SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-7-881-2016Structural geology and geophysics as a support to build a hydrogeologic model of granite rockMartinez-LandaLurdeslurdes.martinez@upc.eduCarreraJesúsPérez-EstaúnAndrésGómezPalomaBajosCarmenDepartment of Civil and Environmental Engineering, Universitat Politècnica de Catalunya (UPC), c/Jordi Girona 1-3, 0803 Barcelona, SpainInstitute of Environmental Assessment and Water Research (IDAEA), CSIC, c/Jordi Girona 18, 08034 Barcelona, SpainAssociated Unit: Hydrogeology Group (UPC-CSIC)Instituto de Ciencias de la Tierra Jaume Almera, CSIC, c/Lluis Solé Sabaris s/n, 08028 Barcelona, SpainCentro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Departamento de Impacto Ambiental de la Energía, 28040 Madrid, SpainEmpresa Nacional de REsiduos (ENRESA), c/ Emilio Vargas 7, 28043 Madrid, Spaindeceased, August 2014Lurdes Martinez-Landa (lurdes.martinez@upc.edu)1June2016738818956February201622February201619April201620April2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://se.copernicus.org/articles/7/881/2016/se-7-881-2016.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/7/881/2016/se-7-881-2016.pdf
A method developed for low-permeability fractured media was applied to
understand the hydrogeology of a mine excavated in a granitic pluton. This
method includes (1) identifying the main groundwater-conducting features of
the medium, such as the mine, dykes, and large fractures, (2) implementing
this factors as discrete elements into a three-dimensional numerical model,
and (3) calibrating these factors against hydraulic data
. A key question is how to identify preferential
flow paths in the first step. Here, we propose a combination of several
techniques. Structural geology, together with borehole sampling, geophysics,
hydrogeochemistry, and local hydraulic tests aided in locating all
structures. Integration of these data yielded a conceptual model of the site.
A preliminary calibration of the model was performed against short-term (< 1 day)
pumping tests, which facilitated the characterization of some of the
fractures. The hydraulic properties were then used for other fractures that,
according to geophysics and structural geology, belonged to the same
families. Model validity was tested by blind prediction of a long-term (4
months) large-scale (1 km) pumping test from the mine, which yielded
excellent agreement with the observations. Model results confirmed the
sparsely fractured nature of the pluton, which has not been subjected to
glacial loading–unloading cycles and whose waters are of Na-HCO3 type.
Introduction
Low permeability fractured media have an important role in
enhanced geothermal energy and waste management. Studies of this media are
hampered by the contrast between conductive fractures and a nearly impervious
matrix, and this heterogeneity must be considered. The characteristics and
challenges of these media have led to studies in a large number of
investigation sites, notably in granites, around the world: Stripa
, Äspö
and Forsmark in Sweden; Grimsel in
Switzerland ; Fanay-Augères
in France ; Mirror Lake in New Hampshire, USA
; Olkiluoto in Finland , and
the Canadian Shield , among others. Most of these sites are located in
zones affected by the last glaciation, which caused severe deformation and
fracturing. In contrast, sites in southern Europe have not been subjected to
glacial unloading stresses. As a result, most fractures have been driven by
much older tectonics and they have
been largely filled by mineral precipitates. In addition, saline water has never
infiltrated these systems, so that the deep water is of Na-HCO3, rather
than Na-Cl type. This group of sites includes El Berrocal pluton
or Ratones mine , both of which
are in Spain. This article is based on work conducted at the Ratones Mine in
Cáceres province of Spain (Fig. 1), and contributes to improve our
understanding of this type of media.
Geological map of Albalá granitic pluton with location of the
Mina Ratones area .
There is currently no generally accepted method for modeling low-permeability
fractured media. While several different approaches have been used, they are
generally combinations of methods of modeling two extremes: continuous medium
and discrete fracture networks. Continuous medium fracture network-based
approaches assimilate the domain to a porous medium, which includes the
effect of fractures. The hydraulic conductivity field is estimated by
geostatistical techniques conditioned to actual measurements of hydraulic
conductivity and pressure at observation points . Discrete fracture network-based approaches represent the medium by
means of fractures networks that are statistically generated. This approach
is based on the assumption that fractures behave as preferential flow paths
.
More sophisticated fracture network models are channel network models, which
represent only the conductive portions of the fractures planes
. We have been using an intermediate mixed approach that
treats the matrix and minor fractures as an equivalent, possibly
heterogeneous, porous medium and deterministically simulates the
hydraulically dominant fractures . The main
drawback of this approach is that these dominant fractures must be identified
and characterized. Strict characterization is only possible in intensely
tested environments. Therefore, the validity of such an approach may be
questionable when flow is predicted at longer scales. This is especially
worrisome in view of the apparent ubiquity of scale effects.
Scale effects refer to the apparent increase of hydraulic conductivity or
transmissivity as rock volume increases . Multiple explanations exist for scale effects.
provides a thorough discussion on scale effects and their
origin, pointing to other issues such as poorly developed wells
and turbulence in the boreholes ().
argue that long-term pumping tests are purposefully
performed at the most conductive intervals, implying that they are not
representative. Other authors attribute scale effects to the connectivity among structures.
Certainly, the hydraulic conductivity derived from interpreting pulse tests
yields information about the closer vicinity of the borehole, which results
in marked differences between values associated with intervals that intersect
any conductive structure and those that are open only in the matrix
(). In cross-hole tests, pumping must be performed
at intervals in high conductivity zones to maintain a significant flow rate
over a long period of time, but pumping also affects points located in the
matrix. If the interpretation of the tests does not take into account the
existence of conductive fractures, the conductive fractures will lead to a
high effective hydraulic conductivity. reported
that this effective conductivity is appropriate for predicting large-scale
tunnel inflows. Such effective conductivity, however, is far larger than the
average of any small-scale hydraulic test, the vast majority of which are
sparsely fractured; hence, the scale effect occurs.
Thus, good representation of fractured media requires identification of the main
water-conducting features. We contend that this is possible by integrating
different types of information (geology, geophysics, hydrochemistry, and
hydraulics). Accounting for the heterogeneity allows for the use of models
for non-trivial predictions under conditions that differ from those during
calibration. This has been demonstrated in previous studies
, but always at scales similar
to those during calibration. Still, the best indicator of model robustness
lies in its ability to predict changes in flow conditions at scales different
from those during calibration. We propose that structural geology and
geophysics can be used to identify large water-conducting fractures that have
not been characterized by direct hydraulic tests. By assigning the hydraulic
parameters of similar fracture types whose water-conducting fractures have
been characterized, we could effectively extend the model scale and
potentially model a large volume of rock. This article has three objectives.
First, we present a method based on the above proposal and test its
predictive capability. Second, we discuss the scale effects observed at the
Ratones mine. Finally, we provide additional information regarding the
low-permeability fractured media of the Iberian Peninsula in southern Europe.
To accomplish these objectives, we analyzed field data sets from the Ratones
Mine (study of the hydrogeology around an old uranium mine excavated in a
granitic pluton). Data sets were obtained from geochemical,
geologic-structural, geophysics, and hydrogeological studies that aided in
identifying the main structures (heterogeneities), including their position,
direction, dip, and extent. A three-dimensional (3-D) numerical model was then
constructed, where the matrix and minor fractures were treated as an
equivalent porous medium, and the identified fractures were implemented as
2-D planes embedded in the matrix. This numerical model was
then used to calibrate cross-hole tests and to predict the results of
long-scale pumping tests from the mine.
Fault zone architecture of Mina Ratones area obtained from
structural, seismic, core, and well log data . The main
identified structures are the North Fault (NF), the South Fault (SF), and the 27 and 27' dykes. Other
relevant brittle structures of minor size are faults 474 and 285.
Test siteGeological and geophysical characterization
The Albalá granitic pluton is located in the southwest sector of the
Iberian Massif (Central Iberian Zone of ). The pluton is
a concentrically zoned body, elongated in an N–S direction, with porphyric
biotite granites in the rim and fine-grained two-mica leucogranites in the
core (Fig. 1). Ratones is an abandoned uranium mine, located in the central
aureoles of the pluton. The fault zone architecture in the Mina Ratones area
was established on the basis of field geology, structural analysis, seismic
experiments, drill cores (Ratones borehole, Sondeo Ratones, SR1 to 5) and sonic well log data
. Surface geology was mapped at a 1 : 1000 scale in a zone
that includes the block where the seismic tomography survey was to be
conducted. The resulting maps included granitic facies, dykes,
ductile-brittle shears, fault zones, and granitic soil cover (lehm). The 3-D
fault distribution obtained for this area is shown in Fig. 2. The
post-Variscan structural evolution of the Albalá granitic pluton was
established on the basis of fault kinematics and paleostress analysis in
superficial outcrops . This evolution
includes three episodes of brittle deformations related to different
stress-field configurations, which cut and reactivated ductile and
ductile-brittle late-Variscan structures.
The first episode was extensional and produced the intrusion of
Jurassic subvertical diabasic dykes, aligned following a NNE–SSW trend. The
constant trend of these dykes at regional scale indicates that σ3
was subhorizontal and WNW–ESE to NW–SE directed.
The second episode was characterized in the Mina Ratones area by
the development of strike-slip faults with E–W direction and kilometer thickness.
The third fracturation episode, post-Hercinian, partially reactivated
the previous WSW–ENE to E–W structures and dykes as normal and normal-slip faults .
The main identified structures in the Ratones area (Figs. 1 and 2) are as
follows.
The North Fault (NF)
has a N70 to N80∘ E
trend and a 55–65∘ S dip, diminishing with depth to
30–40∘.
The South Fault (SF)
has a N64 to N78∘ E
trend and a 68–82∘ N dip. It has a subvertical set of parallel fractures, forming a fragile transcurrent shear
zone.
The 27 and 27'
mineralized subparallel and subvertical dykes have a thickness of 0.4–1.8 m,
are composed of a quartz breccia, and are cemented by sulfides .
The damage zone
is defined by small faults and kinematically related fracture sets and
joints. The dykes and other fractures (as NF and SF) are hosted in an extensively fractured damage zone of hydrothermally altered granite (fractured belts).
Other relevant brittle structures of minor size include the following.
The 474 and 474' faults
are two high-dip subparallel structures that trend N64 to N76∘ E. Both structures connect toward the W with the 27 dyke.
The 285 brittle structure
is also a sinistral strike-slip fault with a N52 to N60∘ E trend and subvertical dip.
Faults 474 and 285 are younger than the NF; they cut and displace it, so
that it remains hydraulically disconnected.
Groundwater hydro-geochemical behavior model of the Ratones aquifer-mine system (modified from ).
Some of the chemical and geophysical logs recorded at borehole SR1,
which helped to identify dyke 27 and fracture SR1-3. The chemical logs only
display the effect of dyke 27, because all upper water (including SR1-3) is
mine water. The effect of both structures can be noticed in other logs,
especially in the gamma log, due to the fracture filling. Flowmeter values,
recorded with an upward flow rate of pumping indicate little water flow
beneath dyke 27. The same applies to a lesser extent to fracture SR1-3.
The last deformational phase indicates that the stress state at the recent
evolution was in a E–W direction. As a result, the north and south family
fractures are oriented in the most favorable direction, making them ideal
candidate water-conducting structures. For the purpose of subsequent
hydrogeological analyses, it is important to highlight that (1) the above
discussion is the result of a structural geological interpretation of 3-D
seismics. (Specifically, direct seismic inversion was often modified after
discussions with structural geologists, who pointed to inversion
inconsistencies. Therefore, it can be stated that seismic inversion was
“structural geologically biased”.) (2) Some water-conducting faults
(specifically 474, and 285) were identified during seismic inversion, but
had not been mapped because they are located in an area covered by meadows.
(3) Geophysics also helped to identify highly fractured zones (e.g.,
fractured granitic units, fractured belts around the dykes and the South Fault),
which provided hydraulic connectivity.
Hydrogeochemical characterization
Rainwater entering into the rock-mine system is saturated with atmospheric
O2, leading to (1) oxidation of metal sulfides present in the dykes and
mineralized fractures, (2) precipitation of metal oxyhydroxides, and (3) the
addition of acid to the medium. Acidic water causes dissolution of carbonates
in fissure fillings, buffers the mine water pH, and promotes the
precipitation of metals released by sulfide oxidation as metal carbonates
(Fig. 3). In parallel, plagioclase weathering promotes kaolinite
precipitation in the shallower areas. The albite becomes smectite clay,
resulting in sodium bicarbonate waters in the deeper areas (400–500 m deep,
borehole SR5) with transit times of tens of thousands of years (16 000 years
dated with noble gases). These sodium bicarbonate waters are typical of
granitic water with relatively high residence times. The groundwater
circulating through granites in the Hesperian Massif is of sodium
bicarbonate type, which may be significant for waste disposal in this type of
rock . This is different from the chemical
composition of the groundwater in granitic formations in other parts of the
world, such as in the Hercinian granitic in the Chardon U mine in France
and in the Canadian ()
and Scandinavian shields, where chloride and sodium are the
major ions. Hydrogeochemical studies help to identify water-conducting
fractures. Water flowing through fractures is chemically marked by water–rock
interactions along the flow path within the medium . This enables the identification of some
fractures and potential connections. Figure 4 shows an example of the
downhole geophysical loggings and records of a multiparametric device (electrical conductivity,
redox potential, resistivity, gamma rays, spontaneous potential, diameter,
and flowmeter) in borehole SR1, down-gradient of the mine. These data were
used to identify the intersection with dyke 27, where water flows from the
mine, as indicated by chemical parameters and changes in the upward velocity
of flow within the borehole (flowmeter log).
Results from preliminary interpretation of the South cross-hole test
(pumping at SR4-1). Transmissivity and storativity are derived by fitting
each borehole drawdowns, one at a time, using the Theis model. The degree of
connectivity was derived from the estimates of storage coefficients. T varies
from 4 to 64 × 10-5 m2 s-1, while S ranges from
1 × 10-7 to 0.43. We take S estimates to reflect connectivity. A
small S (fast response) implies a good hydraulic connection between pumping
and observation wells. This suggests that the best connections occur between
the pumping interval and points S10 and SR1-3, while points S5 and SR1-2,
and, especially mine well (point PM, Pozo Maestro), were damped by the mine influence (which behaves as
a constant head boundary).
Model geometry. The model comprises two areas. The external
matrix area is treated as an equivalent porous media without explicit
fractures, because not all of the main structures have been identified. At
the local level (right picture), the main structures are explicitly taken
into account, including the fractures, dykes 27, 27', and SR3, and the
mine itself, which is excavated in both dykes. The structures are represented
by means of 2-D elements. The Maderos creek is embedded in the SF. Projections
of the boreholes that are closer to the mine are represented by black dots on
the surface and by an arrow pointing to the borehole end.
Hydraulic characterization
As discussed above, geology and geophysics provide insight into the physical
configuration of the fracture network. Structural geology and geochemistry
help to identify which of those fractures may be water-conducting. Simple
borehole hydraulic tests (pulse tests, slug tests, and constant head tests)
provide the location of these structures and their hydraulic conductivity.
Large-scale connectivity can be identified using cross-hole tests and
hydrochemistry if the water contains a chemical tracer. The hydraulic
conceptual model of the system is largely based on the hydraulic extent of
the fractures and their connectivity, which is why a hydraulic testing survey
was designed. Different types of measurements were used.
Development pumping was carried out in all SR boreholes, comprising pumping with an open borehole to withdraw all drilling materials.
Chemical sampling pumping was carried out of NF in borehole SR2, of SF in borehole SR4, and some intervals of
SR5. Time, flow rate, and interval pressure were measured. These data were used as hydraulic test data.
Additionally, specifically designed hydraulic tests were conducted.
Pulse, slug, and constant head single-hole tests were conducted at short intervals in boreholes
SR3, SR4, and SR5. The three tests were performed between two packers separated at a constant distance and
a system of pipes and valves to allow the injection or extraction of very low water volumes with high precision .
Three cross-hole tests were planned. For this purpose, the boreholes were
divided into intervals and hydraulically isolated by packers. The position of the packers was determined
to isolate the structures that had to be tested. Each interval was equipped with a pressure outlet and a water injection/extraction point.
The North test had a pumping interval of S14-1 (first interval of S14 borehole, numbered from bottom
to top) which was designed to characterize the dyke 27 up-gradient of the mine.
The East test had a pumping interval of SR3-1, which intersects the NF, like SR2-2. After pumping for
9 days, there were no responses at any observation point. Interpretation of the seismic profiles in a
subsequent stage revealed the absence of a possible hydraulic
connection between these points because the NF was cut and displaced by faults 474 and 285 (see Fig. 2).
The South test had a pumping interval of SR4-1. This was located in the discharge zone of the mine, pumping
within the SF. This was the only test in which all observation points reacted to the pumping.
Hydraulic characterization begins with a single-hole test (pulse, slug, and
constant head). The calibrated parameters provide insight into the
transmissivity field around the boreholes, and allow for identification of
the most conductive intervals. Cross-hole tests were then conducted in these
boreholes to identify connectivities. All hydraulic tests were interpreted
using the Theis method . This model assumes that the medium
is homogeneous and isotropic, thereby integrating the effect of
heterogeneities into the matrix. In cross-hole tests, the preliminary
interpretations were done separately for each recorded drawdown curve, which
yielded a couple of transmissivity–storage (T-S) values for each observation
point (Fig. 5). Note that estimated Ts range over 1 order of magnitude,
whereas the estimated S's, which are presumed nearly constant, range over
almost 5 orders of magnitude. These highly variable storativities provide
information about the connectivity between pumping and observation points
through fractures. A good connection is reflected by a fast response (i.e.,
high diffusivity, T/S). As discussed in the introduction, the estimated
T reflects large-scale conductivity. Therefore, the estimated storage
coefficient will be low for observation points that are well connected to the
pumping interval . The dykes and NF and SF are accompanied
by systems of minor fracturing that form part of a higher transmissivity zone
situated around these structures. Points S10 and SR1-3, which display the
lowest S values (Fig. 5), are connected to the pumping point through these
fracturing belts. In the 3-D numerical model, this connectivity was
simplified by simulating fracture planes referred to as fracture S10 to
connect point S10 to the pumping interval SR4-1, and fracture SR1-3 to
connect point SR1-3 to SR4-1. The remaining observation points have a lower
response (i.e., higher S) to the pumping. Observation points S5 and SR1-2
intersect dykes 27' and 27, respectively. A priori, these should have a
better response to the pumping. Drawdowns between both points were limited by
the influence of the mine cavity, which acts as a constant head boundary.
Schematic vertical section representing variations in data derived
from the hydraulic characterization of borehole SR5 (500 m deep) indicate that
hydraulic conductivity changes with depth. This fact was studied by
. The model uses a modification of Stober's equation:
hydraulic conductivity is kept constant up to the base of the fractured
granitic unit (200 m), then changes with depth down to 350 m. From there, it
remains constant down to the model bottom.
Numerical model
A 3-D numerical model was constructed to interpret the South cross-hole test,
taking into account the water-conducting features. Figure 6 displays a plan
view of the features introduced in the model. The granitic matrix was divided
into two zones depending on the degree of characterization of the whole area.
On the one hand, the internal matrix represents, in detail, all the fractures
identified around the mine. On the other hand, the external matrix does not
include all of the structures (because they have not all been identified).
The external matrix has larger hydraulic conductivities than the internal
matrix, because it includes the effect of the unidentified fractures. Its
value was assigned by approximately evaluating the effective hydraulic
conductivity (matrix plus fractures) of the area that was characterized in
detail. The model reaches up to 600 m depth to include all measurements (the
SR5 borehole reached a depth of 500 m) and all units. The shallowest layer
is formed by 2-D elements and represents the lehm, which comprises altered,
disaggregated, and washed granite (much like granitic sand) with high
hydraulic conductivity. This layer drains rapidly in humid seasons, and
carries no water in dry seasons. The altered unit reaches a depth of 20 m
according to geophysics and borehole samples. The layer is highly weathered,
but the granite structure is conserved. Its hydraulic conductivity and
porosity are higher than those of unaltered granite due to the decomposition
of feldspar. The fractured granitic unit could be identified by geophysics,
and corroborated by the hydraulic characterization of a borehole 500 m deep
(Fig. 7) that was drilled in the matrix far from the mine. At this depth,
hydraulic conductivity is reduced, and remains almost constant up to this
horizon. In this unit, the granite does not behave like the altered unit, but
its fracturing index is high and, therefore, its effective hydraulic
conductivity is higher than that of non-deformed granite. Finally, the
non-deformed granitic unit, in contrast to the other units, has a lower
fracturing index and effective hydraulic conductivity. Both the matrix
effective hydraulic conductivity and specific storage decrease with depth.
The upper units (lehm and altered granitic unit) are treated as separate
hydraulic conductivity zones. Constant hydraulic conductivity is also adopted
in the bottom portion of the model (below 350 m; Fig. 7). The linear
relationship proposed by was adopted between the upper and
lower units. Storativity drops linearly by an order of magnitude between the
surface and the bottom (600 m deep), based on the results obtained from the
hydraulic tests.
Main structures implemented in the model as planar structures. They
are defined by 2-D elements. The model honors surface traces and
dips. Downwards extension of these structures is performed with the aid of
geophysics, structural geology, and intersections at boreholes. The NF is the
more vertical one at its upper section, and it is cut and disconnected by
faults 474 and 285 towards the south. The mine is also represented by means
of 2-D elements, because it results from the exploitation of the
dykes. The SF is zonated at the surface, in order to reproduce the altered
zone in which the stream is embedded, where most of the water flows.
The magnified image in the central part of the model (Fig. 6, right) shows
the structures included explicitly in the model. Both matrix zones and
fractured belts are simulated by 3-D elements; fractures are reproduced with
2-D elements (lines in the figure); finally, boreholes are introduced as
1-D elements (points in the figure). Structures included in the
model (Fig. 8) through 2-D elements preserve their azimuth, dip, and
interception points with the boreholes. In general, they are subvertical,
except for the NF, which is slanted on the surface and cut, displaced, and
tilted by fractures 474 and 285. The mine is also treated with 2-D elements,
because it corresponds to the mining of part of dykes 27 and 27' (planar
structures).
Results obtained after calibrating the SR4-1 cross-hole test are
indicated by a black line; observed data are indicated with circles using a
3-D model. All graphs maintain the vertical scale to facilitate comparison of
the results.
Parameters obtained after calibrating the South cross-hole test.
Units are meters and seconds, 1-D features are defined in terms of hydraulic
conductivity and specific storativity by assigning them a 1 m2 cross-sectional area. Hydraulic conductivity values of matrix and fractured belts
change with depth: the first value holds for the upper 250 m (constant
parameter), the second value applies to the bottom of the domain – both for
the matrix and the fractured belts. The storativity of the fractured
belts lehm is negligible in the model (1.0 × 10-30), to
prevent the artifact that water might be withdrawn from that zone. Standard
deviations (SDs) of the decimal logarithms of estimated parameters (not shown for
parameters held constant during calibration) are also presented as a measure
of uncertainty.
Permeability values obtained from interpretation of hydraulic tests
performed at varying scales in different holes and intervals. In general,
permeability increased with the increase in scale, except for the cross-hole
test, which was performed in the highly transmissive portion of the site. See
the text regarding the surprising lack of high K observation during small-scale tests.
Model calibration of the cross-hole South test
Once the conceptual model was built and the numerical model implemented, a
cross-hole test conducted downstream of the mine (South test) was calibrated.
As initial parameters, those obtained from interpretation of the test with
the Theis model were taken. The interpretation model of the south cross-hole
test works with drawdowns; that is, initial drawdowns were zero and all
prescribed fluxes, except the pumping, were set to zero. The model boundaries
were placed at a distance large enough to impose a zero drawdown condition.
Table 1 summarizes the results obtained alter calibrating the test. Note that
fully penetrating boreholes are explicitly included in the model to represent
that they connect fractures.
Blind prediction of the long-term pumping of the mine (point PM),
using calibrated with the SR4-1 cross-hole test. This pumping lasted for 4
months, and its effects reached all observation points. Measurements are
represented by circles, and model results are represented by continuous line.
Figure 9 represents the resulting model fits. The pumping point is SR4-1,
intercepting the SF. Observation points S10 and SR1-3 respond rapidly to the
stress, because they are connected to the pumping point through small
structures that form the fractured belts (simplified in the model as fracture
S10 and fracture SR1-3), which intercept the SF. The remaining observation
points were not that well connected with the pumping point. Pozo Maestro (PM) is the mine
well, whose response (changes in pressure due to the injection test, represented as drawdowns)
is to completely damp the poor connection with the pumping point. Observation points S5 and SR1-2 cut dyke 27' and 27,
respectively, close to the mine. The pumping withdraws the water mainly from
the shallow layer (lehm), draining it through the bed of Maderos stream,
where water flows sub-superficially. Water also comes from the mine and the
altered granitic unit, flowing towards the pumping point through the
fractures (SF, fractured belts, and dykes).
Scale effect
Figure 10 represents the hydraulic conductivities obtained from all hydraulic
tests as a function of the scale. Transmissivities derived from the tests are
divided by the length of the pumped interval to convert them into
conductivities. The scale of the test is determined by the rock volume
affected by each test, which ranges between a few centimeters (pulse tests)
to some tens of meters (South cross-hole test). As mentioned above, the
tested intervals did not coincide for all types of tests. In the case of
pulse, slug, and constant head tests, they were conducted sequentially with
the same interpretation, but they do not coincide with the packed off
intervals for the cross-hole tests, which were selected to isolate the main
structures from the matrix and minor fractures. It is still possible,
however, to compare the calibration parameters with the results of
homogeneous and isotropic models (interpreted before using the Theis model),
as larger scale tests also involve the rock volume of lower scale tests. The
dispersion of hydraulic conductivity values for each test diminished with the
scale, because when the medium is treated as homogeneous, it includes the
hydraulic conductivities of the main structures in the effective
conductivity; i.e., it is more homogeneous. The right panel of Fig. 10 shows
the hydraulic conductivity values obtained after interpreting the South
cross-hole test with a 3-D numerical model. This 3-D model represents the
main features of the medium heterogeneity in an explicit way (fractures,
mine, and different units of the granite, depending on its hydraulic
behavior). The points represented in Fig. 10, for this cross-hole test, do not represent different results at different observation
points, but rather the conductivity of the main fractures, fracturing belts,
altered unit, lehm, and matrix. The fracture transmissivities were
transformed into conductivities by assigning them a unitary width (dividing
by interval lengths). This results in larger conductivity values for the
fractures and dykes, whereas the lower values correspond to the matrix
elements whose conductivity depends on the fracturing index. The most
surprising feature of Fig. 10 is the absence of high conductivity values for
small-scale tests. Even if effective permeability increases with scale, some
high permeability values should have been identified by the small-scale tests
coinciding with the intersection of the fractures that dominate the large-scale behavior of the site (see e.g., ). We
attribute this absence to the fact that characterization tests were performed
during an unusually rainy season. The most permeable zones are located at the
discharge zone of the mine, which caused some of the boreholes to become flowing
wells, making it impracticable to perform a short-scale test for the highest
permeable part. As a consequence, the highest values are not available for
the short-scale tests. In summary, the lower values of hydraulic conductivity
are associated with pulse tests for points situated within the matrix. These
values are consistent with the conductivity estimated for the matrix in a 3-D
heterogeneous model. The increase in hydraulic conductivity with scale is
caused by the contribution of fractures to the effective conductivity
obtained when interpreting the tests using homogeneous models.
Blind prediction
A 4-month pumping test from the mine was carried out with a mean flow rate of
0.0025 m3 s-1. All boreholes were used as observation points
(Fig. 11). As for the interpretation of the south cross-hole test, all
observations were introduced as drawdowns calculated from the measurements
registered prior the start of the pumping (assuming steady state). The model
adopted the calibrated parameters of the analysis of the South cross-hole
test (shown in Table 1), and those obtained from the hydraulic
characterization of structures not involved in the cross-hole tests.
Fractures not affected by any test were assigned the transmissivities that
had been obtained during calibration for fractures belonging to the same
family, according to the geophysical and structural analysis. Specifically,
fractures 474, and Fr4 were assigned the values obtained for fracture 285,
and the SR3 dyke was assigned the transmissivity obtained for the non-mined portion of the
27 dyke. Results of the blind prediction (i.e., without calibration) are
shown in Fig. 11. Overall, the fit of this blind prediction was visually
excellent. Nevertheless, it becomes apparent that the estimated specific
yield for the south cross-hole test model was too high, which may reflect
the fact that the hydraulic characterization of the zone was performed entirely during
the winter and within a wet interannual cycle, so that the entire zone was
fully saturated. In the prediction, it can be observed that most of the water
came from the lehm, which is a very conductive layer with a relatively high
specific yield (3.6 × 10-3), while the altered unit specific
yield was set at 4.6 × 10-6 m-1. This artifact reflects
that water is withdrawn from the altered unit, which is the actual storage
unit in dry periods. It is worth pointing out the good quality of the
responses obtained at intervals SR2-1, SR2-2, SR3-1, and SR3-2, which would
have displayed a much more delayed response than observed and
modeled data (perhaps similar to the response observed in SR4-3 or SR3-3) if the untested
(and only observed through geophysics) fractures had not been included.
Perhaps more important, the slope of the late time drawdown in semi-log is
quite accurate in most observation points, which suggest that the overall
parameters are adequate for this test, even though it lasted some 100 times
longer than the calibration tests.
Conclusions
The main objective of this work was to show that structural geology and
geophysical techniques together with hydrochemical and hydraulic data can
help to identify the main fractures, which carry most groundwater flow.
Values of transmissivity were estimated at different scales by considering
the medium as homogeneous. When plotting these values against the
representative field scale, we observed a progressive increase in
transmissivity with an increase in scale. This scale effect is attributed to
the main fractures because all observations can be explained by incorporating
such dominant fractures explicitly into the model. In fact, this model
yielded an excellent fit to a 4-month long pump test that provoked responses
at all observation points. The fact that the results of such a long pump test
could be blindly predicted using only short-term (< 1 day) tests supports
both the use of seismic geophysics and structural geology to identify the
dominant fractures and their explicit incorporation into the groundwater flow
model (mixed approach). The results are consistent with the hypothesis that
southern European granite plutons display low hydraulic conductivity. This,
together with the low aggressivity of their sodium bicarbonate groundwater,
makes them appropriate for hosting nuclear waste.
Acknowledgements
This work was part of a multidisciplinary project funded by ENRESA (Spanish
Nuclear Waste Management Company), through grant no. 770071. Participating
institutions included AITEMIN (in charge of hydraulic testing), CIEMAT
(chemical sampling), and CSIC-IJA (structural geology). The revised text has
benefited from constructive criticism by Christopher Juhlin and Paul
Hsieh.
Edited by: F. Bastida
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