An Introduction to Ground Water Hydrology

Section 4 - Maine's Water Situation

Throughout this handbook examples of hydrogeological situations found in Maine are used to illustrate ground water theory. This chapter presents more specific characteristics of Maine's water situation.

Relative Use of Ground and Surface Waters for Domestic Supplies

Ground water is widely used in Maine for drinking purposes. Nearly all of the rural population depends on wells and springs for potable water, and approximately 50% of the municipal systems use ground water at least in part. In total, about 57% of Maine's people use ground water for their domestic water supply (U.S. Geological Survey, 1984).

Infiltration of Precipitation

As was described earlier, precipitation falling on the earth evaporates back into the atmosphere, runs off to streams and water bodies, and recharges the ground water supply. Numbers representative of contitions in Maine are as follows: 50% of the precipitation runs off as streamflow, 30-40% returns to the atmosphere by evapotranspiration, and 10-20% infiltrates soil and rock and recharges the ground water reservoir (Wagner, 1977). Locally, where permeable glacial sand and gravel deposits are exposed at the surface, recharge may be as much as 50% of the total precipitation (Geraghty and Miller, Inc., 1972).

Total Ground Water in Storage and Annual Recharge

The total ground water in storage in the soils and rocks of Maine can be calculated only crudely. Using some rough estimates of the specific yield of these porous materials, it is calculated that every square foot of land in Maine to a depth of 1,020 feet has 14.5 cubic feet of ground water. Maine, then, has 100 trillion gallons of ground water, or 20 times the total volume of surface water in storage.

Annual ground water recharge in Maine is dependent upon the average annual rainfall, and the percentage of this that infiltrates the ground surface. Both parameters vary considerably over the State. Using the average annual rainfall figure of 41 inches, and an average recharge of 15% of the total rainfall, the annual recharge to ground water reservoirs is calculated as follows:

average annual rainfall x average recharge of total rainfall = annual recharge

41 inches of rain x 0.15 = 6.15 inches of annual recharge

Thus, over every square foot of the State there is an annual recharge of about 0.51 cubic feet of water. Annual recharge, then, is about 3.5% of the volume of ground water available in the upper 1,020 feet of the crust. This is the amount of ground water that is replenished each year; that is, the long term safe yield of Maine's ground water resource, or about 3.5 trillion gallons.

Ground Water Available in Surficial and Bedrock Aquifers

The amount of ground water available from the unconsolidated materials overlying bedrock is highly variable, depending mostly on the texture and saturated thickness of the material. Gravel and sand are permeable, and where saturated serve as good sources of ground water. Tills yield some water to wells, but clays are poorly permeable, usually making poor water sources even when saturated.

Throughout the State, till is generaly present at the surface or underlying younger sediments. Clay of marine origin is extensive in the coastal region and extends up the major river basins for some distance. Farther inland there are small areas of glacial-lake clays. Sand and gravel deposits occur over about 25% of the State. They are most extensive in York, Cumberland, Oxford, Hancock, and Washington Counties. The highest yielding unconsolidated aquifers (500 gallons per minute or more) are the gravel deposits that are saturated, and are in hydraulic contact with a lake or stream.

Quantities of ground water available from the bedrock of Maine are extremely variable locally, but essentially similar regionally. The underlying crystalline rock, which includes granitic and metamorphic types, shows similar yield characteristics throughout the State.

Average yield from most of the bedrock in Maine is less than 10 gpm, and sufficient only for supplying dwellings and limited agricultural needs. The range of domestic well yields (6-inch diameter wells) is from "dry" to about 100 gpm. The highest yields are found where the bedrock is extensively fractured, possibly by faulting. These fracture zones can be located by a combination of aerial photograph, satellite, geophysical, and other hydrogeologic techniques. Sustainable yields of 100 to 500 gpm are obtainable from major zones of fractured bedrock in Maine.

Numerous homes in Maine have wells yielding less that 1 gpm. Usually these wells are relatively deep, more than 200 feet, sometimes up to 500 feet or more. A guide developed by the Water Well Drillers Association of Maine, and shown as Table 4, illustrates the relationship between available well yield and the depth of well required to provide sufficient water storage. The deeper the well, the less the intantaneous flow need be. (A 6-inch diameter well casing stores 1 1/2 gallons per foot of depth below the static, or non-pumping, water level.) The values given in this table are very conservative in that the per-person use is higher than the national average of about 55 gallons per day per person, and the total daily use is obtained in one hour by all five members of the family.

Necessary Well Depths

Domestic wells dug or driven into the overburden are typically between 5 and 25 feet deep depending on the depth to the water table and the depth to bedrock. They usually are just deep enough to penetrate the water table by several feet, and for this reason may go dry during periods of drought. Higher yielding sand and gravel wells, such as those used by municipalities, are generally at least 30 feet deep, and as much as 150 feet deep depending on the overall thickness of the water-bearing formation. These wells are constructed to fully penetrate the water-bearing zone of a significant sand and gravel aquifer and, thereby, to obtain the maximum volume of ground water available on a continuous basis.

Bedrock wells obtain ground water from water-bearing fractures; thus, a well is drilled to a finished depth at which at least one water-bearing fracture is encountered. Records for domestic wells in the mid-coastal part of Maine suggest that the first such fracture is struck most often before drilling 100 feet, and that there is about a 90% chance of encountering water before drilling 100 feet more. The majority of domestic bedrock wells obtain sufficient ground water from three or fewer fractures; thus the typical domestic bedrock-well depth, at least in coastal Maine, is in the 100 to 300-foot range.

A common problem for the well driller and homeowner is the decision to continue drilling in a hole that is several hundred feet deep, but yields insufficient ground water. Many Maine drillers consider a depth of around 300 feet a point to stop and start over if no water has been encountered. The greater difficulty of drilling below 300 feet is in part responsible for the choice of this turning point. Figure 49, however, shows that only about 50% of the available fractures occur in the upper 300 feet of rock. It further shows that the decrease in percentage of fractures, the frequency of change, is nearly the same between about 40 to 500 feet into bedrock. Rather than at 300 feet, a decrease in frequency of water-bearing fractures appears to occur at around 500 feet. Drilling between 300 and 500 feet depth seems to give nearly the same chance of success per foot as drilling from 40 to 300 feet. This is a general observation, however. In practice, test drilling in Maine has found areas where the only water-bearing fractures are above a depth of 100 feet, and has found other areas where water-bearing fractures can be found at depths of 700 to 900 feet.

In some instances, a bedrock well that is 200 or more feet deep but is not yielding enough water can be made to yield sufficient water by the technique of hydraulic fracturing. Water is pumped into the sealed well bore under such high pressure that incipient, or possibly plugged, fractures are opened, allowing ground water to more freely flow into the well. Further deepening of the well is then unnecessary.

Bedrock wells located and drilled specifically for municipal, industrial, or commercial uses typically encounter zones of closely spaced fractures at depths between 50 to 700 feet. Often the location of a production bedrock well is selected on the basis of previously drilled 6-inch diameter test wells to intersect the major fracture zones at depths greater than 200 to 300 feet. The reason for this is to permit a large drawdown under pumping conditions without dewatering the major fractures. Thus in contrast to the typical domestic bedrock well, the necessary depth of the municipal or other production bedrock well is a matter of design rather than accident.

Ground Water Quality

The morst common natural ground water problem in Maine is excessive iron content. Iron is a nuisance, causing poor taste, staining of fixtures, and encrustation of pipes, pumps, and tanks. It is most common in iron-sulfide-bearing rocks that seem to be very numerous throughout the coastal half of the State. Some gravel aquifers also produce water with excessive iron, probably because of the concentration of iron-bearing mineral particles incorporated into the sand and gravel.

Other natural ground water quality problems include excessive hardness, sulfur, sodium chloride, and radon. Hardness causes soap curd and encrustation of pipes and especially boilers, sulfur results in poor taste, salt makes water unfit for most uses, and radon can be dangerous to one's health.

Regional variations in ground water quality are not well-documented in Maine because little study has been done. However, an obvious difference occurs in areas of limestone, such as eastern Aroostook County. Bedrock ground water in this area is almost always hard, compared to that from the crystalline rocks throughout the southern half of the State. Wells in the thin, patchy limestone formations, such as those found in Knox County and parts of Cumberland, Androscoggin, and Kennebec Counties, usually produce hard water.

High levels of sodium chloride may occur in coastal areas due to salt-water intrusion from the ocean. In addition to areas of salt-water intrusion, inland wells with high levels of sodium chloride occur in several areas of Maine. The salt-water concentrations are the result of sea water trapped in bedrock valleys under a layer of marine clay since the close of the last glacial episode, 10,000 to 13,000 years ago (Tepper, 1980).

Radon, a radioactive gas, is present in some quantity in nearly all Maine ground water. Radon concentrations are highest in rocks such as granite and highly metamorphosed sediments which contain uranium.

Bedrock geologic maps of Maine, available from the Maine Geological Survey (Augusta), show the areal distribution of the rock types mentioned above, and may be useful as a rough guide to the quality of ground water available in different parts of the State.

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Last updated on March 25, 2009

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