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Simulation of Groundwater Flow To Evaluate Hydrogeologic Controls on a PFAS Plume, Coakley Landfill Superfund Site, Rockingham County, New Hampshire

Scientific Investigations Report 2026-5008
Prepared in cooperation with the U.S. Environmental Protection Agency
By:  and 
https://doi.org/10.3133/sir20265008

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Abstract

Per- and polyfluoroalkyl substances (PFAS), including perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), have been detected at combined concentrations above 2,000 nanograms per liter (ng/L) at groundwater seep locations near the Coakley Landfill Superfund site, in North Hampton, New Hampshire. The landfill was active from 1972 to 1985. An impermeable cap was placed on the landfill in 1998. The adjacent area to the Coakley Landfill has many water supply wells, and transport of PFAS compounds to the wells is a concern. Fracture anisotropy in the underlying bedrock aquifer complicates the understanding of PFAS transport because groundwater preferentially travels along fractures that may not align with the prevailing groundwater flow direction.

In 2018, the U.S. Environmental Protection Agency and the U.S. Geological Survey began an investigation of the groundwater flow from the Coakley Landfill site. This report describes the modification of a numerical groundwater-flow model for the local area around the Coakley Landfill and summarizes findings of the investigation. In addition, this report includes a brief description of PFOA and PFOS occurrence, a discussion of model construction, evaluation of model performance through calibration, and discussion of simulation results for two periods (before and after capping). Limitations are also discussed.

Results show that simulated groundwater flow moves from the Coakley Landfill to the west and north. Advective transport modeling using particle tracking shows that groundwater from the landfill discharges primarily to streams to the west and north, and a small amount is transported to distal wells. Dilution of contaminants through advection and dispersion likely plays a role in whether PFAS compounds from the landfill will be detected above laboratory reporting levels at distal wells.

Plain Language Summary

A class of chemicals called per- and polyfluoroalkyl substances (PFAS) has been seeping from the Coakley Landfill in southeastern New Hampshire to the local groundwater. The movement of the groundwater is complex because of the local geology, and more information is needed about where PFAS goes after it comes out of the landfill. The U.S. Geological Survey worked with the U.S. Environmental Protection Agency to understand more about how PFAS move from the landfill through the local groundwater and why concentrations are higher in some places than in others. A computer groundwater model of the Coakley Landfill area was developed based on an older groundwater model for southeast New Hampshire, and the new model was used to explore how soil, bedrock, rain or snowmelt infiltration, and bedrock fractures affect the speed and direction of groundwater flow. The new model was refined using recently collected data from the bedrock aquifer, where the greatest contamination from the Coakley Landfill has been detected. A modeling technique called particle tracking was used to estimate where groundwater travels from the landfill. The model shows that groundwater flows primarily to the west, north, and northeast from the landfill, likely following bedrock fractures. Some groundwater flow paths originating at the landfill eventually come to the surface in streams, up to about 3 miles away from the landfill. These flow paths predicted by the model may explain why there have been PFAS detections in wells relatively far from the landfill. However, predicted groundwater flow paths do not account for some factors that could reduce the total travel distance of contaminants, like dilution, mixing, and adsorption. Model results show that an impermeable cap placed on the landfill in 1998 reduces the amount of rain and snowmelt that flow into the landfill.

Suggested Citation

Harte, P.T., and Collins, A.L., 2026, Simulation of groundwater flow to evaluate hydrogeologic controls on a PFAS plume, Coakley Landfill Superfund site, Rockingham County, New Hampshire: U.S. Geological Survey Scientific Investigations Report 2026–5008, 41 p., https://doi.org/10.3133/sir20265008. [Supersedes preprint https://doi.org/10.31223/X53761.]

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Acknowledgments
  • Abstract
  • Plain Language Summary
  • Introduction
  • Design of Numerical Groundwater-Flow Model
  • Simulation of Groundwater Flow
  • Limitations of the Model
  • Evaluation of Hydrogeologic Controls on PFAS Plume
  • Summary and Conclusions
  • References Cited
  • Appendix 1. Supplemental Model Construction Information
Publication type Report
Publication Subtype USGS Numbered Series
Title Simulation of groundwater flow to evaluate hydrogeologic controls on a PFAS plume, Coakley Landfill Superfund site, Rockingham County, New Hampshire
Series title Scientific Investigations Report
Series number 2026-5008
DOI 10.3133/sir20265008
Publication Date May 11, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) New England Water Science Center
Description Report: viii, 41 p.; Data Release
Country United States
State New Hampshire
Other Geospatial Coakley Landfill Superfund Site
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Additional publication details