Site selection

The U.S. Geological survey (USGS), in a cooperative project with the Connecticut Department of Natural Resources, digitized more than 40 map layers for the areas covered by the USGS Broad Brook and Ellington 7.5-minute topographic quadrangle maps.

This information can be combined and manipulated in a GIS to address planning and natural resource issues. GIS information was used to locate a potential site for a new water well within half a mile of the Somers Water Company service area.

To prepare the analysis, digital maps of the water service areas were stored in the GIS. Using the buffer function in the GIS, a half-mile zone was drawn around the water company service area.

This buffer zone was the "window" used to view and combine the various map coverages relevant to the well site selection.

The land use and land cover map for the two areas shows that the area is partly developed.

A GIS was used to select undeveloped areas from the land use and land cover map as the first step in finding well sites. The developed areas were eliminated from further consideration.

The quality of water in Connecticut streams is closely monitored. Some of the streams in the study area were known to be unusable as drinking water sources. To avoid pulling water from these streams into the wells, 100-meter buffer zones were created around the unsuitable streams using the GIS, and the zones were plotted on the map.

The map showing the buffered zone was combined with the land use and land cover map to eliminate areas around unsuitable streams from the analysis.

The areas in blue have the characteristics desired for a water well site.

Point sources of pollution are recorded by the Connecticut Department of Natural Resources. These records consist of a geographic location and a text description of the pollutant.

To avoid these toxic areas, a buffer zone of 500 meters was established around each point.

This information was combined with the previous two map layers to produce a new map of areas suitable for well sites.

The map of surficial geology shows the earth materials that lie above bedrock.

Since the area under consideration in Connecticut is covered by glacial deposits, the surface consists largely of sand and gravel, with some glacial till and fine-grained sediments. Of these materials, sand and gravel are the most likely to store water that could be tapped with wells. Areas underlain by sand and gravel were selected from the surficial geology map

and combined with the results of the previous selections to produce a new overlay map consisting of sites in undeveloped areas underlain by sand and gravel that are more than 500 meters from point sources of pollution and more than 100 meters from unsuitable streams.

A map shows that the thickness of saturated sediments was created by using the GIS to subtract the bedrock elevation from the surface elevation.

For this analysis, areas having more than 40 feet of saturated sediments were selected and combined with the previous overlays.

The resulting site selection map shows areas that are undeveloped, are situated outside the buffered pollution areas, and are underlain by 40 feet or more of water-saturated sand and gravel.

Because of map resolution and the limits of precision in digitizing, the very small polygons (areas) may not have all of the characteristics analyzed, so another GIS function was used to screen out areas smaller than 10 acres. The final six sites are displayed with the road and stream network and selected place names for use in the field.

The process illustrated by this site selection analysis has been used for a number of common applications, including transportation planning and waste disposal site location. The technique is particularly useful when several physical factors must be considered and integrated over a large area.

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The content of this page was copied from USGS Website.
For more information please refer to the site: mapping.usgs.gov