Why It’s Not Physically Feasible to Build 95 Housing Units on 6 Acres
August 23, 2025
By: Madeleine Blake
Note: This analysis is solely my own and any mistakes are attributable only to me.
Summary
It is not possible to site 95 units of housing, assuming 2 bedrooms per unit, on a 6-acre parcel that lacks piped water and municipal sewer.
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The operative constraint is not the cost of a wastewater treatment plant. Rather it is the amount of undeveloped land required to surround the public well,
as well as the prescribed setback from the public well to the effluent field for the wastewater treatment plant.
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For smaller numbers of units, it becomes more difficult to state conclusively that building the housing would not be possible. However, given the applicable rules, as well as Carlisle’s experience, even on an ideal 6-acre site, the likely outcome would be a relatively small number of housing units (18 units with 36 bedrooms) served by a nitrogen-enhanced septic system.
Analysis of Current Regulations
Drinking Water Regulations
Massachusetts DEP regulations provide that a well must be regulated as a public water supply if it supplies more than 24 people. This is equivalent to 12 bedrooms, with an assumption of 2 occupants per bedroom. Public water supplies are regulated by the Massachusetts Drinking Water regulations, 310 CMR 22.00. 1
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Public wells are classified and regulated according to how much water they produce. For purposes of this analysis, it is assumed that the public water supply will be unmetered, and that a reasonable assumption is 165 gallons per day per bedroom of water usage. According to statements by Carlisle’s Health Agent, this 165 gpd figure is used by Carlisle to determine well capacity. 2
Although Title 5 regulations provide that each bedroom should be assumed to generate 110 gpd of discharge, Carlisle uses a figure of 165 gpd for multi-family housing for its local septic regulations; the assumption of 165 gpd of water usage reflects a basic “what goes in must come out” analysis. The 165 gpd figure is consistent with state policy, which assumes 100 gpd per person.3
Public water supplies are required to have Zone 1 areas. Zone 1 areas are protective above-ground areas surrounding the well. The larger the well, the larger the Zone 1 area. Zone 1 areas are strictly regulated as no-build areas. Regulation 310 CMR 22.21 (a)(b)(5) provides that current and future land uses in the Zone 1 area are limited to those directly related to the provision of drinking water or that will have no significant adverse impact on water quality. 4 Zone 1 areas must be owned or controlled by the water supplier. Separately, in a sort of belt and suspenders situation, Title 5 and the Groundwater Discharge Permit rules both provide that septic outflow and/or wastewater treatment effluent cannot be discharged into the Zone 1. 5
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The size of the Zone 1 is a fixed number set forth in the regulation. The fixed number is for a radius extending from the well or wellfield in all directions. For a water supply producing 100,000 gpd, the Zone 1 radius is 400 feet. At 10,000 gpd, it is 250 feet. There is a formula to calculate the radius for other gallons per day figures. Next, there is a calculation to determine how many acres are represented by a particular radius. For example, a radius of 400 feet is 11.5 acres. A radius of 250 feet is 4.5 acres. Note that because this is specified as a radius, the land for the Zone 1 must be roughly circular and thus will fit more easily on regularly-shaped parcels. Long, narrow parcels might not accommodate Zone 1’s even if they have enough acreage; and pieces of parcels such as pork- chop necks might not be useful for meeting the Zone 1 requirement.
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Public wells are also required to have either a Zone 2 or an Interim Wellhead Protection Area (IWPA). Zone 2 refers to a physical area of the aquifer (i.e. it is underground, not above-ground like the Zone 1) that supplies the well. The Zone 2 area is determined by the DEP through underground testing. For smaller wells, IWPA serves as a sort of proxy. Instead of doing the underground testing, the IWPA is determined through a formula based on how much water is pumped per minute. The regulations list various activities and land uses that cannot take place in the Zone 2/IWPA, but this can be achieved through wellhead zoning rather than ownership/control. Examples of activities that cannot take place in Zone 2 areas include landfills, activities related to petroleum, and snow storage. 6
1 Massachusetts’ regulations derive from the federal Safe Drinking Water Act, which was enacted in 1974 and amended during the 1980s and 1990s.
2 See MBTA Communities Subcommittee minutes June 4, 2024 pp. 2-3.
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3 Unlike under Title 5, the necessary volume for a public water supply is determined on a per capita basis, rather than per bedroom. According to Mass. DEP Drinking Water Policy 88-10 Daily Consumption Per Person, for unmetered public water systems, an assumption of 100 gallons per day per person should be used. However, planners may make assumptions that not every bedroom will be occupied by two people. For comparison purposes, if you assume that every bedroom is occupied by two people, that would be an assumption of 200 gallons per day per bedroom. An assumption that half the bedrooms are occupied by two people and half by one person would result in a figure of 150 gallons per day per bedroom. As Carlisle’s preferred figure of 165 gallons per day is reasonable, close to the assumption of 1.5 persons per bedroom and consistent with the state’s drinking water policy of 100 gallons per person per day, it will be used in this memo.
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4 See also Massachusetts Drinking Water Policy 94-03 Implementation of Zone I Requirements, which states that only activities demonstrated to have no substantial adverse impacts on water quality will be allowed in Zone 1 areas; that passive recreation such as walking will be allowed; and that no picnic areas may be established due to “Mass. DEP’s desire to avoid concentrated human activity in the Zone 1.”
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5 310 CMR 22.21(1)(b) contains the Zone 1 prohibitions in the drinking water rules. 314 CMR 5.06(1)(f) contains the prohibition against discharging wastewater into the larger of the Zone 1, or the distance ground water travels in six months.​​​​​
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6 I am uncertain whether a public water supply for 95 two-bedroom apartments would be large enough to require the DEP to delineate an actual Zone 2 area. If an IWPA estimate is used, there is a table in the regulations to determine the radius. If the flow is 10 gallons per minute, a 540 foot IWPA would be applied. This would equate to approximately 21 acres. This topic is outside the scope of this memo but is mentioned here to highlight that there are additional challenges and land use considerations in addition to needing an unbuilt Zone 1 area.
Carlisle’s Wastewater Consultant’s Opinion
At the August 11, 2025 Planning Board meeting, the Planning Board’s wastewater consultant, Joseph Peznola, of Hancock Associates, confirmed that it would not be physically feasible to construct 95 units of housing on a 6-acre site in Carlisle. Mr. Peznola’s analysis was based on Massachusetts’ drinking water regulations, and the need for a large amount of land to buffer the public well. Mr. Peznola’s analysis differed from this memo because he assumed that a developer would construct a series of smaller, clustered public wells, with smaller and overlapping radii, instead of one large public well. If this were to be allowed by the DEP, the amount of land needed to buffer the well would be smaller than set forth in this memo but still too large to make it feasible to construct 95 housing units on 6 acres. 7
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7 310 CMR 22.02 defines “wellfield” as a “series of 3 or more wells that are manifolded together and which meet certain other technical requirements. 310 CMR 22.21, which sets forth the Zone 1 requirements discussed in this memorandum, applies to “any public water supply well, wellfield or spring . . .” Accordingly, the drinking water regulations specifically provide for the possibility of a wellfield of (presumably) smaller wells rather than one large well; however, because certain technical and other requirements pertain, it appears that this would not be possible in all cases but would need to be evaluated through testing. Even if a wellfield rather than a well could be utilized, with a correspondingly smaller Zone 1, based on the Planning Board’s expert’s opinion, this would not materially change the results of this analysis for 95 units on 6 acres.
Wastewater Regulations
The discharge of wastewater into the ground is regulated by Title 5 (310 CMR 15.00)(up to 10,000 gallons per day) and the Groundwater Discharge Permit Rules (314 CMR 5.00) (above 10,000 gpd). The Groundwater Discharge Permit Rules provide that above 10,000 gpd, septic effluent can be discharged into the ground only from a wastewater treatment plant. Accordingly, above 10,000 gpd, a conventional or nitrogen-enhancing
septic system can no longer be used, and a wastewater treatment plant must be used. This is equivalent to 90 bedrooms, or 45 two-bedroom housing units.
According to figures provided by the state, the cost of a wastewater treatment plant, both to construct as well as to operate and maintain, is approximately six times as much as a conventional septic system. 8 Accordingly, although it is permissible to use a wastewater treatment plant for outflows below 10,000 gpd, it is not likely to be cost effective. Joseph Peznola, the Planning Board’s wastewater expert, offered his opinion that wastewater treatment plants are not economically viable for fewer than 300 units. In his estimate, it costs $1.5 to $2 million to build the plant and $100,000 per year in annual maintenance costs.
According to figures provided by the state, the cost of a wastewater treatment plant, both to construct as well as to operate and maintain, is approximately six times as much as a conventional septic system. 8 Accordingly, although it is permissible to use a wastewater treatment plant for outflows below 10,000 gpd, it is not likely to be cost effective. Joseph Peznola, the Planning Board’s wastewater expert, offered his opinion that wastewater treatment plants are not economically viable for fewer than 300 units. In his estimate, it costs $1.5 to $2 million to build the plant and $100,000 per year in annual maintenance costs.9
The Carlisle BOH has enacted local septic regulations that require designers of septic systems for multi-family housing to assume outflows of 165 gpd per bedroom, rather than the 110 gpd in Title 5. The BOH has authority under state law to enact more protective regulations if they deem that local conditions (such as our unique subsurface conditions) require it. Accordingly, for some purposes, such as determining when a groundwater discharge permit is required, the state standard of 110 gpd will apply, while for other purposes under local control, the local Carlisle standards will apply.
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8 Slide 15 of 28 in Achieving Higher Density Development in Areas without Sewer/Water Service, part of the Massachusetts Smart Growth/Smart Energy Toolkit.
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9 Because the relative cost of wastewater treatment plants is about six times higher than septic systems, apparently many projects are sized at 43 or 44 two-bedroom apartments, in order to remain just under the 10,000 gpd figure that triggers a wastewater treatment plant. See Sewage Rules Create Gap in Housing Supply in Massachusetts, July 2015, by Joseph D. Peznola, PE, Hancock Associates (being on either side of the 10,000 gpd bright line translates to hundreds of thousands of dollars in cost and a gap in project sizes)​.​
Regulations Applied to 6 Acre Parcel
95 2-bedroom apartments
95 2-bedroom apartments would include 190 bedrooms. Using the assumption that each bedroom should be allocated 165 gpd of water, this development would need to have a capacity of 31,350 gallons of water per day. The Zone 1 radius for a public well producing 31,350 gpd is approximately 324.4 feet. This equates to 7.59 acres. This means that to accommodate this much housing, the well would require a no-build area of 7.59 acres. As this is larger than the parcel, it would be impossible. Separately, the Groundwater Discharge Permit rules prohibit discharging effluent from the wastewater treatment plant within this 7.59 acres, or within an area determined by the speed with which ground water travels in six months, whichever is larger. Accordingly, the 7.59 acres is a minimum size within which wastewater cannot be discharged, and the area could be increased based on hydrogeologic testing.
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The Planning Board’s wastewater expert, Joseph Peznola, concurred that it is not physically feasible to build 95 housing units on a 6-acre parcel. However, he assumed that a wellfield of smaller wells, rather than one large well, would constitute the public water supply. Mr. Peznola also assumed that the development would use approximately 20,000 gallons of water per day rather than the 31,350 figure used in this memo. According to Mr. Peznola’s opinion, the Zone 1 for the development would be approximately 3-1/2 to 4 acres, which would still be large enough to render a 95-unit development on 6 acres “not physically feasible.”
45 2-bedroom apartments
45 2-bedroom apartments would have 90 bedrooms and use 14,850 gpd of water under Carlisle’s assumptions. At 14,850 gpd of water, the Zone 1 radius would be 274.9 feet, or 5.54 acres, leaving just .46 of an acre remaining on the 6-acre site. If a cluster of wells is used, the wellfield might require a Zone 1 smaller than 5.54 acres, but the result will not be materially different.
The state’s wastewater assumptions are that each bedroom generates 110 gpd of wastewater. Accordingly, the 90 bedrooms, although they would be assumed to use 14,850 gallons per day of water, would be assumed under state standards to generate only 10,000 gpd of wastewater. This is the threshold for when a wastewater treatment plant would be required. However, a project sized at 43 or 44 apartments would remain just under that threshold and would be able to utilize a septic system. As discussed above, because a wastewater treatment plant is so costly, there would be substantial economic incentives to remain just below the 10,000 gallons per day threshold.
The acreage remaining on the site after allowing for the Zone 1 would be only .46 of an acre. This is 20,037 square feet. This would not be sufficient to site the housing, driveway, parking, septic or wastewater treatment facility and leach field. The housing alone would likely need around a footprint of 30,000 square feet (45 apartments at 1200 sf each is 54,000 square feet, divided into two floors of 27,000 square feet each, with some allowance for hallways and mechanical rooms). 10
10 In addition to the housing itself, items that would need to be included on the site include: a 24-foot wide driveway; a paved pull-out area next to the fire cistern; the fire cistern; a cul-de-sac or turn around area to accommodate fire trucks; the septic or wastewater treatment plant; and a leach or drainage field.​
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18 2-bedroom units
In practical terms, the limiting factor on a 6-acre site will not be the size of the Zone 1 but rather the number of bedrooms allowed by Title 5. This is because it would likely not be economically rational to construct a wastewater treatment plant at six times the cost of a septic system, only to achieve 10 to 15 additional bedrooms. There does not seem to be a “sweet spot” where the bedrooms are few enough not to require a prohibitively large Zone 1, but numerous enough to justify the cost of a wastewater treatment plant.
For example, if you assume a project size of 36 bedrooms, or 18 2-bedroom units, this might fit on a 6-acre site. This is the maximum number of bedrooms that could be constructed on a 6-acre site under Title 5. This would equate to a public well of just under 6,000 gpd. A well producing 6,000 gpd of water would need a radius of approximately 217 feet, resulting in a total Zone 1 area of 3.4 acres. This would leave approximately 2-1/2 acres to accommodate the other items on the site, such as the housing structures, the septic and the leach field. However, there would still be a lot of unknowns, such as what hydrogeologic testing would reveal, and whether the required items could be arranged satisfactorily on the site.
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11 This point was confirmed by the Planning Board’s wastewater expert, Joseph Peznola, at the August 11 planning board meeting, where he stated that many people believe it is not economically feasible to construct a wastewater treatment plant for fewer than 300 units.
Carlisle's Past Experience
For Benfield Farms and Woodward Village, the original parcels were very large. The public wells are located in portions of the parcels that were eventually separated and placed under Conservation Restrictions.12
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For two recent 40B projects, Coventry Woods and Brem/Lifetime Green Homes, the applicants proposed using a series of private wells, which would have avoided the regulations that would have accompanied having a public well or wellfield. In both cases, the town was not comfortable with this, and in the case of the Brem project, town counsel wrote a lengthy letter to the DEP arguing that it would be unsafe and contrary to public health to allow a series of private wells that would not be regulated as a public water supply.13 However, both projects were withdrawn before the issue was conclusively settled.
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The Planning Board’s “scattered sites” plan is designed to do something similar – it is designed to avoid the need for a public well by dividing projects up into two-acre lots that would have just under the number of bedrooms that would trigger a public water supply.14
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​11 Note that, if there is sufficient land and the existing conservation restrictions allow for it, the Zone 1’s for the public wells at Benfield Farms and Woodward Village possibly could be expanded.
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12 November 14, 2014 letter from Town Counsel Tom Harrington to the Massachusetts DEP regarding classification of wells at the Lifetime Green Homes site.
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13 MBTA Communities Subcommittee June 4, 2024 minutes at page 2 (discussing goal to “keep developments under the threshold for a public water system, they would be limited to 12 bedrooms (24 people) . . . “).​
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Conclusion
Carlisle Alliance for Preservation supports MBTA Communities solutions that minimize excessive rezoning of land, environmental cost and neighborhood impact.
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We believe it is essential that residents are provided with accurate information about what build-outs are possible given Carlisle’s lack of municipal infrastructure.