Contaminant Hydrogeology
Not all groundwater contamination problems are caused by people. Natural occurrences of groundwater contamination include iron and managanese which may not be a health hazard but cause other problems that restrict groundwater use. Other contaminants including heavy metals, arsenic, and radioactive nuclides (especially radon), are also naturally occurring and pose more of a health threat. There are also organic compounds from natural sources that can be problematic but are generally a nuisance, or indirectly cause other problems.
Sometimes when naturally occurring contaminants begin to increase, it is the result of changes or increases in pumping of the aquifer, or the source is from one layer in the formation penetrated by the well. Treatment at the well or point of use may be a practical solution but in some cases managing or controlling how and when the aquifer is pumped can alleviate the problem. At times reconstructing wells to avoid strata that are the source of a contaminate is also a solution. A key is understanding the source of the contaminant, whether it is from a specific part of the aquifer or because of a change in geochemical processes at work.
By far the most frequent cause of groundwater contamination is from industrial/commercial sites and includes regulated hazardous waste, UST’s, fuels, mining, and industrial/commercial sites; residential areas can also be a source of contamination. There are also a variety of other wastes, typically high volume, that may or may not be subject to regulation.
One important, and often overlooked aspect, is how the presence of contamination in groundwater triggers changes to subsurface oxidation/reduction or redox potentials. In groundwater redox tends to drift lower because of the increased biological activity triggered by contaminated leachate in groundwater. Organic compounds in particular are a problem because they provide a carbon nutrient source to indigenous bacteria which leads to a drop in redox potentials. Lower redox frequently leads to an increase in native metals going into solution (especially iron) that can adversely affect groundwater usability for the long term.