Groundwater hydraulics covers the flow of water in aquifers and involves, more generally, the flow and transport processes in porous media. Water management and the environment are main topics of concern. Groundwater is part of the hydrological cycle and represents a complex ecosystem which needs a multidisciplinary understanding. Groundwater constitutes the most available freshwater resource of the earth, as much as two orders of magnitude larger than the total water volume of rivers and lakes. Because it is less prone to pollution than surface waters, it is most important as a drinking water resource.
Groundwater systems vary greatly, depending on the geological formation of the aquifer (sand and gravel aquifers, fissured rock aquifers, karstic aquifers). Their hydraulics are characterised by large water bodies, very small flow velocities and hence extremely long exchange times. Transport at the regional scales is affected by local heterogeneities of the geological formations, and is governed by advection and molecular diffusion at pore/grain size scale. Chemical reactions of pollutants and bacterial life take place at the pore/grain size scale. Groundwater is recharged by rainfall, when it percolates through the unsaturated zone to the aquifers, and by infiltration of surface water. Across its open boundaries, groundwater can be polluted by local sources of accidentally released contaminants or by pollutants released from diffusely distributed sources like agricultural activities.
Groundwater contamination has developed into one of the key environmental issues in most industrialised countries. Industrial contamination sources include waste sites, leakage, accidental spills and leaking septic tanks. Liquid contaminants (e.g., oil products or halegonated hydrocarbons), which are not miscible with the water can exist underground (in the unsaturated and in the saturated zones) as non-aqueous phase as well as dissolved in the water, adsorbed at the solid phase, and mixed in the soil-air. In such cases the subsoil is a multi-phase system. Agricultural contamination sources are fertilisers and the use of herbicides and pesticides. Air pollution contributes by depositing contaminants that make their way into the groundwater.
Groundwater management: Acute problems for groundwater management are overexploitation, lowering of groundwater tables, water deficits, and water pollution. The evaluation and development of sustainable water resources systems remain important topics in groundwater management. Improved land use management, needed to increase recharge by reducing runoff and evaporation, is currently being studied especially in arid climates. Further problems result from land subsidence caused by overexploitation of groundwater resources. In coastal areas, problems of salt water intrusion are important. Furthermore, groundwater can be used to store and retrieve heat for cooling and heating. However any groundwater management activity has to be based on an adequate description and thorough prospecting. Improved methods have to be developed.
Groundwater monitoring: Monitoring groundwater quantity and quality: Careful monitoring of aquifers, including estimates of discharges in groundwater flows and storage, can allow early recognition of chemical spills and biological activities and enhance timely countermeasures. Remote sensing methods, using new sensors like radar, are being developed for estimates of the soil water balance.
Groundwater remediation: A re-establishment of groundwater quality of polluted aquifers requires the clean-up of contamination in the saturated and the unsaturated zones. The mechanisms of multiphase flow, and the transport, mixing and mass transfer in the multiphase system ”subsurface” are not sufficiently understood and the efficiency of different clean-up techniques is difficult to assess. Therefore, there is a need for basic work concerning the understanding and the mathematical description of the processes, as well as applied research concerning the developed and improvement of remediation techniques and the construction of effective hydraulic barriers for polluted sites.
Political decisions on land use: Groundwater is often considered as a secondary issue in decisions regarding land use in highly populated and industrialised countries. However, the issue of groundwater protection has to establish its value among the conflicting interests of infrastructure, industry and agriculture.
Risk and uncertainty analysis in the decision-making process for sustainable development: The risk of groundwater pollution is ubiquitous, and it is often characterised by extremely long remediation and recovery times. Improved assessment of the vulnerability of groundwater resources is critically important.
Underground space use: Caused by an increased demand for the use of underground space for construction, the development of suitable geotechnical techniques that ensure both quantity and quality of the adjacent groundwater is needed.
Unsaturated Zone: The zone of groundwater table fluctuations is characterised by frequent changes between saturated and unsaturated conditions. Of particular significance are areas such as flood plains or wetlands. Within the unsaturated zone, air is present as a third phase, leading to aerobic processes. It can form an important barrier that prevents pollutants from entering the groundwater. Although models of transport and reaction in the upper soil layer have been developed, they are not sophisticated enough to be able to predict with confidence the impact of new agricultural practices on the watershed. Improper irrigation can cause salination problems. Direct effects of changes in groundwater tables upon vegetation and the terrestrial ecosystem can be observed and should be better understood.
Exchange with surface water: Groundwater is related to the surface terrestrial and aquatic ecosystems in many ways. The directions of the fluxes of water and pollutants vary with the levels of surface water and groundwater, depending on hydrological conditions. These fluxes are controlled by the state of the river bed or lake bottom, which can be clogged by filtering, biological growth or previous chemical reactions. Alternatively these areas can be cleared by a flood with the consequence that pollutants stored in the sediment are released. This interface is a crucial zone where processes like absorption and decay of pollutants need further study.
Coupled flow, transport and bio-geo-chemical processes: In order to model non-conservative contaminant transport, it is essential to couple chemical and biochemical phenomena with flow and transport models. The approach taken by chemists and biologists is to understand the microscopic reactions (redox reaction, sorption, precipitation, biodegradation). The kinetics of the reactions can be controlled either by the actual thermodynamic reaction or by the transport mechanism in the porous media which bring the reactants into contact with one another. It is therefore essential to understand the exact coupling of flow and transport and of bio-geo-chemistry. Aquifers as bacterial ecosystems are poorly described. More effort is needed to understand the dependence of the nature and abundance of bacteria upon factors like temperature, water movement, distribution of organic matter, chemical composition and physical properties of the aquifer. The interaction of these micro-organisms with the chemistry of the groundwater has to be further studied.
Upscaling: The rules and laws that can be used to upscale flow and pollutant transport still need to be further developed, both in terms of the constituent equations and the relevant parameters which have to be measured in the field. Spatial variability of flow and transport parameters and bio-geochemical properties in natural media can be very large. Major multidisciplinary field experiments have to be carried out to study this upscaling.
Mathematical modelling: The purpose of mathematical models is not only to provide instruments to forecast or estimate processes in a real system, but also to relate sub-processes and concepts, e.g., found in laboratory, and to transfer concepts from one system to another. Stochastic approaches are needed to describe the heterogeneity of the aquifers and to develop parameter estimation methods which allow the models to be calibrated against field observations. These issues are addressed by estimating the variance of the predictions or by conditioning based on measured values. Therefore, adequate models should be formulated.