J/~fKRASNY NG U - BU L L 43 9 , 2 0 0 2 - PAGE 7

Quantitative hardrock hydrogeology in a regional scale

JI A' KRAs NY

Krasny, J. 2002: Quant itative hardrock hydrogeology in a regiona l scale.Norgesgeologiske undersekelse Bulletin 439,7-14.

In the hydrogeological characterisation of an area, two quantitat ive aspectsare to be taken into account, namely th egeological sett ing that defines the geometry and anatomy of a hydrogeological environment, and the spatial andt ime variat ions of available natural groun dwater resources, that most ly depend on recharge possibil it ies.Methodological approaches aiming to quanti fy th ese two aspects in hardrock areas at a regional scale are pre­sented. A hardrock environment can be defined as an intrica te hierarchical system consist ing of inhomogeneouselements of different extent.A regionally prevailing transmissivity, mostly in units m'ld with anomalies ranging from0.1 m'ld up to 100 m'ld , seems to be very similar in hardrock environments th roughout th e world.Considering thisregio nal distribu t ion of tran smissivit y, natural groundwater resources typically depend on th e prevail ing climaticconditions.On a backgro und of the Eart h's basic climatic zonation (arid, humid, temperate zones),a vertical climaticdifferenti ation (mountains,lowlands,etc.) is of imp ortance.Under favourable conditi ons in temperate climatic zonesnatural groundwater resources might reach up to 15 Li s km' in the highest parts of hardrock mountainous areas.

JiNKrosny, CharlesUniversity Prague,Faculty of Science,Albertov 6, 128 43 Praha 2, CzechRepublic.

IntroductionMost of th e present-d ay hydrogeological projects in th e

world are of local character. Issues such as cont aminant

hydrogeology, most water supp ly problems, groundwater

assessment in civil eng ineering and mining projects are of

thi s type.However, results of regional hydrogeological stud­ies are indispensable for administrators and decision mak­

ers,and will have a considerable influ ence on any land-useplanning.They also determine th e condit ions of an optimum

sustainable use of groundwater and it s protection in

extended areas. Regional results are also of importance for

hydrogeologists, in enabling them to draw genera l hydroge­

ological conclusions and to compare th e cond itions in dif­ferent areas. In this way, factors causing similarities or differ­

ences in parti cular hydrogeological environments can be

considered and discussed at region al, nationa l or interna­t ionallevels.Methodologies involving hydrogeological data

regionalisation are important especially for hardrock areasas there, in contrast to hydrogeological basins occupied by

stratabound sedimentary aquife rs, regional approaches in

hydrogeolog ical stud ies have not been common.

When characterising the hydrogeology of an area, twoquant itat ive aspects should be taken into account:

• Static properties of the aquifer system: the geometryand anatomy of hydrogeological bodie s and the spatialdist ribution of their hydraul ic characteristics . No hydro­

geologica l environment is homogeneous and isotropicunder natural cond itions ,and the hardrock environment

in particular is to be considered as an int ricate hierarch i­

cal system of hydrogeolog ical inhomogeneit ies on dif­ferent scales.

• Geological sett ing determines the character of a hydro­

geological environ ment within which groundwater, the

dynamic element that is the object of our att ention,moves and is distributed. Groundwater recharge, which

varies in tim e and space,determines our sustainable nat­ural groundwater resources. Under specific conditions,

induced and/or art ificial resources mig ht com plement

the natur al resources.In th is contribution, th e results of various hydrogeolog i­

cal studies are summarised and a meth odology is suggested

which aims to help in th e assessment of reasonable ground­

water development in hardrock areas at a regiona l scale.

General characteristics of hardrockenvironmentsGeometry and anatomy of hydrogeologicalbodiesThe geom etry (horizontal extent and thickness) and

anatomy (interna l character, distrib ution and type of poros­

ity ) of hydrogeologica l bodies (aquifers and aquitards) are

st rongly dependent on the age and character of the last

important orogenesis and on the general geological devel­

opment during the post-orogenic period . The age of therocks and th e tectonic activities control th e degree of diage­nesisand the relationship between the intergranular and fis­

sure porosity of rocks. Consequently, th ese features det er­mine the magnitude and distribution of their hydraulic para­

mete rs (t ransmissivity and permeability, storativi ty ).

Comparatively old and folded sedimentary rocks have gen­

erally lost most of their primary intergranular poro sity.

Instead of this, fracture porosity prevails. This is especially

the case with bedrock, which is usually represented by crys-

NGU - BULL 439 , 200 2 - PAG E 8

talline (igneo us and metamorp hic) or by sedimenta ry, highly

cemented and/or folded rocks. The common feature of a

hardrock environment, designated as a 'hydrogeological

massif', is a vertical sequence of three zones, termed upp erweathered, middle fractu red and lower massive (Krasny

1996a).As in the caseof the intergranular environment, th e per­

meabilit y of fractures and of fault zones typ ically decreases

th rough geolog ical t ime due to different geo log icalprocesses such as hydrothermal alteration, mineral precipi­

tati on and mechanical clogg ing (Mazurek 2000).Therefore,

in any hardrock environment geolog ically young fractures

are the most important for groundwat er flow, but as not ed

by Banks et al. (1993), such fractures might not be recorded

by standard field geological and geophysical meth ods.

Relatively solub le rocks such as carbonates,gypsum and saltdepos it s are the only exceptions to thi s general rule as their

permeabiliti es general ly increase with t ime.

Hydraulic parameters for identifyinghardrock environmentsAquifers and aquitards are dist ingu ished both qualit atively

by th eir geomet ry and anato my, and quant itatively by the

magn itude of representat ive hydraulic paramete rs. Thecom monly used coefficient of hydraulic cond uct ivit y (per­meabi lity ) was orig inally derive d to characterise an inter­

granular and homogeneous environment. Subsequent ly, itwas commonly used in a mechanical senseand wi thout tak­

ing into account the origi nal preconditions in interpreting

the results of aquifer tests in other hydrogeolog ical situa­

tions . In hydrogeological environments with a prevailing

fracture porosity, however, hydraulic conduct ivity can beused for hydrogeological interpretat ions only very cau­

t iously, taking into account scale differences in pathways ofgroundwater flow and the objectives of the hydrogeological

study (Krasny 2001).In contrast to hydraulic conductivity, the coeffic ient of

t ransmissivity is defined as a parameter expressing the

property of the entire thic kness of an aquifer. In a hardrock

environment, permeabil ity typically decreases wi thin a

depth of some tens of metres from the surface. As waterwells sited in such an environment most ly reach depths ofbetween 30 and 60 metres, the ir t ransmissivity can be con­

sidered as representi ng the ent ire hardrock aquifer (Krasny1993 a). Consequent ly, transmissivity may well express th e

capab ility of the hydrogeological environment fo r ground ­

water abstraction. This is generall y th e main objective of

most hydrogeological stud ies regardless as to whet her the

groundwater is considered as a natu ral resource for water

supply,a nuisance factor dur ing diffe rent unde rground con­struct ions, or a t ransport medium for the dispersion of cont­

amin ants.Results of thousands of pumping tests available world­

wide in hardrock environments have enabled a quantitative

and standard ised approach for study ing transmissivity dls-

JlflfKRASNY

t ribut ion under various condi tio ns. In many cases, how ever,

only data of a less reliable character are available in archives.

Such data are typically not suitable for determining the

exact hydraulic parameters as the coefficient of transmissiv­

ity.On the other hand, many of these data are good enough

for providing ideas on t ransmissivity distribution and couldbe treated stat ist ically.On this basis,an 'index of t ransmissiv­

ity Y' [ = log (10· q) where q = specific capacity in Us m] was

intro duced as one of the comparat ive regional parameters(Jetel & Krasny 1968).This facilitated a statistical treatment

of th e available data from pump-tested wells,and the objec­

t ive classification of transmissivity magnitude and variation

is now the principal procedure for draw ing important con­

clusions regarding transmissivity spatial distribution.

Statistica l treatment and classifi­cation of transmissivity dataWhere there exist suffic ient transmissivity data for the distri­

but ion to be examined by stat ist ical techn iques, sample

popul ations based on lithological units, geog raphical areas,

etc., can be characterised by a central value and a stat ist ical

parameter expressing the variation in the distribution. If one

makes no assumpt ions abou t the natu re of the distri buti on,non-paramet ric paramete rs such as 'median' and 'interquar­

ti le range' are best suited for this pur pose. Several authors,however, have noted that distr ibution s of well yield, specific

capacity and transmissivity tend to approxima te to a log­normal distr ibut ion (e.g.,Jetel & Krasny 1968, Banks 1998).lf

one assumes that such distribut ions are log-no rmal, they

may be characterised by the geometric mean and the stan ­

dard deviation of log-t ransformed values. As the index oftransmisivity Y is already log-transfo rmed, its distribution

can be characterised by the arithmetic mean and standarddeviation.

Transmissivity data distr ibut ion of particular statis tical

samples can be graphically represented on probability

paper by cumu lative relative frequencies of t ransmissivity

values (Fig. 1).Treated values can be expressed by the index

of transmissivity Y, by the coeffic ient of t ransmissivity T,or by

the specific capacity q.The last two parameters should then

be expressed in a logarithmic form .

By using th is procedure, the range of prevailing trans­missivity values x ± s ( x = arith met ic mean, s = standarddeviation of a statistical sample) represent the t ransmissivity

backgro und of a stat ist ically treated hydrogeolog ical envi­

ronme nt. Transmissivity values outside the transmissivity

background are considered as out lying data - posit ive or

negative anomalies (+A, -A). The far outl iers, or ext reme

anomal ies, positiv e (++A) and negative (- -A), can be found

outside the interval x± 2s (Fig. 1).

To classify transmissivity occurring in different hydroge­ological environments, the whole range of possible trans­

missivity values was separated into six classes represent ing

the orders of transmissivity magnitude introd uced by

Krasny (1993a) (Table 1). A class (or more than one class) of

JlfiiKRASNY NG U -B UL L 4 3 9 , 2002 - PA G E 9

+A

x +s

'" '"'"+' +' Xrx IX

x-s

-A

z-zs

--A

CLASSES OF TRAN SMISSIVITY MAG NITUDE

VERY LO W LOW INTERMEDI ATE HIGH VERY HIGH

V IV 111 11 I

r 'J

~~Y +17

+A .

4 V

' I :~ :1'" ~- i ~~ , I~V' ,

----:.~, - ~ ,I

~-~- Itv

~/'"- - - I I~ ,1 <0

/ r . ' I V

/ /. rI:JJY ~l/

I Il l. -A ·

I

1 0 35 ' 0 '5 50 55 60 6 5 7.0 1 5 ab

g7. 7

Q5

IlO

' 5

' 0

2.3

Index Y

0001 0002 !0P';i, .9101 I Oj2 ,o p i , .9j1 0 2 I O,5! , " I' f ! ! "'10 20I 5p " WP q [I/s m], ! I , I I , , J

0 1 02 , 0,5 " ,,) f " ,jO zp I ~ I ,,:pO, s " ,!p3 1 L "10' T [m 2/d], , r , , I I ! I I

Fig.l . Prevailing t ransmissivity and it s classification expressed as fields of cumula tive relative frequencies (modified after Krasny 1999). q = specificcapacity in Lis m,T = coeffic ient of t ransmissivity in m'ld, Index Y = index of t ransmissivity Y (Index Y= log 1O'q ,q = specific capacity in Lis m),x = arith ­meti c mean, s = standard deviat ion ,x ± s = int erval of prevailing t ransmissivit y (hydrogeo log ical or t ransmissivity background) compr ising approxi­mately th e centra l 68% of transmissivity values of a stat ist ical sample,++A,+A,-A, - -A =field s of positive and negative anomalies (+A,-A) and extre meanomalies (++A,- -Al, respect ively (outside th e interval x ± s of prevailing t ransmissivity).A - field comprising t ransmissivity values of the majori ty of hardrock types, B - field of t ransmissivity values in crystall ine limestones andlor in ot herhardrocks of higher prevail ing t ransmissivity, C - cumu lative relative frequency of t ransmissivity of fluvial depos its along the Labe River in the CzechRepub lic (for comparison wi th transmissivity of hardro cks expressed by fields A and B), indicating not only high er transmissivity but insig nificant vari­ability (steep slope of the line) aswe ll (t ransmissivity class lIa).Classificat ion of t ransmissivity magnitude and variat ion after Krasny (1993a).

Table 1. Classification of t ransmissivity magnitu de (afte r Krasny 1993a).

Comparative regional parameters VeryCoefficient Classof Designation approximately corresponding to the approximate

of trans- transmis- of trans- coeff icient of transmissivity Groundwater supply expectedmissivity sivity missivity potential discharge in LIs of(m' /d) magnitude magnitude Non-logarithmic: Logarithm ic: a single well at

Specific capacity (LIsm) Index Y 5 m drawdown

Very high Withdrawals of great > 50regional importance

- - 1,000 - - - - - - - - - - - - - - - - - - - - - - - - - - --- - --- - - - - - - - - - - 10 - - - - - - - - - - - - --- ----- 7.0 --- ---- - --- --- - --- - --- - - - - - - - - - - - - - --- - ---------11 High Withd rawals of lesser

regional impor tance 5 - 50- - - - 100 - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - --- 1- - - - - - - - - - - - - - - - - - - - 6.0 - - - - - - - --- - --- - - - - - --- - - - - - - - - - - - - - - - - - - - - - - - - - -

Withdrawals for localIII Intermediate water supply (small 0.5 - 5

communit ies,plants, etc.)- - - - - 10 - ---- --- - - - - - - - - - - - - - - --- --- ------ - - - - - --- - - 0.1- - - - - - - - - - - - - - - - --- 5.0-- - ----- - ---- - ----- ---- - - - - - - - - - - - - - --- ---------

Smaller wi thdrawals forIV Low local water supply 0.05 - 0.5

(private consum pt ion, etc.)------ 1 --- ----- ---- ------ ---- --- - ----- ------ ---- -- 0.01--- - ------------- 4.0 - - --- - - - - --- - - - - - - - - - - - - - - - - - - - - --- - - --- --------

Withd rawals for localV Very lo w water supply wit h 0.005 - 0.05

limited consumption- - - -- 0.1 - - --- ---- ---- ------ ---- ------ ----- - --- - - - - 0.001- ------ - - - ------- 3.0 - - - - - ----- - - - - - - - - - - ---- --- - ----- ------- - - - - ----

Sources for local waterVI Imperceptible supply are diff icult < 0.005

(if possible) to ensure

NGU -B U L L 4 3 9 , 2002 - PAG E 10 JII'i{KRASNY

Or logarithmic tran sformat ion of any parameter exp ressing tran smissivity

.. Usablealsofor hydraulicconductivity evaluation

Designation oftransmissivity

variatio n

Standarddeviation Class ofof transmissivity transmissivity

index y " variation

Table 2. Classification of t ransmissivit y variation (after Krasny 1993a).

Hydrogeological

environmentfrom the point of

view of itshydraulicheteroqene ity"

a Ins ignificant Homogeneous- ---- - - 0.2 --------------------------- --- -------------------

b Small Sligh tlyheter ogeneous

--- --- - 0.4 - - - --- --- ------------------------------------- ---c Moderate Fairly

heterogeneous------- 0.6 - - --- ---- - - - --- ------------------ ---- --- --- --- - --

d Large Considerablyheterogeneous

- - - - - - - 0.8 ------------------------- - - --- - - - - --- --- --- ---- --e Very large Very heterogeneous

- ---- --1 .0 - - - - - - - --- --- - - - - - --------------------- ------ - - - -f Extremely large Extremely

heterogeneous

Scale effect in permeabil ity distri ­bution: hierarchy of inhomogeneityelementsThe above -mentioned conclusions are based on results of

aquife r tests carried out in water wells, mostly to depths of

some tens of metres.The charac ter of a spat ial distribut ion

of inhomogeneity elements, and consequently also of

hydraulic parameters in a fractured environment, strongly

depend on the exten t of a study area and its relation to the

size of respect ive inh omogeneity elements (Rats 1967, Rats

& Chernyshov 1967, Kiraly 1975). Consequent ly, because of

this 'scale effect' values of hydraul ic parameters are typically

influenced by methods used in their determination .

In a hydrogeolog ical environment characterised by

inhomogeneity elements (fracturing) of similar size, trans­

missivi ty mean values becom e closer wi th increasing testing

density wit hin the study area. Finally, t hese values are pract i­

cally the same, irrespective of t he position of a tested area

wi thin th is envi ronment (Krasny 2000a). The exten t of an

area representi ng the lowe st limit above which pract ically

no changes in mean t ransmissivity values occur is desig­

nated as a representative elementary vol ume (REV).

Accord ing to Bear (1994), th e main characteristi c of a REVis

that the average value of fluid and sol id propert ies taken

over it are ind ependent of its size. Occurrences of larger

inhomogeneous elements (e.g., large fractu re zones), how­

ever, may cause other supp lementary differences in t rans­

missivity values and the REV size might expand consider­

ably.

In a fractured hardrock envi ronmen t, permeability and

transmissivity d istribution is commonly considere d dis­

arranged, wi t hout any possibi lit y to be predicted. However,

stud ies on transmissivity distribu t ion in hard rocks of the

transmissivity magnitude is determined aft er th e percent­

age of th e interval x ± s (transmissivity backg round ) belong­

ing to part icular classes (Fig. 1) (for a detailed description of

the classification system, see Krasny 1993a). The part icular

classes mig ht indi cate the prospect of groundwater supply

in different hydrogeological envi ronments (Table 1).

Another important prope rty of a set of t ransmissivity

values (i.e.,of a stati stical sampl e) is their variabili ty.This sug­

gests spat ial transmissivit y changes and, consequently, indi­

cates the int ernal character (anatomy) of a hydr og eologi cal

environment and its degree of hydrau lic heterogeneity.

Similarly as wi th t ransm issivity mag nitude, t ransmissivity

variat ion is also classified by six classes, denominated a to f

(Table 2) and based on a standa rd deviat ion of transm issiv­

it y of a stat istic al sample. Any transmi ssivi ty parameter

expressed in a logarithmic form, but preferably the t ransmis­

sivity index Y, can be used. On probabil ity paper (Fig. 1),sam­

ples wi t h low variation will plot alon g lines wi th steeper

slopes th an samples wi th a large variat ion .

This classificat ion system enables a realistic assessment

of aquifer capabili ty to wi t hd raw groundwater in different

areas,and helps in di scussions on the inf luences causing dif­

ferences in t ransmissivity values. It also makes it possible to

express quant itati vely reg ional hydrogeolog ical cond it ions

in a com pact form and to compare them in tables, figu res

and in hyd rogeo logical maps.

Transmissivity data from aquifer tests have been stati sti­

cally t reated du ring recent years in many hardrock areas of

th e Czech Republ ic, especially wit hin th e framework of the

count ryw ide hydrogeolog ical mapping programm e. This

has brou ght to light important informat ion on transmissiv­

ity distr ibution.Stat ist ical samp les were chosen accordi ng to

hydrogeological units, rock ty pes and th e structu ral, geo­

morph olog ical and hydrogeological position of water well s.

Part icul ar statisti cal samples represent areas rangi ng from

several km ' to tens or hundreds of km', w it h th e least used

data frequency around seven. Prevail ing transmissivity was

mo stly in un its of m'/d up to slightly mor e than 10 m' /d

(Krasny 1999, Fig. 1).

Examples from di fferent count ries show the possib ilit ies

fo r correlati ve transm issivity stud ies in hardrock areas based

on th is stan dardi sed approa ch. In Poland, Stasko & Tarka

(1996) analysed the transrnisslvity distribu t ion of

Precambr ian and Lower Palaeozoic gnei sses, migmatites,

horn fels and amphibol ites. In Sweden , Carlsson & Carlstedt

(1977) determined cumulative relat ive frequencies of t rans­

missivity indices Y from gneisses, granites, Algo nkian and

Cambrian sandstones and Ordovician limesto nes. A similar

proc edure was used for tran smissivity data analysis from

hardrock regions in Ghana (Darko & Krasny 1998, Darko

2001) and in Korea (expressed by Krasny, having used data

from Callahan & Choi 1973). Everywh ere a similar prevailing

transmissivity of hardrocks was determi ned th at can be

expr essed by th e classes IV (V,1I1) c, d (Fig. 1).

JIf/fKRASNY

Bohemian Massif enabled th e definition of a hierarchical

system of inhomogeneity elemen ts of different scalesdesig­nated as local, med ium -scale (sub-regional) and regional­

scale inhomogeneiti es (Krasny 2000b).On a local scale, irregular changes in transmissivity spa­

tial distribution within th e 'near-surface aquifer' of hard

rocks are common, and are marked by a weathered (upper)

zone, often wi th j uxtaposed Quaternary deposits, and by a

fractured (middle) zone.These are indicated by differences

in yields and transmissivity of nearby wells dri lled in th esame rock that mig ht reach several orders of magnitude

(Fig. 1). In more extensive areas, transmissivity determ inedby aquifer tests tend s to attain considerably closer valuesboth in prevailing ranges and in arithmetic means. As men­

tioned above, these prevailing values represent th e trans­

missivity background as ment ioned above.

Superposed upon this background, however, significant

differences in transmissivity magnitude might occur due tothe presence of inhomogeneity elements of a higher scale

level (medium-scale or sub-regional inhomogeneitie s).

These belo ng to tectonically strongly affected zones with aconsiderably higher tran smissivity that might be of impor­

tance for groundwater abstraction in hardrock environ­ments (Krasny 1996c).

Other authors have comm ented upon the higher pre­

vailing permeabili ty of rocks in valleys than on slopes and

topographic elevations (LeGrand 1954, Krasny 1974, 1998,

Henriksen 1995).Following these findings, the hydrogeolog­

ical environment in fractu red rocks should not be consid­ered as regionally homogeneous. It should rather be consid ­

ered as a compl ex system where belts of regionally highe rprevai ling permeability occur, usually fol lowing the valleys

and depressions.These belts can be perceived as manifesta ­

tions of inhomogeneity elements of a higher order superim­

posed on an environme nt where local inhomoge neity ele­

ments can be averaged.The differences in statist ically pre­vailing values between valleysand elevations may reach one

order of magn it ude or even more. The ratio of arithmeticmeans ranges from 1.6 to 38 in different types of hydrogeo­logical environments (Krasny 1998).These differences seem

to be of genera l validi ty, even though they are of distinct

magn itu des in different hydrogeological environme nt. The

overall tectonic pred isposition of valleys or depressions may

well be the main cause of these differences,a lthough, hydro­

geo logica l inf luences wo uld seem to be addi tiona l facto rs

leading to increases in these differences, as reported byKrasny (1974).

Regional-scale transmissivity changes were determined

in extended hardrock areas in South Bohemia (Krasny et al.1984). This regional transmi ssivity distribution evidently

ref lects the intensi ty of so-called neotectonic activity, lasti ng

from the Late Tertiary up to recent times . A lower prevailing

transm issivity is characterist ic of relatively flat areas with

negligible neotectonic activi ty, and higher in zones where

neotectonic deformat ion has been more pronounced (e.g.,

NGU- BULL 4 39 , 2002 - PAGE 11

the Sumava Mts.). The differences in regionally prevailing

t ransmissivit y reach more than one order of t ransmissivity

magn itude.This represents an important shift in a regional

transmissivity background upon which local changes oft ransmissivity (posit ive and negative anomalies) are super­

imposed (Havlfk & Krasny 1998).This might be of practical

inter est for groundwater supply studies and the sit ing of

waste deposit s or deep repositories.

Similar gradual changes in th e regio nally prevailing

transmissivity of hardrocks, caused apparent ly by different

intensiti es of rock fractu ring, have been reported fromNorway by Rohr-Torp (1994). His study showed a wel l yie ld

correlation with th e amount of post-g lacial isostatic upl ift

following the Weichselian glaciation. Even though the rea­

sons for the differen t degrees of fracturing in South

Bohemia and in Norway are dissimilar, th e comparable

effects indicate possibilit ies of regional variations in trans­

missivity also in other fracture-dominated environments.The above-ment ioned hierarchical system of t ransmis­

sivity d istribution due to disparate inhom ogeneity elementsis to be expected in all fractured and dou ble-porosi ty envi­

ronments. Practical conclusions should be drawn for con­

ceptua l model impl ementation, grou ndwater flow model­

ling, safe yield assessment, well sitin g and studies on

groundwater vulnerability.

Influences of lithology on regionaltransmissivity distribution inhardrock areasResults of hundreds of pumping tests carried out in drilled

and dug wells in hard rocks within th e Czech part of theBohemian Massif have enabled a quantitative and standard­

ised approach to transmissivity distribution studies.Irregular

local permeability changes, determin ed from results of par­

ti cular aquifer tests, usually scatter over a wide interval of

several orders of magnitude. On the other hand, regionally

prevai ling transmissivity values (hydrogeological back­

ground) resulting from a statistical t reatment, mostly belongto classes IV(V,III) c,d, i.e. very low to inte rmediate transmis­

sivity with moderate to large t ransmissivity variation - seefield A in Fig. 1. Based on the differences in pet rograph ic

composi tions of rocks and their different ways of fissuring

and weathering, differences in hydrogeological propert ies

have also often been considered and expected.An objective

comparison of the transmissivity magnitude of particular

rocks in the Czech part of the Bohemian Massif, however,

indicated only small differences in the regionally prevailingt ransmissivit y of partic ular areas. Except for some types of

rocks, the influence of petrography on t ransmissivity spatialdistribution cannot be considered important. Areas com­

pr ising crystalline limestones (marbles) represent the most

prominent exception of th is rule. Marble s commonly occur

as intercalations of variable th ickness in bedrock and their

hydrogeological properti es usually differ considerably from

other rock types.The prevailing transmissivity and hydraulic

N GU-BUL L 4 39 , 2 002 - PAG E 1 2

conductivity of marb les is usually a half to one order of mag­

nitu de higher compared with other crystalline rocks (Krasny

1999) and might be expressedschemat ically by field B in Fig.

1 (most ly classes Ill-IV c.d).

There are some indicat ions that relativ ely high er trans­

missiviti es may be expected in areas underlain by basicigneous rocks and also by some types of granit e. On the

other hand, phyllites displayed a lower region ally prevailingtransmissivity in some areas (Krasny 1993b). No sign if icant

transmissivity differences on a regiona l scale, however, have

been demo nstrated between most granites and the majo r­

ity of metamorphic rock types,even though the fo rmer have

oft en been considered more permeable. Possible small dif­

ferences in t ransmissivity caused by dist inct ive petrogra­phies of hardrocks are evident ly masked by th e more impor­

tant influence of fracturing. Differences in the ir geomechan­

ical properties, frequency of fractu ring and th e character ofweathering are obvious, however,and may result in local dif­

ferences in th eir permeab ility. At depth , massive granit ic

rocks seem to be more predisposed to form imp ortant frac­

ture zones as many th ermal waters are usually associated

with them. Thermo-mineral waters in the Czech Republic,

such as those at Karlovy Vary (Carlsbad) Spa and Teplice Spa,

emanate from granite and quartz porphyry (rhyolite),respect ively (Hanzlik &Krasny 1998,Jakes &Krasny 1998).

The influence of weathering and the presence of gener­

ally more highly permeable Quaterna ry deposits, regoli t hs,debris and fluvia l deposit s result in higher transmissivit ies in

wells where hardrocks are covered by these thick, unconsol­

idated depos its. The transmissivity variation of hardrocks

com prising fluvial Quaternary depos its is usually lower than

th at of hardrocks wi thout a Quaternary cover.This suggestsan equalising effect of the hydraulically more homoge­

neous,Quaternary depos it s (Krasny 1993b).

Natural groundwater resourcesAn assessment of groundwater runoff into rivers draining

surroundin g aquifers isgenerally considered the best way to

estimate natura l (renewable) groundwater resources of th e

tempe rate climatic zone on a regional scale. During a map­

ping programme focused on th e assessment of groundwa­

ter runoff in the Czech Repub lic, the to tal, long-term, mean

natu ral grou ndwate r resources of the whole coun t ry wereestimated to be ashigh as 205 m'/s.The result ing map of th e

JlfI{KRAsNY

long-term, mean groun dwater runoff in the former

Czechoslovakia at 1:1,000,000 scale w ith explanato ry notes

(Krasny et al. 1981, 1982), might serve as an examp le of a

national cartographi c representation of natura l groundwa­

ter resources. Previous assessments of natural groundwa ter

resources (=recharge) in hardrock areas of Cent ral Europewere low, mostly up to a maximum of 1-2 Us km' (e.g., Hynie

1961). The results of the above-mentio ned mapping pro­gramme have indicated regionally valid values in the

hardrock environment ranging from 1-2 Us km2 to more

than 10 Us km'.

To compile such a map, a number of methods for

groundwater runoff esti matio n were tested and compared.

For a variety of reasons, mainly due to its obje ct ive data pro­

cessing, th e method of Kille (1970) was chosen andemp loyed by all co-authors.The results were compared with

a number of othe r hydrolog ical methods aiming to separatea ground water component from the total runoff, e.g.,

Castany et al. (1970), Kliner & Knezek (1974) and Makarenko

(1948).An independent met hod applying regionally prevail­

ing t ransmissivity values and morphometri c character istics

of the area (Krasny & Knezek 1977) was also used.

Differences between th e results ob tained by part icularmethods were not significant. In some catchments, in the

vicin ity of mountain summ its, the long -term specificgroundwater runo ff from hardrock areas reaches in excess

of 15 L/s km'.This is mainly because of favourable climat icand geomorpholo gical conditions .The latter result in a rela­

t ively high hydraulic gradient of groundwater flow even in

this less transmissible hardrock aquifer (Krasny 1999). With

decreasing elevation , and mostly due to a decrease in pre­

cip itation, th e rate of grou ndwater runoff generally gradu­ally diminishes down to 1-2 Us km'.The approximate rela­

t ionsh ip bet ween climat ic and hypsometric condit ions andground water runoff (= natu ral groundwater resources) dis­

t ribution in hardrock areasof the Bohemian Massif is shown

in Table 3. Maximum long-term recharge may thus be esti­

mated at mo re th an 300 mm /year.This accounts for more

than 20 % of the mean annual precipitat ion.Natural ground wate r resources in hardrock areas of the

Bohemian Massif were also studied in the neighbouring

regionsof Germany and Poland.Killes (1970) method, part ly

mod ified by Kopf & Rothascher (1980),was used for regiona lrenewab le ground water resource estimation in Bavaria,

Table 3. Relatio nship between climatic and hypsometric cond it ion s and groundwater runoff (natural groundwater resources) distri bution in hardrockareas of the Bohem ian Massif (aft er Krasny 1996b).

Morph ological Approximate Mean annual Mean annual Groundwater runoff(hypsometric) unit elevat ion (m a.s.l.l precipitation (mm) evapot ranspirat ion (natural ground water

(esti mat ion in mm) resour ces- LIs km ')

Mo untains 1,200 - 1,600 1,000 - 1,200 450 10 - 15Lower mountains 800 - 1,200 800 - 1,000 7 -10Piedmo nt areas 300 - 800 600- 800 3 - 7Flat areas, low lands less th an 300 500 - 600 650 1 - 3

)I(fi KRASNY

Germany, by Apel et al. (1996). Results of regional studies on

groundwater runoff distributi on in Poland were presented

by Bocheriska et al. (1997) and Kryza & Kryza (1997). The

results obtained both in Bavaria and in Poland were quite­

comp arable wi th those from the Czech Repub lic.Residence t ime, based on isotope stud ies,was assessed

to between a half to one year for a shallow grou ndwater

flow and up to ten yearsfor a deep one in crystalline rocks of

the Bavarian Forest in Germany (Seiler & Muller 1996).

Previously, Mart inec (1975) had reported a gro undwater res­

idence tim e in crystalline rocks of the Krkonose Mts. (Czech

Republic) of one to two years.

ConclusionsDepending on hydrogeolog ical and climat ic cond itions , the

limitin g factor for groundwater resource develop ment may

be either:

hydraulic prop erti es of rocks,orground water recharge.

A regional assessment and knowledge of the se two

aspects is indispensable for any decision -making on the sus­tainable develop ment and protec tion of groundwater in

more extended areas such as regions, states,or even conti ­

nent s.

In arid and semi-arid regions, due to th e limited precipi­

tation and high evaporation, natural groundwater rechargetypically represents a clear limitation for groundwater

abstraction. Intensive groundwater withdrawals may result

in a long-term overdraft.

On th e other hand, within the Eart h's temp erate climaticzone,a relationship betwee n groundwate r abst ract ion pos­

sibili t ies of different hydrogeolog ical environme nts, givenby their hydraulic parameters, and the available natura l

gro undwater resources determines what is effectively a safe

yield for an area. In hardrock environments where high nat­

ural (renewable) groundwater resources reach up to 10-15

LIs km' in some catchments in the vicinity of mountain sum­mits, recharge is suffic ient to cover abst ract ion possibilities

th at are limited only by the tran smissivit y of th e rocks.

Mountains with th eir surprisingly high natu ral groundwaterresources, in spite of th eir relat ively low t ransmissivity, com­

monly represent source areas high enough to maintain flowin water courses in adjacent piedmont zones du ring dry

periods.Therefore, when considering the format ion of nat­

ural groundwater resources (groundwater recharge) against

a background of th e Earth's clim atic zonation (arid, hum id,temp erate, etc.) th e vert ical climat ic zonat ion mostly arising

from hypsometric differences (mountains, lowlands, etc.),should also be taken into account.

Under the se conditions, and considering the present­

day general t rends towards water manageme nt optimisa­

ti on and also increases in existing wate r demand, ade­quately sited water wells or other water int ake systems in

hardrock areas can cover th e requirements on wate r supp ly

for small communiti es, plants or farms,and also for domestic

N G U -B U L L 439 , 2002 - PAGE 13

water consumpt ion. In areas wi th high t ransmissivity ofhardrocks, th e groundwater abst ract ion possibi lit ies might

even be high enough to supply small towns. Therefore,

within hardrock areas with suffi cient water resources, scat­

tered water intake sites can effective ly cover the water sup­

ply demands of a large number of consumer s. A hydrog eo­

log ical basis, i.e., an adequate understanding of transm issiv­

ity and permeabili ty distribu tion , is an essential factor in

attaining this goal.

AcknowledgementsThe author acknow ledges the suppo rt of the EC Project LOWRGREP(Landuse opti misation w it h respect to groundwater resources prot ec­t ion in the mountain hard rock areas) and of th e Ministr y of Education ofthe Czech Repub lic (Grant No. J13/98: 113100005).Thanks are also dueto th e reviewers, David Banks and Helge Henrik sen, for th eir helpful andconstructive comments on th e manuscript .

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