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Section 5 - Locating Potable Water Supplies
Individual Domestic Wells
For the person wishing to construct a water well on a parcel of land of two acres or less, the published ground water data are generally of little specific use. Other kinds of geologic information and on-site inspection by a hydrogeologist, however, may be of help, most particularly in the case where neighboring wells indicate a problem, or where a dry hole has already been completed on the lot.
For parcels of land greater than two acres, available information relating to surficial geology, gravel aquifers, and overburden thickness can be a useful guide for construction of a domestic dug, driven, jetted, or drilled well in the overburden. Some site-specific information, however, such as a test pit, gravel pit, or existing neighboring well, will give a greater assurance of success.
For parcels of land of 10 or more acres, published hydrogeological information can be helpful in selecting a favorable site for constructing any type of domestic water well, including a drilled bedrock well. Bedrock well yield maps show areas of potentially high yield (10 or more gpm). Overburden thickness maps and bedrock well depth maps at the same scale indicate the likely range of casing length and well depth. Even for land tracts of 10 or more acres, however, it is possible that none of the available hydrogeologic data will guide well location or construction. The bedrock aquifer maps are based on the results of existing wells. In places where there are no such wells, these maps obviously must be blank. It should be clear that many tracts of land in Maine cannot be evaluated fully as to the availablility of potable ground water through use of the published hydrogeological information, but the use of this information will allow the owner or prospective buyer to evaluate the probability of locating potable water supply and the likely type of well required.
A landowner should discuss well location and construction with the driller or digger. It is important to be aware of proposed or existing septic tank and field locations, and of the State regulations pertaining to their location and construction. Most often the well is kept uphill at a horizontal distance of 100 feet from waste disposal facilities and other potential sources of contamination. The homeowner should bear in mind that these State regulations in no way guarantee potability of a domestic water supply, and that in some geological situations additional separation and/or construction features (septic tank or well) may be necessary.
Common problems encountered when drilling domestic wells in Maine include lack of water, water that will not clear, and wells that collapse. Dry holes occur because the well does not intersect a water-bearing fracture. Usually at a depth of around 300 feet, a dry hole is abandoned and a second well begun at a new site 20 to a few hundred feet away. The selection of the new drilling site is often based on a hunch. A hunch, however, has its limitations in Maine because the geology is so highly variable from one place to another. The advice of a hydrogeologist will improve the chances of finding water in a location where a dry hole has been drilled.
In cases where a bedrock well of 200 feet or more in depth yields too little water to be useful, well drillers employ several methods to open fractures around the borehole. The best technique is to build up a sustainable pressure in the well by pumping water into it in a process that was described earlier called "hydraulic fracturing." Another, but less successful, technique is to use dry ice to build up pressure in a capped well. Use of dynamite is discouraged because it rarely works well and occasionally does harm to neighboring ground water sources.
Occasionally a well is drilled that obtains a plentiful supply, but one that will not clear even though the well is pumped for weeks or months. The material clouding the water is fine sand to clay-sized particles that are derived from the bedrock itself. They are not well cuttings, but may be fault gouge (ground up rock) or disaggregated rock left by deep-weathering processes. In a few cases, the fine material has refilled wells to 1/3 or 1/2 the original depth. A possible solution to this problem is to locate the depth at which the material enters the well and seal it off with additional casing or a concrete plug. Raising the pump intake pipe above the source of the fine materials is sometimes effective. Another alternative is to drill a new well where the problem is less likely to occur.
Some bedrock in Maine is soft and oxidized at depths of several hundred feet below ground surface, particularly where there are major fracture zones. Deep weathering is indicative of gound water movement at depth and often accompanies discovery of large yields. In some cases, however, the bedrock is so weathered and soft for a condsiderable depth that well construction is precluded. In these situations, it is best to move and drill another well.
Selecting a new well location when a problem has been encountered, be it lack of sufficient water, cloudy water, or caving bedrock, requires considerable experience and expertise. On-site investigation by the driller and a hydrogeologist is needed if the problem situation is not to be repeated.
Where hydrogeologic conditions permit, a single well can be constructed to supply a group of homes. These so called "community water systems" may be a good choice where low yield or contamination of individual wells is likely. Although all of the available hydrogeologic maps and reports are helpful and should be consulted, the greater water requirements of a community well necessitate the assistance of a hydrogeologist if excessive drawdown, dry well, contaminant mitgration, and similar problems are to be avoided. Furthermore, any shared water system necessitates some form of legal agreement so that the system is maintained for all users, even though property ownership changes from year to year.
Municipal, Industrial, and Commercial Wells
Municipal wells are constructed primarily in gravel deposits from which yields of 500 to 1000 or more gallons per minute are available. Bedrock wells of several hundred gallons per minute might be considered for small communities, or anywhere augmentation is required. Industrial and commercial wells are most often in the range of 50 to 500 gpm, for which bedrock aquifer zones are also suitable for single or multiple-well systems. Available maps are used to locate areas where such high-yield wells are feasible, but a hydrogeologist is needed to evaluate the available data and to conduct field investigations so test wells can be drilled in the best locations within the potential aquifer areas.
Because these wells supply large amounts of water, the possible sources of contamination need careful consideration. Potentiometric surface maps can be used to give the general directions of flow in the bedrock. Surficial geology, overburden thickness, bedrock surface topography, and topographic maps should be consulted. This general information can be reinforced by observations made in any test wells, especially during a pump test of the aquifer. The long-term yield of the aquifer is calculated through analysis of pump test data, and through analysis of the surface area likely to contribute recharge to the aquifer. The ultimate long-term yield cannot be greater than the volume of precipitation that falls on the contributing area, less the runoff and evapotranspiration. Exceptions to the above are planned ground water mining and ground water recharge through inducement from an adjacent water body.
Last updated on March 25, 2009
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