Groundwater aquifers, importance

A further demonstration of the importance of fundamental properties of both pollutants and water bodies is provided by the behaviour of chemicals upon reaching a groundwater aquifer. Soluble chemicals, such as nitrate, move in the same direction as groundwater flow. A poorly soluble liquid which is less dense than water, such as petrol, spreads out over the surface of the water table and flows in the direction of the groundwater. Poorly soluble liquids which are denser than water, such as various chlorinated solvents, sink below the water table and may flow separately along low permeability layers encountered at depth in the aquifer and not necessarily in the same direction as that of the overlying groundwater. ... 

Catalysis of hydrolytic reactions may also occur by surface-bound metals. Although there has been a greater focus on the study of metal catalysis by dissolved metal ions, there is increasing evidence to suggest that catalysis by surface-bound metals may be of greater importance in environmental systems such as groundwater aquifers. Stone (1989) postulates 3 mechanisms for catalysis at the mineral-water... 

Multiple pathways are a major concern since depostion of PIC would have occurred. Specific soil conditions determine attenuation rates of penta PIC leachate. Once penta reaches the water table, other transport and fate processes become important. Penta exists in two forms ionized and non-ionized. The ionized form is soluble in water, while the non-ionized form is not. The ratio of the two forms in water is dependent on the pH of the aquifer. In alkaline environments penta PIC tend to be more soluble and more susceptible to advective transport and biological decay. Half-lives of penta leachate in groundwater have been estimated ranging from 27 days to 58 years. 

Summary. U isotope compositions potentially can provide an important tool for tracing groundwaters from different aquifer conditions. However, clear interpretations of (234u/238y) quantification of the responsible processes, remain difficult. 

For the confined aquifer, the pressure head becomes more important than the elevation head. As can usually be seen in an artesian aquifer condition, the groundwater may flow from a lower elevation to a higher elevation if the water pressure at the lower elevation is higher. 

Important issues in groundwater model validation are the estimation of the aquifer physical properties, the estimation of the pollutant diffusion and decay coefficient. The aquifer properties are obtained via flow model calibration (i.e., parameter estimation see Bear, 20), and by employing various mathematical techniques such as kriging. The other parameters are obtained by comparing model output (i.e., predicted concentrations) to field measurements a quite difficult task, because clear contaminant plume shapes do not always exist in real life. 

An important anomaly in respect of subsurface pesticide mobility is the fact that some otherwise strongly sorbed compounds can be mobile when in the sorbed phase attached to colloidal minerals. This phenomena has been little investigated to date but could be the explanation for the presence of some otherwise highly immobile compounds in groundwater samples, especially from fissured aquifers. 

Water and hydrocarbons occurring together, in shallow aquifer systems, may be considered immiscible for flow calculation purposes however, each is somewhat soluble in the other. Since groundwater cleanup is the purpose behind restorations, it receives greater attention. Definition of water quality based on samples retrieved from monitoring wells relies heavily upon the concentration of individual chemical components found dissolved in those samples. An understanding of the processes that cause concentration gradients is important for the proper interpretation of analytical results. 

Aquifer restoration planning should be based on the degree of restoration required, and a strategy developed based on an evaluation of the risk presented to public health and the environment. This is important since complete aquifer restoration is rarely accomplished despite over two decades of intense groundwater remediation research and activities. Aquifer restoration requires several objectives to be accomplished contaminant plume containment, source containment, contaminant plume removal, and source zone removal. 

Several important parameters control aquifer bioremediation projects. These include hydraulic conductivity, soil structure and stratification, groundwater mineral content, groundwater pH, temperature, microbial presence, and bench-scale testing, as further discussed below. 

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