Geology and ground-water conditions in the Wilmington-Reading area, Massachusetts

Abstract

The Wilmington-Reading area, as defined for this report, contains the headwaters of the Ipswich River in northeastern Massachusetts. Since World War II the growth of communities in this area and the change in character of some of them from rural to suburban have created new water problems and intensified old ones. The purpose of this report on ground-water conditions is to provide information that will aid in understanding and resolving some of these problems. The regional climate, which is humid and temperate, assures the area an ample natural supply of water. At the current stage of water-resources development a large surplus of water drains from the area by way of the Ipswich River during late autumn, winter, and spring each year and is unavailable for use during summer and early autumn, when during some years there is a general water deficiency. Ground water occurs both in bedrock and in the overlying deposits of glacial drift. The bedrock is a source of small but generally reliable supplies of water throughout the area. Glacial till also is a source of small supplies of water, but wells in till often fail to meet modern demands. Stratified glacial drift, including ice-contact deposits and outwash, yields small to large supplies of water. Stratified glacial drift forms the principal ground-water reservoir. It partly fills a system of preglacial valleys corresponding roughly to the valleys of the present Ipswich River system and is more than 100 feet thick at places. The ice-contact deposits generally are more permeable than the outwash deposits. Ground water occurs basically under water-table conditions. Recharge in the Wilmington-Reading area is derived principally from precipitation on outcrop areas of ice-contact deposits and outwash during late autumn, winter. and spring. It is estimated that the net annual recharge averages about 10 inches and generally ranges from 5 inches during unusually dry years to 15 inches during unusually wet years. Ground water withdrawn largely by municipal wells supplies the towns of North Reading, Reading, and Wilmington. In 1957 the average daily withdrawal from these wells was about 2.5 million gallons, of which about half was used outside the Ipswich River drainage basin. The chemical quality of the ground water is generally satisfactory except for local excessive concentrations of iron. The storage capacity of the ground-water reservoir and recharge in the Wilmington-Reading area are large enough to sustain a total withdrawal of ground water at several times the current rate, but the use of the reservoir probably will be limited by the extent to which wells of moderate or large capacity can be dispersed. This will depend upon the distribution of areas of thick permeable materials. Conditions in the Martins Brook-Skug River drainage basin seem generally favorable for increased development of water supplies. In the rest of the Wilmington-Reading area the chances of finding substantial bodies of thick permeable materials probably are small, but further exploration is desirable. Measures proposed to drain swampland by deepening and straightening the Ipswich River and its tributaries will have some effect upon the ground-water conditions. Probably the most obvious effect will be a lowering of water levels in wells near improved reaches of channel. Also important will b the effect of changes in low streamflow conditions on wells that induce infiltration from streams and the effect on well yields of an improved hydraulic connection between streams and the ground-water body. The Reading 100-acre well field, which derives part of its supply by inducing recharge from the Ipswich River, would be affected by the drainage measures. During a dry summer, such as that of 1957, the flow of the Ipswich is fully diverted by pumping at this well field, and drawdowns at some of the wells approach half the saturated thickness of the aquifer there. If the drainage measures are