selected "Key Documents", "Useful Weblinks" "List of Abbreviations" and "Key Technical Terms".

All these resources are available at the end of the "Further Description". They differ to the general resources available

from the topic's homepage in that they are selected by the EUGRIS team rather than being posted by users.

Contaminants can migrate directly into groundwater from below-ground sources that lie within the saturated zone.

Or contaminants can enter the groundwater system from the surface by vertical leakage through the seals around well

casings, through wells abondoned without proper procedures, or as a result of contaminant disposal of improperly

constructed wells.

Three processes can be distinguished which govern the transport of contaminants in groundwater: advection, dispersion

and retardation. Dispersion and density/viscosity differences may accelerate contaminant movement, while retardation

processes can slow the movement rate. Some contamination problems involve two or more fluids. Examples include

air, water and organic liquids in the unsaturated zone, or organic liquids and water in an aquifer. Tracers are useful for

characterizing water flow in the saturated and unsaturated zone.

Ecotoxicology is the study of the effects of anthropogenically derived contaminants on individual organisms, species or

ecosystems.

Physician Paracelsus said in the early 1500's "The poison is in the dose". The study of ecotoxicity if further complicated

by various synergistic and antagonistic effects of mixtures of organic contaminants and heavy metals in environmental

media on a site and species specific basis.

Guidelines for assessing threshold levels of different chemicals in different environmental media and ecosystem types

have been developed by countries and international organisations. Similarly numerous tools for assessing the ecotoxicity

of soils, sediments and water are both commercially available and in the development phase.

The further description provides an overview of the chemical processes, interactions and mass transport that govern the

natural chemical composition of groundwater.

The natural chemical composition of groundwater is governed by the interactions that occur between percolating water

and soils in the unsaturated zone and the redox state and interactions between the groundwater and aquifer matrix.

Groundwater begins as rainfall - a dilute chemical solution with a signature of the atmosphere at the time the rain fell. As

rain soaks in and travels through the soil and rock it is filtered and minerals are dissolved from the rock, with the

concentration of solutes generally increasing the longer groundwater remains in contact with the rock. These processes

give groundwater its variable natural quality characteristic of a particular area and aquifer.

Hydrogeology is the study of groundwater – its origin, occurrence, movement and quality. Groundwater moves through

the sub-surface from areas of greater hydraulic head to areas of lower hydraulic head.

The rate of groundwater movement depends upon the slope of the hydraulic head (hydraulic gradient), and intrinsic aquifer

and fluid properties. The ground-water system must be understood in relation to both surface water and moisture in the

atmosphere. Most addition to ground water come from the atmosphere in the form of precipitation.

Microbiology is the study of microorganisms, comprising prokaryotes (eubacteria, archaea) and eukaryotes (fungi, algae,

protists).

Microorganisms are ubiquitous and degrade numerous pollutants, thus responsible for the most important Natural

Attenuation (NA) and Enhanced Natural Attenuation (ENA) process. The use of different electron acceptors during

pollutant degradation in contaminated lands lead to the development of different redox zones, underlining the importance

of anaerobic microbial processes for NA or ENA.

The term modelling in this context refers to the use of computer-based numerical methods to obtain approximate solutions

to the coupled equations of groundwater flow and solute transport. Groundwater flow simulations require an understanding

of geology and the hydraulics of groundwater flow as well as a command of numerical simulation methods.

When solute movement is to be simulated, the complexity of the problem is increased. The description of the flow regime

may not have the resolution needed to support transport analysis; chemical, physicochemical and biochemical mechanisms

must be represented in the governing equations.