The risk of groundwater pollution is increasing both from the disposal of water materials and from the widespread use by industry and agriculture of potentially polluting chemicals in the environment.
Groundwater pollution can occur either as discrete, point sources (e.g. from landfills), or from the wider, more diffuse use of chemicals, such as the application to and fertilizers and pesticides and the deposition of airborne pollutants in heavily industrialized regions.
Changes in groundwater quality may result from direct or indirect anthropogenic activities. Direct influence occurs as a result of the introduction of natural or artificial substances derived from human activities into groundwater. Indirect influences are those changes in groundwater quality caused by human interference with hydrological, physical and biochemical processes, but without the addition of substances.
The main contaminants of groundwater are heavy metals, organics chemicals, fertilizers, bacteria and viruses. The enormous range of contaminants encounter in groundwater reflects the wide range of human economic activities in the world. The major activities generating contaminants are associated with agricultural, mining, industrial and domestic sectors.
Transport Of Contaminants In Groundwater
The subsurface movement of contaminants is influenced by the moisture content of the unsaturated zone and the volume of groundwater flow in the saturated zone below the water table width of which are determined by climatic and topographic parameters.
The fundamental physical processes controlling the transport of non-reactive contaminants are advection and hydrodynamic dispersion. Advection is the component of solute movement attributed to transport by the flowing groundwater Hydrodynamic dispersion occurs as a result of mechanical mixing and molecular diffusion.
The significance of the dispersive processes is to decrease the contaminant concentration with distance from the source. The idea of homogeneous aquifer in which the hydrogeological properties do not vary in space is a simplification of the real situation in nature. Heterogeneities within the aquifer lithology will create pattern of solute movement considerably different from that predicted by the theory for homogeneous material.
Reactive substances behave similarly to non-reactive substances, but can also undergoes change in concentration resulting from chemical reactions that take place either in the aqueous phase or as a result of adsorption of the solute to the solid matrix of the rock. The chemical and biochemical reactions that can alter contaminant concentrations in groundwater are acid-based reactions solution precipitation reactions, oxidation reduction reactions ion pairing of complexation microbiological processes and radioactive decay, adsorption attenuates, or retards a dissolved contaminant in groundwater.
The processes of advection, dispersion and retardation all influence the pattern of contaminant distribution away from the pollution source. If a pollution sources contains multiple solutes and occurs within a heterogeneous aquifer than there will be a number of contaminant fronts and the morphology of the
resulting plume will be very complex indeed. Consequently prediction of the pollution front will be very difficult.
In fractured media, aquifer properties are spacially variable and are controlled by the orientation and frequency of fractures. Information relating to contaminant migration in fractured rocks is limited. A common approach in field investigations is to treat the problem as if it were a granular medium. When contaminant concentration between the mobile groundwater in the fracture and the static water in the adjacent rock matrix. Under this condition, part of the contaminant mass will migrate by molecular diffusion from the fracture into the matrix, so effectively removing it from the flowing groundwater.