Hydrogeologic Setting for Stratified Drift Aquifers
in the Housatonic Valley Region

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HYDROLOGY OF THE REGION
The entire water resource of the Housatonic Valley Planning Region is derived from precipitation, both that which falls directly on land and water surfaces within the Region and that which falls elsewhere and flows into our area via rivers and streams.

The U.S. Geological Survey has estimated that 30% to 50% of the 46 inches of mean annual rainfall in the local area returns to the atmosphere in the form of evaporation and transpiration from plant life. The remainder enters surface streams and groundwater reserves, eventually flowing out of the Region towards Long Island Sound via the Housatonic, Norwalk, Saugatuck and other rivers.

Groundwater represents that portion of the “hydrologic cycle”– evaporation, condensation, precipitation, runoff – in which water enters the soil and surface rocks and is transmitted to a point of discharge, such as a stream or well. The movement of water through the ground’s unsaturated zone varies in relation to soil characteristics, but is generally downward through permeable soil to the upper boundary of the “saturated zone” or water table.

In permeable soils, the saturated zone may rise and fall in response to recharge from rainfall, or discharge to streams and wells. The configuration of the water table tends to roughly parallel the surface of the ground above it. Consequently, groundwater flows “downhill”: within a drainage basin, just as do the surface waters above, generally rising upward, however, at the bottom of a valley to discharge into a flowing stream or pond through its bottom sediments. Drainage basins are therefore the key land areas in analysis of groundwater aquifers.

An aquifer is a geologic formation or deposit which is capable of yielding usable quantities of groundwater. Groundwater occurs in three types of aquifers: stratified drift aquifer, till aquifer, and crystalline bedrock aquifer.

The stratified drift aquifer, typically a layered deposit of gravel, sand and silt in valleys, is the only formation sufficiently productive to meet large volume water needs such as public water supply wells. While the term "aquifer" is used freely throughout this inventory, it should be remembered that it is short for "stratified drift aquifer."

GROUNDWATER RESOURCES
The Western Highlands of Connecticut date from geologically ancient times before the continent of North America had assumed its present form; its “rock-ribbed” hills and mountains are the eroded stumps of what were once lofty mountain peaks.

As the mountains wore down and subsequent uplifts of the earth’s crust occurred, a carpet of ocean floor was thrust up among older rocks to form the limestone or carbonate rock strata that makes up the limestone valleys: Great Swamp and the Titicus Valley in Ridgefield; Mill Plain in Danbury; East Swamp in Danbury; and the great “rift” or valley which extends north from West Redding through Bethel, Danbury, Brookfield and New Milford.

During the comparatively recent Ice Ages, which affected the Region as recently as 15,000 years ago, great glaciers repeatedly advanced and retreated over the Region, scouring its hills, filling the valleys with “stratified drift” sediments deposited by meltwaters and draping the uplands with glacial till.

The final surface topography and drainage system of the Region were therefore shaped by the moving ice and running water of the last glacier. As the meltwaters moved in temporary channels across the ice and adjacent land, or stood blocked in lakes by obstructions, layers of sand, gravel, silt and clay, but predominantly sand and gravel, were laid down in the various stream, valleys and lowlands.

These stratified drift deposits are capable of holding large quantities of groundwater. Well yields, where the stratified drift has favorable hydraulic characteristics because of coarse texture and major depth or “saturated thickness”, may produce from 50 to 2,000 gallons of water per minute.

Till, the compact sediment covering most hills, slopes and uplands is an unsorted, unstratified material composed or rock particles of all sizes including stones and boulders. This sedimentary material was deposited directly, as a mantle on the bedrock, by the glacier.

Thickness of till varies from 0 to 200 feet, but till in this Region is usually shallow, frequently from 10 to 50 feet deep. Its poor hydraulic conductivity limits wells to very modest yields, typically a few hundred gallons per day in a shallow domestic well.

Bedrock of the Region, principally metamorphic and igneous crystalline rocks such as gneiss, schist and granite, tends to be close to the ground surface. Groundwater is transmitted in these hard rocks through “fracture” systems, or cracks, both horizontal and vertical, within several hundred feet of the surface.

The size and distribution of these water-bearing cracks is irregular, and therefore ater yields will differ significantly from one site to another, but generally do not exceed 100 gallons per minute and frequently yield less than 10 gallons per minute.

Bedrock, the principal aquifer under more than 90% of the Region, is nonetheless highly important as a source of water for small individual uses such as private dwellings and small commercial or institutional establishments. The carbonate or limestone bedrock area typically is more fractured and tends therefore to be a somewhat more productive water source, yielding up to 300 gallons per minute to individual wells, though generally less.