The risk of groundwater pollution is increasing both from the disposal of water materials and from the extensive use by industry and agriculture of potentially polluting chemicals in the ecosystem.
Groundwater pollution can occur either as discrete, point supplies (e.g. from landfills), or from the wider, more travel 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 consequence from direct or indirect anthropogenic activities. Direct influence occurs as a consequence 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 quantity of groundwater flow in the saturated zone below the water table width of which are determined by climatic and topographic parameters.
The basic physical processes controlling the transport of non-reactive contaminants are advection and hydrodynamic dispersion. Advection is the part of solute movement credited to transport by the flowing groundwater Hydrodynamic dispersion occurs as a consequence of mechanical mixing and molecular spread.
The significance of the dispersive processes is to decline the contaminant concentration with distance from the source. The idea of homogeneous aquifer in which the hydrogeological similarities do not vary in space is a simplification of the real situation in character. Heterogeneities within the aquifer lithology will create pattern of solute movement considerably different from that expected by the theory for homogeneous material.
Reactive substances behave similarly to non-reactive substances, but can also undergoes change in concentration resulting from chemical responses that take place either in the aqueous phase or as a consequence of adsorption of the solute to the substantial matrix of the rock. The chemical and biochemical responses that can alter contaminant concentrations in groundwater are acid-based responses solution precipitation responses, oxidation reduction responses 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 dispensing away from the pollution source. If a pollution supplies 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 complicate indeed. consequently prediction of the pollution front will be very difficult.
In fractured media, aquifer similarities are spacially variable and are controlled by the arrangement and frequency of fractures. Information relating to contaminant migration in fractured rocks is limited. A shared 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 nearby rock matrix. Under this condition, part of the contaminant mass will migrate by molecular spread from the fracture into the matrix, so effectively removing it from the flowing groundwater.