U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5148

Prepared in cooperation with the Ohio Water Development Authority

Arsenic in Groundwater of Licking County, Ohio, 

2012—Occurrence and Relation to Hydrogeology
Cover illustration: The design on the cover is based on a geospatial 

coverage of the thickness of glacial deposits created by the Ohio 

Department of Natural Resources Division of Geological Survey (2004).
Arsenic in Groundwater of Licking County, 

Ohio, 2012—Occurrence and Relation to 

Hydrogeology

By Mary Ann Thomas

Prepared in cooperation with the Ohio Water Development Authority

Scientific Investigations Report 2015–5148

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

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

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Suggested citation:

Thomas, M.A., 2016, Arsenic in groundwater of Licking County, Ohio, 2012—Occurrence and relation to hydrogeology:

U.S. Geological Survey Scientific Investigations Report 2015–5148, 38 p., http://dx.doi.org/10.3133/sir20155148.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)
iii

Acknowledgments

The work on which this report is based was part of a larger multi-agency project to increase 

awareness and understanding of arsenic in groundwater of Ohio, which was done in 

cooperation with the Ohio Water Development Authority. The author would like to thank 

members of the project’s advisory committee: Rebecca Fugitt, Russ Smith, and Bob Frey 

(Ohio Department of Health); Mike Slattery, Chris Kenah, and Kathy Pinto (Ohio Environmental 

Protection Agency); Jim Raab (Ohio Department of Natural Resources, Division of Soil and 

Water); Mike Angle and Rick Pavey (Ohio Department of Natural Resources, Division of 

Geological Survey); and Tom Sorg (U.S. Environmental Protection Agency). These professionals 

contributed to (1) planning and oversight of the project, and (2) compilation and review of multiagency arsenic-related data for a Web site (http://ArsenicInOhioGroundwater.info). The author 

would also like to thank Chad Brown and his staff at the Licking County Health Department for: 

(1) collection of water samples for analysis of arsenic during routine well inspection activities, 

and (2) efforts related to planning and implementation of the Test Your Well workshops. The 

author also appreciates assistance with Test Your Well workshops from: members of the 

advisory committee (listed above); Curtis Coe (Ohio Department of Natural Resources, Division 

of Soil and Water); Frank Fugitt, (Ohio Department of Natural Resources, Division of Geological 

Survey); Donna Runkle, Al Dillenburg, Ralph Haefner, and Charles Hart (U.S. Geological Survey). 
iv
v

Contents

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

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

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

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

Background ...........................................................................................................................................3

Previous Studies ...................................................................................................................................3

Methods...........................................................................................................................................................4

Collection and Analysis of Primary Water-Quality Data.................................................................5

Collection and Analysis of Arsenic Speciation Data .....................................................................5

Quality Assurance.................................................................................................................................6

Data Analysis ........................................................................................................................................7

Description of Study Area ............................................................................................................................8

Arsenic Concentrations..............................................................................................................................12

Arsenic Species..................................................................................................................................13

Factors Related to Arsenic Concentrations ............................................................................................14

Geochemical Conditions of Groundwater ......................................................................................14

Redox Conditions Estimated From Concentrations of Iron, Manganese, Sulfate, and 

Nitrate ............................................................................................................................15

Redox Conditions Estimated From Iron Concentrations......................................................16

Comparison of Methods for Estimating Redox Conditions ................................................16

Aquifer and Well Characteristics From Well Logs ........................................................................17

Aquifer Type ...............................................................................................................................17

Well Characteristics..................................................................................................................18

Hydrogeologic Characteristics of Licking County.........................................................................19

Topography .................................................................................................................................19

Lithology of Glacial Deposits ...................................................................................................20

Drift Thickness ...........................................................................................................................21

Hydrogeologic Settings ............................................................................................................22

Bedrock Geology .......................................................................................................................25

Bedrock Lithology ............................................................................................................26

Well Construction Characteristics of Bedrock Wells ................................................27

Preliminary Extrapolation of Results From Licking County to Other Parts of Ohio ...........................28

Summary........................................................................................................................................................30

References Cited..........................................................................................................................................31

Tables 2–5......................................................................................................................................................35
vi

Figures

1. Map showing location of the study area in Licking County, Ohio, 2012. A—A’

indicates the approximate location of the cross section in figure 2....................................8

2. Cross section showing geology of Licking County, Ohio. Approximate location 

of cross section A—A’ is shown on figure 1............................................................................9

3. Maps showing characteristics of glacial deposits of Licking County, Ohio.....................11

4. Map showing arsenic concentrations from 168 domestic wells in Licking County, 

Ohio, 2012 .....................................................................................................................................12

5. Graph showing arsenic concentrations in relation to redox conditions estimated 

from concentrations of iron, manganese, sulfate, and nitrate—Licking County, 

Ohio, 2012 .....................................................................................................................................15

6. Graph showing arsenic concentrations in relation to redox conditions estimated 

from iron concentrations—Licking County, Ohio, 2012........................................................16

7. Graphs showing comparison of glacial and bedrock aquifers, Licking County, Ohio, 

2012 ...............................................................................................................................................17

8. Map showing distribution of arsenic concentrations in Licking County, Ohio 

(2012) in relation to topography................................................................................................19

9. Map showing distribution of arsenic concentrations in Licking County, Ohio 

(2012) in relation to lithology of glacial deposits ...................................................................20

10. Map showing distribution of arsenic concentrations in Licking County, Ohio 

(2012) in relation to drift thickness...........................................................................................21

11. Map showing arsenic concentrations in relation to hydrogeologic settings of 

Licking County, Ohio, 2012 .........................................................................................................22

12. Graphs showing arsenic concentrations in relation to hydrogeologic settings, 

Licking County, Ohio, 2012 .........................................................................................................23

13. Graphs showing redox conditions in hydrogeologic settings, Licking County, 

Ohio, 2012 .....................................................................................................................................24

14. Map showing distribution of arsenic concentrations in bedrock wells in relation 

to bedrock formations, Licking County, Ohio, 2012................................................................26

15. Graph showing relation between arsenic concentrations and bedrock lithology from 

well logs, Licking County, Ohio, 2012 .......................................................................................26

16. Graphs showing arsenic concentrations in bedrock wells in relation to wellconstruction characteristics, Licking County, Ohio, 2012....................................................27

17. Maps showing preliminary extrapolation of results from Licking County, Ohio, 

2012 ...............................................................................................................................................28
vii

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Datum

Vertical coordinate information is referenced to “mean sea level” in Dove (1960) on which 

cross-section elevations are based.

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). Solid-phase concentrations are given in micrograms per kilogram 

(mg/kg).

Tables

1. Methods used for collection and analysis of water-quality data, Licking County, 

Ohio, 2012 .......................................................................................................................................5

2. Water-quality and hydrogeologic data for 168 domestic wells in Licking County, 

Ohio, 2012 .......................................................................................................................................5

3. Results of quality-control samples from Licking County, Ohio analyzed by a private 

lab during Test Your Well workshops, 2012 ..............................................................................6

4. Results of arsenic-speciation quality-control samples from Licking County, Ohio 

analyzed by the U.S. Geological Survey National Water Quality Laboratory, 2012 ...........6

5. Results of quality-control samples from Licking County, Ohio analyzed at private 

lab and U.S. Geological Surve National Water Quality Laboratory, 2013............................6

6. Variables used for analysis of arsenic data from Licking County, Ohio, 2012 ....................7

7. Arsenic species from nine domestic wells, Licking County, Ohio, 2012............................13

8. Framework used to estimate redox conditions from concentrations of iron, 

manganese, sulfate, and nitrate in groundwater, Licking County, Ohio, 2012 ..................13

9. Well-log variables, summary statistics, and results of a statistical test for 

correlation with arsenic concentrations in groundwater, Licking County, Ohio, 

2012 ...............................................................................................................................................17

Abstract

Arsenic concentrations were measured in samples 

from 168 domestic wells in Licking County, Ohio, to document arsenic concentrations in a wide variety of wells and to 

identify hydrogeologic factors associated with arsenic concentrations in groundwater. Elevated concentrations of arsenic 

(greater than10.0 micrograms per liter [μg/L]) were detected 

in 12 percent of the wells (about 1 in 8). The maximum arsenic concentration of about 44 μg/L was detected in two wells 

in the same township. 

A subset of 102 wells was also sampled for iron, sulfate, 

manganese, and nitrate, which were used to estimate redox 

conditions of the groundwater. Elevated arsenic concentrations 

were detected only in strongly reducing groundwater. Almost 

20 percent of the samples with iron concentrations high 

enough to produce iron staining (greater than 300 μg/L) also 

had elevated concentrations of arsenic. 

In groundwater, arsenic primarily occurs as two inorganic species—arsenite and arsenate. Arsenic speciation was 

determined for a subset of nine samples, and arsenite was the 

predominant species. Of the two species, arsenite is more difficult to remove from water, and is generally considered to be 

more toxic to humans. 

Aquifer and well-construction characteristics were compiled from 99 well logs. Elevated concentrations of arsenic 

(and iron) were detected in glacial and bedrock aquifers but 

were more prevalent in glacial aquifers. The reason may be 

that the glacial deposits typically contain more organic carbon 

than the Paleozoic bedrock. Organic carbon plays a role in the 

redox reactions that cause arsenic (and iron) to be released 

from the aquifer matrix. Arsenic concentrations were not significantly different for different types of bedrock (sandstone, 

shale, sandstone/shale, or other). However, arsenic concentrations in bedrock wells were correlated with two well-construction characteristics; higher arsenic concentrations in bedrock 

wells were associated with (1) shorter open intervals and 

(2) deeper open intervals, relative to the water level. 

The spatial distribution of arsenic concentrations was 

compared to hydrogeologic characteristics of Licking County. 

Elevated concentrations of arsenic (and iron) were associated 

with areas of flat topography and thick (greater than 100 feet), 

clay-rich glacial deposits. These characteristics are conducive 

to development of strongly reducing redox conditions, which 

can cause arsenic associated with iron oxyhydroxides in the 

aquifer matrix to be released to the groundwater. 

Hydrogeologic characteristics conducive to the development of strongly reducing groundwater are relatively widespread in the western part of Licking County, which is part of 

the Central Lowland physiographic province. In this area, a 

thick layer of clay-rich glacial deposits obscures the bedrock 

surface and creates flat to gently rolling landscape with poorly 

developed drainage networks. In the eastern part of the county, 

which is part of the Appalachian Plateaus physiographic 

province, the landscape includes steep-sided valleys and 

bedrock uplands. In this area, elevated arsenic concentrations 

were detected in buried valleys but not in the bedrock uplands, 

where glacial deposits are thin or absent. The observation 

that elevated concentrations of arsenic (and iron) were more 

prevalent in the western part of Licking County is true for both 

glacial and bedrock aquifers.

In Licking County, thick, clay-rich glacial deposits (and 

elevated concentrations of arsenic) are associated with two 

hydrogeologic settings—buried valley and complex thick drift. 

Most wells in the buried-valley setting had low arsenic concentrations, but a few samples had very high concentrations 

(30–44 μg/L) and very reducing redox conditions (methanogenic and near-methanogenic). For wells in the complexthick-drift setting, elevated arsenic concentrations are more 

prevalent, but the maximum concentration was lower (about 

21 μg/L). Similar observations were made about arsenic concentrations in parts of southwestern Ohio.

The hydrogeologic settings and characteristics associated with arsenic in Licking County also exist in other parts 

of Ohio. The statewide extent of these characteristics roughly 

corresponds to areas where elevated concentrations of arsenic 

are known to exist. This preliminary conceptual model can be 

tested and revised as additional wells are sampled for arsenic. 

Arsenic in Groundwater of Licking County, Ohio, 2012—

Occurrence and Relation to Hydrogeology

By Mary Ann Thomas 
2 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Introduction

Arsenic is often a minor component of the rocks and sediments that form the solid framework of an aquifer (the aquifer 

matrix). Arsenic can be released from the aquifer matrix 

to the groundwater (mobilized) under certain geochemical 

conditions. Arsenic is a known human carcinogen; long-term 

exposure to arsenic in drinking water can cause cancers of the 

bladder, lung, and skin, and may also be related to cancers 

of the kidney, liver, and prostate (International Agency for 

Research on Cancer, 2004; World Health Organization, 2010).

Long-term exposure to arsenic has also been linked to cardiovascular disease, Type-2 diabetes, developmental effects, 

neurotoxicity, and disruptions of the immune system (U.S. 

Environmental Protection Agency, 2001; Reynolds, 2010; 

World Health Organization, 2010). 

The maximum contaminant level (MCL) for arsenic is 

10.0 micrograms per liter (μg/L); this is the highest concentration allowed in drinking water from public-water systems 

(U.S. Environmental Protection Agency, 2009). In this report, 

the term “elevated arsenic concentrations” refers to arsenic 

concentrations greater than the MCL. In Ohio, public-water 

systems are required to monitor for arsenic on a regular 

basis, and the average annual concentration must meet the 

MCL. About 17 percent of public-water systems in Ohio had 

elevated arsenic concentrations prior to treatment (Ohio Environmental Protection Agency, 2010). 

In contrast, most private residential (domestic) wells are 

not routinely tested for arsenic. About 1.8 million Ohioans rely 

on domestic wells for their water supply (Maupin and others, 

2014), and there is limited public awareness of the relatively 

frequent occurrence of arsenic in Ohio groundwater and (or) 

the potential threat to human health posed by consuming water 

with elevated arsenic concentrations. In addition, scientists 

and managers have a limited understanding of where elevated 

arsenic concentrations are most likely to occur. Arsenic in 

groundwater typically appears to be “spotty;” wells in the 

same neighborhood can have widely varying concentrations. 

Although it is not possible to predict the arsenic concentration 

of an individual well, some groundwater settings are more 

vulnerable to arsenic mobilization than others (Smedley and 

Kinniburgh, 2002). A better understanding of these vulnerable 

Domestic (private) wells are 

used as a source of drinking 

water by about 1.8 million 

Ohio residents (Maupin and 

others, 2014). Many domestic 

wells are never tested for 

arsenic.

In Ohio, public-supply wells are 

routinely monitored for arsenic, 

and about 17 percent (1 in 6) 

produce water that has elevated 

arsenic concentrations prior to 

treatment (Ohio Environmental 

Protection Agency, 2010).

hydrogeologic settings, and where they occur in Ohio, would 

allow well testing and education about water treatment to be 

focused on areas of greatest need. In 2012, the U.S. Geological Survey in cooperation with the Ohio Water Development 

Authority, began a project to increase understanding of arsenic 

in Ohio groundwater, and the work on which this report is 

based was part of that project. 

Purpose and Scope 

This report presents results of an investigation of arsenic 

in groundwater of Licking County, Ohio. The primary goal 

was to document arsenic concentrations in domestic wells 

that tap a range of hydrogeologic settings. A second goal was 

to identify factors associated with elevated arsenic concentrations. A third goal was, to the extent possible, extrapolate 

results to other parts of the State. The study was based in Licking County because use of domestic wells is relatively widespread, a range of hydrogeologic characteristics are present, 

and there was an opportunity to collaborate with the Licking 

County Health Department in a mutually beneficial way. 

Water samples from 168 domestic wells in Licking 

County were collected by homeowners or county sanitarians 

during March through August of 2012. Samples were analyzed 

at MASI Laboratories in Columbus, Ohio. All samples were 

analyzed for arsenic, and 102 samples were also analyzed 

for iron, manganese, sulfate, nitrate, calcium, and sodium. 

In November 2012, nine of the wells were resampled and 

analyzed for arsenic speciation at the USGS National Water 

Quality Laboratory (NWQL).

Statistical, graphical, and spatial (geographic information system [GIS]) methods were used to document arsenic 

concentrations and identify factors associated with elevated 

arsenic concentrations. Information about geology and well 

construction was compiled from well logs, which were available for 99 of the wells. The GIS coverages from the Ohio 

Department of Natural Resources were used to identify spatial 

relations between arsenic concentrations in groundwater and 

geologic/hydrogeologic characteristics of Licking County. The 

GIS coverages were also used to extrapolate results from Licking County to other parts of Ohio. 
Introduction 3

Background 

Arsenic is a component of a wide range of substances, 

both natural (metal sulfides, metal [oxy]hydroxides, coal, ore 

deposits, hydrothermal deposits) and manmade (biocides, 

including wood preservative, glass production, animal feed 

additives) (Welch and others, 2000; Plant and others, 2003). 

Most instances of widespread arsenic contamination of 

groundwater are the result of natural sources of arsenic that 

exist in the aquifer matrix (Smedley and Kinniburgh, 2002). 

Minerals in the aquifer matrix often include minor amounts 

of arsenic, which can be released to the groundwater under 

certain geochemical conditions. There is a wide range of geochemical conditions under which arsenic can be mobilized, so 

results of arsenic investigations from other parts of the Nation 

may not be directly applicable to the geochemical conditions 

that exist in Ohio groundwater. 

In Ohio, elevated concentrations of arsenic are detected 

in groundwater with reducing redox conditions and circumneutral or slightly alkaline pH (Matisoff and others, 1982; 

Petty, 2000; Slattery and others, 2000; Thomas and others, 

2005). Redox conditions of groundwater are the result of 

microbially mediated reactions that involve the exchange 

of electrons between electron donors (commonly organic 

carbon) and electron acceptors. Dissolved oxygen is a common electron acceptor, but if it is not available, other electron 

acceptors are used in a sequential order. If conditions become 

sufficiently reducing, arsenic associated with iron oxyhydroxides can be released to the groundwater. Iron oxyhydroxides 

are a relatively common component of an aquifer matrix; they 

can be dispersed throughout a geologic formation and (or) 

concentrated in a particular horizon that has been subjected to 

weathering or contact with oxygenated water. In oxyhydroxides, arsenic can exist within the mineral structure and (or) 

sorbed onto the mineral surface. Iron oxyhydroxides (and the 

associated arsenic) are stable in oxic groundwater, but if ironreducing conditions develop and (or) pH increases, arsenic can 

be mobilized. Arsenic can also be mobilized by high concentrations of other oxyanions, especially phosphate, which can 

compete with arsenic for binding sites on aquifer matrix. 

Redox reactions, including iron reduction, are enhanced 

at the interfaces between fine- and coarse-grained sediments 

(McMahon, 2001). These interfaces can act as mixing zones, 

where organic acids or humic substances in fine-grained 

sediments (electron donors) can come in contact with coarsegrained sediments that include iron oxyhydroxides (electron 

acceptors and a potential source of arsenic).

Smedley and Kinniburgh (2002) concluded that the onset 

of reducing conditions is one of the most common triggers 

for mobilizing arsenic in low-temperature, nonmineralized 

groundwater. Another factor related to the development of 

high arsenic concentrations is sluggish groundwater flow. 

After arsenic is released from the solid phase by the onset of 

reducing conditions, the rate of groundwater flushing should 

be low enough to allow arsenic to accumulate in the aquifer 

(Smedley and Kinniburgh, 2002). 

Previous Studies

In Ohio, the current (2015) understanding of arsenic in 

groundwater is based on a patchwork of studies by multiple 

agencies that fall into three broad categories: water-quality 

surveys, Test Your Well workshops, and targeted arsenic 

investigations.

Water-quality surveys.—Arsenic is among the constituents measured as part of water-quality surveys done in selected 

geographic areas and (or) aquifer types and (or) well types. 

The Ohio Environmental Protection Agency (Ohio EPA) has 

documented arsenic concentrations in a network of more than 

200 public-supply wells as part of the Ambient Groundwater 

Program (Ohio Environmental Protection Agency, [n.d.]). A 

compilation of the arsenic data revealed that elevated arsenic 

concentrations were detected in the each of the State’s three 

major aquifer types but were more prevalent in sand and 

gravel aquifers than in sandstone or carbonate bedrock aquifers (Slattery and others, 2000; Ohio Environmental Protection 

Agency, 2000). Elevated arsenic concentrations were associated with iron concentrations indicative of reducing redox 

conditions. In general, higher arsenic concentrations were 

associated with public-supply wells that had lower pumping 

rates/volumes (Ohio Environmental Protection Agency, 2002). 

The USGS measured arsenic concentrations as part of 

water-quality surveys, and results varied widely, depending 

on the geographic areas and (or) aquifer types sampled. Breen 

and Dumouchelle (1991) measured arsenic in 82 wells tapping 

the glacial and carbonate bedrock aquifers in Lucas, Sandusky, 

and Wood Counties (northwestern Ohio); results indicated that 

1 percent of the wells had elevated arsenic concentrations. 

Dumouchelle (1998) documented water quality in 25 domestic 

wells tapping the Lockport Dolomite in Darke, Miami, Montgomery, and Preble Counties (southwestern Ohio); results 

indicated that 68 percent of the wells had elevated arsenic 

concentrations. A water-quality survey by the National WaterQuality Assessment (NAWQA) program included arsenic 

determinations for a network of 27 domestic wells tapping the 

unconfined glacial buried-valley aquifer in the Little and Great 

Miami River Basins (southwestern Ohio). Results indicated 

that 7 percent had elevated arsenic concentrations (Shindel 

and others, 2001). Results of NAWQA water-quality studies 

in Ohio were included in compilations of arsenic data from 

the glacial aquifer system of the Midwest (Thomas, 2003) and 

the northern United States (Thomas, 2007). Jagucki and others 

(2015) documented water quality in 16 domestic wells tapping 

glacial and bedrock aquifers in Geauga County (northeastern Ohio); results indicated that 12 percent of the wells had 

elevated arsenic concentrations.

Samples of soil and stream sediments from Ohio were 

analyzed for arsenic concentrations as part of the USGS 

National Geochemical Database (http://mrdata.usgs.gov/

geochemistry/ngs.html). Venturis and others (2014) used 

statistical modeling and mapping methods to conclude that 

the distribution of arsenic in soil and stream sediments varies 

across the State and is related primarily to the distribution of 
4 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

sulfide-bearing bedrock formations; the effects of glaciation 

are considered to be secondary. 

Test Your Well workshops.—The Groundwater Foundation developed guidelines for workshops that allow homeowners to have their well water screened for nitrate by volunteers 

trained to use a colorimeter (Groundwater Foundation, 2007). 

The Miami Conservancy District (MCD) modified this design 

to facilitate testing for arsenic and cosponsored workshops 

in multiple counties of southwestern Ohio. In addition, they 

recruited workshop attendees for additional sampling of their 

wells by MCD. Using these methods, arsenic data were collected from 107 domestic wells in 11 counties of southwestern 

Ohio. Results indicate that about 20 percent of samples had 

elevated concentrations (Miami Conservancy District, 2011). 

A similar type of workshop was held in Union County (westcentral Ohio) during 2008; results indicated that 6 of 24 wells 

(25 percent) had arsenic concentrations greater than the MCL 

(Christina Burri, Union County Soil and Water Conservation 

District, written commun., 2008). 

Targeted arsenic investigations.—A limited number of 

studies have been done specifically for the purpose of investigating the source and distribution of elevated arsenic concentrations in domestic wells. The first investigation of this type 

in Ohio was done by researchers at Case Western Reserve 

University (Matisoff and others, 1982). The goal was to 

determine the source of arsenic detected in two school wells in 

an area of northeastern Ohio. After several potential anthropogenic sources were considered and eliminated, the source was 

determined to be naturally occurring arsenic released from 

iron hydroxides by the onset of reducing conditions, which 

were attributed to a thick layer of till that impedes recharge of 

oxygenated water or possibly to the release of methane from 

an underground storage facility. 

The Ohio Department of Health investigated arsenic in 

domestic wells in parts of Perry, Vinton, and Jackson Counties 

(southeastern Ohio). Results indicated that elevated arsenic 

concentrations were detected in wells tapping a similar hydrogeologic setting—upper Mississippian sandstone immediately 

underlying valley-fill deposits (Petty, 2000). 

Multiple studies have been done in parts of southwestern 

Ohio. The Ohio Department of Health, in cooperation with the 

U.S. Environmental Protection Agency (EPA), investigated 

the source of arsenic detected in domestic wells in the vicinity 

of a waste site in Preble County, Ohio. Investigators sampled 

more than 500 domestic wells and concluded that the arsenic 

contamination was not related to the waste site but was a 

result of geologic materials. Highest arsenic concentrations 

were detected in wells greater than 100 feet (ft) deep within or 

along margins of buried valleys (Ohio Department of Health, 

1999).

The USGS and the Miami Conservancy District investigated factors associated with elevated arsenic in a three-county 

area of southwestern Ohio (Thomas and others, 2005). A network of 57 domestic wells tapping a range of hydrogeologic 

settings and aquifer types were sampled. Elevated arsenic 

concentrations were detected in strongly reducing groundwater 

in glacial deposits as well as carbonate bedrock aquifers. The 

highest arsenic concentrations were in methanogenic groundwater in glacial buried-valley deposits. A followup study in 

southwestern Ohio included detailed analysis of solid phase 

and groundwater samples at two sites in a small watershed 

in southwestern Ohio (Thomas and others, 2008). Results of 

solid-phase (sediment and bedrock) analysis indicate that the 

highest arsenic concentrations were in the vicinity of unconformities, including (1) the boundary between glacial deposits 

and Paleozoic bedrock, and (2) paleosols in the glacial deposits. Analysis of the groundwater indicated that depth-related 

shifts in redox conditions caused dramatic variations in arsenic 

concentrations at a single site. 

Methods

The water-quality data for this study were collected and 

analyzed by several different methods (table 1). A private lab 

in Columbus, Ohio, analyzed samples from 168 wells for arsenic; 102 of the samples were also analyzed for six other waterquality constituents. Of the 168 samples, 125 were collected 

by homeowners and 43 were collected by county sanitarians. 

Because methods of collection and analysis were similar, the 

data were combined into a single dataset that is referred to as 

the “primary water-quality dataset.” In addition, nine wells 

were resampled by USGS personnel and analyzed for arsenic 

species and other constituents at the USGS NWQL. For the 

most part, the speciation data and primary water-quality data 

were analyzed separately due to the collection and analysis 

differences listed in table 1. Water quality data are presented 

in table 2 (at the back of this report or the Excel file can be 

accessed at http://dx.doi.org/10.3133/sir20155148). 

Most of the water samples were 

collected by well owners and analyzed for arsenic at a local private 

laboratory. Bottles and instructions 

for collecting samples were provided to well owners as part of Test 

Your Well workshops.
Methods 5

Collection and Analysis of Primary WaterQuality Data

A total of 125 water samples were collected during three 

Test Your Well workshops held in June, July, and August of 

2012. Workshop attendees were given labeled bottles and 

instructions for collecting unfiltered water samples. The 

instructions included guidance about collecting untreated 

water, purging the tap prior to filling the bottles, and keeping 

the samples chilled. The bottles did not include a preservative, 

so samples had to be returned during specified hours and were 

preserved by acidification upon arrival at the lab. In addition, 

43 water samples were collected by sanitarians from the Licking County Health Department during routine well-inspection 

activities between March and August, 2012. Methods used 

for sample collection were similar to those provided to 

homeowners. 

A total of 168 water samples (125 collected by homeowners and 43 collected by sanitarians) were analyzed at MASI 

Laboratories in Columbus, Ohio. All samples were analyzed 

for arsenic, and 102 samples were also analyzed for iron, 

manganese, sulfate, nitrate, calcium, and sodium. Nitrate was 

analyzed within 48 hours of sample collection, and the other 

constituents were analyzed within 7 days of collection. These 

water-quality data are included in table 2.

Collection and Analysis of Arsenic Speciation 

Data 

After compiling arsenic results from the private laboratory, nine wells were selected for resampling to determine 

arsenic speciation. The criteria for well selection were (1) the 

arsenic concentration from the primary dataset was greater 

than 10.0 μg/L, (2) a well log was on file at ODNR, and 

(3) well owners were willing to participate in the study. 

The nine wells were sampled during November, 2012, by 

USGS personnel using documented methods for well purging, determination of field parameters, sample collection, 

preservation, and quality assurance (U.S. Geological Survey, 

variously dated). Samples were analyzed at the USGS NWQL 

for concentrations of arsenic species, major ions, trace metals, 

nutrients, and dissolved organic carbon (table 1). Determinations of pH, specific conductance, temperature, dissolved oxygen, and alkalinity were made at the time of sample collection. 

The complete dataset is available on the USGS National Water 

Information System (NWIS) (http://nwis.waterdata.usgs.gov/

usa/nwis/qwdata). For two wells, sulfate concentrations from 

the NWQL were substituted for those from the private laboratory in table 2 because the lower detection limits were necessary to estimate redox conditions (see “Redox Conditions 

Estimated From Concentrations of Iron, Manganese, Sulfate, 

and Nitrate” section). 

Table 1. Methods used for collection and analysis of water-quality data, Licking County, Ohio, 2012.

[μg/L, microgram per liter; SM, standard method; mg/L, milligram per liter; EPA, U.S. Environmental Protection Agency method; As, arsenic; 

USGS, U.S. Geological Survey]

Number 

of 

samples

Sample collection Sample analysis

Collected by Date Laboratory Sample 

type

ConstituentsReporting 

limit

Analytical 

method Method reference

Primary water-quality data

125 Well owners 

who attended 

Test Your 

Well workshops

June–

August, 

2012

MASI Laboratories, 

Columbus, Ohio

Unfiltered Arsenic 3 μg/L SM3113B

http://

standardmethods.

org/.

Sulfate 5.0 mg/L SM4500 SO4-E

Nitrate 0.10 mg/L SM4500 NO3-F

Iron 80 μg/L

EPA 200.7

U.S. Environmental 

Protection Agency, 

1994.

43 Licking County 

Health 

Department 

sanitarians

March–

August, 

2012

Manganese 30 μg/L

Calcium 2 mg/L

Sodium 10 mg/L

Arsenic speciation data

9 U.S. Geological 

Survey 

personnel

November, 2012U.S. Geological 

Survey, National 

Water Quality 

Laboratory, 

Denver, Colorado

Filtered Arsenate 0.4 μg/L 

as As

I–2195–05 Garbarino and 

others, 2006.

Arsenite 0.2 μg/L 

as As

Monomethylarsonate3.2 μg/L 

as As

Dimethylarsinate 0.6 μg/L 

as As
6 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Quality Assurance

MASI Laboratories was certified by the Ohio EPA to 

analyze samples from public-supply wells, and at the time of 

this study (2012), the Licking County Health Department used 

MASI Laboratories to analyze water samples collected during 

routine well inspections. A USGS lab-evaluation process was 

used to verify that analytical results from MASI Laboratories 

would meet the data-quality objectives of this study (http://

bqs.usgs.gov/). MASI Laboratories met the criteria of the 

evaluation process. 

A total of 22 quality-control (QC) samples were used to 

assess variability and bias of the water-quality data collected 

during the course of this study. Results of the QC samples are 

presented in tables 3, 4, and 5 (all at the back of this report). 

During Test Your Well workshops in the summer of 2012, 

10 QC samples were submitted to MASI Laboratories along 

with samples collected by homeowners (table 3). The samples included a trip blank, five replicates, and four standard 

reference samples. Results of the trip blank did not indicate 

contamination during transport or analysis. Arsenic concentrations for pairs of replicate samples differed by 18 percent or 

less. Arsenic concentrations determined for standard reference 

samples differed from the most probable values (MPVs) by 31 

percent or less. The MPV is the median concentration determined by multiple laboratories. The results showed no clear 

bias; two of the results were less than the MPV, and two were 

greater. 

During collection of the speciation dataset in November 

2012, three QC samples were submitted to the NWQL 

(table 4). The samples included an equipment blank (and a 

source-solution blank), a field blank (and a source-solution 

blank), and a replicate sample analyzed for arsenic speciation. 

Concentrations of arsenic species for replicate samples differed by 3 percent or less. Results of the equipment blank did 

not indicate contamination related to the sampling equipment. 

In the field blank, arsenic was detected at a concentration 

of 0.0575 μg/L. This concentration is not considered to 

be problematic because it is less than the reporting level of 

0.08 μg/L (U.S. Geological Survey National Water Quality 

Laboratory, 2015). 

For most wells, arsenic concentrations in the speciation 

dataset are different from those in the primary dataset. The 

cause is not known, but it may be related to differences in 

sampling date, type of sample (filtered vs. unfiltered), method 

of collection, or analytical method (table 1). In addition, processing errors in the field or laboratory could have contributed 

to the different results. To investigate further, additional QC 

samples were collected in April 2013, during the data-analysis 

phase of this study (table 5). Four pairs of replicate samples 

were submitted to MASI Laboratories, and results indicated 

that arsenic concentrations for pairs of replicate samples differed by 30 percent or less. In addition, five unfiltered split 

samples were submitted to MASI Laboratories and the USGS 

NWQL to evaluate analytical differences between the labs. 

Results indicated that arsenic concentrations in the laboratory split samples differed by -7–81 percent. Four of the five 

samples analyzed at MASI Laboratories had higher results 

than the samples analyzed at NWQL. The cause of these differences is not known. 

A different type of quality assurance was used to assess 

whether samples collected by homeowners had been affected 

by water treatment (even though homeowners were instructed 

to collect untreated water). Water softening by cation 

exchange is the most common form of water treatment in Ohio 

and is also used as a form of pretreatment for other processes 

used to treat arsenic, such as reverse osmosis and anion 

exchange. During cation exchange, calcium displaces sodium 

on the solid media, and as a result, calcium concentrations 

in the water decrease and sodium concentrations increase. 

Therefore, high sodium/calcium ratios in water can be 

indicative of the effects of water softening. Four out of 

102 samples were suspected of being softened because 

(1) sodium/calcium ratios were very high (98–133), and 

(2) calcium was not detected above the minimum reporting 

level of 2 milligrams per liter (mg/L). In comparison, the 

other 98 samples had sodium/calcium ratios of 0.05–18 and 

calcium concentrations of 9–139 mg/L. Cation exchange has 

little effect on arsenic concentrations because arsenic exists 

in groundwater as a neutral or negatively-charged compound; 

however, cation exchange decreases iron and manganese 

concentrations, which were used to estimate redox conditions 

of water. Therefore, iron and manganese concentrations of 

softened samples were excluded from the dataset, but arsenic 

concentrations were not. None of the samples indicated treatment by reverse osmosis or anion exchange on the basis of 

iron, manganese, nitrate, and sulfate concentrations. The 

66 samples that were not analyzed for additional water-quality 

constituents could not be screened for evidence of water 

treatment. 

Overall, it is likely that the methods used for sample 

collection and analysis added some variability and (or) 

uncertainty to the water-quality results. This uncertainty was 

taken into consideration during data analysis by grouping 

arsenic concentrations into broad concentration classes 

(of <3 ug/L, 3–10.0 ug/L; 10.1–29.9 ug/L, and 30–44 ug/L). 

The effect of the analytical variability on estimation of redox 

conditions is likely minimal because redox categories are 

defined by (1) broad concentration classes and (2) a combination of at least two constituents. 
Methods 7

Data Analysis 

Two types of data were collected as part of this study: 

water-quality data and well-log data (table 6). The waterquality data includes arsenic concentrations for 168 wells, 

concentrations of 6 additional water-quality constituents for 

102 wells, and arsenic species for 9 wells. Well logs, which 

were available for 99 wells, were used as a source of information about aquifer and well characteristics. Arsenic concentrations, additional water-quality constituents, and well logs were 

available for 68 wells. 

Relations among the variables were investigated using 

robust non-parametric statistical methods. The Kruskal-Wallis 

test was used to compare arsenic concentrations among groups 

of samples. The Spearman’s rho test was used to identify 

monotonic correlations between continuous variables. Test 

results with a p-value less than 0.05 were considered to be 

statistically significant. The number of samples used for each 

individual statistical test varied depending on availability of 

the different types of data. 

In addition, GIS coverages from the Ohio Department 

of Natural Resources (ODNR) (http://geospatial.ohiodnr.gov/

gis-home) were used to assess spatial relations between water 

quality and selected geologic/hydrogeologic characteristics of 

Licking County. The same coverages were used to extrapolate 

results from Licking County to the rest of the Ohio. All of the 

variables used for data analysis are included in table 2. 

Information from well logs and geologic maps/coverages was used to help understand results of the water analyses.

Table 6. Variables used for analysis of arsenic data from 

Licking County, Ohio, 2012.

Analysis variables Number of 

samples

Arsenic concentrations 168

Six additional water-quality constituents 1102

Well log variables 299

Arsenic concentrations, 6 additional water-quality 

constituents, and well logs

68

Arsenic species 9

Geospatial coverages:

Bedrock geology of Ohio 

 (Slucher and others, 2006 ).

Unconsolidated aquifers of Ohio 

 (Ohio Department of Natural Resources, 2000).

Drift thickness of Ohio 

 (Ohio Department of Natural Resources, Division of Geological 

Survey, 2004).

Pollution potential of Licking County 

 (Ohio Department of Natural Resources, 1995).

1

Nitrate concentration was not available for 1 sample.

2

Well depths and water levels were known for 100 wells. 
8 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Description of Study Area

Licking County is in central Ohio on the eastern outskirts of the Columbus metropolitan area (fig. 1). The county 

includes a mix of urban and rural land uses, and the population 

during 2010 was 166,492 (U.S. Census Bureau, 2012). 

The hydrogeology of Licking County has been described 

by Dove (1960), Forsyth (1966), Hartzell (1982), Angle 

(1995), Siegrist and others (1997), Frolking and Szabo 

(1998), and Frolking and Pachell others (2006). The geologic 

and hydrogeologic characteristics of Licking County have 

been mapped by the ODNR (http://geospatial.ohiodnr.gov/

gis-home). 

Most of Licking County is within the Muskingham River 

watershed. The principal stream is the Licking River, which 

is formed by the confluence of the North Fork Licking River, 

South Fork Licking River, and Raccoon Creek (fig. 1). The 

water supply for Newark is from the North Fork Licking 

River, but all other public-water systems in the county rely on 

groundwater. About 40 percent of the population have domestic water supply wells that tap bedrock formations and (or) 

unconsolidated glacial deposits (Siegrist and others, 1997). 

OHIO

Columbus Study area

Lake Erie

Hartford

Monroe

Jersey

Saint Albans

Harrison

Etna

Granville

Newark

Madison

Hanover

Franklin Hopewell

Bowling Green

Licking

Union

Hebron

Pataskala

Hanover

Granville Newark

Liberty

Mckean

Newton

Mary Ann Perry

Bennington Burlington

Washington

Eden Fallsbury

Utica

Johnstown

KNOX COUNTY

PERRY COUNTY

FAIRFIELD COUNTY

COSHOCTON

COUNTY

MUSKINGUM COUNTY

DELAWARE COUNTY

FRANKLIN COUNTY

LICKING COUNTY

40°10'

82°40'

40°

82°30'

82°20'

Base from U. S. Geological Survey

Digital data, National Map, 2015

Universal Transverse Mercator projection

Zone 17N

North American Datum of 1983

Boundary of physiographic provinces from Fenneman and Johnson (1946)

Glacial boundary from Ohio Department of Natural Resources (2000)

Appalachian Plateaus

Physiographic province

Central Lowland

A

A A'

A'

South Fork Licking River

Racoon Creek

North Fork

Licking River

Rocky Fork

Licking River

EXPLANATION

Glaciated region

Glaciated region boundary

Unglaciated region

Physiographic province boundary

Line of section

0

0 8 KILOMETERS

6

2

2

6

8 MILES

4

4

Figure 1. Location of the study area in Licking County, Ohio, 2012. A—A’ indicates the 

approximate location of the cross section in figure 2.
Description of Study Area 9

The boundary between two major physiographic provinces is near the middle of Licking County (figs. 1 and 2) 

(Fenneman and Johnson, 1946). The western part of the 

county is in the Central Lowland physiographic province, 

where a thick layer of glacial deposits obscures the underlying bedrock and the land surface is gently undulating plains. 

Surface drainage networks are poorly developed, and most 

of the rivers (except Raccoon Creek) occupy shallow, narrow 

valleys (Dove, 1960). The eastern part of the county is in the 

Appalachian Plateaus physiographic province, where much 

of the landscape consists of steep-sided valleys with bedrock 

uplands. Thick glacial deposits exist in the buried valleys, but 

glacial deposits are thin or absent in the bedrock uplands. 

The bedrock of Licking County consists of Devonian- to 

Pennsylvanian-age sedimentary formations that dip gently to 

the east (Slucher and others, 2006) (fig. 2). The oldest formations—Sunbury Shale, Berea Sandstone, and Bedford Shale—

subcrop near the western boundary of the county. The Berea 

Sandstone is considered a good aquifer but is not frequently 

used for domestic supply because of its depth (Dove, 1960). 

The Mississippian-age Cuyahoga Formation is present over 

most of the county and is considered to be the most productive 

aquifer (Dove, 1960). The formation is a relatively complex 

mix of sandstone, shale, siltstone, and conglomerates that is 

generally more coarse-grained in the east. In parts of central 

and eastern Licking County, the Cuyahoga Formation is overlain by the Logan Formation, which consists of interbedded 

sandstone, shale, siltstone, and conglomerates. In easternmost 

Licking County, the Logan Formation is unconformably overlain by Pottsville and Allegheny Formations of Pennsylvanian 

age. The lithology is interbedded sandstone, shale, siltstone, 

and conglomerate. The Allegheny Formation also includes thin 

beds of limestone, coal, and flint. These formations are considered to be a poor aquifer, largely because of the thickness and 

(or) position relative to the water table (Dove, 1960). 

System Formation Lithology

Quaternary Glacial deposits

Pennsylvanian Allegheny

Pottsville

Mississippian 

Logan Interbedded sandstone, shale, siltstone, conglomerate. 

Cuyahoga

Sunburyshale Brown to black shale. 

Devonian 

Berea Sandstone1 Fine-grained sandstone. 

Bedford shale1 Red or blue shale. 

1,200

800

400

A

Elevation, in feet

A'

1,200

800

400

Physiographic province

Central Lowland Appalachian Plateaus

Unconsolidated till, silt, clay, sand, gravel, and bolders. 

Sandstone, shale, siltstone, conglomerate. Some thin beds 

of limestone, coal, flint. 

Sandstone, shale, siltstone, conglomerate. Generally 

coarser-grained towards the east. 

1

Classified as Mississippian by Dove (1960); later re-classified as Devonian (Slucher and others, 2006).

0 Modified from Dove (1960)

0 8 KILOMETERS

6

2

2

6

8 MILES

4

4

Figure 2. Geology of Licking County, Ohio. Approximate location of cross section A—A’ is shown on figure1.
10 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Glacial deposits overlie bedrock in much of the county 

(fig. 3A). The most widespread type of glacial deposit is till 

(unsorted clay, silt, sand, and gravel), which may include 

lenses of sand and gravel. Fines (clay and silt) with lenses 

of sand and gravel are present in valleys in the southern and 

eastern parts of the county. Sand and gravel, with minor fines, 

are present in valleys in the northern and central parts of the 

county. 

Glacial deposits are 0 to 596 ft thick (fig. 3B). The 

easternmost part of the county is unglaciated, although glacial 

deposits occur in valleys beyond the limit of glacial advance. 

The thickest glacial deposits are in two north-south trending 

valleys cut into bedrock by the preglacial Teays River system. These are referred to as “buried valleys” because they 

eventually filled with glacial sediments. The buried valley in 

the western part of the county is filled with predominantly 

fine-grained sediment; it has no current stream and is not visible from land surface. The buried valley near the center of 

the county partially corresponds to the location of the current 

Licking River; this buried valley has a larger fraction of sand 

and gravel, but fines are also present. 

The glacial deposits of Ohio have been grouped into 

hydrogeologic settings largely on the basis of lithology and 

drift thickness (Ohio Department of Natural Resources, 2000). 

In Licking County, the predominant hydrogeologic settings 

are buried valley, complex thick drift, ground moraine, thin 

upland, and alluvial (fig. 3C). In addition, a small patch of Illinoian end moraine is present in the eastern part of the county 

but is not considered to be one of the predominant hydrogeologic settings.

In Licking County, wells produce water from glacial 

deposits and (or) bedrock at depths of about 40–400 ft. Well 

yields vary from less than 10 to more than 500 gallons per 

minute (Hartzell, 1982). The best water supplies are available from thick deposits of sand and gravel, especially those 

at land surface and (or) in contact with current rivers. In areas 

where surficial glacial deposits consist of till or fines, water 

can be produced from sand and gravel lenses or layers within 

the finer-grained sediment. If sufficient sand and gravel are not 

present in glacial deposits, wells can be drilled to the underlying bedrock. Most wells in upland areas in the central and 

eastern parts of the county are in bedrock. Some areas of the 

county use bedrock and glacial wells almost equally (Angle, 

1995).

Domestic wells in Licking County produce water 

from glacial deposits or bedrock (or both). Minor 

amounts of arsenic in these geologic materials can be released to the groundwater under 

certain geochemical conditions.

Western Licking County is overlain 

by thick layer of glacial deposits, 

which creates a flat to gently-rolling 

land surface (above). Over much of 

eastern Licking County, the layer of 

glacial deposits overlying bedrock is 

much thinner, and the topography is 

more rugged (right). These factors can 

influence the chemical quality of the 

groundwater. 
Description of Study Area 11

40°10'

82°40'

40°

82°30'

82°20' EXPLANATION

Till—Unsorted clay, silt, sand, and gravel. May

 include lenses of sand and gravel 

Fines—Primarily clay and silt with lenses of

 sand and gravel

Sand and gravel—Primarily sand and gravel with

 minor clay and silt

Glacial deposits are thin or absent

40°10'

82°40'

40°

82°30'

82°20' EXPLANATION

Less than 1

1−99

100−199

200−349

350−596

Thickness of glacial deposits, in feet

Lithology of glacial deposits

40°10'

82°40'

40°

82°30'

82°20'

Base from U. S. Geological Survey digital data, National Map, 2015

Universal Transverse Mercator projection, zone 17N

North American Datum of 1983

EXPLANATION

Buried valley—Thick drift (usually less than 100 feet) in existing 

 bedrock valleys; primarily fines with lenses of sand and gravel

Complex thick drift—Thick drift (usually greater than 100 feet); 

 primarily till with lenses of sand and gravel. May consist of a 

 combination of settings such as end moraine overlying buried 

 valley

Ground moraine—Moderately thick drift (25−100 feet); primarily 

 till with lenses of sand and gravel

Thin upland—Thin drift (less than 25 feet); primarily weathered 

 till; not used as an aquifer

Alluvial—Areas of modern streams not associated with buried 

 valley systems; usually less than 100 feet thick; primarily fines 

 with lenses of sand and gravel

Glacial deposits are thin or absent

End moraine—Not included in data analysis

Lithology of glacial deposits from Ohio Department of Natural Resources (2000)

Drift thickness from Ohio Department of Natural Resources, 

 Division of Geological Survey (2004)

Hydrogeologic settings from Ohio Department of Natural Resources (2000)

0

0 8 KILOMETERS

6

2

2

6

8 MILES

4

4

A

B

C

Figure 3. Characteristics of glacial deposits of Licking County, Ohio. A, Lithology. B, Drift thickness. C, Hydrogeologic settings. 
12 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Arsenic Concentrations

Arsenic concentrations were determined for water 

samples from 168 domestic wells in Licking County, and 

concentrations ranged from less than 3 μg/L to 43.8 μg/L 

(fig. 4).The maximum contaminant level (MCL) for arsenic is 

10.0 μg/L, and almost 1 in 8 samples (12 percent) had concentrations greater than the MCL. In this report, “elevated arsenic 

concentrations” refers to concentrations greater than the MCL. 

The spatial distribution of the data (fig. 4) shows that elevated 

arsenic concentrations are more prevalent and widespread in 

the western part of the county than in the eastern part. 

Almost 1 in 8 domestic wells 

had arsenic concentrations 

greater than the maximum 

contaminant level of 10 ug/L. 

In general, elevated arsenic 

concentrations were more 

widespread and prevalent in 

the western half of Licking 

County than in the eastern half.

Figure 4. Arsenic concentrations from 168 domestic wells in Licking County, Ohio, 2012.

40°10'

82°40'

40°

82°30'

82°20'

Base from U. S. Geological Survey digital data,

National Map, 2015

Universal Transverse Mercator projection, zone 17N

North American Datum of 1983

EXPLANATION

Arsenic concentration, in

 micrograms per liter (μg/L)

Less than 3

3−10.0

10.1−29.9

30−44

Number of samples 168

Arsenic concentrations,

in μg/L

Median <3

75th percentile 6

Maximum 43.8

Percentage with arsenic concentrations: 

 <3 ug/L (lower detection limit)

 3−10.0 μg/L

 >10.0 ug/L (maximum contaminant level) 

58

30

12

0

0 8 KILOMETERS

6

2

2

6

8 MILES

4

4
Arsenic Concentrations 13

Arsenic Species

In groundwater, arsenic primarily occurs as two inorganic 

species—arsenite (As3+) and arsenate (As5+). In general, As3+

is considered to be more toxic to humans than As5+. The As5+

is usually the predominant species in oxic redox conditions, 

and As3+ is usually the predominant species in reducing redox 

conditions. At pH values typical of groundwater, As5+ exists in 

solution as a negatively charged ion (HAsO4

2− or HAsO4−

). In 

contrast, As3+ exists as an uncharged species (H3AsO3), which 

makes it more difficult to remove from water; treating water 

to remove As3+ can require multiple steps (U.S. Environmental Protection Agency, 2003). Although arsenic speciation is 

important to know when purchasing a water-treatment system, 

this type of analysis was not available from the private laboratory. Therefore, in November, 2012, a subset of nine wells 

with elevated arsenic concentrations was resampled by USGS 

personnel, and filtered samples were analyzed at the NWQL 

for arsenic species and other constituents (table 1). 

Four arsenic species were analyzed—two inorganic species (arsenate and arsenite) and two organic species (dimethylarsinate and monomethylarsonate) (table 7). The organic 

species were not detected above the minimum reporting levels. 

Arsenite (As3+) was the predominant species in all samples. 

The percentage of As3+ was estimated relative to the sum of 

As3+ and As5+, and values ranged from 55 to 92 percent, with a 

median of 80 percent. 

For most samples, the sum of the two inorganic species 

was greater than total arsenic concentration; however, this was 

not considered to be a problem because the data are used in a 

qualitative way to identify the predominant arsenic species. 

Differences between arsenic concentrations in the primary and 

speciation datasets are briefly discussed in the “Quality Assurance” section. 

The effectiveness of water treatment methods to remove arsenic depends on the type (species) of 

arsenic present in the water. Nine of the wells were resampled for analysis of arsenic species at 

the USGS National Water Quality Laboratory.

Table 7. Arsenic species from nine domestic wells, Licking County, Ohio, 2012.

[Arsenic and arsenic species were analyzed from filtered samples at U.S. Geological Survey National Water Quality Laboratory; mm/dd/yy, month/day/year; 

μg/L, microgram per liter; As, arsenic; <, less than]

Well 

identifier

Date

(mm/dd/yy)

Arsenic

(μg/L)

Arsenite (As3+) 

(μg/L as As)

Arsenate (As5+) 

(μg/L as As)

Dimethylarsinate

(μg/L as As)

Monomethyl 

arsonate 

(μg/L as As)

Percentage of 

As3+ relative to 

(As3++As5+)

LI–40 11/06/12 20.2 16.9 3.9 <.6 <3.2 81

LI–37 11/06/12 20.6 15.8 4.1 <.6 <3.2 79

LI–43 11/07/12 7.1 5.1 .7 <.6 <3.2 88

LI–32 11/07/12 67.5 67.2 6.3 <1.8 <9.6 91

LI–31 11/07/12 51.7 51.2 4.2 <1.2 <6.4 92

LI– 36 11/08/12 31.0 21.8 10.6 <1.2 <6.4 67

LI–49 11/08/12 22.0 21.7 3.3 <.6 <3.2 87

LI–33 11/09/12 12.7 8.5 3.8 <.6 <3.2 69

LI– 38 11/09/12 31.0 17.3 14.2 <1.2 <6.4 55
14 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Factors Related to Arsenic 

Concentrations

Three types of information were used to investigate 

factors related to arsenic concentrations in the groundwater 

of Licking County: (1) water-quality constituents indicative 

of redox conditions, (2) aquifer and well characteristics from 

well logs, and (3) GIS coverages of the spatial distribution of 

hydrogeologic characteristics across the county. 

Geochemical Conditions of Groundwater

Redox conditions and pH are the two most important 

geochemical controls on arsenic concentrations in groundwater (Smedley and Kinniburgh, 2002). In parts of southwestern 

Ohio, elevated arsenic concentrations were detected in 

groundwater with strongly reducing redox conditions 

and circumneutral to slightly alkaline pH (Thomas and 

others, 2005). To investigate whether arsenic is mobilized in 

similar geochemical conditions in Licking County, 102 of 

the168 water samples were analyzed for water-quality constituents used to estimate redox conditions of the groundwater. 

In this section of the report, redox conditions are estimated 

using two methods, and the results are compared. It was not 

practical to determine pH of samples in the primary dataset; 

however, the nine samples in the speciation dataset had pH 

values of 6.9–7.8 (median of 7.2), which is circumneutral to 

slightly alkaline. 

McMahon and Chapelle (2008) developed a method for 

estimating redox conditions of groundwater using concentrations of five water-quality constituents: dissolved oxygen, 

nitrate, manganese, iron, and sulfate. For the current study, 

it was not practical to analyze samples for dissolved oxygen 

because the measurement should be made at the time of sample collection, using equipment not available to homeowners 

or county sanitarians. Therefore, for this report, estimates of 

redox are based on concentrations of four constituents—iron, 

manganese, sulfate, and nitrate. The modified system is shown 

in table 8.

Staining of household surfaces is caused by high-iron 

groundwater, which is found in many parts of Licking County. 

More than 18 percent of the high-iron samples had elevated 

concentrations of arsenic.

Table 8. Framework used to estimate redox conditions from concentrations of iron, manganese, sulfate, and nitrate in 

groundwater, Licking County, Ohio, 2012.

[Modified from McMahon and Chapelle (2008); NO3, nitrate as nitrogen; mg/L, milligram per liter; Mn, manganese; μg/L, microgram per 

liter; Fe, iron; SO4, sulfate; --, not applicable; ≥, greater than or equal to; <, less than]

Redox

Category

Estimated 

redox 

condition

Water-quality criteria

NO3

(mg/L)

Mn

(μg/L)

Fe 

(μg/L)

SO4

(mg/L)

Not strongly reducing Oxic or suboxic <0.5 <50 <100 --

NO3 reducing ≥0.5 <50 <100 --

Mn reducing <0.5 ≥50 <100 --

Mixed1 ≥0.5 -- ≥100 --

Strongly reducing Fe or SO4 reducing <0.5 -- ≥100 ≥0.5

Methanogenic <0.5 -- ≥100 <0.5

1

Other types of mixed redox are possible, but this was the only type detected in this study.
Factors Related to Arsenic Concentrations 15

Redox Conditions Estimated From Concentrations 

of Iron, Manganese, Sulfate, and Nitrate

A total of 101 samples had sufficient data to estimate 

redox conditions by using the modified redox framework 

(fig. 5). Most (75 percent) samples were iron- or sulfatereducing, and one sample was methanogenic. In this report, 

these redox conditions are categorized as “strongly reducing.” 

All samples with elevated concentrations of arsenic were 

strongly reducing; however, not all strongly reducing samples 

had elevated arsenic concentrations. Of the strongly reducing 

samples, 18 percent had arsenic concentrations greater than 

the MCL of 10.0 μg/L. 

About 24 percent of the samples were estimated to be 

oxic, suboxic, nitrate-reducing, manganese-reducing, or 

mixed, and are categorized as “not strongly reducing.” None 

of the samples in this category had elevated arsenic concentrations; 4.9 μg/L was the maximum arsenic concentration for a 

sample that was not strongly reducing. 

As shown in table 8, methanogenic waters differ from 

iron- or sulfate-reducing waters in terms of sulfate concentrations; both types of water have high iron concentrations, but 

methanogenic waters have sulfate concentrations less than 0.5 

mg/L (McMahon and Chapelle, 2008). The reporting level for 

sulfate at the private lab was 5 mg/L, and the two samples with 

the maximum arsenic concentrations had sulfate concentrations less than 5 mg/L; therefore, it was not possible to determine if the redox conditions were methanogenic. However, the 

same two wells were resampled for analysis of arsenic speciation at the NWQL, where the reporting limit for sulfate was 

0.09 mg/L; so sulfate concentrations from the NWQL were 

substituted into the primary dataset for two wells with arsenic 

concentrations of about 44 ug/L. One of the samples had a sulfate concentration of less than 0.09 mg/L and was classified as 

methanogenic. The second sample had a sulfate concentration 

of 4.45 mg/L and was classified as iron- or sulfate-reducing; 

however, in the “Hydrogeologic Settings” section, this sample 

will be referred to as “near-methanogenic.”

Estimated redox conditions

Not strongly reducing Strongly reducing

Number of samples1 7 10 7 76 1

0 0 0 18 1 of 1

Arsenic concentrations,

in μg/L

Median <3 <3 <3 3.7 --

75th percentile <3 <3 <3 8.2 --

Maximum 4.9 <3 <3 43.6 43.8

0

10

20

30

40

50

Methanogenic

Not strongly reducing Strongly reducing

Oxic or

suboxic

Oxic or

suboxic 

Nitrate

reducing

Nitrate

reducing 

Manganese reducing

or mixed

Manganese reducing

or mixed

Iron or sulfate

reducing

Iron or sulfate

reducing

Methanogenic

Percentage of arsenic concentrations 

 >10.0 μg/L

1

Concentrations of nitrate, manganese, iron, and sulfate were known for 101 wells. Arsenic concentration, in micrograms per liter (μg/L)

MCL

Figure 5. Arsenic concentrations in relation to redox conditions estimated from concentrations of iron, 

manganese, sulfate, and nitrate—Licking County, Ohio, 2012. [MCL, Maximum Contaminant Level]
16 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Redox Conditions Estimated From Iron 

Concentrations

A simplified method for estimating redox conditions is 

to consider only iron concentrations. The EPA established a 

secondary MCL (SMCL) of 300 μg/L for iron because concentrations greater than that cause iron staining of plumbing and 

household fixtures (U.S. Environmental Protection Agency, 

2009); therefore, 300 μg/L is used as the boundary between 

two simplified redox categories: “low iron” and “high iron.”

Iron concentrations, which were available for 102 wells, 

ranged from less than 80 μg/L (the minimum reporting level) 

to 6,560 μg/L, with a median of 767 μg/L. Most samples 

(71 percent) had high iron concentrations (fig. 6). Elevated 

arsenic concentrations were detected only in high-iron samples; however, not all high-iron samples had elevated arsenic 

concentrations. About 19 percent of samples with iron concentrations greater than 300 μg/L also had arsenic concentrations greater than 10.0 μg/L. Low concentrations of iron were 

detected in 29 percent of samples, none of which had elevated 

arsenic concentrations. The maximum arsenic concentration 

for a low-iron sample was 4.9 μg/L. 

Comparison of Methods for Estimating Redox 

Conditions 

The two methods of estimating redox conditions led to 

similar conclusions: elevated arsenic concentrations were 

detected only in strongly reducing or high-iron samples; 

18 percent of strongly reducing and 19 percent of high-iron 

samples had arsenic concentrations greater than 10.0 μg/L. In 

contrast, not strongly reducing or low-iron samples had arsenic concentrations less than 4.9 μg/L. 

An advantage to categorizing redox conditions using 

iron concentrations is the relatively wide applicability of the 

method. If a water analysis is not available, iron staining of 

household fixtures (by untreated water) can provide a visual 

clue that the water is strongly reducing and therefore, the arsenic concentration may be greater than the MCL of 10.0 μg/L. 

Based on data from the current study, elevated arsenic concentrations may exist in almost 1 in 5 wells (19 percent) that have 

iron concentrations high enough to cause staining. 

A drawback to estimating redox conditions using only 

iron concentrations is that it is not possible to identify samples 

with mixed or methanogenic redox conditions. In general, 

mixed redox conditions are more likely to be detected in 

public-supply wells than domestic wells, which is because 

public-supply wells are often pumped at relatively high rates 

and (or) have relatively long screened or open intervals and 

are therefore more likely to produce water from multiple 

horizons that may have different redox conditions (Eberts and 

others, 2013). 

An advantage of estimating redox using four water-quality constituents, instead of one, is that methanogenic redox 

conditions can be identified. Results of an arsenic study in 

southwestern Ohio indicated that the highest arsenic concentrations were associated with methanogenic groundwater 

(Thomas and others, 2005, 2008). The presence of methane 

in domestic wells can be a safety concern for homeowners 

because, if not properly vented, methane from groundwater 

could accumulate in confined spaces (wells or basements) and 

pose a risk of explosion or asphyxiation. 

Figure 6. Arsenic concentrations in relation to redox 

conditions estimated from iron concentrations—Licking 

County, Ohio, 2012. [MCL, Maximum Contaminant Level] 

30 72

0 19

<3 4.2

<3 8.3

43.8

0

10

20

30

40

50

10 100 1,000 10,000

Low iron High iron 

Iron secondary maximum 

contaminant level = 300 μg/L

Iron concentration, in micrograms per liter (μg/L)

Arsenic concentration,

in micrograms per liter (μg/L)

MCL

Low iron

≤300 μg/L

High iron

>300 μg/L

Number of samples1

Arsenic concentrations,

 in μg/L

Median

75th percentile 

Maximum

Percentage with arsenic concentrations 

 >10.0 μg/L

1

Iron concentrations were known for 102 wells.
Factors Related to Arsenic Concentrations 17

Aquifer and Well Characteristics From Well 

Logs

Well logs (drillers logs) include information about 

geology, well construction, and well yield. Of the 168 wells 

sampled for this study, 99 well logs were available from the 

ODNR Well Log Database (http://soilandwater.ohiodnr.gov/

search-file-well-logs). In addition, the well depth and water 

level were available for one well that did not have a well log. 

Aquifer Type 

The type of aquifer tapped by the well was noted on 

97 well logs. Forty-six wells tapped glacial deposits (referred 

to as glacial wells) and 51 wells tapped bedrock (bedrock 

wells). Two wells that tapped both aquifers are not included 

in the total. Elevated arsenic concentrations were detected in 

each aquifer type but were more prevalent in glacial aquifers 

than in bedrock aquifers (fig. 7A). Twenty-four percent of glacial wells had arsenic concentrations greater than 10.0 μg/L, as 

compared to 8 percent of bedrock wells. The maximum arsenic concentration for glacial wells was 43.8 μg/L, as compared 

to 19.4 μg/L for bedrock wells. Results of the Kruskal-Wallis 

test indicated that the difference in arsenic concentrations was 

statistically significant (chi-square = 13.8478, p = 0.0002, 

n = 97). 

Glacial and bedrock wells also differed in terms of iron 

concentrations (fig. 7B). Iron concentrations and aquifer type 

were available for 68 wells. High-iron waters were more 

prevalent in glacial wells (94 percent) than in bedrock wells 

(63 percent). The median iron concentration for glacial wells 

was four times higher than for bedrock wells, and results of 

the Kruskal-Wallis test indicate that the difference is statistically significant (chi-square = 20.4711, p < 0.0001, n = 68). 

The difference in iron concentrations (and redox conditions) is 

likely due to the presence of more labile organic carbon in the 

younger glacial deposits than in the older Paleozoic bedrock. 

Organic carbon acts as an electron donor in the redox-related 

processes that mobilize arsenic from iron oxyhydroxides. 

Some well logs also included information about the type 

of bedrock tapped by the well. Bedrock lithology (shale, sandstone, sandstone and shale, or other) was indicated on 48 well 

logs. The relation between arsenic concentrations and bedrock 

lithology will be discussed in the “Bedrock Geology” section 

of this report. 

Figure 7. Comparison of glacial and bedrock aquifers, Licking County, Ohio, 2012. A, Arsenic 

concentrations. B, Iron concentrations. [MCL, Maximum Contaminant Level]

Arsenic

Glacial Bedrock

Number of samples1 46 51

24 8

5.2 <3

9.9 4.3

43.8 19.4

Glacial Bedrock

Aquifer type

Glacial Bedrock

Aquifer type

50

40

30

20

10

0

A

B 6,000

5,000

4,000

3,000

2,000

1,000

0

Median 

75th percentile 

Maximum 

Arsenic concentrations, 

 in μg/L

Arsenic concentration, in micrograms

per liter (μg/L)

Iron concentration, in micrograms per liter (μg/L)

1

Aquifer type was known for 97 wells.

Percentage with arsenic concentrations 

>10.0 μg/L

Iron

Glacial Bedrock

Number of samples2 33 35

94 63

1,760 403

3,130 849

5,400 2,590

Median 

75th percentile 

Maximum 

Iron concentrations, 

 in μg/L

2

Aquifer type and iron concentrations were known for 68 wells.

Percentage with iron concentrations 

>300 μg/L

MCL
18 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Well Characteristics

Well logs include information about well construction 

and well yield, including well depth, water level, top of the 

open interval, length of the open interval, date of construction, 

and results of aquifer tests (pump rate, drawdown of the water 

level, and duration of the test), which can be used to calculate 

specific capacity—a rough indicator of well yield. Summary 

statistics for 10 well characteristics were computed for glacial 

and bedrock wells (table 9). In addition, the Spearman’s rho 

test was used to investigate whether any of the variables were 

correlated with arsenic concentrations. For bedrock wells, two 

variables had statistically significant (p<0.05) relations to arsenic concentrations: (1) the length of the open interval, and 

(2) the top of the open interval above/below the water level. 

Since these relations are only relevant to bedrock wells, they 

will be discussed in the “Bedrock Geology” section of this 

report. 

Table 9. Well-log variables, summary statistics, and results of a statistical test for correlation with arsenic 

concentrations in groundwater, Licking County, Ohio, 2012.

[Bold text indicates that correlation with arsenic concentration is statistically significant based on the p-value <0.05; <, less than; 

ft, foot; gpm/ft, gallon per minute per foot of drawdown]

Aquifer type Number of 

wells Range Median

Result of Spearman’s rho test for correlation with 

arsenic concentration

Spearman’s correlation 

coefficient p-value

Well depth, ft below land surface

Glacial 46 42–327 115 0.06208 0.6819

Bedrock 51 37–400 160 -0.06584 0.6462

Water level, ft below land surface

Glacial 46 1–155 44 0.13670 0.3650

Bedrock 49 1–200 65 -0.07527 0.6072

Top of open interval, ft below land surface

Glacial 43 39–311 107 0.14916 0.3398

Bedrock 50 25–255 100 0.13383 0.3542

Top of open interval, ft below water level

Glacial 43 -20–197 53 0.06274 0.6894

Bedrock 49 -58–128 30 0.36383 0.0102

Length of open interval, ft

Glacial 43 <1–48 3 -0.27033 0.0796

Bedrock 50 <1–293 41 -0.32972 0.0194

Date of construction, year

Glacial 45 1956–2012 1994 -0.00743 0.9587

Bedrock 51 1957–2012 1998 -0.26620 0.0771

Pump rate, gallons per minute

Glacial 45 5–50 12 -0.05303 0.7294

Bedrock 48 4–45 15 -0.22307 0.1275

Drawdown, ft

Glacial 39 <1–150 10 0.20648 0.2073

Bedrock 43 <1–175 20 0.18630 0.2316

Test duration, hours

Glacial 40 1–22 2 -0.14398 0.3754

Bedrock 47 0–24 1 0.12679 0.3958

Specific capacity1, gpm/ft

Glacial 39 0.05–300 1 -0.22243 0.1735

Bedrock 43 0.08–200 1.05 -0.25803 0.0948

1

Computed as pump rate/drawdown.
Factors Related to Arsenic Concentrations 19

Hydrogeologic Characteristics of Licking County

In this section of the report, the spatial distribution of 

arsenic concentrations is compared to hydrogeologic characteristics of Licking County, including topography, thickness 

and lithology of glacial deposits, hydrogeologic settings, and 

bedrock geology (Ohio Department of Natural Resources, 

[n.d.]).

Topography

The topography of Licking County was presented in a 

generalized form by Ohio Department of Natural Resources 

(1995). Land surface slopes of less than 6 percent are relatively widespread in the west, where a thick layer of glacial 

deposits overlies the bedrock surface. In the east, low slopes 

are generally limited to stream valleys; in upland areas, where 

glacial deposits are thin or absent, slopes can be more than 18 

percent. In Licking County, elevated arsenic concentrations 

roughly correspond to areas of low topographic relief (fig. 8). 

Topographic relief is one of the primary drivers of subsurface 

groundwater flow, and flat topography is often associated with 

low hydraulic gradients and sluggish rates of groundwater 

flow. 

Figure 8. Distribution of arsenic concentrations in Licking County, Ohio (2012) in relation to topography.

 [MCL, Maximum Contaminant Level]

EXPLANATION

0−6

6−12

12−18+

Slope of land surface, in percent

40°10'

82°40'

40°

82°30'

82°20'

MCL

Arsenic concentration, in 

 micrograms per liter (μg/L)

Less than 3

3−10.0

10.1−29.9

30−44

Slope of land surface from Ohio Department of Natural 

 Resources, 1995

Base from U. S. Geological Survey digital data, National Map, 2015

Universal Transverse Mercator projection, zone 17N

North American Datum of 1983 0

0 8 KILOMETERS

6

2

2

6

8 MILES

4

4

0−6

0

10

20

30

40

50

6−12

Slope of land surface, in percent

Arsenic concentration, 

in microgrgams per liter

12−18+
20 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Lithology of Glacial Deposits

Glacial deposits of Licking County can be grouped into 

three general categories: fines (predominantly clay or silt with 

lenses of sand and gravel), till (unsorted mixture of clay, silt, 

sand, and gravel with or without lenses of sand and gravel), 

and sand and gravel (predominantly sand and gravel with 

minor fines) (Ohio Department of Natural Resources, 2000). 

In Licking County, elevated arsenic concentrations were 

associated with till or fines (fig. 9). In general, till and fines are 

clay-rich deposits that are often associated with low rates of 

groundwater recharge and (or) flow.

Figure 9. Distribution of arsenic concentrations in Licking County, Ohio (2012) in relation to lithology of glacial deposits. 

[NA, glacial deposits are thin or absent; MCL, Maximum Contaminant Level]

40°10'

82°40'

40°

82°30'

82°20'

EXPLANATION

Till—Unsorted clay, silt, sand, and gravel. 

 May include lenses of sand and gravel 

Fines—Primarily clay and silt with lenses 

 of sand and gravel

Sand and gravel—Primarily sand and gravel 

 with minor clay and silt

Not applicable

Lithology of glacial deposits

Arsenic concentration, in 

 micrograms per liter (μg/L)

Less than 3

3−10.0

10.1−29.9

30−44

Glacial lithology from Ohio Department of Natural Resources, 2000

Fines Till Sand and gravel NA

Lithology of glacial deposits

0

10

20

30

40

50

Arsenic concentration, 

in microgrgams per liter 

Base from U. S. Geological Survey digital data, National Map, 2015

Universal Transverse Mercator projection, zone 17N

North American Datum of 1983

MCL

0

0 8 KILOMETERS

6

2

2

6

8 MILES

4

4
Factors Related to Arsenic Concentrations 21

Drift Thickness 

Drift thickness refers to the thickness of the layer of 

glacial deposits overlying bedrock. In Licking County, drift 

thickness varies from 0 to 596 ft (Ohio Department of Natural 

Resources, Division of Geological Survey, 2004). As shown 

on the generalized map of figure 10, thick glacial deposits are 

widespread in the western part of the county but are limited to 

buried valleys in the east. In Licking County, elevated arsenic 

concentrations generally correspond to areas where glacial 

deposits are thicker than 100 ft.

The hydrogeologic characteristics associated with 

elevated arsenic concentrations—low topographic relief 

(fig. 8) and a thick layer of low-permeability glacial deposits 

(figs. 9 and 10)—are more prevalent in the western part of 

Licking County, which is within the Central Lowland physiographic province (figs. 1 and 2). These characteristics are generally conducive to the development of reducing groundwater. 

The redox condition of groundwater is a balance between the 

supply of dissolved oxygen (and other electron acceptors) as 

compared to the supply of organic carbon (and other electron 

donors). In general, a thick layer of low-permeability glacial 

deposits promotes reducing conditions in the subsurface by 

(1) retarding the movement of dissolved oxygen from the land 

surface to the aquifer, and (2) providing a source of organic 

carbon, which serves as an electron donor in redox reactions. 

If redox conditions become sufficiently reducing, arsenic can 

be released from iron oxyhydroxides in the aquifer matrix to 

the groundwater. In addition, a low-relief and low-permeability land surface is conducive to low rates of recharge and 

sluggish rates of groundwater flow. The observations from 

Licking County are generally consistent with the conceptual 

model of Smedley and Kinniburgh (2002), which states that 

high concentrations of arsenic can develop where (1) the onset 

of reducing conditions causes arsenic to be mobilized from 

the aquifer matrix, and (2) low rates of groundwater flushing 

allow the released arsenic to accumulate in the aquifer. 

Figure 10. Distribution of arsenic concentrations in Licking County, Ohio (2012) in relation to drift thickness. 

[MCL, Maximum Contaminant Level]

40°10'

82°40'

40°

82°30'

82°20'

Base from U. S. Geological Survey digital data, National Map, 2015

Universal Transverse Mercator projection, zone 17N

North American Datum of 1983

EXPLANATION

Less than 1

1−99

100−199

200−596

Thickness of glacial deposit, in feet

Drift thickness from Ohio Department of Natural Resources, 

 Division of Geological Survey, 2004 

Arsenic concentration, in 

 micrograms per liter (μg/L)

Less than 3

3−10.0

10.1−29.9

30−44

0

10

20

30

40

50

Arsenic concentration, 

in microgrgams per liter 

Less than 1 1−99 100−199 200−596

Drift thickness, in feet

MCL

0

0 8 KILOMETERS

6

2

2

6

8 MILES

4

4
22 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Hydrogeologic Settings

Unconsolidated glacial deposits of Ohio have been 

mapped as 10 hydrogeologic settings, 5 of which are present 

over substantial areas of Licking County: buried valley, complex thick drift, ground moraine, thin upland, and alluvial 

(fig. 3C) (Ohio Department of Natural Resources, 2000). 

Elevated arsenic concentrations are generally associated with 

two of the hydrogeologic settings—buried valley and complex 

thick drift (fig. 11A). Buried-valley settings are present in previously existing bedrock valleys; streams overlie some, but not 

all, buried valleys. Complex-thick-drift settings may consist of 

a combination of hydrogeologic settings, such as end moraine 

overlying a buried valley. Both settings are characterized by 

(1) predominantly low-permeability sediment (fines and till) 

that may contain lenses of sand and gravel, and (2) drift thickness greater than 100 ft. Because the buried-valley and complex-thick-drift hydrogeologic settings are both partly defined 

on the basis of drift thickness, the extent of drift thicker than 

100 ft (fig. 10) is somewhat similar to the combined extent of 

the two hydrogeologic settings (fig. 11A). 

Even though elevated arsenic concentrations are associated with buried-valley and complex-thick-drift settings, 

the statistical distribution of arsenic concentrations differs 

between the two settings. Arsenic concentrations measured 

in wells in relation to the hydrogeologic setting extracted 

from the GIS coverage are shown in figure 11B. In buriedvalley settings, most arsenic concentrations are low (less than 

3 μg/L), but a few samples have very high concentrations 

(30–44 μg/L). For the complex-thick-drift setting, elevated 

arsenic concentrations were more prevalent, but all concentrations were less than 21 μg/L. 

Two samples with elevated arsenic concentrations are 

associated with the ground moraine hydrogeologic setting, 

rather than the buried-valley or complex-thick-drift setting 

(figs. 11A and 11B). Ground moraine is defined as primarily till that is 25–100 ft thick (Ohio Department of Natural 

Resources, 2000); however, well logs for these two wells 

indicate that the glacial deposits are 121 and 131 ft thick and 

therefore meet the definition of buried-valley or complexthick-drift settings. 

Figure 11. Arsenic concentrations in relation to hydrogeologic settings of Licking County, Ohio, 2012. A, Spatial relations (see fig. 3 

for more detailed map and definitions of hydrogeologic settings). B, Statistical relations. [MCL, Maximum Contaminant Level]

40°10'

82°40'

40°

82°30'

82°20'

Base from U. S. Geological Survey digital data,

National Map, 2015

Universal Transverse Mercator projection, zone 17N

North American Datum of 1983

EXPLANATION

Arsenic concentration, in

 micrograms per liter (μg/L)

Hydrogeologic setting

Less than 3

3−10.0

10.1−29.9

30−44

Complex thick drift—Thicker than 100

 feet, primarily till with sand and gravel

 lenses

Buried valley—Thicker than 100 feet,

 primarily fines with sand and gravel

 lenses

Ground moraine, thin upland, or alluvial

A

Hydrogeologic settings from Ohio Department of

 Natural Resources (2000)

0

10

20

30

40

B 50

Alluvial Thin 

upland

Ground

moraine

Complex

thick drift

Buried

valley

Hydrogeologic setting

Number of samples2 7 29 11 72 47

Arsenic concentrations,

in μg/L

Median <3 <3 <3 5.3 <3

75th percentile <3 <3 8.3 8.5 3.3

Maximum <3 6.2 37 20.8 43.8

Aluvial Thin

upland

Ground

moraine

Complex

thick drift

Buried

valley

1

Well logs indicate drift thickness of 121 and 131 feet, which is characteristic of complex-thick-drift or 

 buried-valley hydrogeologic settings.

2

Hydrogeologic setting was known for 166 wells. 

Arsenic concentration, 

in micrograms per liter (μg/L)

1

MCL 1

0

0 8 KILOMETERS

6

2

2

6

8 MILES

4

4
Factors Related to Arsenic Concentrations 23

The data from figure 11 are subdivided by aquifer type 

on figure 12. Even though hydrogeologic settings are defined 

by characteristics of the glacial deposits, relations between 

arsenic concentrations and hydrogeologic settings are relatively similar for glacial aquifers (fig. 12A), bedrock aquifers 

(fig. 12B), and for wells for which the aquifer type is unknown 

because no well log was available (fig. 12C). This similarity 

may indicate that arsenic concentrations in bedrock aquifers 

are at least partly related to characteristics of the overlying 

glacial deposits. 

In Licking County, elevated concentrations of arsenic were generally more prevalent in glacial wells than in bedrock wells. Glacial 

aquifers can have widely varying hydrogeologic characateristics, 

and arsenic is more prevalent in some types of glacial deposits 

than in others.

Figure 12. Arsenic concentrations in relation to hydrogeologic settings, Licking 

County, Ohio, 2012. A, Glacial wells. B, Bedrock wells. C, Unknown-aquifer wells. 

[MCL, Maximum Contaminant Level]

A. Glacial

B. Bedrock

C. Unknown aquifer (no well log)

1

1

50

40

30

20

10

0

50

40

30

20

10

0

50

40

30

20

10

0

Hydrogeologic setting

Alluvial Thin

upland

Ground

moraine

Complex

thick drift

Buried

valley

1

Well log indicates drift thickness is 

121 feet, which is characteristic of 

complex-thick-drift or buried-valley 

hydrogeologic settings. 

1

Well log indicates drift thickness is 

at least 131 feet, which is characteristic of complex-thick-drift or buriedvalley hydrogeologic settings. 

Arsenic concentration, in micrograms per liter (μg/L)

MCL

MCL

MCL
24 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

The redox conditions of samples associated with the 

hydrogeologic settings are shown on figure 13. All samples 

from the complex-thick-drift setting are classified as iron- or 

sulfate-reducing (fig. 13A). Redox conditions in the buriedvalley setting are more varied, and the highest arsenic concentrations were associated with methanogenic (or near-methanogenic) redox conditions. These observations are similar 

to observations from arsenic studies in southwestern Ohio 

(Thomas and others, 2005; Thomas and others, 2008). It is 

interesting to note that samples associated with complex-thickdrift and buried-valley hydrogeologic settings have relatively 

similar iron concentrations (fig. 13B) but very different sulfate 

concentrations (fig. 13C). 

Figure 13. Redox conditions in hydrogeologic settings, Licking County, Ohio, 

2012. A, Redox conditions estimated by using nitrate, manganese, iron, and sulfate 

concentrations. B, Iron concentrations. C, Sulfate concentrations.

A

0

2,000

4,000

6,000

8,000

0

100

200

300

400

500

600

Alluvial Thin

upland

Ground

moraine

Complex

thick drift

Buried

valley

6 17 9 36 33

Redox classification (percent)

Not strongly reducing 33 59 33 0 27

Iron or sulfate reducing 67 41 67 100 67

0 0 0 0 6

B

C

1

Hydrogeologic setting and iron or sulfate concentrations were known for 101 wells. Iron concentration, in micrograms per liter (μg/L) Sulfate concentration, in milligrams per liter (mg/L)

Hydrogeologic setting

Alluvial Thin

upland

Ground

moraine

Complex

thick drift

Buried

valley

Number of samples1

Methanogenic or nearmethanogenic
Factors Related to Arsenic Concentrations 25

In Licking County, the two wells with the highest arsenic 

concentrations (43.6 and 43.8 μg/L) were associated with 

the buried-valley setting in the east-central part of the county 

(fig. 11A). For one well, the redox condition was estimated 

to be methanogenic. For the second well, the redox condition was estimated to be “near-methanogenic” (as discussed 

in the “Estimation of Redox Conditions From Concentrations 

of Iron, Manganese, Sulfate, and Nitrate” section). A third 

well that taps buried-valley deposits less than 2 miles to the 

northeast produces water with methanogenic redox conditions 

and an average arsenic concentration of 69 μg/L. This well 

was not sampled as part of the current study (and is not shown 

on fig. 9), but it was the site of an EPA pilot study of arsenicremoval methods for use in small public-water systems (U.S. 

Environmental Protection Agency, 2011).

These three wells, with arsenic concentrations of 

44–69 μg/L and methanogenic or near-methanogenic redox 

conditions, are in an area of predominantly fine-grained glacial deposits in the buried valley between the municipalities of 

Newark and Hanover (figs. 1 and 11A). The glacial geology of 

this area was studied in detail to gain insight into the processes that led to Pleistocene drainage reversals (Frolking and 

Pachell, 2006). During the Late Wisconsinan, a west-flowing 

tributary of the Teays River system was blocked by glacial 

ice advancing from the northeast, which caused ponding of 

the river water and led to deposition of a thick layer of dense, 

laminated lakebed (lacustrine) silt. The Late Wisconsinan 

lacustrine silt overlies Illinoian sand and gravel deposits. 

It is possible that this type of setting may be especially 

conducive to arsenic mobilization. The layer of laminated 

lacustrine silt would presumably be conducive to the development of reducing conditions in an underlying aquifer by 

inhibiting recharge of oxygenated water. In addition, lacustrine sediment is typically rich in organic matter, which serves 

as an electron donor in redox reactions that mobilize arsenic. 

The underlying deposits—Illinoian sand and gravel—may 

be rich in iron oxyhydroxides (and associated arsenic) as a 

result of weathering during the interglacial period. This idea is 

generally consistent with observations from a study in southwestern Ohio, where the highest solid-phase concentrations 

of arsenic, iron, and other metals in glacial sediments were 

in paleosols near the boundary of Illinoian and Wisconsinan 

deposits (Thomas and others, 2008). 

In Licking County, the boundary between Wisconsinan 

lacustrine deposits (presumably rich in organic carbon) and 

weathered Illinoisan paleosol (presumably rich in iron oxyhydroxides and associated arsenic) may have geochemical conditions favorable for arsenic mobilization. A water well that taps 

that particular horizon may be vulnerable to contamination 

from naturally occurring arsenic; however, this hypothesis 

needs to be tested. 

Bedrock Geology 

Bedrock in Licking County is a complex mix of interbedded shales, sandstones, siltstones, and limestones of Devonian 

to Pennsylvanian age (Slucher and others, 2006). The spatial 

distribution of arsenic concentrations in bedrock wells is 

shown in figure 14. Four of the 51 bedrock wells had elevated 

arsenic concentrations, and all are in the subcrop of the 

Cuyahoga and Logan Formations in western Licking County. 

The western part of the county is also where topography and 

glacial characteristics are conducive to arsenic mobilization, 

as discussed in the “Topography, Lithology of Glacial Deposits, and Drift Thickness” section. Therefore, it is difficult to 

determine if higher arsenic concentrations in bedrock wells are 

related to the composition of the bedrock, the characteristics of 

the overlying glacial deposits, or both. The relations illustrated 

in figure 12B—higher arsenic concentrations in bedrock wells 

overlain by complex-thick-drift and buried-valley settings—

may indicate that a thick (greater than 100 ft) layer of till or 

fines could be conducive to development of reducing conditions, and mobilization of arsenic, in the underlying bedrock. 

This idea may warrant further investigation in future studies. 
26 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Bedrock Lithology 

Information about bedrock lithology was available from 48 well logs. Bedrock wells were subdivided into four groups: 

shale, sandstone and shale, sandstone, and other.Arsenic concentrations in sandstone wells were generally lower than in the 

other categories (fig. 15); however, results of the Kruskal-Wallis test indicate that differences in median concentrations among 

the groups are not statistically significant (chi-square = 6.5552; p = 0.0875; n = 48) .

Figure 14. Distribution of arsenic concentrations in bedrock wells in relation to bedrock formations, Licking 

County, Ohio, 2012. 

Figure 15. Relation between arsenic concentrations and bedrock lithology 

from well logs, Licking County, Ohio, 2012. [MCL, Maximum Contaminant 

Level]

Bedrock formations from Slucher and others (2006)

40°10'

82°40'

40°

82°30'

82°20'

Base from U. S. Geological Survey digital data,

National Map, 2015

Universal Transverse Mercator projection, zone 17N

North American Datum of 1983

EXPLANATION

Arsenic concentration, in micrograms

 per liter (μg/L)

Less than 3

3−10.0

10.1−19.6

Allegheny and Pottsville Groups,

 undifferentiated

Logan and Cuyahoga formations,

 undivided

Blackhand Mamber of the Cuyahoga

 Formation

Subury Shale or Berea Sandstone or

 Bedford Shale

Ohio Shale

0

0 8 KILOMETERS

6

2

2

6

8 MILES

4

4

10

MCL

0

20

30

Bedrock lithology

Bedrock lithology

Number of samples2 11 25 10 2

18 4 0 50

Arsenic concentrations,

in μg/L

Median 4.2 <3 <3 -

75th percentile 7.6 3.7 <3 -

Maximum 16.5 19.4 5.7 17.7

Shale Sandstone/shale Sandstone Other1

Shale Sandstone/

shale Sandstone Other1

Percentage with arsenic concentrations 

 >10.0 μg/L

1

Includes sandstone/shale/limestone and blue rock. Arsenic concentration, in micrograms per liter (μg/L)
Factors Related to Arsenic Concentrations 27

Well Construction Characteristics of Bedrock Wells 

As discussed in the “Well Characteristics” section, 

arsenic concentrations in bedrock wells were correlated with 

two well-construction characteristics compiled from well logs 

(table 9). 

Length of open interval.—The term “open interval” is 

used to refer to a well screen or an uncased section of the 

borehole. The length of the open interval was estimated by 

subtracting the top of the open interval (or the bottom of the 

casing) from the bottom of the open interval (or the bottom of 

the well). For bedrock wells, open intervals ranged from <1 

to 293 ft, with a median of 41 ft (fig. 16A; table 9). Results 

of the Spearman’s rho test indicate that the length of the open 

interval was negatively correlated with arsenic concentrations (Spearman’s correlation coefficient= -0.32972; p-value 

= 0.0194; n = 50). A negative correlation coefficient indicates 

shorter open intervals were associated with higher arsenic 

concentrations. Elevated arsenic concentrations were detected 

only in bedrock wells with open intervals less than 40 ft long. 

For glacial wells, open intervals ranged from <1 to 48 ft, 

with a median of 3 ft (table 9). Elevated arsenic concentrations 

were detected only in glacial wells with open intervals <10 ft, 

and the correlation coefficient was negative, but the correlation with arsenic concentrations was not statistically significant (Spearmans’s correlation coefficient = -0.27033; p-value 

= 0.0796; n = 43). 

Long open intervals can allow mixing of water from 

multiple parts of an aquifer that have different water-quality 

characteristics (Eberts and others, 2013). The correlation of 

longer open intervals with lower arsenic concentrations is 

consistent with the idea that high-arsenic water is from a relatively thin horizon of the aquifer. If this were true, a long open 

interval could allow high-arsenic water from a thin horizon to 

be diluted by inflow of low-arsenic water from other parts of 

the aquifer. The idea that high-arsenic water may originate in 

a thin horizon of the aquifer is consistent with observations 

from a study in southwestern Ohio (Thomas and others, 2008). 

Solid-phase analysis of samples from a 100-ft core of carbonate bedrock indicated that, with one exception, all bedrock 

samples had very low concentrations of arsenic (less than or 

equal to 2 milligrams per kilogram [mg/kg]). The only exception was a sample from a thin (less than 5 ft) horizon, just 

below glacial/bedrock boundary, where the arsenic concentration was 42 mg/kg. The same horizon was also rich in iron and 

other metals. 

Top of open interval below/above the water level.—The 

depth to the top of the open interval, relative to the water 

level, was computed by subtracting the water level (in feet 

below land surface) from the top of the open interval (in feet 

below land surface). This variable was correlated with arsenic 

concentrations for bedrock wells only (Spearman’s correlation coefficient = 0.36382; p-value = 0.0102; n = 49) (fig. 16B

and table 9). The positive correlation coefficient indicates that 

higher arsenic concentrations in bedrock wells are generally 

associated with greater depths (relative to the water level); 

however, it is interesting to note that one of the four bedrock 

wells with elevated arsenic concentrations (16.5 μg/L) was 

from a very shallow (40 ft) shale well and the top of the open 

interval was only 21 ft below the water level. Additional 

water-quality data are not available for this well, so redox conditions are unknown. Additional studies may be warranted to 

investigate whether well construction characteristics can affect 

the concentration of arsenic in the produced water. 

One aspect of well construction is determining 

which part of groundwater system is tapped to 

produce water. Different parts of the groundwater 

system can have different chemical characteristics, 

so well construction/design can sometimes have an 

effect on the quality of the water produced from a 

well (Eberts and others, 2013).

Figure 16. Arsenic concentrations in bedrock wells in relation 

to well-construction characteristics, Licking County, Ohio, 2012. 

A, Length of the open interval. B, Top of the open interval, in 

feet above/below water level. Summary statistics and results 

of Spearman’s rho test are shown in table 6. [MCL, Maximum 

Contaminant Level]

-100 -50 0 50 100 150 200

0 100 200 300

Length of open interval, in feet

Top of open interval, in feet below/above water level 

50

40

30

20

10

0

50

40

30

20

10

0

MCL

MCL

Arsenic concentration, in micrograms per liter

A

B
28 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Preliminary Extrapolation of Results 

From Licking County to Other Parts of 

Ohio

In Licking County, elevated arsenic concentrations 

were detected in areas where glacial deposits are thicker than 

100 ft (fig. 10) and in buried-valley or complex-thick-drift 

hydrogeologic settings (fig. 11). Similar hydrogeologic characteristics exist in other parts of Ohio, and it is possible that 

groundwater in these areas also has elevated arsenic concentrations (greater than the MCL of 10.0 μg/L). 

The statewide distribution of hydrogeologic characteristics associated with elevated arsenic concentrations in groundwater of Licking County is shown in figure 17A: (1) buriedvalley and complex-thick-drift hydrogeologic settings, and 

(2) drift thickness greater than 100 ft. The combined extent of 

these characteristics, shown in figure 17B, can be considered 

an initial estimate of areas where groundwater may be vulnerable to contamination by natural sources of arsenic. 

Figure 17. Preliminary extrapolation of results from Licking County, Ohio, 2012. A, Characteristics associated with elevated arsenic 

concentrations (greater than 10.0 micrograms per liter) in groundwater of Licking County, Ohio, 2012. B, Combined extent of the 

characteristics shown in A. C, Approximate location of targeted arsenic investigations in domestic wells. D, Arsenic concentrations in 

public-supply wells of the Ohio Environmental Protection Agency ambient groundwater network, measured in 2013. 

0

Base from U. S. Geological Survey

Digital data, National Map, 2015

Universal Transverse Mercator projection

Zone 17N

North American Datum of 1983

60 KILOMETERS

20 40

0 40

60 MILES

20

84°

83° 82°

81°

41°

40°

39°

Combined extent of characteristics shown in A

B

Lake Erie

Serves as an initial estimate of areas where groundwater may be 

vulnerable to contamination from natural sources of arsenic 

84°

83° 82°

81°

41°

42°

40°

39°

EXPLANATION

Buried valley

Drift thickness (Ohio Department of Natural Resources, 2004)

Greater than 100 feet

Complex thick drift

A

Lake Erie

Hydrogeologic settings (Ohio Department of Natural Resources, 2000)
Preliminary Extrapolation of Results From Licking County to Other Parts of Ohio 29

As an initial test of this hypothesis, the extent of the 

potentially vulnerable areas was compared to the location of 

known elevated arsenic concentrations. Figure 17C shows the 

approximate location of the targeted arsenic investigations 

discussed in the “Previous Studies” section. These studies 

were done in areas where elevated arsenic concentrations in 

domestic wells were interpreted to be from natural sources. 

Figure 17D shows arsenic concentrations in public-supply 

wells sampled during 2013 as part of the Ohio EPA ambient groundwater program (Ohio Environmental Protection 

Agency, 2013). For both maps, there is a relatively good 

spatial correspondence between detections of elevated arsenic 

concentrations and the potentially vulnerable areas. This 

preliminary conceptual model can be tested and refined as 

additional wells are sampled for arsenic and other water-quality constituents.

Figure 17. Preliminary extrapolation of results from Licking County, Ohio, 2012. A, Characteristics associated with elevated arsenic 

concentrations (greater than 10.0 micrograms per liter) in groundwater of Licking County, Ohio, 2012. B, Combined extent of the 

characteristics shown in A. C, Approximate location of targeted arsenic investigations in domestic wells. D, Arsenic concentrations in 

public-supply wells of the Ohio Environmental Protection Agency ambient groundwater network, measured in 2013.—Continued

84°

83° 82°

81°

41°

40°

39°

0

Base from U. S. Geological Survey

Digital data, National Map, 2015

Universal Transverse Mercator projection

Zone 17N

North American Datum of 1983

60 KILOMETERS

20 40

0 40

60 MILES

20

D

Less than 3

Arsenic concentrations in micrograms per liter

3 to 10

Greater than 10

Lake Erie

84°

83° 82°

81°

41°

40°

39°

EXPLANATION

Matisoff and others (1982)

Petty (2000)

Ohio Department of Health (1999)

Thomas and other (2005, 2008)

C

Lake Erie

Approximate location of targeted arsenic investigations in domestic 

wells of Ohio

Arsenic concentration in public-supply well of the Ohio Environmental Protection Agency ambient groundwater network, 2013

Current study in

Licking County
30 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Summary

Rocks and sediments of the aquifer matrix often 

include minor amounts of arsenic, which can be released to 

the groundwater under certain geochemical conditions. In 

groundwater of Ohio and other parts of the Midwest, evidence 

indicates that arsenic associated with metal oxyhydroxides is 

mobilized in groundwater with strongly reducing redox conditions and circumneutral to slightly alkaline pH.

Consumption of drinking water with elevated arsenic 

concentrations (greater than 10.0 micrograms per liter [μg/L]) 

is linked to multiple cancers and other serious health problems, but most domestic wells are never tested for arsenic. 

Understanding which parts of the groundwater system are 

likely to have elevated arsenic concentrations would allow 

well testing and education about water treatment to be targeted 

to areas of greatest need. 

During March–August of 2012, arsenic concentrations 

were measured in water samples from 168 domestic wells in 

Licking County, Ohio. Samples were collected by well owners 

and county sanitarians and were analyzed at a local private 

laboratory. Methods of sample collection and analysis probably introduced some variability/uncertainty into the waterquality dataset, but this was taken into account by grouping 

the data into broad concentration classes for analysis. Elevated 

arsenic concentrations (greater than 10.0 μg/L) were detected 

in 12 percent of the samples (about one in eight). The maximum concentration of about 44 μg/L was detected in two 

wells in the same township. 

In groundwater, arsenic occurs as two inorganic species—arsenite (As3+) and arsenate (As5+). Nine of the wells 

with elevated arsenic concentrations were resampled and 

analyzed for arsenic speciation at the U.S. Geological Survey 

National Water Quality Laboratory. Results indicated that As3+

was the predominant species. The percentage of arsenic in the 

nine samples ranged from 55–92 percent. Arsenite is generally 

considered to be more toxic to humans than As5+. In addition, 

As3+ is more difficult to remove from water because it occurs 

as an uncharged species (whereas As5+occurs as a negatively 

charged ion). Removing high concentrations of As3+ from 

drinking water can require multiple treatment steps. 

A subset of 102 samples were also analyzed for concentrations of iron, manganese, sulfate, and nitrate, which were 

used to estimate redox conditions. Elevated arsenic concentrations were detected only in samples classified as “strongly 

reducing” or “high iron.” About 18 percent strongly reducing 

samples, and 19 percent of high-iron samples, had arsenic 

concentrations greater than 10.0 μg/L. 

The water-quality data were analyzed in relation to aquifer and well-construction characteristics compiled from drillers logs, which were available for 99 wells. Wells in Licking 

County produce water from two types of aquifers—unconsolidated glacial deposits and the interbedded sandstone and shale 

bedrock. Elevated concentrations of arsenic were detected in 

both types of wells, but were more prevalent in glacial wells 

(24 percent) than in bedrock wells (8 percent). The reason may 

be that the younger glacial deposits typically contain more 

organic carbon than the Paleozoic bedrock. Organic carbon 

acts as an electron donor in redox reactions that can mobilize 

arsenic (and iron) from oxyhydroxides in the aquifer matrix. 

Arsenic concentrations were not significantly different for 

different types of bedrock (sandstone, shale, sandstone/shale, 

or other). 

The relation between arsenic concentrations and well 

characteristics was investigated for glacial and bedrock wells 

separately. Variables compiled from well logs included well 

depth, water level, length of the open interval, top of the open 

interval below the land surface, top of the open interval above 

or below the water level, date of construction, and aquifertest variables (pump rate, drawdown of the water level, and 

duration of the test), which were used to calculate specific 

capacity—a rough indicator of well yield. Results of a nonparametric statistical test indicated that higher arsenic concentrations in bedrock wells were associated with (1) shorter 

open intervals and (2) deeper open intervals, relative to the 

water level. Additional study may be warranted to investigate 

the relation between well construction characteristics and the 

concentration of arsenic in the produced water.

The spatial distribution of the water-quality data was 

compared to hydrogeologic characteristics mapped by the 

Ohio Department of Natural Resources. Elevated concentrations of arsenic (and iron) were associated with low topographic relief and a thick (greater than 100 feet) layer of 

clay-rich glacial deposits. In general, these characteristics are 

conducive to the development of redox conditions that are sufficiently reducing to mobilize arsenic from iron oxyhydroxides 

in the aquifer matrix. 

Hydrogeologic characteristics conducive to the development of strongly reducing conditions are relatively widespread 

in the western part of Licking County, which is part of the 

Central Lowland physiographic province. In this area, a thick 

layer of clay-rich glacial deposits obscures the bedrock surface 

and creates a flat to gently rolling landscape with poorly developed drainage networks. The eastern part of the county is in 

the Appalachian Plateaus physiographic province, where topographic relief is greater and much of the landscape consists of 

steep-sided valleys and bedrock uplands. In this area, elevated 

arsenic concentrations were detected in buried valleys but 

not in the bedrock uplands, where glacial deposits are thin or 

absent. The observation that elevated concentrations of arsenic 

(and iron) were more prevalent in the western part of Licking 

County is true for both glacial and bedrock aquifers.

In Licking County, thick, clay-rich glacial deposits (and 

elevated concentrations of arsenic) are associated with two 

hydrogeologic settings—buried valley and complex thick drift. 

In the complex-thick-drift hydrogeologic setting, all samples 

were strongly reducing, 18 percent had elevated arsenic concentrations, and the maximum arsenic concentration was 

about 21 μg/L. In the buried-valley hydrogeologic setting, wells had a wide range of arsenic concentrations; most 

were <10.0 μg/L, but a few samples had very high arsenic 
References Cited 31

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The hydrogeologic settings and characteristics associated with arsenic in Licking County also exist in other parts 

of Ohio. The statewide extent of these characteristics roughly 

corresponds to areas where elevated concentrations of arsenic 

are known to exist. This preliminary conceptual model can be 

tested and revised as additional wells are sampled for arsenic. 

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Venteris, E.R., Basta, N.T., Bigham, J.M., and Rea, Ron, 2014, 

Modeling spatial patterns in soil As to estimate natural baseline concentrations: Journal of Environmental Quality, v. 43, 

no. 3, p. 936–946. 

Welch, A.H., Westjohn, D.B., Helsel, D.R., and Wanty, R.B., 

2000, Arsenic in ground water of the United States—Occurrence and geochemistry: Ground Water, v. 38, no. 4, p. 

589–604. 

World Health Organization (WHO), 2010, Exposure to 

arsenic—A major public health concern: accessed July 2015 

at http://www.who.int/ipcs/features/arsenic.pdf?ua=1.

Tables 2–5
36 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Table 2. Water-quality and hydrogeologic data for 168 domestic wells in Licking County, Ohio, 2012.

Table 2 is an Excel file that can be accessed at http://dx.doi.org/10.3133/sir20155148.

Table 3. Results of quality-control samples from Licking County, Ohio analyzed by a private lab during Test Your Well workshops, 2012.

[All samples are unfiltered; mm/dd/yy, month/day/year; μg/L, microgram per liter; mg/L, milligram per liter; ND, no difference; --, no data;

NA, not applicable; -, negative]

Sample type Sample type 

identifier

Date

(mm/dd/yy)

Arsenic

(μg/L)

Nitrate

(mg/L)

Manganese

(μg/L)

Iron

(μg/L)

Sulfate

(mg/L)

Sodium

(mg/L)

Calcium

(mg/L)

Blank Blank water1

29658

ND -0.001 0.0025 0.013 1.237 0 0.004 0.001

07/10/12 <3.0 <0.10 <30 <80 <5 <5 <2

Relative percent difference ND ND ND ND ND ND ND

Replicates 29659 07/09/12 19.2 <0.10 46 2,810 158 130 19.5

29660 07/09/12 16.7 <0.10 45 2,720 158 131 19.5

Relative percent difference 14 ND 2 4 ND 0.8 ND

Replicates 44990 08/14/12 <3 -- -- -- -- -- --

44982 08/14/12 <3 -- -- -- -- -- --

Relative percent difference - -- -- -- -- -- --

Replicates 44980 08/14/12 4.9 -- -- -- -- -- --

44982 08/14/12 4.1 -- -- -- -- -- --

Relative percent difference 18 -- -- -- -- -- --

Replicates 44966 08/14/12 <3 -- -- -- -- -- --

44976 08/14/12 <3 -- -- -- -- -- --

Relative percent difference - -- -- -- -- -- --

Replicates 44983 08/14/12 33.3 -- -- -- -- -- --

44984 08/14/12 33.8 -- -- -- -- -- --

Relative percent difference 1.5 -- -- -- -- -- --

Standard 

reference 

sample

2

T-171 NA 3.5 -- -- -- -- -- --

44990 08/14/12 <3 -- -- -- -- -- --

Relative percent difference >-15 -- -- -- -- -- --

Standard 

reference 

sample

2

T-183 NA 4.5 -- -- -- -- -- --

44980 08/14/12 4.9 -- -- -- -- -- --

Relative percent difference 9 -- -- -- -- -- --

Standard 

reference 

sample

2

T-193 NA 3.44 -- -- -- -- -- --

44966 08/14/12 <3 -- -- -- -- -- --

Relative percent difference >-14 -- -- -- -- -- --

Standard 

reference 

sample

2

T-201 NA 24.4 -- -- -- -- -- --

44983 08/14/12 33.3 -- -- -- -- -- --

Relative percent difference 31 -- -- -- -- -- -- 1

Blank water (lot 4101271) used in preparation of field blank was obtained from U.S. Geological Survey National Water Quality Laboratory.

2

Standard reference samples obtained from U.S. Geological Survey National Water Quality Laboratory. 
Tables 2–5 37

Table 4. Results of arsenic-speciation quality-control samples from Licking County, Ohio analyzed by the U.S. Geological Survey 

National Water Quality Laboratory, 2012.

[All samples are filtered; mm/dd/yy, month/day/year; hh, hour, mm, minute; μg/L, microgram per liter; As: as arsenic; ND, no difference; --, no data;

Sample type Sample type 

identifier

Date

(mm/dd/yy)

Time

(hhmm)

Arsenic 

(μg/L)

Arsenate 

(As5+)

(μg/L as As)

Arsenite 

(As3+)

(μg/L as As)

Dimethylarsinate

(μg/L as As)

Monomethylarsonate

(μg/L as As)

Equipment blank Blank water1

Li−40−OAQ

11/05/12 1614 <0.04 -- -- -- --

11/05/12 1604 <0.04 -- -- -- --

Relative percent difference -- ND -- ND -- --

Field blank Blank water1

Li−31−OAQ

11/07/12 1716 <0.04 <0.3 <0.4 <0.6 <3.2

11/07/12 1706 20.0575 <0.3 <0.4 <0.6 <3.2

Relative percent difference -- -- ND ND ND ND

Replicates Li−49 11/08/12 1100 -- 3.25 21.74 <0.6 <3.2

Li−49−WGQ 11/08/12 1401 -- 3.32 21.63 <0.6 <3.2

Relative percent difference -- ND 3 0.5 -- --

1

Water used in preparation of blanks was obtained from U.S. Geological Survey National Water Quality Laboratory (lot 127459). 

2

Less than the laboratory reporting level.
38 Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology

Publishing support provided by the U.S. Geological Survey

Science Publishing Network,

Madison and Rolla Publishing Service Centers

Table 5. Results of quality-control samples from Licking County, 

Ohio analyzed at private lab and U.S. Geological Surve National 

Water Quality Laboratory, 2013.

[All samples are unfiltered; mm/dd/yy, month/day/year; μg/L, microgram 

per liter]

Sample type Sample type 

identifier

Date

(mm/dd/yy)

Arsenic

(μg/L)

Replicates 144969

1

44971

04/22/13

04/22/13

44.6

33.4

Relative percent difference 29

Replicates 144977

1

44979

04/22/13

04/22/13

18.8

25.4

Relative percent difference 30

Replicates 144986

1

44987

04/22/13

04/22/13

64.1

56.7

Relative percent difference 12

Replicates 144991

1

44993

04/22/13

04/22/13

40.1

34.0

Relative percent difference 16

Laboratory 

split

2

Li–31–1459

1

44991, 44993

04/23/13

04/23/13

39.9

37.0

Relative percent difference3 -7

Laboratory 

split

2

Li–31–1400

1

44986, 44987

04/23/13

04/23/13

51.0

60.4

Relative percent difference3 29

Laboratory 

split

2

Li–38–1100

1

44969, 44971

04/22/13

04/22/13

29.8

39.0

Relative percent difference3 27

Laboratory 

split

2

Li–38–1159

1

44977, 44979

04/22/13

04/22/13

9.3

22.1

Relative percent difference3 81

Laboratory 

split

2

Li–36–1459 1

44967

04/22/13

04/22/13

5.7

8.5

Relative percent difference3 39

1

Unfiltered sample analyzed at private lab. 

2

Unfiltered sample analyzed at U.S. Geological Survey National Water 

Quality Laboratory.

3

Computed using average of the two replicate samples analyzed by the 

private lab.

Mary Ann Thomas—Arsenic in Groundwater of Licking County, Ohio, 2012—Occurrence and Relation to Hydrogeology—Scientific Investigations Report 2015–5148

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155148