AQUA INDEX: Risk Assessment of Land Use on High Yield Sand and Gravel Aquifers in Maine
Google Earth Interactive Maps page – look for “AQUA Index”
Calculation of the AQUA Index
Findings
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Introduction
High-yield sand and gravel aquifers are a unique and limited resource. They generally contain very clean water, and this water can be extracted from these aquifers at very high rates. Because the rate of recharge to these aquifers is very high, they are also particularly vulnerable to contamination.
The AQUA Index was developed to assess the relative risk due to human activities on high-yield sand and gravel aquifers. The Index is a decimal number between 0 and 1 where 0 represents significant risk and 1 indicates minimal risk . AQUA stands for Aquifer Quantitative Use Assessment.
The purpose of the AQUA Index is to allow water districts, municipalities, planners, consultants, organizations, and private citizens to have a tool to measure the potential risk from development to these unique and valuable resources. Increased awareness of the possible impacts will improve efforts to conserve and to protect these drinking water resources.
Calculation of the AQUA Index
Land-use activities are put into two categories: point sources and non-point sources. The former have specific locations, while the latter generally occur over a wide area. Point sources are the potential or actual sources of contamination to groundwater categorized in the Environmental Geographic Analysis Database (EGAD) database, including gasoline tanks, landfills, junkyards, and floor drains (that presently drain or formerly drained to the ground or to a septic system). Each of the 38 site types defined for the EGAD database is assigned a risk value, based on the nature of the potential contaminants associated with the site. If a site identified in the EGAD database is located on a mapped high yield sand and gravel aquifer, the risk value for that site type is added to a term used to calculate the Index for that aquifer.
Non-point sources of contamination to groundwater (and surface water as well) would include road salt, and oil, grease, and gasoline from roads, driveways, and parking lots. Non-point sources are hard to locate specifically, so it is assumed that the non-point risk was proportional to the area of roads on the aquifer. Since roads allow access to and development of the aquifer, the density of the road network should reflect the potential for impact from that development.
The area of roads over sand and gravel aquifers is expanded (electronically) with a 150 foot buffer, or 75 feet to each side of the road (to account for the influence of spills, drips, and storm runoff from the roads, and development of the adjacent land). The actual acreage of roads and buffer areas over a mapped high-yield sand and gravel aquifer is then calculated, using ArcMap tools. The resulting acreage is then subtracted from the overall aquifer acreage in the AQUA Index formula.
Findings
See also the summary diagram (pdf) for this information.
• 49 high yield aquifer locations (16%) show minimal risk (AQUA Index of 1.0) from land uses (1,168 acres or 4% of total acres)
• 160 (53%) have indexes greater than .50 (moderate to low risk) (11,812 acres or 40% of total acres)
• 91 (30%) have indexes less than .50 (moderate to significant risk) (17,933 acres or 60% of total acres (29,745))
• 18% of the high yield sand and gravel aquifers at minimal risk (AQUA Index = 1) have public water supply wells.
• 33% of the aquifers with AQUA values between 1.0 and 0.5 have public water supply wells
• 39% of the aquifers with AQUA values less than 0.5 have public water supply wells.
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Contacts
