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1
Department of Environmental Engineering and Physics
University
of Basilicata, Potenza, Italy
2
Soil Conservation Department, University of Calabria, Cosenza, Italy
3
Institute of Advanced Methodologies of Environmental Analysis,
National
Research Council, Tito (PZ), Italy
*Corresponding
address: via della Tecnica, 3 - 85100 - Potenza (Italy)
Tel.
+39-0971-474681, fax +39-0971-56537, e-mail telesca@unibas.it
A
geophysical survey, based on geoelectrical and self-potential methods, was
carried out in a test site located in Montalto Uffugo (Cosenza, Southern
Italy), during the injection of a natural tracer. These prospecting techniques
allow us to outline the complex geological features and the hydrogeological
structure of the investigated area. Furthermore, the application of ultra-high
resolution tomographic techniques gives
a contribute to correlate the electrical resistivity values to hydraulic
parameters for the study of the transport processes of pollutants in
groundwaters under controlled conditions.
Keywords: Water pollution,
transport processes, geoelectrical survey
In recent years
many researches have been performed to
improve theoretical and numerical models of mass transport in heterogeneous
aquifers in the framework of monitoring activity in contaminated areas. Furthermore,
great attention has been devoted to the application of monitoring techniques
able, on one side, to provide helpful
information about the study of hydraulic and transport flux processes and, on
the other side, to be linked with the available hydraulic models of simulation
and forecast.
The geophysical
prospecting methods, in particular, seem to satisfy such requests and can be
considered as a powerful tool to better define the spatial variability of
hydraulic parameters and to observe the dynamics of pollutant plumes. More than any other geophysical technique geoelectrical methods are
directly affected by the presence of conductive pore fluids in the subsurface.
The application of electrical methods
had particularly wide use for groundwater exploration, for mapping and
monitoring salt-water incursion in susceptible aquifers and transport processes
of contaminants (LaBrecque, 1989, Berryman and Kohn, 1990, Daily et al., 1992,
Lapenna et al., 1992, Ramirez et al., 1993, Daily et al., 1995; Dahlin, 1996).
In recent years, electrical
resistivity surveys (DC) have progressed from the conventional vertical
soundings to techniques which provide two-dimensional and even
three-dimensional electrical images of the subsurface. This development started
with the introduction of geoelectrical tomography field systems and was soon
followed by post-processing and inversion software to transform the observed
apparent resistivity in real resistivity (Loke and Barker, 1992). In the study
of transport processes and/or hydraulic flows, the DC pseudosections are
generally integrated with self-potential (SP) profiles and/or maps (Coppola et
al., 1994; Di Maio et al., 1995).
In this framework an intensive survey activity in the groundwater experimental site belonging to the Soil Conservation Department of University of Calabria, located in Montalto Uffugo (Cosenza, Southern Italy) has been carried out. The aquifer concerned with the test site is controlled by a system of wells and piezometric sensors.
Fig. 1
Test site. 1) Hydraulic great models
laboratory; 2) office; 3) water analysis laboratory; 4) office; 5) investigated
area 6) wells (dot triangles) area. Geoelectrical tomography profiles are
indicated with the letters AA', BB' and CC'; SP profiles have been carried out
along AA' line.
The investigated area is located
close to the Department of Soil Conservation (University of Calabria), Southern
Italy (fig. 1). From a geological point of view, the investigated area
represents a valley of recent formation with alluvial, conglomeratic and sandy
deposits. In particular, the direct soundings carried out in the wells area
identified the presence of a thick layer of silty sand (from 11 to 40 m below
the earth surface) covered by an alluvial layer (from 0 to 7 m) with the
interpolation of a thin layer of clay (from 7 to 11 m). From this, we have the
presence of a local shallow perched groundwater in the alluvium, sustained by
the clay layer, and a deeper confined groundwater in the silty sand layer. The
aquifer concerned with the test site is controlled by a system of wells and
piezometric sensors.
Of the 11 wells and
piezometric sensors, ten are organized as five monitoring pairs: a central pair
plus four pairs around the former, ranging 10 meters from it and located along
two perpendicular directions, matching groundwater flow main direction and the
right-angled one. Each monitoring pair includes: a) a piezometric sensor, 8
meters in depth down to the thin layer of clay, which can be used for
hydro-geological field tests interesting with the local shallow perched
groundwater system; b) a deep well, 40 meters about, which can be used for
hydro-geological field tests interesting with the confined aquifer (Troisi et
al., 1996). The eleventh well, 57 meters about in depth reaches the
clay bottom of the confined aquifer. It is located downstream 19 meters away
the well of the central monitoring pair, along the groundwater flow main
direction (Fig. 2).
Fig.2
Schematic section of test site with the wells and the piezometric
sensors.
A set of pumping tests,
performed in the site a few years ago, enabled a reliable definition of the
site main hydro-geological parameters (Troisi et al., 1993). In addition, a
careful geophysical investigation, including conventional electrical and
seismic soundings, was performed some years later, achieving new information
about the site hydro-geological characterization and a cross-validation of the
previous studies (Troisi et al., 1995). Recently, the application of advanced
geostatistical techniques to those geophysical data, has allowed the extension
of the hydro-geological parameter definition to a much wider spatial domain
(Troisi et al., 1996).
Aiming to better define the
hydro-dispersive parameters of the groundwater system interested by the above
said experimental site, in past time many tracer tests were carried out, either
under natural flow conditions and under well pumping influence. Hereinafter the
last tracer test, started on July 6th 1998, is referred. Four
hundreds liters of saltwater solution with a concentration of 200 g/l of NaCl
were injected into the confined aquifer through the well nr. 1, while the
central well nr. 5 was pumping 2,3 l/s of water. Solution injection took 15
minutes, the entire test took 10 days about. Hence, the injection may be
considered an impulsive one. Furthermore, in the injection well a water
re-circulation facility carried out continuos mixing of the solution, assuring
the homogenization of the NaCl concentration along the well column. Figure 3
reports the NaCl concentration values vs. time recorded in the confined aquifer
by groundwater sampling at the pumping well.
Fig.3
NaCl concentrations values vs. time recorded in the tracer test.
A
geophysical survey, based on geoelectrical prospecting methods, has been carried out in the investigated area before
and during the tracer test. In particular the geoelectrical dipole-dipole pseudosection technique is integrated with self-potential profiles (Parasnis,
1986).
Technically
during a geoelectrical dipole-dipole pseudosection the electric current is sent
into the ground via two contiguous electrodes x meters apart, and the potential
drop is measured between two other electrodes x meters apart in line with
current electrodes. The spacing between the nearest current and potential
probes is an integer n times the basic distance x. In surveying, several
traverses are made with various values
of n. The value of the apparent resistivity for each traverse are assigned,
along a horizontal axis, at the intersections of two converging lines at 45
degrees from the center of the current dipole and the center of the measuring
dipole. By choosing geophysically significant apparent resistivity classes or
variable contour intervals, one may consider the pseudo-section as a first
tomographic image of the subsurface structure.
SP data are
collected in the field as potential drops across a passive dipole, normally
consisting of a pair of grounded electrodes, moving along a selected direction.
The phenomenon which originated the self-potential anomalous field of interest
in the hydrogeological study, is known as electrofiltration. Basically, it
consists of the generation of an electric field due to the movement of
underground electrolytic waters in a porous permeable system. When an
electrolytic solution moves across a porous membrane a potential difference is
generated between the opposite sides of the membrane. The porous media can be
considered membranes, when they have porosity in the form of a dense network of
capillaries, which the underground waters can permeate. The walls of the pores
have the power of adsorbing only one type of ions, which in turn attract all
around them mobile ions of opposite signs thus forming a stable electric double
layer. The flow of water thus produces a net separation of charges which is
seen in the form of an anomalous SP field.
Furthermore, the injection of a inorganic natural tracer produce an
increase of ionic concentration and, as a consequence, an anomaly behaviour in
SP profiles.
In order to determine the
groundwater electrical characteristics in absence of pumping operations and
thus in steady state conditions, three pseudosections were carried out in
different directions (AA', BB' and CC'): the first one in the same direction of
the underground average water flow (E-W), inferred by direct soundings, and the
other one perpendicularly oriented to the first one (Fig.1). For the sake of
brevity, we report only the pseudosection in direction AA': the enlarged upper
portion (spacing 5m) gives us information about the shallow groundwater, while
the lower part allows us to describe the deeper groundwater (Fig. 4). The
maximum investigation depth was 70 m and all the apparent resistivities
measured values vary in the range 20-80 Ohm×m, showing
very slow fluctuations. This regularity in the values of apparent resistivity
in the shallow and deep groundwaters, is confirmed by the information obtained
from the above mentioned sample log.
Moreover some preliminary results, relating to a continuous monitoring activity carried out during the pumping and the tracer tests are reported and discussed. Firstly, in order to remove the outliers and the possible noise due to the presence of high/low resistivity blocks (pipelines, walls, etc...) a preliminary filtering procedure of the measured data has been performed. In figg. 5 and 6 two resistivity pseudosections along the line AA' are depicted, the first performed before the injection of natural tracer and the second one three days after later.
From the analysis of the first
tomography (Fig.4) is possible to point out
lower apparent resistivity values respect to the pseudosection in steady
state condition (Fig.3) for the
influence of pumping operations. Moreover, observing the second one (Fig.5) we
note a slowly variation of apparent resistivity values probably due to the
presence of the tracer. The difficulty to highlight an evident plume
could be related to the water flow, induced by the well pumping operations, and the very low resistivity values of
subsurface system before the start-up of
injection and pumping operations.
On the contrary, observing the SP
profiles (Fig.7) measured before, during and after the injection along the AA'
line, this ambiguity is not evident.
Before the salt injection, the SP profiles (dot lines) have a time
invariant constant shape related to subsurface resistivity distributions and to
the water flow. Afterwards, the SP profiles (dashed lines) show a more
irregular behaviour and increased
values near the injection point.
Fig. 4 Geoelectrical tomographies related to
the profile AA'.
Fig. 5 DC resistivity pseudosection related
to the profile AA' carried out before the injection of the natural tracer.
Fig. 6 DC resistivity pseudosection related
to the profile AA' carried out after the injection of the natural tracer.
These first preliminary results
allow us to outline the main geological settings of the investigated area and
to test the possible application of DC
and SP techniques in the monitoring activity of contaminated aquifers. The application of these combined
techniques has been able to detect the presence of a natural tracer in the
aquifer, but a clear identification of the plume was not possible. To better
characterise the spatial distribution and the time evolution of the plume the
authors are applying 2D inversion
methods for DC and SP measurements and a multi-channel system (128 electrodes)
in real time. Moreover in future geoelectrical measurements will be used to analyse and
define the correlation between subsurface electrical and hydraulic properties
of the site. Then, it will be possible to compare the latter correlation
results with those obtained in previous studies, on the basis of conventional
geo-electrical soundings carried out in the same site and, therefore, to
improve subsurface hydraulic and transport processes modelling.
We
would like to thank Professor S. Troisi, and Dr. S. Straface for useful discussions and suggestions about
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