Prepared in cooperation with the 

Northern Tier Regional Planning and Development Commission

Drinking Water Health Standards Comparison and 

Chemical Analysis of Groundwater for 72 Domestic Wells 

in Bradford County, Pennsylvania, 2016

Scientific Investigations Report 2018–5170

Version 1.2, May 2019

U.S. Department of the Interior

U.S. Geological Survey
Cover. The landscape of Bradford County, Pennsylvania, is predominately agricultural, forested 

areas, developed/residential land, and unconventional gas wells with related infrastructure. Aerial 

photograph by National Agriculture Imagery Program, 2016.
Drinking Water Health Standards 

Comparison and Chemical Analysis 

of Groundwater for 72 Domestic Wells 

in Bradford County, Pennsylvania, 2016

By John W. Clune and Charles A. Cravotta III

Prepared in cooperation with the 

Northern Tier Regional Planning and Development Commission

Scientific Investigations Report 2018–5170

Version 1.2, May 2019

U.S. Department of the Interior

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

DAVID BERNHARDT, Acting Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2019

First release: 2019

Revised: April 22, 2019 (ver. 1.1)

Revised: May 2019 (ver. 1.2)

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

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

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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:

Clune, J.W., and Cravotta, C.A., III, 2019, Drinking water health standards comparison and chemical analysis of 

groundwater for 72 domestic wells in Bradford County, Pennsylvania, 2016 (ver 1.2, May 2019): U.S. Geological 

Survey Scientific Investigations Report 2018–5170, 66 p., https://doi.org/10.3133/sir20185170.

ISSN 2328–0328 (online)
iii

Acknowledgments

This project was funded by the Appalachian Regional Commission under a cooperative agreement with the Northern Tier Regional Planning Commission (NTRPC). The authors would like 

to thank Kimberly Barnes, Frank Thompson, Rachel Hauser, and Lauren Egleston with NTRPC, 

Matthew Williams with the Bradford County Planning Department, and Laura Fitzgerald with the 

Guthrie Clinic for making this study possible. Sincere appreciation is extended to all the county 

and township officials who helped with reconnaissance and location of eligible wells. Thanks 

are extended to U.S. Geological Survey colleagues for the groundwater sampling conducted by 

Lee Eicholtz, Dennis Low, Dana Heston, Matthew Conlon, Kyle Ohnstad, Frederick Fenner, and 

Kimberly Barry, and for report reviews performed by Tammy Zimmerman, Brandon Fleming, and 

William Kappel. The authors also sincerely appreciate the generosity of local landowners who 

made their wells accessible for sampling.
iv

Contents

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

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

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

Purpose and Scope ..............................................................................................................................3

Description of Study Area ...................................................................................................................3

Study Methods ...............................................................................................................................................8

Selection of Sampling Locations........................................................................................................8

Collection and Analysis of Samples ..................................................................................................8

Graphical and Statistical Analyses....................................................................................................9

Quality Assurance and Quality Control .............................................................................................9

Groundwater Quality and Comparison to Drinking Water Health Standards ......................................9

Physical and Chemical Properties...................................................................................................10

Major Ions............................................................................................................................................10

Nutrients...............................................................................................................................................15

Bacteria................................................................................................................................................15

Metals and Trace Elements...............................................................................................................15

Radionuclides......................................................................................................................................17

Dissolved Methane and Other Naturally Occurring Hydrocarbon Gases.................................17

Man-Made Organic Compounds......................................................................................................18

Chemical Analysis and Relations Among Constituents in Groundwater............................................18

Conceptual Hydrogeochemical Model ...........................................................................................18

Relations Among pH, Specific Conductance, and Constituent Concentrations .............18

Ionic Contributions to Conductivity and Total Dissolved Solids.........................................19

Correlations Among Major and Trace Constituents in Groundwater.........................................23

Relations Between Groundwater Quality, Geology, and Topographic Setting.........................23

Major Ion Compositions Indicated by Trilinear Diagrams...................................................23

Chloride, Bromide, and Sodium in Groundwater..................................................................24

Isotopic Composition, Origin, and Spatial Distribution of Methane ..................................29

Summary and Conclusions.........................................................................................................................31

References Cited..........................................................................................................................................32

Appendix 1.....................................................................................................................................................41

Appendix 2.....................................................................................................................................................42

Appendix 3.....................................................................................................................................................44

Appendix 4.....................................................................................................................................................51
v

Figures

1. Maps showing A, bedrock geology, B, land use, and C, lithology and sampling sites

in Bradford County, north-central Pennsylvania.....................................................................4

2. Graph showing groundwater levels for the period of record (1966–2016) for U.S.

Geological Survey BR 92 Bradford County observation well (414330076280501)...............7

3. Maps showing spatial distribution of A, arsenic concentrations, B, chloride

concentrations, C, pH values, and D, methane concentrations in 72 wells sampled

and bedrock geology in Bradford County, Pennsylvania.....................................................14

4. Graphs showing relation between pH and selected constituents for 

72 groundwater samples in Bradford County, Pennsylvania...............................................16

5. Graphs showing concentrations of selected constituents in groundwater 

from Bradford County, Pennsylvania, 2016, compared to: A–C, pH; 

or D–F, specific conductance...................................................................................................20

6. Graphs showing major ion contributions to specific conductance (SC) for 

72 groundwater samples from Bradford County, Pennsylvania, 2016: 

A, comparison of estimated ionic conductivity contributions by sodium, calcium, 

magnesium, chloride, bicarbonate, and sulfate to measured SC for all 72 samples; 

and B, relative contributions by major ion species to computed SC and selected

groundwater samples of representative water types, expressed in percent, 

in order of increasing SC...........................................................................................................21

7. Pie charts showing water types and ionic contributions to specific conductance 

computed for selected groundwater samples from Bradford County,

Pennsylvania, 2016 .....................................................................................................................22

8. Trilinear diagrams showing A, predominant water types or hydrochemical facies;

and B, data for 72 groundwater samples from Bradford County, Pennsylvania, 2016,

plus median composition of brine from oil and gas wells in western Pennsylvania

and flowback water from Marcellus Shale gas wells..........................................................25

9. Graphs showing chloride concentrations compared to A, chloride/bromide mass 

ratios, B, bromide concentrations, and C, sodium contentrations for groundwater 

from Bradford County, Pennsylvania, 2016, plus median values for Salt Spring, 

flowback waters from Marcellus Shale gas wells, and oil and gas well brines from 

western Pennsylvania................................................................................................................27

10. Graphs showing A, isotopic composition of methane in groundwater samples

collected from five wells in Bradford County, Pennsylvania, 2016; and B, ratio of

methane to higher chain hydrocarbons (C1/C2+) in relation to carbon isotopic

composition for methane in these samples ...........................................................................30

Tables

1. Minimum, median, and maximum values of selected characteristics and

constituents in groundwater samples collected in 72 domestic wells in 

Bradford County, Pennsylvania, May–August 2016..............................................................12
vi

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

Flow rate

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Pressure

inch of mercury at 60oF (in Hg) 3.377 kilopascal (kPa) 

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L)

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

°F = (1.8 × °C) + 32

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

°C = (°F − 32) / 1.8

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

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
Drinking Water Health Standards Comparison and 

Chemical Analysis of Groundwater for 72 Domestic Wells 

in Bradford County, Pennsylvania, 2016

By John W. Clune and Charles A. Cravotta III

Abstract

Pennsylvania has the second highest number of residential wells of any state in the Nation with approximately 

2.4 million residents that depend on groundwater for their 

domestic water supply. Despite the widespread reliance on 

groundwater in rural areas of the state, publicly available data 

to characterize the quality of private well water are limited. In 

Bradford County, more than half of the residents use groundwater from private domestic-supply wells as their primary 

drinking source. The quality of private well water is influenced 

by the regional and local setting, including the surrounding 

soil, geology, land use, household plumbing, and well construction. The groundwater used for domestic water supply in 

Bradford County is obtained primarily from shallow bedrock 

and from unconsolidated (glacial) deposits that overlie the 

bedrock. Historical land use has been predominately forested, 

agricultural, and residential, but more recently unconventional oil/gas development has been distributed throughout the 

landscape. Pennsylvania is one of only two states in the Nation 

without statewide water-well construction standards.

To better assess the quality of groundwater used for 

drinking water supply in Bradford County, data for 72 

domestic wells were collected and analyzed for a wide range 

of constituents that could be evaluated in relation to drinking 

water health standards, geology, land use, and other environmental factors. Groundwater samples were collected from 

May through August 2016 and analyzed for physical and 

chemical properties, including major ions, nutrients, trace 

elements, volatile organic compounds, ethylene and propylene 

glycol, alcohols, gross-alpha/beta-particle activity, uranium, 

radon-222, and dissolved gases. A subset of samples was analyzed for radium isotopes (radium-226 and -228) and for the 

isotopic composition of methane. This study was conducted by 

the U.S. Geological Survey in cooperation with the Northern 

Tier Regional Planning and Development Commission and is 

part of a regional effort to characterize groundwater in rural 

areas of Pennsylvania.

Results of the 2016 study show that groundwater quality generally met most drinking-water standards. However, a 

percentage of samples failed to meet maximum contaminant 

levels (MCLs) for total coliform bacteria (49.3 percent), 

Escherichia coli (8.5 percent), barium (2.8 percent), and 

arsenic (2.8 percent); and secondary maximum contaminant 

levels (SMCL) for sodium (48.6 percent), manganese 

(30.6 percent), gross alpha and beta activity (16.7 percent), 

iron (11.1 percent), pH (8.3 percent), total dissolved 

solids (5.6 percent), chloride (1.4 percent), and aluminum 

(1.4 percent). Radon-222 activities exceeded the proposed 

drinking-water standard of 300 picocuries per liter (pCi/L) in 

70.4 percent of the samples. There were no exceedances of 

drinking water health standards for any volatile organic compounds, and the only detections were for three trihalomethanes 

in one sample.

The pH of the groundwater had a large influence on 

chemical characteristics and ranged from 6.18 to 9.31. Generally, the higher pH samples had higher potential for elevated 

concentrations of several constituents, including total dissolved solids, sodium, lithium, chloride, fluoride, boron, 

arsenic, and methane. For the Bradford County well-water 

samples, calcium/bicarbonate type waters were most abundant, 

with others classified as sodium/bicarbonate or mixed water 

types including calcium-sodium/bicarbonate, calcium-sodium/

bicarbonate-chloride, sodium/bicarbonate-chloride, sodium/

bicarbonate-sulfate, or sodium/chloride types. Six principal components (pH, redox, hardness, chloride-bromide, 

strontium-barium, and molybdenum-arsenic) explained nearly 

78.3 percent of the variance in the groundwater dataset.

Groundwater from 12.5 percent of the wells had concentrations of methane greater than the Pennsylvania action 

level of 7 milligrams per liter (mg/L); detectable methane 

concentrations ranged from 0.01 to 77 mg/L. In addition, low 

levels of ethane (as much as 0.13 mg/L) were present in seven 

samples with the highest methane concentrations. The isotopic 

composition of methane in five of these groundwater samples 

was consistent with the isotopic compositions reported for 

mud-gas logging samples from these geologic units and a 

thermogenic source. Isotopic composition from a sixth sample 

suggested the methane in that sample may be of microbial 

origin. Well-water samples with the higher methane concentrations also had higher pH values and elevated concentrations 

of sodium, lithium, boron, fluoride, arsenic, and bromide. 
2 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

Relatively elevated concentrations of some other constituents, such as barium and chloride, commonly were present 

in, but not limited to, those well-water samples with elevated 

methane. 

Four of the six groundwater samples with the highest methane concentrations had chloride/bromide ratios that 

indicate mixing with a small amount of brine (0.02 percent or 

less) similar in composition to those reported for gas and oil 

well brines in Pennsylvania. In several other eastern Pennsylvania counties where gas drilling is absent, groundwater with 

comparable chloride/bromide ratios and chloride concentrations have been reported, implying a potential natural source 

of brine. Most of Bradford County well-water samples have 

chloride concentrations less than 20 mg/L, and those with 

higher chloride concentrations have chloride/bromide ratios 

that indicate anthropogenic sources (such as road-deicing salt 

and septic effluent) or brine. Brines that are naturally present may originate from deeper parts of the aquifer system, 

whereas anthropogenic sources are more likely to affect 

shallow groundwater because they occur on or near the land 

surface. 

The available data for this study indicate that no one 

physical factor, such as the topographic setting, well depth, 

or altitude at the bottom of the well, was particularly useful 

for predicting those well locations with an elevated dissolved 

concentration of methane. The 2016 assessment of groundwater quality in Bradford County shows groundwater is generally 

of good quality, but methane and some constituents that occur 

in high concentration in naturally occurring brine and also in 

produced waters may be present at low to moderate concentrations in groundwater in various parts of the aquifer.

Introduction

Pennsylvania has the second highest number of residential wells of any state in the Nation with approximately 

2.4 million residents that depend on groundwater for their 

domestic water supply. Despite the widespread reliance on 

groundwater in rural areas of the state, publicly available data 

to characterize the quality of private well water are limited 

(Low and Chichester, 2006; Giddings, 2014; Johnson and 

Belitz, 2017). In Bradford County, more than half of the residents use groundwater from private domestic-supply wells as 

their primary drinking source (Pennsylvania Bureau of Topographic and Geologic Survey, written commun., 2017). 

Groundwater from more than one in five private domestic-supply wells in the Nation potentially exceeds a humanhealth standard and yet, the potential health risks associated 

with domestic groundwater supplies are often understudied 

in comparison to the research and testing of public water supplies (DeSimone, 2009). Unlike public water-supply systems, 

managing domestic-supply wells in Pennsylvania is solely 

the responsibility of the homeowner. Acute gastrointestinal 

illnesses have been attributed to parasitic, viral, and bacterial 

pathogens in rural private well water (Raina and others, 1999; 

Murphy and others, 2016). Metals such as lead can be present in the aquifer or leached from plumbing components by 

corrosive groundwater, causing delayed mental and physical 

development (Brown and Margolis, 2012; Belitz and others, 

2016), and long-term exposure to arsenic has been linked to 

skin and internal cancers (National Research Council, 1999). 

The presence of metals like iron and manganese in drinking 

water can be a nuisance owing to the unpleasant taste, odor, 

and color and often require additional maintenance and treatment of household plumbing systems (Penn State Extension, 

2017). The complex mixture and breakdown of radioactive 

elements and volatile organic compounds (VOCs) in drinking 

water also has been shown to be carcinogenic (Otton, 1992; 

National Research Council, 1998; Hopke and others, 2000). 

Elevated levels of dissolved gases like methane can pose an 

explosive safety risk if private wells are not properly vented 

(Swistock and Sharpe, 2006). 

The quality of private well water is influenced by the 

regional and local setting, including the surrounding soil, geology, land use, and household plumbing. In addition, improper 

well construction (such as, no grouting and [or] sanitary well 

cap) can facilitate the introduction of potential contaminants 

from the surface into private groundwater wells and local 

aquifers (Zimmerman and others, 2001; Simpson, 2004; Swistock and Sharpe, 2005; Giddings, 2014). Despite legislative 

attempts, Pennsylvania is one of two states without statewide 

water-well construction standards (Swistock, 2012;Wagner , 

2012; LaRegina, 2013; Giddings, 2014; Pennsylvania General 

Assembly, 2015a,b, 2016).

Previous national and statewide studies that include Bradford County provide general characterizations of water quality 

for a bedrock and glaciated groundwater system. In glaciated settings of the United States, the quality of groundwater 

is vulnerable to dissolved solids from the natural geology, 

anthropogenic contaminants such as rising chloride levels, 

and nuisance constituents like iron (Warner and Ayotte, 2015). 

Sampling of private wells in Pennsylvania show bedrock 

geology and well construction have a significant influence on 

water quality, and common contaminants often include bacteria, low pH, arsenic, lead, nitrate, and radon (Swistock and 

others, 1993; Swistock and others, 2009). 

Early regional groundwater assessments in the Bradford 

County area show comparatively moderate dissolved solids 

and hardness, with the most troublesome constituents being 

naturally occurring iron and manganese (Taylor and others, 

1983). The predominant hydrogeochemical groundwater zones 

exhibit a sodium-chloride water type in the bedrock (restricted 

flow) and calcium/bicarbonate in the glaciated valleys (unrestricted flow) (Williams and others, 1998). Wells that have 

open holes that have been drilled in the restricted groundwater 

flow of bedrock formations can produce naturally elevated 

levels of chloride, barium, strontium, and radium (Williams 

and others, 1998; Yager and others, 2017). 

More recent research that has furthered the understanding of local groundwater quality in Bradford County and 
Introduction 3

surrounding counties has been a byproduct of the unconventional natural gas development in areas of Pennsylvania 

underlain by the Marcellus Shale since 2005 (U.S. Environmental Protection Agency, 2012b; Vidic and others, 2013; 

Soeder, 2017). Groundwater sampling from a subset of private 

domestic-supply wells before and after gas well drilling did 

not suggest major influences from gas well drilling, but did 

show that 40 percent of the samples failed at least one common drinking water health standard such as coliform bacteria 

(Boyer and others, 2012). Most well water chemistry in the 

region has been shown to be comparable to historical groundwater (American Petroleum Institute/America’s Natural Gas 

Alliance, 2013) with some evidence of animal waste, septic 

effluent, or road salt but no detectable impact of flowback 

waters from gas well development (Reilly and others, 2015). 

Pre- and post-drilling comparisons show the quality of water 

from private domestic-supply wells is largely influenced by 

groundwater contact time (residence time) with the natural 

geology (Weston Solutions, 2012). Freshwater is often found 

where rainwater has more recently flushed the shallow aquifer, 

whereas brackish water may be encountered in deeper formations that have the remains of connate (water trapped in the 

pores of sedimentary sediments when deposited) seawater 

(Lohman, 1939). Analysis of pre-drilling groundwater shows 

the natural migration of Appalachian Basin brines along 

natural pathways—such as faults, fractures, lineaments, and so 

forth—in the bedrock of some shallow aquifers (Warner and 

others, 2012; Llewellyn, 2014). Analysis of robust propriety 

groundwater datasets from the natural gas industry show 

common exceedances of drinking water health standards for 

sodium, manganese, iron, lithium, turbidity, low pH, arsenic, 

lead, and barium that are often related to the geological formation, water type, and topographic position (Siegel and others, 

2015).

Naturally occurring thermogenic- and biogenic-derived 

dissolved hydrocarbon gases such as methane can also be 

common in groundwater from private domestic-supply wells 

(Molofsky and others, 2011, 2013; Heisig and Scott, 2013; 

Botner and others, 2018), although the potential for stray gas 

from natural gas extraction development is a concern (Osborn 

and others, 2011; Jackson and others, 2013; Heilweil and others, 2015; Llewellyn and others, 2015) especially in valleys 

and near faults (Wen and others, 2018). 

The U.S. Geological Survey (USGS) has conducted a 

series of county groundwater studies since 2007 in order to 

provide publicly available data to better characterize the quality of groundwater in rural areas of Pennsylvania. Studies in 

Pike County and Sullivan County indicated few exceedances 

of the drinking water health standards, and samples that had 

elevated pH and methane often showed higher concentrations 

of sodium, lithium, boron, fluoride, and bromide (Senior, 

2009, 2014; Sloto, 2013; Senior and Cravotta, 2017). Studies in Wayne County and Lycoming County presented similar 

results but also included bacteria sampling that indicated 

exceedances of the drinking water health standards and geochemical modeling that offered further insight into the effect 

that water-rock reactions, topographic position, and pH have 

on the regional groundwater chemistry (Senior and others, 

2017; Gross and Cravotta, 2017). 

The research described in this report is a continuation of 

these ongoing USGS county groundwater quality assessments 

in Pennsylvania characterizing the quality of groundwater in 

Bradford County and aims to further the national, statewide, 

and regional understanding of previous groundwater investigations. At the local level, the results will serve the needs of 

resource managers with a comparison of the quality of private 

well water with drinking water health standards and provide a 

conceptual model of the natural and anthropogenic influences 

on local groundwater quality. This study was conducted by the 

U.S. Geological Survey in cooperation with the Northern Tier 

Regional Planning and Development Commission and is part 

of a regional effort to characterize groundwater in rural areas 

of Pennsylvania. 

Purpose and Scope

The purpose of this report is to (1) compare the groundwater quality from private domestic-supply wells to drinkingwater standards, and (2) to provide a chemical analysis of 

the natural and anthropogenic influences on groundwater 

quality in Bradford County, Pennsylvania. These objectives 

were met by the collection of analytical data for groundwater from 72 domestic wells sampled throughout Bradford 

County, Pennsylvania, from May through August of 2016. 

Groundwater samples were analyzed for physical and chemical properties, including major ions, nutrients, trace elements, 

VOCs, ethylene and propylene glycol, alcohols, gross-alpha/

beta-particle activity, uranium, radon-222, and dissolved 

gases. A subset of samples was analyzed for radium isotopes 

(radium-226 and -228) and for the isotopic composition of 

methane. The measured concentrations of analyzed constituents in groundwater were compared to U.S. Environmental 

Protection Agency (EPA) drinking-water standards. The 

groundwater-quality data, summary statistics, and geochemical computations are presented to further the understanding 

of the current (2016) chemical quality of groundwater from 

freshwater aquifers used by private domestic-supply wells in 

Bradford County. The relations among observed groundwaterquality characteristics, geology, topographic setting, land use, 

and other environmental variables associated with sampled 

wells are evaluated to explain the variability in the quality of 

the groundwater. 

Description of Study Area

Bradford County occupies approximately 1,147 square 

miles in the glaciated portion of the Appalachian Plateaus 

Physiographic Province in north-central Pennsylvania 

(fig. 1A). The geology of Bradford County is characterized as 

gently folded clastic sedimentary bedrock that is overlain in 

most places by unconsolidated glacial deposits and alluvium 
4 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

(less than 20 feet thick). The bedrock is composed primarily 

of shale, siltstone, and sandstone strata of Devonian (400 million years ago) to Pennsylvanian (300 million years ago) 

age, including (from youngest to oldest) the Allegheny and 

Pottsville Formation, Burgoon Sandstone, Huntley Mountain 

Formation, Catskill Formation, and Lock Haven Formation 

(Miles and others, 2001). Fractures in the bedrock provide 

water-bearing zones that store and transfer groundwater to 

wells. In the valleys, layers of fluvial outwash and lacustrine 

(lake) deposits from past glaciations have filled the lowlands with mostly unconsolidated silt, clay, sand, and gravel 

(fig. 1C). Figure 1C does not show lithology of surficial material or bedrock for the entire county which is often a mix of 

both. Private domestic-supply wells may be completed in bedrock or the overlying unconsolidated deposits. Wells drilled in 

bedrock typically are cased from the surface down to competent bedrock without grouting and consist of an open hole 

through the water-bearing zone, whereas those constructed in 

EXPLANATION

Allegheny and Pottsville Formations

Pottsville Formation

Burgoon Sandstone

Huntley Mountain Formation

Catskill Formation

Lock Haven Formation

Fault

76°45' 76°30' 76°15'

42°

41°45'

A. Bedrock geology

Base from U.S. Bureau of the Census digital data, 2000

1:100,000-scale Albers equal-area projection

Standard parallels 40 ̊ N and 42 ̊ N, central meridian −78 ̊ W, latitude of origin 39 ̊

0 10 20 KILOMETERS

0 10 20 MILES

Geology from Miles and others, 2001

Physiographic Provinces of Pennsylvania

Central Lowlands

Appalachian Plateaus

Ridge and Valley

New England

Piedmont

Atlantic Coastal Plain

EXPLANATION

Physiographic provinces from Pennsylvania Department of Conservation

and Natural Resources, 2016, 1:50,000-scale digital data

Location of Bradford County

Figure 1. A, bedrock geology, B, land use, and C, lithology and sampling sites in Bradford County, north-central Pennsylvania. Bradford 

County is in a section of the glaciated region of the Appalachian Plateaus Physiographic Province.
Introduction 5

unconsolidated sand/gravel may be through open-ended casing 

or screened in the water-bearing zone.

Land use is predominately forested (56.1 percent), mostly 

on private land except for a small portion of Pennsylvania 

state forest, one state park, and several state game lands. 

Agricultural land use consists mainly of pasture/hay (23.5 percent) and cropland (11.4 percent; fig. 1B) (Multi-Resolution 

Land Characteristics Consortium, 2014). During the summer 

of 2016, Bradford County was second among all counties in 

the state for the most horizontal unconventional gas wells 

drilled (n=1,339; Pennsylvania Department of Environmental 

Protection, 2017), and the related infrastructure (well pads, 

pipelines, and so forth) has become a relatively recent part of 

the landscape. Wetland and water resources compose 2.67 percent and 0.98 percent of the landscape, respectively. The 

remaining land area is developed/residential land (4.95 percent) with a largely rural population of 62,622 recorded in 

2010 (U.S. Census Bureau, 2011). Some of the most densely 

populated areas in the county include the boroughs of Sayre, 

Athens, and Towanda, and the townships of Athens, Canton, 

EXPLANATION

Land-use category (by percentage)—Land use from 

2011 National Land Cover Dataset (NLCD), 

U.S. Geological Survey, 2014

Water (0.98)

Wetlands (2.67)

Barren land (0.36)

Forest (56.11)

Developed (4.95)

Pasture/hay (23.47)

Cropland (11.44)

Natural gas well—Natural gas well locations 

from Pennsylvania Department of 

Environmental Protection, 2016

Land use, by percentage

0 10 20 30 40 50 60 70 80 90 100

76°45' 76°30' 76°15'

42°

41°45'

B. Land use

Base from U.S. Bureau of the Census digital data, 2000

1:100,000-scale Albers equal-area projection

Standard parallels 40 ̊ N and 42 ̊ N, central meridian −78 ̊ W, latitude of origin 39 ̊

0 10 20 KILOMETERS

0 10 20 MILES

Figure 1. A, bedrock geology, B, land use, and C, lithology and sampling sites in Bradford County, north-central Pennsylvania. Bradford 

County is located in a section of the glaciated region of the Appalachian Plateaus Physiographic Province.—Continued
6 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

and Ridgebury, with populations ranging from approximately 

2,000 to 6,000 residents (U.S. Census Bureau, 2011). 

Topography in Bradford County consists mostly of low 

to moderate relief with rounded hills and valleys with elevations ranging from 636 feet (ft) to 2,710 ft above sea level 

(Sevon, 2000; U.S. Geological Survey, 2009). The main 

streams within the county are Towanda, Sugar, Wysox, and 

Wyalusing Creeks that flow into the north branch of the 

Susquehanna River watershed, which eventually drains to 

the Chesapeake Bay. The local climate provides for a wet 

season of increased precipitation from April to June, with a 

mean annual rainfall of 37 inches per year (in/yr), of which 

approximately 10–12 inches recharges the aquifer (Risser and 

others, 2008). Groundwater levels fluctuate seasonally with 

increased recharge in the spring (March to May), a relatively 

stable water table during winter (November to February), 

and a steady decline during the summer (June to August) 

months owing to increased evapotranspiration (fig. 2). Average 

temperatures throughout the year in Bradford County range 

from about 33.6 to 55.6 degrees Fahrenheit (PRISM Group at 

Oregon State University, 2006a,b). 

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BR 896

BR 895

BR 894

BR 893

BR 892

BR 890

BR 428

BR 258

EXPLANATION

BR 952

Mudstone/sandstone bedrock

Unconsolidated silt, clay, sand, and gravel—Sand and gravel 

layer adapted from Sevon and Braun, 1997

Sampled well and identifier

Not to scale

Types of sampling locations

Stream

Sampled 

well

76°45' 76°30' 76°15'

42°

41°45'

C. Lithology and sampling sites

Base from U.S. Bureau of the Census digital data, 2000

1:100,000-scale Albers equal-area projection

Standard parallels 40 ̊ N and 42 ̊ N, central meridian −78 ̊ W, latitude of origin 39 ̊

0 10 20 KILOMETERS

0 10 20 MILES

Figure 1. A, bedrock geology, B, land use, and C, lithology and sampling sites in Bradford County, north-central Pennsylvania. Bradford 

County is located in a section of the glaciated region of the Appalachian Plateaus Physiographic Province.—Continued
Introduction 7

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

EXPLANATION

Groundwater levels—Water table

Sampling period

Data not available

Higher (wetter conditions)

Lower (dryer conditions)

Figure 2. Groundwater levels for the period of record 

(1966–2016) for U.S. Geological Survey BR 92 Bradford 

County observation well (414330076280501). The water 

table in Bradford County fluctuates seasonally with 

increased precipitation/recharge (wetter conditions) in 

the spring (March to May) and declines during the summer 

(June to August) due to increased evapotranspiration 

(drier conditions). Groundwater samples were collected 

across Bradford County, Pennsylvania, from May to August 

2016 (shown in red).
8 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

Study Methods

To provide a characterization of groundwater quality in 

Bradford County, 72 private domestic-supply wells throughout the county (fig. 1) were selected for sampling during the 

summer months of 2016 (fig. 2). One sample from each well 

was analyzed for physical and chemical properties, including major ions, nutrients, trace elements, VOCs, ethylene and 

propylene glycol, gross-alpha/beta-particle activity, uranium, 

radon-222, and dissolved gases. A subset of samples was 

analyzed for radium isotopes (radium-226 and -228) and for 

the isotopic composition of methane. A list of wells sampled, 

ancillary data, and a National Water Information System 

(NWIS) website URL to water quality results used for analysis 

are provided by Clune and Cravotta (2018). Using the station 

identification numbers, the NWIS water quality results can 

also be retrieved with the dataRetrieval R package (Hirsh and 

DeCicco, 2015; Hirsh and others, 2015a, 2015b) or through 

user defined queries using USGS web services (U.S. Geological Survey, 2018). Additionally, quality assurance data results 

collected during sampling are available (Clune and Cravotta, 

2018).

Selection of Sampling Locations

A computerized, stratified random site-selection approach 

was used to design a groundwater-quality network for sampling (Scott, 1990) by creating an equal-area grid of cells with 

a randomly placed population of potential sites across the 

county. The program arbitrarily specified one site from each 

cell as a primary sampling point and three additional sites as 

alternate sampling locations. The best available well record 

data within Bradford County were obtained from the Pennsylvania Groundwater Information System database (Pennsylvania Bureau of Topographic and Geologic Survey, 2014) 

and from local well drillers. Potential wells for sampling were 

selected from these data based on the criteria that the well was 

(1) used for domestic or stock purposes, (2) had an associated 

well drillers record, (3) was drilled after 1970, (4) was not 

hand dug, and (5) the top of the casing was above land surface. 

Wells located within 1 mile of one of the computer-generated 

random sampling points were identified, and well owners were 

contacted for permission to sample and to schedule a plumbing review to verify an untreated sample representative of 

the aquifer could be collected. Private domestic-supply wells 

meeting the criteria and well verification were prioritized for 

sampling.

Collection and Analysis of Samples

Sampling sites included 72 groundwater well locations 

throughout Bradford County (fig. 1C) drilled with an open 

hole interval usually in bedrock and sometimes unconsolidated 

deposits. Groundwater samples were collected from May 

through August 2016 (fig. 2) following protocols outlined in 

the USGS National Field Manual (U.S. Geological Survey, 

variously dated). Groundwater samples were collected using 

Teflon tubing attached to a sampling point (outside spigot, 

pressure tank, and so forth) prior to any water treatment. Wells 

were purged until field parameters (pH, specific conductance, 

water temperature, turbidity, and dissolved oxygen) stabilized 

and the final readings were recorded. Samples were then collected for laboratory analysis. 

The samples for total dissolved solids, major ion, trace 

metals, and nutrient chemistry were filtered onsite through a 

disposable filter with a 0.45-micrometer pore size and analyzed by the National Water Quality Lab in Denver, Colorado. 

Samples for major ions were preserved with nitric acid if the 

pH was greater than 2 and analyzed at the lab by inductively 

coupled plasma atomic emission spectrometry, ion chromatography, or automated colorimetry (EPA Method 524). Nutrient analyses were performed by automated colorimetry with 

persulfate digestion when required (Fishman, 1993; Patton and 

Kryskalla, 2011). 

Radiological samples for alpha/beta particles were 

preserved with nitric acid if pH was greater than 2 and analyzed at ALS Laboratories in Fort Collins, Colorado, by gas 

flow proportional counting (EPA Method 900.0). A subset of 

samples that had gross alpha greater than 5 picocuries per liter 

(pCi/L) were analyzed further for radium-226 and -228 by 

radon emanation, chemical separation, and gas flow proportional counting (EPA Method 903.1 and 904.0). Radon-222 

samples were obtained through an inline septum with a gastight syringe to avoid atmospheric contact and analyzed at the 

lab by liquid scintillation (ASTM Method D 5072-16). 

Dissolved gases sampling used a collection bag, and 

samples were analyzed at Isotech Laboratories, Champaign, 

Illinois, by gas compositional analysis to determine the 

concentration of fixed gases and light hydrocarbon gases 

dissolved in the sample (Dai and others, 2012). Samples 

containing a sufficient concentration of methane were further 

analyzed for stable carbon isotopes 12C and 13C and the stable 

hydrogen isotopes 1H and 2H (deuterium). The 13C/12C and 2

H/1H ratios were determined by an isotope ratio mass spectrometric analysis that compared the sample to a reference standard. The carbon isotope ratio value in methane (δ13C CH4

) 

is reported in terms of per mil (‰) notation with respect to 

the Vienna Peedee belemnite standard. The hydrogen isotope 

ratio value in methane (δD CH4) is reported in terms of per 

mil notation with respect to the Vienna Standard Mean Ocean 

Water standard.

The remaining unfiltered samples were analyzed by Seewald Laboratories, Inc., in Williamsport, Pennsylvania. VOCs, 

dissolved gases, and glycol samples were collected onsite 

using a stainless-steel manifold fitting to fill vials to reduce 

turbulence. VOC vials were preserved with hydrochloric acid 

and analyzed by purge and trap gas chromatography/mass 

spectrometry (EPA Method 524.2). Dissolved gas samples for 

ethane and propane were determined by analyzing a portion 

of the headspace with a gas chromatograph equipped with a 

flame ionization detector (PA DEP Method 3686). Based upon 
Groundwater Quality and Comparison to Drinking Water Health Standards 9

the equilibrium gas concentration, a liquid sample concentration of these gases is calculated. Samples for glycols and 

alcohols were evaluated using the direct injection method, 

which uses a gas chromatograph equipped with a flame ionization detector that is chromatographically optimized for the 

separation and determination of the compounds of interest 

(EPA Method 8015 D). Oil and grease samples were preserved 

with hydrochloric acid and analyzed in the lab using hexane to 

determine the petroleum hydrocarbon fraction (EPA Method 

1664 B). After sterilizing and flaming the sampling point, the 

bacteria sample was collected and analyzed by the lab using 

the Colilert color method in which samples were enumerated 

to give counts of total coliform and Escherichia coli (Standard 

Method 9223).

Graphical and Statistical Analyses

Various graphical and statistical techniques were used 

in this report to compare water-quality data among different sites, to distinguish natural and anthropogenic sources 

of dissolved constituents, and to identify possible factors 

affecting the occurrence or transport of solutes in the aquifers 

in the study area. Techniques included bivariate scatter plots, 

mass ratios of chloride to bromide (Cl/Br), trilinear diagrams, 

nonparametric statistical approaches, correlation coefficients 

(Spearman’s rho), principal components analysis (PCA), and 

aqueous speciation computations. The boxplots in the appendix considered pH (fig. 4.1), specific conductance (fig. 4.2), 

redox (fig. 4.3), bedrock formation (fig. 4.4), and topographic 

position index (fig. 4.5) to classify the data. The boxplots 

show a notched interval around the median that can be used 

by a reader to judge the significance of potential differences 

(Velleman and Hoaglin, 1981; Helsel and Hirsch, 2002). If 

the notched intervals around the medians for sample subsets 

do not overlap, the medians are statistically different at the 

95-percent confidence interval. An extensive description of the 

graphical and statistical methods used in this report and the 

basis for classifications used in the appendix are described in 

Gross and Cravotta (2017). 

Quality Assurance and Quality Control

For quality control (QC), filtered and unfiltered replicate 

samples were collected from wells BR 907, BR 910, BR 913, 

BR 939, and field blanks were processed onsite for BR 921, 

BR 923, and BR 940. All these samples were submitted to the 

laboratories for analysis. These results demonstrate that, for 

most constituents, field equipment or sampling methodology 

did not contaminate the sample. The results of the equipment 

blank indicate that sampling equipment may introduce low 

levels of two constituents into the samples at concentrations of 

as much as about 0.22 micrograms per liter (μg/L) for ammonia and 0.05 μg/L for molybdenum. The results of one of the 

three field blanks (BR 921) indicate that sampling methodology may introduce low levels of four constituents into samples 

at concentrations of as much as about 0.05 milligrams per 

liter (mg/L) for calcium, 0.10 mg/L for sodium, 0.04 mg/L for 

chloride, and 0.03 mg/L for silica. 

Results from three sets of replicate samples indicate 

combined sampling and analytical precision (reproducibility) was within 5 percent and (or) concentrations were less 

than two times the minimum reporting level for most constituents. Within two times the minimum reporting level, the 

concentrations were less precise but still within 20 percent, 

except for gross alpha and beta radioactivity. For the replicate sample collected from BR 939, calcium values were 

77.3 and 66.6 mg/L with a difference of 10.7 mg/L (15 percent), and methane had low detected values of 0.0042 and 

0.0037 mg/L with a difference of 0.0005 mg/L (13 percent). 

For the replicate sample collected from BR 910, bromide was 

0.440 and 0.493 mg/L with a difference of 0.053 mg/L (11 percent difference). For the replicate sample at BR 907, ammonia 

was 0.29 and 0.33 mg/L with a difference of 0.04 (14 percent), 

and radon 222 was 324 and 349 pCi/L with a 25-pCi/L difference (7 percent). The two other replicate samples with values 

for calcium, methane, bromide, ammonia, and radon 222 had 

a less than 3-percent difference or were less than two times the 

minimum reporting level.

For quality assurance (QA) of the inorganic chemical 

analyses, intrasample characteristics were evaluated using 

standard procedures described by Hem (1985) and Fishman 

and Friedman (1989). Evaluations of accuracy and precision 

included comparison of field- and laboratory-measured values 

for pH and specific conductance for each sample. Additional 

QA/QC checks involved comparisons of the computed cation 

and anion equivalents concentrations and the corresponding 

ionic charge balance, comparisons of the ratios of cation or 

anion equivalents to specific conductance, and comparisons of 

total dissolved solids (TDS) computed as the sum of major ion 

concentrations to the measured residue on evaporation (ROE) 

at 180 degrees Celsius (°C). Lastly, the measured specific conductance was compared to the computed specific conductance, 

which was estimated as the sum of ionic conductivities after 

accounting for aqueous speciation (McCleskey and others, 

2012). The values of specific conductance measured in the 

field and laboratory were consistent with one another and with 

the computed values of specific conductance and TDS on the 

basis of measured solute concentrations. The measured TDS 

concentration as ROE by two different laboratories and the 

computed TDS as the sum of constituent concentrations were 

in close agreement. 

Groundwater Quality and Comparison 

to Drinking Water Health Standards

Analytical results for the 72 groundwater samples collected in Bradford County are provided in the following 

sections and compared to EPA drinking-water standards and 

health advisories (U.S. Environmental Protection Agency, 
10 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

2012a). The EPA has established maximum contaminant levels 

(MCLs), secondary maximum contaminant levels (SMCLs), 

and action levels for some constituents in drinking water 

(table 1). MCLs generally are set because elevated concentrations of these constituents may cause adverse health effects. 

SMCLs generally are set for aesthetic reasons; elevated 

concentrations of these constituents may impart an undesirable 

taste or odor to water. These MCLs and SMCLs may be used 

as a guideline for private well owners but are only regulated 

for public drinking-water supplies. 

Physical and Chemical Properties

Physical and chemical properties discussed in this section include temperature, pH, specific conductance, dissolved 

oxygen, and turbidity. Most of these properties are unstable 

and are determined in the field at the time a water sample is 

collected. 

The temperature of the groundwater samples ranged from 

9.4 to 18.0 °C, with a median of 11.5 °C. These temperatures 

generally were less than the daytime air temperatures during 

sampling, which reflects the generally cool groundwater environment. Dissolved gases and carbonate minerals can dissolve 

to a greater extent in cooler water than in warmer water (Hem, 

1985).

Dissolved oxygen (DO) concentrations ranged from 

0.03 to 10.6 mg/L; the median concentration was 1.21 mg/L 

(table 1). Generally, most of the groundwater had DO concentrations that were substantially less than saturation at the sample temperature, indicating the waters had been out of contact 

with the atmosphere. Low DO concentrations are associated 

with higher pH values; 21 of the 30 water samples with a DO 

concentration of less than 1 mg/L had a pH greater than 7.5. 

Low DO concentrations are related to chemical or biochemical reactions that consume oxygen and may be associated with 

chemically reducing conditions that promote the release of 

iron, manganese, and associated metals from rock and other 

solids in contact with water along its flow path. The chemical 

reactions that consume oxygen generally involve organic carbon that can be naturally occurring in soil or aquifer materials, 

or can be introduced from industrial, agricultural, or domestic 

wastes. Of the 72 well samples in this study, 36.1 percent were 

classified as “anoxic” (DO<0.5 mg/L), 50.0 percent as “oxic” 

(DO≥0.5 mg/L), and 13.9 percent as “mixed” (DO>0.5 mg/L 

with manganese (Mn)>0.05 mg/L or iron (Fe)>0.1 mg/L), 

based on criteria of McMahon and Chapelle (2008) (fig. 4.3). 

Of the 26 samples classified as anoxic, 12 had chemical characteristics consistent with MnIV and FeIII reducing conditions 

as defined by McMahon and Chapelle (2008) (DO<0.5 mg/L, 

nitrate (NO3)<0.5 mg/L, Mn>0.05 mg/L, Fe>0.1 mg/L, and 

sulfate (SO4)>0.5 mg/L), and two were identified with more 

strongly reducing conditions necessary for methanogenesis 

(DO<0.5 mg/L; NO3<0.5 mg/L, Mn>0.05 mg/L, Fe>0.1 mg/L, 

and SO4<0.5 mg/L). Both samples identified as methanogenic had elevated concentrations of methane; BR 910 and 

BR 921 had methane concentrations of 77 mg/L and 26 mg/L, 

respectively.

The pH is a measure of acidity and is related to the 

potential corrosivity of the water and its potential to leach 

metals, such as lead and copper, from pipes and plumbing. The 

standard pH scale is 0–14, with 7 being neutral. Generally, pH 

values of 6.5 to 7.5 are considered near neutral, values less 

than 6.5 are considered acidic, and values greater than 7.5 are 

considered basic or alkaline. The field pH of water samples 

collected in Bradford County ranged from 6.18 to 9.31; the 

median pH was 7.47. The pH of 6 of the 72 samples (8.3 percent) was outside the EPA SMCL range of 6.5–8.5 (U.S. 

Environmental Protection Agency, 2012a). Two samples had 

a pH less than 6.5, and four samples had a pH greater than 8.5 

(table 1 and fig. 3C). Generally, the pH varied widely within 

and among the three bedrock aquifers sampled; median pH 

values were greatest for the Lock Haven Formation (7.6), least 

for Huntley Mountain Formation (6.8), and intermediate for 

the Catskill Formation (7.4) (fig. 4.4).

Specific conductance (SC) is a gross measure of the 

ability of ions in water to conduct an electrical current and is 

expressed in units of microsiemens per centimeter (μS/cm). 

The higher the value for specific conductance, the higher the 

concentrations of total dissolved solids and associated ions 

in the water. The field-measured SC for the 72 groundwater samples ranged from 72.4 to 3,410 μS/cm at 25 °C; the 

median specific conductance was 365 μS/cm (table 1). Values 

were similar for the laboratory-measured SC (table 1), which 

is useful because the laboratory measured SC avoids potential 

issues with air bubbles forming on the electrode surfaces when 

the sample is first drawn from the subsurface.

Turbidity is a measure of the amount of solid particles 

suspended in water that block the transmission of light through 

the sample. Turbidity is expressed in nephelometric turbidity 

ratio units (NTRU), which quantify the degree to which light 

is scattered by solid particles suspended in water. The higher 

the NTRU, the more turbid the water. Turbidity concentrations 

ranged from 0.12 to 247 NTRU; the median concentration was 

1.19 NTRU (table 1). In general, samples with high turbidity 

could be expected to have total concentrations of constituents 

that include contributions from suspended particles, in addition to the dissolved concentration. 

Major Ions

Major ions are frequently derived from the dissolution 

of common minerals, including carbonates, silicates, oxides, 

sulfates, and sulfides, and can be influenced by ion-exchange, 

redox processes, and mixing of freshwater with residual brines 

that remain in the aquifer matrix or that could be mobilized 

from deep sources. The concentrations of major ions, TDS, 

salinity, and SC of groundwater are directly related, and 

generally expected to increase with, progressive evaporation 

or dissolution of minerals (Hem, 1985). The concentrations of 

trace elements in solution may increase with TDS or SC, not 
Groundwater Quality and Comparison to Drinking Water Health Standards 11

only because of the release of trace constituents with the major 

ions dissolved from minerals, but because of the potential for 

displacement of adsorbed or exchangeable trace ions from 

mineral surfaces by the major ions.

Major cations (positively charged ions such as calcium, 

magnesium, sodium, and potassium), major anions (negatively 

charged ions such as sulfate, chloride, fluoride, and bicarbonate), and nonionic solutes (uncharged solutes such as silica) 

typically are present in natural waters at concentrations greater 

than 1 mg/L, whereas dissolved trace constituents (such as 

iron, manganese, zinc, lead, copper, nickel, vanadium, molybdenum, arsenic, selenium, radium, uranium, lithium, and bromide) typically are present at concentrations less than 1 mg/L 

(Hem, 1985). Concentrations of DO and nutrients (such as 

nitrogen and phosphorus compounds) commonly range from 

values less than 1 mg/L to values greater than 1 mg/L because 

of biological processes that involve the production or metabolism of organic carbon compounds that contain nitrogen and 

phosphorus. Although biological (biochemical) processes can 

affect the concentrations of nutrients and trace constituents in 

groundwater directly or indirectly because of changes to pH 

and redox, such processes generally have minor effects on 

major ion concentrations. 

Analyses for major ions were conducted on filtered 

samples to represent dissolved concentrations. Major anions, 

in order of decreasing median dissolved concentration, were 

bicarbonate (alkalinity), sulfate, chloride, fluoride, and bromide; major cations were calcium, sodium, magnesium, and 

potassium. These cations and anions, plus uncharged silica, are 

the primary contributors to TDS and SC. Of the major ions, 

only fluoride has an established EPA MCL in drinking water 

(4 mg/L) or SMCL (2 mg/L). Fluoride concentrations ranged 

from 0.03 to 0.78 mg/L, and none of the samples exceeded the 

EPA MCL or SMCL (table 1). 

The concentration of TDS can be measured by evaporating a given volume of water and weighing the ROE or by 

summing the measured concentrations of the major ions in a 

filtered sample (TDScalc). The measured TDS concentrations 

ranged from 52.7 to 2,030 mg/L; the median concentration 

was 218 mg/L. The TDScalc was comparable in value to the 

ROE and ranged from 42.0 to 1,940 mg/L. Out of 72 samples, 

4 samples (5.6 percent) had TDS (ROE and TDScalc) that 

exceeded 500 mg/L, which is the EPA SMCL for TDS in 

drinking water. One of these four samples had elevated concentrations of sulfate (BR 946) that may result from the oxidation of pyrite, whereas three samples (BR 913, BR 921, and 

BR 927) had elevated concentrations of sodium and chloride. 

Elevated sodium and chloride concentrations could be caused 

by connate water in the formations and a zone of restricted 

groundwater flow that limits flushing by fresh recharge; mixing of saline and freshwater; or by the introduction of mineral 

salts from near-surface contaminant sources such as road deicing compounds, water softener additives, effluent from septic 

systems, or animal waste (Mullaney and others, 2009).

Dissolved concentrations of sodium ranged from 

2.18 to 760 mg/L; the median concentration was 19.1 mg/L. 

The EPA has issued a taste threshold for sodium in public 

water supplies that recommends reducing sodium concentrations to between 30 and 60 mg/L (U.S. Environmental Protection Agency, 2003). This recommendation is a guideline and 

is not federally enforceable. Salt used for food, road deicing, 

and other applications is mainly composed of sodium chloride. 

Thus, high concentrations of sodium and chloride may result 

from the addition of salt to groundwater and may be related to 

land use as well as the bedrock and (or) unconsolidated lithology. Dissolved concentrations of chloride ranged from 0.77 to 

1,020 mg/L; the median concentration was 5.28 mg/L (table 1, 

fig. 3B). Out of the 72 samples, only 1 sample (1.4 percent), 

BR 913, exceeded the EPA SMCL of 250 mg/L for chloride in 

drinking water. 

The alkalinity of a solution indicates its capacity to 

neutralize acid and commonly results from dissolved carbonate and bicarbonate ions (Hem, 1985). Alkalinity, also referred 

to as the acid neutralizing capacity, generally increases 

with the pH of a water sample. Alkalinity ranged from 

26.1 to 347 mg/L as calcium carbonate (CaCO3); the median 

concentration was 178 mg/L (table 1). The highest alkalinities 

were from the Lock Haven Formation, and the lowest were 

from Huntley Mountain Formation (fig. 4.4).

The hardness of the 72 well-water samples ranged from 

1.32 to 627 mg/L as CaCO3, with a median value of 134 mg/L 

as CaCO3. Hardness is directly related to the concentrations of 

calcium and magnesium (computed as the sum of calcium, in 

mg/L, multiplied by a factor of 2.5 plus magnesium, in mg/L, 

multiplied by a factor of 4.1) and generally is comparable 

to the alkalinity. The alkalinity and associated cations can 

be released into groundwater by the dissolution of calciumand magnesium-bearing minerals, such as calcite (CaCO3

) 

and dolomite (CaMg(CO3)2), that generally are associated 

with limestone and other calcareous sedimentary rocks. 

Hard water decreases the lathering of soap and increases 

accumulation of mineral deposits in plumbing and cooking 

utensils. Using a common hardness classification (Durfor and 

Becker, 1964), the measured values indicate that 8 (11.1 percent) of the 72 water samples are soft (less than 60 mg/L 

as CaCO3), 19 samples (26.4 percent) are moderately hard 

(61 to 120 mg/L as CaCO3), 27 samples (37.5 percent) are 

hard (121 to 180 mg/L as CaCO3), and 18 samples (25.0 percent) are classified as very hard (greater than 180 mg/L 

as CaCO3). Median hardness values were 52.1, 128, and 

147 mg/L as CaCO3 for the Huntley Mountain, Catskill, and 

Lock Haven Formations, respectively. Hardness varied with 

pH, with hardness generally greatest at near-neutral pH values 

(6.5 to 7.5). Water samples with low pH (less than 6.5) and 

very high pH (greater than 8.0) typically had hardness less 

than 60 mg/L and were considered soft. The high-pH waters 

with low hardness typically have elevated sodium, which 

may result from natural water-softening processes, which are 

described later. There are no health-related standards established specifically for hardness in drinking water. 

Hard water has greater potential than soft water to 

form scale or encrustations on plumbing and fixtures. Water 
12 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

Table 1. Minimum, median, and maximum values of selected characteristics and constituents in groundwater samples collected in 

72 domestic wells in Bradford County, Pennsylvania, May–August 2016. Available U.S. Environmental Protection Agency Maximum 

Contaminant Levels, Action Levels, and Secondary Maximum Contaminant Levels for analyzed constituents (2012 Edition of the Drinking 

Water Standards and Health Advisories [EPA 822-S-12-001])

[n, number of results; EPA, U.S. Environmental Protection Agency; MCL, Maximum Contaminant Level; SMCL, Secondary Maximum Contaminant Level; 

—, no MCL, Action Level, or SMCL established; °C, degrees Celsius; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter; mV, millivolts; 

CaCO3, calcium carbonate; SiO2, silicon dioxide; μg/L, micrograms per liter; <, less than; MPN/100 mL, most probable number per 100 millimeters; >, greater 

than; pCi/L, picocuries per liter; 72-hr, sample analyzed for gross alpha concentration at approximately 72 hours after sample collection as referenced to a detector calibrated using thorium-230 (230Th); 30-d, sample used for the 72-hour gross alpha analysis is counted a second time approximately 30 days after the initial 

count as referenced to a detector calibrated using 230Th; δ, delta; mil, million]

Constituent (units) n Minimum Median Maximum

Results above the 

reporting level Results exceeding standard EPA 

MCL

EPA 

SMCL

Number Percent Number Percent Graph

Well characteristics

Well depth (feet) 72 42 183 600 — — — — — — —

Physical properties

Water temperature (°C) 72 9.40 11.5 18.0 72 100 — — — — —

Dissolved oxygen (mg/L) 72 0.03 1.21 10.6 72 100 — — — — —

Specific conductance, field (μS/cm) 72 72.4 365 3,410 72 100 — — — — —

pH, field (standard units) 72 6.18 7.47 9.31 72 100 6 8.3 — 6.5–8.5

Redox potential (mV) 71 −190 140 370 71 100 — — — — —

Turbidity (Nephelometric Turbidity Ratio Units) 70 0.12 1.19 247 70 100 — — — — —

Laboratory analysis

Specific conductance, lab (μS/cm) 72 79.3 369 3,800 72 100 — — — — —

pH, lab (standard units) 72 6.96 8.09 9.16 72 100 3 4.2 — 6.5–8.5

Alkalinity, lab (mg/L CaCO3) 72 26.1 178 347 72 100 — — — — —

Total dissolved solids (mg/L) 72 52.7 218 2,030 72 100 4 5.6 — 500

Suspended solids (mg/L) 72 15.0 15.0 488 5 6.94 — — — — —

Hardness, total (mg/L CaCO3) 71 1.32 134 627 71 100 — — — — —

Major ions

Calcium (mg/L) 72 0.04 39.6 164 72 100 — — — — —

Magnesium (mg/L) 72 0.01 8.37 51.6 72 100 — — — — —

Sodium (mg/L) 72 2.18 19.1 760 72 100 35 48.6 — 30–60

Potassium (mg/L) 72 0.22 1.40 4.89 72 100 — — — — —

Chloride (mg/L) 72 0.77 5.28 1,020 72 100 1 1.4 — 250

Fluoride (mg/L) 72 0.03 0.12 0.78 71 98.6 0 0 4 2

Sulfate (mg/L) 72 0.06 13.5 418 71 98.6 0 0 — 250

Silica (mg/L as SiO2) 72 6.05 11.1 15.9 72 100 — — — — —

Trace elements

Aluminum (μg/L) 72 < 3.00 < 3.00 771 10 13.9 1 1.4 — 50

Antimony (μg/L) 72 < 0.02 < 0.02 0.30 31 43.1 0 0 6 —

Arsenic (μg/L) 72 < 0.05 0.63 23.2 67 93.1 2 2.8 10 —

Barium (μg/L) 72 < 0.25 142 4,160 71 98.6 2 2.8 2,000 —

Beryllium (μg/L) 72 < 0.01 0.02 0.10 8 11.1 0 0 4 —

Boron (μg/L) 72 < 5.00 68.9 567 70 97.2 — — — — —

Bromide (mg/L) 72 < 0.01 0.02 8.60 58 80.6 — — — — —

Cadmium (μg/L) 72 < 0.03 0.03 0.31 2 2.78 0 0 5 —

Chromium (μg/L) 72 < 0.30 < 0.30 5.00 2 2.78 0 0 100 —

Cobalt (μg/L) 72 < 0.03 0.05 0.75 17 23.6 — — — — —

Copper (μg/L) 72 < 0.20 0.82 82.3 37 51.4 0 0 — 1,000

Iron (μg/L) 72 < 4.00 11.34 1,830 44 61.1 8 11.1 — 300

Lead (μg/L) 72 < 0.02 0.10 2.61 53 73.6 0 0 15 —

Lithium (μg/L) 72 0.41 21.8 1,510 72 100 — — — — —

Manganese (μg/L) 72 < 0.40 10.5 1,120 56 77.8 22 30.6 — 50

Molybdenum (μg/L) 72 < 0.05 0.48 2.42 68 94.4 — — — — —

Nickel (μg/L) 72 < 0.20 < 0.20 2.00 30 41.7 — — — — —
Groundwater Quality and Comparison to Drinking Water Health Standards 13

Table 1. Minimum, median, and maximum values of selected characteristics and constituents in groundwater samples collected in 

72 domestic wells in Bradford County, Pennsylvania, May–August 2016.—Continued Available U.S. Environmental Protection Agency 

Maximum Contaminant Levels, Action Levels, and Secondary Maximum Contaminant Levels for analyzed constituents (2012 Edition of 

the Drinking Water Standards and Health Advisories [EPA 822-S-12-001])

Constituent (units) n Minimum Median Maximum

Results above the 

reporting level Results exceeding standard EPA 

MCL

EPA 

SMCL

Number Percent Number Percent Graph

Trace elements—Continued

Selenium (μg/L) 72 < 0.05 < 0.05 0.56 30 41.7 0 0 50 —

Silver (μg/L) 72 < 0.02 < 0.02 10.0 1 1.39 0 0 — 100

Strontium (μg/L) 72 < 0.80 539 3,860 71 98.6 — — — — —

Zinc (μg/L) 72 < 2.00 3.23 981 47 65.3 0 0 — 5,000

Nutrients

Kjeldahl nitrogen, total (mg/L as Nitrogen) 72 < 0.07 0.07 1.71 33 45.8 — — — — —

Ammonia (mg/L as Nitrogen) 72 < 0.01 < 0.01 1.60 42 58.3 — — — — —

Nitrite (mg/L as Nitrogen) 72 < 0.001 < 0.001 0.23 17 23.6 0 0 1 —

Nitrate + Nitrite (mg/L as Nitrogen) 72 < 0.04 < 0.04 7.01 36 50.0 0 0 10 —

Orthophosphate (mg/L as Phosphorus) 72 < 0.004 0.01 0.16 63 87.5 — — — — —

Bacteria

Total coliform (MPN/100 mL) 71 < 1 < 1 > 2,420 35 49.3 35 49.3 0 —

Escherichia Coli (MPN/100 mL) 71 < 1 < 1 > 2,420 6 8.5 6 8.5 0 —

Radiochemicals

Gross alpha radioactivity, 30-d (pCi/L) 72 −0.89 1.00 8.3 39 54.2 0 0.0 15 —

Gross alpha radioactivity, 72-hr (pCi/L) 72 −0.33 1.70 11.2 52 72.2 0 0.0 15 —

Gross beta radioactivity, 30-d (pCi/L) 72 −0.50 2.34 8.1 57 79.2 12 16.7 4 —

Gross beta radioactivity, 72-hr (pCi/L) 72 0.03 2.40 5.89 64 88.9 12 16.7 4 —

Radon-222 (pCi/L) 71 34.4 614 8,540 71 100 50 70.4 1

300 —

Uranium (pCi/L) 72 < 0.01 0.26 10.4 62 86.1 0 0.0 30 —

Radium-226 (pCi/L) 8 0.16 1.08 2.08 8 100 0 0.0 5 —

Radium-228 (pCi/L) 8 −0.02 0.31 1.54 4 50.0 0 0.0 5 —

Glycols, alcohols and oil/grease

Ethanol (mg/L) 72 < 5.00 < 5.00 < 5.00 0 0 — — — — —

Ethylene glycol (mg/L) 72 < 5.00 < 5.00 < 5.00 0 0 0 0.0 14 —

Isobutyl alcohol (mg/L) 72 < 5.00 < 5.00 < 5.00 0 0 — — — — —

Isopropyl alcohol (mg/L) 72 < 5.00 < 5.00 < 5.00 0 0 — — — — —

Methanol (mg/L) 72 < 5.00 < 5.00 < 5.00 0 0 — — — — —

n-Butanol (mg/L) 72 < 5.00 < 5.00 < 5.00 0 0 — — — — —

n-Propanol (1-Propanol) (mg/L) 72 < 5.00 < 5.00 < 5.00 0 0 — — — — —

Propylene glycol (mg/L) 72 < 5.00 < 5.00 < 5.00 0 0 — — — — —

Oil and grease (mg/L) 72 < 5.00 < 5.40 < 6.00 0 0 — — — — —

Volatile organic compounds2

Bromodichloromethane (CHBrCl2) (μg/L) 72 < 0.50 < 0.50 1.04 1 1.39 0 0 600 —

Dibromochloromethane (μg/L) 72 < 0.50 < 0.50 1.58 1 1.39 0 0 60 —

Tribromomethane (μg/L) 72 < 0.50 < 0.50 0.81 1 1.39 0 0 80 —

Dissolved gases

Methane (mg/L) 72 < 0.0002 0.006 77 64 88.9 9 12.5 3

> 7 —

Ethane (mg/L) 72 < 0.0002 0.0002 0.13 15 20.8 — — — — —

Propane (mg/L) 72 < 0.0002 0.0002 0.001 2 2.78 — — — — —

δ13C (per mil in CH4) 6 −65.05 −44.37 −38.23 6 100 — — — — —

δD (per mil in CH4) 6 −257.2 −203.65 −191.2 6 100 — — — — —

1

Proposed EPA MCL for states without an enhanced indoor air program.

2

Volatile organic compounds (VOCs) with detections are shown. A full listing of all VOCs sampled but not detected are shown in Appendix 4.

3

Pennsylvania action level for methane.

EXPLANATION

Exceeds MCL Exceeds SMCL No exceedance
14 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

resources engineers commonly identify the potential for 

encrustation or corrosion on the basis of the Langelier Index 

(LI), which provides an indication for the potential for lead 

and copper to enter water supplies from pipes and plumbing 

(Snoeyink and Jenkins, 1981). The LI, which is the difference 

between the measured pH and the pH at equilibrium with calcium carbonate (CaCO3

), is equal in value to the calcite saturation index (SICAL), evaluated in more detail later. If the LI or 

SICAL is positive, the pH is greater than that at equilibrium with 

CaCO3 and the water will tend to deposit CaCO3 coatings or 

scale that can insulate pipes, boilers, and other components of 

a system from contact with water; however, if negative, then 

the water is undersaturated with CaCO3 and will tend to be 

corrosive in the distribution system. An optimum condition is 

desired whereby the LI or SICAL is close to zero, whereby the 

water will neither be strongly corrosive nor scale forming. For 

the 72 groundwater samples, SICAL ranged from −2.6 to 0.8. 

Of the 72 samples, 11 (2.8 percent) had SICAL values that were 

EXPLANATION

Allegheny and Pottsville Formations

Pottsville Formation

Burgoon Sandstone

Fault

Huntley Mountain Formation

Catskill Formation

Lock Haven Formation

76°45' 76°30' 76°15'

42°

41°45'

A. Arsenic

76°45' 76°30' 76°15'

42°

41°45'

B. Chloride

76°45' 76°30' 76°15'

42°

41°45'

C. pH 76°45' 76°30' 76°15'

42°

41°45'

D. Methane

Base from U.S. Bureau of the Census digital data, 2000

1:100,000-scale Albers equal-area projection

Standard parallels 40 ̊ N and 42 ̊ N, 

central meridian −78 ̊ W, latitude of origin 39 ̊

Geology from Miles and others, 2001

0 10 20 KILOMETERS

0 10 20 MILES

pH, in standard units

≤ 6.50

> 6.50 to 7.50

> 7.50 to 8.00

> 8.00

Methane concentration, in 

milligrams per liter

≤ 2.0

> 2.0 to 10

> 10 to 26

> 26

Arsenic concentration, in 

micrograms per liter

≤ 0.10

> 0.10 to 5.0

> 5.1 to 10

>10

Chloride concentration, in 

milligrams per liter

≤ 25

> 25 to 100

> 100 to 500

> 500

Figure 3. Spatial distribution of A, arsenic concentrations, B, chloride concentrations, C, pH values, and D, methane concentrations in 

72 wells sampled and bedrock geology in Bradford County, Pennsylvania. (≤, less than or equal to; >, greater than)
Groundwater Quality and Comparison to Drinking Water Health Standards 15

less than −0.5, indicating potentially strongly corrosive characteristics; the remaining samples would be considered neither 

strongly corrosive nor scale forming. Two samples characterized as strongly corrosive, BR 893 and BR 952, had SICAL less 

than −1.0 and hardness less than 30 mg/L as CaCO3, but pH 

values were 8.1 and 7.0, respectively. 

Nutrients

Nutrients include nitrogen and phosphorous species. 

Nitrogen is found in water principally as nitrate (NO3), nitrite 

(NO2), and ammonia (NH4), whereas phosphorus is present mainly as orthophosphate (PO4

). Nutrients are essential 

for plant growth; however, the presence of elevated nutrient 

concentrations generally is an indicator of anthropogenic 

sources, which might include fertilizers, storm runoff, animal 

wastes, and effluent from septic systems. Summary statistics 

for nutrients are given in table 1. Nitrate is the most prevalent 

nitrogen species in oxic groundwater, but all nitrate concentrations were still less than the EPA MCL of 10 mg/L for nitrate 

as N (table 1). Excessive nitrate in drinking water causes a 

health risk, especially in infants, because it disrupts oxygen 

flow in blood. Infants below the age of six months who drink 

water containing nitrate in excess of the MCL could become 

seriously ill, develop symptoms including shortness of breath, 

and blue-baby syndrome. Because concentrations of nitrite are 

so low, nitrate plus nitrite concentrations essentially represent 

nitrate concentrations. The concentration of nitrate in samples 

ranged from less than 0.04 to 7.01 mg/L as N; the median concentration was less than 0.04 mg/L as N. The concentration of 

orthophosphate ranged from less than 0.004 to 0.16 mg/L with 

a median of 0.01 mg/L.

Bacteria

All samples were analyzed for total coliform and Escherichia coli (E. coli) bacteria to determine the associated 

human-health risk of potentially pathogenic (disease-causing) 

waterborne microorganisms. Coliform bacteria are ubiquitous in the environment and are not always pathogenic. Some 

strains of E. coli, such as O157:H7, can cause severe illness, 

and the presence of E. coli may indicate general fecal contamination because of the strong association of E. coli with the 

intestines of warm-blooded animals. Any detection of E. coli

in public drinking-water supplies is considered cause for concern and a violation of health standards (U.S. Environmental 

Protection Agency, 2001), and these EPA criteria can be used 

to provide a reference point for domestic wells.

Total coliform bacteria were detected in 35 of the 

71 groundwater samples (48.6 percent) (table 1). Total coliform concentrations ranged from less than 1 colony (reported 

as most probable number [MPN] per 100 milliliters [mL]) 

to greater than 2,420 MPN/100mL. Of the 35 samples that 

had total coliform bacteria, 6 samples had detectable E. coli, 

with concentrations ranging from 1 to greater than 2,420 

MPN/100mL. The most probable number could not be determined for BR 890, but a present/absent test with the lab determined this sample was absent of total coliform and E. coli

bacteria, and for the summary table (table 1) this sample was 

represented as less than 1 MPN per 100 milliliters. No value 

was available for BR 928.

Metals and Trace Elements

Metals and other trace elements typically are present in 

concentrations less than 1 mg/L in natural waters (Hem, 1985). 

Most metals and trace elements in groundwater are leached 

from soil or dissolved from underlying bedrock in minute 

quantities by groundwater. Some are present in precipitation. 

Summary statistics for metals and trace elements are listed in 

table 1. 

The EPA has established MCLs and SMCLs for various metals and trace elements in drinking water (table 1). 

Two samples (2.8 percent) exceeded the EPA MCL for arsenic 

and barium, 8 samples (11.1 percent) exceeded the SMCL for 

dissolved iron, 22 samples (30.6 percent) exceeded the SMCL 

for dissolved manganese, and 1 sample exceeded the SMCL 

for aluminum with a value of 770.0 μg/L. None of the samples 

exceeded the EPA MCL or SMCL for antimony, beryllium, 

cadmium, chromium, copper, lead, selenium, silver, or zinc 

(table 1). 

Arsenic concentrations ranged from less than 0.05 to 

23.2 μg/L with a median concentration of 0.63 μg/L (table 1). 

Arsenic concentrations were locally elevated and were not 

consistently related to pH (fig. 4A). Two samples, BR 907 and 

BR 929, had arsenic concentrations exceeding the EPA MCL 

of 10 μg/L and had a pH of 7.7 and 8.0, respectively. Possible 

health effects associated with ingestion of drinking water with 

arsenic in excess of the MCL for many years include skin 

damage, circulatory system problems, and increased cancer 

risk. Arsenate (AsVO4

3−) and arsenite (AsIIIO33−), which are the 

predominant forms of arsenic in groundwater, tend to adsorb 

to a variety of aquifer materials, including iron oxides, aluminum oxides, and clay minerals, at mildly acidic to neutral pH, 

but not at alkaline pH conditions (Dzombak and Morel, 1990; 

Smedley and Kinniburgh, 2002). Furthermore, under strongly 

acidic or reducing conditions, oxide minerals may become 

unstable and dissolve, releasing arsenic to solution. More 

detailed explanation of the effects of pH on sorption processes 

and the possible relations between redox state and arsenic 

concentration are presented in a later section of this report. 

Although the highest concentrations of bromide 

(8.60 mg/L) and sodium (760 mg/L) were found in samples 

having pH greater than 7.5 (fig. 4), most high pH samples 

did not have elevated concentrations of these constituents. In 

contrast, the highest concentrations of lead (2.61 μg/L), copper (82.3 μg/L), zinc (981 μg/L), and other trace metals were 

found in samples that had pH less than 7.5 (fig. 4G–H). Generally, decreased concentrations of trace metals with increased 

pH may be explained by their strong adsorption at alkaline pH 
16 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

H

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

80

90

70

60

50

40

30

20

10

0

Copper, in micrograms per liter

pH (standard units)

2.40

2.80

2.00

1.60

1.20

0.80

0.40

0.00

Lead, in micrograms per liter

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

pH (standard units)

G

F 80

70

60

50

40

30

20

10

0

Methane, in milligrams per liter

8

10

6

4

2

0

Bromide, in milligrams per liter

E

D 800

700

600

500

400

300

200

100

0

Sodium, in milligrams per liter

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Fluoride, in milligrams per liter

C

600

500

400

300

200

100

0

Boron, in micrograms per liter

24 B

20

16

12

8

4

0

Arsenic, in micrograms per liter

A

EXPLANATION

Groundwater—Dissolved oxygen ≥ 0.5 mg/L

Groundwater—Dissolved oxygen < 0.5 mg/L

Figure 4. Relation between pH and selected constituents for 72 groundwater samples in Bradford County, Pennsylvania. (≥, greater 

than or equal to; <, less than)
Groundwater Quality and Comparison to Drinking Water Health Standards 17

by iron oxides, whereas the increased concentrations of trace 

anions, such as arsenic or boron (present as borate), may result 

from their desorption at alkaline pH, explained in more detail 

further on. The increased concentrations of sodium could 

result from cation-exchange reactions, also explained further 

on.

Elevated concentrations of iron and manganese in water 

may impart a bitter taste and stain laundry and plumbing 

fixtures with a yellowish or brownish-orange color. Concentrations of dissolved iron ranged from less than 4.0 to 1,830 μg/L 

with a median of 11.34 μg/L. Eight of these samples (11.1 percent) exceeded the EPA SMCL of 300 μg/L for iron. Concentrations of dissolved manganese in water ranged from less than 

0.40 to 1,120 μg/L; the median concentration was 10.5 μg/L. 

Water samples from 22 of the 72 sampled wells (30.6 percent) 

exceeded the EPA SMCL of 50 μg/L.

Although none of the groundwater samples had concentrations of lead or copper in excess of the respective MCL 

and SMCL values of 15 and 1,000 μg/L, corrosive water, as 

previously described by the LI or SICAL less than 0.5, could 

acquire metals from lead or copper pipes in the household 

water system. Long-term exposure to excess copper can cause 

liver and kidney damage, whereas lead can cause neurological 

problems, especially in young children. Depending on water 

treatment, the corrosive characteristics may or may not be 

mitigated. Sampling at the tap would be advisable to evaluate 

the actual concentrations of constituents in household drinking 

water and to evaluate the potential effects of water treatment. 

Lead and copper levels could be much higher if samples were 

collected before the plumbing system is flushed, as in this 

study. 

Radionuclides

Radionuclides naturally present in rocks and soils may be 

dissolved or leached into groundwater. Analyses for radioactivity and radionuclides include gross alpha radioactivity, 

gross beta radioactivity, and dissolved radon-222 (radon gas). 

Uranium, a radioactive element, also was analyzed in the dissolved form. Naturally occurring radioactivity is described in 

detail in previous reports on groundwater quality in Wayne, 

Pike, and Lycoming Counties in Pennsylvania (Senior and 

others, 2017; Senior and Cravotta, 2017; and Gross and Cravotta, 2017, respectively). Summary statistics for radioactive 

constituents in groundwater samples from Bradford County 

as well as the established or proposed EPA MCLs for some of 

these constituents are given in table 1. 

The activities of radon-222 in water from the 72 sampled 

wells ranged from 34.4 to 8,540 pCi/L with a median activity 

of 614 pCi/L (table 1). The EPA does not regulate radon-222 

in drinking water. However, under the framework specified 

by the 1999 Notice for the Proposed Radon in Drinking Water 

Rule (Federal Register, 1999), the EPA proposed an alternative maximum contaminant level (AMCL) of 4,000 pCi/L for 

radon-222 for community water systems that use groundwater 

for all or some of the supply in states with an enhanced indoor 

air radon program. For states without an enhanced indoor air 

program, the EPA proposed an MCL of 300 pCi/L for radon222. Water samples from 50 of the 72 wells sampled (70.4 percent) exceeded the proposed EPA MCL of 300 pCi/L, and only 

one sample exceeded the proposed EPA AMCL of 4,000 pCi/L 

for radon-222.

The gross alpha-particle radioactivity (72-hour count) 

in water from the 72 sampled wells ranged from −0.33 to 

11.2 pCi/L; the median activity level was 1.7 pCi/L (table 1). 

Alpha particles are counted at 72 hours and 30 days because 

some alpha-particle emitters, such as radium-224 (half-life 

of 3.6 days), would not be present in the 30-day count. Gross 

alpha-particle activity in the 30-day count was similar to or 

slightly less than the activity in the 72-hour count. No water 

sample exceeded the EPA MCL of 15 pCi/L for gross alphaparticle activity (table 1).

The gross-beta particle radioactivity (72-hour count) 

ranged from 0.03 to 5.89 pCi/L. Gross beta-particle activity in 

the 30-day count was similar to or slightly less than the activity in the 72-hour count in about half of the samples (table 1). 

The concentration of uranium ranged from less than 0.01 to 

10.4 μg/L. The median concentration was 0.26 μg/L. No water 

samples exceeded the EPA MCL of 30 μg/L for uranium.

A subset of eight samples, which had the highest grossalpha and gross-beta activities of the 72 wells sampled, was 

analyzed for radium-226 and radium-228. Concentrations 

(activities) of radium-226 ranged from 0.16 to 2.08 pCi/L 

in water samples, and activities of radium-228 ranged from 

−0.02 to 1.54 from the eight wells. None of the samples 

exceeded the EPA MCL of 5 pCi/L for combined radium-226 

and radium-228. The highest activities of radium-226 and 

radium-228 were measured for the same sample, BR 921. This 

sample also had the highest 30-day gross alpha and 30-day 

gross beta activities of the 72 samples reported herein.

Dissolved Methane and Other Naturally 

Occurring Hydrocarbon Gases

Water sampled from the 72 wells had concentrations of 

dissolved methane ranging from less than 0.0002 to 77 mg/L, 

plus lower to nondetectable concentrations of other, more 

complex hydrocarbon gases, including ethane, ethylene, 

propane, propylene, iso-butane, and N-butane (in order of 

increasing number of hydrocarbon chains) (table 1). None of 

the samples had detectable concentrations of the dissolved 

hydrocarbons iso-pentane, N-pentane, and hexane.

Detectable levels of methane were found in water from 

64 of the 72 wells sampled (88.9 percent) for which concentrations were greater than the reporting level of 0.0002 mg/L 

(table 1). Nine of the samples had dissolved methane concentrations that exceeded the Pennsylvania action level of 

greater than 7 mg/L (Commonwealth of Pennsylvania, 2014). 

Most samples with detectable concentrations of methane 

had alkaline pH; the 10 water samples with highest methane 
18 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

concentrations had pH greater than 7.5 (fig. 4F). Eight of these 

10 samples also had low concentrations of dissolved oxygen 

(less than 0.5 mg/L) (fig. 4F).

Man-Made Organic Compounds

All samples were analyzed for selected man-made 

organic compounds, including VOCs, glycols and alcohols, 

and oil and grease (table 1). Out of these man-made organic 

compounds, only three were measured in detectable concentrations in 1 of the 72 wells sampled. 

VOCs include a wide range of natural and synthetic 

carbon-based compounds that have high vapor pressure and 

relatively low solubility in water. VOCs are used in industrial, commercial, and domestic applications and can enter the 

groundwater as liquid through spills and leaks or by atmospheric deposition. VOCs typically found in groundwater 

include industrial solvents, fuel hydrocarbons and oxidizers, 

fumigants, organic synthesis compounds, refrigerants, and 

disinfection byproducts (trihalomethanes) (Carter and others, 2010). VOCs are the most commonly found contaminant 

class associated with industrial and commercial sites, dumps, 

landfills, hazardous waste facilities, and military bases (Zogorski and others, 2006) and are widespread where other human 

activities occur in developed countries. Historically, many 

waste chemicals were disposed of indiscriminately, and the 

widespread use of VOCs has resulted in a considerable mass 

of material released to the environment. Localized releases 

of VOCs occur from many sources, including leakage from 

storage tanks, direct application of pesticides containing VOCs 

(Barbash and Resek, 1996), septic systems, and leaking sewer 

systems. VOCs also are released to the atmosphere through 

engine exhausts, aerosol sprays, leakage of refrigerants, and 

application of fumigants and pesticides. Trihalomethanes are 

frequently associated with chlorinated water and chlorinated 

cleaning products.

The reporting level for VOCs was 0.5 mg/L, which is 

lower than drinking-water MCLs established for 14 of the 

68 analyzed VOCs (U.S. Environmental Protection Agency, 

2012a). However, drinking-water MCLs for two compounds—

1,2-dibromo-3-chloropropane (DBCP) and 1,2-dibromoethane 

(EDB)—are less than the method reporting level as performed 

by the laboratory, so for these compounds, a more sensitive 

method is needed to determine if they are present in concentrations greater than the MCLs (although below the reporting 

level of 0.5 mg/L). Therefore, with the exception of these two 

compounds, if VOCs are present in groundwater at concentrations less than the reporting level of 0.5 mg/L, those concentrations would not exceed an established drinking-water 

standard. MCLs have not been established for VOCs where 

data are insufficient to evaluate human-health effects.

Of the 68 VOCs analyzed in the 72 groundwater samples, 

only 3 trihalomethanes were detected above the reporting 

level: bromodichloromethane (1.04 μg/L), dibromochloromethane (1.58 μg/L), and tribromomethane (0.81 μg/L). All 

three VOCs were detected in samples from the same well, 

BR 913, but concentrations of the detected VOCs did not 

exceed any established EPA drinking-water standard. Although 

the measured VOC concentrations do not pose any established 

health risk, the detection of these man-made compounds in 

groundwater indicates a possible low level of groundwater 

contamination by human activities.

Glycols and alcohols, and oil and grease, were less than 

the 5 mg/L reporting level. Although lower concentrations of 

these constituents may be present in the sampled groundwater, 

at the time of this study, a more sensitive approved method 

with lower reporting levels was not available. As of 2018, 

there are no established EPA drinking-water standards for oil 

and grease.

Chemical Analysis and Relations 

Among Constituents in Groundwater

Dissolved constituents in groundwater may be derived 

from atmospheric, geologic, biologic, and anthropogenic 

sources as the aqueous solution interacts with various materials along transport pathways. Solute concentrations can range 

widely depending on the presence of constituent elements 

in the source(s), the extent of contact between water and the 

source, the aqueous solubility and interactions among the 

dissolved elements, plus geochemical conditions such as pH 

and oxidation-reduction (redox) state that affect element form, 

mobility, and transport in the aqueous environment.

Conceptual Hydrogeochemical Model

A conceptual hydrogeochemical model that indicates 

spatial relations among the salinity, water types, redox, and 

methane sources associated with shallow, intermediate, and 

deep groundwater flow systems for Bradford and surrounding counties is described in Gross and Cravotta (2017). Most 

wells constructed for domestic use are completed within the 

local freshwater-flow system. The wells sampled in Bradford County for this study were completed to depths ranging 

from 42 to 600 feet, with half of the wells drilled to depths of 

175 to 400 feet. Several wells with depths ranging from 165 to 

300 feet exhibited characteristics of brine-influenced waters 

with characteristics of elevated SC and TDS dominated by 

sodium and chloride, plus elevated concentrations of bromide, 

lithium, and methane, as explained in more detail below. 

Relations Among pH, Specific Conductance, and 

Constituent Concentrations

Evaluation of the correlations among chemical constituents and environmental variables provides insight on hydrochemical processes affecting groundwater chemistry in the 

study area. The pH, SC, and associated chemical compositions 
Chemical Analysis and Relations Among Constituents in Groundwater 19

of the 72 groundwater samples collected in Bradford County 

during 2016 ranged widely, as indicated by pH values from 

6.18 to 9.31 and SC from 72.4 to 3,410 μS/cm (fig. 5). Most of 

the samples had pH values from 7 to 8.2 and SC from 300 to 

550 μS/cm. Only 3 of the 72 well-water samples in this study 

had SC values greater than 1,000 μS/cm. In addition to figure 

5, which shows the relations among pH, SC, and multiple 

inorganic constituents, boxplots are included in the appendix 

that show the overall relations among individual constituents 

with pH class interval (fig. 4.1) and SC class interval (fig. 4.2). 

Additional boxplots show the same constituents as a function 

of redox class interval (fig. 4.3), bedrock formation (fig. 4.4), 

and topographic position index (fig. 4.5). 

The pH for the 72 groundwater samples was positively 

correlated with SC and other measures of ionic strength, 

including TDS (TDScalc and ROE), whereas the pH and SC 

were negatively correlated with the concentration of DO. With 

increased pH, the concentrations of several major and trace 

constituents generally increased, including alkalinity, sodium 

(Na), lithium (Li), boron (B), fluoride (F), and arsenic (As) 

(fig. 5, fig. 4.1). Despite positive correlations between SC and 

the concentrations of chloride (Cl) and bromide (Br), (fig. 5), 

those constituents were not positively correlated with pH over 

the entire range of measured values (table 3.2). Concentrations 

of potassium (K) positively correlated with SC, but were not 

correlated with pH (table 3.2).

The relation between the pH and concentrations of Ca, 

Mg, and hardness changes from positive to negative at about 

pH 7.5 (fig. 5). For pH values less than 7.5, the concentrations of hardness, Ca, Mg, and other cations, including Sr, Na, 

K, Li, and alkalinity, generally increased with pH. However, 

for pH values greater than 7.5, the concentrations of hardness, Ca, Mg, Sr, and Ba decreased with increased pH, while 

SC and concentrations of Na, alkalinity, and TDS continued to increase. Such patterns in pH, SC, and constituent 

concentrations are consistent with the dissolution of calcite 

(CaCO3 + H+ = Ca2+ + HCO3

−

) over the range of pH, combined 

with cation exchange at pH greater than 7.5. Through cationexchange reactions, the hardness is removed, or softened, as 

Ca, Ba, Mg, and Sr ions displace Na ions from exchange sites 

on clay minerals (0.5Ca2+ + NaX = 0.5CaX2 + Na+) (Appelo 

and Postma, 2005), as explained in more detail below.

Boxplots show the constituent concentrations for four different pH class intervals (fig. 4.1). As indicated by the scatter 

plots that showed constituent concentrations as a function of 

pH for all 72 groundwater samples (fig. 5), most constituent 

concentrations increased with pH to values of 7.4 (acidic to 

neutral range). The most alkaline pH class (8.0<pH<9.4) had 

higher median values for SC, TDS, Na, K, alkalinity, Cl, Br, F, 

B, Li, and methane than the acidic (6.1<pH<6.4) or near-neutral (6.5<pH<7.4) pH classes. In contrast, the most alkaline pH 

class had lower or equal medians for hardness, Ca, Mg, nitrate 

(NO3), dissolved oxygen, Zn, U, and Rn-222 compared to the 

acidic to near-neutral pH classes. 

Boxplots also show constituent concentrations for four 

different SC class intervals (fig. 4.2). As indicated by the 

scatter plots that showed constituent concentrations as a function of SC for all 72 groundwater samples (fig. 6), most constituent concentrations increased with SC to values approaching 700 μS/cm. The highest SC class (600<SC<3,800 μS/cm) 

had higher median values for pH, SC, TDS, Na, K, alkalinity, 

Cl, Br, F, B, Li, methane, ammonia (NH3), and phosphate 

(PO4), and lower median values for uranium and Rn-222 than 

the lower SC classes. Constituents that did not vary with SC 

and (or) pH may be controlled by other factors, such as redox 

state.

The constituent concentrations for anoxic, mixed, and 

oxic redox class intervals also are illustrated as boxplots 

(fig. 4.3). Although most constituent concentrations did not 

vary with redox, the medians for several constituents were significantly different between the anoxic and oxic redox classes. 

The anoxic samples had higher median values for pH, Na, Ba, 

Sr, Br, F, B, NH3, Mo, Fe, Mn, and methane, and lower median 

values for DO, NO3, U, and Rn-222 than the oxic samples.

Ionic Contributions to Conductivity and Total 

Dissolved Solids

Despite general correlations between SC and major ion 

concentrations, the predominance of various ionic contributions to the SC varied widely for samples with comparable 

values of SC (fig. 6). Calcium and bicarbonate were the predominant ions in most samples, considering the ionic contributions to conductivity or considering the ion concentrations 

in units of mass, moles, or equivalents. Although sodium and 

chloride were subordinate in most samples, these constituents 

were the predominant ions in a few samples, particularly those 

with SC greater than 1,000 μS/cm, but also in several samples 

with relatively low values of SC. Sulfate was subordinate to 

bicarbonate and chloride as a source of conductivity in the 

samples; its importance generally decreased with increased 

SC. Contributions by magnesium were comparable to sulfate; however, contributions by nitrate, potassium, hydrogen, 

hydroxyl, or carbonate ions were relatively minor.

Variations in the major ion species contributions to the 

laboratory measured SC values for six selected groundwater 

samples are illustrated in figure 7. To provide context, the samples illustrated in figure 7 are identified among all 72 samples 

in figure 6. Calcium, bicarbonate, magnesium, sodium, sulfate, 

and chloride ions are the predominant sources of conductivity for the average of the 72 samples and most other samples 

having low to moderate SC values (fig. 7A, BR 952; fig. 7B, 

BR 963), with minor contributions from nitrate, potassium, 

and hydrogen ions. Samples with elevated SC values generally have increased contributions from sodium, bicarbonate, 

and chloride (fig. 7D, BR 941; fig. 7F, BR 913) and, in some 

cases, sulfate (fig 7E, BR 946).

Sample BR 952 (fig. 7A) is representative of minimally 

evolved groundwaters with a relatively low SC value of 

72 μS/cm. The conductivity of this sample can be produced as 

the result of rainwater evaporation plus dissolution of various 
20 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

0.0001

0.001

0.01

0.1

1

10

100

Concentration, in milligrams per liter

F

10 100 1,000 10,000

Specific conductance, in microsiemens per centimeter

K Li

Br F

As B

SO4 Hrd

Ca Mg

Sr Ba

0.0001

0.001

0.01

0.1

1

10

100

1,000

Concentration, milligrams per liter

E

10 100 1,000 10,000

Specific conductance, in microsiemens per centimeter

TDS ALK Cl

Na O2

0

1

2

3

4

5

6

7

8

9

10

11

12

13

Dissolved oxygen, in milligrams per liter

10 100 1,000 10,000

Specific conductance, in microsiemens per centimeter

0.1

1

10

100

1,000

10,000

Concentration, in milligrams per liter

D

K Li Br F As B

pH

6.0 6.4 6.8 7.2 7.6 8.0 8.4 8.8 9.2 9.6

0.0001

0.001

0.01

0.1

1

10

100

Concentration, in milligrams per liter

C

pH

6.0 6.4 6.8 7.2 7.6 8.0 8.4 8.8 9.2 9.6

0.0001

0.001

0.01

0.1

1

10

100

1,000

Concentration, in milligrams per liter

B

SO4 Hrd Ca

Mg Sr Ba

0.1

1

10

100

1,000

10,000

A

Concentration, in milligrams per liter

or conductivity, in microsiemens per centimeter

pH

6.0 6.4 6.8 7.2 7.6 8.0 8.4 8.8 9.2 9.6

0

1

2

3

4

5

6

7

8

9

10

11

12

13

Dissolved oxygen, in milligrams per liter

SC ALK Cl

Na O2

EXPLANATION

SO4—Sulfate

Hrd—Hardness

Ca—Calcium

Mg—Magnesium

Sr—Strontium

Ba—Barium

SC—Specific conductance

ALK—Alkalinity

Cl—Chloride

Na—Sodium

O2—Oxygen

TDS—Total dissolved solids,

computed as sum 

of constituents

K—Potassium

Li—Lithium

Br—Bromide

F—Fluoride

As—Arsenic

B—Boron

Figure 5. Concentrations of selected constituents in 

groundwater from Bradford County, Pennsylvania, 2016, 

compared to: A–C, pH; or D–F, specific conductance 

measured in the laboratory. The detection limits for bromide 

and arsenic were 0.01 and 0.05 micrograms per liter, 

respectively; symbols plotted at those values were below 

detection.
Chemical Analysis and Relations Among Constituents in Groundwater 21

0

10

20

30

40

50

60

70

80

90

B 100 Cumulative percentage contribution to computed conductivity

Well Samples

BR952BR963BR940BR941

BR946BR913

H+ (Hydrogen)

Ca+2 (Calcium)

Mg+2 (Magnesium)

Na+ (Sodium)

K+ (Potassium)

SO4

−2

 (Sulfate)

Cl− (Chloride)

HCO3

−

 (Bicarbonate)

CO3

−2

 (Carbonate)

NO3

−

 (Nitrate)

OH− (Hydroxide)

Major ions

EXPLANATION

Sodium

Calcium

Magnesium

Chloride

Bicarbonate

Sulfate

EXPLANATION

0.01

0.1

1

10

100

1,000

A 10,000 Ion conductivity, in microsiemens per centimeter

10 100 1,000 10,000

Specific conductance, measured in the laboratory,

in microsiemens per centimeter

Figure 6. Major ion contributions to specific conductance (SC) for 72 groundwater samples from Bradford County, Pennsylvania, 

2016: A, comparison of estimated ionic conductivity contributions by sodium, calcium, magnesium, chloride, bicarbonate, and sulfate to 

measured SC for all 72 samples; and B, relative contributions by major ion species to computed SC and selected groundwater samples 

of representative water types, expressed in percent, in order of increasing SC. Individual ion conductivities estimated from dissolved 

constituent concentrations as the transport number (the relative contribution of a given ion to the overall conductivity, using the 

methods of McCleskey and others [2012]) after aqueous speciation calculations with PHREEQC (Parkhurst and Appelo, 2013).
22 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

H+ (Hydrogen)

Ca+2 (Calcium)

Mg+2 (Magnesium)

Na+ (Sodium)

K+ (Potassium)

SO4

−2

 (Sulfate)

Cl

−

 (Chloride)

HCO3

−

 (Bicarbonate)

CO3

−2

 (Carbonate)

NO3

−

 (Nitrate)

OH− (Hydroxide)

EXPLANATION

F. Sodium-chloride type

BR913

SC = 3,800 μS/cm

pH = 8.7

H+

0.0% Ca+2

0.4%

Mg+2

0.2%

Na+

37.9%

K+

0.2%

SO4

−2

0.0%

Cl

−

56.2%

HCO3

−

4.8%

CO3

−2

0.4%

NO3

−

0.0%

OH−

0.0%

E. Calcium/sulfate-bicarbonate type

BR946

SC = 1,210 μS/cm

pH = 7.4

H+

0.0%

CO3

−2

0.1%

Ca+2

26.9%

Mg+2

13.1%

Na+

8.1% 

K+

0.4%

SO4

−2

34.2%

Cl

−

1.1% HCO3

−

16.1%

NO3

−

0.0%

OH−

0.0%

D. Sodium-calcium/chloride type

BR941

SC = 850 μS/cm

pH = 6.4

H+

0.0%

Ca+2

19.7% Mg+2

2.8%

Na+

18.6%

K+

0.3%

SO4

−2

1.5%

Cl

−

51.0%

HCO3

−

5.0%

CO3

−2

0.0%

NO3

−

1.0%

OH−

0.0%

C. Sodium/bicarbonate type

BR940

SC = 567 μS/cm

pH = 9.2 H+

0.0% Ca+2

0.1%

Mg+2

0.0%

Na+

49.8%

K+

0.2%

SO4

−2

5.4%

Cl

−

0.3%

CO3

−2

7.4%

NO3

−

0.0%

OH−

0.2%

HCO3

−

36.5%

B. Calcium-sodium/bicarbonate type

BR963

SC = 298 μS/cm

pH = 9.3

H+

0.0%

Ca+2

20.2%

Mg+2

10.1%

Na+

22.8%

K+

1.3%

SO4

−2

3.6%

Cl

−

1.5%

HCO3

−

32.3% 

CO3

−2

7.7%

NO3

−

0.1%

OH−

0.5% 

A. Calcium/bicarbonate type

BR952

SC = 72 μS/cm

pH = 6.2

H+

0.3%

Ca+2

32.4%

Mg+2

9.5%

Na+

6.9%

K+

1.6%

SO4

−2

11.9%

Cl

−

6.0%

HCO3

−

29.1% 

CO3

−2

0.0%

OH−

0.0%

NO3

−

2.3%

Figure 7. Water types and ionic contributions to specific conductance (SC) computed for selected groundwater samples from 

Bradford County, Pennsylvania, 2016. Individual ionic conductivities estimated from dissolved constituent concentrations as the 

transport number (the relative contribution of a given ion to the overall conductivity, using the methods of McCleskey and others [2012]) 

after aqueous speciation calculations with PHREEQC (Parkhurst and Appelo, 2013). (μS/cm, microsiemens per centimeter)
Chemical Analysis and Relations Among Constituents in Groundwater 23

minerals and salts. Sample BR 963 (fig. 7B) had higher pH of 

9.3 and SC of 298 μS/cm compared to BR 952; this would be 

classified as a mixed calcium-sodium/bicarbonate water type. 

In contrast, sample BR 940 (fig. 7C), which had pH of 9.2 and 

SC of 567 μS/cm, had predominant ionic conductivity contributions by sodium and bicarbonate, with lesser contributions 

by chloride, carbonate, and sulfate ions. The mixed calciumsodium/bicarbonate and the sodium/bicarbonate waters can 

be produced as a result of cation exchange processes, where 

sodium ions in the mineral are exchanged for calcium ions 

from the water. Chloride is the dominant anion in sample 

BR 941 (fig. 7D), whereas sulfate predominates in BR 946 

(fig. 7E). These samples have relatively high SC values resulting from the addition of these ions to the groundwater. In 

the extreme, sample BR 913 (fig. 7F), which had pH of 8.7 

and the highest measured SC of 3,800 μS/cm, had predominant contributions by sodium and chloride, which indicate a 

salt source. The predominance of sodium and chloride ionic 

conductivities was exhibited mainly by those samples with 

elevated SC but also by a few samples with intermediate SC 

values, such as sample BR 926, which had pH of 8.2 and SC 

of 646 μS/cm.

Correlations Among Major and Trace 

Constituents in Groundwater

Constituents that were correlated with one another were 

“loaded” together and separated from other constituents in 

a principal component analysis (PCA) model (appendix 3). 

Six principal components (pH, redox, hardness, chloridebromide, strontium-barium, molybdenum-arsenic) explain 

nearly 78.3 percent of the variance in the groundwater dataset 

and consist of 26 routinely detected constituents (table 3.1). 

The association of pH with total dissolved solids, alkalinity, 

sodium, and other parameters is consistent with the progressive reaction of groundwater with minerals along flow paths, 

where the older water farther from recharge areas becomes 

more mineralized, alkaline, and softened. The association 

of redox-sensitive constituents, including dissolved oxygen, 

nitrate, iron, manganese, and methane, is interpreted to indicate isolation from the atmosphere, the depletion of oxygen, 

and the development of reducing conditions that may be 

attributed to the microbial decomposition of organic matter 

in the aquifer or introduced to the groundwater. The association of hardness, calcium, magnesium, alkalinity, and sulfate 

results from the dissolution of calcite, dolomite, gypsum, and, 

possibly, pyrite or other sulfide minerals. Chloride-bromide 

associations could indicate the influence of Appalachian Basin 

brine or other sources of salinity, such as sewage, fertilizer, 

and (or) road-deicing salt, on the groundwater of associated 

samples. Lastly, associations of the strontium-barium cations 

and silica could indicate a common origin or geochemical 

control, such as silicate or carbonate minerals. Associations 

of molybdenum-arsenic and lack of correlation with other 

constituents indicates that variations in the trace-element 

concentrations are independent of variations in the major ions, 

SC, pH, or redox.

Relations Between Groundwater Quality, 

Geology, and Topographic Setting

Groundwater acquires solutes through natural and anthropogenic loading of constituents in the recharge area from 

precipitation, weathering reactions of minerals in the soil and 

aquifer materials, and constituents applied by human activities 

at or near the land surface. Additional solutes may be acquired 

as groundwater flows through the aquifer. Groundwater supplying most domestic wells completed in shallow fracturedbedrock aquifers is derived principally from local recharge and 

will be influenced by land use and geology near the well. 

The bedrock underlying Bradford County mainly consists 

of clastic sedimentary lithologies that include shale, siltstone, 

and sandstone. Such clastic rocks are mainly composed of silicate and aluminosilicate minerals, including quartz, feldspar, 

chlorite, muscovite, and illite, plus minor carbonate, sulfate, 

sulfide, and oxide minerals that occur as clasts, fracture filling, 

and cements. Although mineralogy is expected to vary locally, 

the carbonate, sulfate, and sulfide minerals in the siliciclastic 

bedrock are prone to weathering in near-surface environments 

where they will affect pH, hardness, alkalinity, sulfate, and 

associated solutes. Likewise, the chlorite, muscovite, illite, 

and various other clay minerals readily accommodate ionic 

substitutions and are widely recognized to be involved in 

cation-exchange and sorption processes (Hem, 1985; Appelo 

and Postma, 2005). The glacial lacustrine and alluvial deposits 

are generally made up of the same constituents as the local 

bedrock but also may include some minerals from distant 

locations.

The pH, SC, and associated constituent concentrations 

for the Bradford County groundwater samples, classified 

by the source aquifer bedrock formations, are illustrated as 

boxplots (fig. 4.4). Of the 72 samples, 47 were obtained from 

the Lock Haven Formation, 22 from the Catskill Formation, 

and 3 from the Huntley Mountain Formation. Generally, the 

medians for pH, SC, TDS, hardness, and most other constituents were significantly lower and the median dissolved oxygen 

was significantly higher for the waters from the Huntley 

Mountain Formation compared to those from the Catskill and 

Lock Haven Formations. Although median values for pH, SC, 

TDS, hardness, Ca, Cl, and Br were equivalent for the Catskill 

and Lock Haven Formations, samples from the Lock Haven 

Formation had slightly higher median values than the Catskill 

Formation for Mg, Na, alkalinity, SO4, F, B, NH3, Fe, Mn, and 

methane, and lower median values for NO3, U, and Rn-222.

Major Ion Compositions Indicated by Trilinear 

Diagrams

Trilinear diagrams indicate the percentage contributions of the major cations and anions, in equivalents (molar 
24 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

concentration multiplied by ionic charge), relative to the total 

equivalents for cations and anions in a sample. Although 

similar in concept to the pie graphs showing ionic conductivity 

contributions to the SC (fig. 7), the factors used to compute the 

equivalents do not consider ion size and mobility, which are 

incorporated with the transport numbers for ionic conductivity 

(giving somewhat different ionic proportions). The corresponding water type is identified on the basis of the predominant (greater than 50 percent) cation(s) and anion(s) shown 

on the trilinear diagrams (fig. 8). For the Bradford County 

well-water samples, calcium/bicarbonate type waters were 

most abundant, with others classified as sodium/bicarbonate 

or mixed water types, including calcium-sodium/bicarbonate, 

calcium-sodium/bicarbonate-chloride, calcium/sulfate-bicarbonate, sodium/bicarbonate-chloride, sodium/bicarbonatesulfate, or sodium/chloride types (fig. 8).

Representative samples are identified on the trilinear 

diagram in figure 8 in order to explain possible origins of 

the observed water types. Well-water sample BR 952 has 

near-neutral pH and is classified as calcium/bicarbonate type 

(fig. 8A). A majority of the Bradford County groundwater 

samples could be classified as calcium/bicarbonate type, 

which can be produced by the dissolution of calcite (CaCO3) 

by rainwater (recharge) or groundwater. Sample BR 963 is 

a mixed calcium-sodium/bicarbonate type, whereas sample 

BR 940 is a sodium/bicarbonate type. Such water types are 

somewhat common in Bradford County and likely to form 

by the dissolution of calcite and (or) dolomite combined 

with cation exchange; these processes are described in more 

detail below. Samples plotting away from an apex or within 

the center part of the diagram are classified as mixed hydrochemical types. For example, sample BR 946 is classified as a 

mixed calcium/sulfate-bicarbonate type, BR 941 is classified 

as a mixed sodium-calcium/chloride type, and sample BR 921 

is a mixed calcium-sodium/chloride-bicarbonate type. The 

mixed water types imply that multiple sources of constituents 

or processes may be important. Finally, sample BR 913 was a 

sodium-chloride type, which is unlike any other sample. The 

predominance of chloride implies a substantial contribution of 

salt (NaCl, CaCl2) from road-deicing compounds, sewage or 

animal waste, or possibly brine of geological origin. Predominant water types or hydrochemical facies (Back, 1966) and 

evolution pathways for mixing of dilute calcium/bicarbonate 

(Ca/HCO3) groundwater with road salt, brine, brine combined 

with cation exchange, or with brine plus calcite dissolution are 

presented in detail in Gross and Cravotta (2017).

Chloride, Bromide, and Sodium in Groundwater

The concentrations of chloride (0.77–1,020 mg/L), 

bromide (less than 0.01–8.60 mg/L), and sodium 

(2.18–760 mg/L) for the 72 groundwater samples collected 

for this study ranged widely and were positively correlated 

with one another. In Bradford County, chloride concentrations 

greater than a few milligrams per liter in shallow groundwater may result from human activities or contributions 

from naturally occurring deeper, more saline groundwater of 

regional extent. The elevated chloride concentrations may 

be associated with elevated concentrations of sodium and, in 

some cases, nitrate and sulfate, which could indicate influence from human or animal waste. Elevated concentrations 

of chloride also may be associated with elevated (but two or 

more orders of magnitude smaller) concentrations of bromide, 

which could indicate influence from residual brine of geologic 

origin.

Chloride/bromide ratios can be useful to distinguish 

different sources of chloride (Davis and others, 1998; 

Whittemore, 2007; Mullaney and others, 2009). Bromide, like 

chloride, is a soluble anion that exhibits conservative transport 

properties and can be used as a tracer, if present at detectable 

concentrations. Some sources of chloride introduced by human 

activities into the environment, such as salt (sodium chloride) 

used for road deicing or present in septic effluent, typically 

have low amounts of bromide and relatively high chloride/

bromide mass ratios. Recent studies of groundwater quality 

in nearby Susquehanna County in northeastern Pennsylvania 

(Warner and others, 2012; Llewellyn, 2014; Siegel and others, 

2015) have reported groundwater that has concentrations of 

chloride and chloride/bromide ratios that indicate possible 

mixing with higher salinity or brine-type waters; these brinetype waters are postulated to be discharging at the surface 

from undetermined depths below the freshwater aquifer and 

mixing with shallow, more dilute groundwater.

The chloride/bromide mass ratios for the 72 Bradford 

County well-water samples are shown in relation to chloride 

concentrations in figure 9, which also shows curves representing compositions resulting from mixing of different proportions of dilute groundwater with (1) relatively pure sodium 

chloride (NaCl) salt containing only a trace of bromide (such 

as salt used for road deicing, in water softeners, and present 

in human and animal waste), or (2) bromide-rich oil and gas 

well brines (such as those from wells producing gas from the 

Marcellus Shale). The majority of Bradford County wellwater samples, including the samples (BR 941 and BR 957) 

with the second and third highest chloride concentrations 

(218 and 215 mg/L, respectively) plot on or near the mixing 

curve for deicing salt, which may enter groundwater from 

road runoff or septic effluent. However, several well-water 

samples (BR 913, BR 921, BR 926, BR 907, and BR 910) had 

relatively high bromide concentrations (greater than 0.4 mg/L) 

with low chloride/bromide mass ratios compared to their chloride concentrations of 50 mg/L and higher (fig. 9). These wellwater samples plot near or on the mixing curve for oil and gas 

well brines (fig. 9), suggesting a possible small contribution of 

chloride from a brine-like source (0.02 percent or less). Also 

plotting along the brine mixing curve is Salt Spring, a naturally occurring saline spring in Susquehanna County. 

Of the 8 water samples that had chloride concentrations 

greater than 20 mg/L and methane concentrations less than 

0.5 mg/L, 2 samples had chloride/bromide ratios that plotted on the road-deicing salt mixing curve, and 4 samples had 

chloride/bromide ratios that plotted on the brine mixing curve 
PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

A

M

M

J

J

J

J

J

J B

B

B

B

B

B

B

B

B

BB

B

B

B

B

B

B

BB

B

B B

B B B

E

E

E

EE E EEE E E EEEE E E EE E EE EPM CC A A A A A AAAAAA E 20 E 0

40

60

80

100

Mg

100 80 60 40 20 0 Ca

CATIONS

20

0

40

60

80

100

M

M

J J J J J

J

BB

BB

B

B

B

B

B

B

BBB

BB

B

B

B

BB

B

B

B

B B

B

EE

E

E

E

E

E

EE

E

EEEE

E

E

EE

E

E

E

E

C

C

PM

A

A

A

A

A

A

A

A

A

A A

E

E

Na + K

SO4 + Cl

20

0

40

60

80

100

20

0

40

60

80

100

80

100

60

40

20

0

Ca + Mg

HCO3 + CO3

80

100

60

40

20

0 913

946

952

941

940

963

M

M

J

J

J J J

J

BB

B B B BBB BBBBBB BBBB BBB B

B

B

EE E E E E

E

EEEEEEEEEEECEE C PM AAA AEE0AAAA20 A40A 60 80 100

Cl

ANIONS

80

100

60

40

20

0

20

0

40

60

80

100

SO

4

PERCENTAGE OF TOTAL MILLIEQUIVALENTS 

B

J JJ

J

J

J

BBBBB B BB

B BB

BB

B

B

B

BB B

BBBB BB

B

B B

B

B

B

BB B

B

B

BB

B

B BB

B

B

B B BBBBB

B B

B

BA

AA

A A

E

EE

E

PM E

Mg

20

0

40

60

80

100

20

0

40

60

80

100

Na + K

CATIONS

100 80 60 40 20 0 Ca

J

J

J

J

J

B

J

B

B B

B

B

B

B

B

B

B

BB

BB

B

B

B

B

BB

BB

B

B

B

B

B

B

B

BB

B B

B

B

BB

B

BB

B

B

B

B

B

B

BB

B

B

B

B

A

AA

A

A

E

E

E

E

E

PM

913

946

940

941

952

963

SO4 + Cl

0

20

0

40

60

80

100

20

40

60

80

100

Ca + Mg

80

100

60

40

20

0

80

60

40

20

0

100

J

JJ

JJ BJBB B BBBBBBB B BBB B BBBBB B BBB B BBBB B B

BBBBBBBB BBBBBBBBB A

AA

EA 

E

20 0 

HCO 

3 + CO3

80

100

60

40

EXPLANATION

pH

 6.0 < pH < 6.5 6.5 < pH < 7.0 7.0 < pH < 7.5 7.5 < pH < 8.0 8.0 < pH < 8.5 8.5 < pH < 9.0 9.0 < pH < 9.5

MJ

B

EA

C

E

Chemical abbreviations

Bicarbonate HCO3

CaCO3ClNaMgKSO4

CalciumCarbonateChlorideSodiumMagnesiumPotassiumSulfate

PM

Oil and gas brineFlowback water

941 Domestic well

EXPLANATION

PM

Oil and gas brineFlowback waterDomestic well

Specific conductance (SC)

70 < SC < 200 μS/cm200 < SC < 370 μS/cm370 < SC < 600 μS/cm600 < SC < 1,000 μS/cm1,000 < SC < 3,800 μS/cm

J

B

B

A

E

C

941

Figure 8. A, Predominant water types or hydrochemical facies, and B, data for 72 groundwater samples from Bradford County, Pennsylvania, 2016, pbrine from oil and gas wells in western Pennsylvania (Dresel and Rose, 2010) and flowback water from Marcellus Shale gas wells (Hayes, 2009). Symbby A, pH and B, specific conductance (SC). (<, less than; μS/cm, microsiemens per centimeter)
A 10 100 1,000 10,000 100,000

Chloride/bromide mass ratio

Chloride, in milligrams per liter

0.1 1 10 100 1,000 10,000 100,000 1,000,000

Salt spring

Brine

Seawater

Deicing salt

925 913 960

903

924

910 921

937

959

907

941

926

258

895

945 901

930915

917

928

957 912

Deicing salt mixing curve, upper and lower bounds

Deicing salt mixing curve, median

Oil and gas brine mixing curve, upper and lower boundsOil and gas brine mixing curve, median

Oil and gas brine (Dresel and Rose, 2010)Oil and gas brine, median (Dresel and Rose, 2010)

Marcellus Shale flowback, median (Hayes, 2009)

Seawater, average (Hem, 1985)

Salt spring (Llewellyn, 2014)

Bradford groundwater (metBradford groundwater (metBradford groundwater (6.0 ≤ Bradford groundwater (6.5 ≤ Bradford groundwater (7.5 ≤ Bradford groundwater (8.0 ≤ 

EXPLANATION

Figure 9. Chloride concentrations compared to A, chloride/bromide mass ratios, B, bromide concentrations, and C, sodium concentrations for grounCounty, Pennsylvania, 2016, plus median values for Salt Spring (Llewellyn, 2014), flowback waters from Marcellus Shale gas wells (Hayes, 2009), and ofrom western Pennsylvania (Dresel and Rose, 2010). Mixing curves computed for initial freshwater with chloride of 0.5 to 0.6 milligrams per liter (mg/L0.025 to 0.035 mg/L mixed with road deicing salt having a composition of NaCl0.99996Br0.00004 (upper curve, after Llewellyn [2014]) or with a median compowell brine (lower curve, after Dresel and Rose [2010]). Selected (numbered) samples have methane concentrations greater than 0.5 mg/L, alkaline pH, characteristics that illustrate the variable compositions of the samples from Bradford County. (<, less than; ≥, greater than or equal to; ≤, less than or 
B

0.001

0.01

0.1

1

10

100

1,000

10,000

Bromide, in milligrams per liter

Chloride, in milligrams per liter

0.1 1 10 100 1,000 10,000 100,000 1,000,000

Salt spring Seawater

907

921

913

910

924

960 903

937

959 925

926

945

895

258

901 957

941

952

Deicing

salt

Brine

Deicing salt mixing curve, upper and lower boundsDeicing salt mixing curve, median

Oil and gas brine mixing curve, upper and lower boundsOil and gas brine mixing curve, median

Oil and gas brine (Dresel and Rose, 2010)Oil and gas brine, median (Dresel and Rose, 2010)

Marcellus Shale flowback, median (Hayes, 2009)Seawater, average (Hem, 1985)

Salt spring (Llewellyn, 2014)

Bradford groundwater (metBradford groundwater (met

Bradford groundwater (6.0 ≤ Bradford groundwater (6.5 ≤ Bradford groundwater (7.5 ≤ 

Bradford groundwater (8.0 ≤ 

EXPLANATION

Figure 9. Chloride concentrations compared to A, chloride/bromide mass ratios, B, bromide concentrations, and C, sodium concentrations for grounCounty, Pennsylvania, 2016, plus median values for Salt Spring (Llewellyn, 2014), flowback waters from Marcellus Shale gas wells (Hayes, 2009), and ofrom western Pennsylvania (Dresel and Rose, 2010). Mixing curves computed for initial freshwater with chloride of 0.5 to 0.6 milligrams per liter (mg/L0.025 to 0.035 mg/L mixed with road deicing salt having a composition of NaCl0.99996Br0.00004 (upper curve, after Llewellyn [2014]) or with a median compowell brine (lower curve, after Dresel and Rose [2010]). Selected (numbered) samples have methane concentrations greater than 0.5 mg/L, alkaline pH, characteristics that illustrate the variable compositions of the samples from Bradford County. (<, less than; ≥, greater than or equal to; ≤, less than or 
Deicing salt mixing curve, upper and lower bounds Deicing salt mixing curve, median

Oil and gas brine mixing curve, upper and lower boundsOil and gas brine mixing curve, median

Oil and gas brine (Dresel and Rose, 2010)Oil and gas brine, median (Dresel and Rose, 2010)

Marcellus Shale flowback, median (Hayes, 2009)Seawater, average (Hem, 1985)

Salt spring (Llewellyn, 2014)

Bradford groundwater (metBradford groundwater (met

Bradford groundwater (6.0 ≤ Bradford groundwater (6.5 ≤ Bradford groundwater (7.5 ≤ 

Bradford groundwater (8.0 ≤ 

EXPLANATION

C

0.1

1

10

100

1,000

10,000

100,000

Sodium, in milligrams per liter

0.1 1 10 100 1,000 10,000 100,000 1,000,000

Chloride, in milligrams per liter

Excess sodium

Seawater

Salt spring

Brine

952

941

913

700

921

926

910

945

925

959

940

902 905

946 960

963

927 895

258 957

893

917

907

924

901

900

908 918

Deicing Salt

NaCl

Figure 9. Chloride concentrations compared to A, chloride/bromide mass ratios, B, bromide concentrations, and C, sodium concentrations for grounCounty, Pennsylvania, 2016, plus median values for Salt Spring (Llewellyn, 2014), flowback waters from Marcellus Shale gas wells (Hayes, 2009), and ofrom western Pennsylvania (Dresel and Rose, 2010). Mixing curves computed for initial freshwater with chloride of 0.5 to 0.6 milligrams per liter (mg/L0.025 to 0.035 mg/L mixed with road deicing salt having a composition of NaCl0.99996Br0.00004 (upper curve, after Llewellyn [2014]) or with a median compowell brine (lower curve, after Dresel and Rose [2010]). Selected (numbered) samples have methane concentrations greater than 0.5 mg/L, alkaline pH, characteristics that illustrate the variable compositions of the samples from Bradford County. (<, less than; ≥, greater than or equal to; ≤, less than or 
Chemical Analysis and Relations Among Constituents in Groundwater 29

(fig. 9A–C). Most of these wells were located in valley or 

valley sides and had depths ranging from 42 to 250 feet below 

land surface. Nevertheless, many other wells in the valley settings did not exhibit elevated methane concentrations. 

Many of the Bradford County groundwater samples had 

chloride/bromide ratios that plot along the mixing curve for 

road-deicing salt—which is essentially common salt (NaCl)—

that is also present in human and animal waste. For example, 

the water sample from well BR 941, which had the second 

highest concentration of chloride (218 mg/L=6.15 millimoles 

per liter (mmol/L)), had a similar molar concentration of 

sodium (84.6 mg/L=3.68 mmol/L), which could result from 

the dissolution of pure NaCl by groundwater or recharge water 

(fig. 9C). Nevertheless, compared to the stoichiometry for 

NaCl, many other samples that had chloride and bromide compositions plotting on the chloride/bromide mixing curve for 

salt also contained excess sodium (greater than 1:1 [Na]:[Cl] 

molar ratio) (fig. 9C). The excess sodium, derived from mineral sources that do not contain chloride, may result from the 

release of residual sodium on cation-exchange sites on clay 

minerals to the groundwater. 

The chloride/bromide relations for the Bradford County 

well-water samples collected for this study generally are 

similar to those for 20 groundwater samples collected in 2012 

in Sullivan County (Sloto, 2013), 95 groundwater samples 

collected in 2012 and 2015 in Pike County (Senior, 2014; 

Senior and Cravotta, 2017), and 121 groundwater samples collected in 2013–14 in Wayne County (Senior and others, 2017). 

Although drilling to access natural gas in the Marcellus Shale 

produces methane and brine, gas wells have not been drilled 

to this formation in Pike or Wayne Counties, which are within 

the Delaware River Basin. The regional, localized occurrence 

of groundwater throughout northeastern and north-central 

Pennsylvania that has elevated chloride and chloride/bromide 

ratios that plot along the mixing curve for brine implies a natural origin for most of the observed chloride, bromide, methane, 

and associated constituents. However, in rare instances, such 

as well BR 913, the presence of VOCs and other man-made 

constituents could indicate an anthropogenic source of the 

observed methane and brine constituents or connectivity with 

a deep fracture.

Isotopic Composition, Origin, and Spatial 

Distribution of Methane

Only the six samples (BR 910, BR 921, BR 903, BR 924, 

BR 925, and BR 937) with the highest concentrations of 

methane (13–77 mg/L) were analyzed for the stable isotopic 

compositions of methane and the associated concentrations of 

higher-chain hydrocarbon gases (fig. 10). Ethane (C2H6) was 

detected at concentrations ranging from 0.01 to 0.13 mg/L in 

4 of 6 samples, but no detectable amount of propane (C3H8) or 

butane (C4H10) was detected in any samples. 

The methane in 5 of these 6 groundwater samples 

(BR 924, BR 903, BR 910, BR 937, and BR 921) had δ13CCH4

values ranging from −38.23 to −65.05 per mil and δDCH4

values ranging from −191.2 to −257.2 per mil, which are consistent with the isotopic compositions reported by Baldasarre 

and others (2014) for mud-gas logging samples from these 

geologic units. However, the methane in these groundwater 

samples has lower (lighter) δ13CCH4 values than those reported 

by Reese and others (2014) for Marcellus Shale methane gas 

samples from Lycoming County. The sixth sample (BR 925) 

had a methane concentration of 14 mg/L, with an isotopic 

composition, δ13CCH4 of −65.1 and δDCH4 of −257.2 per mil, 

and is associated with the higher-chain hydrocarbon ethane that is consistent with methane of microbial origin. The 

ratios of methane to ethane plus higher-chain hydrocarbons 

(C1/C2+) for the other five samples are intermediate between 

values identified by Reese and others (2014) for microbial or 

thermogenic methane (fig. 10B). These characteristics could 

indicate a mixed thermogenic and microbial source (carbondioxide reduction process) of methane and also could imply 

that methane of microbial origin has been oxidized (becoming 

isotopically heavier). 

Groundwater with relatively elevated methane concentrations (near or above 1 mg/L) also had a chemical composition that differed in some respects (pH, selected major ions, 

and inorganic trace constituents) from groundwater with low 

methane concentrations. The five groundwater samples with 

the highest methane concentrations also had among the highest pH values (greater than 8.2) and elevated concentrations of 

sodium, lithium, boron, fluoride, arsenic, and bromide. Relatively elevated concentrations of some other constituents, such 

as barium, strontium, and chloride, commonly were present 

in, but not limited to, those well-water samples with elevated 

methane.

Although Llewellyn (2014) described possible associations between elevated methane or brine signatures with deep 

faulting and associated structural features, the available data 

for this study indicate that no one physical factor, such as the 

topographic setting, well depth, or altitude at the bottom of the 

well, was particularly useful for predicting those well locations with an elevated dissolved concentration of methane. A 

few parameters stand out as related to topographic position 

index (TPI) (fig. 4.5); in particular, chloride, bromide, and 

methane generally increase in the downslope direction. Also, 

PC4 (table 3.1, Cl-Br) has progressively higher scores in the 

downslope direction. These general trends are displayed in 

the conceptual model that indicates discharge of older, deeper 

groundwater into the valleys (Gross and Cravotta, 2017).
30 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

A

−300

−250

−200

−150

−100

−50

0

Hydrogen isotope ratio delta value (δDCH4), 

relative to Vienna Standard Mean Ocean Water,

in parts per thousand

Carbon isotope ratio delta value (δ13CCH4)

relative to Vienna Peedee Belemnite,

in parts per thousand

−80 −70 −60 −50 −40 −30 −20

Microbial

(carbon dioxide reduction)

Mixed

Thermogenic

(dry gas)

Thermogenic

(wet gas)

Maturation/oxidation

925

921

937 910

903,924

1

10

100

1,000

10,000

100,000

BMethane to higher-chain hydrocarbons ratio, 

calculated for mole percents in gas

−80 −70 −60 −50 −40 −30 −20

Carbon isotope ratio delta value (δ13CCH4)

relative to Vienna Peedee Belemnite,

in parts per thousand

Microbial

(carbon dioxide reduction)

Thermogenic

925

937

910

903

921

924

EXPLANATION

Hamilton Group

Marcellus Shale

Marcellus Shale

Isotopic composition of methane gas 

in groundwater

Bradford County 

groundwater, 2016

Mean isotopic composition of methane gas 

from mud-gas logging of shale-gas 

wells in Pennsylvania (Baldassare 

and others, 2014) 

Catskill/Lock Haven Formation

Brallier Formation

Isotopic composition of methane gas, 

Bradford County (Reese and 

others, 2014)

EXPLANATION

Bradford County 

groundwater, 2016

Isotopic composition of carbon in methane 

gas and ratio of methane to higher chain 

hydrocarbons (C1/C2+)

Bradford County Marcellus Shale

methane gas samples (Reese

and others, 2014)

Figure 10. A, Isotopic composition of methane in groundwater samples collected from five wells in Bradford County, Pennsylvania, 

2016; and B, ratio of methane to higher-chain hydrocarbons (C1/C2+) in relation to carbon isotopic composition for methane in these 

samples. Boundaries for microbial and thermogenic gas types and compositional shift related to gas maturation/oxidation, shown by 

arrow, from Reese and others (2014, fig. 30, p. 38). Marcellus Shale methane gas data for Bradford County from Reese and others (2014, 

table 7) and mud-gas logging data by geologic formation from Baldassare and others (2014, table 1).
Summary and Conclusions 31

Summary and Conclusions

In Bradford County, more than half of the residents use 

groundwater from private domestic-supply wells as their 

primary drinking source. The quality of private well water is 

influenced by the regional and local settings, including the 

surrounding soil, geology, land use, household plumbing, and 

well construction. The groundwater used for domestic water 

supply in Bradford County is obtained primarily from shallow bedrock and from unconsolidated (glacial) deposits that 

overlie bedrock. Historical land use has been predominately 

forested, agricultural, and residential, but more recently 

unconventional oil/gas development has also been distributed 

throughout the various land uses in the county. Despite legislative attempts, Pennsylvania is one of two states in the Nation 

without statewide water-well construction standards

To assess the quality of groundwater used for drinking 

water supplies in Bradford County, samples were collected 

from 72 domestic wells from May through August 2016 

and analyzed for a wide range of constituents that could be 

evaluated in relation to drinking water health standards, geology, land use, and other environmental factors. Groundwater 

samples were analyzed for physical and chemical properties, including major ions, nutrients, bacteria, trace elements, 

volatile organic compounds, ethylene and propylene glycol, 

alcohols, gross-alpha/beta-particle activity, uranium, radon222, and dissolved gases. A subset of samples was analyzed 

for radium isotopes (radium-226 and -228) and for the isotopic 

composition of methane. 

Results of the 2016 study showed that groundwater 

quality generally met most drinking-water standards. A 

subset of samples exceeded primary maximum contaminant 

levels for total coliform bacteria (49.3 percent), Escherichia 

coli (8.5 percent), and arsenic (2.8 percent); and secondary 

maximum contaminant levels for sodium (48.6 percent), manganese (30.6 percent), gross alpha and beta activity (16.7 percent), pH (8.3 percent), iron (11.1 percent), total dissolved 

solids (5.6 percent), chloride (1.4 percent), and aluminum 

(1.4 percent). Generally, samples that had elevated TDS, 

chloride, or arsenic concentrations had high pH. Radon-222 

activities exceeded the proposed drinking-water standard of 

300 pCi/L in 70.4 percent of the samples. 

The pH of the groundwater had a large influence on 

chemical characteristics and ranged from 6.18 to 9.31. Generally, the higher pH samples had higher potential for elevated 

concentrations of several constituents, including total dissolved solids, sodium, lithium, chloride, fluoride, boron, 

arsenic, and methane. For the Bradford County well-water 

samples, calcium/bicarbonate type waters were the most abundant, with others classified as sodium/bicarbonate or mixed 

water types, including calcium-sodium/bicarbonate, calciumsodium/bicarbonate-chloride, sodium/bicarbonate-chloride, 

sodium/bicarbonate-sulfate, or sodium/chloride types. Six 

principal components (pH, redox, hardness, chloride-bromide, 

strontium-barium, and molybdenum-arsenic) explain nearly 

78.3 percent of the variance in the groundwater dataset.

Groundwater from 12.5 percent of the wells had concentrations of methane greater than the Pennsylvania action level 

of 7 mg/L; detectable methane concentrations ranged from 

0.01 to 77 mg/L. Low levels of ethane (as much as 0.13 mg/L) 

were present in seven samples with the highest methane 

concentrations. The isotopic composition of methane in five 

of these groundwater samples was consistent with the isotopic compositions reported for mud-gas logging samples from 

these geologic units and a thermogenic source of the methane, and a sixth sample suggests the methane in that sample 

may be of microbial origin. Well-water samples with the 

higher methane concentrations also had among the higher pH 

values and elevated concentrations of sodium, lithium, boron, 

fluoride, arsenic, and bromide. Relatively elevated concentrations of some other constituents, such as barium and chloride, 

commonly were present in, but not limited to, those well-water 

samples with elevated methane. 

Four of six groundwater samples with some of the highest methane concentrations had chloride/bromide ratios that 

indicated mixing with a small amount of brine (0.02 percent or 

less) similar in composition to those reported for gas and oil 

well brines in Pennsylvania. In several more eastern Pennsylvania counties where gas drilling is absent, groundwater with 

comparable chloride/bromide ratios and chloride concentrations have been reported, implying a potential natural source 

of brine. Most of Bradford County well-water samples have 

chloride concentrations less than 20 mg/L, and those with 

higher chloride concentrations have chloride/bromide ratios 

that indicate anthropogenic sources (such as road-deicing salt 

and septic effluent) or brine. Brines that are naturally present may originate from deeper parts of the aquifer system, 

whereas anthropogenic sources are more likely to affect 

shallow groundwater because they occur on or near the land 

surface. 

The available data for this study indicate that no one 

physical factor such as the topographic setting, well depth, 

or altitude at the bottom of the well was particularly useful 

for predicting those well locations with an elevated dissolved 

concentration of methane. The 2016 assessment of groundwater quality in Bradford County shows groundwater is generally of good quality, but methane and some constituents that 

occur in high concentration in naturally occurring brine and 

produced waters from gas and oil wells may be present at low 

to moderate concentrations in groundwater in various parts of 

the aquifer.
32 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

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Appendixes 1–4

Appendix 1. 41

Appendix 1.

Table 1.1. Compilation of data not available in the National Water Information System, including station name, station identification 

number, lithologic class, and topographic position index for domestic wells sampled by the U.S. Geological Survey in Bradford County, 

Pennsylvania, May–August 2016 (available online as an Excel file at https://doi.org/10.5066/P9VRV6US).

Station 

name

Station 

identification number

Lithologic 

class

Topographic 

position index

BR 943 413407076180601 Sandstone Gentle Slope

BR 956 413515076191101 Sandstone Lower Slope

BR 961 413531076323901 Sandstone Ridges

BR 958 413602076320501 Sandstone Valley

BR 941 413635076255101 Sandstone Lower Slope

BR 942 413647076253101 Sandstone Upper Slope

BR 924 413658076521001 Mudstone Lower Slope

BR 944 413708076131901 Sandstone Gentle Slope

BR 932 413743076240801 Sandstone Upper Slope

BR 959 413828076484101 Sandstone Valley

BR 919 413834076165001 Sandstone Ridges

BR 915 413931076120301 Sandstone Gentle Slope

BR 939 413931076470401 Mudstone Gentle Slope

BR 952 413955076305801 Sandstone Valley

BR 957 414037076421101 Mudstone Valley

BR 895 414037076501601 Mudstone Gentle Slope

BR 936 414107076185301 Sandstone Gentle Slope

BR 914 414115076091901 Sandstone Valley

BR 912 414123076244001 Sandstone Gentle Slope

BR 923 414138076350601 Sandstone Gentle Slope

BR 937 414153076375401 Mudstone Gentle Slope

BR 960 414158076403901 Mudstone Upper Slope

BR 901 414200076264001 Sandstone Upper Slope

BR 890 414209076452501 Mudstone Ridges

BR 896 414234076501301 Sandstone Valley

BR 906 414319076454401 Mudstone Gentle Slope

BR 913 414327076140401 Sandstone Lower Slope

BR 913 414327076140401 Sandstone Lower Slope

BR 900 414418076313901 Mudstone Ridges

BR 907 414517076391801 Mudstone Lower Slope

BR 911 414522076230801 Mudstone Lower Slope

BR 258 414542076102801 Sandstone Lower Slope

BR 892 414552076182201 Mudstone Upper Slope

BR 902 414608076255701 Mudstone Upper Slope

BR 903 414647076224901 Mudstone Lower Slope

BR 928 414710076320301 Mudstone Gentle Slope

Station 

name

Station 

identification number

Lithologic 

class

Topographic 

position index

BR 897 414717076501801 Sandstone Valley

BR 954 414747076292001 Mudstone Gentle Slope

BR 963 414747076462201 Sandstone Valley

BR 951 414904076412101 Mudstone Valley

BR 927 414917076324601 Mudstone Upper Slope

BR 962 414923076190001 Mudstone Gentle Slope

BR 894 414930076133001 Sandstone Gentle Slope

BR 931 414930076240201 Mudstone Gentle Slope

BR 935 415029076100101 Mudstone Valley

BR 935 415029076100101 Mudstone Valley

BR 945 415035076493101 Mudstone Valley

BR 950 415116076324301 Mudstone Upper Slope

BR 929 415118076292401 Mudstone Gentle Slope

BR 920 415130076185401 Mudstone Ridges

BR 908 415137076410001 Mudstone Upper Slope

BR 904 415141076462001 Sandstone Gentle Slope

BR 898 415215076493001 Mudstone Valley

BR 893 415256076112901 Mudstone Upper Slope

BR 925 415353076412201 Mudstone Valley

BR 946 415411076175501 Mudstone Gentle Slope

BR 948 415414076225901 Mudstone Lower Slope

BR 949 415430076333901 Mudstone Steep Slopes

BR 899 415449076513801 Mudstone Ridges

BR 947 415508076251901 Mudstone Upper Slope

BR 922 415608076083801 Mudstone Valley

BR 909 415609076294001 Mudstone Upper Slope

BR 955 415619076475301 Mudstone Lower Slope

BR 930 415646076202301 Mudstone Lower Slope

BR 916 415650076532801 Mudstone Lower Slope

BR 940 415726076362801 Mudstone Steep Slopes

BR 926 415729076353901 Mudstone Valley

BR 921 415807076154401 Mudstone Valley

BR 918 415819076363401 Mudstone Lower Slope

BR 917 415921076450601 Mudstone Ridges

BR 428 415935076184901 Mudstone Gentle Slope

BR 953 415937076502901 Mudstone Gentle Slope
42 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

Appendix 2.

Table 2.1. Minimum, median, and maximum values of selected characteristics and constituents in groundwater samples collected 

from 72 domestic wells in Bradford County, Pennsylvania, May–August 2016. Available U.S. Environmental Protection Agency Maximum 

Contaminant Levels, Action Levels, and Secondary Maximum Contaminant Levels for analyzed constituents (2012 Edition of the Drinking 

Water Standards and Health Advisories [EPA 822-S-12-001]).

[n, number of results; EPA, U.S. Environmental Protection Agency; MCL, Maximum Contaminant Level; SMCL, Secondary Maximum Contaminant Level; 

μg/L, micrograms per liter; <, less than; —, no MCL, Action Level, or SMCL established]

Constituent (units) n Minimum Median Maximum

Results above the 

reporting level Results exceeding standard EPA 

MCL

EPA 

SMCL

Number Percent Number Percent Graph

Volatile Organic Compounds

1,1,1,2-Tetrachloroethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

1,1,1-Trichloroethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 200 —

1,1,2,2-Tetrachloroethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

1,1,2-Trichloroethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 5 —

1,1-Dichloroethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

1,1-Dichloroethene (1,1-Dichloroethylene) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 7 —

1,1-Dichloropropene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

1,2,3-Trichlorobenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

1,2,3-Trichloropropane (1,2,3-TCP) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

1,2,4-Trichlorobenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 70 —

1,2,4-Trimethylbenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

1,2-Dichlorobenzene (o-Dichlorobenzene) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 600 —

1,2-Dichloroethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 5 —

1,2-Dichloropropane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 5 —

1,3,5-Trimethylbenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

1,3-Dichlorobenzene (m-Dichlorobenzene) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

1,3-Dichloropropane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

1,4-Dichlorobenzene (p-Dichlorobenzene) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 75 —

2,2-Dichloropropane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

2-Butanone (Methyl ethyl ketone, MEK) (μg/L) 72 < 1.0 < 1.0 < 1.0 0 0.00 — — — — —

2-Chloroethyl vinyl ether (μg/L) 72 < 1.0 < 1.0 < 1.0 0 0.00 — — — — —

2-Chlorotoluene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

4-Chlorotoluene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Acetone (μg/L) 72 < 1.0 < 1.0 < 1.0 0 0.00 — — — — —

Benzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 5 —

Bromobenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Bromochloromethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Bromomethane (Methyl bromide) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Carbon Disulfide (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Chlorobenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 100 —

CHBrCl2, wu, μg/L (32101) 72 < 0.5 < 0.5 1.04 1 1.39 0 0 — 80

Chloroethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —
Appendix 2. 43

Table 2.1. Minimum, median, and maximum values of selected characteristics and constituents in groundwater samples collected 

from 72 domestic wells in Bradford County, Pennsylvania, May–August 2016. Available U.S. Environmental Protection Agency Maximum 

Contaminant Levels, Action Levels, and Secondary Maximum Contaminant Levels for analyzed constituents (2012 Edition of the Drinking 

Water Standards and Health Advisories [EPA 822-S-12-001]).—Continued

Constituent (units) n Minimum Median Maximum

Results above the 

reporting level Results exceeding standard EPA 

MCL

EPA 

SMCL

Number Percent Number Percent Graph

Volatile Organic Compounds—Continued

Chloromethane, wu, μg/L (34418) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

cis-1,2-dichloroethene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 70 —

cis-1,3-dichloropropene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Dibromochloromethane (μg/L) 72 < 0.5 < 0.5 1.58 1 1.39 0 0 80 —

Dibromomethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Ethylbenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 700 —

Hexachlorobutadiene (1,3-Hexachlorobutadiene) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Iodomethane (Methyl iodide) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Isopropylbenzene (Cumene) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

m-Xylene plus p-xylene (μg/L) 71 < 1.0 < 1.0 < 1.0 0 0.00 0 0 10,000 —

Methyl chloride (Chloromethane) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Methyl tert-butyl ether (MTBE) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Naphthalene (μg/L) 72 0.5 < 0.5 < 0.5 0 0.00 — — — — —

n-Butylbenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

n-Butyl methyl ketone (μg/L) 72 < 1.0 < 1.0 < 1.0 0 0.00 — — — — —

n-Propylbenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

o-Xylene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 10,000 —

p-Isopropyltoluene (4-Isopropyltoluene) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

sec-Butylbenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Styrene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 100 —

tert-Butylbenzene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Tetrachloroethene (Perchloroethylene, PCE) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 5 —

Tetrachloromethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Toluene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 1,000 —

trans-1,2-Dichloroethene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 100 —

trans-1,3-Dichloropropene (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Tribromomethane (μg/L) 72 < 0.5 < 0.5 0.81 1 1.39 0 0 60 —

Trichloroethene (TCE, Trichloroethylene) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 5 —

Trichloromethane (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 80 —

Trihalomethanes (THMs), summation (μg/L) 72 < 2.0 < 2.0 3.43 1 1.39 — — — — —

Vinyl Acetate (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 — — — — —

Vinyl Chloride (Chloroethene) (μg/L) 72 < 0.5 < 0.5 < 0.5 0 0.00 0 0 2 —

Xylene (μg/L) 72 < 1.5 < 1.5 < 1.5 0 0.00 — — — — —

EXPLANATION

No exceedance
44 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

Appendix 3.

Principal component analysis (PCA) provides insight on 

hydrochemical processes affecting groundwater chemistry in 

the study area by indicating intercorrelations among chemical constituents and environmental variables. Six principal 

components (PCs) explain nearly 78.3 percent of the variance 

in the groundwater dataset and consist of 26 routinely detected 

constituent loadings (table 3.1). As explained by Thyne and 

others (2004), the loading value is comparable to the correlation coefficient between a constituent and the PC, whereas the 

communality and eigenvalues indicate the degree to which 

the PCA explains the total variability of the elements considered in the analysis. Associations of additional chemical and 

physical variables excluded from the model are indicated by 

the Spearman-rank coefficient of correlation of these variables with the PC scores (table 3.2); significant correlations 

are listed below the six PCs included in table 3.1, which are 

identified as principal component 1 (PC1) through principal 

component 6 (PC6).

PC1 (pH), which explains 36.1 percent of the variance 

in the data, has positive loadings by boron, lithium, sodium, 

alkalinity, pH (field), fluoride, specific conductance (SC) 

(laboratory), ammonia, and methane, with negative loading by 

radon-222, nitrate, calcium, and uranium (table 3.2). Scores on 

PC1 are positively correlated with total dissolved solids (TDS; 

table 3.2). The negative associations of PC1 with calcium 

and radon 222 and positive associations with pH, alkalinity, 

lithium, and sodium are consistent with the progressive reaction of groundwater with minerals along flow paths, where 

the older water further from recharge areas becomes more 

mineralized, alkaline, and “softened.” Samples classified as 

anoxic generally had higher, positive scores on PC1 compared 

to those classified as mixed or oxic, plus PC1 scores were 

generally greatest for samples from the Lock Haven formation and least for Huntley Mountain, as shown by Appendix 

figures 4.3 and 4.4. High positive scores on PC1 generally 

may be attributed to the progressive weathering of calcite and 

dolomite (carbonate minerals) combined with cation-exchange 

processes. The naturally occurring cation-exchange reactions 

liberate sodium and other alkali earth cations, such as lithium, 

while removing calcium and magnesium (hardness), much like 

a water-softening treatment system (Hem, 1985; Poth, 1962). 

The removal of calcium and magnesium from solution leads to 

undersaturation of the groundwater with respect to calcite and 

dolomite, thus promoting additional dissolution of the carbonate minerals and progressive increases in pH and alkalinity 

along the flow path. The resultant sodium-bicarbonate waters 

tend to have alkaline pH (>8) values and high positive scores 

on PC1 (fig. 3.2). Scores on PC1 also increased with SC class 

interval (fig. 4.2).

PC2 (Redox), which explains 13.5 percent of the variance in the data, has positive loadings by manganese, iron, 

ammonia, and methane, and negative loadings by dissolved 

oxygen, nitrate, and radon-222 (table 3.2). Scores on PC2 

are negatively correlated with selenium, copper, and redox 

potential (Eh). High scores on PC2 are interpreted to indicate 

isolation from the atmosphere, the depletion of oxygen, and 

the development of reducing conditions that may be attributed 

to the microbial decomposition of organic matter in the aquifer 

or introduced to the groundwater. Samples classified as having 

anoxic or mixed redox characteristics generally had positive 

scores on PC2, whereas samples classified as oxic generally 

had negative scores (fig. 4.3). The negative correlations of 

copper and selenium with PC2 indicate decreased mobility 

of these constituents where concentrations of dissolved iron 

and manganese are elevated, possibly because of adsorption 

by hydrous ferric oxide (HFO) minerals, including goethite 

(FeOOH) or ferrihydrite (Fe(OH)3), that could be present 

along with the dissolved iron. 

PC3 (Hardness), which explains 8.7 percent of the variance in the data, has positive loadings by sulfate, magnesium, 

calcium, SC (laboratory), silica, and alkalinity. Scores on PC3 

are positively correlated with hardness, nickel, and TDS. High 

scores on PC3 can be attributed to the dissolution of calcite, 

dolomite, gypsum, and, possibly, pyrite or other sulfide minerals, without the cation-exchange softening effects (alkaline 

pH) indicated by high scores on PC1. Median PC3 scores 

decreased in the order of Lock Haven, Catskill, and Huntley 

Mountain Formations, as shown by Appendix figure 4.4. 

PC4 (Chloride-Bromide), which explains 7.2 percent 

of the variance in the data, has positive loadings by chloride, 

bromide, SC (laboratory), and sodium and is positively correlated with TDS (table 3.2). The positive association between 

chloride, bromide, and sodium could indicate the influence of 

Appalachian Basin brine or other sources of salinity, such as 

sewage, fertilizer, and (or) road-deicing salt, on the groundwater of associated samples. Scores on PC4 increased progressively with SC class interval (fig. 4.2) and generally were 

greater for samples from the Catskill Formation than those 

from the Lock Haven or Huntley Mountain bedrock formations (fig. 4.4). 

PC5 (Strontium-Barium), which explains 6.7 percent of 

the variance in the data, has positive loadings by strontium, 

potassium, barium, silica, and lithium (table 3.2) and was 

positively correlated with gross beta activity. The positive 

associations of the cations and silica could indicate a common 

origin or geochemical control, such as silicate or carbonate 

minerals. For example, potassium, barium, and strontium 

commonly substitute for calcium in carbonate (aragonite) and 

for one another in sulfate (barite-celestine) minerals, which 

could be possible sources or sinks of the cations (Hanor, 1968; 

Hanshaw and Back, 1979). Additionally, these cations may be 

retained by clay minerals involved in cation exchange (Appelo 

and Postma, 2005) and, thus, may be released as the exchange 

sites become depleted in sodium and enriched in calcium and 

magnesium.

PC6 (Molybdenum-Arsenic), which explains 6.0 percent of the variance in the data, has positive loadings by 

molybdenum, arsenic, uranium, and radon-222. The positive 
Appendix 3 45

associations of these trace elements and lack of correlation 

with other constituents, indicates that variations in the traceelement concentrations are independent of variations in the 

major ions, SC, pH, or redox. 

References Cited

Appelo, C.A.J., and Postma, D., 2005, Geochemistry, 

groundwater and pollution: Leiden, The Netherlands, A.A. 

Balkema Publishers, 649 p.

Hanor, J.S., 1968, Frequency distribution of compositions in 

the barite-celestite series: American Mineralogist, v. 53, 

p. 1215–1222.

Hanshaw, B.B., and Back, W., 1979, Major geochemical 

processes in the evolution of carbonate—Aquifer systems: 

Contemporary Hydrogeology—The George Burke Maxey 

Memorial Volume, v. 43, no. 1, p. 287–312.

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water: U.S. Geological Survey 

Water-Supply Paper 2254, 263 p., [Also available at https://

doi.org/10.3133/wsp2254.]

Poth, C.W., 1962, The occurrence of brine in western Penn-

sylvania: Pennsylvania Geological Survey 4th ser., Mineral 

Resources Report 47, 53 p.

SAS Institute, Inc., 2012, SAS 9.4 for Windows: Cary, N.C., 

USA, SAS Institute, Inc.

Thyne, G., Guler, C., and Poeter, E., 2004, Sequential analy-

sis of hydrochemical data for watershed characterization: 

Ground Water, v. 42, no. 5, p. 711–723.
46 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

Table 3.1. Principal components analysis model of major factors controlling the chemistry of groundwater and statistically significant 

Spearman rank correlations of other variables with those factors determined from chemical properties of and dissolved concentrations 

of constituents in groundwater samples from 72 wells, Bradford County, Pennsylvania, 2016.

[PC1–PC6, principal components 1 through 6 with major factor indicated in parentheses; Varimax rotation pattern for rank-transformed data (SAS, 2012); 

minimum eigenvalue > 1; loading values for constituents included in model and Spearman correlation coefficients for constituents excluded from model multiplied by 100 and rounded; *, indicates highly significant loading or correlation (p < 0.001); + indicates significant loading or correlation (p < 0.01); -, indicates 

insignificant correlation or not applicable]

Principal component PC1

(pH)

PC2

(Redox)

PC3

(Hardness)

PC4

(ChlorideBromide)

PC5

(StrontiumBarium)

PC6

(MolybdenumArsenic)Communality

Constituent loadings

Boron (B) 87 * 26 + −7 9 26 + 1 0.912

Lithium (Li) 84 * 9 −13 16 36 * 3 0.891

Sodium (Na) 79 * 25 + −4 39 * 10 2 0.855

Alkalinity (ALK) 70 * 31 + 41 * 21 3 5 0.805

pH, Field (pHF) 69 * 30 + −32 + −13 10 20 0.735

Fluoride (F) 68 * 22 −35 + 11 3 −10 0.660

Specific conductance, lab (SCL) 56 * 21 50 * 53 * 0 −11 0.907

Radon 222 (Rn222) −68 * −37 * −11 4 3 37 * 0.756

Manganese (Mn) 6 85 * 31 17 8 3 0.863

Iron (Fe) 24 76 * 4 22 −3 −17 0.710

Ammonia (NH3N) 55 * 68 * 0 20 19 −15 0.868

Methane (Methane) 41 * 67 * −16 26 24 −9 0.781

Nitrate (NO3N) −46 * −66 * 2 8 −16 5 0.681

Dissolved oxygen (DOX) −22 −74 * 13 1 −16 −18 0.679

Sulfate (SO4) 7 −9 84 * −2 −22 7 0.779

Magnesium (Mg) −18 9 80 * 1 28 2 0.767

Calcium (Ca) −42 * 5 73 * 20 22 21 0.853

Chloride (Cl) 0 14 3 89 * 10 −6 0.833

Bromide (Br) 31 + 14 −1 81 * 10 −1 0.779

Strontium (Sr) 29 + 32 + 1 6 77 * 10 0.793

Potassium (K) 31 + −3 20 18 72 * −4 0.696

Barium (Ba) 3 19 −46 * 35 + 62 * 3 0.755

Silica (SiO2) −1 20 48 * −26 57 * 2 0.657

Molybdenum (Mo) −2 15 4 −4 4 93 * 0.886

Arsenic (As) 8 −9 8 −4 6 78 * 0.628

Uranium (U) −53 * −31 + 11 −2 −15 65 * 0.832

Eigenvalue

Eigenvalue 9.40 3.52 2.27 1.88 1.74 1.56 -

Percent Variance Explained 36.1 13.5 8.7 7.2 6.7 6.0 -

Cumulative Percent Variance Explained 36.1 49.7 58.4 65.7 72.3 78.3 -

Significant Spearman correlations (p < 0.001)

Total dissolved solids (TDS) 57 - 46 48 - - -

Phosphate (PO4P) 38 - - - - - -

Ethane (Ethane) 37 43 −33 - - - -

Selenium (Se) - −48 - 36 - - -

Copper (Cu) −44 −57 - - - - -
Appendix 3 47

Table 3.1. Principal components analysis model of major factors controlling the chemistry of groundwater and statistically significant 

Spearman rank correlations of other variables with those factors determined from chemical properties of and dissolved concentrations 

of constituents in groundwater samples from 72 wells, Bradford County, Pennsylvania, 2016.—Continued

Principal component PC1

(pH)

PC2

(Redox)

PC3

(Hardness)

PC4

(ChlorideBromide)

PC5

(StrontiumBarium)

PC6

(MolybdenumArsenic)Communality

Significant Spearman correlations (p < 0.001)—Continued

Redox potential (EHmv) - −64 - - - - -

Hardness (HRDcalc) - - 79 - - - -

Nickel (Ni) - - 48 - - - -

Gross beta radioactivity, 30-day count, water, filtered, 

Cesium-137 curve, picocuries per liter (BetaCs30)

- - - - 51 - -

Gross beta radioactivity, 72-hour count, water, filtered, 

Cesium-137 curve, picocuries per liter (BetaCs72)

- 30 - - 47 - -

Gross alpha radioactivity, 30-day count, water, filtered, 

Th-230 curve, picocuries per liter (AlphaTh30)

- - - - 38 - -

Gross alpha radioactivity, 72-hour count, water, filtered, 

Th-230 curve, picocuries per liter (AlphaTh72)

- - - - - 34 -

Cadmium (Cd) −32 - - - - - -

Lead (Pb) −32 −31 - - - - -

Antimony (Sb) - −31 - - - -

Well depth (WELLZ) 31 −32 - - - - -

pH, Lab (pHL) 69 - −33 - - - -

Residue on evaporation 180C (ROE180) 58 - 43 48 - - -

Specific conductance, field (SCF) 53 - 47 53 - - -

Aluminum (Al) - - - - - - -

Beryllium (Be) - - - - - - -

Cobalt (Co) - - - - - - -

Land-surface altitude (LSALT) - - - - - - -

Propane (Propane) - - - - - - -

Total coliform (Tcoli) - - - - - - -

Temperature (TEMPC) - - - - - - -

Depth to water (WATRZ) - - - - - - -

Well bottom altitude (WELLZALT) - - - - - - -

Zinc (Zn) - - - - - - -
Table 3.2. Spearman rank correlation coefficient (r) matrix for groundwater chemical data Bradford County, Pennsylvania, 2016. [r-values multiplied by 100 and rounded; only values significant at p < 0.01 shown; PC, principle component; -, not significant; abbreviations are defined in table 3.1] Parameter name Parameter abbreviation

PC1

PC2

PC3

PC4

PC5

PC6

TEMPC

DOX

EHmv

LSALT

WELLZALT

WELLZ

WATRZ

pH

pHL

SCL

SCF

ROE180

TDS

HRDcalc

Ca

Mg K

Na

ALK

Br

Cl F

SiO

2

SO

4

NHN

3

NON

3

POP

4

Tcoli

Al

Ba

Be

Cd

Co

Cu

Fe

Pb

Li

Mn

Mo

Ni

Sr

Zn

Sb

As 

Principal component 1 PC1 100 - - - - - - - - - - 31 - 69 69 55 53 58 57 - −35 - 40 79 72 32 - 66 - - 55 −48 38 - - - - −32 - −44 - −32 85 - - - 39 - - - Principal component 2 PC2 - 100 - - - - - −75 −64 - - −32 - 30 - - - - - - - - - 30 32 - - - - - 70 −66 - - - - - - - −57 76 −31 - 87 - - 33 - −31 - Principal component 3 PC3 - - 100 - - - - - - - - - - −33 −33 46 47 43 46 79 72 80 - - 36 - - - 45 85 - - - - - −44 - - - - - - - - - 48 - - - - Principal component 4 PC4 - - - 100 - - - - - - - - - - - 51 53 48 48 - - - - 37 - 83 89 - - - - - - - - 31 - - - - - - - - - - - - - - Principal component 5 PC5 - - - - 100 - - - - - - - - - - - - - - - - - 71 - - - - - 59 - - - - - - 58 - - - - - - 42 - - - 77 - - - Principal component 6 PC6 - - - - - 100 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 93 - - - - 77 Temperature TEMPC - - - - - - 100 - - −36 −35 - - - - - - - - - - - - - - - - - - - - - - 32 - - - - - - - - - - - - - - - - Dissolved oxygen DOX - −75 - - - - - 100 35 - - - - −48 −43 - - - - - - - - −38 −38 - - −36 - - −59 55 - - - −31 - - - 55 −42 - −34 −56 - - −40 - - - Redox potential EHmv - −64 - - - - - 35 100 - - - - - - - - - - - - - - - - - - - - - −51 42 - - - - - - - 40 −65 - - −57 - - - - - - Land-surface altitude LSALT - - - - - - −36 - - 100 92 30 - - - - - - - - - - - - - - - - - - - - - - - −36 - - - - - - - - - - - - - - Well bottom altitude WELLZALT - - - - - - −35 - - 92 100 - - - - - - - - - - - - - - - - - - - - - - - - −34 - - - - - - - - - - - - - - Well depth WELLZ 31 −32 - - - - - - - 30 - 100 63 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - −38 - - - - - - Depth to water WATRZ - - - - - - - - - - - 63 100 - - - - - - - - - - - - - - - - - - - - - - −58 - - - - −51 - - - - - - - - - pH, Field pH 69 30 −33 - - - - −48 - - - - - 100 88 - - - - −37 −41 - - 54 45 - - 64 - - 50 −50 - - - 32 - - - −57 31 −35 66 - - −39 36 - - - pH, Lab pHL 69 - −33 - - - - −43 - - - - - 88 100 - - - - −41 −42 - - 60 44 - - 66 - - 50 −48 - - 31 38 - −31 −33 −50 - −37 66 - - −35 36 −32 - - Specific conductance, lab SCL 55 - 46 51 - - - - - - - - - - - 100 98 96 100 31 - 31 32 70 83 58 50 41 - 37 56 −36 41 - - - - - - −43 40 - 48 44 - - - - - - Specific conductance, field SCF 53 - 47 53 - - - - - - - - - - - 98 100 95 98 33 - 32 - 67 81 58 52 41 - 40 53 −32 40 - - - - - - −42 41 - 46 43 - - - - - - Residue on evaporation 180C ROE180 58 - 43 48 - - - - - - - - - - - 96 95 100 97 - - - 35 72 81 56 46 46 - 37 53 −33 42 - - - - - - −41 38 - 49 43 - - - - - - Total dissolved solids TDS 57 - 46 48 - - - - - - - - - - - 100 98 97 100 - - - 32 71 84 56 48 43 - 38 58 −37 42 - - - - - - −44 43 - 49 45 - - - - - - Hardness HRDcalc - - 79 - - - - - - - - - - −37 −41 31 33 - - 100 98 88 - - - - - −33 41 53 - - - - - - - - - - - - - - - 46 - - - - Calcium Ca −35 - 72 - - - - - - - - - - −41 −42 - - - - 98 100 77 - - - - - −36 38 51 - - - - - - - - - - - - −33 - - 46 - - - - Magnesium Mg - - 80 - - - - - - - - - - - - 31 32 - - 88 77 100 31 - 31 - - - 44 49 - - - - - - - - 30 - - - - 37 - 36 - - - - 
Table 3.2. Spearman rank correlation coefficient (r) matrix for ground-water chemical data Bradford County, Pennsylvania, 2016.—Continued

Parameter name Parameter abbreviation

PC1

PC2

PC3

PC4

PC5

PC6

TEMPC

DOX

EHmv

LSALT

WELLZALT

WELLZ

WATRZ

pH

pHL

SCL

SCF

ROE180

TDS

HRDcalc

Ca

Mg K

Na

ALK

Br

Cl F

SiO

2

SO

4

NHN

3

NON

3

POP

4

Tcoli

Al

Ba

Be

Cd

Co

Cu

Fe

Pb

Li

Mn

Mo

Ni

Sr

Zn

Sb

As 

Potassium K 40 - - - 71 - - - - - - - - - - 32 - 35 32 - - 31 100 38 32 - - - 31 - 35 - - - - 31 - - - - - - 52 - - - 58 - - - Sodium Na 79 30 - 37 - - - −38 - - - - - 54 60 70 67 72 71 - - - 38 100 69 60 36 56 - - 68 −51 56 - - - - - - −47 46 −34 78 34 - - 41 - - - Alkalinity ALK 72 32 36 - - - - −38 - - - - - 45 44 83 81 81 84 - - 31 32 69 100 41 - 49 - - 63 −54 44 - - - - −31 - −53 40 −40 61 45 - - 34 - - - Bromide Br 32 - - 83 - - - - - - - - - - - 58 58 56 56 - - - - 60 41 100 70 - - - 41 - 32 - - - - - - −36 32 - 45 - - - - - - - Chloride Cl - - - 89 - - - - - - - - - - - 50 52 46 48 - - - - 36 - 70 100 - - - 33 - - - - 38 - - - - - - - - - - - - - - Fluoride F 66 - - - - - - −36 - - - - - 64 66 41 41 46 43 −33 −36 - - 56 49 - - 100 - - 58 −38 42 - - - - −33 - −57 36 −40 60 - - - - - - - Silica SiO2 - - 45 - 59 - - - - - - - - - - - - - - 41 38 44 31 - - - - - 100 31 - - - - - - - - - - - - - - - - 42 - - - Sulfate SO4 - - 85 - - - - - - - - - - - - 37 40 37 38 53 51 49 - - - - - - 31 100 - - - - - −52 - - - - - - - - - 43 - - - - Ammonia NH3N 55 70 - - - - - −59 −51 - - - - 50 50 56 53 53 58 - - - 35 68 63 41 33 58 - - 100 −72 38 - - 31 - - - −67 71 −43 61 62 - - 56 - −36 - Nitrate NO3N −48 −66 - - - - - 55 42 - - - - −50 −48 −36 −32 −33 −37 - - - - −51 −54 - - −38 - - −72 100 - - - - - - - 64 −52 39 −48 −55 - - −42 - 38 - Phosphate PO4P 38 - - - - - - - - - - - - - - 41 40 42 42 - - - - 56 44 32 - 42 - - 38 - 100 - - - - - - - 36 - - - - - - - - - Total coliform Tcoli - - - - - - 32 - - - - - - - - - - - - - - - - - - - - - - - - - - 100 - - - - - - - - - - - - - - - - Aluminum Al - - - - - - - - - - - - - - 31 - - - - - - - - - - - - - - - - - - - 100 - 35 - - - - - - - - - - - 31 - Barium Ba - - −44 31 58 - - −31 - −36 −34 - −58 32 38 - - - - - - - 31 - - - 38 - - −52 31 - - - - 100 - - - - - - 36 - - - 53 - - - Beryllium Be - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 35 - 100 - 32 - - - - - - - - - - - Cadmium Cd −32 - - - - - - - - - - - - - −31 - - - - - - - - - −31 - - −33 - - - - - - - - - 100 - - - - −32 - - - - 33 - - Cobalt Co - - - - - - - - - - - - - - −33 - - - - - - 30 - - - - - - - - - - - - - - 32 - 100 - - - - - - 50 - - - - Copper Cu −44 −57 - - - - - 55 40 - - - - −57 −50 −43 −42 −41 −44 - - - - −47 −53 −36 - −57 - - −67 64 - - - - - - - 100 −53 54 −44 −55 - - −40 - 41 - Iron Fe - 76 - - - - - −42 −65 - - - −51 31 - 40 41 38 43 - - - - 46 40 32 - 36 - - 71 −52 36 - - - - - - −53 100 −40 32 70 - - 36 - - - Lead Pb −32 −31 - - - - - - - - - - - −35 −37 - - - - - - - - −34 −40 - - −40 - - −43 39 - - - - - - - 54 −40 100 −32 −38 - - - - 46 - Lithium Li 85 - - - 42 - - −34 - - - - - 66 66 48 46 49 49 - −33 - 52 78 61 45 - 60 - - 61 −48 - - - 36 - −32 - −44 32 −32 100 - - - 56 - - - Manganese Mn - 87 - - - - - −56 −57 - - −38 - - - 44 43 43 45 - - 37 - 34 45 - - - - - 62 −55 - - - - - - - −55 70 −38 - 100 - - 34 - - - Molybdenum Mo - - - - - 93 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 100 - - - - 63 
Table 3.2. Spearman rank correlation coefficient (r) matrix for ground-water chemical data Bradford County, Pennsylvania, 2016.—Continued Parameter name Parameter abbreviation

PC1

PC2

PC3

PC4

PC5

PC6

TEMPC

DOX

EHmv

LSALT

WELLZALT

WELLZ

WATRZ

pH

pHL

SCL

SCF

ROE180

TDS

HRDcalc

Ca

Mg K

Na

ALK

Br

Cl F

SiO

2

SO

4

NHN

3

NON

3

POP

4

Tcoli

Al

Ba

Be

Cd

Co

Cu

Fe

Pb

Li

Mn

Mo

Ni

Sr

Zn

Sb

As 

Nickel Ni - - 48 - - - - - - - - - - −39 −35 - - - - 46 46 36 - - - - - - - 43 - - - - - - - - 50 - - - - - - 100 - - 43 - Strontium Sr 39 33 - - 77 - - −40 - - - - - 36 36 - - - - - - - 58 41 34 - - - 42 - 56 −42 - - - 53 - - - −40 36 - 56 34 - - 100 - - - Zinc Zn - - - - - - - - - - - - - - −32 - - - - - - - - - - - - - - - - - - - - - - 33 - - - - - - - - - 100 - - Antimony Sb - −31 - - - - - - - - - - - - - - - - - - - - - - - - - - - - −36 38 - - 31 - - - - 41 - 46 - - - 43 - - 100 33 Arsenic As - - - - - 77 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 63 - - - 33 10Boron B 89 30 - - 33 - - −45 - - - - - 66 71 54 52 56 55 −31 −35 - 43 84 69 43 - 64 - - 72 −63 38 - - - - −33 - −55 41 −38 90 31 - - 56 - - - Selenium Se - −48 - 36 - - - 36 - - - - - - - - - - - - - - - - - - - - - - - 45 - - - - - - - 39 - 35 - −46 - - - - 32 - Gross alpha radioactivity, 30-day count, water, filtered, Th-230 curve, picocuries per liter

AlphaTh30 - - - - 38 - - - - - - - - - - - - - - - - - - - - - - - - −32 - - - - - 40 - - - - - - - - - - 37 - - - 

Gross alpha radioactivity, 72-hour count, water, filtered, Th-230 curve, picocuries per liter

AlphaTh72 - - - - - 34 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 36 - - - - - 

Gross beta radioactivity, 30-day count, water, filtered, Cs-137 curve, picocuries per liter

BetaCs30 - - - - 51 - - - - - - - - - - - - - - - - - 44 - - - - - - - 30 - - - - 45 - - - - - - - - - - 61 - - - 

Gross beta radioactivity, 72-hour count, water, filtered, Cs-137 curve, picocuries per liter

BetaCs72 - 30 - - 47 - - - - - - - - - - - - - - - - - 58 32 - - 31 - - - 49 - - - - 46 - - - −32 43 −31 35 40 - - 58 - - - 

Radon 222 Rn222 −66 −38 - - - 36 - - 32 - - - - −54 −53 −55 −53 −54 −58 - - - - −59 −55 - - −63 - - −66 46 −33 - - - - - - 56 −55 36 −51 −37 - - - - - - Uranium U −52 −34 - - - 65 - - 39 - - - - −34 −37 −37 −34 −37 −39 34 39 - - −49 −31 - - −58 - - −62 47 - - - - - - - 50 −46 34 −47 - 55 - −31 - - 33 Methane Methane 43 71 - - - - - −59 −52 - - - - 44 47 42 38 43 45 - - - 34 65 46 46 33 44 - - 83 −62 34 - - 40 - - - −58 62 −40 55 61 - - 49 - - - Ethane Ethane 37 43 −33 - - - - −46 −36 - - - - 41 44 - - - - - - - - 46 36 32 - 46 - −39 54 −41 43 - - 48 - - - −39 39 −34 46 30 - - - - - - Propane Propane - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
Appendix 4 51

Appendix 4.

Distributions of continuous variables were compared 

among different sample classifications using notched box-

plots (Velleman and Hoaglin, 1981; Helsel and Hirsch, 2002). 

Where the median for a group is greater than the common 

reporting limit, it is displayed as a horizontal line within the 

box that is defined by the 25th and 75th percentiles for that 

group; otherwise, the median is displayed at the reporting 

limit. If the notched intervals around the medians for sample 

subsets do not overlap, the medians are statistically different at 

the 95-percent confidence interval.

References Cited

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods 

in water resources: U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 4, chap. A3, 

510 p., accessed February 2019 at https://doi.org/10.3133/

twri04A3.

McMahon, P.B., and Chapelle, F.H., 2008, Redox processes 

and water quality of selected principal aquifer systems: 

Groundwater, v. 46, no. 2, p. 259–271, accessed October 16, 

2017, at http://onlinelibrary.wiley.com/doi/10.1111/j.1745-

6584.2007.00385.x/abstract.

Pennsylvania Bureau of Topographic and Geologic Survey, 

2001, Bedrock geology of Pennsylvania: Pennsylvania 

Bureau of Topographic and Geologic Survey, Department of 

Conservation and Natural Resources, digital data, accessed 

October 2017 at http://www.dcnr.state.pa.us/topogeo/map1/

bedmap.aspx.

Velleman, P.F., and Hoaglin, D., 1981, Applications, basics, 

and computing of exploratory data analysis: Boston, Mass., 

Duxbury Press, 354 p.
52 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

2 35 21 14

Methane,

in milligrams per liter

 0.0001

 0.001

 0.01

 0.1

 1

 10

 100

 1,000

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

2 35 21 14

Lithium,

in micrograms per liter

 0.1

 1

 10

 100

 1,000

 10,000

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

2 35 21 14

 Boron,

in micrograms per liter

 1

 10

 100

 1,000 2 35 21 14

Fluoride,

in milligrams per liter

 0.01

 0.1

 1

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

2 35 21 14

Bromide,

in milligrams per liter

 0.001

 0.01

 0.1

 1

 10

 100 2 35 21 14

 Chloride,

in milligrams per liter 

 0.1

 1

 10

 100

 1,000

 10,000 2 35 21 14

Sulfate,

in milligrams per liter

 0.01

 0.1

 1

 10

 100

 1,000 2 35 21 14

 Alkalinity, 

in milligrams per liter

as calcium carbonate

 10

 100

 1,000

2 35 21 14

Sodium,

in milligrams per liter 

 1

 10

 100

 1,000

 10,000 2 35 21 14

 Magnesium,

in milligrams per liter 

 0.001

 0.01

 0.1

 1

 10

 100 2 35 21 14

 Calcium,

in milligrams per liter 

 0.01

 0.1

 1

 10

 100

 1,000 2 35 21 14

 Hardness, 

in milligrams per liter

as calcium carbonate 

 0.01

 0.1

 1

 10

 100

 1,000

 Total dissolved solids,

in milligrams per liter 

2 35 21 14

 10

 100

 1,000

 10,000

 Specific conductance,

in microsiemens per centimeter 

 10

 100

 1,000

 10,000 2 35 21 14

 0 

 70 

 140 

 210 

 280 

 350 

 420 

 490 

 560 

630 

 Reduction-oxidation 

(redox) potential,

in millivolts

2 35 21 14

 pH,

in standard units 

 6 

 6.5 

 7 

 7.5 

 8 

 8.5 

 9 

 9.5 2 35 21 14

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

35 Number of samples

Figure 4.1. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified by pH class 

interval. The pH range is indicated on the x-axis (5.3 < pH < 6.4, 

n = 2; 6.5 < pH < 7.4, n = 35; 7.5 < pH < 7.9, n = 21; 8.0 < pH < 9.4, 

n = 14). (<, less than; n, number of samples; see table 3.1 for 

descriptions of principal components)
Appendix 4. 53

 Radon 222,

in picocuries per liter 

 10

 100

 1,000

 10,000

 100,000

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

2 35 21 14

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

 0.001

 0.01

 0.1

 1

 10

 100 2 35 21 14

 Uranium,

in micrograms per liter

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

2 35 21 14

 Zinc,

in micrograms per liter

 1 

 10 

 100 

 1,000 

 10,000 

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

2 35 21 14

Manganese, 

in micrograms per liter

 0.1

 1

 10

 100

 1,000

 10,000

2 35 21 14

Iron, 

in micrograms per liter

 1

 10

 100

 1,000

 10,000 2 35 21 14

Molybdenum, 

in micrograms per liter

 0.01

 0.1

 1

 10 2 35 21 14

Selenium, 

in micrograms per liter

 0.01

 0.1

 1 2 35 21 14

Arsenic, 

in micrograms per liter 0.1

 0.01

 1

 10

 100

2 35 21 14

Orthophosphate,

in milligrams per liter 

as phosphorus

 0.001

 0.01

 0.1

 1 2 35 21 14

Ammonia,

in milligrams per liter 

as nitrogen

 0.001

 0.01

 0.1

 1

 10 2 35 21 14

Nitrate plus nitrite,

in milligrams per liter 

as nitrogen

 0.01

 0.1

 1

 10 2 35 21 14

Dissolved oxygen,

in milligrams per liter

 0.01

 0.1

 1

 10

 100

2 35 21 14

Silica,

in milligrams per liter

 1

 10

 100 2 35 21 14

Potassium,

in milligrams per liter

0.1

 1

 10 2 35 21 14

Strontium,

in micrograms per liter

 1

 10

 100

 1,000

 10,000 2 35 21 14

Barium,

in micrograms per liter

 1

 10

 100

 1,000

 10,000

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

35 Number of samples

Figure 4.1. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified by pH class 

interval. The pH range is indicated on the x-axis (5.3 < pH < 6.4, 

n = 2; 6.5 < pH < 7.4, n = 35; 7.5 < pH < 7.9, n = 21; 8.0 < pH < 9.4, 

n = 14). (<, less than; n, number of samples; see table 3.1 for 

descriptions of principal components)—Continued
54 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater Principal component 6

−2

−2.5

−1.5

−1

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

 3

Principal component 5

−2

−3

−1

 0

 1

 2

 3

Principal component 4

−2

−3

−1

 0

 1

 2

 3

Principal component 3

−2

−3

−1

 0

 1

 2

 3

Principal component 2

−2

−2.5

−1.5

−1

−0.5

 0

 0.5

 1

 1.5

 2

Principal component 1

−1

−1.5

−2

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

 3

Temperature,

in degrees Celsius

 8

 9.5

 11

 12.5

 14

 15.5

 17

 18.5

Well depth,

in feet below land surface

 10

−70

 80

 150

 220

 290

 360

 430

 500

 570

640

Altitude of land surface,

in feet above NAVD 88

 600

 800

 1,000

 1,200

 1,400

 1,600

 1,800

 2,000

 2,200

Altitude of well bottom,

in feet above NAVD 88

 400

 550

 700

 850

 1,000

 1,150

 1,300

 1,450

 1,600

 1,750

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

5.3–6.46.5–7.47.5–7.98.0–9.4

pH range,

in standard units

2 35 21 14 2 35 21 14

2 35 21 14 2 35 21 14 2 35 21 14 2 35 21 14

2 35 21 14 2 35 21 14 2 35 21 14 2 35 21 14

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

35 Number of samples

Figure 4.1. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified by pH class 

interval. The pH range is indicated on the x-axis (5.3 < pH < 6.4, 

n = 2; 6.5 < pH < 7.4, n = 35; 7.5 < pH < 7.9, n = 21; 8.0 < pH < 9.4, 

n = 14). (<, less than; n, number of samples; see table 3.1 for 

descriptions of principal components)—Continued
Appendix 4. 55

 6

 6.5

 7

 7.5

 8

 8.5

 9

 9.5

 10

 0

 70

 140

 210

 280

 350

 420

 490

 560

 630

 10

 100

 1,000

 10,000

 10

 100

 1,000

 10,000

 0.01

 0.1

 1

 10

 100

 1,000

 0.01

 0.1

 1

 10

 100

 1,000

 0.001

 0.01

 0.1

 1

 10

 100

 1

 10

 100

 1,000

 10,000

 10

 100

 1,000

 0.01

 0.1

 1

 10

 100

 1,000

 0.1

 1

 10

 100

 1,000

 10,000

 0.001

 0.01

 0.1

 1

 10

 100

 0.01

 0.1

 1

 1

 10

 100

 1,000

 0.1

 1

 10

 100

 1,000

 10,000

 0.0001

 0.001

 0.01

 0.1

 1

 10

 100

 1,000

6 30 26 10 6 30 26 10 6 30 26 10 6 30 26 10

6 30 26 10 6 30 26 10 6 30 26 10 6 30 26 10

6 30 26 10 6 30 26 10 6 30 26 10 6 30 26 10

6 30 26 10 6 30 26 10 6 30 26 10 6 30 26 10

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

Methane,

in milligrams per liter

Lithium,

in micrograms per liter

 Boron,

in micrograms per liter

Fluoride,

in milligrams per liter

Bromide,

in milligrams per liter

 Chloride,

in milligrams per liter 

Sulfate,

in milligrams per liter

 Alkalinity, 

in milligrams per liter

as calcium carbonate

Sodium,

in milligrams per liter 

 Magnesium,

in milligrams per liter 

 Calcium,

in milligrams per liter 

 Hardness, 

in milligrams per liter

as calcium carbonate 

 Total dissolved solids,

in milligrams per liter 

 Specific conductance,

in microsiemens per centimeter 

 Reduction-oxidation 

(redox) potential,

in millivolts

 pH,

in standard units 

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

30 Number of samples

Figure 4.2. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified by specific 

conductance (SC) class interval. The SC range is indicated on the 

x-axis (70 < SC < 200 μS/cm, n = 6; 200 < SC < 370 μS/cm, n = 30; 

370 < SC < 600 μS/cm, n = 26; 600 < SC < 3,800 μS/cm, n = 10). 

(<, less than; μS/cm, microsiemens per centimeter; n, number of 

samples; see table 3.1 for descriptions of principal components)
56 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

 1

 10

 100

 1,000

 10,000

 1

 10

 100

 1,000

 10,000

 0.1

 1

 10

 1

 10

 100

 0.01

 0.1

 1

 10

 100

 0.01

 0.1

 1

 10

 0.001

 0.01

 0.1

 1

 10

 0.001

 0.01

 0.1

 1

 0.1

 1

 10

 100

 0.01

 0.1

 1

 0.01

 0.1

 1

 10

 1

 10

 100

 1,000

 10,000

 0.1

 1

 10

 100

 1,000

 10,000

 1

 10

 100

 1,000

 10,000

 0.001

 0.01

 0.1

 1

 10

 100

 10

 100

 1,000

 10,000

 100,000

6 30 26 10 6 30 26 10 6 30 26 10 6 30 26 10

6 30 26 10 6 30 26 10 6 30 26 10 6 30 26 10

6 30 26 10 6 30 26 10 6 30 26 10 6 30 26 10

6 30 26 10 6 30 26 10 6 30 26 10 6 30 26 10

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

 Radon 222,

in picocuries per liter 

 Uranium,

in micrograms per liter

 Zinc,

in micrograms per liter

Manganese, 

in micrograms per liter

Iron, 

in micrograms per liter

Molybdenum, 

in micrograms per liter

Selenium, 

in micrograms per liter

Arsenic, 

in micrograms per liter

Orthophosphate,

in milligrams per liter 

as phosphorus

Ammonia,

in milligrams per liter 

as nitrogen

Nitrate plus nitrite,

in milligrams per liter 

as nitrogen

Dissolved oxygen,

in milligrams per liter

Silica,

in milligrams per liter

Potassium,

in milligrams per liter

Strontium,

in micrograms per liter

Barium,

in micrograms per liter

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

30 Number of samples

Figure 4.2. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified by specific 

conductance (SC) class interval. The SC range is indicated on the 

x-axis (70 < SC < 200 μS/cm, n = 6; 200 < SC < 370 μS/cm, n = 30; 

370 < SC < 600 μS/cm, n = 26; 600 < SC < 3,800 μS/cm, n = 10). 

(<, less than; μS/cm, microsiemens per centimeter; n, number of 

samples; see table 3.1 for descriptions of principal components)—

Continued
Appendix 4. 57

 400

 550

 700

 850

 1,000

 1,150

 1,300

 1,450

 1,600

 1,750

 1,900

 2,050

 600

 800

 1,000

 1,200

 1,400

 1,600

 1,800

 2,000

 2,200

 10

 80

 150

 220

 290

 360

 430

 500

 570

 640

 8

 9.5

 11

 12.5

 14

 15.5

 17

 18.5

−1

−1.5

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

 3

−2

−2.5

−1.5

−1

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

-2

-1

 0 

 1 

 2 

 3 

−2

−3

−1

 0

 1

 2

 3

−2

−3

−1

 0

 1

 2

 3

−2

−2.5

−1.5

−1

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

6 30 26 10 6 30 26 10 6 30 26 10 6 30 26 10

6 30 26 10 6 30 26 10 6 30 26 10 6 30 26 10

6 30 26 10 6 30 26 10

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

70–200

200–370370–600

600–3,800

Specific conductance,

in microsiemens 

per centimeter

Principal component 5

Principal component 6

Principal component 1Principal component 2Principal component 3

Principal component 4

Temperature,

in degrees Celsius

Well depth,

in feet below land surface

Altitude of land surface,

in feet above NAVD 88

 Altitude of well bottom,

in feet above NAVD 88

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

30 Number of samples

Figure 4.2. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified by specific 

conductance (SC) class interval. The SC range is indicated on the 

x-axis (70 < SC < 200 μS/cm, n = 6; 200 < SC < 370 μS/cm, n = 30; 

370 < SC < 600 μS/cm, n = 26; 600 < SC < 3,800 μS/cm, n = 10). 

(<, less than; μS/cm, microsiemens per centimeter; n, number of 

samples; see table 3.1 for descriptions of principal components)—

Continued
58 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

 6

 6.5

 7

 7.5

 8

 8.5

 9

 9.5

 10

 0

 70

 140

 210

 280

 350

 420

 490

 560

 630

 10

 100

 1,000

 10,000

 10

 100

 1,000

 10,000

 0.01

 0.1

 1

 10

 100

 1,000

 0.01

 0.1

 1

 10

 100

 1,000

 0.001

 0.01

 0.1

 1

 10

 100

 1

 10

 100

 1,000

 10,000

 10

 100

 1,000

 0.01

 0.1

 1

 10

 100

 1,000

 0.1

 1

 10

 100

 1,000

 10,000

 0.001

 0.01

 0.1

 1

 10

 100

 0.01

 0.1

 1

 1

 10

 100

 1,000

 0.1

 1

 10

 100

 1,000

 10,000

 0.0001

 0.001

 0.01

 0.1

 1

 10

 100

 1000 26 10 36 26 10 36 26 10 36 26 10 36

26 10 36 26 10 36 26 10 36 26 10 36

26 10 36 26 10 36 26 10 36 26 10 36

26 10 36 26 10 36 26 10 36 26 10 36

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

Methane,

in milligrams per liter

Lithium,

in micrograms per liter

 Boron,

in micrograms per liter

Fluoride,

in milligrams per liter

Bromide,

in milligrams per liter

 Chloride,

in milligrams per liter 

Sulfate,

in milligrams per liter

 Alkalinity, 

in milligrams per liter

as calcium carbonate

Sodium,

in milligrams per liter 

 Magnesium,

in milligrams per liter 

 Calcium,

in milligrams per liter 

 Hardness, 

in milligrams per liter

as calcium carbonate 

 Total dissolved solids,

in milligrams per liter 

 Specific conductance,

in microsiemens per centimeter 

 Reduction-oxidation 

(redox) potential,

in millivolts

 pH,

in standard units 

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

36 Number of samples

Figure 4.3. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified as anoxic (n = 26), 

mixed (n = 10), and oxic (n = 36) on the basis of dissolved oxygen 

concentration and other water-quality criteria of McMahon 

and Chapelle (2008). (n, number of samples; see table 3.1 for 

descriptions of principal components)
Appendix 4. 59

 1

 10

 100

 1,000

 10,000

 1

 10

 100

 1,000

 10,000

 0.1

 1

 10

 1

 10

 100

 0.01

 0.1

 1

 10

 100

 0.01

 0.1

 1

 10

 0.001

 0.01

 0.1

 1

 10

 0.001

 0.01

 0.1

 1

 0.1

 1

 10

 100

 0.01

 0.1

 1

 0.01

 0.1

 1

 10

 1

 10

 100

 1,000

 10,000

 0.1

 1

 10

 100

 1,000

 10,000

 1

 10

 100

 1,000

 10,000

 0.001

 0.01

 0.1

 1

 10

 100

 10

 100

 1,000

 10,000

 100,000 26 10 36 26 10 36 26 10 36 26 10 36

26 10 36 26 10 36 26 10 36 26 10 36

26 10 36 26 10 36 26 10 36 26 10 36

26 10 36 26 10 36 26 10 36 26 10 36

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

 Radon 222,

in picocuries per liter 

 Uranium,

in micrograms per liter

 Zinc,

in micrograms per liter

Manganese, 

in micrograms per liter

Iron, 

in micrograms per liter

Molybdenum, 

in micrograms per liter

Selenium, 

in micrograms per liter

Arsenic, 

in micrograms per liter

Orthophosphate,

in milligrams per liter 

as phosphorus

Ammonia,

in milligrams per liter 

as nitrogen

Nitrate plus nitrite,

in milligrams per liter 

as nitrogen

Dissolved oxygen,

in milligrams per liter

Silica,

in milligrams per liter

Potassium,

in milligrams per liter

Strontium,

in micrograms per liter

Barium,

in micrograms per liter

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

36 Number of samples

Figure 4.3. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified as anoxic (n = 26), 

mixed (n = 10), and oxic (n = 36) on the basis of dissolved oxygen 

concentration and other water-quality criteria of McMahon 

and Chapelle (2008). (n, number of samples; see table 3.1 for 

descriptions of principal components)—Continued
60 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

 400

 550

 700

 850

 1,000

 1,150

 1,300

 1,450

 1,600

 1,750

 600

 800

 1,000

 1,200

 1,400

 1,600

 1,800

 2,000

 2,200

 10

 80

 150

 220

 290

 360

 430

 500

 570

 640

 8

 9.5

 11

 12.5

 14

 15.5

 17

 18.5

−1

−1.5

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

 3

−2

−2.5

−1.5

−1

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

−2

−1

 0

 1

 2

 3

−2

−3

−1

 0

 1

 2

 3

−2

−3

−1

 0

 1

 2

 3

−2

−2.5

−1.5

−1

−0.5

 0

 0.5

 1

 1.5

 2

 2.5 26 10 36 26 10 36

26 10 36 26 10 36 26 10 36 26 10 36

26 10 36 26 10 36 26 10 36 26 10 36

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

Anoxic

Mixed

Oxic

Reduction-oxidation

classification

 Principal component 5

 Principal component 6

 Principal component 1 Principal component 2 Principal component 3

 Principal component 4

Temperature,

in degrees Celsius

Well depth,

in feet below land surface

Altitude of land surface,

Altitude of well bottom,

in feet above NAVD 88

in feet above NAVD 88

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

36 Number of samples

Figure 4.3. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified as anoxic (n = 26), 

mixed (n = 10), and oxic (n = 36) on the basis of dissolved oxygen 

concentration and other water-quality criteria of McMahon 

and Chapelle (2008). (n, number of samples; see table 3.1 for 

descriptions of principal components)—Continued
Appendix 4. 61

 6

 6.5

 7

 7.5

 8

 8.5

 9

 9.5

 10

 0

 70

 140

 210

 280

 350

 420

 490

 560

630

 10

 100

 1,000

 10,000

 10

 100

 1,000

 10,000

 0.1

 1

 10

 100

 1,000

 0.01

 0.1

 1

 10

 100

 1,000

 0.001

 0.01

 0.1

 1

 10

 100

 10

 100

 1,000

 10,000

 10

 100

 1,000

 0.01

 0.1

 1

 10

 100

 1,000

 0.1

 1 

 10

 100

 1,000

 10,000

 0.01

 0.001

 0.1

 1

 10

 100

 0.01

 0.1

 1

 1

 10

 100

 1,000

 0.1

 1

 10

 100

 1,000

 10,000

 0.0001

 0.00001

 0.001

 0.01

 0.1

 1

 10

 100

 1,000 3 22 47 3 22 47 3 22 47 3 22 47

3 22 47 3 22 47 3 22 47 3 22 47

3 22 47 3 22 47 3 22 47 3 22 47

3 22 47 3 22 47 3 22 47 3 22 47

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

Methane,

in milligrams per liter

Lithium,

in micrograms per liter

 Boron,

in micrograms per liter

Fluoride,

in milligrams per liter

Bromide,

in milligrams per liter

 Chloride,

in milligrams per liter 

Sulfate,

in milligrams per liter

 Alkalinity, 

in milligrams per liter

as calcium carbonate

Sodium,

in milligrams per liter 

 Magnesium,

in milligrams per liter 

 Calcium,

in milligrams per liter 

 Hardness, 

in milligrams per liter

as calcium carbonate 

 Total dissolved solids,

in milligrams per liter 

 Specific conductance,

in microsiemens per centimeter 

 Reduction-oxidation 

(redox) potential,

in millivolts

 pH,

in standard units 

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

47 Number of samples

Figure 4.4. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified on the basis 

of bedrock map unit identified by Pennsylvania Bureau of 

Topographic and Geologic Survey (2001) as Huntley Mountain 

Formation (HNTLYMT; n = 3), Catskill Formation (CATSKIL; n = 22), 

and Lock Haven Formation (LOCKHAV; n = 47). (n, number of 

samples; see table 3.1 for descriptions of principal components)
62 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

 1

 10

 100

 1,000

 10,000

 1

 10

 100

 1,000

 10,000

 0.1

 1

 10

 1

 10

 100

 0.01

 0.1

 1

 10

 100

 0.01

 0.1

 1

 10

 0.001

 0.01

 0.1

 1

 10

 0.001

 0.01

 0.1

 1

0 .1

 1

 10

 100

 0.01

 0.1

 1

 0.01

 0.1

 1

 10

 1

 10

 100

 1,000

 10,000

 0.1

 1

 10

 100

 1,000

 10,000

 1

 10

 100

 1,000

 10,000

 0.001

 0.01

 0.1

 1

 10

 100

 10

 100

 1,000

 10,000 3 22 47 3 22 47 3 22 47 3 22 47

3 22 47 3 22 47 3 22 47 3 22 47

3 22 47 3 22 47 3 22 47 3 22 47

3 22 47 3 22 47 3 22 47 3 22 47

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

 Radon 222,

in picocuries per liter 

 Uranium,

in micrograms per liter

 Zinc,

in micrograms per liter

Manganese, 

in micrograms per liter

Iron, 

in micrograms per liter

Molybdenum, 

in micrograms per liter

Selenium, 

in micrograms per liter

Arsenic, 

in micrograms per liter

Orthophosphate,

in milligrams per liter 

as phosphorus

Ammonia,

in milligrams per liter 

as nitrogen

Nitrate plus nitrite,

in milligrams per liter 

as nitrogen

Dissolved oxygen,

in milligrams per liter

Silica,

in milligrams per liter

Potassium,

in milligrams per liter

Strontium,

in micrograms per liter

Barium,

in micrograms per liter

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

47 Number of samples

Figure 4.4. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified on the basis 

of bedrock map unit identified by Pennsylvania Bureau of 

Topographic and Geologic Survey (2001) as Huntley Mountain 

Formation (HNTLYMT; n = 3), Catskill Formation (CATSKIL; n = 22), 

and Lock Haven Formation (LOCKHAV; n = 47). (n, number of 

samples; see table 3.1 for descriptions of principal components)—

Continued
Appendix 4. 63

 400

 550

 700

 850

 1,000

 1,150

 1,300

 1,450

 1,600

 1,750

 1,900

 2,050

 600

 800

 1,000

 1,200

 1,400

 1,600

 1,800

 2,000

 2,200

 2,400

 2,600

 10

 80

 150

 220

 290

 360

 430

 500

 570

640

710

 8

 9.5

 11

 12.5

 14

 15.5

 17

 18.5

−1

−1.5

−2

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

 3

−2

−2.5

−1.5

−1

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

−2

−1

 0

 1

 2

 3

−2

−3

−1

 0

 1

 2

 3

−2

−3

−1

 0

 1

 2

 3

−2

−2.5

−1.5

−1

−0.5

 0

 0.5

 1

 1.5

 2

 2.5 3 22 47 3 22 47

3 22 47 3 22 47 3 22 47 3 22 47

3 22 47 3 22 47 3 22 47 3 22 47

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

HNTLYMT

CATSKIL

LOCKHAV

Bedrock map unit

 Principal component 5

 Principal component 6

 Principal component 1 Principal component 2 Principal component 3

 Principal component 4

Temperature,

in degrees Celsius

Well depth,

in feet below land surface

Altitude of land surface,

Altitude of well bottom,

in feet above NAVD 88

in feet above NAVD 88

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

47 Number of samples

Figure 4.4. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified on the basis 

of bedrock map unit identified by Pennsylvania Bureau of 

Topographic and Geologic Survey (2001) as Huntley Mountain 

Formation (HNTLYMT; n = 3), Catskill Formation (CATSKIL; n = 22), 

and Lock Haven Formation (LOCKHAV; n = 47). (n, number of 

samples; see table 3.1 for descriptions of principal components)—

Continued
64 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

 6

5.5

 6.5

 7

 7.5

 8

 8.5

 9

 9.5

 10

 0

 70

 140

 210

 280

 350

 420

 490

 560

630

700

 10

 100

 1,000

 10,000

 10

 100

 1,000

 10,000

 0.01

 0.1

 1

 10

 100

 1,000

 10,000

 0.01

 0.1

 1

 10

 100

 1,000

 10,000

 0.001

 0.01

 0.1

 1

 10

 100

 1

 10

 100

 1,000

 10,000

 10

 100

 1,000

 0.1

 1

 10

 100

 1,000

 0.1

 1

 10

 100

 1,000

 10,000

 0.001

 0.01

 0.1

 1

 10

 100

 0.01

 0.1

 1

 1

 10

 100

 1,000

 10,000

 0.1

 1

 10

 100

 1,000

 10,000

 0.0001

 0.001

 0.01

 0.1

 1

 10

 100

 1,000

7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16

7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16

7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16

7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

Methane,

in milligrams per liter

Lithium,

in micrograms per liter

 Boron,

in micrograms per liter

Fluoride,

in milligrams per liter

Bromide,

in milligrams per liter

 Chloride,

in milligrams per liter 

Sulfate,

in milligrams per liter

 Alkalinity, 

in milligrams per liter

as calcium carbonate

Sodium ,

in milligrams per liter 

 Magnesium ,

in milligrams per liter 

 Calcium ,

in milligrams per liter 

 Hardness, 

in milligrams per liter

as calcium carbonate 

 Total dissolved solids,

in milligrams per liter 

 Specific conductance,

in microsiemens per centimeter 

 Reduction-oxidation 

(redox) potential,

in millivolts

 pH,

in standard units 

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

12 Number of samples

Figure 4.5. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified on the basis 

of topographic position index (TPI). The TPI classes are ridge 

(RIDGE; n = 7), upper slope (UPSLOPE; n = 12), steep slope 

(STSLOPE; n = 2), gentle slope (GNSLOPE; n = 20), lower slope 

(LOSLOPE; n = 13), and valley (VALLEY; n = 16). (n, number of 

samples; see table 3.1 for descriptions of principal components)
Appendix 4. 65

 1

 10

 100

 1,000

 10,000

 1

 10

 100

 1,000

 10,000

 0.1

 1

 10

 1

 10

 100

 0.01

 0.1

 1

 10

 100

 0.01

 0.1

 1

 10

 0.001

 0.01

 0.1

 1

 10

 0.001

 0.01

 0.1

 1

 0.1

 0.01

 1

 10

 100

 0.01

 0.1

 1

 0.01

 0.1

 1

 10

 1

 10

 100

 1,000

 10,000

 0.1

 0.01

 1

 10

 100

 1,000

 10,000

 1

 10

 100

 1,000

 10,000

 0.001

 0.01

 0.1

 1

 10

 100

 10

 100

 1000

 10,000

 100,000 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16

7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16

7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16

7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

 Radon 222,

in picocuries per liter 

 Uranium,

in micrograms per liter

 Zinc,

in micrograms per liter

Manganese, 

in micrograms per liter

Iron, 

in micrograms per liter

Molybdenum, 

in micrograms per liter

Selenium, 

in micrograms per liter

Arsenic, 

in micrograms per liter

Orthophosphate,

in milligrams per liter 

as phosphorus

Ammonia,

in milligrams per liter 

as nitrogen

Nitrate plus nitrite,

in milligrams per liter 

as nitrogen

Dissolved oxygen,

in milligrams per liter

Silica,

in milligrams per liter

Potassium,

in milligrams per liter

Strontium,

in micrograms per liter

Barium,

in micrograms per liter

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

12 Number of samples

Figure 4.5. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified on the basis 

of topographic position index (TPI). The TPI classes are ridge 

(RIDGE; n = 7), upper slope (UPSLOPE; n = 12), steep slope 

(STSLOPE; n = 2), gentle slope (GNSLOPE; n = 20), lower slope 

(LOSLOPE; n = 13), and valley (VALLEY; n = 16). (n, number of 

samples; see table 3.1 for descriptions of principal components)—

Continued
66 Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater

 400

 550

 700

 850

 1,000

 1,150

 1,300

 1,450

 1,600

 1,750

 600

 800

 1,000

 1,200

 1,400

 1,600

 1,800

 2,000

 2,200

 10

 80

 150

 220

 290

 360

 430

 500

 570

640

710

 8

 9.5

 11

 12.5

 14

 15.5

 17

 18.5

−1

−1.5

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

 3

−2

−2.5

−1.5

−1

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

−2

−1

 0

 1

 2

 3

−2

−3

−1

 0

 1

 2

 3

−2

−3

−1

 0

 1

 2

 3

−2

−2.5

−1.5

−1

−0.5

 0

 0.5

 1

 1.5

 2

 2.5

 3

7 12 2 20 13 16 7 12 2 20 13 16

7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16

7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16 7 12 2 20 13 16

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

RIDGE

UPSLOPE

STSLOPE

GNSLOPE

LOSLOPE

VALLEY

Topographic

position index

Principal component 5

Principal component 6

Principal component 1Principal component 2Principal component 3

Principal component 4

Temperature,

in degrees Celsius

Well depth,

in feet below land surface

Altitude at land surface,

Altitude of well bottom,

in feet above NAVD 88

in feet above NAVD 88

EXPLANATION

25th percentile

Median

Upper notch

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile

Outlier data value less than or equal to 3 

and more than 1.5 times the interquartile

range outside the quartile 

Outlier data value more than 3 times the

interquartile range outside the quartile 

75th percentile

Lower notch

12 Number of samples

Figure 4.5. Composition of 72 groundwater samples from 

Bradford County, Pennsylvania, 2016, classified on the basis 

of topographic position index (TPI). The TPI classes are ridge 

(RIDGE; n = 7), upper slope (UPSLOPE; n = 12), steep slope 

(STSLOPE; n = 2), gentle slope (GNSLOPE; n = 20), lower slope 

(LOSLOPE; n = 13), and valley (VALLEY; n = 16). (n, number of 

samples; see table 3.1 for descriptions of principal components)—

Continued
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://pa.water.usgs.gov/

Publishing support provided by the West Trenton Publishing 

Service Center
Clune and Cravotta—Drinking Water Health Standards Comparison and Chemical Analysis of Groundwater—Scientific Investigations Report 2018–5170, ver. 1.2

ISSN 2328–0328 (online)

https://doi.org/10.3133/sir20185170