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Simulated Hydrologic Responses to Proposed Wastewater-Return-Flow Scenarios in Falmouth, Massachusetts

Scientific Investigations Report 2025-5066
Prepared in cooperation with the Town of Falmouth
By:  and 
https://doi.org/10.3133/sir20255066

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Abstract

The Cape Cod aquifer is the sole source of drinking water for communities on Cape Cod, Massachusetts, including the Town of Falmouth, where the aquifer is currently threatened by contamination from septic-system-derived nitrogen. To address this problem, the Town is proposing to sewer areas of Falmouth, treat the wastewater at the Town’s Main Wastewater Treatment Facility (a nitrogen removing/tertiary treatment facility), and discharge the treated wastewater to an ocean outfall pipe in Nantucket Sound.

The U.S. Geological Survey, in cooperation with the Town of Falmouth, updated a three-dimensional steady-state groundwater flow model to represent current (defined as 2019–23) average hydrologic conditions and to simulate the long-term average freshwater hydrologic response to two wastewater-return-flow scenarios. Scenario 1 involves the sewering of all properties south of Route 28 in Falmouth, which approximates the Town’s possible sewer expansion over the next 20–30 years. Scenario 2 involves sewering of all properties in Falmouth to demonstrate the maximum potential effect of sewering on the aquifer.

Overall, the simulated hydrologic response of water-table altitudes and streamflow in both scenarios was relatively small compared to fluctuations from natural recharge. In scenario 1, the water-table altitude decreased by about 0.1 feet south of Route 28, where the conversion to municipal sewers removed wastewater-return flow from onsite septic systems. The water-table altitude decreased by about 0.1–0.2 feet over a larger area in Falmouth under town-wide sewering in scenario 2. The greatest decrease in water-table altitude in both scenarios occurred near the Main Wastewater Treatment Facility, with a decrease of about 1.1 feet in scenario 1 and about 1.3 feet in scenario 2.

Simulated decreases in streamflow also were estimated for six selected streams in Falmouth and Mashpee. In both scenarios, the largest simulated decreases in streamflow were at the Coonamessett River, which is the closest stream to the Main Wastewater Treatment Facility. In scenario 1, the average annual decrease in flow at the Coonamessett River was 0.1 cubic feet per second, a 1.1 percent decrease from current (2019–23) conditions. In scenario 2, streamflow at the Coonamessett River decreased by 0.6 cubic feet per second, a 5.4 percent decrease from current (2019–23) conditions.

Suggested Citation

Goldstein, K.M.F., and McCobb, T.D., 2025, Simulated hydrologic responses to proposed wastewater-return-flow scenarios in Falmouth, Massachusetts: U.S. Geological Survey Scientific Investigations Report 2025–5066, 19 p., https://doi.org/10.3133/sir20255066.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Acknowledgments
  • Abstract
  • Introduction
  • Updates to the Existing Regional Groundwater Flow Model
  • Wastewater-Return-Flow Scenarios and Simulation Approaches
  • Simulated Responses to Changes in Wastewater-Return Flow
  • Limitations of the Study
  • Summary
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Simulated Hydrologic Responses to Proposed Wastewater-Return-Flow Scenarios in Falmouth, Massachusetts
Series title Scientific Investigations Report
Series number 2025-5066
DOI 10.3133/sir20255066
Publication Date August 08, 2025
Year Published 2025
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) New England Water Science Center
Description Report: vii, 19 p.; Data Release
Country United States
State Massachusetts
City Falmouth
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Additional publication details