A Water Balance Approach

BO OLOFSSON N GU - BU L L 4 3 9 , 2 002 - PAG E 15 Estimating groundwater resources in hardrock area s - a water balance approach BOOLOFSSON Olofsson, B. 2001: Estimating groundwater resources in hardrock areas - a water balance approach. Norgesqeoio giskeundersekelseBulletin 439,15-20. Among decision makers in counties and municipalities, the re is a great need for simple but hydrogeologically cor rect estimations of the groundwater resources.Today's methods are ofte n based on extensive field investigations and compli cated mathe matical flow modell ing or too simpli fied annually based water balances. In this paper, a groundw ater balance meth od is presented that is especially adapted to small and com plex aquife rs, such as in hardrock, in wh ich the storage capacity of the aquife rs gives the extraction limit s.The balance is focused on the development over ti me of the storage, to which groundwater recharge and withdrawal from well users are added and drawn . A compu terised system has been develop ed as an expert system for users, who are not professional hydrogeo logists.The balance can be used for calculating th e maximum acceptable numb er of consumers in an area or for the set of a maximum acceptab le sanitary standard, based on available ground water resources.The balance method hasso far been used asa planning tool within several areasaround Stockholm, Sweden.The results are rea sonable,compared to experiences of t he real groundwater situations in the areas,although a tru e validat ion has not yet been carried out. Bo Oloisson, Dept.of Land and Water ResourcesEngineering.Roya/ln stituteof Technology.SE-lOO44Stockholm, Sweden, e-mail:[email protected] Introduction t ive methods previously used for t he estimation of ground Many densely populated areas in th e Scandinav ian coun wate r qu ant it ies and vuln erability.The qu alitativ e meth ods trie s are located in areas w ith limited groundwater sources. do not give absolute values of discharge possibil ities but Some of the biggest citi es, such as Stockho lm, Goth enburg, instead give a relative measure.Such met hod s include local Helsinki and Bergen are located in hardrock terrain with a expe riences and some variable methods, th e DRASTIC large surface area of bare outcrop and in t he lower parts method (AIIer et al. 1987) and th e LeGrand method (LeGrand soils, mainly consisting of till, clay and in places sand and 1983), w hich are vulnerability met hod s developed in th e gravel. The outer areas, which commonly have a mi xture of USA for comparison between different areas. A specifi c vari summer housing and permanent hou sing, generally rely on able meth od, adapte d to Swedi sh cond itions, th e Risk a small-scale water supply based on private dri lled we lls. Variable method (t he RV-met hod), has been deve lop ed and There is also a hard exploitati on pressure on those areas, tested for t he occurrence of salt groundwater in Sweden w hich are usually located in attractive surroundings. The using GIS in Norrtalje municipality, Stockholm coun ty areas are slowly converting from pu re summer hou sing to (Lindberg & Olofsson 1997). Although the qualita tive vari perma nent housing. In addition, the sanitary standards in able methods can sometimes give valuable information, th e the areas are ste adily improving, which in turn increases the results are not always easily understandable, which can lead specific demand for fresh water and increases th e amount of to inconsistent decisions. Quant itative methods, on t he sewage water produced.A too high groundwater ext raction other hand, give calculated values for th e possib ilities of sometimes leads to saltwate r intrusion or other groundwa extracti on of groundwater. The met hod s are commonly ter quality problems, such as increased nit rate content and based on direct measurements of physical prope rt ies, often bacteria. The municipal and county administ rations often hydraulic measurements in we lls and mo re or less complex have to take decisions wheth er an increased exploitation and sophi sticated calculations using water balances or can be accepted, based on the available water sources and math ematical modelling .The latter has commonly been car t he possibilities to handl e the sewage wat er.There are acute ried out during big con struction projects in some areas, e g., quality prob lems in some areas in municipal ities along the wit hin th e nuclear waste repository program me in Sweden Swedish eastern coastline, such as in Varmdo, Nacka, (for examp le, Follin 1995).However, math ematical modellin g Osteraker and Norrta lje municipal iti es in th e county of requires an exte nsive amo unt of data, especially in hydroge Stockholm. ologically heterog eneou s areas, and can usually only be car ried out by experts. Therefore, th e costs of such investiga Basis for municipal decisions t ion s are quite high. The ground water balances t hat have There isa la rge number of different qualitati ve and quant ita- been carried out, on t he oth er hand,have gen erally been too NGU-BU LL 439, 2002 - PAGE 16 BOOLOFSSON simple, and have usually not taken th e arti ficial and time under fully saturated conditions. The flow from soil to rock is dependent discharge as well as the heterogeneous rock an even more uncertain variable.It can only occur at specific characteristics into consideration . hydrogeologically con ditions where a conductor in the rock Groundwat er balances have been used asa decision tool is hyd raulically connected to a conductive reservoir in the in mun icipal planning in Swed en since th e beginning of th e soil (Olofsson 1994). 1980s (Sund & Bergman 1980).The balances were orig inally In vegetation-covered areas of southern and centra l based on a simplified recharge-d ischarge approach in whi ch Sweden and Finland, groundwater recharge is assumed as t he annu al groundwater recharge (strict ly formed from pre small or negligible during the summer period, because th e cipitation) in an area was compared to t he annua l wi t h potent ial evapotranspiration greatly exceeds the precip ita drawal of ground water by house hold consumption. tion (Fig. 1).Therefo re, groundwater recharqe in those areas usually occurs during the spring and autumn. P'C 'A=Q 'n ±r (1) During winter, a considerable part of the precipitation is P=precipitation C=coefficient of infiltration A=area sometimes stored in snow and added to the infil tration dur Q=specific withdrawal n=number of persons r=rest ter m ing th e snow -melting period. Although th e potential The part played by precipitation, w hich cou ld infiltrate groundwater recharge in the county of Stockholm is as high into th e ground, the coefficient ofinfiltration (C), is assumed as 270 mm per year, t he amount of groundwater that can be to vary between different geological terrains (usually a frac stored in t he hardrock is mu ch lower in reality. In practice, tion of 0.OS-0.2). If t he artificial discharge from the area (by maximum discharge from t he hardrock is lim ited by the pumping) was equal or higher t han the recharge,water sup kinematic porosity of the geological material, since a hetero ply problems could not be avoid ed. The method was fre geneous fractu red rock and a heterogeneous till comprise quent ly used during th e 1980s (Sund & Bergman 1983, drainable as well as non-drainable pores (Fig. 2). Eriksson & Tilly 1984, among others). The limitations of recharge and extraction The recharge and discharge of groundwater are highl y vary ing functions of the spatiall y and time-dependent distribu t ion of precipita tion, geo logical conditions, topography, typ e of land use and vegetation, housing and sanitary stan dards. Methods for recharge estimations in the most com mon soils in Swed en (in till) are given by Johansson (1987). Very little research, however, has been carried out regarding the recharge to rock. Bergman (1972) measured the loss of water during art ificial sprink ling of water on bare hardrock Fig. 2. Fractures in hardrock:A= drainable,flowing.B= drainable,no flow outcrop s, which only can give an approximate measure C= not drainable, no flow (Olofsson et al. 2000). 160 140 120 100 E E 80 60 40 20 o • Precipitation (mm) fZj PoLevapotranspiration (mm) 0 PoLgrw.recharge (mm) Fig. 1. Monthly available amount of water for infiltration in Stockholm county (based on data from Swedish Meteorological Hydrologica l Institute, SMHI). BOOLOFSSON NGU-BU LL 439, 2 0 0 2 - PAGE 17 Fig.3. A new ground water balance approach; important factors. Q1-36S(withdrawal , permanent) _ Q1-36S(withdrawal , summer) tt - c=J Rock Ilffijjfijgm......= rmI _ Clay c:=J Sand iura' discharge .... The kinematic porosity in hardrock is very small, usually The balance can be mathematically formulated as: 365 m m much less than 0.05% (Carlsson & Olsson 1981, Olofsson ~S g, i (2) I n:N, ' l{Jg • Ag • Tg = Qi - n,± 2: 1991); therefore, in practice, only a minor part of the poten 1= 1 9=1 9=1 tial recharge can be stored. In soil, however, th e kinematic N,= Pi- ET; + R,(if Ni<O then Ni= 0) (3) poro sity (ofte n similar to th e effective porosity) is a hundred h S g = 2: t/Jj•d, (4) (in till) to a thousand (in sand) tim es higher,i.e.a 1 drn-th ick i=l layer of well-sorted saturated sand may cont ain as much where: drainable water as a one hund red metre th ick sequence of P =Precipitation (mm) ET= Evapotranspiration (mm ) hard crystalline rock.In areasof bare outcrops,such as along N = Nett infiltration (mm) R =Artificial recharge (mm) a great part of th e Swedish coastlin e, th is is a fundamental q> = Infiltration factor (%) A = Area (m') limitation for withdrawal of groun dwater.Since much of th e r =Homogeneity factor (%) Q = Specific withdra wal (rn') n = Numb er of users liS= change in storage (m') annual extracti on within th ese particular areas is focused to !/J = kinematic porosity (%) d = thicknessof the soil and the summer season, th e gro undwater resources are heavily rock layers (m) stressed dur ing th is perio d.

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