| GIS Analysis to Assess where Shallow Ground Water Supplies in the United States are Vulnerable to Contamination by Releases of MotorFuel from Underground Storage Tanks-A (EPA/600/R-11/108) December 2011
Geographic Information Systems (GIS) were used to assess the vulnerability of ground water supplies to contamination. The analysis was conducted for the 48 contiguous United States, and then again for groups of states corresponding to the EPA Regions. The long form of the 1990 census asked the respondents where they got the water for their home. The choices were: (1) a public system such as a city water department or private company; (2) an individual drilled well; (3) an individual dug well; or (4) some other source such as a spring, creek, river, cistern, etc. The reported estimates for the numbers of drilled wells, dug wells, and other supplies of water were summed to obtain an estimate of the number of households in each census block group that obtained water from a private source. The 1990 census also reported the surface area [square miles] of each census block group. A data file was purchased from ESRI Business Solutions that contained the latitude and longitude of active retail gasoline service stations in the United States. Using Geographical Information System tools (GIS tools) and geo-referenced GIS coverage files on each census block group, the latitude and longitude of each active service station was used to assign the service station to a census block group. Then the number of service stations in each census block group was summed. A simple probability analysis was performed based on the distribution of service stations and the distribution of the households that obtained water from a private supply. Three separate indices were calculated. Each index was calculated for those census block groups that had at least one service station and at least one household that obtained water from a private source.
Vulnerability Index 1 is simply the density of service stations in each census block group. It is calculated as the number of service stations in each census block group divided by the area of each census block in square miles. It provides an estimate of the possibility that the water supplied to a household from a private source will be impacted by a service station. Vulnerability Index 1 describes the consumerís risk of having his water supply impacted.
Vulnerability Index 2 is the density of households in each census block group that obtain water from a private source. It is calculated as the number of households in each census block group that obtain water from a private source divided by the surface area of the block group in square miles. It provides an estimate of the possibility that a release from a particular service station will impact the water supplied to a household that obtains water from a private source. Vulnerability Index 2 describes the risk to the service station owner that a release from his station will impact someoneís private water supply.
To describe the risk to the entire community that obtains ground water from shallow sources, the index that describes the possibility that a single household might be impacted was multiplied by the number of households that are at risk. Vulnerability Index 3 was calculated by multiplying Vulnerability Index 1 for each block group by the number of households in each block group that obtain water from a private source. Vulnerability Index 3 describes the resource managerís risk that a release from a gasoline service station in their geographic area will impact the private water supply of a household in their geographic area.
The report provides maps showing the distribution of census block groups that fell into the highest 30%, the highest 10%, the highest 3% and the highest 1% of census block groups for each Vulnerability Index.
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