Deb Anderson
Express contributor
The Town of Plympton has prevailed in defending its implementation of the Five-Acre Rule, requiring new construction in Plympton to use a mounded septic system on a lot of at least five acres.
The Plymouth County Superior Court, in Civil Action 2283CV00462, in a Memorandum dated June 30, 2026, found in favor of the Town of Plympton, upholding the Five-Acre Rule.
The suit was brought against the Town by Henry J. O’Hearn, III, concerning a 2.14 acre parcel of land at 10 Mayflower Road, Plympton. Development of the land requires an on-site septic system and an on-site well. The property, currently undeveloped, has no approved septic system, and lacks access to municipal water or sewer service.
According to the ruling from the Court, O’Hearn wishes to construct a single-family dwelling on the property. On Dec. 5, 2020, he applied for a permit to install an on-site system designed to handle 564 gallons of water a day, enough for a five-bedroom home. His application was denied by the Plympton Board of Health on January 14, 2021, and this case followed, challenging the Five-Acre Rule.
Both plaintiff and defendant called expert witnesses. O’Hearn relied on the testimony of Robert F. Silva, who testified that discharges from septic systems pose a major environmental concern and health risk to drinking water wells. Silva told the Court that it is reasonable to regulate on-site septic systems to achieve the MassDEP recommended five milligrams per liter.
Plympton’s expert witness, Scott Horsley, a hydrologist, explained the importance of the recharge area. Horsley explained that, because on-site septic systems contribute nitrogen and other pollutants to ground water, the recharge will dilute their concentrations by adding more clean water to the area.
Silva, in his testimony, acknowledged that a larger land area leads to higher volume of recharge as compared to the pollutants entering the system, leading to greater dilution of water containing septic system discharge.
By considering published data from the United States Geological Survey, Horsley estimated the recharge rate would require at least 4.6 acres of land to sufficiently dilute nitrogen from a septic system so that the groundwater does not exceed a nitrogen concentration of five milligrams per liter, a concentration.
O’Hearn asserted that Plympton is arbitrarily using the Five-Acre Rule to impose an impermissible limit of growth in the Town.
The Court found that the plaintiff, O’Hearn, did not meet the burden of proving that the regulation is “illegal, arbitrary, or capricious”, nor did he submit credible evidence to prove the Title 5 Regulation has anything to do with public health and safety.”
The Court noted that the stated purpose of the five-acre rule is “to protect, preserve, and maintain the existing and potential groundwater supply and recharge areas of the town, and to promote the health, safety, and general welfare of the community by establishing limitation on the installation of, or updates that increase capacity of, on-site septic systems in certain areas with high groundwater elevations…action must be taken to protect the Town’s environmental resources and drinking water supply.”