Simulation of zones of groundwater contribution to wells south of the Naval Weapons Industrial Reserve Plant in Bethpage, New York

Scientific Investigations Report 2017-5161
Prepared in cooperation with the Naval Facilities Engineering Command
By:

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Abstract

A steady-state three-dimensional groundwater-flow model that simulates present conditions was coupled with the particle-tracking program MODPATH to delineate zones of contribution to wells pumping from the Magothy aquifer near a chlorinated volatile organic compound (VOC) plume. This modeling was part of a study by the U.S. Geological Survey in cooperation with the Naval Facilities Engineering Command to delineate groundwater near the Naval Weapons Industrial Reserve Plant in Bethpage, New York. Because rates of advection within the coarse-grained sediments typically exceed 0.1 foot per day, transport by dispersion and (or) diffusion was assumed to be negligible. Resulting zones of contribution are complex shapes, influenced by hydrogeologic features including confining beds and a basal gravel zone, and the interplay of nearby hydrologic stresses. The use of two particle tracking techniques identified zones of contribution to wells. Particles are backtracked from pumping well screens, and particles are forward tracked from the location of a VOC plume, as defined by surfaces of equal total VOC concentration. During any period of 5 years or less, about 1 to 3 percent of particles backtracked from pumping wells within a focus area intersected the 5-part per billion (ppb) VOC plume shell, indicating that the vast majority of particles were not sourced from the plume. During 5 years or less, none of the particles backtracked from pumping wells intersected the 50-ppb VOC plume shell. Forward-tracking techniques identified the fate of water within the VOC plume after 5 years as it moves toward ultimate well capture and (or) discharge to model constant head and drain boundaries. Out of 4,813 forward tracked particles started within the 50-ppb VOC plume shell, 1 forward-tracked particle was captured by well ANY8480. Out of 22,958 forward tracked particles started within the 5-ppb VOC plume shell, 100 were captured by production wells (less than 1 percent). The subset of forward pathlines that represent well plume capture are similar in number and shape to those of backtracked pathlines.

Model simulations were conducted to assess uncertainties and improve understanding of how variability in hydraulic properties, pumpage rates, and maximum particle traveltime affect delineation of zones of contribution. By use of driller’s’ logs, a transitional probability approach generated nine alternative realizations of heterogeneity within the Magothy aquifer to assess uncertainty in model representation. Fine-grained sediments with low hydraulic conductivity were realized as laterally discontinuous, thickening towards the south, and comprising about 27 percent of the total aquifer volume within the transitional probability subgrid. Model simulations with alternative pumpage rates, porosity terms, and alternative maximum particle traveltime were also used to demonstrate how the size and shape of zones of contribution may vary.

Suggested Citation

Misut, P.E., 2018, Simulation of zones of groundwater contribution to wells south of the Naval Weapons Industrial Reserve Plant in Bethpage, New York: U.S. Geological Survey Scientific Investigations Report 2017–5161, 45 p., https://doi.org/10.3133/sir20175161.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Methods
  • Analysis of Zones of Contribution to Wells
  • Discussion
  • Summary and Conclusion
  • References Cited
  • Glossary
  • Appendix 1. List of Wells Within the Study Area South of the Naval Weapons Industrial Reserve Plant in Bethpage, New York

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Simulation of zones of groundwater contribution to wells south of the Naval Weapons Industrial Reserve Plant in Bethpage, New York
Series title Scientific Investigations Report
Series number 2017-5161
DOI 10.3133/sir20175161
Year Published 2018
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) New York Water Science Center
Description Report: vii, 45 p.; Data release
Country United States
State New York
City Bethpage
Online Only (Y/N) Y
Additional Online Files (Y/N) N