A primary reason to remove the Milltown Dam and reservoir sediments was to remove the source of contamination that leaked arsenic into the groundwater in Milltown.  The area of contaminated groundwater is called the arsenic plume.  As part of ongoing monitoring during the Milltown project, groundwater is sampled from 10 contaminated wells within the plume, from 14 locations in nearby monitoring wells (not contaminated), and from 20 private and public water supply wells.  The clean wells serve as an early warning system in case the arsenic plume were to expand.  Fortunately this hasn't happened, and in fact, most wells within the arsenic plume are becoming cleaner.

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This first map shows results since spring 2004 for water samples from monitoring wells inside the arsenic plume.  The charts show arsenic concentrations in parts per billion (ppb).  For reference, the drinking water standard is 10 ppb and is shown with a red line.  Of the 10 contaminated wells in the arsenic plume, all but one are substantially cleaner since the start of the project.  In several cases, arsenic is now 10 times lower than it was, and two of these wells now meet drinking water standards.

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This second map shows no real change in arsenic concentration in early warning wells beyond the arsenic plume - and that's good, because these wells were mostly clean before the project started.  The drinking water standard of 10 ppb is shown with a red line.