Is there anything unusual about the soil in West Acton? A map shown by Joseph Shea, Senior Vice President, and Jack Troidl, Project Manager, of the town's sewer company Woodard & Curran, suggests there is something different about the soil in West Acton. But the map is misleading.
Above, Woodard & Curran's map identifies the area shown in red as "very limited" for use with septic systems, and cites the Natural Resources Conservation Service (NRCS) as its source.
In trying to get a better understanding of this map, we looked at the same NRCS ratings over a larger area. We found that virtually all of Acton, including the leaching field of Acton's wastewater treatment plant, has the same "very limited" rating as West Acton:
Grey regions indicate no rating, while yellow areas are rated “somewhat limited.” Note that no regions are green, which would indicate a rating of “not limited.” The above map of Acton led us to ask the question, Is there something unusual about Acton's soil? To find out, we expanded the plotting region to an even greater region -- all of Middlesex Country:
As it turns out, approximately 450,000 acres in Middlesex Country are rated “very limited,” while ~15,000 acres are rated “somewhat limited.” Not a single acre in the county is given a green rating of “not limited.” This can be seen in the table below.
From the expanded map it is clear the original map from Woodard & Curran, a sewer company, was misleading. Their map didn't make clear that the red region around West Acton is the result of a hand-drawn border by Woodard & Curran. Moreover, the soil rating shown in the map is irrelevant, as a metric that lumps West Acton with 97% of rated land in the county is not a meaningful measure of West Acton’s suitability for septic systems.
Why is most of Massachusetts considered "very limited?" As it turns out, what the NRCS considers suitable for septic systems is very different from what Massachusetts' Title V considers suitable. In short, if soil parameters are anything other than exactly optimal, the soil is rated as "limited." And since currently little granularity has been implemented in the NRCS rating, almost all soils have been assigned the severest rating of "very limited," and are therefore shown in NRCS maps as red.
In summary, the original map from Woodard & Curran gives West Acton homeowners the mistaken impression that they need sewers due to supposedly bad soil in their neighborhood. We have seen no indication other than this misleading map that West Acton soil is unsuitable for septic systems.