Just because you have a well that yields plenty of water doesn’t mean you can go ahead and just take a drink. Because water is such an excellent solvent it can contain lots of dissolved chemicals. And since groundwater moves through rocks and subsurface soil, it has a lot of opportunity to dissolve substances as it moves. For that reason, groundwater will often have more dissolved substances than surface water will.
Even though the ground is an excellent mechanism for filtering out particulate matter, such as leaves, soil, and bugs, dissolved chemicals and gases can still occur in large enough concentrations in groundwater to cause problems. Undergroundwater can get contaminated from industrial, domestic, and agricultural chemicals from the surface. This includes chemicals such as pesticides and herbicides that many homeowners apply to their lawns.
Major contamination of groundwater by winterizted road salt is of great concern in the northeast regions of the United States. Salt is spread on roads to melt ice, and, with salt being so soluble in water, excess sodium and chloride is easily transported into the subsurface groundwater. The most common water-quality problem in rural water supplies is bacterial contamination from septic tanks, which are often used in rural areas that don’t have a sewage-treatment system. Effluent (overflow and leakage) from a septic tank can percolate (seep) down to the water.
Contaminants can be natural or human-induced and are present in the rocks and sediments. As groundwater flows through sediments, metals such as iron and manganese are dissolved and may later be found in high concentrations in the water. Industrial discharges, urban activities, agriculture, groundwater pumpage, and disposal of waste all can affect groundwater quality. Contaminants from leaking fuel tanks or fuel or toxic chemical spills may enter the groundwater and contaminate the aquifer. And most importantly, pesticides and fertilizers applied to lawns and crops can accumulate and migrate to the water table.