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An aquifer is a geologic formation or deposit which is capable of yielding usable quantities of groundwater. Groundwater is found 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 reviewed in depth on this section of the HVCEO web site. While the term "aquifer" is used freely throughout this inventory, it is in every case the short term for "stratified drift aquifer."

Of interest will be the U. S. Geological Survey's definition of sand and gravel aquifers of alluvial and glacial origin. See these deposits mapped for area towns on the 1992 USGS Connecticut Surficial Materials Map.

Yields from bedrock wells, common for single family homes on larger lots, are usually limited by the inability of bedrock fractures to transmit much water. But the gravel in a stratified drift aquifer does not have this limitation; there is much storage space between the gravel particles and water can travel towards the well relatively easily.

In the early 1970's the U.S. Geological Survey first mapped the stratified drift deposits in this planning region. See as an example the legend for the mapping of the Region's northern aquifers. Many of these valley locations, having major roadways and gentle slopes, had already been zoning for commerce and industry, before their water supply potential was recognized.

In the late 1970's HVCEO and the other regional planning agencies, with detailed guidance from the CT DEP and the USGS, processed the stratified drift mapping and created proposed aquifer protection districts on town zoning maps. Industrial and commercial zoning was not to be stopped, just modified to reduce risk to groundwater.

It was determined that the potential value of an aquifer as a major water source depends on its storage capacity in extent and thickness, its hydraulic conductivity or ability to transmit water through layers of sedimentary material to a well, its lack of contamination from man-made or other pollutants, and the capacity of its recharge area.

But again, there is a caution against over regulating. The location of a major aquifer is apt to greatly influence its value and need for immediate protection, especially if it lies miles away from an area needing additional water supply.

Very importantly, the mapping criteria allowed for some of the stratified drift to be "written off" as not of value for future water supply. This was a very serious determination. But some stratified drift was so thin or fine grained that there could be no conceivable useful public well yield, thus they were dropped from the proposed regulatory system.

This geographically focused the protection strategy on the most important areas and reduced criticism. HVCEO Aquifer Mapping Criteria were created in conjunction with USGS and CT DEP. The use of these criteria has withstood the test of time and the resulting maps are today part of the zoning regulations for Brookfield, Newtown and Ridgefield.

The 26 major aquifers in the ten town Housatonic Valley Region, as identified and evaluated by HVCEO studies, are those land areas of unconsolidated glacial stratified drift comprising at least 100 acres of area and possessing saturated water thickness of 10 feet or greater. The hydrogeologic setting for this Region's aquifers is of relevance in understanding local stratified drift aquifer resources.

As of 2004, wells in stratified drift aquifers augment the public water supplies of Bethel, Danbury, New Fairfield, New Milford, Newtown and Ridgefield, Connecticut. Additional towns, such as Brookfield which protects a future well field, may be added to the list in the future.

Many aquifers across the state remain threatened by contamination due to potentially contaminating land uses. To address this problem, Connecticut has established the Aquifer Protection Program (CT General Statutes§ 22a-354a et. sec.). The purpose of this program is to identify critical groundwater water supply areas and to protect them from pollution by managing the land use above them.

To satisfy their statutory mandate, the DEP, in conjunction with the local/regional water authorities, have identified over 120 Aquifer Protection Areas (APA) across the State. APA's must meet the criterion of containing a public water supply well that is located in stratified drift and services more than 1000 people. DEP is in the process of drafting regulations that would limit the types of land uses that could be sited in the “protection area,” which coincides with the well recharge area of the aquifer.

CT DEP Proposed Aquifer Regulations focus is on siting controls in sensitive aquifer areas for new high-risk uses and best management practices for existing high-risk uses.

There are 83 towns with aquifer protection areas in CT as shown on this statewide DEP map with links on the page to larger maps of aquifers in Bethel, Danbury, New Milford, Newtown and Ridgefield.