenthewaterconsumption in summer cottagesandin permanent houses,based on literature values andlocalexperience(eq. 5).

A computerised system

The computer program consists of an 'expert system' in which hydrometeorolog icaldata and typicalhydrogeologi- cal informa tion for the part icular region hasbeen set by an 'expert; based on existin g databases and general experiences.Theoperatorof the system,usually withinthe municipal admini st rat ion,only has to add thedistribut ion of soils and outcrop, andinformati onregarding the population and the sanitarysta ndard of the actu alhousingareas.

The program calculate sthedevelop ment of the storage during the year.Aspecific balance modelling can also be carried out in order to calculate the maximum numberof houses ina specific area,orthe acceptabledegree of permanent housing and sanitary standard wit h respect to the actu al groundwaterresour ces.

N

*"

_ = _:=::::=::=J

Okm 20km 40km 60km 80km

4 3 Ska lsma ra Eks koge n 2 Alga

Fig.4 .Thelocati on ofsomeexamples of groundwater balances in the Stockho lm area.

ings, a typical stratigraphy is built up representing a stratig- raphy which is common in the surrounding areas (a hydrogeological type setting). In eastern central Sweden, for example ,clay usually covers a coarser soil, such as sandysilty till,which in turn is resting on the fractured hard crys- talline rock.Each stratigraphical unit is given typical values of kinematic porosity taken from the literature,a typical thick- ness,based on drillholesor general experiences and a typical depth to the groundwater level.However, all units beneath the uppermost unitare usually assumed as fully saturated. The homogeneity factor is a rough measure of the percent- age of stored groundwater,which in reality can be re-cap- tured.Someof the groundwater will usually be lost due to the naturalgroundwater discharge. Decreasing homogeneity meansthat it is much more uncertain whether a specific infil t rated amount of water really can be re-usedfor water supply.The factor depends on the geologicalmaterial andis usually given values between 50 and 100%,the latter for a homogeneous sand and gravel. For hard rock the value is also affected by the fracture pattern,and several fracture sets in various orientationsincrease the homogeneity factor.

When the storageis filled up, no further recharge occurs and whenit isempty no discharge is possible.

Recharge to the storage

The recharge to the storage is given by the precipitation when it is exceeding the potential evapotranspiration.

Therefore,in principal thereis no rechargein areaswit h vegetation during the summer season.However,itis notimpos- sible that some recharge may occur during the summerin hardrock outcrop areas, if the fractures are locally saturated during summer rains. Recharge occurs as long as the tot al storage isnot filled up.When a state of full saturation is reach,rechargemay continuebut onlywith thespeed of the naturalandind uced groundwater flow.In large parts of the Scandinavian countries, surface runoff is usually of mino r impo rtance exceptin hardrock outcrop areas,especiallyon steep slopes and on natural discharge areas, e.g. fens.

However,after intensive rainfall and snow melting,thesur- face runoff may be considerable.Rodhe(1987) has shown that most of the water found in minorst ream s in Sweden emanates from groundwater.Hence,the infiltration factor given in the water balance (eq. 2),usually varies between 60%of thepotential infiltration for a terrain with some parts consisting of clay,and 100%for areas consisting ofsand and gravel. Different infiltration factors are usuallyset for various soils depending on the local conditions.

The values of precipitation and potentialevapotranspi- ration are obtained from hydrometeorological data basesas mean monthly values,but also values representing special dry conditions,e.g.,20 or 50 years ret urn time,are used.

Furthermore,there is a possibility to include artificial recharg einto the balance,such as collection of precipitation and percolation systems as well asre-infiltrationof treated sewagewater.

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BOOLOFSSON NGU-BULL 439,2002 - PAGE 19

Table1.Resultsfromgroundwaterbalancecalculatio nswit hinso me areas in thevicinityof Stockh olm .

Variable Algo Ekskogen Saltaro Skiilsm ara

Reference Olofsson (2000) Jacksetal (2000) VaiVa-pr oj ekt(2000) (unpubl. 1997)

Municipality Nacka Vallentu na Varmd o Varrnd o

Area (ha) total(net to) 200(160) ha 160(160) ha 510(400) 350(200)

No ofhouses 540 269 763 387

No of Person sper house 2.S 2.3 2.3 2.5

Degreeof perm anenthousing (%) 35% 16% 28% 40%

Specific wit hd raw al(m') Summer100L/p,d Summer100Up,d Summer150 Up,d Summer 150Up,d Perm.180L/p,d Perm.150L/p,d Perm.200L/p,d Perm. 200L/p,d

Geol ogy Rock 80% Rock 70% Rock 57% Rock 61%

Sand y silty till10% Sandysilty till 10% Gravelly sandy till9% Sand 1%

Clay10% Clay20% Clay 32% Clay33%

Org.soil s 2% Org.soils5%

Top og raphy 0-50rn.a.s.l 30-70 rn.a.s.l, 0-30 m.a.s.1. 0-30 m.a.s.1.

fairly rough moderate moderate mod erate

Mini m umfill of storage

Normalyear(d ry year) 39%(20%) 78% (73%) 78%(73%) 69%(60%)

Minimumfill of sto rageat 100% 0%(0%)

permanenthousing mod elstopsat81%

Norma lyear (dryyear) (51%)perm.houses 51% (31%) 60% (44%) 46%(25%)

Balanceresult(ann.)

(Recharge/wit hd rawal) 1.17 1.51 1.52 1.31

Groun d waterqualit y sit uat ion Moderatequality Noquality Smallandlocal Moderatequality

today problemstoday problemstoday qualit yproblems problems today

(Cl",microbes) (Cl",N03",PO. 3")

«n

^(Cl",^microbes)

Alt houg h a true validation of the balance estimations has not,and probably cannot be carriedout,theresults from the balance calculations clearly correlate with the actual groundwatersituation withinthe four presented areas.If the storage situation is nottaken intoconsideration,an overesti- mation of the groundwater resources isthelikely outcome. Such calculations using the simplifie dbalanceequation(eq.

1) at Alga have given much higher recharge values and extractionpossibilities (VIAK 1989).TheHBV-model (Sanner 1995)at Ekskogengaveatheoret ical groundwa terrecharge of 210 mmcompared with the calculatedavailableground- water recharge of only 41 mm using the above balance approach(eq.2-5).Still,the extractionof water from thearea was less than 15% of the potent ial recharge (Jacks et al.

2000).

One area, Alga, isalready todayvery close to overex- ploitation.The balance modellingat Algaindicatesthatthe groundwater resourcescan also sustain specific dry years (wit h a recovery time of 20 years)wit h the current housing situation (65% summer houses).However,the proximity to St ockhol m favours an increasingtend ency towards permanent housing, which will definitely lead to stress on the waterresources.Using awit hd rawal valueof 180litres per person per day,thegroundwater sit uati onmay collapsedur- ing specific dry years,and already when the perm anent housingreaches50%(Fig.5).The situationcan then only be solved by artificiallyincreasing the storage,decreasingthe water consumptionper person, desalination ofseawate ror connecting to the mainland district water supply.Thetwo last-mentioned methods are very expensive since they

Actual month Iy storage situation (0.10)

~Normal values ~Extreme values ^Fig.5.Result s fromwater

balance calcu latio ns at Algo.Developmentover tim e of the groun d water storagefor norm al years and specific dry years.

The sto rage is emp ty when the perm anent ho using reaches 81%

(norm alyears)and 51%

(dry years).

September November

August October December

June May April

'jil

Ja nu a ~ Mareh

February

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NGU-BULL439,2002 - PAGE 20

require construction of a distribution network, mainly through rock blasting, instead of relying on today'sprivate wells.

Re-infiltrationof sewage water(grey water)was not con- sidered here since withdrawal of groundwater was made in drilled wells andinfilt rat ion is usually carried out in the most superficial and unsaturated parts of the soil. Microbe analy- ses on groundwater from Ekskogen and Alga indicate that there are no shortcuts between infiltrated sewage wat er and the wells(Jacks et al. 2000).

Discussion and conclusions

The groundwater balance approach presented above gives reasonable values of groundwater recharge and withdrawal possibilities in areas with limited groundwater resources, although a true validation is very difficult to carry out. The simplicity of the balance makes it easily understandable and it requires a limited amount of locally measured data,since much of the information needed can be collected from hydrometeorological data bases, maps, general literatureval- ues and from local experience.Therefore,it can be used as a tool in municipal planning but only as an aid in decision making.

There are, however, several uncertaint ies, especially related to representative values of the homogeneity and infiltration factors.Further experiences from application of the water balance in various geologicalterrains willimprove the selection of representative values.

Considerable emphasis should also be given to the typical geological sections for each type of soil. Stratigraphywill be the most important singlefactor,because the kinematic porosity varies with a factor of up to 10' between different geological materials.The stratigraphy must therefore be defined by an experiencedhydrogeologist,and all available drillholes and previous experiences should be im ple- mented. Another important factor,which requires detailed hydrogeological knowledge, is the delineat ion of the recharge area. The hydrologically defined drainage basins are generally dissimilar to the recharge areas of the specific wells, especially in hard rockareaswit h well-developed fracture zones.In many cases,the study area shouldalso be further subdivided into sub-areasbased on changes in the geological conditions.In all theexamples presented above, no such subdivision was made since the terraintypes were rather similar within the differentareas.

Acknowledgements

Thework was financedby internalfundi ngat KTHand carriedoutinco- operat ion with the Nacka,Vallentuna and Varrnd o municipal it iesin Swedenas well as withVaiVa-proje ct AB(Crist ina Frycklund, nowat VIAK AB).Prof.GertKnutsson,KTH,has provided helpfuland valuabl ecom - mentsonthe manu script.

BOOLOFSSON

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Bergman,G.1972:Bestarnninqavinfilt rations-koeffi cienter far bergytor och perkolation sbanor i jordlage r(Est imat ion of infiltration coeffi- cientsin the bedrock andpercolationpathways in soil).Universityof Stockholm, Dept.oi Quaternary Geology, Final repo rt STU 69- 519/U386.(InSwedish)

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Estimating groundwater resources in hardrock area s - a water balance approach