Nuclear Metals, Inc. was a company that made depleted uranium munitions for the Department of Defense at a 46-acre site on Rt. 62 in Concord, just across the Concord–Acton town line. These activities resulted in significant contamination of the soil, sediment, and groundwater, and the site is now part of the federal Superfund program. Of concern to Acton, there is a plume of 1,4-Dioxane traveling through the groundwater, passing underneath the Assabet River, and reaching the Assabet 1 public water supply well in the southeast corner of Acton.
The map below shows the geometry of the plume relative to the NMI site, the Assabet River, and the wells. This particular map shows the situation before the intervention to remediate the dioxane began. You can see a larger version as a pdf.
The map shows a small portion of southeast Acton and an adjacent area in Concord. For scale and orientation, note the large, tan-colored building in the southwest corner of the map; that is the Stop & Shop on Rt. 62.
The conspicuous, light blue curve wending its way across the map is the Assabet River. In the southeast corner of the map, outlined in a heavy black line, is the boundary of the NMI property itself, located at 2229 Main Street, Concord.
The Acton Water District’s Assabet 1 water supply well is on the west end of the map, marked by a small red circle. A larger red, dotted circle represents the 400-foot radius around the well, the “Zone 1” water protection area.
The bright pink contour lines depict the problem — the concentration of 1,4-Dioxane in the groundwater. Groundwater is water in tiny pore spaces between grains of sand, silt, and gravel. The pink contours map out a swath or “plume” of elevated dioxane level extending from the NMI site northwestward to the Assabet 1 well. Envision water moving very slowly down hill, though little interconnected pore spaces, carrying the contaminant from the NMI site toward the well. It seems surprising that the plume would travel underneath the Assabet River, but it does.
If you download the higher resolution pdf version of the map, you’ll be able to read the numbers on the contours. At the leading edge of the plume, farthest from NMI and closest to the Assabet well, the dioxane concentration was 1 ppb (parts per billion) when this map was made. For comparison, the Massachusetts Department of Environmental Protection (MassDEP) guideline for dioxane in drinking water is 0.3 ppb. In the most polluted spot, in the axis of the plume just east of the river, the dioxane concentration was 40 ppb.
The take-home message from this map, as far as understanding the nature of Acton’s problem, is the geometry of the dioxane plume. The contaminant is traveling underground, directly from the Superfund site to one of Acton’s public water supply wells. The dioxane concentration at the tip of the the plume — which has reached the Assabet 1 well — is still pretty low, but higher concentrations lurk upstream. An intervention has begun to intercept and treat the dioxane-bearing water in the plume before even more dioxane can reach the Assabet well.
Green Acton and the League of Women Voters are sponsoring an expert panel discussion about 1,4-Dioxane, in groundwater and drinking water, on Thursday, October 25, 7–9 pm at the Acton Public Safety Building, 371 Main Street, Acton. In addition, Green Acton has assembled information about 1,4-Dioxane, both general and Acton-specific, available here.
P.S. If you are wondering about other things represented on the map, here is further explanation (feel to free skip this). The map is labeled “overburden,” which means we are looking at data from groundwater in the pore spaces between loose grains of sand, silt, and gravel — as opposed to groundwater in fractures in “bedrock,” the underlying solid rock of the Earth’s crust.
The blue contours map the top of the water table, and are labeled in “feet above sea level.” You can see that the water table slopes downhill from the NMI site (at 128 feet) to the Assabet well (117 feet). The bull’s-eye of blue contours around the Assabet well is called the “cone of depression,” which occurs because pumping water out of a production well pulls down the local water table around the well.
The blue and pink contours come from a combination of modeling and measurements made at wells. Many monitoring wells were drilled as part of the Superfund work. Well locations are shown as circles with black and white quadrants. Close inspection of the large version of the map (download the pdf), shows a well labeled EW-1 in the axis of the plume just west of the river. That is the extraction well for the intervention. The heavy dashed line, tracing out a parabola around the extraction well, shows the limit of the area from which water is expected to be drawn toward the extraction well, the so-called “capture zone.”