IN-SITU AND LABORATORY INVESTIGATIONS INTO CONTAMINANT MIGRATION

Predicting the spread of groundwater pollution demands a detailed unde rstanding of the physical, chemical and microbial processes that contr ol contaminant mobility in aquifers. Many field studies have been carr ied out around pollutant sources in an attempt to understand these pro cesses, but quantitative results are often difficult to obtain because of the number of assumptions that have to be made about the flow regi me or the source term which has given rise to the pollution. Models ca n be constructed with emphases on different processes to describe the known distribution of contaminants at any one time. However, if these models are to be used for predictive purposes, or to help remediation, it is important to identify and quantify individual processes precise ly by independent or direct methods and not to rely on inference alone . Laboratory tests suffer from the fact that aquifer material has to b e sampled and transferred to the laboratory. In the process, the sampl e may be disturbed physically thus altering its porosity, permeability and dispersive properties. It may be oxidised, thereby altering its c hemistry, and the numbers, activity and character of any microbial pop ulation may change. In situ tracer experiments attempt to overcome the limitations of the laboratory by maintaining natural conditions, but at the same time allowing the injection of solute to be accurately def ined and the hydraulic regime to be well controlled and monitored.Exam ples are given showing how integrated laboratory and field approaches have been used to study: (1) organic degradation in a pollution plume resulting from the disposal of industrial wastes and (2) the role of c olloids in transporting radionuclides in an intergranular aquifer.