Prepared in cooperation with the Connecticut Department of Public Health

Arsenic and Uranium Occurrence in Private Wells in 

Connecticut, 2013–18—A Spatially Weighted and Bedrock 

Geology Assessment

Open-File Report 2020–1111

Version 1.1, November 2020

U.S. Department of the Interior

U.S. Geological Survey
Cover. Farmington River in Collinsville, Connecticut. Photo by Tiziana Shea.
Arsenic and Uranium Occurrence in 

Private Wells in Connecticut, 2013–18—

A Spatially Weighted and Bedrock 

Geology Assessment

By Eliza L. Gross and Craig J. Brown

Prepared in cooperation with the Connecticut Department of Public Health

Open-File Report 2020–1111

Version 1.1, November 2020

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

First release: 2020

Revised: November 2020 (ver. 1.1)

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 https://www.usgs.gov or call 1–888–ASK–USGS.

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

visit https://store.usgs.gov.

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Gross, E.L., and Brown, C.J., 2020, Arsenic and uranium occurrence in private wells in Connecticut, 

2013–18—a spatially weighted and bedrock geology assessment: U.S. Geological Survey Open-File Report 

2020–1111 (ver. 1.1, November 2020), 13 p., https://doi.org/10.3133/ofr20201111.

Associated data for this publication:

Gross, E.L, 2020, Data in support of “Arsenic and Uranium Occurrence in Private Wells in Connecticut, 2013–18, 

a Spatially-Weighted and Bedrock Geology Assessment”: U.S. Geological Survey data release, https://doi.org/

10.5066/P9KOU55K. 

ISSN 2331-1258 (online)
iii

Acknowledgments

This study was conducted in cooperation with the Connecticut Department of Public Health 

(DPH) as part of their cooperative agreement with the National Center for Environment Health 

of the Centers for Disease Control and Prevention of the U.S. Department of Health and Human 

Services to provide support for public health drinking water programs to reduce drinking water 

exposures to contaminants. The authors thank Laura Apinis and Sue Isch of the DPH Laboratory 

for sample analyses and Gary Archambault, Ryan Tetreault, and Tiziana Shea of the DPH for the 

site location data used in this study.
iv

Contents

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

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

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

Arsenic and Uranium Concentration Data Sources ................................................................................1

Arsenic and Uranium Concentrations in the State ..................................................................................3

Arsenic and Uranium Spatially Weighted Assessment...........................................................................3

Arsenic and Uranium Occurrence in Relation to Bedrock Geology .....................................................6

References Cited..........................................................................................................................................13

Figures

1. Concentrations of A, arsenic in water samples from 2,433 private wells and B, 

uranium in water samples from 2,191 private wells in Connecticut, 2013-18 .....................2

2. Spatially weighted statewide-scale proportion of wells within grid cells for 

arsenic and uranium ....................................................................................................................4

3. Spatial distribution and range of percentages of private wells in Connecticut, 

grouped by geologic units, in which A, arsenic concentrations exceeded the 

maximum concentration level (MCL) of 10 micrograms per liter (μg/L), and B, 

uranium concentrations exceeded the MCL of 30 μg/L .......................................................11

Tables

1. Inventory of standard reference samples for arsenic and uranium concentrations, 

2015–18 ...........................................................................................................................................3

2. Arsenic concentrations in water samples from 2,433 private wells and uranium 

concentrations in water samples from 2,191 private wells in Connecticut, 2013–18........5

3. Spatially weighted statewide-scale proportions for arsenic and uranium during 

2013–18 from the Connecticut Department of Public Health database...............................5

4. Arsenic and uranium concentrations that exceed U.S. Environmental Protection 

Agency’s maximum contaminant levels in samples from private wells in Connecticut, 

by geologic unit and major bedrock category, 2013–18.........................................................7
v

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

mile (mi) 1.609 kilometer (km)

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

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

Altitude, as used in this report, refers to distance above the vertical datum.

Supplemental Information

Concentrations of chemical constituents in water are given in micrograms per liter (μg/L).

Abbreviations

DPH Connecticut Department of Public Health 

EPA U.S. Environmental Protection Agency

MCL maximum contaminant level

USGS U.S. Geological Survey
vi
Arsenic and Uranium Occurrence in Private Wells in 

Connecticut, 2013–18—A Spatially Weighted and Bedrock 

Geology Assessment

By Eliza L. Gross and Craig J. Brown

Abstract

The U.S. Geological Survey, in cooperation with the 

Connecticut Department of Public Health, conducted a study 

to determine the presence of arsenic and uranium in private 

drinking water wells in Connecticut. Samples were collected 

during 2013–18 from wells completed in 115 geologic units, 

with 2,433 samples analyzed for arsenic and 2,191 samples 

analyzed for uranium. The study concluded four major 

findings.

• In a spatially weighted analysis of groundwater 

samples collected from more than 2,000 private 

wells in bedrock aquifers in Connecticut, 3.9 percent 

of collected samples contained arsenic concentrations greater than the U.S. Environmental Protection 

Agency’s (EPA) maximum contaminant level (MCL) 

of 10 micrograms per liter (μg/L), and 4.7 percent of 

collected samples contained uranium concentrations 

greater than the EPA MCL of 30 μg/L. 

• Of the 2,433 water samples collected and analyzed 

from bedrock aquifers in Connecticut, 4.2 percent 

(102) contained arsenic concentrations at greater than 

10 μg/L, and of the 2,191 water samples collected and 

analyzed from bedrock aquifers in Connecticut, 5.4 

percent (118) contained uranium concentrations greater 

than 30 μg/L.

• Uranium concentrations greater than or equal to 1 μg/L 

are relatively ubiquitous across the State of Connecticut, with these concentrations present in 44.9 percent 

of the State, according to spatially weighted statewidescale proportion analysis.

• Of the 115 geologic units studied, 44 had at least one 

sample with arsenic or uranium concentrations that 

exceeded the respective constituent’s EPA MCL.

Introduction

Arsenic and uranium occurrence in groundwater used for 

drinking has been a concern owing to the potential health risks 

that can occur when concentrations of these constituents in 

drinking water meet or exceed established human health standards. Previous studies in New England (Ayotte and others, 

2003, 2006; Montgomery and others, 2003; Colman, 2011; 

Flanagan and others, 2014) have shown a strong association 

between geologic setting and arsenic and uranium concentrations in groundwater. A previous study (Flanagan and Brown, 

2017) examined arsenic and uranium concentrations in water 

samples collected from 674 wells completed in 81 of the 

156 geologic units in Connecticut and found that 7 percent 

of samples collected from 19 geologic units contained either 

arsenic or uranium concentrations exceeding the U.S. Environmental Protection Agency’s (EPA) maximum contaminant 

levels (MCLs) for drinking-water supplies of 10 micrograms 

per liter (μg/L) for arsenic or 30 μg/L for uranium. This study, 

completed by the U.S. Geological Survey (USGS) in cooperation with the Connecticut Department of Public Health (DPH) 

during 2013–18, expands on the 2017 study with the inclusion 

of additional samples and a focus on areas of the State with 

few or no samples. Additionally, this study provides a statewide spatially weighted assessment and updated bedrock geology occurrence information for arsenic and uranium across the 

State and supporting datasets (Gross, 2020).

Arsenic and Uranium Concentration 

Data Sources

The objective of this study was to analyze new data on 

arsenic and uranium concentrations of samples collected from 

private wells throughout areas of Connecticut that have been 

poorly represented in past studies (Flanagan and Brown, 2017) 

in order to fill data gaps across the State. Several towns in central Connecticut were omitted from sample collection efforts 

and the subsequent statewide-scale proportion assessment 

because these towns were assumed to be serviced primarily by 

public water supply and would contain no or few actively used 

domestic wells (fig. 1). Groundwater-quality data that were 

collected, geo-coded, and compiled by the DPH were used in 

the assessment. Sample collection methods are described in 

Flanagan and Brown (2017). In figure 1, well locations were 

offset by one-fourth mile to maintain the confidentiality of the 

well owner’s identity.
2 Arsenic and Uranium Occurrence in Private Wells in Connecticut, 2013–18

Base from Connecticut Department of Environment Protection, 2005

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central meridian 72.75° W, latitude of origin 40.83°

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Towns assumed to be serviced by

public water supply

Uranium concentrations, in

micrograms per liter

Less than 1

1 to 10

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D

Towns assumed to be serviced by

public water supply

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micrograms per liter

Less than 3

3 to 5

Greater than 5 to 10

Greater than 10

D

Figure 1. Concentrations of A, arsenic in water samples from 2,433 private wells and B, uranium in water samples from 2,191 private 

wells in Connecticut, 2013-18. Well locations have been offset by one-fourth mile to maintain the confidentiality of the well owner’s 

identity. [<, less than; >, greater than; μg/L, micrograms per liter]
Arsenic and Uranium Spatially Weighted Assessment 3

As part of the quality assurance plan for this study, the 

USGS submitted 11 standard reference samples to the DPH 

Laboratory with known concentrations of arsenic and uranium 

spanning the range of expected environmental concentrations. 

The standard reference samples were supplied by the USGS 

Branch of Quality Systems in Denver, Colorado, and provided 

an independent analysis of measurable bias. Results from the 

analysis of the standard reference samples (table 1) indicated 

no measurable bias, with all but two relative percent difference 

values less than 10 percent (Mueller and others, 2015).

Arsenic and Uranium Concentrations 

in the State

Arsenic concentrations ranged mostly (95th percentile) 

from less than 3 to 8.3 μg/L (table 2). Uranium concentrations 

ranged mostly (95th percentile) from less than 1 to 32 μg/L 

(table 2). Arsenic concentrations at or greater than the minimum reporting level (MRL) of 3 μg/L were measured in 248 

(10.2 percent) of 2,433 samples, whereas uranium concentrations at or greater than the MRL of 1 μg/L were measured 

in 1,016 (46.4 percent) of 2,191 samples (table 2). Without 

adjustment by spatial weighting, arsenic concentrations equal 

to or exceeding the MCL of 10 μg/L were measured in 102 

(4.2 percent) samples, and uranium concentrations equal to or 

exceeding the MCL of 30 μg/L were measured in 118 (5.4 percent) samples (table 2). 

Arsenic and Uranium Spatially 

Weighted Assessment

Areas across Connecticut have variable amounts of data 

representing concentrations of arsenic and uranium in water 

from private bedrock-aquifer wells (fig. 1). In some parts of 

the State, there are relatively few sample locations, whereas in 

other areas, sample locations are clustered (fig. 1). In order to 

delineate the occurrence and distribution of arsenic and uranium in private wells in an unbiased way, a spatially weighted 

assessment method was used to represent the proportion of 

affected areas across the State. The spatially weighted assessment describes the proportion of the State, excluding towns 

assumed to be served primarily by public water supply, that 

contains concentrations of a constituent (arsenic or uranium) 

greater than a specified threshold and is nondimensional and 

spatially unbiased (Belitz and others, 2015). This statistical approach was based on a set of randomized equal-area 

grid cells (see fig. 2); one grid was created for arsenic, with 

130 grid cells, and one was created for uranium, with 110 grid 

cells (Scott, 1990). Separate grids were created for arsenic and 

uranium so that the number of grid cells could be maximized 

and account for the differing amounts of samples available for 

each constituent. There are 242 more samples and 20 more 

grid cells for arsenic than uranium (tables 2 and 3). Each grid 

cell contains between 1 and 86 wells with sample concentrations for the arsenic set of 130 grid cells and between 1 and 

102 wells with sample concentrations for the uranium set of 

Table 1. Inventory of standard reference samples for arsenic and uranium concentrations, 2015–18.

[SRS, standard reference sample; #, number; μg/L, micrograms per liter; MPV, most probable value; DPH, Connecticut 

Department of Public Health Laboratory in Rocky Hill, Connecticut; RPD, relative percent difference; <, less than; NA, 

not applicable]

SRS lot # Submission 

date

Arsenic concentration, in μg/L Uranium concentration, in μg/L

MPV DPH RPD1 MPV DPH RPD1

T-219 6/30/2015 3.51 <3.0 NA 1.58 1.5 5.2

T-201 7/28/2015 24.4 23 5.9 9.22 9.2 0.2

T-201b 8/18/2015 24.4 23 5.9 9.22 9.1 1.3

T-217 8/31/2015 5.99 5.8 3.2 1.78 1.7 4.6

T-217B 9/29/2015 5.99 5.2 14.1 1.78 1.7 4.6

T-201c 9/29/2015 24.4 22 10.3 9.22 9.3 0.9

T-221a 9/3/2017 17.7 17 4.0 1.49 1.4 6.2

T-217a 9/11/2017 5.99 5.7 5.0 1.78 1.7 4.6

T-193a 10/3/2017 3.44 3.2 7.2 1.7 1.7 0

T-201b 10/24/2017 24.4 24 1.7 9.22 9.2 0.2

T-217b 4/30/2018 5.99 5.5 8.5 1.78 1.8 1.1

1RPD = [|(Sample 1 – Sample 2)| / Average (Sample 1 + Sample 2)] × 100, where sample 1 is the MPV value and 

sample 2 is the DPH value.
4 Arsenic and Uranium Occurrence in Private Wells in Connecticut, 2013–18

0'

Arsenic greater than 3 micrograms per liter Uranium greater than 1 microgram per liter

Uranium greater than 10 micrograms per liter

Uranium greater than 30 micrograms per liter

Arsenic greater than 5 micrograms per liter

Arsenic greater than 10 micrograms per liter

Base from Connecticut Department of Environment Protection, 2005

1:24,000 Lambert Conformal Conic projection: standard parallels 41.20° N and 41.87° N,

central meridian 72.75° W, latitude of origin 40.83°

0 10 20 MILES

0 10 20 KILOMETERS

73° 72°30' 72°

42°

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EXPLANATION

Towns assumed to be serviced by

public water supply

Proportion of wells in grid cell with values

above noted concentration, in percent

Less than 1

 1 to 10

Greater than 10 to 20

Greater than 20 to 30

Greater than 30

Figure 2. Spatially weighted statewide-scale proportion of wells within grid cells for arsenic and uranium.
Arsenic and Uranium Spatially Weighted Assessment 5

West Mountains in West Simsbury, Connecticut. Photo by Tiziana Shea.

110 grid cells. For each constituent, spatially weighted, cellwide proportions were determined by computing the proportion of wells with sample concentrations exceeding specific 

thresholds for each grid cell (Belitz and others, 2010). The 

specific thresholds include greater than 3 μg/L, greater than 

5 μg/L, and greater than 10 μg/L for arsenic and greater than 

1 μg/L, greater than 10 μg/L, and greater than 30 μg/L for 

uranium, representing low, moderate, and high value thresholds, respectively, for each constituent (fig. 2). The calculated 

cell proportions for low, moderate, and high value thresholds 

for arsenic and uranium were then averaged across the State 

to determine one spatially weighted statewide-scale proportion value for each of the three thresholds associated with each 

constituent (table 3; Belitz and others, 2010).

The spatially distributed, randomized equal-area grid 

approach yields a view of groundwater quality in which all 

areas are weighted equally, and regions with a high density 

of groundwater use or with high density of potential arsenic 

or uranium in groundwater are not preferentially represented 

(Belitz and others, 2010; Scott, 1990). Arsenic was present at 

high concentrations (greater than 10 μg/L) in 3.9 percent of 

the State of Connecticut, whereas uranium was present at high 

concentrations (greater than 30 μg/L) in 4.7 percent of the 

State (table 3). Figure 2 illustrates that low uranium concentrations (greater than 1 μg/L) are relatively ubiquitous across 

the State of Connecticut, with these concentrations present in 

44.9 percent of the State (table 3), and account for greater than 

30 percent of the proportion of wells in most grid cells (fig. 2). 

Low arsenic concentrations (greater than 3 μg/L) were not as 

common or widespread (fig. 2) as that for uranium concentrations, perhaps in part, owing to the higher reporting level 

(table 2).

Table 2. Arsenic concentrations in water samples from 2,433 private wells and uranium concentrations in water samples from 

2,191 private wells in Connecticut, 2013–18.

[MRL, minimum reporting level; ≥, greater than or equal to; μg/L, micrograms per liter; <, less than; >, greater than; Min., Minimum; Max., Maximum; 

MCL, maximum contaminant level]

Constituent Number of 

samples

MRL, 

in 

μg/L

Number of wells 

with concentration 

≥ MRL (percent)

Concentration, in μg/L MCL, 

in 

μg/L

Number of wells 

with concentration 

> MCL (percent)

Percentile

Min. 50 75 90 95 Max.

Arsenic 2,433 3 248 (10.2) < 3.0 < 3.0 < 3.0 3 8.3 1,650 10 102 (4.2)

Uranium 2,191 1 1,016 (46.4) < 1.0 < 1.0 3.6 12 32 3,170 30 118 (5.4)

Table 3. Spatially weighted statewide-scale proportions for 

arsenic and uranium during 2013–18 from the Connecticut 

Department of Public Health database. 

[>, greater than; μg/L, micrograms per liter]

Number 

of grid 

cells

Spatially weighted statewide-scale proportion

Low values

(percent)

Moderate values

(percent)

High values

(percent)

Arsenic

130 >3 μg/L

(8.7)

>5 μg/L

(6.6)

>10 μg/L

(3.9)

Uranium

110 >1 μg/L

(44.9)

>10 μg/L

(10.1)

>30 μg/L

(4.7)
6 Arsenic and Uranium Occurrence in Private Wells in Connecticut, 2013–18

Penwood State Park in Simsbury, Connecticut. Photo by Tiziana Shea.

Arsenic and Uranium Occurrence in 

Relation to Bedrock Geology

For this study, it was assumed that each sampled well was 

drilled and completed in the mapped bedrock geologic unit 

(referred to as “geologic unit” in this report) represented at the 

well’s location, as identified on the bedrock geological map 

of Connecticut (Rodgers, 1985; Connecticut Department of 

Environmental Protection, 2000). Corresponding arsenic and 

uranium samples were grouped according to the geologic units 

in which the sampled wells were located (table 4). 

Of the 156 geologic units in the State, 115 units (covering 

98.1 percent of the land area) were represented by at least one 

water sample analyzed for arsenic and (or) uranium (table 4). 

Of the 115 geologic units represented, 21 geologic units had 

only 1 arsenic sample, 45 geologic units had 2 to 10 arsenic 

samples, and 49 geologic units had more than 10 arsenic 

samples. Of the 113 geologic units with uranium samples, 20 

geologic units had only 1 uranium sample, 44 geologic units 

had 2 to 10 uranium samples, and 49 geologic units had more 

than 10 uranium samples. The 115 geologic units with samples 

were organized under 11 different major categories of bedrock, which are listed in table 4 and can be viewed in figure 

1 of Flanagan and Brown (2017). These major categories of 

bedrock are based on groups of individual geologic units with 

similar geochemical and lithological properties (Robinson 

and Kapo, 2003). The percentage of arsenic and uranium 

samples from each geologic unit containing concentrations 

exceeding MCLs was computed, and geologic units were 

grouped and shaded on the basis of percentage ranges (fig. 3). 

MCL exceedance percentages computed for geologic units for 

this study provide new and updated information on arsenic 

and uranium concentrations in Connecticut groundwater; 

however, they may not represent the actual hazard for existing 

and future wells in geologic units. Some of the local geologic 

maps for New England have been revised since the publication of the Rodgers (1985) geological map compilation and 

the Robinson and Kapo (2003) lithogeochemical map (M.A. 

Thomas, Connecticut Geological Survey, written commun., 

2019), but these revisions are not reflected in the geological 

map versions used in this study. In figure 3, well locations are 

offset by one-fourth mile to maintain the confidentiality of the 

well owner’s identity.

The results show that geologic units had widely differing 

rates of arsenic and uranium concentrations exceeding MCLs 

(table 4). Twenty-two of 115 geologic units had at least one 

sample with arsenic concentrations that exceeded the MCL 

of 10 μg/L, and 32 of 113 geologic units had at least one 

sample with uranium concentrations that exceeded the MCL of 

30 μg/L. The pelitic rocks category had 7 (out of 28) geologic 

units with at least one arsenic concentration that exceeded 

the MCL of 10 μg/L and 8 geologic units with at least one 

uranium concentration that exceeded the MCL of 30 μg/L. 

The mafic rocks category contained 10 (out of 25) geologic 

units with at least one uranium concentration that exceeded 

the MCL.
Arsenic and Uranium Occurrence in Relation to Bedrock Geology 7

Table 4. Arsenic and uranium concentrations that exceed U.S. Environmental Protection Agency’s maximum contaminant levels in 

samples from private wells in Connecticut, by geologic unit and major bedrock category, 2013–18.

[Geologic unit names are the Connecticut Department of Environmental Protection preferred names as modified from Rodgers (1985). Major categories 

of bedrock (subheadings) are modified from Robinson and Kapo (2003). Color shadings indicate the percentage of wells with exceedances greater than 

concentration thresholds in ranges of: , no data; , less than (<) 1 percent; , 1 to 10 percent; , greater than (>) 10 to 20 percent; , greater than (>) 20 to 

30 percent; and , greater than (>) 30 percent. MCL, U.S. Environmental Protection Agency maximum contaminant level enforceable for public water supplies; 

μg/L, micrograms per liter; NA, not available; −, no uranium samples from geologic unit]

Geologic unit name1 Geologic 

unit code1

Number of samples

Percentage of water 

samples with concentrations 

exceeding MCL, in μg/L

Percentage 

of study area 

underlain by 

geologic unit2 Arsenic Uranium Arsenic 

>10 μg/L

Uranium 

>30 μg/L

Avalon Granites

"Scituate" Granite Gneiss Zss 16 16 0 0 0.7

Hope Valley Alaskite Gneiss Zsh 30 24 3.3 0 2.3

Mamacoke Formation Zwm 6 6 0 16.7 0.8

Plainfield Formation Zp 25 24 0 0 1.6

Plainfield Formation, Stony Creek Granite Gneiss and 

Narragansett Pier Granite undivided Zp+Zsc+Pn 10 11 10 0 <0.2

Ponaganset Gneiss Zsp 3 3 0 0 0.3

Porphyritic phase of Potter Hill 

Granite Gneiss Zspp 4 5 0 0 <0.2

Potter Hill Granite Gneiss Zsph 23 21 0 0 1.5

Potter Hill Granite Gneiss and Narragansett Pier Granite 

undivided Zsph+Pn 1 1 0 0 <0.2

Rope Ferry Gneiss Zwr 18 18 0 0 1.2

Stony Creek Granite Gneiss and Narragansett Pier 

Granite undivided Zsc+Pn 3 2 0 0 <0.2

Basalt

Buttress Dolerite Jb 1 1 0 0 0.2

Hampden Basalt Jha 3 3 0 33.3 0.3

Holyoke Basalt Jho 5 5 0 0 1.2

Talcott Basalt Jta 1 1 0 0 0.2

Calcgranofels

Fly Pond (Calc-Silicate) Member of Tatnic Hill 

Formation Otaf 21 19 0 0 0.5

Hawley Formation (Carbonaceous Schist Facies) Ohc 3 3 0 0 <0.2

Hebron Gneiss SOh 290 199 13.4 4.5 5.2

Lower Member of Bigelow Brook Formation SObl 25 25 0 0 0.7

Southbridge Formation SOs? 6 6 0 0 0.3

Southbridge Formation SOs 60 58 0 0 0.8

Carbonate-Bearing Metasedimentary Rocks

Basal Marble Member of Walloomsac Schist Owm 11 11 9.1 0 0.6

Stockbridge Marble OCs 14 13 7.1 0 0.9

Unit a of Stockbridge Marble Csa 1 1 0 0 <0.2

Unit b of Stockbridge Marble Csb 9 9 0 0 0.4

Unit c of Stockbridge Marble Csc 9 9 0 0 0.6

Units e and d of Stockbridge Marble Ose 2 2 0 0 <0.2
8 Arsenic and Uranium Occurrence in Private Wells in Connecticut, 2013–18

Table 4. Arsenic and uranium concentrations that exceed U.S. Environmental Protection Agency’s maximum contaminant levels in 

samples from private wells in Connecticut, by geologic unit and major bedrock category, 2013–18.—Continued

[Geologic unit names are the Connecticut Department of Environmental Protection preferred names as modified from Rodgers (1985). Major categories 

of bedrock (subheadings) are modified from Robinson and Kapo (2003). Color shadings indicate the percentage of wells with exceedances greater than 

concentration thresholds in ranges of: , no data; , less than (<) 1 percent; , 1 to 10 percent; , greater than (>) 10 to 20 percent; , greater than (>) 20 to 

30 percent; and , greater than (>) 30 percent. MCL, U.S. Environmental Protection Agency maximum contaminant level enforceable for public water supplies; 

μg/L, micrograms per liter; NA, not available; −, no uranium samples from geologic unit]

Geologic unit name1 Geologic 

unit code1

Number of samples

Percentage of water 

samples with concentrations 

exceeding MCL, in μg/L

Percentage 

of study area 

underlain by 

geologic unit2 Arsenic Uranium Arsenic 

>10 μg/L

Uranium 

>30 μg/L

Granite, other

Canterbury Gneiss Dc 49 48 0 4.2 1.3

“Eastford Gneiss phase” of Canterbury Gneiss Dce 27 27 3.7 25.9 0.6

Felsic Gneiss Member of Quinebaug Formation Oqf 3 3 0 0 <0.2

Glastonbury Gneiss Ogl 57 41 1.8 14.6 1.8

Maromas Granite Gneiss Dm 1 1 0 0 <0.2

Nonewaug Granite Dng 19 19 0 10.5 0.6

Ordovician? Granitic Gneiss Og 106 86 14.2 25.6 2.3

Porphyritic Member of Southbridge Formation SOsp 2 2 0 0 <0.2

Trap Falls Formation and Ordovician? Granitic Gneiss 

undivided Otf+Og 5 11 0 9.1 1.1

Waterbury Gneiss Cwb 11 10 0 0 0.9

Waterford Group, Stony Creek Granite Gneiss and 

Narragansett Pier Granite undivided Zw+Zsc+Pn 2 2 0 0 <0.2

Grenville Granites

Augen Gneiss Yga 4 4 0 0 <0.2

Gneiss of Highlands Masifs Yg 17 17 0 5.9 0.7

Pink Granitic Gneiss Ygr 32 33 0 3.0 1.2

Mafic Rocks

Amphibolite unit in Rutlum Mountain Schist Ora 2 2 0 0 <0.2

Amphibolite-Bearing unit of Manhattan Schist Cma 8 8 0 0 0.5

Beardsley Member of Harrison Gneiss Ohb 9 9 0 0 0.5

Black Hill Member of Quinebaug Formation Oqb 2 3 0 0 <0.2

Brookfield Gneiss Ob 78 78 5.1 12.8 1.4

Dioritic phase of Lebanon Gabbro Dld 9 6 0 0 <0.2

Gneiss (Metavolcanic) Member of Brimfield Schist Obrg 8 8 0 0 0.3

Harrison Gneiss Oh 7 15 14.3 46.7 0.9

Hornblende Gneiss and Amphibolite Ygh 27 27 0 0 0.8

Hornblende Gneiss Member of Collinsville Formation Ocg 8 8 0 12.5 0.4

Lebanon Gabbro Dl 15 8 13.3 0 0.3

Litchfield Norite Ol 1 1 0 100.0 <0.2

Lower Member of Middletown Formation Oml 5 12 0 25.0 0.2

Massive Mafic Rock in Middletown Formation Omm 1 1 0 0 <0.2

Middletown Formation Om 38 45 0 4.4 1.1

Monson Gneiss Omo 115 94 0 8.5 2.6
Arsenic and Uranium Occurrence in Relation to Bedrock Geology 9

Table 4. Arsenic and uranium concentrations that exceed U.S. Environmental Protection Agency’s maximum contaminant levels in 

samples from private wells in Connecticut, by geologic unit and major bedrock category, 2013–18.—Continued

[Geologic unit names are the Connecticut Department of Environmental Protection preferred names as modified from Rodgers (1985). Major categories 

of bedrock (subheadings) are modified from Robinson and Kapo (2003). Color shadings indicate the percentage of wells with exceedances greater than 

concentration thresholds in ranges of: , no data; , less than (<) 1 percent; , 1 to 10 percent; , greater than (>) 10 to 20 percent; , greater than (>) 20 to 

30 percent; and , greater than (>) 30 percent. MCL, U.S. Environmental Protection Agency maximum contaminant level enforceable for public water supplies; 

μg/L, micrograms per liter; NA, not available; −, no uranium samples from geologic unit]

Geologic unit name1 Geologic 

unit code1

Number of samples

Percentage of water 

samples with concentrations 

exceeding MCL, in μg/L

Percentage 

of study area 

underlain by 

geologic unit2 Arsenic Uranium Arsenic 

>10 μg/L

Uranium 

>30 μg/L

Mafic Rocks—Continued

Mylonite along Paleozoic Faults Pzmy 3 3 0 0 <0.2

Preston Gabbro Op 1 1 0 0 0.2

Pumpkin Ground Member of Harrison Gneiss Ohp 4 4 0 0 0.5

Quinebaug Formation Oq 89 76 1.1 0 2.0

Rowe Schist OCr? 2 2 0 0 <0.2

Rowe Schist OCr 54 55 1.9 1.8 1.9

Rowe Schist and Amphibolite unit in Rowe Schist 

undivided OCr+OCra 2 2 0 0 <0.2

Sweetheart Mountain Member of Collinsville Formation Ocs 2 2 0 50.0 <0.2

Waterford Group Zw 36 30 0 3.3 0.9

Mesozoic Basin Sediments

East Berlin Formation Jeb 14 14 0 0 1.3

New Haven Arkose TRnh 61 59 0 1.7 6.3

Portland Arkose Jp 57 45 8.8 0 8.0

Shuttle Meadow Formation Jsm 14 13 0 0 0.5

Metamorphic Rocks, other

Bristol Gneiss Obs 3 3 0 0 0.4

Clough Quartzite Sbc 1 1 0 0 <0.2

Dalton Formation Cd 13 12 0 8.3 0.7

Fitch Formation Sbf 1 1 0 0 <0.2

Layered Gneiss Ygn 36 37 0 0 1.9

New London Gneiss Zwn 1 1 0 0 0.4

Quartzite unit in Plainfield Formation Zpq 14 14 0 0 0.6

Pelitic Rocks

Basal Member of Taine Mountain Formation 

around Waterbury Dome Otb 3 3 0 0 0.4

Cobble Mountain Formation Ocm 1 1 0 0 0.4

Collinsville Formation Oc 33 32 12.1 3.1 1.1

Everett Schist Ce 1 1 0 0 0.2

Golden Hill Schist Ogh 3 3 0 0 0.3

Hoosac Schist Ch 40 40 0 20.0 1.1

Littleton Formation Dbl 7 7 0 0 0.4

Manhattan Schist Cm 37 37 0 8.1 2.0

Mount Pisgah Member of Littleton Formation Dblm 4 3 0 0 <0.2

Oronoque Schist Oo 1 1 0 0 0.5
10 Arsenic and Uranium Occurrence in Private Wells in Connecticut, 2013–18

Table 4. Arsenic and uranium concentrations that exceed U.S. Environmental Protection Agency’s maximum contaminant levels in 

samples from private wells in Connecticut, by geologic unit and major bedrock category, 2013–18.—Continued

[Geologic unit names are the Connecticut Department of Environmental Protection preferred names as modified from Rodgers (1985). Major categories 

of bedrock (subheadings) are modified from Robinson and Kapo (2003). Color shadings indicate the percentage of wells with exceedances greater than 

concentration thresholds in ranges of: , no data; , less than (<) 1 percent; , 1 to 10 percent; , greater than (>) 10 to 20 percent; , greater than (>) 20 to 

30 percent; and , greater than (>) 30 percent. MCL, U.S. Environmental Protection Agency maximum contaminant level enforceable for public water supplies; 

μg/L, micrograms per liter; NA, not available; −, no uranium samples from geologic unit]

Geologic unit name1 Geologic 

unit code1

Number of samples

Percentage of water 

samples with concentrations 

exceeding MCL, in μg/L

Percentage 

of study area 

underlain by 

geologic unit2 Arsenic Uranium Arsenic 

>10 μg/L

Uranium 

>30 μg/L

Pelitic Rocks—Continued

Ratlum Mountain Schist Or? 12 6 8.3 0 0.3

Ratlum Mountain Schist Or 114 104 0 9.6 4.2

Ratlum Mountain Schist and Amphibolite unit 

in Rutlum Mountain Schist undivided Or+Ora 1 1 0 0 <0.2

Schist and Granulite Member of Trap Falls Formation Otfg 16 13 0 0 0.5

Scotland Schist DSs 96 87 16.7 1.1 1.0

Scranton Mountain Member of Taine Mountain 

Formation Ots 1 1 0 0 <0.2

Shelton (White Gneiss) Member of Trap Falls Formation Otfs 3 3 0 0 0.3

Southington Mountain Member of The Straits Schist DSts 3 3 0 0 0.4

Taine Mountain and Collinsville Formation undivided Ot+Oc 6 6 16.7 33.3 0.3

Taine Mountain Formation Ot 20 20 5.0 0 0.9

Tatnic Hill Formation Ota 80 68 0 1.5 3.1

The Straits Schist DSt 24 24 12.5 0 2.1

Trap Falls Formation Otf 3 − 0 − 0.3

Upper Member of Middletown Formation Omu 20 9 0 11.1 0.2

Walloomsac Schist Ow 11 11 0 0 0.5

Wepawaug Schist DSw 10 8 10.0 0 0.8

Whigville Member of Taine Mountain Formation Otwv 1 1 0 0 <0.2

Yantic Member of Tatnic Hill Formation Otay 40 36 0 0 0.8

Sulfidic Schists

Basal Member of upper slice of Canaan Mountain Schist Cmcub 1 1 0 0 <0.2

Brimfield Schist Obr? 6 6 0 0 <0.2

Brimfield Schist Obr 113 111 0 0 3.4

Carringtons Pond Member of Trap Falls Formation Otfc 3 3 33.3 0 0.5

Collins Hill Formation Och 23 22 0 4.5 0.8

lower slice of Canaan Mountain Schist Cmcl 1 − 0 − <0.2

Rusty Mica Schist and Gneiss Ygs 8 8 0 0 1.0

Upper Member of Bigelow Brook Formation SObu 9 9 0 0 0.3

upper slice of Canaan Mountain Schist Cmcu 1 1 0 0 0.4

Unmapped Areas l 1 1 0 0 1.1

Overall for the Study Area NA 2,433 2,191 4.4 5.4 98.1

1

Geologic units from Connecticut Department of Environmental Protection, 2000; names may not conform to usage by the U.S. Geological Survey. Queries 

indicate uncertainty about geologic unit identification.

2

About 1.9 percent of the study area (State of Connecticut) was underlain by 41 individual geologic units from which no water samples were collected.
Arsenic and Uranium Occurrence in Relation to Bedrock Geology 11

Base from Connecticut Department of Environment Protection, 2005

1:24,000 Lambert Conformal Conic projection: standard parallels 41.20° N and 41.87° N,

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73° 72°30' 72°

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A. Percentage of wells, by grouped geologic units with arsenic concentrations greater than 10 micrograms per liter (MCL)

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Range in percentages of wells, or grouped

geologic units, in which arsenic

concentrations are greater than

10 micrograms per liter (MCL)—

Individual geologic unit boundaries

within a group are not shown. White

areas within the map, representing

1.9 percent of the State, indicate

geologic units from which no arsenic

samples are available.

Less than 1

1 to 10

Greater than 10 to 20

Greater than 20 to 30

Greater than 30

Arsenic concentration in sampled well,

in micrograms per liter

Less than 3

3 to 5

Greater than 5 to 10

Greater than 10

Range in percentages of wells, or grouped

geologic units, in which uranium

concentrations are greater than

30 micrograms per liter (MCL)—

Individual geologic unit boundaries

within a group are not shown. White

areas within the map, representing

2.3 percent of the State, indicate

geologic units from which no uranium

samples are available.

 Less than 1

1 to 10

Greater than 10 to 20

Greater than 20 to 30

Greater than 30

Uranium concentration in sampled well,

in micrograms per liter

Less than 1

1 to 10

Greater than 10 to 30

Greater than 30

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Figure 3. Spatial distribution and range of percentages of private wells in Connecticut, grouped by geologic units, in which A, arsenic 

concentrations exceeded the maximum concentration level (MCL) of 10 micrograms per liter (μg/L), and B, uranium concentrations 

exceeded the MCL of 30 μg/L. Well locations have been offset by one-fourth mile to maintain the confidentiality of the well owner’s 

identity. Geologic units are listed in table 4. See Rodgers (1985) for the location and description of individual geologic units.
12 Arsenic and Uranium Occurrence in Private Wells in Connecticut, 2013–18

Kent Falls State Park in Kent, Connecticut. Photo by Tiziana Shea.

The Carringtons Pond Member of Trap Falls Formation was the only geologic unit with greater than 30 percent 

occurrence of arsenic concentrations (1 out of 3 samples) 

exceeding the MCL of 10 μg/L (table 4). Five geologic units 

(Hampden Basalt, Harrison Gneiss, Litchfield Norite, Sweetheart Mountain Member of Collinsville Formation, and Taine 

Mountain and Collinsville Formation undivided) had greater 

than 30 percent of uranium concentrations (1 of 3, 7 of 15, 

1 of 1, 1 of 2, and 2 of 6 samples, respectively) exceeding the 

MCL of 30 μg/L.

The well containing the highest measured arsenic concentration (1,650 μg/L) in this study is completed in the Basal 

marble member of Walloomsac Schist geologic unit of the 

carbonate-bearing metasedimentary rocks bedrock category. 

Of the 11 samples in this geologic unit, this was the only well 

containing an arsenic concentration exceeding the MCL of 

10 μg/L. Eleven geologic units (table 4) in the State had a 

higher percentage (greater than 9.1 percent) of samples with 

arsenic exceeding 10 μg/L than the Basal marble member of 

Walloomsac Schist geologic unit. The well with the highest 

uranium concentration (3,170 μg/L) is completed in the Harrison Gneiss geologic unit of the mafic rocks bedrock category. 

This geologic unit had 15 samples, and 46.7 percent of the 

samples had uranium concentrations exceeding the MCL of 

30 μg/L. Two other geologic units (table 4) in the State had a 

higher percentage (greater than 46.7 percent) of samples with 

uranium exceeding the MCL. These findings demonstrate that 

wells with arsenic or uranium concentrations exceeding their 

respective MCLs were not always in the same geologic unit.

The findings of this study also indicate that, for a household in Connecticut relying on a private well for drinking 

water, the likelihood of the water containing an arsenic or uranium concentration exceeding the constituent’s MCL is related 

to the particular geologic unit in which the household’s well 

is completed. Other factors that can affect arsenic or uranium 

concentrations are geochemical conditions or residence time 

for water-rock reactions in the local groundwater flow system. 

Testing well water is the only way to know whether a contaminant is present. For more information about well testing and 

treatment guidelines in Connecticut, readers should contact 

the Connecticut Department of Public Health Private Well 

Program using the information provided at http://www.ct.gov/

dph/privatewells.
References Cited 13

References Cited

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For additional information, contact::

Director, Pennsylvania Water Science Center

U.S. Geological Survey

215 Limekiln Road

New Cumberland, PA 17070

or visit our website at: https://www.usgs.gov/centers/pa-water

Publishing support provided by the 

West Trenton Publishing Service Center
Gross and others—Arsenic and Uranium Occurrence in Private Wells in Connecticut, 2013–18—Open-File Report 2020–1111, ver 1.1

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20201111