Project objectives:
The effort to establish an assessment methodology integrates analysis of the following:
Characterization of aquifer heterogeneities, with special emphasis on key features such as low permeability lenses and high permeability fractures
Quantification of aquifer heterogeneity structures, with consideration of:
- reconstruction of three-dimensional structures based on two-dimensional maps
- dynamic (evolving) physico-chemical changes (including deformation, fracture growth, clay swelling, precipitation and dissolution)
Quantification of fluid flow and salt/chemical transport, with consideration of:
- fingering and density-driven flows
- sharp interface vs. mixing zone approximations (saltwater/freshwater interface)
- reactive transport (adsorption, precipitation and dissolution)
- Fickian and non-Fickian transport behaviour
Laboratory and field scale flow and tracer tests, and geophysical monitoring.
Development of analytical and numerical models for fluid flow and salt transport, which account for the above conceptual analyses and laboratory and field
scale experiments.
Integration of these characterizations, concepts and models into an overall environmental risk assessment methodology for saline intrusion into aquifers
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Achieved Objectives:
SALTRANS contributed enhanced basic understanding of issues related to characterization and quantification of aquifer heterogeneity, and of flow and transport
processes within such systems. SALTRANS used these results to formulate and test innovative methods, combining laboratory, field and modelling studies.
These methods were integrated into tools for quantification and prediction of saline intrusion patterns, with specific emphasis on providing means to more
accurately manage exploitation of coastal and inland groundwater resources. Scientific achievements are described according to the methodology adopted:
ü Characterization of heterogeneities and field sites: SALTRANS has extensively investigated three experimental field sites. One study site was established
at Wilfholme, in the Chalk aquifer of the Holderness Peninsula, E. Yorkshire, UK. Here, effects of both ancient and modern salt intrusion affect the utilization
of the chalk aquifer. A second field site was established in a granitic formation at Ploemeur, in Brittany, France, and a third site was developed together with
the ALIANCE project in a coastal region of Mallorca, Spain. At these sites, boreholes were drilled and instrumented, together with existing boreholes. Data
were collected from observations of field exposures, cores, geophysical logging (including acoustic imaging, natural gamma ray, neutron-neutron, resistivity,
fluid temperature, fluid conductivity and flow logging), and hydraulic and tracer tests. Other studies were carried out using data from the Llobregat delta
in Barcelona, and at various sites in Uzbekistan and Botswana. These study sites represent a cross-section of typical geological formations, with representative
saline intrusion problems. These studies yielded databases of information and means to characterise and quantify heterogeneities at different scales.
ü Modelling heterogeneity, flow and chemical transport: SALTRANS has developed theories and accompanying numerical models to simulate fracture formation
and propagation, and to quantify fluid flow and chemical transport in heterogeneous formations. These methods were further developed to allow efficient
incorporation of different types of data in modelling saline intrusion. The influence of different geological and hydrogeological settings on fluid flow
and saline transport was analyzed. These tools and results enable identification of the key hydrogeological parameters that influence saline intrusion
processes. It was found that fracture connectivity, as a function of scale and resolution of measurements, and other high- and low-permeability features,
play a critical on overall transport behaviours. Moreover, for modelling transport, there is often a subtle yet critically important interplay among these
different features.
ü Laboratory and field experiments: In a series of laboratory scale experiments, SALTRANS has demonstrated key features controlling flow and transport
behaviours in heterogeneous systems. Experiments at the scale of single fractures, in core samples, and in meter-scale flow cells also provided an integrated
set of measurements for use in investigation of upscaling procedures, and for testing of numerical models developed in SALTRANS. Some of these experiments
were representative of the key features in the experimental field sites, and aided in design of flow and tracer tests carried out at the field sites. It has been
demonstrated that such small-scale experiments are indeed capable of mimicking transport behaviours observed at much larger scales. A major advantage of
such experiments is that boundary and initial conditions, as well as the system heterogeneity, are well defined and controlled.
ü Implications of site assessment and monitoring: The field sites investigated by SALTRANS are representative of a wide range of aquifer systems in Europe.
All of these aquifers are currently suffering from, or under threat of, intrusion of saline water. The integrated use of laboratory-scale models, theoretical
analysis, and numerical simulation of the formations themselves, and flow and transport phenomena with them, allows assessment of monitoring, management
and possible remediation strategies within the studied field sites. The resulting assessment methodology, completed in the SALTRANS project, is applicable
to the studied field sites, as well as to the many other similar sites throughout Europe and the world. Results of SALTRANS have led to understanding of the key
features that control saline intrusion, and to identification of the data that are required in any field analysis. Specific, calibrated numerical models
have been completed for the various field sites, and another model which accounts for density-dependent flows has been applied to several smaller sites.
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Dr Stefan Gödeke