How can the NMI dioxane plume go underneath the Assabet River?

One of the more surprising aspects of Acton’s 1,4-dioxane situation is that the contaminant plume coming down from the NMI Superfund site goes underneath the Assabet River and thus reaches the Acton Water District’s Assabet 1 public water supply well.

This wasn’t supposed to happen; it was thought that that the river would provide a “hydrologic barrier” that would intercept polluted groundwater before it could reach the Assabet well field. However, when systematic mapping of dioxane distribution was eventually carried out, it showed that a dioxane-rich plume passed directly under the river and made a bee-line for the Assabet 1 well.

On this map, the light blue arc is the Assabet River and the bundle of pink contours mark the plume. Further explanation of this map can be found here. 

The method for finding out if and where there is dioxane in the groundwater is to drill an array of monitoring wells, take samples from various depths in the wells, and send the samples off to a lab for analysis. When the results from those test are plotted in a vertical profile view rather than in map view, it becomes easier to understand how the plume can go underneath the river.

Below is a profile approximately perpendicular to the plume. (To see more detail: download pdf of cross-plume profile and pdf of map showing profile location as a yellow line.)

Profile view, roughly perpendicular to the plume, from http://www.nmisite.org/download/Remedial%20Investigation/NMI-GW-Figures.pdf

The profile extends from SW (on the left) to NE (on the right), and depicts a vertical slice down into the Earth. We are looking into the core of the plume, and our view direction is the direction of groundwater travel. The horizontal dimension of the diagram is about 1800 feet, and the vertical dimension is 150 feet; the vertical dimension has been greatly exaggerated to let us see more detail.

At this scale, the river is just a small blue dimple in the land surface, up at the top of the diagram. This profile cuts across a meander of the river, so that the river actually shows up twice on this profile.

Below the river is 60-70 feet of loose sediment, colored tan and labelled “overburden.” This “overburden” comprises layers of sand, gravel, silt and clay deposited from meltwater during the glacial retreat at the end of the last ice age. A geologist would call this “stratified glacial deposits.” The stratified glacial deposits here in the Assabet Basin are highly permeable and a very effective aquifer for groundwater transport (deSimone, 2004). Below the overburden, is a thin layer shown with stripes and labelled “till.” Till is also a glacial deposit, a hodgepodge of densely-packed sediments deposited from the base of the ice sheet. Till is generally not permeable and not a good aquifer. Below that is the “bedrock,” shown in grey on the diagram, ancient igneous and metamorphic rocks.

Each vertical black line on the diagram show where a well or cluster of wells has been drilled and water has been sampled. Many of the monitoring wells go all the way down into the bedrock. The bulges on each well symbol show the levels at which water has been sampled, and the little numbers next to the bulges show the concentration of various contaminants in the samples. The green numbers are the 1,4-dioxane concentrations in units of parts per billion (ppb). These samples shown here were all collected before the intervention to pump and treat contaminated groundwater began.

Now, finally, to the plume: In the center of the profile, note the orange dashed line surrounding a large irregular blob-shape. All the dioxane concentrations (green numbers) inside the orange dashed line are > 0.46 parts per billion (ppb). All the dioxane concentrations outside the orange dashed line are < 0.46 ppb. In other words, the orange line maps the core of the plume. Why 0.46 ppb? This is the number that was set forth in the Record of Decision as an “acceptable risk-based cleanup number for 1,4-dioxane in groundwater.”

The top of the plume, as defined by that 0.46 ppb boundary, is more than 30 feet below the bottom of the river. Because the overburden here is so thick, there is plenty of room for the plume to pass between the river and the bedrock.

It is also interesting to look along the plume. (Download profile as pdf; location is green line on pdf map.)

Profile view, roughly along the axis of the plume, from http://www.nmisite.org/download/Remedial%20Investigation/NMI-GW-Figures.pdf

This second profile goes from the NMI site at the left end of the profile to the Assabet 1 well on the right. The colors and symbols are the same as on the previous profile. The numbers beneath the profile are distance in feet; it’s only about 2800 feet, or a shade over half a mile, from NMI to Assabet 1. Both the land surface and the top of bedrock slope downward from the NMI site towards the Assabet River, thus favoring flow in that direction.

Want to know more? 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. 

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