In cooperation with the

Delaware Department of Natural Resources and Environmental Control

and the

Delaware Geological Survey

Scientific Investigations Report 2006-5011

U.S. Department of the Interior

U.S. Geological Survey

Occurrence and Distribution of Mercury

in the Surficial Aquifer, Long Neck 

Peninsula, Sussex County, Delaware, 

2003--04

i g

Cre k

L

n

o e

N

E

S

W

Indian River Bay

Pot Nets 

Point
U.S. Geological Survey satellite photograph of the Pot Nets Bayside community, Long Neck Peninsula, 

Sussex County, Delaware. 

Cover.
Occurrence and Distribution of Mercury

in the Surficial Aquifer, Long Neck 

Peninsula, Sussex County, Delaware, 

2003--04

By Michael T. Koterba , A. Scott Andres , Joseph Vrabel , 

Dianna M. Crilley , Zoltan Szabo , John F. DeWild ,

George R. Aiken, and Betzaida Reyes-Padro

U.S. Geological Survey

Delaware Geological Survey

In cooperation with the Delaware Department of Natural Resources and Environmental 

Control and the Delaware Geological Survey

Scientific Investigations Report 2006-5011

U.S. Department of the Interior

U.S. Geological Survey

1

1

2

2

1

1 1

1 1

1
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply

endorsement by the U.S. Government.

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:

Koterba, M.T., Andres, A.S., Vrabel, J., Crilley, D.M., Szabo, Z., DeWild, J.F., Aiken, G.R., and Reyes-Padro, B., 2006, 

Occurrence and distribution of mercury in the surficial aquifer, Long Neck Peninsula, Sussex County, Delaware, 

2003-04: U.S. Geological Survey, Scientific Investigations Report 2006-5011, 163 p.
iii

CONTENTS

Abstract .............................................................................................................................. 1

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

Purpose and scope ......................................................................................................... 4

Description of study area............................................................................................... 5

Hydrogeology................................................................................................................ 6

Data collection and data quality........................................................................................ 12

Well selection and installation..................................................................................... 13

Selection of Long Neck Peninsula production wells............................................. 13

Installation of Pot Nets Bayside and Lakeside monitoring wells.......................... 15

Field data and sample collection ................................................................................. 16

Ground-water data and water samples................................................................... 16

Soil, fill, and aquifer-sediment data and samples .................................................. 19

Data quality ................................................................................................................. 21

Mercury ................................................................................................................. 21

Major ions, alkalinity, acid-neutralization capacity, and bicarbonate ................... 25

Chlorofluorocarbon apparent recharge dates and ages of ground water ............... 27

Mercury in the surficial aquifer......................................................................................... 29

Long Neck Peninsula .................................................................................................. 29

 Forms of mercury ................................................................................................. 29

 Relations between mercury and other chemical constituents............................... 32

Pot Nets Bayside and Lakeside ................................................................................... 37

 Mercury in sediment............................................................................................. 37

Relations between mercury and other sediment constituents ...................... 37

Carbon .................................................................................................. 37

Sulfur and trace elements ..................................................................... 37

Sediment as a source of mercury in ground water........................................ 37

 Mercury in ground water ...................................................................................... 39

Forms of mercury.......................................................................................... 39

Particulate and non-particulate mercury............................................... 40

Methyl mercury .................................................................................... 41

Inorganic and organic complexes......................................................... 42

Hydrophobic and hydrophilic acid carbon fractions ............................ 44

Vertical distribution of mercury .................................................................... 49

Distribution of mercury in relation to sediment type ........................... 49

Distribution of mercury in relation to apparent age of

 ground water......................................................................................... 49

Relations between mercury and other ground-water constituents ................ 51

Specific conductance and major ions ................................................... 51

Volatile organic compounds ................................................................. 55

Combined indicators of ground-water contamination.......................... 55

Summary and conclusions ................................................................................................ 56

Acknowledgments............................................................................................................. 58

Selected references............................................................................................................ 59
iv

Contents–—Continued

Appendixes—(at the end of the report)

A. Data on well characteristics and water quality, and analytical methods

used for water samples for the Long Neck Peninsula mercury study .....................A0

B. Data on borehole characteristics, soil, fill, and aquifer sediment, and

analytical methods used for sediment samples for the Long Neck

Peninsula mercury study .........................................................................................B0

C. Data on cation and anion balances for individual water-quality samples

used for the Long Neck Peninsula mercury study ..................................................C0

D. Antecedent sampling pumping conditions, sampling set-up, preliminary

design study, and integrity checks for selected monitoring wells used

for Long Neck Peninsula mercury study.................................................................D0

Figures

 1. Map showing location and land use and cover of the Long Neck Peninsula,

 Sussex County, Delaware..........................................................................................3

 2a. Map showing surficial geology and line of section A–A′ in the vicinity of

 Pot Nets Lakeside and Bayside communities, Long Neck Peninsula,

 Sussex County, Delaware..........................................................................................7

2b. Geologic cross section A–A′ through Pot Nets Lakeside and Bayside

 communities, Long Neck Peninsula, Sussex County, Delaware............................... 8

 3. Map showing relative recharge potential, showing the ability of the upper

 20 feet of the surficial material to transmit recharge to the surficial aquifer,

 Long Neck Peninsula, Sussex County, Delaware ......................................................9

4a. Map showing location of the Pot Nets Bayside production well (Pi31-01)

 and either formerly (Pi31-13, 14, and 22) or recently (2004) installed 

(Pi31-15, 16, 17, 18, 19, 20, and 21) monitoring wells.............................................10

4b. Map showing location of the Pot Nets Lakeside production well (Ph35-25) 

 and either formerly (Ph35-35, 36, and 37) or recently (2004) installed

 (Ph35-29, 30, 31, 32, 33, and 34) monitoring wells.................................................11

 5. Map showing locations of production wells used in the source-water quality

 survey of the shallow surficial aquifer, Long Neck Peninsula, Sussex 

 County, Delaware.....................................................................................................14

 6. Diagram showing dissolved organic carbon and acid-neutralization 

 capacity for ground-water samples from monitoring wells at the Pot Nets

 Bayside (Pi31-01) and Lakeside (Ph35-25) sites, Long Neck Peninsula, 

 Sussex County, Delaware.........................................................................................26

 7. Map showing total mercury concentrations in ground-water samples from

production wells in the surficial aquifer, Long Neck Peninsula, Sussex

County, Delaware, 2004............................................................................................30
v

Figures—Continued

 8. Map showing total mercury concentrations in ground-water samples

from production wells in the surficial aquifer near Pot Nets Bayside

well (Pi31-01) and Lakeside well (Ph35-25), Long Neck Peninsula,

Sussex County, Delaware, 2004...............................................................................31

 9. Diagram showing relative difference in total mercury concentrations

between filtered and raw water samples from production wells, Long

Neck Peninsula, Sussex County, Delaware, 2004....................................................33

10. Diagram showing turbidity and total mercury concentrations for

ground-water samples from production wells, Long Neck Peninsula,

Sussex County, Delaware, 2004...............................................................................33

11. Diagram showing concentrations of total mercury and major ions in

ground-water samples from production wells, Long Neck Peninsula,

Sussex County, Delaware, 2004...............................................................................35

12. Diagram showing composite lithologic profiles and total mercury

concentrations for sediment at the Pot Nets Bayside (Pi31-01) and

Lakeside (Ph35-25) sites, Long Neck Peninsula, Sussex County,

Delaware, 2005 ........................................................................................................38

13. Diagram showing concentrations of total mercury and total organic

carbon in soil and fill near production well Pi31-01, Pot Nets Bayside,

Long Neck Peninsula, Sussex County, Delaware, 2005 ..........................................39

14. Diagram showing depth distribution of total mercury concentrations

in ground water at the Pot Nets Bayside well (Pi31-01) and Lakeside

well (Ph35-25) sites, Long Neck Peninsula, Sussex County, Delaware,

2005..........................................................................................................................40

15. Diagrams showing (A) concentrations of total mercury in raw and

filtered (0.4-micrometer absolute pore size) ground-water samples

from the Pot Nets Bayside well (Pi31-01) and Lakeside well (Ph35-25)

sites, 2005, and (B) concentrations of total mercury in raw and filtered

(0.1-micrometer absolute pore size) ground-water samples from the

Pots Net Bayside well (Pi31-01) site, Long Neck Peninsula, Sussex

County, Delaware, 2005...........................................................................................41

16. Diagrams showing dissolved organic carbon and specific ultraviolet

absorbance (A) at 280 nanometers and (B) at 254 nanometers for

filtered (0.4-micrometer absolute pore size) samples from Pot

Nets Bayside well (Pi31-01) and Lakeside well (Ph35-25) sites,

Long Neck Peninsula, Sussex County, Delaware, 2005 ..........................................47

17. Diagram showing apparent ages of ground water from three chlorofluorocarbon compounds, and depths from land surface to the tops

of the screened intervals of monitoring wells with high or low

concentrations of total mercury at the Pot Nets Bayside (Pi31-01)

and Lakeside well (Ph35-25) sites, Long Neck Peninsula, Sussex

County, Delaware, 2005...........................................................................................50
vi

Tables

 1. Land use and cover for the Long Neck Peninsula, Sussex County,

Delaware, 2002 ........................................................................................................ 6

 2. Types of wells and ground-water data and samples collected by the

U.S. Geological Survey from production and monitoring wells, Long

Neck Peninsula, Sussex County, Delaware, 2003–04............................................ 17

 3. U.S. Geological Survey laboratories and sample type, handling, and

delivery................................................................................................................... 18

 4. Dissolved organic carbon fractions obtained from dual-column resin

extractions .............................................................................................................. 20

 5. Mercury concentrations in field and source-water blanks and

corresponding ground-water samples for major sampling activities on

Long Neck Peninsula, Sussex County, Delaware, 2003–04 .................................. 22

6. Mercury concentrations in sequentially collected ground-water samples

at Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25) production

wells, Sussex County, Delaware, 2003–04 ............................................................ 23

7. Methyl mercury concentrations and recoveries from background and

spiked filtered ground-water samples from Pot Nets Bayside (Pi31-01)

and Lakeside (Ph35-25), Sussex County, Delaware, 2003–04 .............................. 24

8. Qualified raw data for apparent dates (ages) of recharge for groundwater samples from production and vertically nested monitoring wells

at Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25) sites, Sussex

County, Delaware, 2004......................................................................................... 28

 9. Frequently detected volatile organic compounds in ground-water

samples from production wells on the Long Neck Peninsula,

Sussex County, Delaware, 2003............................................................................. 34

10. Production wells that exceeded water-quality criteria—for elevated

specific conductance, nitrate-nitrogen concentrations, and detectable

volatile organic compounds—and total mercury concentrations,

Long Neck Peninsula, Sussex County, Delaware, 2003 ........................................ 36

11. Relative difference in total mercury concentrations between raw and

filtered (0.1-micrometer absolute pore size) samples for the production

and each vertically nested monitoring well at the Pot Nets Bayside

(Pi31-01) site, Long Neck Peninsula, Sussex County, 

Delaware, 2004 ...................................................................................................... 41

12. Turbidity and total mercury concentrations in ground-water samples

from Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25) sites, Long

Neck Peninsula, Sussex County, Delaware, 2003.................................................. 42

13. Geochemical conditions related to the occurrence or formation of

mercuric sulfide (metacinnabar) in ground water at Pot Nets Bayside

(Pi31-01) and Lakeside (Ph35-25) sites, Long Neck Peninsula, Sussex

County, Delaware, 2004......................................................................................... 43
vii

Tables—Continued

14. Geochemical conditions related to the occurrence or formation of

 mercuric chloride or organomercuric complex formation in ground

 water at Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25) sites,

 Long Neck Peninsula, Sussex County, Delaware, 2004 ....................................... 44

15. Total mercury, dissolved organic carbon, and ultraviolet absorbance

 in ground-water samples from Pot Nets Bayside (Pi31-01) and

 Lakeside (Ph35-25) sites, Long Neck Peninsula, Sussex County,

 Delaware, 2004 ..................................................................................................... 45

16. Total mercury, dissolved organic carbon, and specific ultraviolet

 absorbance (SUVA) in ground-water samples from Pot Nets

 Bayside (Pi31-01) and Lakeside (Ph35-25) sites, Long Neck

 Peninsula, Sussex County, Delaware, 2004 .......................................................... 46

17. Fractional components of dissolved organic carbon in selected

 ground-water samples with elevated total mercury concentrations

 from Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25) sites,

 Long Neck Peninsula, Sussex County, Delaware, 2004 ....................................... 48

18. Total mercury and dissolved organic carbon concentrations, and

 acid-neutralization capacity in ground-water samples from Pot Nets

 Bayside (Pi31-01) and Lakeside (Ph35-25) sites, Long Neck Peninsula,

 Sussex County, Delaware, 2004............................................................................ 48

19. Ground-water samples with (or without) elevated specific conductance,

 increased relative abundance of selected cations, elevated nitrate-

 nitrogen concentrations, detectable volatile organic compounds, and

 total mercury concentrations collected from production and monitoring

 wells at Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25) sites,

 Long Neck Peninsula, Sussex County, Delaware, 2004 ....................................... 52

20. Concentrations of major ions (in milliequivalents per liter) for natural

 or nearly natural ground-water samples and ground-water samples from

 monitoring wells at Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25)

 sites, Long Neck Peninsula, Sussex County, Delaware, 2004 .............................. 53

21. Relative percentages of major ions (in milliequivalents per liter,

 expressed in percent) for ground-water samples from Pot Nets 

 Bayside (Pi31-01) and Lakeside (Ph35-25) sites, Long Neck

 Peninsula, Sussex County, Delaware, 2004 .......................................................... 54

22. Volatile organic compounds detected in ground-water samples from

 Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25) sites, Long

 Neck Peninsula, Sussex County, Delaware, 2004................................................. 55

23. Number of detectable volatile organic compounds and total mercury

 concentrations in ground-water samples from Pot Nets Bayside

 (Pi31-01) and Lakeside (Ph35-25) sites, Long Neck Peninsula,

 Sussex County, Delaware, 2004............................................................................ 56
viii

Conversion Factors, and Vertical Datum 

Multiply By To obtain

Length

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

square mile (mi2) 259.0 hectare 

square mile (mi2) 2.590 square kilometer 

Flow rate

gallon per minute (gal/min) 3.785 liter per minute (L/min)

Mass

pound, avoirdupois (lb) 0.4536 kilogram 

Temperature in degrees Celsius ( °C) can be converted to degrees Fahrenheit ( °F) 

as follows:

°

F = (1.8 x °C) + 32

Vertical datum: In this report, vertical coordinate information is referenced to the 

North American Datum of 1983 (NAD83).

Altitude, as used in this report, refers to distance above or below sea level.

Specific conductance is given in microsiemens per centimeter at 25 degrees 

Celsius (μS/cm at 25 °

C).

Concentrations of chemical constituents in water are given either in milligrams 

per liter (mg/L), micrograms per liter (μg/L), or nanograms per liter (ng/L).

Concentrations of chemical constituents in sediment are given in micrograms 

per kilogram (μg/kg), or as a percentage by weight, in percent (%).
Occurrence and Distribution of Mercury in the Surficial 

Aquifer, Long Neck Peninsula, Sussex County, Delaware, 

2003-04

By Michael T. Koterba, A. Scott Andres, Joseph Vrabel, Dianna M. Crilley, Zoltan Szabo, 

John F. DeWild, George R. Aiken, and Betzaida Reyes-Padro

Abstract

In January 2001, mercury (Hg) was detected (500 nanograms per liter, ng/L, or greater) in the distribution system of 

the Long Neck Water Company (LNWC), Pot Nets, Delaware. By April 2001, two LNWC production wells had been 

taken off-line because discharge concentrations of total mercury (HgT) either had exceeded or approached the Federal 

limit of 2,000 ng/L. From October 2003 through January 

2005, the U.S. Geological Survey, Delaware Geological Survey, and Delaware Department of Natural Resources and 

Environmental Control conducted a cooperative study to 

(a) determine if the Hg contamination was widespread, (b) 

identify possible forms of Hg in ground water, and (c) examine Hg occurrence in relation to (geo)chemical conditions 

and characteristics of ground water and sediment in the surficial aquifer on the Long Neck Peninsula, Sussex County, 

Delaware.

An initial water-quality survey conducted with samples 

from 22 production wells revealed that concentrations of 

HgT in ground water in the surficial aquifer ranged from 

0.11 to 1,820 ng/L. Shallow ground water (less than 120 feet 

below land surface) throughout most of the peninsula, 

including that which contained elevated concentrations of 

HgT (exceeding 100 ng/L), appeared to be affected by 

human activities. All samples contained volatile organic 

compounds (VOCs) and elevated nitrate-nitrogen (NO3-N, 

exceeding 0.4 milligrams per liter, mg/L). Most (16 of 22) 

samples had elevated specific conductance (SC, in excess of 

100 microsiemens per centimeter at 25 degrees Celsius). 

Elevated concentrations of HgT, however, only occurred in 

five production wells in the Pot Nets Bayside and Lakeside 

communities.

The vertical distribution of HgT in shallow ground water 

(less than 80 feet below land surface) was determined with 

samples collected at 5 to 6 vertical-nest short-screened (2–

5-foot length) monitoring wells installed near Bayside and 

Lakeside production wells with the highest HgT concentrations (exceeding 1,000 ng/L). Elevated concentrations of 

HgT (100–6,380 ng/L) occurred in the shallow aquifer near 

each well at different depths.

Chemical analyses of selected soil, fill, and aquifer sediment samples, obtained during the installation of nested 

wells, indicated that little HgT occurred in soil or fill at 

either site (40 micrograms per kilogram, μg/kg, or less). No

HgT was detected (less than 20 μg/kg) in aquifer sediment 

samples. These low HgT concentrations imply that neither 

the soil, fill, nor aquifer sediment was a likely source of the 

elevated Hg in ground water.

Given Hg occurrence appeared to be a ground-water 

transport phenomenon, the forms of Hg in transport were 

investigated. Differences in HgT concentrations between 

raw and filtered (0.1- and (or) 0.4-absolute micrometer pore 

size) samples from nested wells were random in sign and 

similar in magnitude to the variability in measuring HgT 

attributed to field and laboratory methods (± 5–10 percent, 

for HgT concentrations exceeding 100 ng/L). Thus, Hg 

transport likely occurred in a dissolved or fine-colloidal nonparticulate phase.

Methyl mercury (HgMe) only was detected at low concentrations (0.06 ng/L or less) in nested-well samples with 

low to moderate concentrations of HgT (less than 366 ng/L). 

Whether HgMe occurred at similar concentrations in samples with high HgT concentrations was unresolved due to a 

sample-matrix interference problem.

Potential complex forms of Hg were investigated in relation to the occurrence of selected ligands (organic carbon, 

sulfide, and chloride concentrations) and geochemical conditions (for example, pH and dissolved oxygen concentrations). Only dissolved organic carbon (DOC) appeared 

directly related to Hg occurrence. Elevated concentrations of 

HgT and DOC co-occurred in ground water at both Pot Nets 

sites. The average concentration of DOC was about four 

times greater in samples from the Pot Nets wells with the 

highest HgT concentrations (exceeding 1,000 ng/L) than in 

most Pot Nets or peninsula samples with low HgT concentrations (less than or equal to 20 ng/L).
2 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

The relation between Hg and DOC does not appear to be 

coincidental. Nested-well samples with elevated concentrations of HgT and DOC generally had a higher specific ultraviolet absorption at 280 nanometers than samples with low 

concentrations of DOC and HgT. The highest acid-neutralization capacities (ANCs) occurred in ground-water samples 

with the highest concentrations of DOC and HgT at both 

sites, and DOC and ANC were positively correlated 

(r2=0.90). In addition, the fraction of hydrophobic carbon, 

measured for samples with the highest concentrations of 

HgT at each site, equaled or exceeded by several fold the 

total DOC found in samples with low concentrations of HgT 

at each site. These findings indicated the possible presence 

of humic aromatic and aliphatic carbon compounds in the 

hydrophobic carbon fraction. Other studies in different locations, but with similar hydro-chemical conditions, have 

shown such carbon fractions can strongly interact with simple chemical forms of Hg –ionic (HgII), sulfide (HgS), or 

methyl (HgMe).

Shallow ground water at each Pot Nets site appeared 

affected by human activities. Detectable VOCs, and elevated 

SC, NO3

-

-N, and (or) magnesium occurred at all sampled 

depths, including those depths that corresponded to elevated 

concentrations of HgT. In addition, the apparent dates of 

recharge and ages of ground water, estimated from sample 

concentrations of chlorofluorocarbon compounds (CFCs), 

indicated that ground water with elevated concentrations of 

HgT at both Pot Nets sites was similar in relation to apparent 

recharge dates and ages (circa 1970 to 1985, or 20–35 years). 

Ground-water samples with low concentrations of HgT, 

which were obtained from nested wells screened at deeper 

and shallower depths than the wells with high concentrations, of HgT, had apparent recharge dates that corresponded 

to times before and after this period, respectively.

The elevated concentration of Hg in ground water near 

the Pot Nets wells appears to be of local, anthropogenic, and 

relatively recent origin, and possibly from a single source. 

High concentrations of HgT (exceeding 1,000 ng/L) first 

occurred at the Lakeside production well just 2 months after 

high concentrations led to the shutdown of the Bayside well. 

Elevated concentrations of HgT only have been found in 

shallow ground water in the vicinity of these two Pot Nets 

production wells. No measurable Hg occurred in the aquifer 

sediment at either production-well site. Ground water with 

elevated concentrations of HgT appeared relatively young 

and similar in apparent age at both sites. In addition, its quality at all sampled depths at both sites reflected chemical indicators of recent human activities typically associated with 

residential, or agricultural followed by residential, development.

The occurrence of elevated Hg at both Pot Nets community sites in ground water that also appeared affected by residential and (or) agricultural development also has been 

widely observed in similar settings and coastal aquifers in 

New Jersey. It is not possible, however, to determine from 

the limited historical data on Hg in the shallow aquifer, and 

this single study, whether or not there could be a potentially 

more widespread but scattered pattern of Hg occurrence in 

the shallow surficial aquifer in Delaware.

Introduction

This study focused on shallow ground water on the 

Long Neck Peninsula, Sussex County, Delaware (fig.1). 

In January 2001, mercury (Hg) was detected at selected 

points in the distribution system of the Long Neck Water 

Company (LNWC), Pot Nets, Delaware (detection level 

500 nanograms per liter, ng/L, Department of Natural 

Resources and Environmental Control, 2002). In February 

2001, a Hg concentration of 3,000 ng/L at LNWC production well Pi31-01 in the Pot Nets Bayside (hereafter, Bayside) community (fig. 1) exceeded the U.S. Environmental 

Protection Agency (USEPA) Maximum Contaminant Level 

(MCL) of 2,000 ng/L (U.S. Environmental Protection 

Agency, 2002)1, and this well was taken off-line. Shortly 

thereafter, in April 2001, elevated concentrations of Hg 

occurred at a second LNWC production well, Ph35-25, in 

the nearby Pot Nets Lakeside (hereafter, Lakeside) community (fig. 1). The Hg concentration eventually increased to 

1,900 ng/L, and in August 2001, this well was taken off-line. 

Subsequent preliminary studies were conducted in relation to each contaminated well by the Delaware Department 

of Natural Resources and Environmental Control, (DNREC) 

(2002) and McCrone, Inc. and the LNWC (James Mooney, 

Operations Manager, LNWC, written commun., 2004). In 

part, the DNREC study defined wellhead protection areas for 

LNWC wells at Pot Nets Bayside and Lakeside, and found 

the following:

(a) Production wells and the aquifer in the Pot Nets 

community areas appeared vulnerable to surficial 

contamination;

(b) Source-water protection areas for the Pot Nets 

Bayside and Lakeside communities did not overlap; 

and

(c) Hg contamination was not from well construction 

or pump materials.

McCrone, Inc. and the LNWC (James Mooney, Operations Manager, LNWC, written commun., 2004) found the 

following:

(a) Elevated Hg concentrations persisted in production 

wells (for example, Pi31-01 or Ph35-25) that are 

pumped routinely (daily); and

(b) No Hg was detected in shallow ground water (less 

than 40 feet, ft, below land surface) at Bayside 

(Pi31-01) site.

1. Hereafter, references to Federal drinking-water standards or the U.S. 

Environmental Protection Agency Maximum Contaminant Levels relate to 

this reference, which will not be parenthetically cited.
Introduction 3
4 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

Independent of these studies, there also has been no documented release of Hg on the Long Neck Peninsula that could 

indicate a potential recent and local Hg source (Department 

of Natural Resources and Environmental Control, Toxic 

Release Inventory Database, Delaware, 1995–2003).

Before the occurrence of elevated concentrations of 

Hg in Pot Nets production wells, mercury had not been 

measured above the USEPA MCL in any Delaware public 

production well since the mid–1980s (Blair Venables, 

Hydrologist, DNREC, written commun., 2001). Historically, 

mercury has not been reported for the surficial aquifer in 

Delaware, but (pre–1989) data were compiled for the confined aquifers of the Delmarva Peninsula, and although the 

number of analyses were limited, mercury was detected 

(detection limit 100 ng/L) in 31 of 45 well samples, with a 

maximum concentration of 1,200 ng/L (Hamilton and others, 

1989, Table 11, p. 64). Historical data, however, could be 

positively biased. Recent (post–1989) ground-water-quality 

assessments that included Delaware also have not included 

data on Hg (Hamilton and others, 1993; Ferrari, 2002; Denver and others, 2004). Thus, readily available knowledge of 

the distribution of Hg in shallow ground water is lacking for 

Delaware, including the Long Neck Peninsula.

The lack of detectable total mercury concentrations in 

ground water (HgTgw) in the surficial aquifer of Delaware 

is in sharp contrast to shallow ground water in a similar 

coastal aquifer setting in New Jersey. Beginning in the 

1990s, a series of studies of the unconfined and extensive 

(7,770 square kilometers, km2) Kirkwood-Cohansey Aquifer 

(KCA) led to the following conclusions: (a) background

concentrations for HgTgw were 10 ng/L or less; (b) Hg contamination was widespread, but locally and regionally heterogeneous; and (c) the highest concentrations (2,000 ng/L–

72,000 ng/L) of Hg reflected samples collected from hundreds of domestic production wells located chiefly in residential areas developed on, or adjacent to and downgradient 

from, agricultural lands (Murphy and others, 1994; Barringer 

and Macleod, 2001; Barringer and others, 2005).

The contrast in Hg occurrence and distribution in shallow 

ground water given the similar types of surficial aquifers, 

coastal settings, and patterns in development in Delaware 

and New Jersey raised several questions. Did elevated Hg 

concentrations occur elsewhere in the surficial aquifer on the 

Long Neck Peninsula? Did the high concentrations of 

HgTgw at the two Pot Nets production wells reflect a pattern 

of anthropogenic ground-water contamination on the Long 

Neck Peninsula similar to that found in New Jersey? How 

was mercury likely transported, and was it from a single 

native or anthropogenic source or multiple sources?

In October 2004, the U.S. Geological Survey (USGS), 

Delaware Geological Survey (DGS), and DNREC entered 

into a cooperative agreement to determine if the extent 

(occurrence and geographic distribution) of Hg contamination in shallow ground water possibly was more widespread 

than just the Pot Nets Lakeside and Bayside community sites 

near the two production wells that were taken off-line. To 

address whether Hg in the vicinity of the two contaminated 

Pot Nets production wells likely was from an anthropogenic 

source or sources, the vertical distribution of Hg in ground 

water and sediment were characterized, chemical or physical 

form(s) of mercury in sediment and ground water were identified, and other ground-water quality, geochemical, or aquifer characteristics were determined and evaluated in relation 

to the occurrence or transport of Hg in ground water. Results 

from all of the above also were used to aid in the identification of possible source(s) of Hg contamination.

Purpose and Scope

The purpose of this report is to provide descriptions of 

the occurrence and distribution (geographical and vertical) 

of total Hg, other selected forms of Hg, and their relation to 

other determined ground-water physical or chemical characteristics or conditions in the shallow surficial aquifer, on the 

Long Neck Peninsula, Sussex County, Delaware. These 

descriptions were derived from the analyses of ground water 

and surficial-sediment data, which were obtained in relation 

to sequential studies, as follows:

(a) The occurrence and geographical distribution of 

Hg, its possible forms, and the relations between Hg 

and other selected ground-water-quality 

constituents are described on the basis of data and 

samples obtained in November and December, 

2003 from 22 production wells (13 public and 9 

private wells), geographically distributed 

throughout the study area, and screened in the 

surficial aquifer.

(b) The occurrence and vertical distribution of Hg in 

the surficial aquifer, its possible forms in ground 

water, and relations between Hg and other selected 

surficial-sediment or ground-water characteristics 

are described on the basis of surficial-sediment and 

ground-water data and samples obtained in the 

vicinity of the Bayside (Pi31-01) and Lakeside 

(Ph35-25) production wells as part of a second 

study, which consisted of three phases:

Phase 1: Determine the short-term (approximately 

24-hour) variability in the bulk and selected 

chemical and physical characteristics of 

production-well discharge, and given a suitable 

period of stability in these properties in 

production-well discharge, determine if bulk 

physical and chemical properties of shallow 

ground water in the vicinity of each pumping 

production well differ with depth.

Phase 2: Determine the occurrence and vertical 

distribution of Hg and other selected chemical and 

physical characteristics in the surficial sediment in 

the vicinity of each production well. (Total 

mercury concentrations in sediment hereafter are 

referred to as HgTsed).
Introduction 5

Phase 3: Determine the occurrence and vertical 

distribution of total Hg in ground water, selected 

physical and chemical forms of Hg, and their 

relation to other water-quality characteristics and 

conditions, including geochemical characteristics, 

with depth in the vicinity of each production well. 

Given the results from Phases 2 and 3, identify 

possible forms of Hg in transport and probable 

source(s) of elevated Hg. (Total mercury 

concentrations in ground water hereafter are 

referred to as HgTgw ).

For (a) above, concentrations of HgTgw in samples 

obtained from 22 production wells distributed throughout the 

Long Neck Peninsula and screened in the shallow surficial 

aquifer were used to describe the spatial occurrence and distribution of total mercury in raw water (HgTgw-raw) in the 

surficial aquifer. Data used to identify and describe the possible forms of Hg, and ground-water characteristics or 

geochemical conditions that possibly related to the occurrence and distribution of HgTgw, included the following: 

concentrations of total mercury and methyl mercury in filtered samples (0.4-micrometer (μm) absolute pore size 

encapsulated nylon pleated filter), hereafter referred to as 

HgTgw-f0.4 and HgMegw-f0.4, respectively, bulk properties 

(pH, specific conductance (SC), and concentrations of dissolved organic carbon (DOC) and dissolved oxygen (DO), 

and concentrations of major ions, selected nutrients (nitrogen and carbon), trace elements, and volatile organic compounds (VOCs).

For (b) Phase 1 above, concentrations of HgTgw, bulk 

properties, major ions, nutrients, and selected trace elements 

from samples collected at different times during a 26-hour 

period in February 2004 at the Bayside production-well 

(Pi31-01) site were used to describe short-term variations in 

the bulk chemistry of well discharge. Bulk-chemistry data 

obtained at about 1-ft depth intervals in two nearby monitoring wells, and water-quality samples similar to those above 

but obtained at different depths from one monitoring well, 

were used to provide a preliminary description of changes in 

ground-water chemistry with depth in the shallow aquifer. At 

the Lakeside site, data obtained from the production well 

(Ph35-25) and a nearby monitoring well, before and during 

pumping of the production well, were limited to bulk properties. Knowledge of temporal variations (or lack thereof) in 

the bulk chemistry of production-well discharge through 

time, and changes in the chemistry in monitoring well discharge with depth, were used to develop the sampling 

approach, and to identify screen depths for the new vertically 

nested monitoring wells, respectively, at each productionwell site.

For (b) Phase 2 above, concentrations of HgTsed, total 

sulfur, and carbon (total, organic, and inorganic), and trace 

elements were obtained from soil, apparent fill, and aquifer 

sediment samples collected during installation of vertical 

nests of monitoring wells in June and July 2004. These data 

were used to describe the vertical distribution of Hg and its 

relation to other chemical constituents in sediment in the 

aquifer near each contaminated production well.

For (b) Phase 3 above, concentrations of HgTgw-raw, 

HgTgw-f0.4, and additional filtered samples for total mercury 

to better assess Hg occurrence in the particulate phase 

(0.1-μm absolute pore size nylon pleated filter, hereafter 

referred to as HgTgw-f0.1), as well as data on bulk properties, 

major ions, nutrients (carbon, sulfur, and nitrogen), selected 

trace elements, VOCs, chlorofluorocarbon compounds 

(CFC-11, -12, and -113), and dissolved gases (methane, carbon dioxide, nitrogen, oxygen, and argon) were obtained 

from ground-water samples collected from production and 

nested monitoring wells during an approximately 30-hour 

period at each Pot Nets site in September 2004. These data 

were used to describe the vertical distribution of HgTgw in 

the surficial aquifer at selected depths, to compare Hg concentrations in ground water (HgTgw) and sediment (HgTsed), 

to describe different possible forms of Hg in ground water 

(for example, by comparing HgTgw-raw to HgTgw-f0.4, 

HgTgw-f0.1, and HgMegw-f0.4), and to relate Hg occurrence 

to that of other ground-water contaminants, constituents, or 

geochemical conditions, and the apparent recharge dates and 

ages of ground water at different depths. Collectively, these 

data were used to help identify possible sources of Hg contamination.

Description of Study Area

The Long Neck Peninsula is a small (about 9.9-squaremile) coastal area in Sussex County, Delaware (fig. 1). The 

year-round resident population of the peninsula is less than 

10,000, but increases to between 20,000 and 30,000 during 

the summer months—Memorial Day through Labor Day 

(U.S. Census Bureau, 2000).

The Long Neck Peninsula is a low-lying area (elevation 

less than 25 ft) with topography that is relatively flat and 

gently rolling with small streams (fig. 1). Residential and 

commercial development is concentrated along the main 

thoroughfares—the John Williams Highway (State Road 

(SR) 24) and Long Neck Road (SR 23), with SR 23 generally following the topographic divide from west to east along 

the peninsula.

As of 2002, about 40 percent of the Long Neck Peninsula 

could be classified as urban (chiefly residential and light 

commercial) with about 3 percent of the land area in transition, mainly toward urban use (table 1). Before the development of summer and retirement communities that began in 

the early 1960s, predominant land uses were agriculture, forest, and wetland (U.S. Geological Survey, 1954); as of 2002, 

however, each of these land uses comprised less than about 

20 percent of the total land area.
6 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

 

Hydrogeology

The Long Neck Peninsula lies within the Atlantic 

Coastal Plain Physiographic Province, and is underlain by a 

wedge of mainly unconsolidated sediment that extends 

throughout the Delmarva Peninsula. Cushing and others 

(1973) described this wedge as a series of confined aquifers 

and associated confining beds overlain by a surficial aquifer 

that generally is unconfined.

 The aquifer on the Long Neck Peninsula on the basis of 

the formations described can be referred to as the surficial 

aquifer. The major near-surface geologic units for this aquifer on the peninsula are from oldest to youngest as follows 

(Andres, 2005)2: (a) the Bethany Formation of Miocene to 

Pliocene age (Andres, 2004), (b) the Beaverdam Formation 

of Pliocene age (Groot and Jordan, 1990), and (c) the Lynch 

Heights and Scotts Corners Formations of Quaternary age 

(Ramsey, 1993, 1997). In the Bayside and Lakeside areas 

(figs. 2a and 2b), fine-grained beds of the Bethany 

Formation, which commonly form the base of the aquifer, 

were apparently not penetrated by nested-well boreholes of 

up to 80 ft in depth, but Andres (1987) and Talley (1987) 

found the depth to the base of the aquifer can exceed 100 ft. 

The Lynch Heights and Scotts Corners Formations are relatively thin (less than 15-ft-thick) deposits that overlie the 

Beaverdam Formation. The Lynch Heights Formation is the 

surficial unit at land-surface elevations above 15 ft.

The coarsest aquifer sediment, chiefly well-sorted sand 

and gravel, and the most permeable part of the aquifer occur 

in the lower part of the Beaverdam Formation (fig. 2b). 

Drillers for production wells on the peninsula often seek this 

part of the aquifer, and production wells used in this study 

were screened in this part of the aquifer. Above this interval, 

sediments of the upper Beaverdam Formation tend to fine 

upward, and grade into interbedded sand, silty sand, and 

sandy silt and clay, which in turn are overlain by the Lynch 

Heights and Scotts Corners Formations. 

Depending on surficial materials, the surficial aquifer 

could be unconfined, semi-confined, or confined. Surficial 

materials comprised mainly of sand and gravel can function 

as aquifers; however, materials comprised of silts and clays 

can function as semi-confining or confining units.

Andres (1991a, 2004) and Andres and others (2002) 

mapped surficial materials (within 20 ft of the land surface) 

in relation to their potential to transmit water into the surficial aquifer (fig. 3). Recharge potential for surficial sediments ranged from poor to excellent, with poor-to-fair 

potentials in the vicinity of the Lakeside (Ph35-25) and Bayside (Pi31-01) production wells. Samples and geophysical 

logs collected during and after drilling of nested wells at 

these sites confirmed the presence of silts and clays, but their 

thickness was highly variable over short distances. Overall, 

the heterogeneity in recharge potential throughout the study 

area (fig. 3), which also was confirmed by drilling conducted 

during nested-well installations at the Bayside and Lakeside 

sites, indicated that shallow ground-water-flow patterns 

could be complex. 

Surface materials of generally undocumented origin, 

hereafter designated as fill, also can overlie surficial formations in or near areas of urban development on the Long 

Neck Peninsula. Historical topographic maps (U.S. Geological Survey, 1954) indicate sources of fill include borrow pits 

in or near the Lakeside and Bayside Communities. Andres 

(1987) and Talley and Andres (1987) also identified several 

abandoned borrow pits that likely were sources for fill. Most 

of these pits currently (2005) are covered by residential 

developments and related infrastructure.

Fill materials were identified during drilling of nested 

wells at both Pot Nets community sites (Appendix B, figs. 

B1 and B2). At the Bayside production-well (Pi31-01) site, 

which is largely covered by a secondary mixed hardwood 

forest (fig. 4a), from 0.5 to l.5 ft of apparent fill overlies the 

original forest soil (organic and mineral horizons). At the 

Lakeside production-well (Ph35-25) site, a mainly grassy 

area between a landscaped swimming pool complex and residential area (fig. 4b), about 2 ft of new soil and surface fill 

appears to overlie aquifer sediment. The surficial fill at both 

Pot Nets sites likely reflects cut-and-fill operations during 

development construction (James Mooney, Operations Manager, LNWC, written commun., 2004). Given no chemical 

analysis for Hg had been conducted on soil, fill, or aquifer 

sediment in either community before this study, it needed to 

be determined whether the fill was from local sources or was 

Hg-contaminated material brought in from other areas. 

Table 1. Land use and cover for the Long Neck Peninsula, Sussex 

County, Delaware, 2002

[Modified from 2002 coverage, Delaware Office of State Planning 

Coordination, Projection UT Zone 18 NAD 83—see figure 1]

Use category Area 

(square miles)

Area

(percent)

Urban 3.85 38.8

Agricultural 1.84 18.5

Forest 1.87 18.9

Wetland 1.94 19.6

Water 0.10 1.1

Transitional 0.31 3.1

Total 9.91 100

2. The stratigraphic nomenclature used is that of the Delaware Geological 

Survey. It includes formation nomenclature, for example, Beaverdam, 

Bethany, Scotts Corners, and Lynch Heights Formations different than that 

used by the U.S. Geological Survey. Use of the terms upper and lower 

Beaverdam Formations are informal stratigraphic units.
Introduction 7
8 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04
Introduction 9
10 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04
Introduction 11
12 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

All documented water supplies for Sussex County utilize 

ground water (Wheeler, 2003). Supply-well locations and 

depths (Blair Venables, Hydrologist, DNREC, written commun., 2004) indicate that most Long Neck Peninsula wells 

withdraw water from the surficial aquifer. Records also indicate that most residences along SR23 and SR24 on the western end of the peninsula (fig. 1) are served by private 

production wells, whereas the bulk of the water for the large 

(post–1960) developments on the peninsula chiefly is supplied by commercial (public) production wells. The LNWC 

operates 10 public wells in the Bayside (8) and Lakeside 

(2) communities. These wells withdraw water from the surficial aquifer for most major residential areas at and east of 

Lakeside (James Mooney, Operations Manager, LNWC, 

written commun., 2004).

The surficial aquifer throughout much of the State of 

Delaware is unconfined and vulnerable to contamination by 

human activities. Agricultural and residential-commercial 

developments, via septic systems and leachate from applications of fertilizer, lime, and manure, have been considered 

the causes of elevated concentrations of nitrate-nitrogen or 

NO3

-

-N (0.4 milligrams per liter or more, or mg/L) in shallow ground water beneath or downgradient of these areas 

throughout the Delmarva Peninsula, with the highest concentrations (greater than 3 mg/L) of NO3

-

-N having been 

found near or beneath agricultural areas (Shedlock and others, 1999; Hamilton and others, 1989). At least 20 percent of 

the samples from wells used in these studies exceeded the 

USEPA MCL for NO3

-

-N (10 mg/L). Concentrations of 

NO3

-

-N also have increased since the late 1980s at moderate 

depths in well-oxygenated parts of the surficial aquifer, but 

have remained low (less than 1 mg/L) in poorly oxygenated 

ground water (Denver and others, 2004). Low concentrations 

of herbicides (less than 1 microgram per liter, or μg/L), 

below Federal drinking-water standards, and used mainly on 

corn and soybean crops (for example, triazines, alachlor, and 

metolachlor), have occurred in shallow ground water near 

agricultural fields, as well as in nontidal streams during 

base-flow conditions (Koterba and others, 1993; Denver and 

others, 2004). Agricultural and residential land uses also 

have been associated with elevated concentrations of calcium, magnesium, sodium and (or) chloride attributed to 

liming, road salt, and septic effluent (Shedlock and others, 

1999). Local studies of the surficial aquifer in eastern Sussex 

County, including parts of the Long Neck Peninsula, also 

have found elevated concentrations of dissolved solids, 

selected ions, and NO3

-

-N associated with residential and 

agricultural land uses (Denver, 1986, 1989; Andres, 1991b; 

Delaware Department of Natural Resources and Environmental Control, 2002).

Persistent elevated concentrations of Hg in the surficial 

aquifer on the Delmarva Peninsula have not been reported 

(Delaware Department of Natural Resources and Environmental Control, 2002). Mercury, however, has not been part 

of regional or local ground-water-quality studies on the Delmarva Peninsula, or in Delaware or Sussex County, including Long Neck Peninsula (Hamilton and others, 1993;

Ferrari, 2002; Denver and others, 2004).

North of Delaware, and assuming background concentrations of 10 ng/L or less, mercury has been found at elevated 

concentrations in the surficial KCA in the Atlantic Coastal 

Plain of southern New Jersey (Murphy and others, 1994; 

Barringer and others, 2005). Concentrations of Hg in filtered 

samples (0.45-μm capsule) in excess of the USEPA MCL 

(2,000 ng/L) occurred in samples from hundreds of private 

production wells in at least 70 residential areas that draw 

their water from the KCA (Barringer and others, 1997). The 

spatial distribution of Hg-contaminated ground water has 

been described as highly irregular, and extensive contamination plumes of Hg-contaminated ground water have not been 

found (Barringer and others, 2005). There also has been no 

clear evidence of point sources. Mercury concentrations near 

or exceeding the USEPA MCL mainly have occurred in samples from either private wells in residential and mixed residential and agricultural land-use areas, or from monitoring 

wells in residential areas. These mercury-contaminated wells 

have been associated with areas where ground-water quality 

also appeared to have been affected by agricultural and (or) 

residential development; the documented effects have 

included elevated concentrations of NO3

-

-N, other selected 

ions, pesticides, and VOCs (Stackelberg and others, 1997, 

2000; Barringer and others, 2005). These effects appeared 

similar to those noted earlier for agricultural and residential 

land uses on the surficial aquifer of the Delmarva Peninsula, 

including Delaware. Given the similarity in coastal settings, 

land-use patterns, and related types of contamination in the 

surficial aquifers in portions of Delaware and New Jersey, 

and the widespread and elevated concentrations of Hg in the 

KCA in New Jersey, the appearance of high concentrations 

of HgTgw in the LNWC Bayside Pi31-01 and Lakeside 

Ph35-25 wells raised the possibility of widespread Hg contamination in the surficial aquifer on the Long Neck Peninsula.

Data Collection and Data Quality

This study included three major data-collection activities: (a) a source-water-quality survey, which involved sampling at production wells geographically distributed 

throughout the Long Neck Peninsula; (b) a preliminary timeand depth-series study to aid in the design of vertically 

nested monitoring wells, which mainly involved repeated 

water-quality data collection and sampling at the Bayside 

production well (Pi31-01) and at two nearby monitoring 

wells; and (c) a vertical distribution study that required the 

collection of soil, fill, and aquifer-sediment samples during 

the installation of vertical nests of monitoring wells near 

each Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25) 

production well sites, and the subsequent collection of waterquality data and samples at each production and monitoring 

well at these two sites. Methods of data collection relevant to 
Data Collection and Data Quality 13

these activities—including (a) the selection and installation 

of production and monitoring wells, respectively, and (b) the 

collection, processing, and chemical analyses of soil, fill, 

and aquifer-sediment samples and ground-water samples—

are described below. The quality of the data collected and 

whether it bears on the interpretation of the data also are discussed.

Well Selection and Installation

Production wells for sampling were identified that provide a broad geographic distribution of ground-water samples for Hg and other water-quality constituents in that part 

of the surficial aquifer commonly used to obtain drinkingwater supplies. Boreholes also were drilled and nests of 

short-screened monitoring wells installed to characterize the 

vertical distribution of Hg and relate this distribution to other 

chemical constituents in sediment and ground water in areas 

of known Hg contamination.

Selection of Long Neck Peninsula Production Wells

Production wells initially were selected with the assistance of DNREC and DGS from State well-completion 

records, and on the basis of the following guidelines:

(a) The well was to be located within the study area 

(fig. 1), and recently, if not currently, in use;

(b) The well was to be constructed of relatively inert 

materials—preferably polyvinyl chloride (PVC) 

casing and screen with screw rather than glued 

joints, and have a sampling spigot close to the 

wellhead and before any treatment or storage tanks;

(c) Well discharge was to be generated by a 

submersible positive displacement pump to reduce 

the potential for volatile constituent losses—for 

example, CFCs, VOCs, or dissolved gases; and 

(d) Drilling notes were to indicate that the well likely 

was screened in the surficial aquifer in the 

Beaverdam Formation, which would imply that all 

or most of the screen interval occurred within 40–

100 ft below land surface.

About 100 production wells were initially identified in 

the study area. Most of the identified wells occurred on commercial and residential properties, either along one of the 

two main thoroughfares (fig. 1, SR23 and SR24) or on secondary roads just off these thoroughfares. To enhance geographical coverage of the aquifer, the main thoroughfares 

and major secondary roads were subdivided into roughly 

equidistant segments, and each candidate well was assigned 

by proximity to one road segment. Data from DNREC drilling records for wells assigned to a given segment were evaluated with the above guidelines. Wells that met the 

guidelines were considered candidate wells for sampling for 

that segment. One well was randomly selected from among 

the candidate wells for that segment. The owner of that well 

was contacted during an onsite visit to verify that the well 

was operational and could be sampled, and to obtain written 

permission to collect water-quality data and samples. The 

above process was repeated for each segment until wells 

were obtained for each segment or no candidate wells 

remained for a segment.

Geographical coverage was sparse along some road segments. Permission could not be obtained to sample wells 

located along the southeastern two-thirds of Bay Farm Road 

(fig. 1), which eliminated a large area of mixed forest and 

agricultural use from the study area. Permission denials also 

reduced the number of candidate wells in the central part of 

the Peninsula along SR23. Therefore, all 10 LNWC production wells were included to provide coverage of the aquifer 

in the area of known Hg contamination.

Maximizing geographical coverage required the modification of some plumbing systems to include wells that otherwise would have been unsuitable for sampling. Four selected 

wells (Appendix A, Table A1, footnote a) each were fitted 

with a hose bib on a PVC compression coupling that was 

installed between the wellhead and either treatment or storage tanks and multiple distribution lines. Before installation, 

the bib and coupling were cleaned—initially soaked for 

24 hours in a 0.1 percent by volume phosphate-free detergent 

solution, flushed with clean tap water for at least several 

days, and flushed with deionized, low-mercury, active-carbon filtered (DACF) water3 for at least 24 hours. Each 

cleaned hose bib and coupling was double-bagged in sealable polypropylene bags, and shipped to and installed by a 

contract plumber, who was trained to reduce equipment contamination during its installation. Installation occurred about 

2 weeks before sampling in order to initially and repeatedly 

condition the bib and coupling with well water.

With the modifications in guidelines and plumbing, a 

total of 22 production wells, geographically distributed 

throughout the study area (fig. 5), were selected for sampling. Owner, construction, and depth information for these 

wells (Appendix A, table A1), chiefly compiled from 

DNREC well-completion reports, indicated: 

Nine wells supplied private homes or businesses, one of 

which solely was used for residential irrigation (Ph34-24). 

Thirteen wells were public-supply wells.

(a) Most (18) wells were constructed of threaded PVC; 

four wells were made of steel. Each well had a 

smooth or hose-threaded brass bib located between 

the wellhead and any tanks and major distribution 

lines. 4

3. DACF water has a specific conductance of less than 1 microsiemen per 

centimeter at 25 degrees Celsius, and a total mercury concentration of less 

than 1 nanogram per liter.

4. Except for four wells, all wells have brass hose bibs that were installed 

during initial well construction, and presumably for sample collection on 

the main distribution line between the wellhead and storage or treatment 

tanks. Consistent with the above, similar hose bibs were installed on the 

main distribution line of each of the remaining four wells.
14 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04
Data Collection and Data Quality 15

(b) Except for well Ph34-23, all wells had submersible 

positive displacement pumps. Well Ph34-23 was a 

shallow well with a surface suction pump.

(c) Wells were screened in the surficial aquifer and 

Beaverdam Formation. For private wells, the depth 

from land surface to the top of the screened interval 

averaged 55 ft and ranged from 30 to 73 ft. Screen 

length ranged from 5 to 10 ft with a median and 

mode of 10 ft. Public wells generally were deeper 

and had longer screens than private wells. Depth 

from the land surface to the top of the screened 

interval averaged 67 ft and ranged from 42 to 98 ft. 

Screen length ranged from 6 to 40 ft with a median 

and mode of 15 ft.

Installation of Pot Nets Bayside and Lakeside 

Monitoring Wells

During previous studies (James Mooney, Operations 

Manager, LNWC, written commun., 2004), three monitoring 

wells were installed in the vicinity of each of the contaminated Pot Nets production wells (figs. 4a and 4b). Well-completion records from DNREC indicated the screened 

intervals (12 to 15 ft) of these monitoring wells were similar 

in length and depth to the screened interval of the nearby 

production well at each site (Appendix A1, table A1, Pi31-

01 and Ph35-25). Water-quality and geophysical data, 

obtained with a submersible multi-parameter sonde and 

gamma and resistivity tools in selected monitoring wells 

near each pumping production well, indicated potential 

differences in ground-water bulk chemistry or lithologic 

materials with depth (Appendix D; Andres, 2005). The differences observed in bulk chemical properties and geophysical logs were of sufficient magnitude to warrant the 

installation of vertical nests of monitoring wells with short 

screens at each production-well site.

In late June and early July 2004, the DGS and USGS 

installed vertical nests of wells that consisted of one longscreen (70-ft length) and five or six short-screen (2-to-5 ft 

length) monitoring wells in the vicinity of each contaminated 

production well (figs. 4a and 4b). Summary well construction and depth characteristics of the installed monitoring 

wells for each vertical well nest are provided (Appendix A, 

table A2).

Borehole drilling for monitoring wells was conducted 

with a DGS truck-mounted drill rig with an 8-in. (inch) hollow-stem auger (Andres, 2005). During drilling, expelled 

cuttings were placed on polypropylene sheets. After well 

installation was complete, all residual cuttings were removed 

from the site. Between boreholes, auger flights and other 

wetted drilling equipment were cleaned with a power washer 

and water obtained from well Ph31-12 (fig. 5), which drew 

water from formations similar to those being drilled but had 

low concentrations of HgTgw.

5

Vertical-nest boreholes at each site were placed along an 

arc about 15 to 25 ft from the production well with adjacent 

boreholes separated by about 5 ft (figs. 4a and 4b). The use 

of an arc pattern, the distance of the arc from the production 

well, and distance between adjacent boreholes along the arc 

were chosen to ensure all monitoring wells were within the 

zone of influence of the pumping production well, and 

located about the same distance from the production well, 

but with sufficient space between adjacent borehole locations to reduce potential effects of drilling or installation of 

one monitoring well on ground-water flow or quality in the 

vicinity of adjacent installed monitoring wells (Appendix 

D). The arc compass bearing in relation to the production 

well (figs. 4a and 4b) was chosen to lie in the general direction of the formerly installed long-screen monitoring well(s) 

that had most consistently yielded samples with the highest 

Hg concentrations as determined from previous studies 

(Delaware Department of Natural Resources and Environmental Control, 2002; James Mooney, Operations Manager, 

LNWC, written commun., 2004). Well depth, and screen 

depth and length for each well, were selected to intercept 

ground water traveling toward the pumping production well 

from parts of the aquifer where it appeared ground-water 

chemistry and possibly Hg concentrations differed given 

previously observed differences in the bulk properties of 

ground water and in the characteristics of aquifer sediment 

with depth (Appendix D).

Monitoring wells were constructed with 10-ft lengths of 

2-in. nominal-outside diameter (OD) schedule 40 PVC casing with screw-thread joints and O-ring seals, and 0.01-in. or 

0.02-in. slotted 2 to 5 ft schedule 40 PVC screen. Well components were assembled on clean polypropylene sheets 

placed onsite; the assembler wore powderless nitrile gloves. 

After placing the casing and screen in the borehole, the 

annular space was backfilled in the order listed with commercially produced clean commercial fine gravel to a depth 

about 1 ft above the screened interval, about 1 to 2 ft of clean 

filter sand, and at least 2 ft of bentonite pellets. Water from 

LNWC well Ph31-12 was added as necessary to reduce sediment heaving in the borehole. Except for the infrequent collapse of sediments around the borehole, remaining annular 

space was backfilled with a combination of commercial bentonite, fine-to-medium sand, and gravel to the depth of the 

water table, and then granular bentonite, from the water table 

to land surface.

Wells were developed within several days of installation. 

Development involved surging while simultaneously withdrawing water from the well with a 1.75-in. OD PVC perforated tube coupled to 1-in OD polypropylene tubing that was 

attached to a portable centrifical pump. Development was 

terminated at each well after about 45 to 60 minutes of 

pumping at which point discharge was clear, and measured 

bulk chemical properties (pH, temperature, DO, and SC) 

were relatively stable compared to initial readings. Final discharge rates differed with well depth and lithologic material. 

5. Historically (2000–2004), total mercury was not detected in this well 

water, given detection limits that ranged from 0.1 to 0.5 microgram per liter 

(James Mooney, Operations Manager, LNWC, written commun., 2004) . 

Data from this study indicated the total mercury concentration in this well 

water was about 1.5 nanograms per liter (Appendix A, table A6).
16 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

The lowest discharge (about 0.5 gallons per minute) 

occurred at the shallowest well at each site, which was 

screened in fine-silty sand with discontinuous clay lenses 

(Appendix B). The highest discharge (about 8 gallons per 

minute) at each site occurred at the deepest well, which was 

screened in medium-to-coarse sand (Appendix B).

To complete installation, each well casing was cut to 

within several inches of land surface, the top of the fresh-cut 

casing permanently notched to establish a measuring point to 

water level, and the casing sealed with a lockable cap. The 

assigned State well permit tag was attached to the exposed 

casing, and a bolted steel cover was installed over the well. 

The final well elevation was obtained from a laser-level survey privately contracted by the LNWC, which used an established USGS benchmark for reference. Surveys were 

repeated until the difference in elevation at each well was 

less than 0.02 ft for two sequential surveys of all wells at a 

site.

Well development was repeated several weeks after 

installation to enhance the flow of ground water through the 

well screen, and to remove any fines introduced during final 

stage of well installation. Surging first was performed along 

the entire length of the wetted casing with a 1.75-in. OD 

solid cylinder of schedule 80 PVC fitted to 0.75-in. OD 

schedule 40 PVC tubing. This surging immediately was followed by simultaneous surging and water withdrawal with 

the previously described PVC development tool and dedicated polypropylene tubing. This entire process was 

repeated, and the well then pumped until discharge was 

clear. Well depth was re-measured and compared to the 

depth taken during installation to verify that no fines 

remained at the bottom of the well.

After development was completed at each well, reused 

down-hole development tools were cleaned with a 0.5 percent by volume phosphate-free detergent and water (well 

Pi31-12) solution, and then rinsed with well Pi31-12 water to 

remove detergent. All used polypropylene discharge tubing 

was replaced with new tubing for each well.

Field Data and Sample Collection

Field data and samples for ground water (Appendix A) 

and sediment (Appendix B) as well as selected quality-assurance measures (Appendix C) were obtained for each major 

study objective at different geographical scales as follows:

(a) A peninsula-wide source-water-quality survey of 

Hg and other water-quality characteristics in the 

shallow surficial aquifer, which involved the 

collection of bulk chemical and physical property 

data and water samples from 22 (9 private and 13 

public) production wells during November and 

December 2003 (fig. 5), and 

(b) A local Pot Nets Bayside-Lakeside community 

vertical distribution study of Hg in ground water 

and sediment which required:

(1) A preliminary study to aid in the design and 

installation of, as well as development of a data 

and sample-collection procedure for, vertically 

nested monitoring wells, which was conducted 

in February 2004. This study involved the 

collection of bulk property (chemical and 

physical) data and water samples from Bayside 

production well Pi31-01 and nearby monitoring 

wells Pi31-13 and Pi31-17 (fig. 4a), and the 

collection of data from the Lakeside productionwell site at monitoring well Ph35-35 (fig. 4b).

(2) A sediment and water-quality study at the 

Bayside and Lakeside production-well sites 

(figs. 4a and 4b), with sediment data and sample 

collection conducted in late June to early 

July 2004 from boreholes made during the 

installation of vertical-nest monitoring wells; 

and with ground-water data and sample 

collection at each of these wells and the nearby 

pumping production well at each site in 

September 2004.

Ground-Water Data and Water Samples

Detailed descriptions of activities antecedent to sampling—including antecedent production-well pumping, 

equipment cleaning and set-up, and well discharge and sampling rates, are provided (Appendix D). Except for filter 

pore sizes, collection of ground-water data and samples 

(table 2), whether from production or monitoring wells, followed USGS National Field Manual (NFM) protocols 

(Wilde and Radtke, 2003) modified for low-flow trace element and Hg sampling (Lewis and Brigham, 2004; Ivahnenko and others, 1996, 2001; Olson and DeWild, 1999). 

For this study, encapsulated nylon pleated filters with absolute pore sizes of either 0.4 μm or 0.1 μm were used. Dissolved-gas (DG) and CFC samples were collected in 

accordance with USGS CFC-DG Laboratory protocols 

(http//water.usgs.gov/lab/cfc/sampling/newmethod.htm).

Sampling at each well was started with continuous-flow 

discharge measurements of bulk properties—pH, SC, DO, 

turbidity, temperature, and oxidation-reduction potential—

monitored to assess chemical stability of well discharge. 

Except for turbidity, stability was considered achieved when 

a measured bulk property was relatively constant for three 

successive readings taken at 3- to-10 minute intervals (Wilde 

and Radtke, 2003).

Turbidity is a key parameter for trace-element sampling 

(Gibs and others, 2000). Achievement of low turbidity values (less than 1 Nephelometric Turbidity Unit, or NTU) was 

emphasized. Once low turbidity and stability in other bulk 

properties were achieved, which typically took an hour or 

more at each well, bulk-property data were collected (table 2 

and Appendix A, tables A3, A9, A10, and A11) and sampling was initiated.

Data and sample collection differed depending upon the 

sampling activity (tables 2 and 3). In relation to the buffering 

capacity of ground water (Rounds and Wilde, 2001), and for 

the source-water-quality survey and the local preliminary 

time- and depth-series study, alkalinity (ALK) data were
Data Collection and Data Quality 17

 

Table 2. Types of wells and ground-water data and samples collected by the U.S. Geological Survey from production and monitoring wells, 

Long Neck Peninsula, Sussex County, Delaware, 2003–04

[X, sample collected; --, no sample collected; raw, unfiltered water; 0.4-μm filt. or 0.1-μm filt., filtrate water from a 0.4- or 0.1-micrometer (μm) absolute pore 

size pleated and encapsulated nylon filter; 0.45-μm filt., filtrate from a 25-millimeter-diameter, 0.45-μm nominal pore size, glass-fiber plate filter; bulk 

properties include temperature, pH, oxidation-reduction potential, dissolved-oxygen concentration, specific conductance at 25 degrees Celsius, turbidity, and 

either acid-neutralization capacity (ANC) or alkalinity (ALK), as determined by field titrations of raw or 0.4-μm filt. samples, respectively.] 

Data and sample collection for 

Sample type

Source-waterquality survey

Preliminary

time- and depth-series

studies

Vertical profile

distribution

studies

Types of wells

Peninsula

public or private

production wells

Bayside

production

and monitoring wells

Bayside–Lakeside

production and 

monitoring wells

Types of data and samples collected

Mercury, total, raw X X X

Mercury, total, 0.4-μm filt. XXX

Mercury, total, 0.1-μm filt. -- -- X

Mercury, methyl, 0.4-μm filt. X a -- X

Bulk properties, raw X X X

Major ions, 0.4-μm filt. XXX

Nitrogen, raw and 0.45-μm filt. XXX

Carbon, raw and 0.45-μm filt. XXX

Trace elements, 0.4-μm filt. X X --

VOCs, raw X -- X

Chlorofluorocarbon compounds, raw -- -- X

Dissolved gases, raw X -- X

Oxygen-deuterium isotopes, raw -- X --

Hydrogen-sulfide species, raw -- -- X

Ultraviolet absorbance, at 254 and 280 nanometers, raw -- -- X

Dissolved organic carbon fractions, 0.45-μm filt. -- -- X a

a Fourteen of 22 wells were selected for methyl mercury sampling, with an emphasis on wells located in the vicinity of areas with known

mercury contamination in ground water; a monitoring well with elevated mercury at each Pot Nets community site was selected for fractionated-carbon 

sampling.
18 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

 

obtained from filtered samples (0.4-μm absolute pore size 

nylon pleated filter in nylon capsule, hereafter 0.4-μm capsule) with the Gran titration method. For the vertical distribution study, ANC, rather than ALK, data were obtained in a 

similar fashion, with raw, rather than filtered, water samples 

in order to reduce total sampling times at each Pot Nets production-well site while the production well was pumping. 

Given no evidence of a particulate buffering phase, bicarbonate concentrations were computed directly from either 

ANC or ALK (Appendix A, tables A3, A9, A10, and A11). 

Hydrogen sulfide also was determined with raw-water samples (table 2, and Appendix A, table A11) within 10–15 minutes of sample collection with the methylene blue method 

(American Public Health Association, 1981) and color spectrophotometer (Carmody and others, 1998).

Except for CFC and DG samples (see below), all raw and 

filtered samples were collected in a portable sample chamber 

that was designed and cleaned between uses in accordance 

with USGS NFM protocols (Wilde and Radtke, 2003). Most 

filtered samples (table 2) were collected from a 0.4-μm capsule that was inserted directly into the sample feed line. For 

the vertical distribution study, additional filtered samples for 

mercury (HgTgw-f0.1) were obtained from Bayside wells 

with a nylon encapsulated 0.1-μm absolute pore size nylon 

pleated filter (hereafter 0.1-μm capsule) to better assess 

potential particulate phases of Hg in known areas of Hg 

ground-water contamination.

Raw-water samples for nitrogen (total particulate) and 

carbon (total, inorganic, and organic particulate) were collected and chilled to 5 degrees Celsius (or °C) or less (tables 

2 and 3). Each raw-water sample was suction-filtered 

through a baked, 25-millimeter (or mm)-diameter, 0.45-μm 

glass-fiber, plate filter, and the filter and filtrate chilled until 

analyzed.

Four raw-water samples for the fractionation of DOC 

(table 3) were collected sequentially in 1-liter amber baked 

glass bottles from one nested monitoring well at each site. 

Each 1-liter sample was suction filtered as described above, 

Table 3. U.S. Geological Survey laboratories and sample type, handling, and delivery

Laboratory Sample type, handling, and delivery

National Water Quality Laboratory, 

Water Resources Division, Denver, Colorado

Ground water: Major ions, selected nutrients (nitrate-nitrite nitrogen, nitrite nitrogen, 

dissolved organic carbon (DOC) and particulate inorganic and organic carbon), volatile 

organic compounds (VOCs), and DOC ultraviolet absorbance at 254 and 280 nanometers. 

Except for major ions and trace elements, samples were chilled to less than 5 degrees 

Celsius (5 °C) without freezing. Samples were shipped overnight from the field during the 

week samples were collected.

Mercury Research Laboratory,

Middleton, Wisconsin

Ground water: Total mercury in raw and 0.4-micrometer (μm) absolute pore size filter 

samples from each production and monitoring well, total mercury in 0.1-μm absolute pore 

size filter samples from monitoring wells near well Pi31-01, and methyl mercury 

in selected 0.4-μm filtered samples from all production and selected monitoring wells at 

both sites. Samples acidified and chilled to less than 5 °C, and shipped overnight within 

2 weeks of collection. (The laboratory also provided analyses of selected split samples 

spiked with either methyl mercury, or methyl mercury and VOCs.)

Chlorofluorocarbon and Isotope Laboratory, 

Reston, Virginia

Ground water: Chlorofluorocarbons and dissolved gases from samples collected at 

selected production and monitoring wells. Samples were hand-delivered to the laboratory 

within a week of collection.

National Research Program, Boulder, Colorado Ground water: Fractionation of dissolved organic carbon into neutral and reactivecarbon fractions from samples collected at two monitoring wells with high mercury 

(greater than 1,000 nanograms per liter). Four-liter 0.4-μm absolute pore size filtered 

samples chilled to about 5 °C, and shipped overnight within 10 days of collection.

National Water Quality Laboratory, Geologic 

Division, Denver, Colorado

Sediment: Total mercury, selected trace elements, total sulfur, and particulate organic and 

inorganic carbon. Samples were chilled to less than 5 °C without freezing until and after 

processing. Selected processed samples were shipped on dry ice overnight within 3 weeks 

of collection.
Data Collection and Data Quality 19

the filtrates combined in a 4-liter baked amber glass bottle, 

and the composite sample was chilled until analyzed.

Sampling for CFCs and DGs (table 2) required split-flow 

sampling, with an additional sampling line (refrigerant-grade 

copper tubing) dedicated to the collection of these samples 

(Appendix D). Due to waste overflow, sampling for CFCs 

and DGs was conducted in a clean area with drainage, either 

inside or outside the well house.

All samples were physically or chemically preserved as 

soon as possible during or after collection in accordance with 

the USGS NFM (Wilde and Radtke, 2003). Samples were 

delivered or sent to select USGS Laboratories for analysis 

(table 3).

Documentation of all laboratory analytical methods, 

including methods for Hg, is provided (Appendix A, tables 

A18 and A19). Laboratory analytical methods for Hg and the 

separation of DOC into fractional components are briefly 

described below.

All Hg analyses of water samples were performed at the 

USGS Mercury Research Laboratory (MRL), Middleton, 

Wisconsin. Analysis of total Hg in raw or filtered samples 

(table 3) involved three major steps: (a) oxidation of Hg 

species to reactive mercury (Hg(II), (b) reduction of Hg(II) 

to elemental mercury (Hgo), and (c) quantification of Hgo by 

cold vapor atomic fluorescence spectroscopy—CVAF 

(Olson and DeWild, 1999). The analysis of HgMegw-f0.4

required sample distillation to reduce interferences, aqueousphase ethylation of Hg species in the distillate, separation of 

ethylated species by gas chromatography, and quantification 

as methy-ethyl mercury by CVAF (DeWild and others, 

2002). Analysis of HgMegw-f0.4 samples for the peninsula 

source-water-quality survey revealed a matrix-interference 

problem—CVAF detector quenching for samples with high 

HgTgw concentrations (greater than 1,000 ng/L). Subsequent 

quantification for the vertical distribution study (table 3) utilized inductively coupled plasma-mass spectroscopy—

ICPMS.

The fractionation procedure of Aiken and others (1992) 

isolates and quantifies carbon fractions (table 4—hydrophobic and hydrophilic) linked to the complexation and transport of inorganic (Hg(II), methylated (HgMe), or sulfide 

(HgS, colloidal) mercury (Ravichandran and others, 1998, 

1999; Haitzer and others, 2002, 2003; Qian and others, 2002; 

Aiken and others, 2003; Waples and others, 2005). To obtain 

these fractions for ground water, a large volume of total sample often must be collected, filtered, and chilled in the field. 

Sampling at each Pot Nets site had to be completed within a 

30-hour period (Appendix D), which limited fractionation 

sampling to one sample from each site, and total sample volumes to 4 liters. The USGS National Research Program in 

Boulder, Colorado analyzed the samples (table 3). The analysis required passing an acidified (pH 2) filtrate sequentially 

through two resin columns, which were back eluted with 

sodium hydroxide (pH 13) to obtain the reactive organic 

fractions (table 4). The concentration of each fraction was 

obtained by DOC analysis.

Soil, Fill, and Aquifer-Sediment Data and Samples

Surficial soil and fill samples were collected at each production-well site with a hand-screwed steel post auger at 

0.5-ft depth intervals to a total depth of about 2–3 ft, below 

which sediment were operationally defined as aquifer sediment. Subsamples of selected auger material—(relic) soil O, 

A, B, and upper C horizons and apparent fill)—were identified on site, and individually double-bagged in re-sealable 

polypropylene bags and chilled (to about 5 °C). Aquifer 

sediment cores were collected during drilling with 2-ft-long, 

2-in.-OD, steel split-barrel samplers equipped with cleaned6

transparent acetate barrel liners. At each site and first borehole, continuous cores were obtained during drilling from 

the land surface to the water table, and thereafter at roughly 

5-ft intervals. Additional cores were obtained at subsequent 

boreholes at selected and final depths.

Between uses, each core sampler was scrubbed with a 

plastic bristle brush and a 0.5 percent by volume phosphatefree detergent and (well Ph31-12) water solution, rinsed with 

this water to remove detergent, and rinsed several times with 

DACF water. A new cleaned liner then was installed, and the 

sampler reassembled.

As each core sampler was retrieved, the acetate liner was 

removed and sealed with cleaned polypropylene caps and 

placed on a polyethylene sheet. Core depth, orientation, and 

useable portion were recorded on the side of the liner and on 

field forms, which also were used to note sediment composition. Each capped liner was placed in a waxed core box that 

was enclosed in nested polypropylene bags and chilled on 

ice.

Individual soil or fill samples and aquifer-sediment cores 

were transported to the laboratory and refrigerated at about 

5 °C. Cores remained chilled until processed, at which time 

the core was inspected to obtain final data for lithologic 

descriptions, and subdivided to provide subsamples for laboratory analysis (Appendix B, figs. B1 and B2, and tables B1 

through B10). Sample processing was conducted at the 

Patuxent Wildlife Research Center, Laurel, Maryland, in the 

laboratory on a clean dedicated polyethylene sheet, and with 

nonmetallic equipment—cleaned plastic spatulas and tweezers, glass bowls, and polyfluorocarbon subsample containers. Between processing samples, reused equipment was 

washed in a 0.2 percent by volume phosphate-free detergent 

and tap water solution, rinsed with tap water to remove 

detergent, and rinsed several times with DACF water.

Processing of individual sediment cores generally took 

about 20–30 minutes. It began with the initial visual and 

textural identification and documentation (Appendix B, 

tables B1 through B10) of lithologic characteristics (relative 

gravel, sand, silt, and clay content), relative sand size 

6. For sediment collection, cleaned implies equipment soaked for at least 

24 hours in a 10 percent by volume solution of hydrochloric acid and 

deionized activated-carbon filtered water, rinsed at least three times with 

this water, double-bagged in polypropylene bags, and chilled (5 degrees 

Celsius or lower) until used.
20 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

 

 Table 4. Dissolved organic carbon fractions obtained from dual-column resin extractions (modified from Ravichandran and others, 1998)
Data Collection and Data Quality 21

(coarse, medium, or fine), and color (Munsell Color, 1990). 

Extraneous materials, including woody debris (for example, 

sticks and roots) or large pebbles and stones were removed 

with tweezers. Bottom (1-in.) core-end segments and sediment that appeared mixed during drilling or coring also were 

described but discarded. Useable parts of each core that visually and texturally were different in lithologic composition 

were identified, separated from other materials with a dedicated thin-bladed plastic spatula, and trimmed to remove and 

discard thin (0.25-in.) sections from each end. If sediment of 

similar composition appeared as a continuum in two or three 

depth-sequential cores, then subsections were taken from 

individual cores and combined into a composite sample.

Each lithologic sample was transferred to a dedicated 

glass bowl, where it was homogenized by hand with a 

dedicated spatula until uniform in appearance. From 40–

80 grams wet weight of the homogenized sample was transferred into an acid-rinsed tared 500-milliliter (or mL) polyethylene sediment container, and the container re-weighed 

to determine the wet weight of the sample. Another 20–

40 grams of homogenized sample was transferred to an acidrinsed 80-mL polyethylene container for archival. Sample 

containers were refrigerated at about 5 °C.

Lithologic samples were selected for chemical analyses 

after processing all samples, and reviewing lithologic data 

and sample-depth location. Selected samples included soil 

(various horizons), fill, and clay materials near the land surface, and different lithologic materials at depths corresponding to locations above, within, and below the screened 

interval of each contaminated production well (Appendix B, 

table B11). Samples in 500-mL polyethylene containers 

were shipped on dry ice within 3 weeks of their collection 

for sediment chemical analysis to the USGS NWQL, Geologic Division, Denver, Colorado (table 3).

Documentation of laboratory analytical methods for sediment analysis is provided (Appendix B, table B12). In brief, 

sediment analyses included total sulfur, and organic, inorganic, and total carbon on sediment dried (96 hours at 23 °C) 

to constant weight in a convection oven, and lightly ground 

to a uniform texture with mortar and pestle. Analyses for 

HgTsed and selected trace elements (TEs) were conducted on 

digested 7dried-and-ground sediment. Total mercury in the 

digestion solution was quantified by cold-vapor atomic 

absorption spectrometry.

Composite lithologic profiles were prepared by the 

USGS and DGS from core descriptions obtained from each 

site (Appendix B, figs. B1 and B2, and tables B1 through 

B10). These profiles reflected soil, fill, and aquifer sediment 

from the Beaverdam, Lynch Heights, and Scotts Corners 

Formations in the vicinity of each contaminated production 

well, as described earlier in this report (see Introduction, 

Hydrogeology).

Data Quality

Data provided by USGS laboratories from sample analyses (table 3) underwent routine systematic reviews by the 

laboratories, and were externally reviewed by project staff. 

Data-quality issues that resulted from these reviews were 

addressed in discussions between USGS laboratory and 

project staff. Data obtained through direct measurements or 

analyses of samples in the field were quality-assured in 

accordance with the USGS NFM Protocols (Wilde and 

Radtke, 2003). Additional measures, including the collection 

of blank, replicate, and spiked samples, were used to assess 

the quality of selected data.

Quality-assurance reviews indicated that most data 

obtained from sampling activities (table 1) were interpretable for the purposes of this study without reservation. The 

following describes limitations in the quality of selected data 

for Hg, major ions (bicarbonate), ALK, ANC, and the apparent recharge dates (ages) of ground water.

Mercury

Quality-control (QC) samples (blanks and duplicate 

ground-water samples) were collected in the field during 

sampling to help evaluate the potential bias and variability 

in HgTgw concentrations in raw or filtered ground-water 

samples because of field and laboratory equipment or methods. Additional QC samples (ground-water samples spiked 

with different chemical forms of Hg and other constituents) 

were prepared and analyzed by the USGS MRL, Middleton, 

Wisconsin, to evaluate HgMegw-f0.4 analytical methods.

To determine if ambient conditions and (or) field and laboratory methods or equipment led to sample contamination, 

source-water and field blanks were obtained during major 

sampling activities (table 5). Concentrations of HgTgw-raw

in NWQL source water for field blanks ranged from 0.03–

0.05 ng/L or near the average 0.04 ng/L MRL daily detection 

level for mercury. Except for a field blank obtained during 

the vertical distribution study, HgTgw concentrations in raw 

or filtered field blanks were low—less than 0.35 ng/L, and 

no measurable HgMegw-f0.4 (0.04 ng/L or more) was found 

in field blanks (table 5).

The magnitude of HgTgw in field blanks could have 

reflected ambient site or antecedent sampling-equipment 

conditions. Concentrations of HgTgw (in raw and filtered 

samples) were highest in those field blanks processed after 

sampling at wells with high HgTgw concentrations (table 5, 

footnote a), and generally were similar in magnitude to the 

low concentrations of HgTgw found in some ground-water 

samples (Appendix A, tables A6 and A14). Barringer and 

others (2005) also found higher Hg concentrations in blanks 

obtained at monitoring-well sites compared to productionwell sites. This bias could have resulted from the additional 

equipment (for example, portable submersible pump) and 

(or) additional field activity associated with the collection of 

samples at monitoring wells. 7. For total mercury: Nitric acid-sodium dichromate, 2 hours at 

110 degrees Celsius; for trace elements: Hydrochloric-Nitric-PerchloricHydrofluoric acids, 3 hours at 110-150 degrees Celsius.
22 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003-04 Table 5. Mercury concentrations in field and source-water blanks and corresponding ground-water samples for major sampling activities on Long Neck Peninsula, Sussex County, Delaware, 2003–04 [DGS, Delaware Geological Survey; μm, micrometer, absolute pore size of nylon-encapsulated nylon pleated filter; ng/L, nanograms per liter; Min, minimum; Max, maximum; <, less than; ---, no value; n/a, not applicable] Blank concentration Ground-water concentration, range a Blank sample type Ground-water sampling activity DGS well-site identifier Date Time Total mercury, unfiltered (ng/L) Total mercury, 0.4-μm filtered (ng/L) Methyl mercury, 0.4-μm filtered (ng/L) Total mercury, unfiltered (ng/L) Total mercury, 0.4-μm filtered (ng/L) Methyl mercury, 0.4-μm filtered (ng/L) Min Max Min Max Min Max Field Peninsula survey Ph24-01 11/25/2003 1600 0.12 0.12 < 0.04 0.11 1,820 0.09 1,790 < 0.04 < 0.04 Pi31-12 12/17/2003 1009 --- 0.09 < 0.04 Field Time-depth series Pi31-01 02/12/2004 1030 0.32 0.23 --- 1,140 1,560 1,260 1,550 --- --- Field Vertical well nest Ph35-25 09/16/2004 0930 1.62 2.37 < 0.04 0.89 6,380 1.07 6,100 <0.04 0.06 Source water For all of above Ph35-25 04/23/2004 1035 0.05 0.03 < 0.04 n/a n/a n/a n/a n/a n/a

a Range is for ground-water samples from all wells at which samples were collected for that activity. Total mercury concentrations found in raw well water sampled just before field blank was collected were as follows: Ph24-01 (0.18 ng/L), Ph31-12 (1.49 ng/L), Ph31-01 (1,830 ng/L), and Ph35-25 (1,000 ng/L). 
Data Collection and Data Quality 23

 

For the purposes of this study, background concentrations in ground water—geographically for the Long Neck 

Peninsula or vertically at the local Bayside and Lakeside 

sites—were described conservatively as being 10 ng/L or 

less. This concentration was 5 to 10 times greater than the 

actual concentrations in field blanks (table 5) and some 

ground-water samples (Appendix A, tables A6 and A14), 

and thus, could be higher than the actual HgTgw, concentrations in ground water unaffected by human activity.

Data obtained from sequentially collected samples of 

ground water from a given well, hereafter referred to as 

duplicate samples, provided an indication of the variability 

in HgTgw measurements as a result of field and laboratory 

methods—the repeated field processing, handling, and shipping, and laboratory processing and analysis of samples 

(table 6). The relative difference between duplicate-sample 

concentrations for HgTgw in raw (or filtered) samples collected at a given well was about 6–7 percent, and was similar 

to the variation in duplicate-sample Hg concentrations 

reported by others (Barringer and others, 2005) for similar 

data-collection and analytical methods. It also was relatively 

small in magnitude compared to the variability in HgTgw

concentrations found in ground water (table 5), which span 

several orders of magnitude.

For the purposes of this study, relative differences in elevated HgT concentrations of about 6–7 percent or less 

between ground-water samples collected at the same well at 

different times, or at different wells, were considered similar 

in magnitude. Such small relative differences fall within the 

variability in measurement in HgTgw concentrations attributable to field and laboratory methods.

Laboratory recoveries of total Hg from reference and 

internal standards, or raw or filtered ground-water samples 

spiked with Hg by the USGS MRL, fell within the acceptable ranges. Data-quality concerns, however, arose in relation to HgTgw concentrations in survey samples from two 

production wells, and for methyl mercury in all productionor monitoring-well samples with HgTgw concentrations in 

excess of 366 ng/L, and are described below.

Sampling protocols were adopted to help ensure the quality of data collected (see Field Data and Sample Collection). 

These protocols were not met, however, during the sampling 

at two production wells for the peninsula source-water-quality survey (Ph24-02 and Pi32-14). Despite attempts to obtain 

a low turbidity before sampling, discharge from well Ph24-

02 (fig. 5) had the highest turbidity recorded (2.6 NTU) during sampling. The pumping rate for this production well was 

inadvertently set by the operator to about 500 gallons per 

minute (or gpm), instead of the requested 150–200 gpm 

(Appendix D). Small visible solids (angular flakes) were 

observed in discharge, and appeared to be scale or corrosion 

deposits from the well casing or screen. The higher-thanrequested flow rate was discovered after sampling was completed.

Well Ph32-14, located just east of Pot Nets Bayside (fig. 

5), was the only well in this area for which permission was 

obtained to sample. The well hose bib, however, was difficult to access, and located in close proximity to several distribution lines and storage tanks of different age and 

composition. Permission to install a new spigot closer to the 

wellhead was denied, so the bib was cleaned. The bib, or 

backflow from distribution lines or storage tanks, could have 

been a low-level source of contamination.

Measurable concentrations of HgTgw in raw and filtered 

samples for wells Ph24-02 and Ph32-14 were qualified to 

reflect that sampling protocols were violated (Appendix A, 

table A6, footnote a). The qualification notes that these violations could have led to a small positive bias in data for 

HgTgw and (or) other TEs.

Table 6. Mercury concentrations in sequentially collected ground-water samples at Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25) production 

wells, Sussex County, Delaware, 2003–04 

[DGS, Delaware Geological Survey; μm, micrometer, absolute pore size of nylon-encapsulated nylon pleated filter; ng/L, nanograms per liter; <, less than; 

---, no value]

Sample

designation

DGS well 

site identifier Date Time

Total 

mercury,

unfiltered

(ng/L)

Total mercury,

 0.4-μm filtered

(ng/L)

Methyl mercury,

0.4-μm filtered

(ng/L)

Primary Pi31-01 12/10/2003 1230 1,820 1,790 < 0.04

Duplicate Pi31-01 12/10/2003 1345 1,940 1,920 ---

Primary Ph35-25 02/12/2004 1425 --- --- < 0.04

Duplicate Ph35-25 02/12/2004 1426 --- --- < 0.04

 
24 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

 

In addition to the minor data-quality problems described 

for HgTgw, a problem also was encountered in the quantification recovery of HgMegw-f0.4 in selected samples. Detector 

quenching occurred during the analysis of survey production-well samples with the standard CVAF method (DeWild 

and others, 2002). Use of different detection instrumentation 

(ICPMS) eliminated this problem, but revealed an underlying sample-matrix problem. No HgMe could be recovered 

from either background or spiked survey samples (chiefly 

wells Pi31-01 and Ph35-25) that had elevated concentrations 

of HgTgw (greater than 1,000 ng/L).

Several VOCs (polychlorinated alkenes, tetra- and trichloroethylene, and several possible degradation products) 

found in samples from Bayside and Lakeside production 

wells were evaluated as potential sources of the matrix interference. A filtered sample for HgMegw-f0.4 was obtained 

from each monitoring well at the Bayside and Lakeside sites 

during the vertical-distribution study, and split into three 

subsamples. One subsample was used to obtain background 

HgMegw-f0.4 concentrations, a second subsample was spiked 

with HgMe, and a third subsample was spiked with HgMe 

and selected VOCs.

Analytical results indicated that the selected VOCs did 

not inhibit spiked HgMe recovery with the ICPMS methods 

in ground-water samples that contained up to 366 ng/L of 

HgTgw (table 7). Recoveries also were near 100 percent for 

samples spiked with only HgMe. A similar study to assess 

the recovery of HgMe from samples that contained HgTgw in 

excess of 366 ng/L was not performed because of funding 

limitations.

Implications for the interpretation of the methyl mercury 

data obtained for this study were as follows:

(a) All HgMegw-f0.4 concentration data were provided 

(Appendix A, tables A6 and A14), but selected data 

were qualified;

(b) Acceptable recoveries of HgMe from spiked 

HgMegw-f0.4 samples with low to moderate HgTgw

concentrations (366 ng/L or less) were deemed 

applicable to all ground-water samples with similar 

concentrations of HgTgw (Appendix A, tables A6 

and A14, methyl mercury data), and the HgMegwf0.4 concentration data were considered suitable for 

interpretation; and

(c) For the source-water-quality survey, and groundwater samples with HgTgw concentrations that 

exceeded 366 ng/L, the HgMegw-f0.4 data, which 

reflected concentrations that all were less than 

Table 7. Methyl mercury concentrations and recoveries from background and spiked filtered ground-water samples from Pot Nets Bayside (Pi31-01) 

and Lakeside (Ph35-25), Sussex County, Delaware, 2003–04

[DGS, Delaware Geological Survey; μm, micrometer absolute pore size filter; ng/L, nanograms per liter; spiking procedures and calculations are described in 

DeWild and others (2002); recovery is the ratio of the difference between measured concentrations in the spiked and unspiked samples divided by the 

theoretical concentration of the spiked sample, expressed in percent (%); volatile organic compounds in the spike solution (and theoretical spiked sample 

concentration in micrograms per liter) are as follows: Tetrachloroethylene (0.042), Trichloroethylene (0.42), 1,1,1-Trichloroethane (0.042), 

1,1-Dichloroethane (0.042), Diethyl ether (0.016), Trichloromethane (0.8), and Benzene (0.008)]

DGS

well 

identifier

Mercury,

total,

0.4-μm filtered

water

(ng/L)

Mercury,

methyl,

0.4-μm filtered

water

(ng/L)

Mercury,

methyl,

0.4-μm filtered,

methyl mercury

spiked water

(ng/L)

Spiked 

sample

recovery

 (%)

Mercury,

methyl,

0.4-μm filtered,

methyl mercury

and VOCspiked water

(ng/L)

Spiked 

sample

recovery

(%)

Bayside

Pi31-19 366 0.017 0.937 101 1.104 108

Pi31-20 2.46 0.004 1.043 106 1.082 106

Pi 31-21 234 0.062 0.759 85 1.065 98

Lakeside

Ph35-30 1.72 0.003 0.887 94 1.031 104

Ph35-31 1.07 0.007 0.866 90 0.957 95

Ph35-34 1.52 0.015 0.997 106 1.106 100
Data Collection and Data Quality 25

0.04 ng/L, were considered uncertain and qualified 

as such (Appendix A, table A6, footnote b). For the 

vertical-distribution study, and ground-water 

samples with HgTgw concentrations that exceeded 

366 ng/L, no data were obtained for either 

background concentrations or recoveries of HgMe.

In relation to (c) above, there could have been no measurable HgMe in either simple forms, such as methyl mercuric chloride or hydroxide (CH3HgCl or CH3HgOH), or 

weakly to moderately bound HgMe in complex forms, in 

samples with concentrations of HgTgw concentrations in 

excess of 366 n/L. It also could be that HgMe was present, 

but was not measurable. For example, Qian and others 

(2002) found that virtually all HgMe added in simple neutral 

forms to water that contained elevated concentrations of natural DOC appeared tightly bound to humic-carbon reduced 

sulfur groups. Little if any measurable HgMe was found in 

solution. Samples that contained elevated concentrations of 

HgTgw in this study also possibly contained similar humic 

groups (see Mercury in the Surficial Aquifer, Pot Nets Bayside and Lakeside, Mercury in Ground Water, Forms of 

Mercury). Thus, if HgMe in ground-water samples from the 

Bayside and Lakeside sites with high concentrations of 

HgTgw also was tightly bound, and the simple forms of 

HgMe added during spiking underwent similar complexation, then little to no HgMegw-f0.4 possibly was recoverable 

in these samples. Another plausible explanation was that 

simple-neutral or weakly complexed forms of HgMe could 

have been present, but were immeasurable because of a 

chemical interference that simply prevented the ethylation of 

HgMe species—a required step in analysis (see Field Data 

Collection and Sample Collection, Ground-Water Data and 

Water Samples). Thus, studies of HgMe recovery were 

deemed necessary to determine if HgMe was present and 

adequately recoverable from all samples with measurable 

HgTgw concentrations.

Major Ions, Alkalinity, Acid-Neutralization Capacity, 

and Bicarbonate

The USGS NWQL used routine quality-assurance-andcontrol (QAC) samples and procedures for all non-Hg constituents (including major ions and nutrients, TEs, and 

VOCs), and found no analytical problems with individual 

data. In addition, project field blanks contained no measurable constituents except for trace amounts of several major 

ions—for example, calcium, sodium, and chloride concentrations at less than tenths of a milligram per liter. Additional 

QAC procedures used by the USGS NWQL and project, 

however, identified analytical anomalies for selected data 

that warranted discussion.

Given the principle of electroneutrality, the theoretical 

difference between the sum of the cationic and anionic constituent concentrations, expressed in milliequivalents per 

liter (meq/L), in a water sample is zero. Random errors in 

analytical measurements, however, can result in a computed 

balance that is near, but seldom, zero. In relation to the 

above, cation and anion concentrations and sums (in meq/L), 

were calculated for each ground-water sample (Appendix C, 

tables C1 through C3). The relative difference between 

cation and anion sums, expressed in percent, and hereafter 

referred to as the balance error, also was calculated for each 

sample, and compared to the theoretical value (zero), as well 

as with the balance errors of other samples, to help identify 

measurement biases or analytical omissions (the inadvertent 

exclusion of a major ionic constituent).

The balance error was small (about 10 percent or less) 

and positive for most individual samples, and averaged less 

than 5 percent for all samples for the peninsula source-waterquality survey and local vertical-distribution studies (Appendix C, Table C4). The magnitude of these balance errors was 

attributed to small biases in measurement of one or more 

major ions. Only balance errors for samples from two production wells (Ph34-23 and Pi31-01), and one monitoring 

well (Pi31-13) warrant further discussion.

For production well Ph34-23, the balance error was 

–32.5 percent, and contrary in sign, and notably greater in 

magnitude, than the balance errors for other peninsula survey production-well samples (Appendix C, table C4). The 

negative difference indicated that measured anionic concentrations exceeded measured cationic concentrations. Other 

than an unusually high nitrate-plus-nitrite nitrogen concentration (Appendix A, table A5, 12.3 mg/L), this sample 

exhibited no distinguishing chemical characteristics. The 

USGS NWQL confirmed that the concentration value was 

valid, but that the analysis required sample dilution, which 

likely increased the imprecision, and possibly contributed to 

the bias, in measurement. No further action was taken to 

confirm this result, as resampling was not an option. The 

uncertainty and possible bias in the measurement of nitrateplus-nitrite-nitrogen was documented (Appendix A, table 

A5, footnote a).

As part of the preliminary design study, three time-series 

samples were collected from production well Pi31-01 after at 

least 2 hours of pumping, and all three samples had high balance errors (Appendix C, table C4, Bayside time-series sampling, Pi31-01, 11–14 percent). These errors were two to 

three times greater than the balance error for the sample collected immediately after pumping began at this well (4.7 percent). Also, the three depth-series samples collected from 

monitoring well Pi31-13 after 24–26 hours of pumping at 

well Pi31-01 had high balance errors (28–32 percent). These 

balance errors were two to three times greater than the balance errors described above for the three time-series samples 

obtained at the nearby production well (Pi31-01) before and 

after sampling at well Pi31-13.

The potential source of the relatively large positive balance errors for selected samples from wells Pi31-01 and 

Pi31-13 was investigated but remains unknown. A review of 

USGS NWQL analytical control data taken during the analysis of the Pi31-01 and Pi31-13 samples indicated no major 

bias in measurement for individual major ions. Thus, majorion data obtained from the USGS NWQL were deemed suit- 
26 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

able for interpretation.

Given bicarbonate concentrations were derived from 

field ALK and ANC data (see Field Data and Sample Collection), and were relevant in the calculated equivalent balances (Appendix C), data for this ion, as well as ALK and 

ANC, were reviewed by the project staff.8 For the peninsula 

source-water-quality survey, and the local preliminary timeand depth-series study, the ALK and bicarbonate data were 

obtained from titrations of 0.40-μm capsule-filtered samples. 

For the local vertical-distribution study, ANC and bicarbonate data were derived from titrations of raw water. Regardless of whether samples were filtered or unfiltered, however, 

it appeared unlikely that titration results, including estimated 

bicarbonate concentrations, were influenced by sample type. 

There was no evidence of particulate matter in ground-water 

samples. Turbidity of raw water (well discharge) also was 

low (0.7 NTU or less) at the time of sample collection for 

most (41of 42) samples (Appendix A, tables A3, A9, A10, 

and A11), including the samples with large balance errors. 

There was no measurable particulate (in)organic carbon in 

any sample (Appendix A, tables A5, A9, A10, and A13). 

Thus, field titrations of raw or filtered samples were considered equivalent means to estimate bicarbonate.

Bicarbonate data were generated from the Gran titration 

method in accordance with USGS NFM protocols (Rounds 

and Wilde, 2001). An initial review of bicarbonate data 

revealed titration end-point anomalies—the theoretically calculated and actual bicarbonate endpoints differed for some 

ground-water samples. The problem initially was assumed to 

reflect difficulties in the titration of weakly buffered samples 

given the low ALK (ANC) values of 26 mg/L or less as calcium carbonate (Appendix A, tables A3, A9, A10, and A11). 

Subsequent analysis, however, revealed that DOC was positively correlated with ANC (r2 = 0.90), and that the Bayside 

samples with elevated HgTgw-f0.4 also likely contained elevated amounts of DOC (fig. 6). The DOC concentration seldom exceeded 0.4 mg/L in ground water, except for samples 

from the Pot Nets Bayside and Lakeside community wells 

that generally contained elevated concentrations of HgTgwf0.4, DOC, ANC, and bicarbonate (fig. 6 and Appendix A). 

The positive correlation between DOC and ANC indicated that organic ligands could presumably chemically 

buffer changes in sample pH during titrations. This would be 

the first reported instance of DOC compounds at elevated 

levels acting as buffering agents in shallow residential area 8. No titration yielded measurable carbonate (estimated reporting level is 

1 milligram per liter). 
Data Collection and Data Quality 27

ground water on the Delmarva Peninsula. Hem (1985) notes 

that DOC can contain organic compounds that are important 

contributors to the buffering capacity. Kharaka and others 

(1986) describe acetate, succinate, and oxalate compounds in 

DOC as possible contributors.

Assuming organic ligands were present and acted as conjugate bases to buffer ground water, these compounds could 

account in part for the net positive bias in balance errors 

noted above for selected samples from wells Pi31-01 and 

Pi31-13. If a DOC of 1 mg/L reflected acetate with a single 

negative charge (Kharaka and others, 1986), this would 

equate to 0.083 meq/L, which could be considered a relevant 

contribution to the anionic balance (Appendix C). Samples 

for DOC, however, were not collected during the preliminary 

study. Thus, it was not possible to estimate the possible DOC 

contribution, nor the concomitant bicarbonate reduction to 

the anionic balance, for samples with large anionic deficits.

Given the results of the review of the bicarbonate, ALK, 

and ANC titrations, the following data qualifications were 

adopted for this report:

(a) Bicarbonate concentrations derived from titrations 

with end-point anomalies were qualified as 

estimated values (Appendix A, tables A3, A9, A10 

and A11—Bicarbonate, E-prefix code), and

(b) Titration end-point anomalies for wells with 

elevated DOC concentrations (exceeding 0.4 mg/L) 

and HgTgw concentrations (exceeding 100 mg/L) 

were assumed to reflect DOC contributing to the 

buffering capacity of these samples. The ALK or 

ANC and bicarbonate data for these wells were 

qualified to indicate that the DOC could be a 

contributing factor (Appendix A, tables A6, A9, 

A10, and A11—ALK or ANC, and Bicarbonate, 

footnoted), which for bicarbonate, implied that 

computed concentration values likely were 

positively biased.

Chlorofluorocarbon Apparent Recharge Dates and 

Ages of Ground Water

Sampling at the Bayside (Pi31-01) and Lakeside 

(Ph35-25) production wells and vertically nested monitoring 

wells included DGs and CFCs (Appendix A, tables 

A16 and A17). These data were used to determine the 

apparent recharge dates (ages) of ground water (http://

water.usgs.gov/lab/chlorofluorocarbons/background/,—

accessed December 21, 2005). The quality of recharge date 

(age) data was judged by (a) consistency between duplicatesample dates (ages) for a given CFC compound (CFC-11, 

CFC-12, or CFC-113), and (b) consistency among dates 

(ages) obtained among different CFC compounds. To avoid 

rounding error bias, raw data were used to evaluate consistency (table 8).

Most recharge dates (ages) met both of the above consistency criteria. Duplicate-sample recharge dates for each well 

and CFC compound were similar or differed by at most a 

few years (table 8), and were within the expected range in 

variability attributable to analytical methods for duplicate 

samples (Eurybiades Busenberg, Chief Supervisor, USGS 

CFC Laboratory, written commun., 2004). There also was 

close agreement among recharge dates derived for the same 

well but with different CFC compounds, with the proviso 

that CFC-12 dates occurred slightly later in time than either 

CFC-11 or CFC-113 dates. Samples with reliable dates were 

assumed valid for interpretive use (table 8).

An apparent date of recharge (age) was considered unreliable if the sample CFC concentration either exceeded the 

estimated historical atmospheric concentrations for that 

compound, or appeared markedly younger than the apparent 

dates (ages) from the other CFC compounds. Several wells 

were identified for which the recharge dates (and ages) 

appeared unreliable (Table 8, data coded as “ *** ”). The 

CFC-12 dates for wells Ph35-25, Ph35-32, and Ph35-34, for 

example, likely reflected inclusion of a local CFC-12 source, 

which has been noted in ground water affected by residential-commercial development (Szabo and others, 1996; Ho 

and others, 1998; Plummer and others, 1998). Unreliable 

estimates of apparent recharge dates (ages) also can occur if 

ground waters recharged at different times were mixed 

because of ground-water pumping—a situation which is 

more likely to arise if long, rather than short, screened wells 

are used for sampling (Eurybiades Busenberg, Chief Supervisor, USGS CFC Laboratory, written commun., 2004).

Apparent dates of recharge for the Bayside productionwell discharge were deemed unreliable (table 8, Pi31-01). 

Dates for all three CFC compounds for this production well 

were either modern or 1–2 decades younger than the apparent dates (ages) determined from nearby monitoring-well 

samples obtained from samples at depths (screened intervals) that bracket the depth (screened interval) of this production well. The manner of CFC contamination was 

unknown, but results could reflect samples that were 

exposed to air (in soil or from current atmosphere). Although 

similar sampling methods and equipment were used at all 

wells, the potential for contamination by air was greater for 

production than monitoring well samples. At each production well, the discharge passed from the well through about 

100 ft of buried multi-joint steel pipe to a well-house brass 

spigot, and then through three sample-tubing connections 

(Appendix D). For each monitoring well, the discharge 

passed through only one gas-compression fitting—that 

which connects the submersible pump outlet and CFC-DG 

sampling tubing.

For the purposes of this study, only reliable raw recharge 

(age) data were reported (table 8, and Appendix A, table 

A17). For the interpretation of these data, however, raw 

duplicate-sample values of the apparent recharge date (age) 

for a given CFC compound and well (table 8) were averaged; 

the average date (age) rounded to the nearest whole number 

was used as the estimated date of apparent recharge (age). 

This approach allowed the variability attributable to variations in field and laboratory methods to be incorporated into 

data interpretation. For a given well and CFC compound, 

the relative difference in percent between whole-number 
28 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

 

Table 8. Qualified raw data for apparent dates (ages) of recharge for ground-water samples from production and vertically nested monitoring wells 

at Pot Nets Bayside (Pi31-01) and Lakeside (Ph35-25) sites, Sussex County, Delaware, 2004

[DGS, Delaware Geological Survey; raw data, values in tenths may or may not be significant; CFC, chlorofluorocarbon; ***, apparent date (age) estimate 

cannot be reliably determined because of sample exposure to atmospheric conditions at time of sampling, ground-water contamination by non-atmospheric 

local anthropogenic source, or the mixing of waters with different CFC and (or) dissolved gas concentrations]

DGS

well

identifier

Apparent recharge, date Apparent age, years Well 

type and

depth

(feet below

land

surface)

CFC-11 CFC-12 CFC-113 CFC-11 CFC-12 CFC-113

Bayside

Pi31-01 *** *** *** *** *** *** Production well (66.0)

*** *** *** *** *** ***

Pi31-16 1974.0 1977.0 1977.0 30.7 27.7 27.7 Monitoring wells (79.5)

1974.0 1977.0 1977.0 30.7 27.7 27.7

Pi31-17 1978.0 1983.0 1984.0 26.7 21.7 20.7

 1979.5 1983.5 1984.5 25.2 21.2 20.2 (65.0)

Pi31-18 1971.5 1977.5 1979.0 33.2 27.2 25.7

 1971.0 1977.0 1978.5 33.7 27.7 26.2 (62.0)

Pi31-19 1988.5 1990.5 *** 16.2 14.2 ***

 1986.5 1990.5 *** 18.2 14.2 *** (55.0)

Pi31-20 1984.0 1990.0 *** 20.7 14.7 ***

 1984.0 1991.0 *** 20.7 13.7 *** (48.0)

Pi31-21 1985.5 *** 1989.5 19.2 *** 15.2

1985.5 *** 1989.0 19.2 *** 15.7 (26.0)

Lakeside

Ph35-25 1971.5 *** 1975.0 33.2 *** 29.7 Production well (73.0)

1971.5 *** 1974.0 33.2 *** 30.7

Ph35-30 1968.5 1973.5 1964.5 36.2 31.2 40.2 Monitoring wells (78.5)

1968.5 1973.0 1965.5 36.2 31.7 39.2

Ph35-31 1973.0 1979.0 1976.0 31.7 25.7 28.7

1973.0 1977.5 1973.0 31.7 27.2 31.7 (68.0)

Ph35-32 1976.5 *** 1979.0 28.2 *** 25.7

1977.0 *** 1979.0 27.7 *** 25.7 (60.0)

Ph35-33 1978.5 1987.5 1985.5 26.2 17.2 19.2

1980.0 1987.0 1985.5 24.7 17.7 19.2 (46.5)

Ph35-34 1985.5 *** 1987.5 19.2 *** 17.2

1985.0 *** 1987.5 19.7 *** 17.2 (24.5)
Mercury in the Surficial Aquifer 29

 

apparent recharge dates (ages) was computed from the reliable duplicate sample data (table 8) for each well and CFC 

compound (see table above). This variability, which was presumed to reflect the operational variability inherent in field 

and laboratory methods, was relatively small in magnitude 

(maximum 0.15 percent), random in sign, and similar among 

CFC compounds. For comparative purposes, a difference in 

the average apparent recharge dates of 3 years, for example, 

for 1970 and 1973, between two different ground-water samples also yielded a relative difference equal to 0.15 percent. 

Hence, average apparent recharge dates (ages) among different samples computed from the same CFC compound were 

considered to reflect different apparent recharge dates if they 

differed by at least 3 years.

Mercury in the Surficial Aquifer

The conditions and characteristics of ground-water quality in the surficial aquifer mainly were determined from samples collected from production wells as part of the sourcewater-quality survey for the Long Neck Peninsula, and from 

vertically nested monitoring wells near each contaminated 

production well as part of the local vertical-distribution 

study in the Bayside and Lakeside communities. Results for 

each of these studies are described below.

Long Neck Peninsula

Concentrations of HgTgw-raw ranged from 0.11 to 

1,820 ng/L for the 22 Peninsula survey production wells 

(Appendix A, table A6). The wide range in concentrations 

indicated differences occurred in the geographical distribution of Hg in the used portion of the surficial aquifer at 

depths of 30 to 120 ft below land surface. Throughout most 

of the peninsula, the concentrations for HgTgw-raw appeared 

low, 10 ng/L or less (fig. 7), and similar to background concentrations described for production wells in similar coastal 

aquifer settings (KCA) in New Jersey (Barringer and others, 

2005; Murphy and others, 1994) and ground water elsewhere 

(Krabbenhoft and Babiarz, 1992). Slightly elevated concentrations of HgTgw-raw occurred in two production wells that 

were located in relatively diverse locations (fig. 7, Ph24-02, 

13.9 ng/L and Pi32-14, 20.6 ng/L), and reflected low-level 

ground-water contamination or sampling artifacts (see Data 

Quality, Mercury). Truly elevated concentrations of HgTgwraw (greater than 100 ng/L) chiefly occurred in the vicinity of 

the Bayside and Lakeside Communities (fig. 8), and were 

highest at wells Pi31-01 and Ph35-25, which had known histories of elevated Hg (Department of Natural Resources and 

Environmental Control, 2002).

The limited spatial extent of elevated concentrations of 

HgTgw-raw in the surficial aquifer implied that the source 

(origin) of the Hg likely occurred in the vicinity of the Bayside and Lakeside communities. The heterogeneous pattern 

to the spatial occurrence of elevated concentrations of 

HgTgw-raw in peninsula ground water also was similar to 

geographic patterns in the occurrence of Hg that have been 

observed in ground water in the other similar coastal aquifer 

settings (KCA) in New Jersey (Barringer and others, 2005).

Forms of Mercury

Other studies have found that Hg in similar coastal settings and aquifers could exist in particulate (filter) and dissolved (filtrate) forms (Barringer and MacLeod, 2001; 

Barringer and Szabo, 2001). Mercury in shallow ground 

water in the surficial aquifer on the Long Neck Peninsula, 

however, mainly occurred in a dissolved or fine-colloidal, 

Summary statistics for the relative difference in apparent recharge dates (ages) between duplicate ground-water samples collected from verticalnest monitoring wells at the Bayside (Pi31-01) and Lakeside (Ph35-25) sites, Long Neck Peninsula, Sussex County, Delaware

[Relative difference is computed as the difference between recharges dates (ages) divided by their average for duplicate ground-water samples collected from 

a given well and for a given chlorofluorocarbon compound, expressed in percent; Min, minimum; Max, maximum]

Chlorofluorocarbon (CFC) 

compound

Relative difference, in percent

Number of 

duplicate

sample pairs

Range

Average Standard

deviation Min Max

CFC-11 0.00 0.05 -0.08 0.10 11

CFC-12 0.02 0.05 -0.03 0.10 9

CFC-113 0.02 0.06 -0.05 0.15 8
30 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04
Mercury in the Surficial Aquifer 31
32 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

rather than particulate, phase. The absence of a particulatephase Hg was inferred from the similarity in HgTgw-raw and 

HgTgw-f0.4 or -f0.1 concentrations for duplicate samples of 

ground water from each peninsula survey production well 

that were either unfiltered (raw) or filtered before analysis 

(fig. 9). The relative differences in HgTgw concentrations 

between raw and filtered duplicate samples with elevated 

HgTgw (in excess of 100 ng/L) also appeared random in sign, 

small in magnitude (averaged 5.9 percent), and fell within 

the range of the relative difference in HgTgw attributable to 

variations in Hg measurement associated with field and laboratory methods (see Data Quality, Mercury).

Absence of particulate Hg was further supported by turbidity and particulate-carbon data. Except for one production-well sample (Ph24-02), turbidity was low (less than 

1 NTU), and, if particulates were present, their magnitude 

bore no relation to HgTgw-raw (fig. 10). There also was no 

measurable particulate carbon (Appendix A, table A5), a 

potential means of Hg transport. 

Despite the elevated concentrations of HgTgw found in 

selected production wells (fig. 8), no HgTgw concentration

in any production well sampled during this study exceeded 

the Federal standard for public drinking-water supplies 

(2,000 ng/L). The presence of Hg as HgMe, however, was 

investigated because the latter was known to readily bioaccumulate (Fitzgerald and Lamborg, 2005). No HgMegwf0.4 was detected (USGS MRL detection level 0.04 ng/L) in 

ground-water samples from the 14 peninsula production 

wells selected for this analysis (table 3, and Appendix A, 

table A6). This result, however, only was validated for those 

production wells that contained less than 366 ng/L of HgTgw. 

For production wells with HgTgw in excess of 366 ng/L, the 

results (sample concentrations reported as less than 

0.04 ng/L) for HgMegw-f0.4 could not be validated (see Data 

Quality, Mercury).

Relations Between Mercury and Other Chemical 

Constituents

Bulk chemical properties (Appendix A, table A3) indicated ground water in the used resource portion of the surficial aquifer on the Long Neck Peninsula was moderately 

acidic (pH ranged from 5.1–5.9), weakly buffered (ALK was 

less than 18 mg/L), suboxic-to-oxic (DO was between 2.9–

8.5 mg/L), and was low to moderate in dissolved solids 

(SC ranged from 69–262 microsiemens per centimeter at 

25 degrees Celsius, or μS/cm at 25°C). Major-ion data 

revealed that sodium and chloride generally are the dominant 

ions (Appendix C, table C1). Except for barium, TEs, if measurable, generally occurred at low concentrations of about 

10 μg/L or less) (Appendix A, table A6). There was no measurable nitrite or particulate nitrogen; nitrate was the major 

form of nitrogen, and NO3

-

-N concentrations ranged from 

0.7–12.3 mg/L (Appendix A, table A5). There also was no 

measurable particulate carbon (inorganic or organic); carbon 

chiefly occurred as DOC (0.45-μm capsule filter) and, 

except for well Pi31-01 (DOC, 1.2 mg/L), at low concentrations—0.4 mg/L or less. One or more VOCs occurred in all 

22 survey production wells (Appendix A, table A7); the 

most frequently detected VOCs were chloroform, methyl 

tert-butyl ether, and 1,1,1-trichloroethane (table 9).

Federal drinking-water standards, although promulgated 

for public-supply production wells, also were used as guidelines in this study to indicate concentrations of constituents 

that could pose a health risk to privately owned and consumed supplies. Whereas no standard was exceeded for any 

sample collected from peninsula production wells used for 

public supplies, two standards were exceeded in samples 

obtained from two private production wells. For well Ph34-

23, the concentration of NO3

-

-N was 12.3 mg/L, which 

exceeded the Federal standard of 10.0 mg/L. This NO3

-

-N 

concentration, however, reflected a higher-than-normal 

degree of imprecision (see Data Quality, Major Ions...). For 

well Pi32-14, the concentration of tetrachloroethylene was 

7.29 μg/L, and exceeded the Federal standard of 5.0 μg/L. 

This result was verified by re-sampling and independent 

analyses of samples collected by the USGS and DNREC.

There was ample evidence among production-well survey data that ground-water quality in the shallow aquifer on 

the Long Neck Peninsula appeared affected by human activities. Denver (1989) noted that ground water affected by agriculture and (or) septic-system drainage in eastern Sussex 

County had elevated dissolved solids—SC in excess of 

100 μS/cm. Seventy-two percent (16 of 22) of the peninsula 

survey samples had SC values that exceeded 100 μS/cm 

(table 10). Hamilton and others (1993) concluded that concentrations of nitrate-plus-nitrite nitrogen above 0.4 mg/L 

reflected contamination from septic systems and (or) agricultural or residential applications of fertilizer or manure. All 

peninsula survey samples had NO3

-

-N concentrations that 

exceeded 0.4 mg/L (table 10). Ferrari (2002) described 

VOCs in the surficial aquifer in Delaware associated with 

gasoline or other petroleum fossil fuels, by-products from 

chlorination of drinking water, agricultural applications (as 

pesticides or solvents), industrial uses (dry cleaning, solvents, and degreasers), or chemical manufacturing. He found 

all 30 production-well samples had at least 1 detectable 

VOC, and 93 percent (28 of 30) of the samples had at least 

2 detectable VOCs. For the peninsula survey, all 22 production-well samples had at least 1 detectable VOC, and 59 percent (13 of 22) of the samples had at least 2 VOCs (table 10). 

He also noted that chloroform (28 of 30), tetrachloroethylene 

(20 of 30), methyl tert-butyl ether (17 of 30), and 1,1,1-

trichloroethane (10 of 30) were among the most commonly 

detected VOCs in shallow ground water in Delaware. These 

VOCs also were the most commonly detected VOCs in peninsula ground water (table 9).

Elevated concentrations of HgTgw occurred in ground 

water that also appeared affected by human activities on the 

basis of one or more indicators (table 10). This result was 

consistent with the source of Hg being of anthropogenic origin. These indicators also illustrated, however, that ground 

water that contained background concentrations of HgTgw

also was similarly affected by human activities. Thus, 
Mercury in the Surficial Aquifer 33
34 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

 

ground water affected by human activity appeared a necessary but insufficient condition to uniquely identify areas with 

elevated HgTgw in the shallow aquifer on the peninsula. 

Other studies in similar coastal settings also have found Hg 

occurrence can be related to but not uniquely defined by 

anthropogenic indicators of ground-water contamination 

(Barringer and others, 2005).

Although the occurrence of elevated HgTgw can be associated with ground water affected by human activities, 

bivariate plots and correlation analyses revealed none of the 

aforementioned indicators correlated with concentrations of 

HgTgw in ground water. Neither did any other bulk property 

or chemical constituent measured in production-well samples. There also was no discernible difference in the majorion chemistry among ground waters with and without elevated HgTgw (fig. 11).

The only observed relations between elevated HgTgw

and other ground-water constituents of note were as follows:

(a) The highest concentrations of HgTgw-f0.4

(1,790 ng/L) and DOC (1.2 mg/L) occurred at the 

Bayside production well Pi31-01 (Appendix A, 

tables A5 and A6). This DOC concentration was 

three to four times greater than the DOC 

concentration in other wells (Appendix A, table 

A5).

(b) Except for Ph3525 and Pi31-02, Pot Nets wells 

with the highest concentrations of HgTgw

(Appendix A, table A6), also contained 

tetrachloroethylene and (or) likely degradation 

products—such as trichloroethylene and 1,1,1-

trichloroethane (Appendix A, table A7).

The lack of relations between concentrations of HgTgw

and other chemical parameters or constituents in peninsula 

shallow ground water has been observed in other Hg studies 

conducted in similar coastal aquifers and settings. Although 

Barringer and others (2005) found HgTgw (filtered) concentrations that ranged from 0.1 to 100 ng/L were positively 

correlated with concentrations of calcium, chloride, sodium, 

nitrate, and DOC, no significant correlations were found at 

elevated HgTgw concentrations (greater than 100 ng/L).

Results from this study and that by Barringer and others 

(2005) implied that the occurrence of elevated HgTgw was 

only marginally related to widespread patterns in land use 

related to human activity. In both studies, the latter had a 

widespread effect on the quality of ground water, including 

areas where the ground water also contained elevated 

HgTgw. Thus, the occurrence of elevated concentrations of 

HgTgw could have reflected selected human activities that 

occurred only on a limited spatial scale—for example, the 

inadvertent loss or the inappropriate disposal of Hg in 

selected locations—that were embedded within a large spatial backdrop of agricultural and (or) residential land use. 

Both coastal studies illustrated that the occurrence of elevated concentrations of HgTgw in ground water was geographically heterogeneous—a pattern consistent with small 

but widely dispersed and relatively thin contaminant plumes. 

Therefore, the use of production wells could have been of 

limited value in the identification of factors that relate to the 

occurrence of elevated HgTgw. By design, most production 

wells have long screened intervals and relatively high pumping rates (see Selection of Long Neck Peninsula Production 

Wells and Appendix D). These design characteristics are best 

suited for supplying water, but are not necessarily well suited 

for the isolation and identification of chemical characterisTable 9. Frequently detected volatile organic compounds in ground-water samples from production wells on the Long Neck Peninsula, 

Sussex County, Delaware, 2003

[Relative number of detections is ratio of the number of wells for which compound was detected to the total number of wells from which samples were 

collected (22); relative number of detections also is expressed in percent; μg/L, micrograms per liter; E, laboratory estimated value]

Compound Relative number

of detections

Relative number

of detections

(%)

Maximum

concentration

(μg/L)

Chloroform 21/22 95 8.16

Methyl tert-butyl ether 7/22 32 5.5

1,1,1-Trichloroethane 6/22 27 E 0.8

Tetrachloroethylene 4/22 18 7.29

Trichloroethylene 2/22 9 0.40

1,1-Dichloroethane 2/22 9 E 0.6

1,4-Dichlorobenzene 2/22 9 0.12

Bromodichloromethane 2/22 9 7.17

Dibromochloromethane 2/22 9 4.9

Methyl tert pentyl ether 2/22 9 0.65
Mercury in the Surficial Aquifer 35
36 Occurrence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware, 2003–04

 

Table 10. Production wells that exceeded water-quality criteria—for elevated specific conductance, nitrate-nitrogen concentrations, and 

detectable volatile organic compounds—and total mercury concentrations, Long Neck Peninsula, Sussex County, Delaware, 2003

[DGS, Delaware Geological Survey; >, greater than; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; ng/L, 

nanograms per liter; VOCs, volatile organic compounds; YES, exceeds stated threshold value; NO, at or below stated threshold value; shaded data, total 

mercury exceeded 100 ng/L; ---, total mercury less than 100 ng/L; affected well ratio, number of wells that had indicator that exceeded the stated threshold, or 

had more than one detectable VOC, divided by the total number of wells]

DGS

well

identifier

Elevated specific 

conductance

(>100 μS/cm

at 25 oC)

Elevated

nitrate-nitrogen 

concentration

(>0.4 mg/L)

Number of

 detectable

 VOCs

Number of 

criteria 

exceeded

Elevated

total mercury

concentration

(>100 ng/L)

 Bayside Wells 

Pi31-01 YES YES 4 3 1,820

Pi31-11 NO YES 3 2 276

Pi31-02 YES YES 1 2 230

Pi31-05 YES YES 3 3 ---

Pi32-15 YES YES 2 3 ---

Pi41-02 NO YES 1 1 ---

Pi41-03 NO YES 2 2 ---

Pi31-12 YES YES 2 3 ---

Lakeside Wells

Ph35-25 NO YES 1 1 524

Ph35-24 YES YES 1 2 104

Other Wells

Ph24-01 NO YES 1 1 ---

Ph24-02 YES YES 4 3 ---

Ph25-17 YES YES 2 3 ---

Ph34-18 YES YES 1 2 ---

Ph34-22 NO YES 2 2 ---

Ph34-23 YES YES 1 2 ---

Ph34-24 YES YES 5 3 ---

Ph35-28 YES YES 3 3 ---

Ph44-07 YES YES 1 2 ---

Pi32-14 YES YES 4 3 ---

Pi34-09 YES YES 1 2 ---

Pi43-03 YES YES 4 3 ---

Affected 

well ratio 16/22 22/22 13/22 5/22

 
Mercury in the Surficial Aquifer 37

tics or conditions associated with contaminant plumes of 

limited areal and vertical extent. In addition, the mixing of 

ground waters whose chemistry differs with depth in long 

screened intervals during the pumping of these production 

wells to obtain samples also could have masked chemical 

constituents or conditions uniquely associated with a contaminant plume that contained elevated HgTgw. To address 

the above concerns, a vertical distribution study of Hg in the 

shallow aquifer was conducted with vertically nested shortscreened monitoring wells in the vicinity of each of the Bayside (Pi31-01) and Lakeside (Ph35-25) production wells 

with the highest concentrations of HgTgw.

Pot Nets Bayside and Lakeside

The vertical distribution of HgTgw was characterized at 

selected depths in the shallow surficial aquifer at the Bayside 

(Pi31-01) and Lakeside (Ph35-25) production-well sites. 

Chemical characteristics and geochemical settings that 

potentially related to the occurrence, form, transport, and 

source of Hg also were determined for local aquifer sediment 

and ground water at selected depths. The characterization 

and assessment were conducted on the basis of analyses of 

soil, fill, and aquifer sediment data obtained during the 

installation in 2004 of the vertical nests of short-screened 

monitoring wells, and analyses of ground-water data 

obtained in the field and from samples collected from the 

contaminated production wells and the newly installed shortscreened monitoring wells at the Pot Nets Bayside and Lakeside sites.

Mercury in Sediment

Lithologic sediment descriptions (Appendix B, figs. B1 

and B2) were combined with the concentrations of HgT for 

selected sediment samples (HgTsed, Appendix B, table B11) 

to illustrate the vertical (depth) profile of HgTsed in the surficial sediment at each site (fig. 12). This illustration shows 

that slightly elevated concentrations of HgTsed (40 micrograms per kilogram, μg/kg, or less) occurred only in surficial 

soil and fill. No measurable HgTsed (reporting level 

20 μg/kg) was found in aquifer sediment. 

Relations Between Mercury and Other Sediment Constituents

Samples with measurable HgTsed at the Bayside and 

Lakeside sites also were analyzed for other chemical constituents often associated with elevated Hg, including organic 

carbon, sulfur, and selected TEs.

Carbon: Samples of soil and fill that contained measurable 

HgTsed also had measurable organic carbon (fig. 13). 

Organic carbon (OC) was the predominant form of carbon 

(Appendix B, table B11) and, in soil and fill, ranged from 

0.18 to 1.79 percent by weight. It also was visible in surficial 

soil and fill, but not in the underlying sediment samples 

(Appendix B, tables B1 through B10). The co-occurrence of 

HgTsed and OC in soil and fill indicated that Hg possibly was 

bound to OC (Barringer and others, 1997).

Sulfur and Trace Elements: Elevated sulfur in conjunction 

with Hg in sediment is a potential indicator of solid mercuric 

sulfide (Lowry and others, 2004). No measurable sulfur was 

found in samples (Appendix B, table B11, sulfur, less than 5 

percent by weight); thus, no discernible relation was found 

between HgTsed and sulfur in soil, fill, or sediment, and there 

was no evidence to support the occurrence of solid mercuric 

sulfide (metacinnabar) in sediment.

Several TEs were found at measurable and elevated concentrations in soil, fill, or aquifer sediment from both sites 

(Appendix B, table B11—for example, aluminum, iron, and 

titanium). These TEs, however, appeared ubiquitous, and 

occurred in all or most lithologic materials and neither exclusively, nor at elevated concentrations, solely in soil or fill 

samples that contained elevated HgTsed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT sed.

occurred in all or most lithologic materials and neither exclu

-

sively, nor at elevated concentr ations, solely in soil or fill 

samples that contained elevated HgT