AUBURN — The project manager behind Lake Auburn watershed modeling said the city’s proposed ordinance changes would decrease the number of projected new homes in Auburn’s section of the watershed.

Laura Diemer, project manager for FB Environmental, presented a new round of modeling during a City Council workshop Tuesday. The updated model looks at the projected impacts from proposals to change the septic system standard in the watershed, as well as move to a three-acre minimum lot size from a one-acre minimum.

The proposal to lower the allowed density came in response to concerns that the updated septic ordinance would allow for more residential development.

Diemer said taken together, the changes would decrease the buildable acreage and projected new houses in the watershed. She said the intent of the proposed rezoning of 873 acres was to “counteract the relaxation of development restrictions with the septic system change.”

The new septic rules, if approved, would allow property owners to utilize alternative soils in designing septic systems — something that has not been allowed in the watershed, but has also restricted development. The initial report from FB Environmental recommended updating to a better performing septic design, but acknowledged the change would make more lots available to be developed.

Diemer said combined with the lower density, the changes would lower the number of projected buildings by 155. Mayor Jason Levesque said the “max buildout” with the changes is projected at 85 new buildings, which is “fewer than if we don’t change the watershed standards.”

“It was deemed a successful outcome because the decrease in lot size more than offset the septic changes,” Diemer said. “The proposed changes were effective in achieving a net reduction in development potential within the Auburn portion of the watershed.”

The modeling also showed the changes would decrease the amount of phosphorus entering the lake annually, but only by a small margin. Diemer said in order to meet the larger phosphorus goals, Auburn will need help from the “headwater” towns in the upper watershed: Turner, Minot and Buckfield.

The projected buildout is close to 700 combined in the upper watershed towns.

When asked, Diemer said that even if the upper watershed towns implemented half of the low impact development standards that Auburn has or is considering, it would likely help meet the annual phosphorus goal.

The septic rules and zone change to a lower density will be reviewed by the Planning Board before going to the City Council.

The original report found “no net environmental, economic or social benefit supporting expansion of development in the Lake Auburn watershed.” However, among its recommendations was amending the watershed boundary along Gracelawn Road, and updating the septic ordinance.

The new Lake Auburn modeling can be found at https://tinyurl.com/3zhjnenw.