In some areas, chronic overpumpage can cause salt water intrusion . Salt water intrusion takes place in areas where fresh water removal from the aquifer permits saline water from below to intrude into the aquifer. Most human activities at the land surface cause some change in the quality of water in the aquifer beneath them. The importance of the effect of a particular activity is related to the amounts and types of contaminants released. The severity of an occurrence is also related to the ability of the soil and groundwater system to degrade or dilute the contaminants and the degree to which the contamination will interfere with uses of the water.
Contamination is usually more serious in a drinking water supply than in water for other uses.
Except where contaminated water is injected directly into an aquifer, essentially all groundwater pollutants enter the aquifer through recharge water from the land surface. These are considered harmful if ingested with drinking water and may be carried into surface water bodies by groundwater. Each human activity has a particular impact on groundwater. Some agricultural activities add nitrate nitrogen and pesticides to groundwater. Residential areas with septic systems usually add nitrate nitrogen, bacteria, viruses and synthetic organics used in household cleaning products and septic tank cleaners.