PRIVATE WATER SYSTEMS - Completion - Sanitary Protection

Sanitary Protection

Any water well design should be aimed at the extraction of high quality ground water without contaminating it in the process. Locating and sealing the well properly, as well as limiting the sources of pollution should help keep the quality of ground water high.

Locating the Well

Locating a well too close, or downslope from, a pollution source invites pollution of the well. Wells should be located on the highest ground possible and far enough from the pollution sources to insure that contaminated water flowing from the source will not intercept the well's zone of contribution (zoc). Information concerning the general direction of ground-water flow and the zoc of the well can be supplied by the driller or can be determined from well pump tests. See (12) for more on determining the direction of ground water movement.

Most states recommend that wells be sited at least from 100 to 150 feet from a pollution source. Increase that distance in sandy soils. For more specific recommendations, contact the state or county health department.

Sealing the Well

The penetration of a water-bearing formation by a well provides two main routes for possible contamination of ground water by both surface and subsurface sources. These are the open, top end of the casing and annular space between the casing and the borehole. Prevention of contamination through these two routes is carried out both at the upper terminal and lower terminal of the casing.

Upper Terminal

Well casing should extend at least 1 foot above the general level of the surrounding land surface; or 1 foot above the highest expected level of water during a one-hundred year flood where the well is near a body of water. It should be surrounded at the ground surface by a 4-inch thick slab extending at least 2 feet in all directions. The upper surface of the slab and its immediate surroundings should also be sloping so as to drain water away from the well. It is also good practice to place a drain around the edge of the concrete apron and extend it to a discharge point at some distance from the well. Lastly, a sanitary seal should be provided at the top of the well to prevent the entrance of contaminated water or other objectionable material directly into the well.

Sealing the Well
Sealing the Well

Lower Terminal and Casing

For artesian aquifers, the water-tight casing should be extended downwards into the impermeable formation which caps the aquifer.

In water-table aquifers, the casing should be extended at least 5 feet below the lowest expected pumping level. This distance should be extended to 10 feet where the pumping level is less than 25 feet from the surface.

The above are general rules which should be applied with some flexibility where geologic conditions so require.

In addition, wells showing the presence of colliform bacteria have not been properly sealed. Although every precaution may have been applied on the ground surface and upper terminal, vertical seepage can occur readily though the annular space between the well casing and the borehole.

Grouting and Sealing the Casing

Grouting or cementing well casing involoves filling the space around the pipe, usually between the pipe and the drilled hole, with a suitable slurry of cement or clay. If pressure grouting is used the cracks and spaces of the borehole are filled in too. In the figure on the right, one side of the well is properly grouted, while the other side is not allowing water to flow from the surface and other water-bearing layers into the well screen.

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