Untreated non-saline groundwater is normally much safer to drink than any untreated surface water, since the ground itself provides an effective purifying medium; such water constitutes a major source of drinking-water supplies. Nevertheless, groundwater can be polluted by domestic and industrial waste waters, by soluble materials leached out of tips for municipal refuse or industrial waste, by accidental spillage of other liquids, especially oil, and by intrusion of highly saline water.
Groundwater in its percolation through soil and rocks, leaches out soluble salts; it is thus typically mineralized, and sometimes heavily so. The vulnerability of groundwater to contamination is determined by the hydrological setting of the aquifer, the nature of the contaminant and the effectiveness of regulatory action. Of all the activities of man that influence the quality of groundwater, agriculture is probably the most important, as a diffuse source of pollution from fertilizers, pesticides and animal wastes. Industrial wastes include a wide spectrum of materials from all types of industry, and contain many organic and inorganic chemicals which are potential pollutants. Industrial wastes reach groundwater from impoundments or lagoons, spills, pipeline breaks and land disposal sites.
Septic tanks and cesspools contribute filtered sewage effluent directly to the ground, and are the most frequently reported sources of groundwater contamination, especially in rural, recreational and suburban areas. In many areas, the solid residual material known as sewage sludge - which contains a large number of potential contaminants - is spread on agricultural land. In some regions liquid sewage that has not been treated or that has undergone partial treatment is sprayed on the land surface. Such application of liquid sewage and sewage sludge to the land provides valuable nutrients such as nitrogen and phosphorus to the soil, with benefits to agriculture. However, the waste water or sludge can add to the contamination of groundwater. The soil profile shows a considerable ability to remove or detoxify several of the compounds found in the waste water, but some may nonetheless affect groundwater quality. The soil may also effectively eliminate the pathogenic bacteria through filtration and soil microbiological processes, but survival of viruses is still an open question.