Present uplift rates and groundwater potential in Norweg ian hard rocks
ERIKROHR-TORP
Rohr-Torp,E.1994:PresentupliftratesandgroundwaterpotentialinNorwegian hardrocks.Nor.geol. unders.
Bull.426,47-52.
The postglacial isostaticuplift ofFennoscandiais hereregarded asthemost important factor in keepingfractures open for groundwaterflow inNorwegian hard rock aquifers.The presentrate ofupliftis assumedto representa measure ofthetotal upliftof an area.The greater the uplift. themoretectonicdisturbance is created.andthe more open arethefractures.Totestthetheory.five areas inthePrecambrian ofsouthern Norway withdifferentyearly uplifts.and containingatotal of 1278drilledwellshavebeen considered.A linearrelationship isfoundbetween depthandwateryield in the wellsandtheyearlyisostaticuplift.Thisishardly coincidental,anditisproposedthat further workshould be performedas a joint projectbetweentheScandinaviancountries.
ErikRohr-Torp.Norges geologiske undersok etseOslokontoret,P.o.Box5348Majorstua,N-0304Oslo.Norway.
Presentaddress: Asp/an Viak AS.Storgaten8.3600 Kongsberg,Norway.
Introduct ion
The matrix ofunweathered hard rocks,bet- ween faults, fractures, joints and fissures (henceforth called fractures) is regarded as impermeable for practical water-resources purposes.The exploitable porosity and per- meability of hard rocks is thus overwhel- minglycontrolled by the existence of fractu- resystems.Naturalfractu re systems canbe exceedingly complex and have been crea- ted during various periods of tectonic distur- bance throughout the Earth's history.
Furthermore, most of the systems have been reactivated several times due to younger disturba nces . Marked periods of tectonic activity younger than the Precam- brian in Norway occurred during: (1) The Caledonian orogeny, approximately 425- 400 Ma B.P.; (2) Permian activity in Southeast Norway, c. 250 Ma; (3) Tertiary uplift, mostprominentin western Norway,at c.60Ma.
It is a common opinion among Norwegian hydrogeologists that the younges t fractu re systems are the most permeable. In other words,a Permian fracture system is consi- dered more open than a Caledo nian fractu- re system,and is generally believed to give higheryields in drilledwells (Englund 1980).
This may be partly true, but even 'young' fractures formed during the Tertiary uplift have existed for approximately 60 million years. Fractures from this period have had the possibility of transporting solutions over an extremely long period of time, during
which they may have been subject to e.g.
chemical alteration or precipitation. The possibilities of being tightened by second- ary mineralisations are equally as great as fora Precambrian fracture system.
Glac iation and isostasy
The Weichselian glaciation, the last of at least four glaciations in Fennoscandia,star- ted more than 100,000 years ago. Climatic changes caused large variations in the thickness and extent of the ice, but at c.
17,000 - 21,000 B.P. the ice-sheet reached itsmaximum extentwith its southern margin in northern Germany and Denmark. In its central parts, the thickness of the ice was probably up to 3,000 m. From that maxi- mum,theice-sheet started to melt,the mar- gin withdrew,and the thickness decreased.
The coastal areas of Norway were the first to become deglaciated some 10,000-11,000 years ago.The final deglaciationofthe cen- tral parts happened rapidly,and at c.8,500 years ago most of Fennoscandia was ice- free(Lebesbye 1989).
The weightof the ice had caused an isosta - tic depression of the Fennoscan dian crust.
As the ice melted, this depression was graduallycompensated by an isostatic uplift or 'rebound'. In general, contours of the postglacial uplift have a dome-like shape with its maximum located in the Gulf of Bothn ia,coincidingwith themaximum thick- ness of the ice-sheet. Thus, the uplift was
48 ErikRohr-Torp NGU•BULL 426.1994
greater in the eastern parts than in the coastalareasofsouthernNorway.
Fig.1.100 m contoursfor theFennoscandian'postglacial' abso- lute uplift. Minor irregularities occur, especially in the Skagerrak-SouthNorway regionand in thecentralBalticSea.
AfterM6rner(1980).
Forthis firstapproach inlooking fora possi- ble correlation between postglacial uplift and groundwater potential,the above gene- ral picture of the uplift seems relevant.
However,the form and nature of the upliftis more complex, due to the fact that also a non-glacial component may be incorporated in the crustal movements,as pointedoutby Anundsen (1989), Roberts (1991) and Olesen et al. (1992).
Fracturing
less than 10 mm a year today. M6rner (1980) claimed that the total uplift at the centrein the Gulf ofBothnia is800-850 m, and that the uplift started approximatel y 13,000yearsago. Thisisadramatic geody- namic process occurring over a very short periodoftime.
The very recent and brief glacio-isostatic uplift musthave beenassociated withconsi- derable changes in stress and strain rates in the crust,and seismic activity, fracturing and reactivation of oldfracture systems are to be expected. The mostactive period was atthe time ofdeglaciation,and the presen- ce of several late- or postglacial faults are welldocumen ted inFennoscandia(Madsen 1917, Gronlie 1922, Du Rietz 1937, De Geer 1938,1940,Bergsten 1943,Kujansuu 1964,Feyling-Hanssen 1966,M6rner1969, 1972, 1975, 1977, Lundqvist & l.aqerback 1976, Floden 1977, Lagerlund 1977, Lagerback 1979, 1990, Bakkelid 1986, Olesen 1988, Soliid & Tolgensbakk 1988, Anundsen 1989, Backblorn & Stanfors 1989, Roberts 199 1, Olesen et. al. 1992).
Most of these faults are old regional fault zones which have been reactivated.
Postglacial displaceme nts of up to 30 m have been described (Muir Wood 1989).
Johnston (1989) concluded that: "An ice- sheet will inhibit earthquakes by stabilising potentiallyseismogen ic faults in the under- lying brittle crust. This same mechanism may also provide an explanation for the intense late-glacia l faulting in Fennoscandia".
lO' 40' 10' lO'
10' 10' 0'
10'
By extrapolating the 13,000 B.P. palaeo- shoreline curves from the margins of Fennoscandia into the centre of the uplift, and by assuming a global uniformeustasy of 120 m,M6rner(1980) constructeda con- tour map of the absolute 'postglacial' uplift in Fennoscandia.His map is shownin Fig. 1.
The glacio- isostaticupliftstarted well before the land was icefree, reached itsmaximum of up to 0.5 m ayearin the Gulfof Bothnia when the land was deglaciated (M6rner 1978, 1980), and has since decreased to
Reactivation of ordinary fracture systems andformation ofnewfractures are more dif- ficulttoidentify, but thereis every probabili- ty that such processes took place quite extensivelyduringthe postglacial uplift.The dome-shaped uplift must have created an horizontal extension within the crust, both radially and concentrically with respect to the centre of uplift. According to M6rner (1978, 1980),the late glacio-isostatic uplift of Fennoscandia was drastic and rapid compared to long-term events like, for
NGU - BULL 426.1994 ErikRohr-Torp 49
The tableshows that the higherthe isostatic uplift is for an area,the higher are the yields in drilled wells, and the shallower are the well depths. These trends are plotted grap- hically in Figs. 3 and 4. The almost linear trends for wateryield and well depth plotted against yearly uplift are not likely to beacci- dental. The observed decline in well depth with increasing yield is at first sight a puzz- ling phenomenon. It has, however, been uplift,small in the costal areas and high in the eastern parts, the greatest postglacial changes in stress and strain are to be expected in the eastern parts. This again should be reflected in a higher density of new and reactivated fractures,and general- ly higher yields in drilled wells in the eastern than in the coastal parts of Norway.
The Geological Surveyof Norwayhas infor- mation on approximately 20,000 drilled wells in Norway. To test the above theory, five areas in southern Norway with a high density of drilled wells and with different yearly uplifts were selected, all of them in Precambrian rocks,mostly gneisses, grani- tes and amphibolites. The areas (A - E), outlined as standard 1:50,000 map-sheet areas, are shown in Fig. 2. Some of the map-sheetscontain minorareas covered by metasedimentary rocks of Late Proterozoic age and rocks younger than the Precambrian. Wells in such lithologies are omitted. The selected areas are not ideal.
Area B has no uplift data, and area A has few such data. With the exception of areas surrounding the Oslo region, however, the selected areas are the only ones with suffi- cient concentrations of drilled wells within Precambrian rocks in Norway. Areas adja- cent to the Oslo region have been omitted to preclude interference from the intense Permian igneous activity in this region. Admittedly, the Permian activity may also have had someinfluence on areas B,D and E.For this first approximationto the theory, and with limited available information on wells and uplift, the selected areas are at present regarded as the best available.
Further information on the areas (A - E) is given in Table 1 along with some statistics on the drilled wells.
,.,o -, I
I I I I I I I I
, I\
-O.b. \ '-0 9 \
-0:3 ...•04.,1.3
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~0 2- ,..._:~5__ -1()
~ 1Zr'l
'O~__~6_-0.0
Relevance to hydrogeology
instance,the Caledonian orogeny.He rela- ted the intense fracturing of Swedish bedrock to the deglaciation period,with its peak activity at c. 8,000 B. P. Furthermore, similar deglaciations and glaciations took place at several times during the last 100,000 years, thus providing possibilities for the repeated reactivation and formation offractures over this periodof time.
Fig.2.Estimated annualland uplift(mm/year) relativetomean sealevelfor southernNorway.AfterSo rensenet al.(1987).A- Eshow map-sheets forthe fiveareas considered inthetext.
It seems to be generallyaccepted thatthere is a good correlation between the general uplift pattern for the last c.8,000 years and the present uplift rates in Fennoscandia (Balling 1980, Bjerhammar 1980, M6rner 1980). Fig.2 shows the presentestimated crustaluplift in mm/year in relationto mean sea level for southern Norway (Serensen et al. 1987). By assuming that the present uplift reflects the total amount of postglacial
50 ErikRohr-Totp NGU-BULL 426.1994
AREA MAPS (M711) APPROX. NmffiE R TOTAL TOTAL YIELD(I/h) DEPTH(m) YIELDPR.
UPLlIT OF YIEL D DEPT H DRll..LE D M(11h .rn)
(m m/yellr) WELLS (I/h) [rn] ~!EAl> ~!ED. MEAl> MED. ME Al> MED. ---- - _. _- -- _.._..
A 1115 IV1116I 0.4 263 15300 8 21904 582 250 83.5 81.5 7.0 3.1
111711 12 17 11. III
IJ 15151151611 3.5 454 4·15301 265 15 98 1 58.5 57 16,5 8.8
161611.III
C 1319I1419III 4.0 197 24686 0 11189 1253 600 57 55 22.1 10.9
1518 1
D 19161 12015 I.IV 5.8 239 329330 14428 1378 800 60.5 55 22.8 14.6
20 16 11.III
E 20 1711 7.5 125 178290 49 94 1426 1000 40 35 35.7 28.6
2117I.11, Ill.IV
Table 1.Annual land upliftand some statistics(meanand medianvalues)on drilledwellsforfiveareasinsouthern Norwa y.A - Bergen- Hoyan ger area;B- Gal-Geilo area ;C- Lesja-Skjak area;D- Finnskog- Tangen area; E- Trysilarea.
I:,UU
1200
00 2 4 5
Aoor ax.uoli l Cmm/v r)
x
Mean yi e I d R:K:K2 .9 05
Coeff'C lell'LS
AO 595.17 9 A1 124.48 6
0 - - - - - -
Medi an yi eId R:K:K2 .988
Coeff.cie nts
AO 177. 228 A1 106. 786 8
Fig.3.Mean and median valuesfor yield (Vh) plotted against annual land uplift(mmlyear) for fivearea s
in southern Norway.A=Bergen-
Hoyanger area, B =Gal - Geilo area.C =Lesja -Skjakarea,D=
Finnskog -Tangenarea.E=Trysil area.
c
x
Mean dept h R;K;K2 . 853
Coeff Clenl.S
AO 82. 707 A1 -5. 3 7 9
0 - - - - - - Me di o n d e pl h
R;K;K2 .9 15
Coefficients
AO 81. 837
A1 -5. 92 8 Fig.4.Meanand median valuesfor welldepth(m)plottedagainstannu- al landuplift(mm/year).The five are- as A- Earethesame asinFig.3.
NGU•BULL426.1994 ErikRohr-Torp 51
observed by many hydrogeologists in Norway (Rohr-Torp 1987, Banks et al.
1992) and is ascribed to a well-driller dis- continuing drilling once he has found a satisfactory wateryield.At'dry' sites,howe- ver, hemay continue to great depth before abandoningthe borehole.Dividing the yield by thewell-depthgivesthe yield per drilled metre.Thisparameter,as expected,has an even stronger positive relationship with yearlyupliftthanthewell yield (Fig. 5).
Conclusions
median (see Figs. 3 and 4), indicating a symmetricaldistributionof well depth.
The graphs indicate a very simple rule of thumb forpredictingthe'typical'yieldof ran- domly placed wellsin Precambrian rocksof Fennoscandia. Starting at 0 mm yearly uplift,a well can beexpectedto yield 180I/h at 80- 85 m depth.For each mmof yearly uplift, 100I/h can be added,and the depth required to achievethisdecreases by6m.It should be mentioned that most Norwegian wells are drilled more or less at random, withouttheuseof hydrogeologists.
Very young tectonic events may have reju- venated old fractures,andsuchevents are probablymore important for the permeabili- ty of old fracture systems than the original properties of these systems. This brief in- vestigation supports the theory that the postglacial isostatic uplift has reactivated old fracture systems, and most probably also created new fractures. Furthermore, the magnitude of this uplift seems to be decisive for the degree of fracturing. The greater the uplift, the more intense is the reactivationandfracturing.
The median value for water yield in drilled wells avoidsplacing unduestatisticalweight on the few unrepresentative very high-yiel- ding wells, and this value is generally acceptedasmore usefulfor predicting 'typi- cal' yields than the mean value. For well depth,the meanvalueisvery similartothe
Naturally there are several other factors such as rock-type, topography, infiltration area,type of fractures,hydraulic connectiv- ity, etc. which control the water yield in a well. Nevertheless,the above rule of thumb can be useful in giving a simple rough esti- mate forexpected yield and well-depthsin a given area. Furthermore,at an early stage in planning man-madecaverns and tunnels, it may provide a roughindication of expec- ted leakages; and it should also be taken into consideration when planning sites for hazardous waste.
It is proposed that further work should be done on the practicalapplication of the the- ory, preferably as a joint project between the Scandinaviannations.
Fig.5.Meanand median values for yieldperdrilled metre(Vh'm) plot- ted against annual land uplift (mm/year).Thefive areasare the sameas thoseinFig.3.
Coe .;le Ien 5
AD -.974
A1 3.343
AD 4. 86 2
A1 3.778
Coefficients
x
Mean yld/d r iI I ed m RH2 .934
0 - - - -
Median y I d / d r iI led RH2 .8 6 8
Cl
o
5 8E 30 .
L
52 ErikBohr-Torp GU.BULL~26.1994
Acknowledgements
The author is gratefulto DavidBanks forhiscriticalreadingof the manuscript and improving boththe English text and the content. The staff at the Geological Survey'sOslo office is thankedfortheir assistan ceatvarious stages ofthiswork:and HelgeSkarphag en especially has broug h about many fruitful discussions.
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Manuscript receivedOctober 1993;revisedm/sreceivedand accepted Ap ril1994.
