Prepared in cooperation with the Connecticut Department of Public Health

Arsenic and Uranium in Private Wells in Connecticut, 

2013–15

Introduction

The occurrence of arsenic and uranium in groundwater at 

concentrations that exceed drinking-water standards is a concern 

because of the potential adverse effects on human health. Some 

early studies of arsenic occurrence in groundwater considered 

anthropogenic causes, but more recent studies have focused on 

sources of naturally occurring arsenic to groundwater, such as 

minerals within aquifer materials that are in contact with groundwater. Arsenic and uranium in groundwater in New England 

have been shown to have a strong association to the geologic 

setting (Ayotte and others, 2003, 2006) and nearby streambed 

sediment concentrations (Robinson and Ayotte, 2006). In New 

Hampshire and Massachusetts, arsenic and uranium concentrations greater than human-health benchmarks have shown distinct 

spatial patterns when related to the bedrock units mapped at 

the local scale (Montgomery and others, 2003; Colman, 2011; 

Flanagan and others, 2014).

The Connecticut Department of Public Health (DPH; 2016) 

reported that there are about 322,600 private wells in Connecticut serving approximately 823,000 people, or 23 percent of 

the State’s population. The State does not require that existing 

private wells be routinely tested for arsenic, uranium, or other 

contaminants; consequently, private wells are only sampled at 

the well owner’s discretion or when they are newly constructed. 

The U.S. Geological Survey (USGS), in cooperation with the 

DPH, completed an assessment in 2016 on the distribution of 

concentrations of arsenic and uranium in groundwater from bedrock in Connecticut (fig. 1). This report presents the major findings for arsenic and uranium concentrations from water samples 

collected from 2013 to 2015 from private wells. 

Sources of Data on Arsenic and Uranium 

Concentrations

The main objective of this study was to compile and 

analyze arsenic and uranium concentrations from private wells 

throughout Connecticut (fig. 1). In cooperation with the DPH, 

local health departments and districts used volunteers to visit 

randomly selected houses in their districts and collect an unfiltered water sample from an untreated source in the home. Staff 

from DPH also distributed bottles to homeowners at four agricultural fairs in Bethlehem, Durham, Goshen, and Woodstock; 

at water fair events in East Hampton and Colchester; and one 

home show in Hartford. These homeowners were instructed to 

collect an unfiltered water sample prior to any existing treatment 

system. The water samples, collected from 2013 to 2015, were 

submitted to the DPH Laboratory in Hartford for trace-element 

analysis according to EPA method 200.8 (U.S. Environmental 

Protection Agency, 1994). 

The towns of Stamford, Weston, and Wilton have substantially more water samples and wells than the other studied towns. On average, 7 wells were sampled in most towns, 

whereas, 81 wells were sampled in Wilton; 110 wells, in Weston; 

and 732 wells, in Stamford, resulting in an unbalanced distribution of water samples in the State. To balance the distribution for 

the purposes of this study, 7 to 14 wells from Stamford, Weston, 

and Wilton were randomly selected for inclusion in the statewide 

dataset. This subset of samples represented less than 5 percent 

of the final statewide dataset, which consisted of 660 arsenic 

samples and 589 uranium samples collected from 674 wells. 

Data for these 674 wells are available in Flanagan and Brown 

(2017). All available data from these three towns are discussed 

in the “Comparison of Arsenic and Uranium Exceedance Rates 

in Three Towns” section.

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

USGS submitted six standard reference samples to the DPH 

Laboratory with concentrations of arsenic and uranium spanning the range of expected environmental values. These samples 

were supplied by the USGS Branch of Quality Systems (U.S. 

Geological Survey, 2016) in Denver, Colo., and provided an 

independent analysis of measurable bias. Results from the 

analysis of the standard reference samples (table 1) indicated 

no measurable bias.

Major Findings

• Nearly 1 out of 15 (7 percent) water samples from 674 private wells tested in Connecticut contained either arsenic or 

uranium at concentrations that exceed the U.S. Environmental Protection Agency’s (EPA) maximum contaminant levels 

(MCLs) enforceable for drinking-water supplies of 10 micrograms per liter (μg/L) for arsenic or 30 μg/L for uranium.

• Of the 81 geologic units studied, 19 had at least one sample with arsenic or uranium concentrations that exceeded the MCL.

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2017–1046

May 2017
pem16-1082_flanagan_fig01

Figure 1. Locations of major bedrock categories in Connecticut and areas where the majority of sampled wells are located.

The major bedrock categories are described in detail in Robinson and Kapo (2003). 

Calcpelite

Avalon granites

Basalt

Calcgranofels

Carbonate-bearing

metasedimentary rocks

Granite, other

Grenville granites

Mafic rocks

Mesozoic basin sediments

Metamorphic rocks, other

Pelitic rocks

Peraluminous granites

Sulfidic schists

Ultramafic rocks

EXPLANATION

Bedrock category

Approximate area where majority

of sampled wells are located

Long Island Sound

73°30' 73° 72°30' 72°

42°

41°30'

41°

Town boundaries from U.S. Geological Survey digital data, 1:24,000, 1968–88

Lambert Conformal Conic projection

Central meridian –72.75°W

0 10 15

15

5 20 MILES

0 5 10 20 KILOMETERS

0 100 KILOMETERS

0 100 MILES

Study 

area

ME

VT

NH

MA

CT RI

Stamford

Wilton

Weston

Woodstock

Goshen

Hartford

Bethlehem

Colchester

East Hampton

Brookfield

Durham

Oxford

Figure 1. Locations of major categories of bedrock in Connecticut and areas where the majority of sampled wells are located. Modified 

bedrock categories are described in Robinson and Kapo (2003).

Arsenic and Uranium Concentrations in the State

Arsenic concentrations ranged mostly (95th percentile) 

from less than (<) 3 to 7.1 micrograms per liter (μg/L). Uranium concentrations ranged mostly (95th percentile) from <1 

to 23 μg/L (table 2). Arsenic at concentrations at or above the 

minimum reporting level (MRL) of 3 μg/L was measured in 9.1 

percent of samples. Uranium at concentrations at or above the 

MRL of 1 μg/L was measured in 42.1 percent of the samples. 

Statewide, 3.6 percent of samples had arsenic concentrations that 

exceeded the MCL of 10 μg/L, and 3.9 percent had uranium concentrations that exceeded the MCL of 30 μg/L (table 2). Overall, 

about 7 percent of the samples had concentrations of arsenic or 

uranium that exceeded an MCL.

Arsenic and Uranium Occurrence in Relation to 

Bedrock Geology

There are distinct areas in Connecticut where one or more 

groundwater samples have arsenic and uranium concentrations 

greater than their respective MCLs (fig. 2). Data were grouped 

in relation to mapped bedrock units (referred to as geologic 

units in this report) identified on the bedrock geological map 

of Connecticut (Rodgers, 1985). It was assumed that each well 

was drilled and completed in the geologic unit represented at the 

Table 1. Inventory of standard reference samples for arsenic 

and uranium concentrations, in 2015.

[SRS, standard reference sample; #, number; μg/L, microgram per liter; 

MPV, most probable value; DPH, Connecticut Department of Public Health 

Laboratory in Hartford, Conn.; <, 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 <3.00 na 1.58 1.50 5.2

T–201 7/28/2015 24.4 23.0 5.9 9.22 9.20 0.2

T–201b 8/18/2015 24.4 23.0 5.9 9.22 9.10 1.3

T–217A 8/31/2015 5.99 5.80 3.2 1.78 1.70 4.6

T–217B 9/29/2015 5.99 5.20 14.1 1.78 1.70 4.6

T–201c 9/29/2015 24.4 22.0 10.3 9.22 9.30 0.8

1

Relative percent difference = [|(Sample 1 – Sample 2)| / Average 

(Sample 1 + Sample 2)] × 100, where sample 1 is the MPV value and 

sample 2 is the DPH value.
well’s location on the geologic map. Geologic units are rocks 

of a specific geologic age that have unique mineral and physical characteristics, varying degrees of resistance to weathering, 

and similar processes of rock formation. Arsenic and uranium 

samples were grouped according to the geologic units in which 

the sampled wells are located (table 3).

Of the 156 geologic units in the State, 81 units (covering 82.6 percent of the land area), were represented by at least 

one water sample analyzed for arsenic and (or) uranium (table 

3). Twenty-one geologic units had only 1 sample, 43 geologic 

units had 2 to 10 samples, and 17 geologic units had more than 

10 samples. The 81 geologic units were organized under 10 

different bedrock categories and are listed in table 3. These 

bedrock categories (fig. 1) are based on groups of individual 

geologic units with similar geochemical and lithological properties (Robinson and Kapo, 2003). The percentage of samples in 

each geologic unit with arsenic and uranium concentrations that 

exceeded MCLs was computed and then geologic units were 

grouped and colored based on percentage ranges (fig. 2). The 

exceedance percentages for geologic units computed for this 

study may not represent the actual hazard for existing and future 

wells in these units. Nonetheless, this report provides new information on arsenic and uranium contamination at the State scale.

Results of this analysis indicate that the geologic units 

were markedly different in terms of arsenic and uranium concentrations that exceeded MCLs (table 3). Nine of 81 geologic 

units had at least one sample with arsenic concentrations that 

exceeded the MCL of 10 μg/L. Fourteen geologic units had at 

least one sample with uranium concentrations that exceeded the 

MCL of 30 μg/L. None of the geologic units in the carbonatebearing metasedimentary rocks, Grenville granites, mafic 

rocks, or metamorphic rocks, other bedrock categories had 

arsenic concentrations that exceeded the MCL (table 3). None 

of the geologic units in the Avalon granites, carbonate-bearing 

metasedimentary rocks, Grenville granites, Mesozoic basin 

sediments, or sulfidic schists bedrock categories had uranium 

concentrations that exceeded the MCL. The pelitic rocks bedrock category had three geologic units with at least one arsenic 

concentration that exceeded the MCL, and the granite, other 

bedrock category had seven geologic units with at least one uranium concentration that exceeded the MCL—the most among 

the 10 bedrock categories.

In the greater than (>) 20 to 30 percent group (table 3), the 

Taine Mountain and Collinsville Formation undivided (4 samples) and the Glastonbury Gneiss (20 samples) were the only 

geologic units with arsenic and (or) uranium concentrations 

that exceeded MCLs. In the >30 percent group, the Carringtons 

Pond Member of the Trap Falls Formation (3 samples) was the 

only geologic unit with concentrations that exceeded the MCL 

for arsenic, and the Dalton Formation and the Harrison Gneiss 

(3 samples each) were the only geologic units with concentrations that exceeded the MCL for uranium (table 3).

The well with the highest arsenic concentration (470 μg/L), 

from the town of Oxford, is completed in The Straits Schist geologic unit of the Pelitic rocks bedrock category. Although 8.3 

percent of the 12 samples in this geologic unit also have high 

arsenic (>10 μg/L), six other geologic units (table 3) in the State 

have higher percentages of samples with high arsenic. This finding shows that wells with elevated arsenic are not always in the 

most high-risk areas.

The well with the highest uranium concentration 

(766 μg/L), from the town of Brookfield, is completed in the 

Dalton Formation geologic unit of the Metamorphic rock, other 

bedrock category. In a neurotoxicity case study in Brookfield, it 

was determined that a family was unknowingly exposed to well 

water containing 866 to 1,166 μg/L of uranium (Magdo and 

others, 2007). The private well in the case study was located 

in the Brookfield Gneiss geologic unit, a mafic rock common 

throughout the Appalachian ridges of western Connecticut. 

Magdo and others (2007) sampled 10 other wells in close 

proximity to the case-study well and discovered widely variable 

uranium concentrations ranging from 0.21 to 521 μg/L. Altogether, 4 of the 11 wells in Magdo and others (2007) exceeded 

the MCL for uranium. However, none of the 12 water samples 

from the Brookfield Gneiss geologic unit of the Mafic rocks 

bedrock category in this study exceeded the MCL for uranium 

(table 3). This finding shows that uranium concentrations can be 

highly variable, even in wells in close proximity to each other 

or in similar geologic settings. These two examples for wells 

with elevated arsenic and uranium concentrations highlight the 

importance of individual well testing for naturally occurring 

contaminants.

Comparison of Arsenic and Uranium Exceedance 

Rates in Three Towns

A select group of geologic units have concentrations of arsenic and (or) uranium that exceed MCLs 

(fig. 2; table 3). Therefore, a town’s vulnerability to arsenic 

and uranium contamination may depend greatly on the extent 

Table 2. Arsenic and uranium concentrations in water samples from 674 private wells in Connecticut, 2013–15.

[No., number; MRL, minimum reporting level; μg/L, microgram per liter; MCL, U.S. Environmental Protection Agency maximum contaminant level for 

public water supplies; Min, minimum; Max, maximum; <, less than]

Trace 

element

No. of 

samples

MRL, 

in μg/L

Percentage of 

samples with 

concentrations 

equal to or 

exceeding MRL

Concentration, in μg/L

MCL,

in μg/L

Percentage of 

samples with 

concentrations 

exceeding MCL

Min.

Percentile

Max.

50 75 90 95

Arsenic 660 3 9.1 <3 <3 <3 <3 7.1 470 10 3.6

Uranium 589 1 42.1 <1 <1 3.2 9.3 23 766 30 3.9
Figure 2. Range of percentages of private wells in Connecticut, by

grouped 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. 

Concentrations of arsenic and uranium in water samples collected 

from 674 individual wells and the spatial distribution of the wells are 

also shown. 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 3. See Rodgers (1985) for the location and 

description of individual geologic units. <, less than; > greater than.

>10

>5 to 10

3 to 5

<3

<1

1 to 10

>10 to 20

>20 to 30

>30

Arsenic concentration in sampled well, in μg/L

EXPLANATION

>30

>10 to 30

1 to 10

<1

Uranium concentration in sampled well, in μg/L

Range in percentages of wells, by 

grouped geologic units, in which 

arsenic concentrations are > 10 μg/L 

(MCL)— Individual geologic unit 

boundaries within a group are not shown. 

White areas within map, representing 17.6 

percent of the State, indicate geologic 

units from which no samples are available

<1

1 to 10

>10 to 20

>20 to 30

>30

EXPLANATION

Range in percentages of wells, by grouped 

geologic units, in which uranium 

concentrations are > 30 μg/L (MCL)— 

Individual geologic unit boundaries within a 

group are not shown. White areas within 

map, representing 17.6 percent of the 

State, indicate geologic units from which no 

samples are available

0 10 15

15

5 20 MILES

0 5 10 20 KILOMETERS

0 10 15

15

5 20 MILES

0 5 10 20 KILOMETERS

41°

42°

73° 72°

41°

42°

73° 72°

B. Percentage of wells, by grouped geologic units with uranium concentrations > 30 μg/L (MCL)

STAMFORD

WILTON

WESTON

STAMFORD

WILTON

WESTON

42°

A. Percentage of wells, by grouped geologic units with arsenic concentrations > 10 μg/L (MCL)

Long Island Sound

Long Island Sound

Town boundaries from U.S. Geological Survey digital data, 1:24,000, 1968–88

Geologic units from Connecticut Geological and Natural History Survey digital data, 1:125,000, 1985

Town boundaries from U.S. Geological Survey digital data, 1:24,000, 1968–88

Geologic units from Connecticut Geological and Natural History Survey digital data, 1:125,000, 1985

BROOKFIELD

OXFORD

BROOKFIELD

OXFORD
Table 3. Arsenic and uranium concentrations that exceed maximum contaminant levels from 674 private wells in Connecticut, by geologic 

unit and major bedrock category, 2013–15.

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

(subheadings) are modified from Robinson and Kapo (2003). Color shadings indicate the percentage of wells with exceedances above concentration thresholds in ranges 

of , no data (—); , less than (<) 1 percent; , 1 to 10 percent; , more than (>) 10 to 20 percent; , >20 to 30 percent; and , >30 percent. MCL, U.S. Environmental Protection Agency maximum contaminant level enforceable for public water supplies; μg/L, microgram per liter; NA, not available]

Geologic unit name Geologic 

unit code

Number of samples

MCL, percentage1

 of 

water samples with 

concentrations, in 

micrograms per liter

Percentage 

of study area 

underlain 

by geologic 

unit2 Arsenic Uranium Arsenic

>10 μg/L 

Uranium

>30 μg/L

Avalon granite

“Scituate” Granite Gneiss Zss 1 1 0 0 0.7

Hope Valley Alaskite Gneiss Zsh 5 3 20.0 0 2.1

Plainfield Formation Zp 3 2 0 0 1.4

porphyritic phase of Potter Hill Granite Gneiss Zspp 1 1 0 0 <0.2

Potter Hill Granite Gneiss Zsph 2 2 0 0 1.3

Potter Hill Granite Gneiss and Narragansett Pier Granite undividedZsph + Pn 1 1 0 0 <0.2

Rope Ferry Gneiss of the “Waterford Group” Zwr 5 5 0 0 1.1

Calcgranofels

Fly Pond (calc-silicate) member of Tatnic Hill Formation Otaf 1 — 0 — 0.4

Hebron Gneiss SOh 45 24 17.8 4.2 4.7

lower member of Bigelow Brook Formation SObl 2 2 0 0 0.6

Southbridge Formation SOs 5 2 0 0 1.0

Carbonate-bearing metasedimentary rocks

basal marble member of Walloomsac Schist Owm 4 4 0 0 0.5

Stockbridge Marble OCs 3 2 0 0 0.8

unit b of Stockbridge Marble Csb 6 6 0 0 0.4

unit c of Stockbridge Marble Csc 3 3 0 0 0.5

units e and d of Stockbridge Marble Ose 1 1 0 0 <0.2

Granite, other

“Eastford gneiss phase” of Canterbury Gneiss Dce 2 1 0 0 0.5

Canterbury Gneiss Dc 2 1 0 0 1.2

Glastonbury Gneiss Ogl 22 20 4.5 25.0 1.7

lower member of Middletown Formation Oml 3 3 0 0 <0.2

Middletown Formation Om 10 11 0 0 1.0

Monson Gneiss Omo 47 45 0 4.4 2.4

Nonewaug Granite Dng 10 10 0 10.0 0.6

Ordovician granitic gneiss Og 56 41 14.3 9.8 2.1

Trap Falls Formation and Ordovician granitic gneiss undivided Otf + Og 3 9 0 11.1 1.0

upper member of Middletown Formation Omu 9 9 0 11.1 0.2

Waterbury Gneiss Cwb 1 — 0 — 0.8

Waterford Group Zw 26 25 0 4.0 0.8

Grenville granite

Gneiss of Highlands massifs Yg 4 4 0 0 0.6

pink granitic gneiss Ygr 2 2 0 0 1.1

Mafic rocks

amphibolite-bearing unit of Manhattan Schist Cma 3 3 0 0 0.4

Beardsley Member of Harrison Gneiss Ohb 9 8 0 0 0.5

Brookfield Gneiss Ob 14 12 0 0 1.2

dioritic phase of Lebanon Gabbro Dld 1 1 0 0 <0.2

gneiss (metavolcanic) member of Brimfield Schist Obrg 5 5 0 0 0.3

Harrison Gneiss Oh 3 3 0 33.3 0.9
Table 3. Arsenic and uranium concentrations that exceed maximum contaminant levels from 674 private wells in Connecticut, by geologic 

unit and major bedrock category, 2013–15.—Continued.

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

(subheadings) are modified from Robinson and Kapo (2003). Color shadings indicate the percentage of wells with exceedances above concentration thresholds in ranges 

of , no data (—); , less than (<) 1 percent; , 1 to 10 percent; , more than (>) 10 to 20 percent; , >20 to 30 percent; and , >30 percent. MCL, U.S. Environmental Protection Agency maximum contaminant level enforceable for public water supplies; μg/L, microgram per liter; NA, not available]

Geologic unit name Geologic 

unit code

Number of samples

MCL, percentage1

 of 

water samples with 

concentrations, in 

micrograms per liter

Percentage 

of study area 

underlain 

by geologic 

unit2 Arsenic Uranium Arsenic

>10 μg/L 

Uranium

>30 μg/L

Mafic rocks—Continued

hornblende gneiss member of Collinsville Formation Ocg 3 3 0 0 0.4

Lebanon Gabbro Dl 2 2 0 0 0.3

massive mafic rock in Middletown Formation Omm 1 1 0 0 <0.2

Pumpkin Ground Member of Harrison Gneiss Ohp 4 4 0 0 0.4

Quinebaug Formation Oq 11 5 0 0 1.8

Mesozoic basin sediments

East Berlin Formation Jeb 5 4 0 0 1.2

Hampden Basalt Jha 1 1 0 0 0.3

New Haven Arkose TRnh 20 20 0 0 5.7

Portland Arkose Jp 43 41 4.7 0 7.3

Shuttle Meadow Formation Jsm 1 1 0 0 0.4

Metamorphic rocks, other

Bristol Gneiss Obs 2 2 0 0 0.4

Clough Quartzite Sbc 1 1 0 0 <0.2

Dalton Formation Cd 3 3 0 33.3 0.7

Fitch Formation Sbf 1 1 0 0 <0.2

hornblende gneiss and amphibolite Ygh 1 1 0 0 0.7

Llayered gneiss Ygn 3 3 0 0 1.7

quartzite unit in Plainfield Formation Zpq 1 1 0 0 0.6

Tatnic Hill Formation Ota 22 20 0 5.0 2.8

Pelitic rocks

amphibolite unit in Ratlum Mountain Schist Ora 1 1 0 0 <0.2

basal member of Taine Mountain Formation around Waterbury 

dome

Otb 2 2 0 0 0.4

Collins Hill Formation Och 4 4 0 0 0.8

Collinsville Formation Oc 20 20 0 0 1.0

Golden Hill Schist Ogh 3 3 0 0 0.3

Hoosac Schist Ch 10 10 0 10.0 1.0

Littleton Formation Dbl 2 2 0 0 0.4

Manhattan Schist Cm 5 5 0 0 1.8

Maromas Granite Gneiss Dm 1 1 0 0 <0.2

Oronoque Schist Oo 1 1 0 0 0.4

Ratlum Mountain Schist Or 47 40 0 2.5 4.0

Rowe Schist OCr 14 14 0 0 1.7

schist and granulite member of Trap Falls Formation Otfg 17 14 0 0 0.5

Scotland Schist DSs 3 3 0 0 0.9

Shelton (white gneiss) Member of Trap Falls Formation Otfs 3 3 0 0 0.3

Southington Mountain Member of The Straits Schist DSts 1 1 0 0 0.3

Taine Mountain Formation Ot 9 9 0 0 0.8

Taine Mountain Formation and Collinsville Formation undivided Ot + Oc 4 4 25.0 25.0 0.3

The Straits Schist DSt 12 12 8.3 0 1.9

Walloomsac Schist Ow 6 6 0 0 0.5

Wepawaug Schist DSw 9 8 11.1 0 0.7

Whigville Member of Taine Mountain Formation Otwv 1 1 0 0 <0.2
to which these geologic units are within its borders. This is 

demonstrated by examining all available water-quality data for 

three towns with the largest number of sampled wells (table 

4). In Stamford, which is in southwestern Connecticut (fig. 2), 

0.1 percent of 732 samples had high arsenic concentrations 

(>10 μg/L) and 11.2 percent of 731 samples had high uranium 

concentrations (>30 μg/L; table 4). This difference in occurrence rates between the two elements is likely explained by 

the geologic setting of the town. Much of Stamford is underlain by the Harrison Gneiss geologic unit of the Mafic rocks 

bedrock category and the Trap Falls Formation and Ordovician 

granitic gneiss undivided geologic unit of the Granite, other 

bedrock category. For these two units, none of the samples in 

the statewide dataset had high arsenic concentrations, yet 33.3 

and 11.1 percent, respectively, of samples had high uranium 

concentrations (table 3).

The towns of Weston and Wilton are adjacent to each 

other in southwestern Connecticut (figs. 1 and 2). However, 

more than 47 percent of samples in Weston had high arsenic 

concentrations, but only 2.5 percent of samples in Wilton had 

high arsenic concentrations (table 4). In Weston, the dominant 

geologic unit is the Ordovician granitic gneiss of the Granite, 

other bedrock category, and 14.3 percent of 56 samples from 

this unit in the statewide dataset had high arsenic concentrations (table 3). In Wilton, the dominant geologic units are the 

Harrison Gneiss and the Pumpkin Ground Member of Harrison 

Gneiss (Mafic rocks bedrock category), and the Trap Falls Formation and Ordovician granitic gneiss undivided of the Granite, 

other bedrock category; none of the wells from these units in the 

statewide dataset had high arsenic concentrations (table 3).

Human Health Implications

High concentrations of arsenic and uranium in drinking 

water have been associated with increased risk of lung, bladder, and skin cancers for arsenic and adverse effects on kidney 

function for uranium (Magdo and others, 2007). For households 

in Connecticut that rely on private wells for drinking water, this 

study has shown that the likelihood of having arsenic or uranium concentrations that exceed human-health benchmarks may 

depend in large part on the particular geologic unit in which the 

household’s well is located. Other factors can affect arsenic 

or uranium concentrations, such as geochemical conditions or 

residence time for water-rock reactions in the local groundwater 

system. For more information about well testing and treatment 

guidelines in Connecticut (Connecticut Department of Public 

Health, 2013), contact the Connecticut Department of Public 

Health Private Well Program by calling (860) 509–7296 or visit 

their website at http://www.ct.gov/dph/privatewells.

Table 3. Arsenic and uranium concentrations that exceed maximum contaminant levels from 674 private wells in Connecticut, by geologic 

unit and major bedrock category, 2013–15.—Continued.

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

(subheadings) are modified from Robinson and Kapo (2003). Color shadings indicate the percentage of wells with exceedances above concentration thresholds in ranges 

of , no data (—); , less than (<) 1 percent; , 1 to 10 percent; , more than (>) 10 to 20 percent; , >20 to 30 percent; and , >30 percent. MCL, U.S. Environmental Protection Agency maximum contaminant level enforceable for public water supplies; μg/L, microgram per liter; NA, not available]

Geologic unit name Geologic 

unit code

Number of samples

MCL, percentage1

 of 

water samples with 

concentrations, in 

micrograms per liter

Percentage 

of study area 

underlain 

by geologic 

unit2 Arsenic Uranium Arsenic

>10 μg/L 

Uranium

>30 μg/L

Sulfidic schists

Brimfield Schist Obr 33 32 0 0 3.2

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

rusty mica schist and gneiss Ygs 2 2 0 0 0.9

upper slice of Canaan Mountain Schist Cmcu 1 1 0 0 0.3

unmapped areas NA 1 1 0 0 1.0

Overall for the study area NA 660 589 3.6 3.9 82.4

1

The percentage (of exceedance) values for geologic units computed for this study may not represent the actual risk for existing and future wells in these units.

2

About 17.6 percent of the study area (State of Connecticut) was underlain by 75 individual geologic units from which no water samples were collected.

Table 4. Arsenic and uranium concentrations that exceed 

maximum contaminant levels from private wells in Connecticut, 

for three towns, 2013–15.

[MCL, U.S. Environmental Protection Agency maximum contaminant 

levels enforceable for public water supplies; μg/L, microgram per liter; > 

greater than; —, no data]

Town

Number of samples

Percentage of samples with 

concentrations that exceed 

MCL

Arsenic Uranium Arsenic 

>10 μg/L

Uranium 

>30 μg/L

Stamford 732 731 0.1 11.2

Weston 110 — 47.3 —

Wilton 81 80 2.5 7.5

Statewide 

dataset 660 589 3.6 3.9
Acknowledgments

This study was prepared in cooperation with the 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. The authors thank Laura 

Apinis and Jack Bennett at the DPH Laboratory for sample 

analysis; Brian Toal, Gary Archambault, Sharee Rusnak, Kenny 

Foscue, Ryan Tetreault, and Tiziana Shea at the DPH for the 

trace-element and site location data used in this study; and Connecticut State Geologist Margaret Thomas for her input.

References Cited

Ayotte, J.D., Montgomery, D.L., Flanagan, S.M., and Robinson, 

K.W., 2003, Arsenic in groundwater in eastern New England: 

occurrence, controls, and human health implications: Environmental Science and Technology, v. 37, no. 10, p. 2075-

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For additional information, contact

Director, New England Water Science Center

U.S. Geological Survey

331 Commerce Way, Suite 2

Pembroke, NH 03275

Telephone: (603) 226–7800

Email: dc_nh@usgs.gov

Or visit our website at

https://newengland.water.usgs.gov/

By Sarah M. Flanagan and Craig J. Brown

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20171046