U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2016–1143

Prepared in cooperation with the Delaware Department of Natural Resources and 

Environmental Control (DNREC) Water Supply Section, Groundwater Protection Branch

Occurrence and Distribution of Arsenic and Radon in Water 

from Private Wells in the Rancocas Aquifer, Southern New 

Castle and Northern Kent Counties, Delaware, 2015

39°15'

39°30'

75°45' 75°30'

NEW JERSEY

DELAWARE

MARYLAND

MARYLAND

DELAWARE

DELAWARE

DELAWARE RIVER

C&D CANAL

NEW

CASTLE

KENT

Smyrna

Middletown

Dover

Kenton

Odessa

RAS-43

RAS-34

RAS-38

RAS-37

RAS-13

RAS-39

RAS-33

RAS-7

RAS-41

RAS-40

RAS-14

RAS-16

RAS-18

RAS-17

RAS-22

RAS-11

RAS-47

RAS-8

RAS-51

RAS-49

RAS-10

RAS-12

RAS-15 RAS-56

RAS-19 RAS-20

RAS-21 RAS-24

RAS-26

RAS-29 RAS-1

RAS-31

RAS-53

RAS-3 RAS-5

RAS-6
Cover. Study area for the Rancocas aquifer in Delaware showing sampled wells and well numbers. Refer to figure 2.
Occurrence and Distribution of Arsenic and 

Radon in Water from Private Wells in the 

Rancocas Aquifer, Southern New Castle 

and Northern Kent Counties, Delaware, 

2015

By Judith M. Denver

Open-File Report 2016–1143

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the Delaware Department of Natural Resources 

and Environmental Control (DNREC) Water Supply Section, 

Groundwater Protection Branch
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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

natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://store.usgs.gov

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, product, or firm names 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:

Denver, J.M., 2016, Occurrence and distribution of arsenic and radon in water from private wells in the Rancocas 

aquifer, southern New Castle and northern Kent Counties, Delaware, 2015: U.S. Geological Survey Open-File Report 

2016–1143, 15 p., http://dx.doi.org/10.3133/ofr20161143.

ISSN 0196-1497 (print)

ISSN 2331-1258 (online)

ISBN 978-1-4113-4086-2
iii

Acknowledgments

Thanks are extended to the private well owners who allowed us onto their property to sample their wells and made this project possible. Funding for the U.S. Geological Survey (USGS) 

part of this project is provided by the U.S. Environmental Protection Agency’s Drinking Water 

State Revolving Loan Fund 15 Percent Set Aside for Source Water Protection and was carried 

out through a cooperative agreement between USGS, the Delaware Department of Natural 

Resources and Environmental Control (DNREC), and the Delaware Geological Survey (DGS).

DNREC scientists Joshua Kasper, Amber Batille, and Anita Beckel provided considerable 

support for this project including an extensive database of wells in the study area, and assistance with study design, field reconnaissance of sampling sites, and obtaining permissions 

for groundwater sampling. Deborah Bringman, Michael Brownley, Jessica Teunis, and Mark 

Nardi of the USGS provided office and technical support for network design, well sampling, and 

database activities.

v

Contents

Acknowledgments........................................................................................................................................iii

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

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................2

Methods of Study...........................................................................................................................................3

Occurrence and Distribution of Arsenic and Radon................................................................................6

Arsenic....................................................................................................................................................6

Radon......................................................................................................................................................8

References Cited.................................................................................................................................11

Appendix 1. Groundwater-quality data for private wells sampled in the 

Rancocas aquifer, Delaware, June through August 2015 .......................................................14

Figures

1. The Rancocas aquifer includes sediments of the Manasquan, 

Vincentown, and Hornerstown Formations in Delaware. A, map 

showing area of use of the Rancocas aquifer, and B, cross section 

showing position of geologic units in the Rancocas aquifer ................................................3

2. Maps of study area for the Rancocas aquifer in Delaware showing 

A, selection grid and the population of Rancocas aquifer wells, and 

B, sampled wells with well numbers.........................................................................................4

3. Boxplots showing range in concentrations of A, arsenic, and B, radon 

in water from 36 wells in the Rancocas aquifer, Delaware...................................................6

4. Map showing distribution of arsenic in water from the Rancocas 

aquifer, Delaware .........................................................................................................................7

5. Map showing combined distribution of arsenic concentrations from 

water in the adjacent Aquia aquifer of Maryland and the Rancocas 

aquifer of Delaware......................................................................................................................9

6. Map showing distribution of radon in groundwater from the Rancocas 

aquifer, Delaware .......................................................................................................................10

Tables

1. Well depths and concentrations of arsenic and radon in water from 

private wells in the Rancocas aquifer in Delaware sampled from June 

through August 2015.....................................................................................................................5

2. Kendall’s Tau correlation coefficients for relations between arsenic 

and radon with dissolved constituents or physical characteristics in 

groundwater from the Rancocas aquifer, Delaware ..............................................................8
vi

DGS Delaware Geological Survey

DNREC Delaware Department of Natural Resources and Environmental Control

EPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

Abbreviations

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Conversion Factors

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

Datum

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C = (°F - 32) / 1.8

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 in either milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Activities for radioactive constituents in water are given in picocuries per liter (pCi/L).

Supplemental Information
Occurrence and Distribution of Arsenic and Radon 

in Water from Private Wells in the Rancocas Aquifer, 

Southern New Castle and Northern Kent Counties, 

Delaware, 2015

By Judith M. Denver

Abstract

Water samples were collected and analyzed for arsenic 

and radon from 36 private, mostly domestic wells that tap the 

Rancocas aquifer in southern New Castle and northern Kent 

Counties, Delaware, during the summer of 2015. Both arsenic 

and radon are from natural mineral sources, in particular 

glauconitic and other marine-derived sediments, which are 

important components of the geologic formations comprising 

the Rancocas aquifer. Routine testing of domestic wells is 

not required in Delaware; as a result, many homeowners are 

not aware of potential water-quality problems with these 

chemicals in their well water. Arsenic has previously been 

detected at levels of potential concern for human health in 

this aquifer in adjacent parts of Maryland where it is referred 

to as the Aquia aquifer. Arsenic and radon also have previously been detected in several Rancocas aquifer wells in 

Delaware. The Delaware Department of Natural Resources 

and Environmental Control intends to use the data from this 

project to better identify areas with potential for levels of concern for domestic well owners. This report includes chemical 

results and maps showing the distribution of sampled wells 

and concentrations of arsenic and radon. All data collected for 

this study also are available in the U.S. Geological Survey’s 

National Water Information System database. 

Arsenic was detected above the minimum reporting 

limit of 0.1 micrograms per liter (μg/L) in 34 of the 36 wells 

sampled with concentrations ranging from about 0.11 to 

27 μg/L. In 15 of the samples, arsenic concentrations were at 

or above the U.S. Environmental Protection Agency (EPA) 

Maximum Contaminant Level (MCL) of 10 μg/L for public 

wells. Most of the higher concentrations are clustered along a 

band running from the southwest to northeast in the southern 

part of the study area. 

Radon, which is an inert gas derived from radium, was 

detected in all water samples with concentrations ranging from 

85 to 1,870 picocuries per liter (pCi/L). Currently, the EPA 

has not set a MCL for radon in public water systems. There 

were no samples where radon was detected at a concentration 

exceeding the proposed alternative MCL of 4,000 pCi/L. 

Samples from 16 of 36 wells were above the lower proposed 

MCL of 300 pCi/L. Most of these samples were from wells 

greater than 200 feet deep located in a similar part of the 

aquifer as the higher concentrations of arsenic along an eastnortheasterly line in the southern part of the study area.

Introduction

Groundwater is the sole source of potable water supply in 

the rapidly growing area of southern New Castle and northern 

Kent Counties, Delaware, where a large part of the population relies on private wells (Maupin and others, 2014). The 

Rancocas aquifer is an important source of water supply in 

this area and the quality of this resource is of growing concern. 

The availability of water also is of growing concern in this 

area, which is estimated to triple in population between 2000 

and 2030 (He and Andres, 2011). Arsenic and radon are natural contaminants from geologic sources that are commonly 

present in groundwater. One or both of these chemicals have 

been detected at levels of concern in groundwater in previous 

analyses of water from the Rancocas aquifer in Delaware 

(Bachman and Ferrari, 1995; McLaughlin and Velez, 2006; 

A.S. Andres, Delaware Geological Survey, written commun., 

2014) as well as through an extensive sampling program 

for arsenic in adjacent parts of Maryland, where the aquifer 

is referred to as the Aquia aquifer (Drummond and Bolton, 

2010). 

Ingestion of arsenic in drinking water can cause a variety 

of human health problems, including skin, liver, lung, bladder, 

and kidney cancer (World Health Organization, 2001). 

The current U.S. Environmental Protection Agency (EPA) 
2 Occurrence and Distribution of Arsenic and Radon in Water from Private Wells in the Rancocas Aquifer, Delaware, 2015

Maximum Contaminant Level (MCL) for arsenic in public 

water systems is 10 micrograms per liter (μg/L). The presence 

of arsenic has been documented in groundwater from several 

wells in Delaware in the Rancocas aquifer but the issue 

became of greater concern when the MCL was lowered from 

50 μg/L to 10 μg/L on January 23, 2006 (Federal Register, 

2001). Also, recent documentation of arsenic concentrations 

above the new MCL in the adjacent State of Maryland 

(Drummond and Bolton, 2010) in the same aquifer and sampling by the Delaware Geological Survey (DGS) indicated that 

a similar pattern was likely in Delaware. 

Radon, an inert gas derived from radium, is primarily of 

concern through inhalation of indoor air in living spaces after 

the radon degasses from water into air, which could occur in 

a shower, for example (National Research Council, 1999). 

There is currently no EPA drinking water standard for radon. 

Radon has two proposed drinking water standards: a proposed 

alternative MCL of 4,000 picocuries per liter (pCi/L) and 

a proposed lower MCL of 300 pCi/L (U.S. Environmental 

Protection Agency, 2012). A previous study in southern New 

Castle County found that the seven wells sampled in the 

Rancocas aquifer all had concentrations of radon above the 

proposed lower MCL of 300 pCi/L, although none of the concentrations were above the higher proposed alternative MCL 

of 4,000 pCi/L (Bachman and Ferrari, 1995).

Although both chemicals have been previously detected 

in the Rancocas aquifer, the distribution of these chemicals 

in the use area of the Rancocas aquifer of Delaware, where 

many domestic wells tap this aquifer, has not been adequately 

determined. Whereas the quality of public water supplies is 

regulated under the Safe Drinking Water Act, routine sampling 

of domestic wells is not required and it is the homeowner’s 

responsibility to monitor their drinking water. The results of 

this project will provide information to State officials that can 

be used to inform well owners about potential health risks 

from arsenic or radon. These data also will be useful in combination with other data for future studies of the distribution 

of aquifer lithologic and hydrochemical factors affecting the 

occurrence and distribution of these chemicals in groundwater.

Purpose and Scope

The purpose of this report is to document the occurrence 

and distribution of arsenic and radon in groundwater from 

wells located throughout the area of use for domestic water 

supply in the Rancocas aquifer in Delaware (fig. 1). Wells 

identified as tapping the Rancocas aquifer were sampled 

for arsenic and radon, along with field parameters (specific 

conductance, temperature, dissolved oxygen, and pH), major 

ions, nutrients, and some trace elements. Maps showing the 

distribution of sampled wells and the concentrations of arsenic 

and radon are provided. A complete table of chemical results 

is included in Appendix 1 of this report. All data collected for 

this study also are available in the U.S. Geological Survey 

(USGS) National Water Information System (NWIS) database. 

Description of Study Area

The study area includes the area underlain by the 

Rancocas aquifer from its subcrop area beneath the Columbia 

aquifer in southern New Castle County, Delaware near 

Middletown and Odessa, to the southernmost extent of the 

aquifer’s use in northern Kent County along an east-northeasterly line from Kenton to just south of Smyrna (fig. 1). 

This area has been determined in previous studies to be the 

major area of use of this aquifer (Dugan and others, 2008; 

McLaughlin and Velez, 2006). The Rancocas aquifer thickens 

from 0 feet (ft) at its northernmost extent to over 100 ft near 

Middletown where there is significant interconnection with the 

overlying Columbia aquifer and the Rancocas is under watertable, or semi-confined conditions (He and Andres, 2011) 

(fig. 1). In the southern, confined part of the aquifer, it ranges 

from 100 ft thick in northeastern Kent County to 200 ft thick 

in northwestern Kent County, with its top less than 100 ft 

below sea level in northwest Kent County (McLaughlin and 

Velez, 2006).

The Rancocas aquifer is composed of glauconitic sands 

of marine origin that have been placed in the Manasquan 

(Paleocene to Eocene) and Vincentown (Paleocene) 

Formations and the underlying Hornerstown (Paleocene) 

Formation (Benson and Spoljaric, 1996; Dugan and others, 

2008). The Manasquan Formation is glauconitic and shelly 

fine sand. In less silty locations it can function as part of the 

Rancocas aquifer; otherwise it functions as a leaky confining 

unit. The Vincentown Formation is the primary unit making 

up the Rancocas aquifer (Dugan and others, 2008). It is 

described as glauconitic sand composed primarily of medium 

to fine-grained quartz sand (90–95 percent) and fine-grained 

glauconite, and is roughly equivalent to the Aquia Formation 

of Drummond (1998) in Maryland (Dugan and others, 2008). 

Sandier facies of the Hornerstown Formation also function 

as part of the Rancocas aquifer. The Hornerstown Formation 

is composed of glauconitic sand that is silty and slightly to 

moderately clayey with scattered shell beds and a calcareous 

silt and clay matrix. Glauconite forms 90 to 95 percent of the 

sand fraction in the Hornerstown Formation (Dugan and others, 2008). 

Both arsenic and radon are associated with glauconitic 

sediments (Woodruff and others, 1992; Barringer and others, 

2014). In addition to glauconite, geologic materials common 

in marine-derived sediments that are likely to include arsenic 

include other phyllosilicates (such as mica and clay minerals); 

arsenic also is commonly associated with iron in these sediments (Barringer and others, 2014). Radon is very soluble in 

groundwater and usually associated with nearby enrichment 

of its parent element, radium, which is a decay product of 

uranium. Higher concentrations of radioactive elements can be 

concentrated in glauconite and other marine sediments such as 

apatite in phosphate nodules and in fossilized bone materials 

where uranium can substitute for calcium (Woodruff and others, 1992; Schumann, 1993). Low radon values are typically 

associated with non-marine fluvial and deltaic quartz sands.
Methods of Study 3

Base from U.S. Geological Survey digital data, 2004,

1:24,000 projected to NAD 1983 State Plane

Delaware FIPS 0700 from GCS NAD 1983

0

0

10 MILES

10 KILOMETERS

DELAWARE RIVER

C&D CANAL

NEW

CASTLE

KENT

SUSSEX

Wilmington

Dover

Georgetown

DELAWARE

DELAWARE

MARYLAND

MARYLAND

NEW JERSEY

DELAWARE

B AY

AT L A N T I C

OC E A N

38°30'

39°30' Fa24-01

Fb51-12

Smyrna

Kenton

Middletown

Odessa

Gb22-06

Gb24-19

Gb45-01

Gc52-10

Hc14-16

75°30' 75°

39°

PENNSYLVANIA

Indian River

Bay

Rehoboth

Bay

FALL LINE

A

A'

100

Fa24-01 Fb51-12

Gb22-06 Gb24-19

Gb45-01 Gc52-10

Hc14-16

0

-100

Elevation, in feet

Distance, in miles

EXPLANATION

Key to lithologic units

-200

-300

0 2 4 6 8 10 12 14

Qcl

Qcl

Tsr Tc Tmq

Tvt

TKht

Knv

Kml

Kmt

Kmv

Kpt

Ket

A A'

Km

A B

Qcl Columbia Fm.

Hornerstown Fm.

Merchantville Fm.

Calvert Fm.

Navesink Fm.

Magothy Fm.

Shark River Fm.

Mt. Laurel Fm.

Potomac Fm.

Vincentown Fm. Rancocas

aquifer

Manasquan Fm. Englishtown Fm.

Marshalltown Fm.

Tc

Tsr

Tmq

Tvt

TKht

Knv

Kmv

Km

Kpt

Kml

Kmt

Ket

Fa24-01

A A'

Atlantic Coastal Plain

in Delaware

Area of use of the

Rancocas aquifer

Line of section A-A'

Data point and Delaware

Geological Survey identifier

EXPLANATION

Methods of Study

The sampling network for this study was developed 

to obtain information on the occurrence and distribution 

of arsenic and radon in the area of use for drinking water 

of the Rancocas aquifer (fig 2). Delaware Department of 

Natural Resources and Environmental Control (DNREC) 

and USGS staff worked together to identify potential wells 

for sampling in the Rancocas aquifer, design the sampling 

network, and obtain permission for sampling. The area of 

use of the Rancocas aquifer was identified using the hydrogeologic framework from the DGS (Dugan and others, 2008; 

McLaughlin and Velez, 2006). The population of Rancocas 

aquifer wells in the study area, most of which were domestic, 

was identified by intersecting the screened intervals of wells 

from the DNREC well-permitting database with data grids 

from He and Andres (2011) representing the top and bottom of 

the Rancocas aquifer (fig 2a). A grid was overlain on the area 

of use of the aquifer in order to create an equal area distribution for sampling. A subset of 55 wells that cover the aquiferuse area both aerially and at different depths in the aquifer 

were selected as potential sites for sampling. Wells in each 

grid cell were visited to determine if the wells were suitable 

for sampling and to request the owner’s permission. After 

permission for sampling was obtained, information relevant to 

sample collection, such as the location and type of raw water 

tap, was documented. Of the wells originally selected, 

36 wells screened in the Rancocas aquifer were sampled 

(fig. 2b; table 1). Several of the wells that were initially 

Figure 1. The Rancocas aquifer includes sediments of the 

Manasquan, Vincentown, and Hornerstown Formations in 

Delaware. A, area of use of the Rancocas aquifer, and 

B, position of geologic units in the Rancocas aquifer.

(Map and cross section modified from Dugan and 

others, 2008.)
4 Occurrence and Distribution of Arsenic and Radon in Water from Private Wells in the Rancocas Aquifer, Delaware, 2015

Base from U.S. Geological Survey digital data,

2004, 1:24,000 projected to NAD 1983 State Plane

Delaware FIPS 0700 from GCS NAD 1983

39°15'

39°30'

75°45' 75°30'

0 3

0 3

6 MILES

6 KILOMETERS

NEW JERSEY

DELAWARE

B

MARYLAND

DELAWARE

DELAWARE RIVER

C&D CANAL

NEW

CASTLE

KENT

Smyrna

Middletown

Dover

Sampled well and well number

EXPLANATION

Kenton

RAS-14

Base from U.S. Geological Survey digital data, 2004,

1:24,000 projected to NAD 1983 State Plane

Delaware FIPS 0700 from GCS NAD 1983

39°15'

39°30'

75°45' 75°30'

0 3

0 3

6 MILES

6 KILOMETERS

!

NEW JERSEY

DELAWARE

MARYLAND

DELAWARE

DELAWARE RIVER

C&D CANAL

NEW

CASTLE

KENT

Smyrna

Middletown

Odessa Odessa

Kenton

Dover

Rancocas wells (active)

Well selection grid

EXPLANATION

A

!

! !!!

!

!

!

!

!

!!!

!

! !!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!!

!

!

!

! !

!

! !

!

!

!

!

!!!

!

!

!

! !! !

!

! !!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

! ! ! !

!

!

!

!

!

!

!

!

!

!

!

!

!

! !! !!!!

! !

! !

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

! ! !

!!

!

!

!

! !

!

!

!

!

!

!

! ! ! !!!

!

!!

!

!

!!!

!

!

!

! !

!

!

!!

!

!!!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!!

!

! !

! !

! ! !

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

! !

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

! !

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

! !

!

!

!

!

!

!!!

!!

!

!

!

!!!!!

!

!

!

!

!

!

!

! ! !

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

! !

!

!

!!!

! ! !

!

!

!

!!

!

! !

!

!

!! !

!

!

!

!

!

!

! !

!

!

!

!

!

! !

!

!

!

!

!

! ! !

!

!

!

!

!

!!

!

! !

!

! ! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

! !

!

!

!

!

!

!

!

!

!

! !

! !

!

!

!!

!

! ! !

!

!

!

! !

!

!

!

!

!

!

!

! !

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

! ! !

!

! !

!

! ! !

!

! !

!

! !

!

!

!

!

!

! !

!

!

!

!

!

!

!

! !

!!

!

!

!

!

! !

! !!

!

!

! !

!

!

!

!

!

!

!

!! !

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

! !

!

!

!

!

!

! !

! !

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

! ! !

!

!

!

!

!

!

!

!

! ! ! ! !! !

! !

!

! !

!

RAS-43

RAS-34

RAS-38

RAS-37

RAS-13

RAS-39

RAS-33

RAS-7

RAS-41

RAS-40

RAS-14

RAS-16

RAS-18

RAS-17

RAS-22

RAS-11

RAS-47

RAS-8

RAS-51

RAS-49

RAS-10

RAS-12

RAS-15 RAS-56

RAS-19

RAS-20

RAS-21 RAS-24

RAS-26

RAS-29

RAS-1

RAS-31

RAS-53

RAS-3

RAS-5

RAS-6

Figure 2. Study area for the Rancocas aquifer in Delaware showing A, selection grid and the population of Rancocas aquifer wells, 

and B, sampled wells with well numbers.
Methods of Study 5

Table 1. Well depths and concentrations of arsenic and radon in water from private wells in the Rancocas aquifer in Delaware 

sampled from June through August 2015. (Well locations shown on figure 2b.)

[USGS, U.S. Geological Survey; μg/L, micrograms per liter; pCi/L, picocuries per liter; RAS-31*, replicate sample for RAS-31; <, less than]

Local 

well identifier

USGS 

site identifier

Well depth 

(feet) Date sampled Arsenic (μg/L)

Radon-222 

(pCi/L)

RAS-1 391922075425701 140 7/29/2015 14.98 366

RAS-3 391727075411901 190 8/20/2015 25.77 384

RAS-5 391759075451001 155 7/21/2015 11.50 285

RAS-6 391903075421201 180 8/18/2015 15.59 277

RAS-7 391520075440401 210 8/6/2015 27.27 530

RAS-8 392013075353501 165 7/30/2015 3.12 358

RAS-10 392321075381701 161 8/18/2015 0.10 101

RAS-11 392301075320501 300 6/11/2015 3.80 493

RAS-12 392212075362001 120 7/29/2015 3.40 168

RAS-13 391106075435301 380 7/30/2015 11.55 351

RAS-14 392726075371501 66 6/17/2015 0.36 350

RAS-15 392402075403601 75 6/15/2015 0.46 123

RAS-16 392536075392101 100 6/17/2015 1.13 98

RAS-17 392359075370301 95 6/10/2015 1.34 138

RAS-18 392457075374601 135 8/24/2015 <0.1 162

RAS-19 392245075440401 80 6/11/2015 0.30 212

RAS-20 392215075421801 160 7/28/2015 0.69 140

RAS-21 392217075402201 120 7/7/2015 <0.1 163

RAS-22 392312075342201 120 6/10/2015 6.30 254

RAS-24 392228075382201 80 7/22/2015 <0.1 133

RAS-26 392110075454201 145 7/7/2015 4.03 127

RAS-29 391949075450501 118 6/24/2015 4.88 223

RAS-31 391906075420101 142 6/24/2015 10.18 85

RAS-31* 391906075420101 142 6/24/2015 10.35 90

RAS-33 391536075443901 258 7/8/2015 20.15 126

RAS-34 391535075420401 260 7/21/2015 27.23 484

RAS-37 391202075421301 360 7/13/2015 1.68 366

RAS-38 391247075414401 355 7/22/2015 9.96 396

RAS-39 391055075441901 360 8/19/2015 16.29 580

RAS-40 391205075442801 300 7/13/2015 27.14 635

RAS-41 391356075441101 280 8/6/2015 25.11 395

RAS-43 391611075432501 180 7/9/2015 14.49 214

RAS-47 392119075353101 160 7/28/2015 13.20 316

RAS-49 391857075354701 254 6/23/2015 0.60 1,870

RAS-51 391914075353401 238 7/9/2015 2.35 684

RAS-53 391734075401101 225 7/16/2015 26.84 286

RAS-56 392344075375101 119 8/20/2015 0.11 150
6 Occurrence and Distribution of Arsenic and Radon in Water from Private Wells in the Rancocas Aquifer, Delaware, 2015

selected were not appropriate or available to sample for a variety of reasons, including the location or availability of sample 

taps prior to water-treatment systems, difficulty contacting and 

arranging sampling dates with well owners, and in one case, 

misidentification of the aquifer. 

Water samples were collected using standard USGS 

protocols for groundwater sampling for wells with permanent 

pumps (Koterba and others, 1995; U.S. Geological Survey 

National Field Manual, variously dated). Samples for arsenic, 

radon, major ions, some trace elements, and nutrients were 

analyzed at the USGS National Water Quality Laboratory 

(NWQL) in Denver, Colorado. Specific conductance, temperature, dissolved oxygen, and pH were measured in the field 

using a multi-parameter meter. Bicarbonate concentrations 

were calculated on the basis of field titrations for alkalinity 

(Radtke and others, 1998). 

An office blank was collected prior to sampling and one 

replicate sample was collected and analyzed for all waterquality parameters measured to ensure quality control. The 

office blank was collected to ensure sample equipment and 

sampling methods did not cause contamination. All values for 

the office blank were below their respective reporting levels, 

with the exception of chloride, that was at the reporting level 

of 0.2 milligrams per liter (mg/L), which is well below any 

environmental concentrations. A replicate sample measures 

the combined precision of sampling and laboratory analysis 

procedures. The replicate sample had results consistent with 

its respective environmental sample (see Appendix 1). For 

arsenic and radon, the relative percent differences between 

replicate and environmental values were less than 2 percent 

and 6 percent, respectively. 

After data were received from the laboratory, they were 

summarized in the tables and maps that are included in this 

report. Results from individual private wells were sent to well 

owners. A complete table of data is included in Appendix 1. 

All data collected for this project are available online in the 

USGS NWIS database (http://waterdata.usgs.gov/nwis). The 

well identification numbers listed in Appendix 1 can be used 

to retrieve these data. 

Occurrence and Distribution of Arsenic 

and Radon

Arsenic and radon were widely detected in water samples 

from the Rancocas aquifer (table 1). Arsenic was detected 

above the minimum reporting limit of 0.1 μg/L in 34 of the 

36 wells sampled with concentrations ranging from about 

0.11 to 27 μg/L and a median concentration of about 4 μg/L. 

Radon was detected in all water samples with concentrations 

ranging from 85 to 1,870 pCi/L, with a median of about 

260 pCi/L. Median concentrations of both chemicals were 

below the 10 μg/L MCL for arsenic and the proposed lower 

300 pCi/L MCL for radon (fig. 3). 

Arsenic

The concentrations of arsenic in groundwater from sampled wells were lowest in the northernmost part of the study 

area and highest in the southern part of the aquifer use area in 

a band that approximately follows the southwest to northeast 

strike of the geologic formations (fig. 4). With the exception 

of the northernmost sample (fig. 2, RAS-14), samples for 

this study were from confined parts of the Rancocas aquifer. 

Correlation coefficients between arsenic and well depth show 

a strong positive correlation with weaker correlations indicated 

between arsenic and several other dissolved constituents and 

characteristics (table 2). Other constituents with a positive 

correlation with arsenic include pH, total dissolved nitrogen, 

magnesium, and potassium; arsenic is negatively correlated 

with orthophosphate, manganese, chloride, and silica. These 

potential differences in characteristics of water chemistry in 

relation to arsenic occurrence likely relate to the distribution 

of lithologic facies and hydrochemical conditions in the 

aquifer system. 

The area with the highest arsenic concentrations is also 

the area with the highest reported well yields and modeled 

hydraulic conductivity (Dugan and others, 2008; He and 

Andres, 2011). Not surprisingly, the concentrations of arsenic 

collected for this study in Delaware follow a similar trend to 

the one seen in in the adjacent area of Maryland (Drummond 

and Bolton, 2010), and in data recently collected by DGS in 

the area of highest use of the Rancocas aquifer (Scott Andres, 

Delaware Geological Survey, written commun, 2014) (fig. 5). Arsenic concentration, in micrograms per liter Radon concentration, in picocuries per liter

30

25

2,000

1,000

800

600

500

400

300

200

100

80

60

50

20

15

10

5

0

-5

Maximum

Contaminant

Level

Proposed

lower

Maximum

Contaminant

Level

EXPLANATION

Data value less than or equal to 1.5 times

the interquartile range outside the quartile

75th percentile

25th percentile

Median

A B

Figure 3. Range in concentrations of A, arsenic, and B, radon in 

water from 36 wells in the Rancocas aquifer, Delaware.
Occurrence and Distribution of Arsenic and Radon 7

!

!

!

!

Base from U.S. Geological Survey digital data, 2004,

1:24,000 projected to NAD 1983 State Plane

Delaware FIPS 0700 from GCS NAD 1983

39°30'

39°20'

39°10'

75°45' 75°30'

0 2.5

0 2.5

5 MILES

5 KILOMETERS

NEW JERSEY

DELAWARE

MARYLAND

MARYLAND

DELAWARE

DELAWARE

DELAWARE RIVER

NEW

CASTLE

KENT

Middletown

Smyrna

Kenton

Dover

Odessa

!

EXPLANATION

Arsenic concentration,

in micrograms per liter

Cities and towns

!

!

!

!

!

Less than (<) 5

5 to <10

10 to <15

15 to <20

Greater than 20

State capital

Towns

!

!

!

!

!

3.4

13.2

3.12

2.35

0.6

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

4.88

14.98

10.18

15.59

11.5

25.77

20.15

14.49

27.27

27.23

25.11

9.96

1.68

27.14

11.55

16.29

26.84

! !

!

! !

4.03

<0.1

<0.1

6.3

3.8

!

!

!

!

!

!

!

!

!

0.36

1.13

0.46

0.3

0.69

<0.1

0.1

1.34

0.11

Figure 4. Distribution of arsenic in water from the Rancocas aquifer, Delaware.
8 Occurrence and Distribution of Arsenic and Radon in Water from Private Wells in the Rancocas Aquifer, Delaware, 2015

Arsenic distribution is controlled by the distribution of 

lithologic components related to differences in the environment of deposition of sediments that provide a potential source 

for arsenic, several hydrochemical mechanisms that control 

the mobility of arsenic, or both (Drummond and Bolton, 

2010). In nearby Maryland, the coincidence of elevated 

arsenic concentrations with a high-energy offshore bank 

deposit provided evidence of lithologic controls on arsenic 

distribution in the Aquia aquifer (Drummond and Bolton, 

2010). The extension of the area with higher arsenic concentrations in Maryland into Delaware implies that similar aquifer 

conditions are causing arsenic occurrence above the 10 μg/L 

MCL. Redox reactions also were thought to be an important 

mobilization control in the Maryland study; arsenic concentration, however, can also be affected by pH, reductive dissolution, and competition for adsorption sites, and the precise 

hydrochemical control could not be determined (Drummond 

and Bolton, 2010). 

Radon

Concentrations of radon in groundwater of the Rancocas 

aquifer sampled for this study also showed a strong positive 

correlation with well depth and typically were highest in the 

same parts of the aquifer where arsenic concentrations were 

highest (fig. 6; table 2). Radon also had strong positive correlations with several other ions including pH, total dissolved 

nitrogen, magnesium, potassium, and fluoride, and a weaker 

positive correlation with sodium. Radon had a strong negative 

correlation with orthophosphate, calcium, iron, manganese, 

and silica, and weaker negative correlations with chloride and 

sulfate. The occurrence of high concentrations of radioactive 

minerals, such as those found in glauconite and associated 

marine sediments such as apatite and phosphate nodules, is 

of primary importance in determining the potential for radon 

occurrence. Higher concentrations of magnesium, potassium, 

sodium, and fluoride in groundwater are likely associated 

with the glauconitic and other marine sediments. The negative 

correlation between radon and silica may be indicative of 

sediments with higher percentages of quartz sands, which are 

not typically associated with radioactive minerals. Correlation 

with other negatively charged ions, such as phosphate, iron, 

and manganese, also may be related to differences in aquifer 

mineralogy. 

Water from several wells sampled in the northern part 

of the Rancocas aquifer by Bachman and Ferrari (1995) also 

had concentrations of radon similar to those in the southern 

part of the study area (fig. 6). Measurements of gamma radiation at the land surface in the Coastal Plain of Delaware by 

Woodruff and others (1992) found that the highest gamma 

radiation measurements were associated with the Hornerstown 

Formation, which subcrops in the northern part of the study 

area where these higher concentrations were measured 

(fig. 1). These results indicated that this formation was the 

source of the gamma radiation and, possibly, radon gas in that 

area. It is unknown if the higher concentrations in the southern 

part of the study area are related to radioactive minerals in 

the Hornerstown Formation. Further work comparing local 

lithologic knowledge would be useful for comparison to radon 

concentrations in the Rancocas aquifer.

Table 2. Kendall’s Tau correlation coefficients for relations 

between arsenic and radon with dissolved constituents or 

physical characteristics in groundwater from the Rancocas 

aquifer, Delaware.

[Results shown in bold with p<0.01; results shown for p<0.05; --, indicates 

not significant]

Dissolved constituent or 

characteristic

Arsenic 

(n=36)

Radon-222 

(n=36)

Depth 0.42 0.46

Specific conductance -- --

pH 0.24 0.39

Orthophosphate -0.28 -0.52

Total dissolved nitrogen 0.23 0.46

Calcium -- -0.42

Magnesium 0.33 0.42

Sodium -- 0.28

Potassium 0.31 0.52

Iron -- -0.44

Manganese -0.31 -0.41

Bicarbonate -- --

Chloride -0.31 -0.27

Sulfate -- -0.34

Fluoride -- 0.48

Silica -0.25 -0.52

Aluminum -- --
Occurrence and Distribution of Arsenic and Radon 9

Base from U.S. Geological Survey digital data, 2004,

1:24,000 projected to NAD 1983 State Plane

Delaware FIPS 0700 from GCS NAD 1983

0 2.5

0 2.5

5 MILES

5 KILOMETERS

39°15'

75°45' MARYLAND

MARYLAND

DELAWARE

DELAWARE

!

!

Smyrna

Kenton

NEW

CASTLE

CECIL

KENT

QUEEN ANNE'S

CAROLINE

KENT

DELAWARE

RIVER

EXPLANATION

Arsenic concentration in water,

in micrograms per liter

Data providers

0 to less than (<) 5

U.S. Geological Survey

Maryland Geological Survey

Delaware Geological Survey

5 to <10

10 to <15

15 to <20

Greater than 20

Approximate area of arsenic

concentrations greater than

10 micrograms per liter in the

Rancocas aquifer

"

"

"

"

"

"

"

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

"

"

"

"

"""

"

"

"

" "

"

"

"

"

"

"

"

"

" "

"

""

"

"

"

"

"

"

"

"

"

"

"

"

Figure 5. Combined distribution of arsenic concentrations from water in the adjacent Aquia aquifer of 

Maryland and the Rancocas aquifer of Delaware.
10 Occurrence and Distribution of Arsenic and Radon in Water from Private Wells in the Rancocas Aquifer, Delaware, 2015

!

!

!

!

Base from U.S. Geological Survey digital data, 2004,

1:24,000 projected to NAD 1983 State Plane

Delaware FIPS 0700 from GCS NAD 1983

0 2.5

0 2.5

5 MILES

5 KILOMETERS

NEW

CASTLE

KENT

Middletown

Smyrna

Kenton

Dover

Odessa

NEW JERSEY

DELAWARE

MARYLAND

MARYLAND

DELAWARE

DELAWARE

DELAWARE RIVER

39°30'

75°45' 75°30'

39°15'

!

EXPLANATION

Radon concentration,

in picocuries per liter

Cities and towns

!

!

!

Less than (<) 300

Current study

1992 study, Bachman and Ferrari (1995)

(Samples were collected in 1992.)

Greater than (>) 300

to <600

>600

State capital

Towns

"

!

!

!

!

!

! !

395

396

366 635

351

580

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

316

358

684

1,870

127

223

366

85

277

285

384 286

214

126

530

484

98

!

!

!

! !

! !

!

!

!

! !

!

!

"

" ""

"

"

"

820

1,300

920

350

330

162

138

150

101

133

254

493

168

123

212

140

163

540

470

490

Figure 6. Distribution of radon in groundwater from the Rancocas aquifer, Delaware.
References Cited 11

References Cited 

Bachman, L.J. and Ferrari, M.J., 1995, Quality and geochemistry of groundwater in southern New Castle 

County, Delaware: Delaware Geological Survey Report 

of Investigations No. 52, 31 p., accessed July 22, 2016, 

at http://www.dgs.udel.edu/publications/ri52-quality-andgeochemistry-ground-water-southern-new-castle-countydelaware.

Barringer, J.L., Reilly, P.A., Eberl, D.D., Mumford, A.C., 

Benzel, W.M., Szabo, Zoltan, Shourds, J.L., and Young, 

L.Y., 2014, Arsenic in New Jersey Coastal Plain streams, 

sediments, and shallow groundwater: Effects from different 

geologic sources and anthropogenic inputs on biogeochemical and physical mobilization processes: U.S. Geological 

Survey Scientific Investigations Report 2013–5107, 

37 p., accessed July 22, 2016, at http://dx.doi.org/10.3133/

sir20135107.

Benson, R.N., and Spoljaric, N., 1997, Stratigraphy of 

the post-Potomac Cretaceous-Tertiary rocks of central 

Delaware: Delaware Geological Survey Bulletin No. 20, 

28 p., accessed July 22, 2016, at http://www.dgs.udel.edu/

publications/b20-stratigraphy-post-potomac-cretaceoustertiary-rocks-central-delaware.

Drummond, D.D., 1998, Hydrogeology, simulation of 

ground-water flow, and ground-water quality of the upper 

Coastal Plain aquifers in Kent County, Maryland: Maryland 

Geological Survey Report of Investigations No. 68, 76 p.

Drummond, D.D., and Bolton, D.W., 2010, Arsenic in ground 

water in the Coastal Plain aquifers of Maryland: Maryland 

Geological Survey Report of Investigations No. 78, 72 p.

Dugan, B.L., Neimeister, M.P., and Andres, A.S., 2008, 

Hydrogeologic framework of southern New Castle County: 

Delaware Geological Survey Open File Report No. 49, 

22 p., accessed July 22, 2016, at http://www.dgs.udel.edu/

publications/ofr49-hydrologeologic-framework-southernnew-castle-county.

Federal Register, 2001, The arsenic and clarifications to compliance and new source contaminants monitoring final rule: 

Published in the Federal Register (66 FR 6976) on 

January 22, 2001.

He, Changming, and Andres, A.S., 2011, Simulation of 

groundwater flow in southern New Castle County, 

Delaware: Delaware Geological Survey Report of 

Investigations No. 77, 18 p., accessed July 22, 2016, at 

http://www.dgs.udel.edu/publications/ri77-simulationgroundwater-flow-southern-new-castle-county-delaware.

Koterba, M.T., Wilde, F.D., and Lapham, W.W., 1995, 

Ground-water data-collection protocols and procedures for 

the National Water-Quality Assessment Program: Collection 

and documentation of water-quality samples and related 

data: U.S. Geological Survey Open-File Report 95–399, 

113 p., accessed July 22, 2016, at http://pubs.usgs.gov/

of/1995/ofr-95-399/.

Maupin, M.A., Kenny, J.F., Hutson, S.S., Lovelace, J.K., 

Barber, N.L., and Linsey, K.S., 2014, Estimated use of 

water in the United States in 2010: U.S. Geological Survey 

Circular 1405, 56 p., accessed July 22, 2016, at http://

dx.doi.org/10.3133/cir1405.

McLaughlin, P.P., and Velez, C.C., 2006, Geology and 

extent of the confined aquifers of Kent County, Delaware: 

Delaware Geological Survey Report of Investigations 

No. 72, 46 p., accessed July 22, 2016, at http://www.dgs.

udel.edu/publications/ri72-geology-and-extent-confinedaquifers-kent-county-delaware.

National Research Council, 1999, Arsenic in drinking water: 

Washington, D.C., National Academy Press, 310 p.

Radtke, D.B., Wilde, F.D., Davis, J.V., and Popowski, T.J., 

1998, National field manual for collection of waterquality data—Alkalinity and acid neutralizing capacity: 

U.S. Geological Survey Techniques for Water-Resources 

Investigations, book 9, chap. A6.6, 33 p.

Schumann, R.R., ed., 1993, Geologic radon potential of 

EPA Region 3: U.S. Geological Survey Open-File Report 

93–292–C, 206 p.

U.S. Environmental Protection Agency, 2012, Report to 

Congress: Radon in Drinking Water Regulations, May 2012, 

accessed May 19, 2014, at https://archive.epa.gov/water/

archive/web/html/index-9.html.

U.S. Geological Survey, variously dated, National field manual for the collection of water-quality data: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

book 9, chaps. A1–A9, accessed July 22, 2016, at http://

water.usgs.gov/owq/FieldManual/index.html.

Woodruff, K.D., Ramsey, K.W., and Talley, J.H., 1992, 

Radon potential for glauconitic sediments in the coastal 

plain of northern Delaware, Final Report to the Delaware 

Department of Health and Social Services, Division of 

Public Health, Health Systems Protection, Radiation 

Control, contract no. 92–105: Newark, Delaware, 

Delaware Geological Survey, 43 p.

World Health Organization, 2001, Arsenic: Fact Sheet 

No. 372, accessed May 19, 2014, at http://www.who.int/

mediacentre/factsheets/fs372/en/.

Appendix 1 13

Appendix 1
14 Occurrence and Distribution of Arsenic and Radon in Water from Private Wells in the Rancocas Aquifer, Delaware, 2015

Appendix 1. Groundwater-quality data for private wells sampled in the Rancocas aquifer, Delaware, June through August 2015.[USGS, U.S. Geological Survey; DNREC, Delaware Department of Natural Resources and Environmental Control; samples analyzed at the USGS National Water-Quality Laboratory in Denver, Colorado; RAS-31*, replicate sample for RAS-31; pCi/L, picocuries per liter; °C, degrees Celsius; μg/L, micrograms per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; N nitrogen; CaCO3, calcium carbonate; SiO2, silica; <, less than; --, not done]

Local 

well 

identifier

Date 

sampled

USGS 

site 

identifier

DNREC 

well 

identifier

Well 

depth 

(feet)

Arsenic 

(μg/L)

Radon222 

(pCi/L)

Field parameters Nutrients

Water 

temperature 

(°C)

Dissolved 

oxygen 

(mg/L)

pH

Specific 

conductance 

(μS/cm at 

25°C) 

Alkalinity 

(mg/L as 

CaCO3

)

Total 

dissolved 

nitrogen 

(mg/L)

Dissolved 

inorganic 

nitrogen 

(mg/L)

Ammonia 

(mg/L)

Nitrite + 

nitrate 

(mg/L 

as N)

Nitrate 

(mg/L 

as N)

Orthophosphate 

(mg/L)

Nitrite 

(mg/L 

as N)

RAS-1 7/29/2015 391922075425701 94381 140 14.98 366 14.96 0.3 7.42 301 113 0.160 0.169 0.129 <0.04 <0.04 0.043 <0.001RAS-3 8/20/2015 391727075411901 90488 190 25.77 384 15.89 0.3 7.51 304 150 0.271 0.298 0.258 <0.04 <0.04 0.039 <0.001RAS-5 7/21/2015 391759075451001 55591 155 11.50 285 15.93 1.3 7.28 300 129 0.065 0.096 0.056 <0.04 <0.04 0.035 <0.001RAS-6 8/18/2015 391903075421201 203146 180 15.59 277 16.55 0.9 7.43 312 144 0.090 0.125 0.085 <0.04 <0.04 0.039 <0.001RAS-7 8/6/2015 391520075440401 105466 210 27.27 530 15.86 1.5 7.48 301 129 0.287 0.305 0.265 <0.04 <0.04 0.030 <0.001RAS-8 7/30/2015 392013075353501 204944 165 3.12 358 16.11 0.3 7.53 306 155 0.431 0.422 0.382 <0.04 <0.04 0.019 <0.001

RAS-10 8/18/2015 392321075381701 80799 161 0.11 101 14.73 0.3 7.38 260 116 0.057 0.095 0.055 <0.04 <0.04 0.117 <0.001

RAS-11 6/11/2015 392301075320501 222243 300 3.79 493 16.29 0.3 7.92 314 150 0.443 0.507 0.467 <0.04 <0.04 0.005 0.002

RAS-12 7/29/2015 392212075362001 187131 120 3.40 168 16.42 0.3 7.52 305 150 0.222 0.241 0.201 <0.04 <0.04 0.037 <0.001RAS-13 7/30/2015 391106075435301 213047 380 11.55 351 17.01 0.3 7.78 309 142 0.608 0.630 0.590 <0.04 <0.04 0.029 <0.001RAS-14 6/17/2015 392726075371501 217867 66 0.36 350 15.44 7.9 6.69 261 75 8.765 8.103 0.010 8.09 8.09 0.021 <0.001RAS-15 6/15/2015 392402075403601 153948 75 0.46 123 16.32 0.2 7.64 249 134 <0.05 0.094 0.054 0.04 0.04 0.024 <0.001RAS-16 6/17/2015 392536075392101 44001 100 1.13 98 16.72 1.4 7.26 477 196 0.055 0.070 0.030 0.04 0.04 0.039 <0.001RAS-17 6/10/2015 392359075370301 204160 95 1.34 138 16.58 0.1 7.35 382 171 0.086 0.105 0.065 0.04 0.04 0.150 <0.001RAS-18 8/24/2015 392457075374601 205717 135 <0.1 162 14.79 0.3 7.19 401 162 <0.05 0.066 0.026 0.04 0.04 0.146 <0.001RAS-19 6/11/2015 392245075440401 106200 80 0.30 212 15.71 0.4 7.52 258 121 <0.05 0.050 <0.01 <0.04 <0.04 0.125 <0.001RAS-20 7/28/2015 392215075421801 247483 160 0.69 140 16.39 2.5 7.34 255 149 <0.05 0.054 0.014 <0.04 <0.04 0.047 <0.001RAS-21 7/7/2015 392217075402201 104272 120 <0.1 163 19.03 0.4 7.16 308 148 0.210 0.201 0.161 <0.04 <0.04 0.433 <0.001RAS-22 6/10/2015 392312075342201 221150 120 6.30 254 17.77 1.9 7.68 420 211 0.372 0.418 0.378 <0.04 <0.04 0.025 <0.001RAS-24 7/22/2015 392228075382201 94074 80 <0.1 133 18.41 1.3 7.36 291 129 <0.05 0.054 0.014 <0.04 <0.04 0.258 <0.001RAS-26 7/7/2015 392110075454201 219729 145 4.03 127 19.72 2.2 7.01 464 219 0.182 0.185 0.145 <0.04 <0.04 0.047 <0.001RAS-29 6/24/2015 391949075450501 227789 118 4.88 223 15.56 0.2 7.41 286 153 <0.05 0.090 0.050 <0.04 <0.04 0.061 <0.001RAS-31 6/24/2015 391906075420101 200740 142 10.18 85 15.55 0.3 7.65 314 177 0.131 0.173 0.133 <0.04 <0.04 0.042 <0.001

RAS-31* 6/24/2015 391906075420101 200740 142 10.35 90 -- -- -- -- -- 0.113 0.174 0.134 <0.04 <0.04 0.042 <0.001

RAS-33 7/8/2015 391536075443901 246395 258 20.15 126 16.78 0.5 7.55 286 130 0.102 0.130 0.090 <0.04 <0.04 0.039 <0.001RAS-34 7/21/2015 391535075420401 185423 260 27.23 484 17.05 0.6 7.54 299 155 0.363 0.403 0.363 <0.04 <0.04 0.018 <0.001RAS-37 7/13/2015 391202075421301 168854 360 1.68 366 18.19 0.5 8.06 302 162 0.489 0.511 0.471 <0.04 <0.04 0.034 <0.001RAS-38 7/22/2015 391247075414401 169628 355 9.96 396 18.22 1.1 7.87 300 162 0.552 0.586 0.546 <0.04 <0.04 0.018 0.001RAS-39 8/19/2015 391055075441901 195517 360 16.29 580 16.67 0.4 7.78 338 166 0.054 0.086 0.046 <0.04 <0.04 0.029 <0.001RAS-40 7/13/2015 391205075442801 171979 300 27.14 635 17.21 3.3 7.69 301 173 0.557 0.600 0.560 <0.04 <0.04 0.031 <0.001RAS-41 8/6/2015 391356075441101 104607 280 25.11 395 16.78 0.3 7.65 294 140 0.370 0.385 0.345 <0.04 <0.04 0.023 <0.001RAS-43 7/9/2015 391611075432501 234735 180 14.49 214 16.21 0.4 7.41 297 146 0.186 0.213 0.173 <0.04 <0.04 0.030 <0.001RAS-47 7/28/2015 392119075353101 217067 160 13.20 316 20.14 1.6 7.42 313 149 0.404 0.392 0.352 <0.04 <0.04 0.039 <0.001RAS-49 6/23/2015 391857075354701 104153 254 0.60 1,870 16.36 0.2 7.93 383 167 0.396 0.427 0.387 <0.04 <0.04 0.007 <0.001RAS-51 7/9/2015 391914075353401 159245 238 2.35 684 18.87 0.5 7.58 310 155 0.475 0.486 0.446 <0.04 <0.04 0.008 <0.001RAS-53 7/16/2015 391734075401101 211827 225 26.84 286 16.16 0.5 7.43 301 133 0.300 0.342 0.302 <0.04 <0.04 0.025 0.001RAS-56 8/20/2015 392344075375101 171332 119 0.11 150 17.29 1.6 7.39 274 114 <0.05 0.054 0.014 <0.04 <0.04 0.080 <0.001
Appendix 1 15

Appendix 1. Groundwater-quality data for private wells sampled in the Rancocas aquifer, Delaware, June through August 2015.—Continued[USGS, U.S. Geological Survey; DNREC, Delaware Department of Natural Resources and Environmental Control; samples analyzed at the USGS National Water-Quality Laboratory in Denver, Colorado; RAS-31*, replicate sample for RAS-31; pCi/L, picocuries per liter; °C, degrees Celsius; μg/L, micrograms per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; N nitrogen; CaCO3, calcium carbonate; SiO2, silica; <, less than; --, not done]

Local 

well 

identifier

Major ions and trace elements

Calcium 

(mg/L)

Magnesium 

(mg/L)

Sodium 

(mg/L)

Potassium 

(mg/L)

Iron 

(μg/L)

Manganese 

(μg/L)

Aluminum 

(μg/L)

Bicarbonate 

(mg/L)

Chloride 

(mg/L)

Sulfate 

(mg/L)

Fluoride 

(mg/L)

Silica as SiO2 (mg/L)

RAS-1 47.3 6.7 5.9 3.3 352 6 <3 137.7 2.1 6.8 0.2 35.1

RAS-3 46.3 7.8 5.0 6.0 251 8 <3 181.9 1.9 4.5 0.3 24.2RAS-5 54.6 5.0 5.2 2.4 496 10 <3 157.1 3.0 6.3 0.2 31.7RAS-6 56.6 4.1 4.0 4.4 289 2 4 174.3 2.1 5.3 0.1 27.2RAS-7 44.0 7.8 5.1 6.5 275 5 <3 157 1.8 5.0 0.5 19.5RAS-8 34.6 11.8 11.9 9.7 255 7 <3 187 1.3 3.2 0.8 14.4

RAS-10 46.8 4.0 4.9 1.7 729 21 <3 141.2 2.0 4.6 0.2 28.1

RAS-11 23.0 10.7 25.8 11.8 108 0 147 181.3 5.1 2.4 0.6 11.3

RAS-12 44.7 7.6 5.6 5.3 266 6 <3 181.7 1.9 2.9 0.2 32.0RAS-13 19.8 9.6 28.8 12.9 79 1 <3 170.6 1.1 2.8 0.8 12.7RAS-14 41.4 2.5 3.1 7.3 4 5 <3 91.8 8.6 2.6 0.1 16.4RAS-15 44.1 3.8 3.5 3.3 357 9 <3 162.9 1.8 2.7 0.2 16.3RAS-16 94.8 3.3 4.5 2.2 2,115 71 <3 238.6 15.0 37.9 0.4 43.3RAS-17 72.5 3.7 5.7 1.6 978 18 <3 207.9 10.1 12.2 0.2 34.5RAS-18 74.6 4.0 5.9 2.4 1,608 96 <3 196.5 3.7 30.7 0.3 34.1RAS-19 46.8 2.3 4.7 1.5 992 128 <3 147.5 4.3 10.6 0.2 31.1RAS-20 19.4 1.5 39.1 1.9 24 9 58 180.5 2.0 4.5 0.1 29.4RAS-21 45.8 4.1 9.3 1.9 4,696 198 <3 179.5 6.6 3.1 0.2 42.4RAS-22 54.5 16.3 10.8 8.4 297 3 4 255.8 9.3 3.9 0.2 25.6RAS-24 0.4 0.0 77.5 0.4 9 1 4 156 3.0 4.6 0.2 36.5RAS-26 87.9 5.0 6.4 3.5 2,595 33 4 266.8 4.7 17.3 0.3 49.7RAS-29 47.5 6.4 5.4 2.2 1,007 41 <3 185.6 1.9 0.3 0.2 39.7RAS-31 49.8 7.1 5.0 3.9 373 8 <3 214.7 2.1 5.7 0.2 31.0

RAS-31* 50.6 7.2 4.9 3.9 368 8 <3 -- 2.1 5.7 0.2 30.8

RAS-33 52.7 3.4 2.9 4.1 252 2 <3 158 1.9 5.1 0.2 23.0RAS-34 39.4 10.2 6.1 9.9 209 3 <3 187.6 1.7 3.8 0.6 13.5RAS-37 12.0 4.9 51.3 10.0 6 0 <3 193.9 1.0 2.1 0.9 12.1RAS-38 16.9 7.4 38.1 11.7 68 1 <3 195.4 0.9 2.2 0.8 12.4RAS-39 0.0 0.0 85.4 1.8 4 0 <3 199.9 1.4 1.3 0.9 17.2RAS-40 29.5 11.6 16.7 13.3 118 4 <3 209 1.3 3.1 0.9 14.5RAS-41 39.2 8.7 5.0 9.4 241 1 <3 169.6 1.6 3.3 0.6 12.8RAS-43 51.0 6.4 4.5 5.2 317 4 8 176.7 2.0 6.0 0.2 26.8RAS-47 40.5 11.2 5.6 9.2 269 7 9 180.4 2.0 6.0 0.3 15.6RAS-49 19.0 7.7 50.3 10.6 65 3 4 201.6 25.5 1.9 1.1 12.8RAS-51 28.7 12.2 16.2 12.0 61 2 <3 187.2 1.6 2.8 0.9 13.2RAS-53 48.2 7.5 3.8 7.8 169 2 <3 162.3 2.0 4.7 0.2 20.5RAS-56 47.5 4.9 5.3 1.9 447 11 3 138.9 2.2 2.7 0.1 33.8

Prepared by USGS West Trenton Publishing Service Center.

Edited by Valerie M. Gaine.

Graphics and layout by Timothy W. Auer.

For additional information, contact:

Director, MD-DE-DC Water Science Center

U.S. Geological Survey

5522 Research Park Drive

Baltimore, MD 21228

or visit our Web site at:

http://md.water.usgs.gov
Denver, J.M.—Occurrence and Distribution of Arsenic and Radon in Water from Private Wells in the Rancocas Aquifer, Southern New Castle and Northern Kent Counties, Delaware, 2015—Open-File Report 2016–1143

Printed on recycled paper

ISSN 0196-1497 (print)

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20161143