4d monitoring of sea water intrusion by electrical resistivity

GNGTS 2014
Sessione 3.2
4D Monitoring of sea water intrusion by Electrical Resistivity
Tomography: case study in the coastal alluvial plain
of the Volturno River, Italy
D. Tarallo, V. Di Fiore, G. Cavuoto, N. Pelosi, M. Punzo, L. Giordano, E. Marsella
IAMC - CNR, Institute for Coastal Marine Environment, National Research Council, Naples, Italy
Introduction. A common problem of coastal aquifers is saltwater intrusion, induced by
the flow of seawater into freshwater aquifers due to the groundwater development near the
coast. Several factors affect the ingression of sea water. Among these, the most important are
the coastal subsidence, the lowering of the sea level, coastal erosion and excessive pumping
of groundwater��
. In
��
fact, where the groundwater is pumped from coastal aquifers, the induced
gradients may cause the migration of salt water from the sea to the well, making the freshwater
unusable; being the fresh water less dense than salt water, it floats on top.
According to the Integrated Coastal Zone Management (ICZM) of the European Commission,
coastal areas are of great environmental, economic, social and cultural relevance. Therefore,
the implementation of suitable monitoring and protection actions is fundamental for their
preservation and for assuring the future use of this resource. Such actions have to be based
on an ecosystem perspective for preserving coastal environment integrity and functioning and
for planning sustainable resource management of both the marine and terrestrial components.
Planning and management of natural resources through a dynamic process has to set, as its
objective, the promotion of economic and social welfare of coastal zones.
Unfortunately coastal plains are often contaminated by sea water intrusion, and the
vulnerability to salinization is probably the most common and diffused problem in an aquifer.
The boundary between salt water and fresh water is not distinct; the dispersion and transition
zone, or salt-water interface are brackish with salt water and fresh water mixing. Under normal
conditions fresh water flows from inland aquifers and recharge areas to coastal discharge areas
to the sea. In general, groundwater flows from areas with higher groundwater levels (hydraulic
head) to areas with lower groundwater levels. This natural movement of fresh water towards the
sea prevents salt water from entering freshwater coastal aquifers (Barlow, 2003).
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Sea water intrusion in the water table can
cause significant worsening in vegetation
status. A likely related soil salinization would
cause detrimental environmental and socioeconomic impacts. Hence, monitoring the sea
water intrusion represents a priority for the
safeguard of coastal areas. Identify changes
in the freshwater-saltwater interface position
can be a useful element for the rationalization
of water resources and to guide the choice of
use of these areas.
Among the geophysical method applied
for the location and movement of saltwater
intrusion, best results were obtained by
electrical methods (Al-Sayed and AlQuady, 2007; Chitea et al., 2011). Many
hydrological processes can be expected to
provide significant contrasts in resistivity,
consequently,
Electrical
Resistivity
Tomography (ERT) has been adopted as a
Fig. 1 – Location of the study area. Red lines are the 3D
tool for new research within the hydrology
ERT grid.
field. Previous workers have demonstrated
the ability of ERT to visualize hydrological structure within laboratory cores (Binley et al.,
1996a, 1996b), monitor fluid or contaminant migration at the field scale (Daily et al., 1992,
1995; Schima et al., 1993), and to ascertain the efficiency of new contaminant remediation
processes (Daily and Ramirez, 1995; LaBrecque et al., 1996).
This paper outlines the results of a 3D ERT experiment obtained in the coastal alluvial plain
of the Volturno river to assess changes in the freshwater-brine interface. The main aim was
to investigate spatial and temporal variations of groundwater salinity. Acquisitions have been
carried out in the months of May and October 2013 and in May 2014. This acquisition has
allowed to obtain a monitoring 4D salt wedge in a “volume” specific subsurface.
Geological and hydrogeological setting. The investigated area is located in the northern
sector of the Campania Plain, near to the mouth of Volturno river (Fig. 1). It is the main river
in southern Italy: it crosses Molise and Campania regions, for a total length of 175 km with a
watershed of 545 km2. The Volturno is characterized by minimum flow in summer and overflow
in autumn and spring. The area is characterized by a sub-horizontal morphology with main
level similar to the sea level.
The geomorphology of the Campania Plain corresponds to a structural depression which is
formed during the Lower Pleistocene. Until to 130.000 years ago the plain of the Volturno was
regulated by a phenomenon of subsidence (Cinque and Romano, 2001) and fell in a marine
environment. Afterwards, the area was affect by pyroclastic fall-out and flow from Campi
Flegrei and Roccamonfina (Ortolani and Aprile, 1978, 1985).
In general, the stratigraphic sequence of the area is characterized by continental deposits
(Romano et al., 1994; Corniello et al., 2010), a surface layer of silt and clay, an underlying sand
layer and a basal layer characterized by clayey peat and reworked pyroclastics. River-borne
sediments discharged in the sea in the past caused the river mouth to prograde. However, in the
last 150 years, the latter has progressively retreated due to the dams built along the river and the
extraction of gravel from the river bed (Biggiero et al., 1994).
All these sediments have a lenticular pattern that determines a groundwater flow in layered
aquifers. This determines, in conditions of excessive pumping and for a greater thickness of the
sedimentary body that houses the sweet aquifer, a mixture of fresh water with the salt water. In
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addition, the rising salt water also occurs along the bed of the river Volturno as the bottom of the
river is located at the mouth, about 3.5 m below the sea level (Corniello et al., 2010).
Geophysical methods and data processing. The Electrical Resistivity Tomography (ERT)
consists of the experimental determination of the apparent resistivity ρ of a given material, by
joint measurements of electric current intensity and voltage introduced into the subsoil through
separate couples of electrodes, driven in the ground surface. All natural rocks can conduct
electricity when subjected to an electric field. The measure with which the rocks are crossed
through the current depends on the type of resistivity that they present. The resistivity parameter
is influenced by: texture and porosity, degree of cementation, the temperature of the rock, clay
content, water content and its temperature and salinity. Furthermore, under equal lithological
conditions, there are some geological processes that cause an immediate variation of resistivity
because they change the porosity. In general, many of these processes lead to a reduction of the
resistivity as: clay alteration, dissolution, billing rock, saltwater intrusion.
The instrumentation used for the measurement of the resistivity consists of two parts: one
for the measurement of the current intensity I injected into the ground through the electrodes
A and B and one for the measurement of the potential difference ΔV between the electrodes M
and N. In the experimental surveys reported hereafter, the ERT data have been gathered through
electrodes of length equal to 40 cm.
The electrodes were then connected through multichannel cables, adopting the WennerSchlumberger array configuration. This type of arrangement is hybrid between the Wenner
and Schlumberger arrays (Pazdirek and Blaha, 1996): during the acquisition, the wiring is
continuously changed so that the spacing a between the ‘potential electrodes’ remains constant,
while that between the ‘current electrodes’ increases as a multiple n of a. The value of n, in this
case is given by the ratio between the distance of the electrodes A-M (or N-B) and the spacing
between the electrodes of potential M-N. For this array the distribution of the measurements
is comparable with the Wenner array, but the horizontal coverage is better. The choice of such
arrangement was due to the necessity to study areas in which both lateral and vertical variations
of resistivity are present. The resulting horizontal distribution of the underground data points
in the pseudo-section, in fact, is comparable with that typical of the Wenner array, but their
vertical resolution is better. Moreover, this type of array is a fair compromise between the
device Wenner and the dipole-dipole. The intensity of the signal is smaller than the Wenner
but is higher than the dipole-dipole axial. At constant distance between the current electrodes,
the depth of investigation that can be achieved with the device Wenner-Schlumberger is 10%
higher than the Wenner device.
The geoelectric measurements of resistivity were executed with the georesistivimeter
“SYSCAL Pro” of Iris Instrument. Within the chosen area for 4D monitoring of the salt wedge,
geometrically similar to a rectangle of about 4600 square meters (length 115m and width 40
m), the geoelectric surveys performed by acquiring 9 geoelectric profiles. These profiles are
arranged parallel to each other and with a spacing of 5 m and the multi-electrode resistivity
measurements used 24 electrodes, for a total of 216 electrodes. Data acquired have been
processed using 3D inversion technique performed with ERTlabplus software.
Data inversion started from a discretized model of the investigated area, constructed starting
from average apparent resistivities on measured pseudosection. The inversion procedure uses
a smoothness-constrained least-squares routine implemented into Occam’s optimization
algorithm (La Brecque et al., 1996b), which allows determining iteratively a 3D resistivity
model for the subsoil.
Result and discussion. The results of the inversion procedure are three high resolution ERT
3D models in different periods, more specifically in May and October 2013 and in May 2014.
These acquisitions have allowed to obtain a 4D monitoring of saline intrusion in a specific
subsurface “volume”. Resistivity data processing within that volume has therefore defined the
electrical characteristics and geometry of the subsurface going to spatially delimit the intrusion
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Fig. 2 – 3D ERT tomography models determined for
different periods: a) May 2013; b) October 2013; c) May
2014.
Sessione 3.2
Fig. 3 – 3D resistivity contour plot referred to salt water
(resistivity 4-15 Ωm): a) May 2013; b) October 2013; c)
May 2014.
of salt water and the relationship with the sweet water. 3D models obtained by the inversion
also showed seasonal variation (in time) of these relationships. Analyzing the three models
(Fig. 2) it is possible to notice variations of resistivity very small (a) 4,36-31,2 Ωm; b) 4,66
– 30,36 Ωm; c) 7,45 – 32,7 Ωm).The low resistivity zone (Fig. 3), located in the lower and
side part of the models, is interpreted as salt water related to marine intrusion. The middle
sector of the models presents resistivity values compatible with the fresh aquifer. In detail, it
is possible to observe changes between the volumes of brackish water and fresh-water during
the spring and autumn seasons. In particular, the results highlight lower resistivity values in
autumn (October 2013) in the shallow sector, ascribed to an increase of salt intrusion. This
phenomenon is attributed to a lower contribution of fresh water in the aquifer of the plain
due to the reduction of rainfalls during the summer season. The comparison, instead, with
the two resistivity models acquired in May 2013 and 2014 showed no significant changes.
The results of this study have led to a reconstruction of a three-dimensional model of the water
bodies in the areas of flat in order to understand the extent of the phenomenon of saltwater intrusion
in time and space, resulting in qualitative and quantitative analysis of the volume of water used.
This analysis shows that the shallow fresh-water should not be used because the pumping could
determine greater intrusion of sea water, not allowing the accumulation of fresh water necessary
in the natural suction of native species of plants. This monitoring program, supported by a
careful management of resources, would prevent the worsening of the intrusion of the wedge
with a difficult chance to return to the initial equilibrium conditions.
Acknowledgements. The authors wish to thank Dr. Paolo Bonasoni, Scientific Responsible of I-AMICA project
(High technology infrastructure for Integrated Climatic-Environmental monitoring), PON a3_00363.
In addition, the authors wish to thank Paolo Scotto di Vettimo, Michele Iavarone, Dr. Rodolfo Baculo and Dr. Ivan
Granata, for their help during data acquisition.
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