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Selecting a point for analysis

You can either select a point by entering an address or by clicking on the map. When you have successfully selected a point a green dot will appear on the map and information about the point will be displayed to the right of the map pane.

Setting a buffer size

This analysis is based on a model of risk factors that are estimated for each location. The risk estimate is for wells in a "buffer" area around your location. A meaningful risk value is obtained only when the buffer is 1,000 feed or more. You can set the buffer size based on your preference. Smaller buffer sizes give you more detail but more caution should be used when interpreting the results as uncertainty in the model will have a greater impact on the results.

Running the analysis

You must select a point and buffer size before an analysis can be run. When an analysis runs successfully you will see the buffered area appear on the map around the point you selected and you will see a chart appear at the bottom of the map. Summary results will also appear in the Results area to the right of the map window.


The results provided give you the maximum and 90th percentile probability of finding arsenic over 1 or 10 micrograms per liter (ug/L).

The current health based standard is set to 10ug/L while most recent evidence shows health effects at much lower concentrations. A proposal has been made to lower the health based standard to 1ug/L.

The maximum probability within the buffer area gives you the highest value for any of the points in the area. This number helps explain the the potential risk. If any of the points in the area have a high probability then it's very likely the point you selected may also have a high risk. Since the data being analysed are the results of a statistical model this method can help you understand the general risk in the area and is better than looking at the absolute value of a single point.

The 90th percentile is similar to the maximum probability and can help you understand whether the maximum probability value may be a statistical outlier. If the 90th percentile and the maximum value are close then the maximum is probably a reasonable estimate for the area. If the maximum is much greater than the 90th percentile then the data might not be reliable enough in this area to understand specific risk.

About the selected point

This area helps you understand about the point. Provided are basic information about where the point is, including the closest matching address should you want to look the point up again by address rather than clicking on the map.

The "Community water system the point is in" will tell you whether the selected point is likely to be on a private well (if there is no community water system nearby) or may be receiving water from a community water system. If the point you selected is within a named community water system then you should verify the source of water for the location. If it's coming from a community water system then the water company is required to test and treat the water for arsenic and other contaminants regularly. You can request a report of the test results from the water system or from the NH Department of Environmental Services. Each year community water systems are required to provide a consumer confidence report to customers which outlines the test results. If the water is from a private well then the property owner is responsible for testing and treating the water. About half the people in the State of NH are served by community water systems and half private wells. The water system distribution boundaries are not shown on the map for security reasons. It may take a few seconds for the results to be returned.

If you live in NH and drink water from a private well then you need to have the water tested. This tool can help you understand whether arsenic, a harmful contaminant in much of NH's groundwater, is likely to be a problem in your area. High arsenic levels can occur anywhere in NH. This tool uses information about NH's geology, topology, and other things, to make an educated guess at what the chance of high levels of arsenic may be. Even though it is an educated guess, it is still a guess and the only way for you to know for sure is to have your water tested.

What is Arsenic?

Arsenic is a colorless and odorless substance in your water which can cause you to get sick. Short term high level exposures can cause skin problems and learning disabilities. Long term exposures can cause bladder cancer and other conditions.

What can you do to protect you and your household?

Having your water tested for arsenic and other contaminants is the most direct way to understand your risk and what to do. To test your water you can either hire a company to do it for you or you can pick up a test kit, take a sample, and send it to the laboratory yourself. At a minimum, it is recommended to test for the standard set of contaminants which includes arsenic. The NH Department of Environmental Services has created a tool to help interpret the test results and help you understand what type of treatment, if any, you should use.

If you can't afford the recommended treatment methods to make your water safe please contact the NH Department of Environmental Services to discuss your options.

How this tool can help

Step 1

Select a point for analysis by either:

Entering an address


Step 2

Choose a buffer size:

Step 3