Ground-water resources of north-central Connecticut

Abstract

The term 'north-central Connecticut' in this report refers to an area of about 640 square miles within the central lowland of the Connecticut River basin north of Middletown. The area is mostly a broad valley floor underlain by unconsolidated deposits of Pleistocene and Recent age which mantle an erosional surface formed on consolidated rocks of pre-Triassic and Triassic age. The mean annual precipitation at Hartford, near the center of the area, is 42.83 inches and is uniformly distributed throughout the year. The average annual streamflow from the area is about 22 inches or about half the precipitation. The consolidated water-bearing formations are crystalline rocks of pre-Triassic age and sedimentary and igneous rocks of the Newark group of Triassic age. The crystalline rocks include the Middletown gneiss, the Maromas granite gneiss, the Glastonbury granite-gneiss of Rice and Gregory (1906), and the Bolton schist which form the basement complex and the Eastern Upland of north-central Connecticut. Enough water for domestic, stock, and small commercial use generally can be obtained from the crystalline rocks. Recoverable ground water occurs in the interconnected joints and fracture zones and is yielded in amounts ranging from 29 to 35 gpm (gallons per minute) to wells ranging in depth from 29 to 550 feet. The sedimentary rocks of Triassic age underlie all the Connecticut River Lowland and are predominantly arkosic sandstone and shale. Water supplies sufficient for domestic, stock, and small commercial use can be obtained from shallow wells penetrating these rocks, and larger supplies sufficient for industries and smaller municipalities can probably be obtained from deeper wells. Reported yields range from ? to 578 gpm; the larger yields are generally obtained from wells between 300 and 600 feet in depth. Yields are larger where the overlying material is sand and gravel or where the rocks are well fractured. The igneous rocks of Triassic age are basalt and have water-bearing characteristics similar to the crystalline rocks. The unconsolidated deposits comprise ground-moraine and drumlin deposits, ice-contact deposits, outwash-plain and valley-train deposits, and glaciolacustrine and associated delta deposits of Pleistocene age, as well as dune deposits, good-plain deposits, and swamp deposits of Recent age. Ground-moraine deposits occur throughout the area but yield only small quantities of water. The ice-contact deposits consisting mostly of sand and gravel form kames, kame terraces, and crevasse fillings and are the surface deposits in three extensive areas along the eastern margin of the Connecticut River Lowland. The deposits in most places are saturated and, where they consist of well-sorted material, are highly permeable, yielding as much as 750 gpm to properly constructed wells. Outwash-plain and valley-train deposits and bodies of undifferentiated outwash underlie the surface in the eastern and southern parts of the area. These deposits consist of well-sorted sand and silt and some pebble gravel ranging in thickness from nearly zero to more than 225 feet in places. The thicker deposits are an important source of moderate supplies of ground water. Screened wells of moderate depth commonly yield about 150 gpm, but some yield as much as 400 gpm. Bodies of buried outwash deposits of irregular size and shape occur in the bottoms of some of the filled bedrock valleys. They seldom are more than 20-30 feet thick. Their permeability Is generally low because of the high percentage of silt; yields as much as 30 gpm to domestic wells are reported. Under most favorable conditions, these deposits yield as much as 500 gpm. The glaciolacustrine and associated delta deposits occur in nearly all parts of north-central Connecticut and are potential sources of moderate supplies of ground water. They consist of well-sorted sand which generally grades downward into varved clay and silt. The deposits of sand are thic