A variety of human activites stemming from agricultural, industrial, community and residential sources, as well as natural processes can contaminate ground water. Sources of contamination are referred to as point or nonpoint sources. Point sources are localized in areas of an acre or less, and nonpoint sources are dispersed over broad ares.

Agricultural sources of contaminants include the use and storage of fertilizers and pesticides and the disposal of animal and agricultural waste. Contaminants enter ground water from industrial sources such as poor housekeeping practices in the handling and transporting of materials, and the use of surface impoundments to store, treat, and dispose of wastewater and liquid wastes. Mining operations, leaking underground storage tanks, and improperly managed hazardous waste sites are significant sources of groundwater contamination. Community and residential waste disposal, including septic systems and improper storage and disposal of chemicals in our homes also contribute to ground water contamination. A major cause of groundwater contamination comes from residential effluent, or outflow from septic tanks and cesspools. Finally, natural substances found in rocks or soils such as iron, manganese, chlorides, flourides, and sulfates can become dissolved in and contaminate groundwater.

Shallow injection wells are a primary route of contamination from agricultural, industrial, and community sources. Shallow injection wells are typically "low-tech" holes in the ground that rely on gravity to drain or "inject" liquid wastes into the ground.

Shallow injection wells are mainly used for drainage, discharge, or disposal of unwanted fluids. The broad definition of an injection well, which is basically any hole in the ground whose depth is greater than the largest surface dimension, brings a number of underground systems under this classification. Except for disposal systems serving single families or systems serving less than 20 persons, sumps, septic systems, cesspools, and drain fields are classified as injection wells. Misuse of these systems for disposal of anything other than domestic or sanitary waste can pose a substantial threat to ground water.

The simple construction of shallow injection wells provides little or no protection against possible ground water contamination. Shallow injection wells often discharge waste fluids into or above drinkable ground water which can then become polluted as contaminants percolate through the soil and eventually to the aquifers below. This practice may not only upset the natural recharge volume to aquifers, but may also introduce both thermally and chemically distinct fluids into the ground water resources which could endanger human life and/or the safety of the ecosystem which depends upon the health of the impacted aquifer. In general, shallow injection wells are usually located in rural and/or unsewered areas where people depend on ground water for their drinking water.

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