Prepared in cooperation with the U.S. Environmental Protection Agency

Simulation of Groundwater Flow in a Volatile Organic 

Compound-Contaminated Area near Bethpage, 

Nassau County, New York—A Discussion of 

Modeling Considerations

Open-File Report 2011–1128

U.S. Department of the Interior

U.S. Geological Survey
This page has been left blank intentionally.
Simulation of Groundwater Flow in a 

Volatile Organic Compound-Contaminated 

Area near Bethpage, Nassau County, 

New York—A Discussion of Modeling 

Considerations

By Paul E. Misut

Prepared in cooperation with the U.S. Environmental Protection Agency

Open-File Report 2011–1128

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Misut, P.E., 2011, Simulation of groundwater flow in a volatile organic compound-contaminated area near Bethpage, 

Nassau County, New York—A discussion of modeling considerations: U.S. Geological Survey Open-File Report 

2011–1128, 19 p., at http://pubs.usgs.gov/of/2011/1128/.
United States Department of the Interior 

U. S. GEOLOGICAL SURVEY 

NEW YORK WATER SCIENCE CENTER 

2045 Rt. 112

Coram, NY 11727

(631) 736-0783

June 3, 2011

Mr. Robert Alvey, P.G.

Technical Support Team

U.S. Environmental Protection Agency, Region II

Emergency and Remedial Response Division

Program Support Branch

290 Broadway, 18th Floor

New York, NY 10007-1866

Subject: Simulation of Groundwater Flow in a VOC-Contaminated Area near Bethpage, 

Nassau County, New York—A Discussion of Modeling Considerations

Dear Mr. Alvey: 

The U.S. Geological Survey (USGS) was requested by U.S. Environmental Protection 

Agency (USEPA) to conduct a limited review and assessment of groundwater model(s) 

developed by consultants for Northrup Grumman Corporation in connection with the Grumman 

and Navy sites in Bethpage, New York, and contaminated groundwater flow potentially 

affecting public-supply wells. The focus of this review and assessment was necessarily 

limited and could not be conducted by fully utilizing the 2004 USGS Guidance document 

(Reilly, T.E., and Harbaugh, A.W., 2004, Guidelines for evaluating ground-water flow models: 

U.S. Geological Survey Scientific Investigations Report 2004–5038; http://pubs.usgs.gov/

sir/2004/5038/), because apparently a comprehensive calibration document has not been 

produced. Nevertheless, reviewers and users of this study have a need to evaluate the accuracy 

and reasonableness of the current groundwater model. Thus, the review is intended to identify 

ways of improving these aspects of the model, and to explain the limitations of the modeling 

analysis and models in general. This report, USGS Open-File Report 2011–1128, represents 

USGS publication of the subject review submitted to USEPA on April 4, 2011. If you have 

any questions or comments about the information I have provided, please do not hesitate to 

telephone me at the above number or email me at pemisut@usgs.gov.

Sincerely, 

Paul E. Misut

Hydrologist
This page has been left blank intentionally.
v

Contents

Executive Summary.......................................................................................................................................1

Introduction.....................................................................................................................................................2

History of Modeling Efforts at Bethpage ...................................................................................................3

Considerations in Groundwater-Flow Simulation.....................................................................................5

Conclusion.....................................................................................................................................................12

References Cited..........................................................................................................................................12

Glossary of Terms and Acronyms .............................................................................................................14

Figures

1. Map showing location of Public Water Districts surrounding Naval Weapons

Industrial Reserve Plant, Bethpage, Nassau County, New York...........................................2

2. Graph showing total volatile organic compounds (TVOC) at onsite containment

system (ONCT) pumping wells, 1998 to 2010 ............................................................................4

B2–1. Diagram showing conceptual overview of representing complex hydrogeological

conditions through use of pilot points.......................................................................................6

3. Graph showing monthly rate of pumpage at Bethpage Well 6-2..........................................7

4. Graphs showing groundwater level at Well N1259 in Bethpage and precipitation at

National Weather Service station at Mineola, Nassau County, New York, 1950–2000.....9

5. Cross section showing groundwater ages along Nassau-Suffolk County border

prior to development..................................................................................................................10

6. Map showing predicted maximum declines in the water table as a result of sewer

installation during the 1980s .....................................................................................................11

Table

1. History of Bethpage groundwater model applications and updates.................................18
vi

Conversion Factors and Datums

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m) 

mile (mi) 1.6093 kilometer (km)

Volume

gallon (gal) 3.785 liter (L)

Flow Rate

foot per year (ft/yr) 0.3048 meter per year (m/yr)

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Concentrations of chemical constituents in water are given either in parts per billion (ppb) or 

micrograms per liter (μg/L).
Simulation of Groundwater Flow in a Volatile Organic 

Compound-Contaminated Area near Bethpage, 

Nassau County, New York—A Discussion of 

Modeling Considerations

Executive Summary

The 2010 Bethpage groundwater-flow model (ARCADIS, 2010) was based on a steady state1 assumption. Although it 

is widely acknowledged that significant water-level changes have occurred in the past, the reviewed model does not represent 

changing water levels. The steady state approach limits the effectiveness of the following:

1. identification of sources of contamination,

2. analysis of model accuracy,

3. model calibration, and 

4. simulations of future scenarios. 

Future plume movement was simulated in an incomplete manner through an unchanging groundwater-flow field. Available 

time-series information on temporal variation of factors affecting groundwater-flow dynamics includes:

1. public-supply pumping, 

2. groundwater discharges from systems remediating volatile organic compound (VOC) plumes, 

3. recharge and precipitation rates, and

4. water levels and streamflows.

Transient phenomena that might be useful in future hypothetical simulations include pumping variations, redirection of 

containment-system waters for industrial use, and climate-change scenarios. Public-domain computer programs, U.S. Geological 

Survey guidance reports on transient-state calibration and uncertainty methods (Doherty and Hunt, 2010), and additional 

local and regional datasets are available to provide additional confidence in model evaluations and allow better evaluation of 

their limitations.

By Paul E. Misut

1

Glossary terms and acronyms in bold are defined in the glossary and list of acronyms on page 14.
2 

Introduction

The U.S. Geological Survey (USGS) was requested by the U.S. Environmental Protection Agency (USEPA) to provide 

an opinion as to whether a consultant’s current groundwater model of plumes emanating from the Northrup Grumman and 

Navy sites at Bethpage, New York (fig. 1) is sound and appropriate. The product of this review is the following technical 

memo, intended to assist modelers in improving the accuracy and reliability of their models, and to explain the limitations of 

such models. This review provides (1) a brief history of modeling efforts at Bethpage, and (2) describes the use of techniques, 

existing datasets, and data-collection activities for the purpose of gaining additional confidence in model evaluations and better 

judging the uncertainty level and limitations inherent in model evaluations. 

EXPLANATION

Northrup Grumman facility (1997)

Naval Weapons Industrial Reserve 

 plant (1997)

USGS long-term monitoring 

 well N1259

County boundary

Production wells (2010)

0

0

5,000

1,500

10,000 FEET

3,000 METERS

73°31’51”

40°46’22”

73° 25'12"

40°40'37”

BETHPAGE

LEVITTOWN

AQUA NY

S. FARMINGDALE

MASSAPEQUA

HICKSVILLE

AQUA NY

FARMINGDALE

PLAINVIEW Nassau County

Suffolk County

NEW YORK

Long Island

0 50 100 MILES

0 50 100 KILOMETERS

Bethpage

Base digitized from New York State Department of Transportation,1:24,000 quadrangles,

Projection Universal Transverse Mercator, Zone 18, NAD 83

Figure 1. Location of Public Water Districts surrounding Naval Weapons Industrial Reserve Plant, Bethpage, Nassau County, 

New York.
 3

History of Modeling Efforts at Bethpage

USGS modeling of the groundwater-flow system of Nassau County began prior to the advent of computers and modeling of 

the Bethpage area culminated with a 1992 particle tracking analysis (Smolensky and Feldman, 1995) conducted in cooperation 

with the Nassau County Department of Health (NCDH). Water particles were depicted moving (1) towards deep industrial 

pumping wells located on the Bethpage Northrup Grumman site, and (2) away from surface recharge basins where particles 

representing water captured by industrial wells were reintroduced. This pumping-recharge cycle resulted in rearrangement and 

partial containment of a plume of volatile organic compounds (VOCs) generally migrating in a southward direction toward 

the Levittown area at a rate of about 200 feet per year (Smolensky and Feldman, 1995). The analysis also indicated that some 

particles applied at the water table upgradient of onsite recharge basins were drawn into the deep zones of onsite well influence 

but not captured, and ultimately discharged at the southern model boundary in the Levittown area in the bottom 10 percent of the 

lowest model layer.

From 1995 to the present, consultants have developed a series of models that are generally consistent with the earlier USGS 

work but depict greater containment of onsite VOCs. In 1995, a flow model was used to help design a complete onsite hydraulic 

containment system (ONCT) at the Northrup Grumman facility. In 1998 analysis of field data concluded that ONCT was 

operating effectively (Smith, 2011a2); however, estimation was needed of when solutes already downgradient of ONCT at the 

time of its initiation would reach production wells. In response, additional flow modeling was used in 2002 to identify locations 

for monitoring wells to provide a 5-year early warning to downgradient public-water suppliers potentially impacted (Smith, 

2011a2). In 2010, a solute transport model was used to evaluate potential remediation actions at offsite “hot spots” where 

elevated levels of contamination were observed southeast of ONCT (Smith, 2011a2). Although a thorough calibration document 

has not yet been produced for the 2010 model and its reliability cannot be thoroughly assessed, this 2010 model has become an 

indispensible tool for planning future remedial action (ARCADIS, 2010) due to the complexity inherent in system response to 

proposed stresses.

The history of model application at the Bethpage site is tempered by unanticipated observations and revisions including 

the following: 

1. A second groundwater remedial investigation of operable unit 3 (OU3), begun in 2004, delineated a separate plume 

that commingled with the original operable unit 2 (OU2) plume and the capture zone of ONCT Well 19 (Smith, 2011b3). 

The increasing concentration of contaminated water captured at ONCT Well 19 (fig. 2) was contrary to the predicted 

(ARCADIS, 2003) decreasing trend for that site and may be attributed to the OU3 source.

2. Modeled trends in ONCT capture of contaminants originating from the USEPA Hooker/Ruco Superfund site (Occidental 

Chemical Corporation, 1996) did not match observations (Smith, 2011b3); stated lack of formal model documentation 

(Occidental Chemical Corporation, 1996) may have contributed to this discrepancy.

3. The outpost monitoring well associated with an Aqua, New York wellfield, sited through modeling, failed to provide early 

warning of VOC arrival in 2009; that is, VOCs were detected in the production well pumpage before they were detected at 

the associated upgradient outpost monitoring well. This failure has been attributed to inaccurate delineation of the leading 

edge of the plume (Smith, 2011c4).

4. VOC detection at an outpost well associated with the South Farmingdale production Wellfield #1 occurred sooner 

than predicted (Smith, 2011c4). A water sample collected immediately after outpost well installation contained VOCs 

(R. Humann, H2M, oral commun., 2011). Uncertainty of estimated arrival time may limit the usefulness of outpost wells to 

provide sufficient notification to plan and construct treatment for the wellfield. 

5. The models did not accurately reproduce the pumpage rate-related pattern of VOC concentration observed at Bethpage 

Water District Well 6–2 (R. Humann, H2M, oral commun., 2011; Smith, 2011d5)

2

 Smith, K., 2011a, “History of Bethpage Groundwater Model Applications and Updates”, Northrup Grumman, written commun., 01/31/2011, see appendix A.

3

 Smith, K., 2011b, Presentation to Navy Remedy Validation and Optimization Team, Northrup Grumman, written commun., 02/8/2011.

4

 Smith, K., 2011c, Subject: December 14, 2010 Memo “Preliminary Review of Model Used for Comprehensive Feasibility Study”, Northrup Grumman, 

written commun., 02/10/2011.

5

 Smith, K., 2011d, “Responses to 9 Questions Received December 10, 2010 from the USGS on the Northrup Grumman Regional Groundwater Flow and 

Transport Model”, Northrup Grumman, written commun., 1/28/2011.
4 

6. Observed total VOC concentrations above 100 parts per billion (ppb) near the bottom of vertical profile boring VP–47 

(below a depth of 710 feet) are not represented in the model; this is attributed to the age and relative isolation of the VOC 

observations (Smith, 2011d5).

7. A model layering scheme was locally updated with the addition of a confining layer overlying the screen zone of Bethpage 

Water District Well 5–1 (Smith, 2011d5).

8. Models required revisions to steady state pumping and recharge rates (Smith, 2011a2) yet do not address stressed conditions 

that affect the hydraulics of the region. Steady state configurations imply time-invariant pumping and recharge rates; 

however, model changes have been necessary. 

Uncertainties inherent in model simulations, such as those conducted for the Bethpage site, have been widely documented 

(Konikow, 2010) and will likely lead to further model updates as new field data and modeling methods become available. It may 

be important to consider the effects of model revision on previously simulated results. Some model uncertainties are expected to 

continue and may be addressed through presentation of alternate model configurations and parameter-sensitivity analysis. TVOC concentration, in micrograms per liter

5,000

4,500

4,000

3,500

3,000

2,500

2,000

1,500

1,000

500

0

Jan. 1998Jan. 1999Jan. 2000

Dec. 2000

Jan. 2002Jan. 2003Jan. 2004

Dec. 2004

Jan. 2006Jan. 2007Jan. 2008

Dec. 2008

Jan. 2010

Sample date

Well 1

Well 3

Well 17

Well 18

Well 19

Startup of

OU2 onsite

system

Nov. 1998

wells, 1998 to 2010. (From Smith, 2011b3

)

Figure 2. Total volatile organic compounds (TVOC) at onsite containment system (ONCT) pumping 
 5

Considerations in Groundwater-Flow Simulation

The following section describes the use of techniques, existing datasets, and data-collection activities for the purpose of 

gaining additional confidence in model evaluations and (or) better judging the uncertainty level and limitations inherent in 

model evaluations. Topics include (1) the complex heterogeneous nature of the Magothy aquifer, (2) parameter estimation 

methods including pilot points, (3) types of dynamic response present in the Bethpage groundwater-flow system, (4) steady- and 

transient-state simulation methods, and (4) migration of contaminants across zones of contrasting permeability.

Aquifer Complexity—The Magothy aquifer and the upper zone of the Raritan formation are heterogeneous mixtures of highand low-permeability materials through which groundwater flows preferentially within the permeable materials. Thus, the failure 

of an outpost monitoring well installed to provide early warning of VOC arrival at the Aqua New York production well (Smith, 

2011c4) may have resulted from movement of solutes into the pumping well through a highly permeable zone that was not 

tapped by the outpost monitoring well. The fact that many of the low-permeability zones observed in boreholes do not extend 

from one location to another led Kent Smith (Northrup Grumman Corporation) to the conclusion that bulk anisotropy should be 

specified in models:

“Recognizing the variability that has been observed in individual boreholes only tends to emphasize the lack of 

defined variability between boreholes. As employed in the model, use of assigned anisotropy values to represent 

lithologic variability (which accounts in a bulk fashion for the vertical variability in aquifer material) is a tested and 

reliable modeling technique” (Smith, 2011c4)

Models were divided into minimal anisotropy zones. Five zones were included in the 1998 model (ARCADIS, 2003) and 

later studies added zones, among which were a localized confining layer at Bethpage plant 5 (Smith, 2011d5), which lies in 

an area that had apparently been mapped by the modeler in greater detail than other potential zone candidates. The approach, 

however, generally remained one of establishing a minimal number of zones, then manually adjusting associated parameters 

through trial and error to match a simplified steady state approximation of a subset of water levels. Thus, the result is a 

simplified representation of Magothy aquifer heterogeneity that differs fundamentally from that suggested by gamma logs, 

water-level hydrographs, outpost monitoring well failure, and other lines of evidence. 

The large amount of computational work inherent in solving the flow and transport equations requires that modelers 

strive for parsimony—that is, the maximum simplicity attainable without ignoring or excluding critical factors. For example, 

minimally-zoned models may have greater parsimony than models featuring complex zonations in simple homogeneous

settings. Conversely, in complex settings, a simple minimally-zoned model is not parsimonious. In order to judge model 

parsimony, it is useful to consider those factors critically affecting simulation results, and discuss how to improve their 

representation.
6 

Parameter estimation—In models striving to represent heterogeneous mixtures of high- and low-permeability materials, an 

alternative to pre-assigning a minimal zonal structure to a model would be to implement a more flexible, highly parameterized 

approach such as provided by the public domain code PEST (acronym for Parameter ESTimation, Doherty and Hunt, 2010). 

PEST reformulates the groundwater calibration problem such that large numbers of parameter zones become tractable and yield 

a parsimonious model. Among the benefits of PEST is the ability to provide sophisticated estimates of parameter and predictive 

uncertainty. The following excerpt from USGS guidelines (Doherty and Hunt, 2010) describes and illustrates PEST pilot points, 

a method in which traditional parameter zones are replaced with a matrix of points:

“The general goal of pilot points is to provide a middle ground between cell-by-cell variability and reduction to a 

few homogeneous zones for characterizing natural-world heterogeneity in groundwater models. Figure B2–1 depicts 

a schematic representation of the pilot-point implementation. In figure B2–1A, a heterogeneous field is depicted 

overlain by a model grid. This illustrates that, even at the model-cell scale, the representation of heterogeneity requires 

simplification. In figure B2–1B, a network of pilot points is shown in which the size of the circle is proportional to 

the parameter value and the color represents the value on the same color scale as in figure B2–1A. The general pattern 

of variability in the true field is visible in this image, but the resolution is much coarser than reality. Figure B2–1C 

shows the pilot-point values interpolated onto a very fine grid and illustrates that much of the true heterogeneity 

can be reconstructed from a subset of sampled values provided that appropriate interpolation is performed. Figure 

B2–1D shows the interpolated version of the pilot-point values in figure B2–1B on the model-cell grid scale, which 

represents the version of reality that the model would actually see. In reality, rather than directly sampling the true 

field as in this illustration, the pilot points are surrogates for the real parameter field estimated from observations in the 

calibration dataset and are therefore likely to include some error not depicted on this figure. However, the schematic 

representation depicts the best possible representation of the real field given the displayed density of pilot points.”

A B

C D

600

500

400

300

200

100

Figure B2–1. Conceptual overview of representing complex hydrogeological conditions through use of pilot points. 

Panel A, shows the inherent property value overlain by the model grid in gray. Panel B is a representation of the 

true property values by a grid of pilot points in which symbol size indicates value. Panel C shows an interpolated 

representation of panel B on an arbitrarily fine grid scale. Panel D shows the value from the pilot points interpolated 

to the computational-grid scale. Interpolation in all cases was done by using ordinary kriging. The same color scale 

applies to all four panels. (From Doherty and Hunt, 2010)
 7

Seasonal pumping stresses—Pumping by Nassau County water suppliers varies by season and other factors such as shutdown/

rebalancing due to water quality degradation. For example, summer pumpage at Bethpage production Well 6–2 (figures 1, 3) 

may be more than 10 times greater than winter pumpage, as illustrated in the following 1982–2009 hydrograph showing 

summer. For nine months starting in August 2007, Well 6–2 was taken offline due to elevated summer VOC concentrations 

(greater than 800 ppb total VOC, R. Humann, H2M, oral commun., 2011). Temporal variations in pumping rates and other 

stresses enhance plume spreading and may explain why solute concentrations increase at production wells that are pumped 

heavily in summer. 

0

10

20

30

40

50

60

70

80

1980 1990 2000 2010

Millions of gallons pumped

Year

Monthly rate of pumping

July rate of pumping

Bethpage well 6–2

Geological Survey, written commun., 2011)

Figure 3. Monthly rate of pumpage at Bethpage Well 6-2. (Data may be unavailable for 1989–90) (J. Monti, Jr. and J. Hock, U.S. 
8 

Steady state and transient representation—The consultant (Smith, 2011d5) states: 

‘With the exception of local variations (i.e., proximal to production wells), temporal or cyclical changes in water 

levels (both historical and for long-term predictions) have not, nor will they significantly impact long-term average 

groundwater-flow directions and rates. This concept is discussed in detail in “Effect of Seasonal and Long-Term 

Changes in Stress on Sources of Water to Wells, WSP–2445.”

The referenced USGS Water-Supply Paper 2445 (Reilly and Pollock, 1995) presents an analysis of simplified, hypothetical, 

steady state and transient-state models and concludes that the particle pathlines generated by transient-state simulations are 

smoothed and become similar to those generated under averaged steady state conditions when the simulation time includes 

several stress cycles. However, Reilly and others further conclude the following two points: 

1. The differences between a steady state and transient-state estimate of the capture zone “could be significant” (Reilly and 

Pollock, 1996) near the boundaries of the capture zone or at small distances from the sources of transient effects.

2. The area contributing recharge to a well (for the given system geometry and boundary conditions) is independent of 

porosity in a steady state simulation but not in a transient-state simulation (Reilly and Pollock, 1996). 

Comparison of particle pathlines within steady state and transient-state flow systems by Festger and Walter (2002) 

demonstrated further significant differences between the two approaches. 

Aquifer tests—Aquifer tests are controlled, transient-state experiments in which pumpage is intentionally varied while waterlevel dynamics in nearby monitoring wells are observed through continuous water-level recorders, and aquifer hydraulic 

properties are estimated through model parameter inversion. The testing period is typically a few days to minimize the effect of 

other factors that affect water levels over longer time scales. The inversion process uses least squares minimization to identify 

parameter values that provide the closest match between simulated and observed water levels. Production well test stress may 

be generally anticipated to radiate laterally at least 3,000 ft in the semiconfined Magothy aquifer (Misut and Busciolano, 2009), 

such that multiple monitoring points are instrumented for most wellfields. The cessation of supply-well pumping during an 

aquifer test commonly requires that demand in the water-supply system be accommodated through additional pumpage at a 

different wellfield. This can potentially lead to well interference; therefore, careful test planning includes collection of pumpage 

data at all affected wellfields.

Estimates of hydraulic properties from site-specific aquifer tests can be expected to be more accurate than values estimated 

through minimally-zoned regional model calibration; through use of PEST (Doherty and Hunt, 2010), model calibration may be 

conducted such that aquifer test estimates locally outweigh other tendencies for parameter adjustment. As yet, no aquifer tests 

are known to have been conducted at any wellfields in the area of interest; regional models could benefit from such data.

Correlation of water levels and precipitation—Nearly all water entering the groundwater-flow system of the Bethpage area 

(and Nassau County) consists of recharge from precipitation. Groundwater levels reflect the seasonal and long-term variations 

in precipitation rate, as illustrated by the correlation between water levels and precipitation in the hydrographs for 1950–2000, 

(fig. 4). Water levels at Well N1259 are not in a steady state condition; rather, they fluctuate seasonally and show trends over 

longer periods. Model representation of these transient conditions can serve many purposes and may improve estimates of 

the following:

1. Hydraulic property variation and aquifer geometry: Among the hydraulic properties whose values could be better estimated 

through transient-state than through steady state simulation are porosity, horizontal and vertical hydraulic conductivity, 

and storativity. 

2. Model boundary flows: The spatial and temporal distribution of recharge may be obtained in a manner similar to estimation 

of the spatial distribution of hydraulic properties. 

Examples of distinct spatial recharge zones, some of whose values may vary in time, include stormwater recharge-basin 

zones, zones of treated water discharge, residential zones, and impervious zones. The public domain code PEST includes 

powerful transient-state calibration and uncertainty analysis techniques for complex models, and USGS guidelines are available 

(Doherty and Hunt, 2010).
 9

fig 04

Water level, in feet NGVD 29 Precipitation, in inches

Water year

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Mineola

Water-table aquifer well N 1259

41

43

45

47

49

51

53

55

57

59

0

2

4

6

8

10

12

14

16

Autoregressive 

integrated

moving average

Autoregressive 

integrated

moving average

Figure 4. Groundwater level at Well N1259 in Bethpage and precipitation at National Weather 

Service station at Mineola, Nassau County, New York, 1950–2000. (Location of well shown in 

figure 1.) (Modified from Busciolano, 2005) 
10 

Migration of contaminants into poorly-permeable zones—Solutes migrate into poorly-permeable zones through diffusion

and slow advection, and are stored in dissolved and sorbed phases. When dissolved-phase concentrations in adjacent, more 

permeable zones decrease, the diffusion gradient reverses and allows the slow release of stored material from the poorlypermeable zones (Newell, 2011). Adjacent contrasting permeability zones are present within the upper glacial and Magothy 

aquifers, and within the Raritan confining unit. Analytical models are available to simulate the full range of diffusion and 

slow advection mechanisms, but numerical models are not currently practical. However, a “dual domain porosity” process 

has been tractable in numerical models and assumes diffusion-only mass transfer without slow advection. The 2010 Bethpage 

model was updated to simulate the dual-domain porosity process. The resulting summary report concluded that implementation 

of the dual domain porosity would not affect the conclusions reached in the OU3 feasibility study (FS) (ARCADIS, 2010). 

In further discussions, it was stated that no significant change in simulated results was evident during sensitivity analysis of 

dual domain porosity parameters (Smith, 2011c4). However, it was also stated that “lack of sensitivity does not necessarily 

equate to non-improvement in predictions,” and not accounting for dual domain processes would not allow representation of 

the physical processes that are occurring within the groundwater system (Smith, 2011c4). Although the dual domain process has 

become tractable in numerical models, in practice, it has been found that dual domain porosity parameters are scale dependent, 

lack physical significance, and are used primarily as a data-fitting, as opposed to a predictive tool (Flach and others, 2004). 

Dual-domain porosity parameters used in the Bethpage model are an incomplete representation of the processes by which 

contaminants migrate between contrasting permeable zones, because slow advective movement is not represented. 

Prior to any attempt to represent matrix diffusion, a more complete calibration and sensitivity analysis of porosity—of 

the traditional single-domain “effective porosity” type in a fully transient state analysis—should be considered. This parameter 

controls the rate of advective movement and may be adjusted and zoned to improve match to field observations of the rate of 

plume movement, thus (1) building confidence in the ability of the model to predict plume arrival times at water-supply wells, 

(2) leading to discoveries about the uncertainties inherent in plume arrival time prediction, and (3) providing guidance for future 

data collection activities.

Model bottom—The Raritan confining unit (fig. 5) includes an upper zone that has greater heterogeneity and permeability than 

the full thickness of the formation. Water enters this upper zone and constituents such as VOCs can migrate into this upper zone. 

The lower portion of the Raritan confining unit can reasonably be used as a bottom boundary even though there is obviously 

some pathway within the formation that results in recharge of the underlying Lloyd aquifer.

EXPLANATION

Line of equal age of water—

 Interval in years, is variable

300

600

200

NGVD 29

-200

-600

-1,000

-1,400

-1,800

-2,200

FEET

Vertical scale greatly exaggerated Modified from Franke and Cohen, 1972

0

0

5

5

10 MILES

10 KILOMETERS

Regional groundwater divide Great 

South 

Bay

Barrier

Island

Atlantic Ocean

Long Island

Sound

development. (Location of border shown in figure 1. Modified from Olcott, 1995)

Figure 5. Cross section showing groundwater ages along Nassau-Suffolk County border prior to 
 11

Sewer installation—Model parameter estimates—including workable estimates of VOC source loading terms—may be 

improved through transient-state calibration. Contemporary calibration methods, including PEST (Doherty and Hunt, 2010), 

facilitate the transient-state approach. The contaminant plumes appear to have migrated through a dynamic transient-state flow 

field. Previous sections treated dynamic changes resulting from pumping well stresses over several time scales, and variation in 

precipitation. Another cause of change, and opportunity for improved model calibration, is historical analysis of the installation 

of sewer systems in Nassau County during the 1980s, which resulted in complex changes in head surfaces during a period of 

significant VOC loading at Bethpage. Maximum water table declines that were predicted by the USGS Long Island regional 

model to result from 1980s sewer installation are depicted in figure 6. The size and shape of current plume have been influenced 

by sewer installation.

0

0

5

5

10

10

15 MILES

15 KILOMETERS

EXPLANATION

Drawdown, in feet—Contour 

 interval is 2 feet

2

Atlantic Ocean

Long Island Sound

SUFFOLK COUNTY

KINGS 

COUNTY

QUEENS 

COUNTY

NASSAU 

COUNTY

N

74 ̊ 73°

40°45’

40°30’

Figure 6. Predicted maximum declines in the water table as a result of sewer installation during the 1980s. (From Reilly 

and others, 1983)

Interpolation during calibration—Model calibration entails comparison of simulated to observed values and typically entails 

interpolation of discretized model results to the precise location of field observations. The cell center method has been used and 

the following is stated by the consultant:

“Residuals are calculated by comparing the model-predicted head (at the cell center) to the target value (field 

measured value-not necessarily at the cell center). Given the larger size of the cells away from the areas of greatest 

interest and the likelihood that the largest head changes occur within some of the largest model cells (due to a 

relatively uniform gradient across the entire model domain) it would be expected that a target not located coincident 

with the center of the model cell (where the model head is calculated) could exhibit an increase in calibrated residual 

simply as a function of cell size” (Smith, 2011d5

)

Among other capabilities mentioned previously, PEST (Doherty and Hunt, 2010) methods typically interpolate simulated 

values to the precise location inside a cell, resulting in more accurate calculation of residual values than the cell center method. 
12 

Conclusion

An understanding of how plumes evolve over time may be useful in assessing questions as to whether (1) all sources of 

contamination been identified, (2) a model plausibly generates the current plume configuration, and (3) rates of plume migration 

toward production wells are constant. Review of the model and discussion with the Bethpage model consultants indicates an 

assumption was made that significant water-level changes occurred only in the past, and that the flow system is now stable and 

may be modeled as a steady state system starting from a hand-drawn initial condition of plume state, subject to revisions as 

necessary. The consultant approach ignores information on temporal variation of several factors, including

• public-supply pumping, 

• groundwater discharges from systems remediating VOC plumes,

• recharge and precipitation rates, and

• water levels and streamflows.

Such an approach also ignores the potential for future variation in the above factors. Transient phenomena that may lend 

themselves to prediction in future hypothetical simulations include pumping cycles, redirection of containment-system waters 

for industrial use, and climate-change scenarios. Public-domain computer programs, USGS guidance reports on transient-state 

calibration and uncertainty methods, and additional local and regional datasets are available to gain additional confidence in 

model evaluations and better judge their limitations. 

References Cited

ARCADIS, 2003, Comprehensive groundwater model report: US NWIRP/Northrup Grumman, April 28, 2003.

ARCADIS, 2010, Comprehensive Feasibility Study Volume 2–Study Area Feasibility Study, Operable Unit 3 (former Grumman 

Settling Ponds): Bethpage, New York, DRAFT, November 10, 2010.

Busciolano, R.J., 2005, Statistical analysis of long-term hydrologic records for selection of drought-monitoring sites on Long 

Island, New York: U.S. Geological Survey Scientific Investigations Report 2004–5152, 47 p., at http://ny.water.usgs.gov/

pubs/wri/sir045152/.

Doherty, J.E., and Hunt, R.J., 2010, Approaches to highly parameterized inversion—A guide to using PEST for groundwatermodel calibration: U.S. Geological Survey Scientific Investigations Report 2010–5169, 59 p.

Festger, A., and Walter, G., 2002, The capture efficiency map: the capture zone under time-varying flow: Ground Water, 

v. 40(6), p. 619–628. 

Flach, G.P., Crisman, S.A., and Molz, F.J. III, 2004, Comparison of single-domain and dual-domain subsurface transport 

models: Ground Water, v. 42 (6–7), p. 815–28.

Franke, O.L., and Cohen, Philip, 1972, Regional rates of ground-water movement on Long Island: U.S. Geological Survey 

Professional Paper 800–C, p. C271–C277.

Konikow, L., 2010, The secret to successful solute-transport modeling: Ground Water, v. 49(2), p. 144–159.

Misut, P.E., and Busciolano, R.J., 2009, Hydraulic properties of the Magothy and Upper Glacial aquifers at Centereach, Suffolk 

County, New York: U.S. Geological Survey Scientific Investigations Report 2009–5190, 23 p., at http://pubs.usgs.gov/

sir/2009/5190.

Newell, C., 2011, “Overview of Matrix Diffusion Processes and its effects on managing chlorinated solvent sites: Groundwater 

Resources Association of California, podcast accessed March 1, 2011, at http://grac.org/GRACast_MatrixDiffusion_Mar2011.

pdf.

Occidental Chemical Corporation, 1996, Prediction of chlorinated solvent migration in the Bethpage Regional Aquifer, October 

1996: ref# 6883(11).

Olcott, P.G., 1995, Ground water atlas of the United States: U.S. Geological Survey Hydrologic Atlas HA 730–M.
 13

Reilly, T.E., Buxton, H.T., Franke, O.L., and Wait, R.L., 1983, Effects of sanitary sewers on ground-water levels and streams in 

Nassau and Suffolk Counties, New York, Part 1—Geohydrology, modeling strategy, and regional evaluation: U.S. Geological 

Survey Water-Resources Investigations Report 82–4045, 45 p. 

Reilly, T.E., and Harbaugh, A.W., 2004, Guidelines for evaluating ground-water flow models: U.S. Geological Survey WaterResources Investigations Report 2004–5038, 28 p. 

Reilly, T.E., and Pollock, D.W., 1995, Effect of seasonal and long-term changes in stress on sources of water to wells: U.S. 

Geological Survey Water-Supply Paper 2445, 25 p.

Reilly, T.E., and Pollock, D.W., 1996, Sources of water to wells for transient cyclic systems: Ground Water, v. 34(6), p. 979–

988.

Smolenksy, D.A., and Feldman, S.M., 1995, Three-dimensional advection transport of volatile organic compounds in ground 

water beneath an industrial/residential area of Nassau County, New York: U.S. Geological Survey Water-Resources 

Investigations Report 92–4148, 53 p. 
14 

Glossary of Terms and Acronyms

Specialized technical terms are defined below. Definitions pertaining to regulation are taken from http://www.nysdec.gov

and http://www.epa.gov.

A

Advection A transport mechanism of a substance by a fluid, due to the fluid’s bulk motion in a particular direction.

Anisotropy the property of being directionally dependent, as opposed to isotropy which implies identical properties in all 

directions. An example of an anisotropic material is wood, which is easier to split along its grain than against it.

B

BWD Bethpage Water District.

C

Confining unit A layer of geologic material which impedes the movement of water into and (or) out of an aquifer.

D

Diffusion The spread of particles through random motion from regions of higher concentration to regions of lower 

concentration. 

Dual domain porosity An aggregated porosity of media consisting of slowly and rapidly conducting pore sequences.

F

FS Feasibility study.

G

Gardiners Clay A shallow marine or brackish-water deposit of late Pleistocene age, underlying the upper glacial aquifer on 

parts of the south shore of Long Island. Its northern extent varies from 3 to 5 miles inland and is indented by long, narrow northsouth channels.

H

Heterogeneity Lack of uniformity in composition or character. 

Homogeneous Uniform in composition or character.

Hot spot A small area of intense or important activity embedded in a larger area of relative calm.

I

Interim remedial measure (IRM) A discrete set of activities to address both emergency and non-emergency site conditions, 

which can be undertaken without extensive investigation and evaluation, to prevent, mitigate, or remedy human exposure and 

(or) environmental damage or the consequences of human exposure and (or) environmental damage attributable to a site.

Interpolation calculation of the value of a function between the values already known.

K

Kriging A form of statistical modeling that interpolates data from a known set of sample points to a continuous surface.

L 

Least squares minimization The best fit in the least-squares; minimizes the sum of squared residuals, a residual being the 

difference between an observed value and the fitted value provided by a model.

Lloyd aquifer The Lloyd Aquifer is the deepest and oldest of Long Island’s aquifers, composed of fine to coarse sand and 

gravel with a clayey matrix and some layers of silty or solid clay. Derived from stream erosion of areas to the north and west 

during the late Cretaceous age (60–100 million years ago).
 15

M

Magothy aquifer The largest of Long Island’s aquifers, composed of river delta sediments that were deposited on top of the 

Raritan Formation during the late Cretaceous age (60–100 million years ago). Fine to medium sand is interbedded with clay 

and sandy clay of moderate permeability and silt and clay of low to very low permeability. The basal zone commonly contains 

coarse sand and gravel. 

MODFLOW U.S. Geological Survey groundwater-flow modeling software.

MODPATH U.S. Geological Survey particle tracking analysis software.

Monitoring wells Wells (often small-diameter wells) drilled for the purposes of measuring water levels and testing water 

quality. Monitoring wells are not typically used to supply water.

MT3D Solute transport modeling software developed at University of Alabama.

N

NCDH Nassau County Department of Health.

NYSDEC New York State Deparatment of Environmental Conservation

NWIRP Naval Weapons Industrial Reserve Plant.

O

ONCT Onsite containment system.

Operable Unit (OU) Addresses geographical portions of a site, specific site problems, or initial phases of an action, or may 

consist of any set of actions performed over time or any actions that are concurrent but located in different parts of a site.

Outpost monitoring well An early warning system.

Oxy site USEPA Hooker/Ruco superfund site.

P

Park Bethpage Community Park.

Parsimony Use of the simplest or most frugal route of explanation available.

Particle tracking analysis Water particles tracked through time assuming they are transported by advection using a simulated 

flow field. Particles can be tracked either forward in time or backward in time. This qualitative approach does not conserve mass.

Permeability A measure of the ability of a material (such as rocks) to transmit fluids.

PEST software package for parameter estimation and uncertainty analysis of complex environmental and other computer 

models.

Pilot points Interpolation points used in parameter estimation, may replace parameter zones.

Plume Body of contaminated groundwater flowing from a specific source. The movement of the groundwater is influenced by 

such factors as local groundwater-flow patterns, the character of the aquifer in which groundwater is contained, and the density 

of contaminants.

Porosity Is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 

0–1, or as a percentage between 0–100 percent.

PPB parts per billion.

R

Raritan confining unit Underlies the Magothy aquifer, derived from stream erosion of areas to the north and west during the 

late Cretaceous age (60–100 million years ago).

Record of decision (ROD) A public document that explains which cleanup alternatives will be used to clean a 

contaminated site.

Remedial Investigation (RI) Determines the nature and extent of contamination. Assesses the treatability of site contamination 

and evaluates the potential performance and cost of treatment technologies.

Residual The error in a result.
16 

S

Sensitivity analysis A technique for systematically changing parameters in a model to determine the effects of such changes.

Sorbed The physical adherence or bonding of ions and molecules onto the surface of another phase.

Steady state A system is in steady state if its behavior is unchanging.

T

TVOC Total volatile organic compounds (VOCs).

U

Upper glacial aquifer An unconfined aquifer directly underlying the Long Island ground surface. The upper glacial aquifer was 

formed during the last ice age. 

USEPA U.S. Environmental Protection Agency. The mission of USEPA is to protect human health and the environment.

USGS U.S. Geological Survey, a science agency that provides impartial information on the health of our ecosystems and 

environment, the natural hazards that threaten us, the natural resources we rely on, the impacts of climate and land-use change, 

and the core science systems that help us provide timely, relevant, and usable information.

V

VOC Volatile organic compound.

VPB Vertical profile borings.

W

Water table The top of an unconfined aquifer below which the pore spaces are generally saturated; the level in the saturated 

zone at which the pressure is equal to the atmospheric pressure.
 17

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18 

Table 1. History of Bethpage groundwater model applications and updates.—Continued

(K. Smith, Northrup Grumman, written commun., 2011a2)

Model applications and updates Purpose Results 

1995: Original Bethpage groundwaterflow model developed, consisting of 

groundwater-flow model (MODFLOW) 

and particle tracking model (MODPATH) 

Design onsite groundwater containment 

system to prevent offsite migration of 

contaminated groundwater from former 

Grumman & NWIRP facilities.

• Designed and installed onsite containment 

system (ONCT), which became fully 

operational as an interim remedial 

measure (IRM) in 1998, and has 

removed over 150,000 pounds of VOCs 

through 2010.

• Ongoing monitoring shows ONCT has 

been highly effective in preventing offsite 

migration of contaminated groundwater. 

2000: Model used to support evaluation of 

groundwater remedial alternatives in OU2 

Feasibility Study (FS).

Model updated to add solute transport code 

(MT3D) 

• Evaluate groundwater alternatives in 

OU2 FS. 

• Predict contaminant mass loadings, 

maximum concentrations, and cleanup 

time frames at remedial wells under 

various cleanup alternatives.

• Validated continued use of ONCT system 

IRM as permanent remedy. 

• 8 alternatives presented in FS for 

remediating onsite and offsite impacted 

groundwater from former Grumman 

& NWRIP facilities (including 

GM–38 Area). 

• NYSDEC OU2 ROD, issued in 

2001, selected Alternative 3 from FS, 

consisting of continued operation of 

ONCT, installation of GM–38 area 

system, and implementation of stateapproved contingency plan for municipal 

wellhead treatment. 

2002: Model used to support 

implementation of OU2 ROD.

Model updated to:

• Expand model domain laterally in all 

directions 

• Add additional vertical model layers

• Update hydraulic conductivity zonation, 

natural recharge, and well pumping/

recharge rates

• Design remedial wells for GM–38 area.

• Design outpost wells to serve as early 

warning of potential VOC impacts to 

public-supply wells. 

• Determined number, locations, depths, and 

required flow rates for remedial wells in 

GM–38 area to achieve maximum mass 

removal in this contamination “hotspot”. 

• Determined locations and screen depths 

for outpost wells to serve as early warning 

of potential VOC impacts to public-supply 

wells (S. Farmingdale Wellfields 1 and 

3, Aqua, New York Wells 3S and 4S, and 

Town of Hempstead Well 13). 

2003: Model used to evaluate modification 

of proposed GM–38 area system design

Evaluate functional feasibility of adding 

a 3rd remedial well to GM–38 area 

proposed 2-well system (to address 

Bethpage Water District input).

Determined that addition of 3rd remedial 

well offered no additional benefits for 

mass removal or reducing concentrations 

at downgradient supply wells.

2004: Model used to evaluate modifications 

to ONCT system

Evaluate if recharge from recovery wells 

to onsite basins could be reduced (to 

supply treated water to Calpine, and 

reduce potable water consumption) while 

effectively maintaining overall ONCT 

recovery well capture zone.

• Results indicated flow to recharge basins 

could be reduced without compromising 

effectiveness of ONCT system, while 

saving potable water. 

• Northrop Grumman provides treated water 

to Calpine in exchange for electric power 

for portion of ONCT system. 
 19

Table 1. History of Bethpage groundwater model applications and updates.—Continued

(K. Smith, Northrup Grumman, written commun., 2011a2)

Model applications and updates Purpose Results 

2005: Model used to evaluate modifications 

to ONCT system

Determine if migration of vinyl chloride 

from Oxy site could be prevented 

downgradient from northernmost ONCT 

recovery well (Well 3), while maintaining 

overall capture zone.

• Model indicated higher pumping rate at 

Well 3 and lower pumping rate at Well 

1 could be sustained while successfully 

maintaining overall ONCT capture zone. 

• Changes in Wells 1 and 3 pumping rates 

resulted in preventing migration of vinyl 

chloride beyond Well 3.

2006: Model used to evaluate modification 

of GM–38 area system (2 recovery wells 

installed but not operating) 

Evaluate feasibility of replacing RW–2 with 

new well at location west of RW–2 (to 

address Bethpage Water District (BWD)

concerns regarding GM–38 design). 

• Model predicted that replacement of 

Well RW–2 with new well (Well RW–3) 

was viable. 

• Well RW–3 installed in 2009 and is 

operating (in place of Well RW–2) along 

with Well RW–1. The 2 operating wells 

are successfully removing VOC mass. 

2006–2009: Model used to support OU3 

Remedial Investigation (RI) 

Select locations and depths of vertical 

profile borings (VPBs) to characterize 

VOC groundwater contamination. 

VPB data from various depths used to 

interpret contaminant distribution in 

groundwater for RI and estimate VOC 

mass to support contaminant transport 

modeling for OU3 FS. 

2008: Model used to design IRM for 

Bethpage Community Park (Park) area 

Sub-model extracted from regional model to 

refine model grid in Park area

Design groundwater containment system to 

prevent offsite migration of contaminated 

groundwater from Park area. 

• Determined number, locations, depths, 

and flow rates for remedial wells in 

Park area to prevent offsite migration 

of contamination.

• Designed and installed Park groundwater 

IRM, which became operational in 2009.

• Ongoing monitoring shows IRM is 

effectively preventing offsite migration of 

contaminated groundwater. 

2009–2010: Model used to support 

evaluation of groundwater remedial 

alternatives in Comprehensive FS.

Model updated with: 

• Current pumping rates

• Current VOC mass distribution

• Dual domain transport capability

• Evaluate groundwater alternatives in 

Comprehensive FS. 

• Predict contaminant mass loadings, 

maximum concentrations, and cleanup 

time frames at remedial wells under 

various cleanup alternatives. 

• Predict VOC concentration reductions in 

BWD Plants 4 and 5.

• 6 alternatives presented in Volume 2 

of Comprehensive FS for remediating 

impacted groundwater. 

• Recommended Alternative 3, consisting 

of source remediation, installation of new 

remedial well, and continued operation of 

BWD wellhead protection, ONCT, Park 

IRM, and GM–38 systems. 
This page has been left blank intentionally.
Prepared by the Pembroke Publishing Service Center 

For additional information write to:

New York Water Science Center

U.S. Geological Survey

2045 Route 112, Bldg. 4

Coram, NY 11727

Information requests:

(518) 285-5602

or visit our Web site at:

http://ny.water.usgs.gov
Misut—Simulation of Groundwater Flow in a Volatile Organic Compound-Contaminated Area near Bethpage, Nassau County, New York —Open-File Report 2011–1128