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Scientific Investigations Report 2007-5068

National Water Quality Assessment Program
Transport of Anthropogenic and Natural Contaminants (TANC) to Public-Supply Wells

Simulations of Ground-Water Flow, Transport, Age, and Particle Tracking near York, Nebraska, for a Study of Transport of Anthropogenic and Natural Contaminants (TANC) to Public-Supply Wells

By Brian R. Clark, Matthew K. Landon, Leon J. Kauffman, and George Z. Hornberger

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Scientific Investigations Report
2007-5068 PDF (8.5 MB)
Abstract

Contamination of public-supply wells has resulted in public-health threats and negative economic effects for communities that must treat contaminated water or find alternative water supplies. To investigate factors controlling vulnerability of public-supply wells to anthropogenic and natural contaminants using consistent and systematic data collected in a variety of principal aquifer settings in the United States, a study of Transport of Anthropogenic and Natural Contaminants to public-supply wells was begun in 2001 as part of the U.S. Geological Survey National Water-Quality Assessment Program.

The area simulated by the ground-water flow model described in this report was selected for a study of processes influencing contaminant distribution and transport along the direction of ground-water flow towards a public-supply well in southeastern York, Nebraska. Ground-water flow is simulated for a 60-year period from September 1, 1944, to August 31, 2004. Steady-state conditions are simulated prior to September 1, 1944, and represent conditions prior to use of ground water for irrigation.

Irrigation, municipal, and industrial wells were simulated using the Multi-Node Well package of the modular three-dimensional ground-water flow model code, MODFLOW-2000, which allows simulation of flow and solutes through wells that are simulated in multiple nodes or layers. Ground-water flow, age, and transport of selected tracers were simulated using the Ground-Water Transport process of MODFLOW-2000. Simulated ground-water age was compared to interpreted ground-water age in six monitoring wells in the unconfined aquifer. The tracer chlorofluorocarbon-11 was simulated directly using Ground-Water Transport for comparison with concentrations measured in six monitoring wells and one public supply well screened in the upper confined aquifer.

Three alternative model simulations indicate that simulation
results are highly sensitive to the distribution of multilayer well bores where leakage can occur and that the calibrated model resulted in smaller differences than the alternative models between simulated and interpreted ages and measured tracer concentrations in most, but not all, wells. Results of the first alternative model indicate that the distribution of young water in the upper confined aquifer is substantially different when well-bore leakage at known abandoned wells and test holes is removed from the model. In the second alternative model, simulated age near the bottom of the unconfined aquifer was younger than interpreted ages and simulated chlorofluorocarbon-11 concentrations in the upper confined aquifer were zero in five out of six wells because the conventional Well Package fails to account for flow between model layers though well bores. The third alternative model produced differences between simulated and interpreted ground-water ages and measured chlorofluorocarbon-11 concentrations that were comparable to the calibrated model. However, simulated hydraulic heads deviated from measured hydraulic heads by a greater amount than for the calibrated model. Even so, because the third alternative model simulates steady-state flow, additional analysis was possible using steady-state particle tracking to assess the contributing recharge area to a public supply well selected for analysis of factors contributing to well vulnerability.

Results from particle-tracking software (MODPATH) using the third alternative model indicates that the contributing recharge area of the study public-supply well is a composite of elongated, seemingly isolated areas associated with wells that are screened in multiple aquifers. The simulated age distribution of particles at the study public-supply well indicates that all water younger than 58 years travels through well bores of wells screened in multiple aquifers. The age distribution from the steady-state model using MODPATH estimates the youngest 7 percent of the water to have a flow-weighted mean age of 16 years. In comparison, interpretations of age tracer and stable isotope data indicate water samples from the study public-supply well are a mixture of 7 to 14 percent water with a mean age of 14 years or less mixed with 86 to 93 percent old, upper confined aquifer water.

Long-term projections of nitrate-nitrogen concentrations and the effects of denitrification were made using simulated ages from the steady-state model with MODPATH, estimates of historical and projected nitrate-nitrogen concentrations in recharge, and estimated denitrification rates. The calculated concentration of nitrate-nitrogen of 0.19 milligrams per liter for 2003 of the projection is similar to measured values of nitrate-nitrogen at the study public-supply well, which range from 0.17 to 0.20 milligrams per liter. The projections indicate that nitrate-nitrogen concentrations will remain near steady-state values for the duration of the 100-year projection period as long as denitrification rates remain constant and steady-state flow conditions occur. While simplistic, the conceptual calculations indicate the importance of denitrification as a mechanism influencing nitrate-nitrogen concentrations in the study area. The peak value of decayed nitrate-N appears to lag behind the input concentration in recharge by approximately 30 years.

Young, potentially contaminated water in the unconfined aquifer can move downward into the confined aquifers of the ground-water system where public-supply wells are screened. Water chemistry results and results of Ground-Water Transport age and tracer simulations indicate that wells screened through multiple aquifers can introduce flow through the well bores, thus providing a “short circuit” from the unconfined aquifer to the upper confined aquifer. Simulations indicate that chlorofluorocarbon-11 concentrations in the upper confined aquifer originate at wells screened through multiple aquifers and are drawn toward other pumping wells. This mechanism for movement of younger water through the well bores of wells screened through multiple aquifers is thought to be the primary process affecting the presence and distribution of contaminants in the confined aquifers and is in agreement with independent geochemical data.

Version 1.0

Posted April 2008


Suggested citation:

Clark, B.R., Landon, M.K., Kauffman, L.J., and Hornberger, G.Z., 2008, Simulations of ground-water flow, transport, age, and particle tracking near York, Nebraska, for a study of transport of anthropogenic and natural contaminants (TANC) to public supply wells: U.S. Geological Survey Scientific Investigations Report 2007–5068, 48 p.



Contents

Abstract

Introduction

Purpose and Scope

Description of Local-Scale Model Area

Public-Supply Well Selection

Preferential Flow through the Upper Confining Unit

Methods for Simulation of Ground-Water Flow and Age

Updates to the Regional Model

Local-Scale Model Geometry and Discretization

Boundary Conditions, Model Stresses, and Initial Conditions

Model Hydraulic Parameters

Model Parameters for Simulating Ground-Water Age and CFC-11 Concentrations

Alternative Model Construction

Design of Particle-Tracking Simulations

Flow and Transport Model Uncertainties and Limitations

Flow and Transport Model Calibration Results

Flow Model Calibration Results

Simulated Flows and Heads

Simulated Water Budget

Simulated Transport and Particle-Tracking Results

Contributing Recharge Area Summary

Comparison of Simulated Transport and Particle Tracking Age Distribution

Long-Term Concentrations of Nitrate in the Study Public-Supply Well

Implications of Well-Bore Leakage on Ground-Water Quality

Summary and Conclusions

References Cited

Appendix 1. Computation of MODPATH Particle Traveltimes Through Well Bores

Appendix 2. Digital Three-Dimensional Animations of CFC-11 Transport Simulations

 

 


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