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Drinking water can become contaminated from natural sources or from numerous types of human activities. Residential, municipal, commercial, industrial and agricultural activities can all affect surface and groundwater quality. Contaminants may reach water sources from activities on the land surface, such as industrial waste storage or spills; from sources below the land surface but above the water table, such as septic systems; and from structures beneath the water table, such as wells. Some of these sources are discussed in the next few pages.