Applied Geochemistry 60 (2015) 29–36

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Applied Geochemistry

journal homepage: www.elsevier .com/ locate/apgeochem

Iodine as a sensitive tracer for detecting influence of organic-rich shalein shallow groundwater

http://dx.doi.org/10.1016/j.apgeochem.2014.10.0190883-2927/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: zunlilu@syr.edu (Z. Lu).

Zunli Lu a,⇑, Sunshyne T. Hummel a, Laura K. Lautz a, Gregory D. Hoke a, Xiaoli Zhou a, James Leone b,Donald I. Siegel a

a Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, USAb Reservoir Characterization Group, New York State Museum, Albany, NY 12230, USA

a r t i c l e i n f o

Article history:Available online 28 November 2014

a b s t r a c t

Public and regulatory agencies are concerned over the potential for drinking water contamination relatedto high-volume hydraulic fracturing (hydrofracking) of the Marcellus shale in Pennsylvania and in NewYork State (NYS), where exploitation of Marcellus gas has not yet begun. Unique natural tracers arehelpful for distinguishing the influence of formation water and/or flow-back water. Here we use halogenconcentrations, particularly bromine and iodine, to characterize natural variability of baseline waterchemistry in the southern tier of NYS. Majority of streams and drinking water wells have Br and I con-centrations below 1 and 0.1 lM, respectively, a range typical for relatively pristine surface water andshallow groundwater. Wells that have higher Br and I concentrations are likely affected by formationwaters. Br/I ratios indicate two different sources of formation waters in these wells, possibly controlledby geologic settings. Our results suggest that iodine, combined with other halogens, may be a novel andsensitive tool for fingerprinting trace levels of formation water signal in drinking water sources.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Shale gas extraction from the Marcellus formation in the Appa-lachian Basin has increased with technological improvements inhorizontal drilling and hydraulic fracturing (Kerr, 2010; Soeder,2010). The increase in hydraulic fracturing has led to environmen-tal concerns regarding potential shallow aquifer contamination.Primary concerns are contamination from hydraulic fracturing flu-ids, and also from produced brines during their transport, drilling,and disposal (Dresel and Rose, 2010; Rowan et al., 2011), includingstray gas migration to shallow aquifers (Osborn et al., 2011).

Hydrocarbon bearing formations in sedimentary basins typi-cally contain water with high salinity, which can exceed 100 g/L,due to seawater evaporation and halite precipitation during thebasin’s evolution (Dellwig and Evans, 1969; Dollar et al., 1991;Hanor, 1994). With compaction, residual brines disperse into otherstratigraphic layers of the sedimentary basin as it evolves throughgeologic time. Hydraulic fracturing of organic-rich shale produceswaters, called flow-back, with high salinity and high concentra-tions of dissolved solids, trace metals, and organic compounds.

Chloride and bromine have been used to trace groundwater andsurface water flow for decades and distinguish different sources of

salinity, such as road salt and septic effluent in streams andgroundwater. For example, chloride and bromine were used to dis-tinguish contamination of drinking water from oil-field brines,halite dissolution, and other sources (Townsend and Whittemore,2005). Other studies show that Cl/Br ratios can effectively distin-guish saline water coming from road salt, halite dissolution brines,and household water softeners (Alley et al., 2011; Davis et al.,2004, 1998; Knuth et al., 1990; Panno, 2006; Walter et al., 1990).Halogen ratios are potentially effective for distinguishing forma-tion water from other sources of salinity because halite precipitateexcludes bromine from its crystal structure, causing the remainingformation waters to have low Cl/Br ratios compared to the seawa-ter they originated from (Herrmann, 1972).

Fresh water overlies saline water in many sedimentary basins(Feth, 1970). In particular, water wells in valleys, where groundwa-ter discharges from local to regional scale flow systems, candevelop naturally high salinity because of their close proximityto the saltwater–freshwater interface (Panno, 2006; Warneret al., 2012). This saline groundwater can have about the sameCl/Br value as deeper groundwater in formations that commer-cially produce gas and oil (Haluszczak et al., 2013). For example,bromine and chloride could not distinguish between AppalachianBasin brines as a whole from Marcellus formation or older brines(Warner et al., 2012). Another non-reactive solute, in addition tobromine and chloride, needs to be considered that can potentially

30 Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36

distinguish and ‘‘fingerprint’’ brines from deep gas producing for-mations from those in shallow gas producing formations.

Iodine has been used to trace water movement in methane-richsettings, because pore water iodine and methane mostly derivefrom organic matter (Fehn et al., 2000; Lu et al., 2008a,b, 2011;Martin et al., 1993; Moran et al., 1995; Muramatsu et al., 2007;Scholz et al., 2010). Organic-rich rocks and associated pore watersin marine sedimentary basins have elevated iodine concentrationsand low Br/I ratios (Worden, 1996). Therefore iodine is apotentially sensitive tracer for sedimentary basin brines where itis associated with geologic formations enriched in organic matterdeposited in marine environments, such as the Marcellus shale.

In this paper, we present the results of an initial geochemicalbackground study and show the utility of iodine as a tracer for flu-ids potentially associated with the Marcellus formation of theAppalachian Basin of the eastern United States. Iodine concentra-tions, in conjunction with other halogens and isotopic systems,may be able to distinguish the presence of deep fluids that migrateupwards into shallow groundwater or spilled on the land surfacefrom other sources of salinity.

1.1. Iodine geochemistry

Similar to other halogens, iodine is a conservative solute andiodine concentrations in freshwater are lower than those in themarine environment (Fig. 1) because iodine is virtually absent fromterrestrial igneous rocks (Fig. 1a). Iodine concentrations in shallowground water and surface waters are generally <0.05 lM (Pannoet al., 2006), whereas the average iodine concentration in seawateris ten times higher, about 0.5 lM (GERM, 2012). Some rare con-taminated surface water and shallow ground water have iodineconcentrations of �10 lM derived from municipal landfill leachateincorporating disinfectants, medical products, and animal feeds/additives (Panno et al., 2006). Iodized salt is a potential anthropo-genic source of iodine; however, elevated iodine concentrations inground water due to such contamination have not been reported,to our knowledge.

Marine origin organic-rich sedimentary rocks and halite are thetwo major natural sources of iodine in the terrestrial environment,and these have distinctive Br/I ratios (Fig. 1a). Iodine is a biophillicelement, strongly enriched in marine organic matter, such as kelp,and consequently accumulates in marine sediments (Elderfield and

I Br Cl

Marine organicma�ers

Marine sediments

Salt Terrestrial sediments

Igneousrock

10-3

10-2

10-1

100

101

102

103

104a

Fig. 1. Comparison of halogen concentrations in solid materials and fluids (Bottomley e1995; Muramatsu et al., 2007; Muramatsu and Wedepohl, 1998; Osborn et al., 2012; Pa

Truesdale, 1980; Muramatsu and Wedepohl, 1998). When marineorganic matter is buried, microbial/thermal decomposition pro-duces methane and releases iodine. This process leaches out iodinefrom the sediments, and concentrates it in interstitial fluids, alongwith methane. These methane-rich and iodine-rich fluids escape tothe surface, in modern seeps/gas fields, or get trapped within sed-imentary strata which are now located in terrestrial settings. Highconcentrations of iodine in groundwater occur in oil/gas fieldbrines (Moran et al., 1995). Iodine concentrations are enriched inthe Appalachian Basin brines (Fig. 1b), similar to those found inother oil/gas fields (Bottomley et al., 2002; Dresel and Rose,2010; Moran et al., 1995; Osborn et al., 2012).

1.2. Geologic settings

The Appalachian Basin formed as elongated foreland basin westof the ancestral Appalachian Mountains with relativelyun-deformed sedimentary rocks deposited throughout the Paleo-zoic era (Colton, 1970). Repeating sequences of carbonates, shales,siltstones, and sandstones occur through the Paleozoic sectionreflecting several marine transgressions and regressions. The blackshales present throughout the Appalachian Basin contain highlevels of organic matter (Roen, 1983).

A stratigraphic cross-section for the southern tier NYS is shownin Fig. 2. The lithofacies most relevant for halogens are the shalesand evaporites of Ordovician to Devonian ages. The Marcellussub-group is Middle Devonian organic-rich black shale of the Ham-ilton Group. The remainder of the Hamilton group consists of int-erbeded shale, mudstone, sandstone, and limestone, depositedduring several cycles of sea-level and redox changes (Sagemanet al., 2003; Straeten et al., 2011). The Geneseo, Middlesex andRhinestreet are additional black shale formations in the southernNYS (Fig. 2). The Steuben County overlaps with all of these shaleformations, while other counties towards the east overlaps mainlywith the Marcellus and Geneseo. The Middlesex and Rhinestreetare both black shale formations of the Frasnian age (Rickard,1975). No euxinic condition was suggested for the deposition ofGeneseo formation which has lower TOC than the Marcellus and(Sageman et al., 2003). The Upper Silurian Salina group consistsof inter-bedded shale, dolomites, and salt deposits (Fig. 2). UticaShale is an organic-rich, thermally-mature, black to gray shale,

Oil/gas field

brines

Appala.brines

Seawater

Groundwater

Surfacewater

Contam-inated shallow

water

10-3

10-2

10-1

100

101

102

103

104

105

106

107

μM

/l)

b

t al., 2002; Davis et al., 1998; Dresel and Rose, 2010; Lu et al., 2008a; Moran et al.,nno et al., 2006; Warner et al., 2012; Worden, 1996).

L O D IG E NE S E O

T UL L Y

MO T T E VIL L E _MB RS T AF F OR D

MAR C E L L USO AT K A_C R E E K

C HE R R Y _VAL L E YUNIO N_S P R ING S

O NO NDAG A

THR UWAY _D IS C ONF OR MITY

DOLGEVILLE

TR E NTON

Present day Sea Level

3000 Meters Below Present day Sea Level

Silurian Salt of the Salina Group Interbedded with Shales

Steuben County Broome CountyChemung County Tioga County

Trenton LimestoneU

Queenston Group

Clinton Group

Lockport Limetone

Grey Hamilton Group and Marcellus Subgroup some black shale and limestone beds

Rhinestreet Shale

Middlesex Shale Geneseo Shale

Marcellus Shale

Medina Sandstone

nainoveD

nair uli Snai ci vodr

O

Tully Limestone

Onondaga Limestone

W E

Sandstone and Shales

Fig. 2. Simplified geologic cross section across southern tier NYS.

Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36 31

derived from the erosion of the Taconic Mountains during theOrdovician, overlying the Trenton Group strata (SGEIS, 2011).

2. Methods

2.1. Sampling strategy and protocol

We completed a systematic sampling of landowner drinkingwater wells and streams from four counties located in the southerntier of NYS (Fig. 3). Waters from 60 drinking water wells and 20headwater streams were sampled. An aerial grid of 49 km2 wasplaced over the counties of interest (Steuben, Tioga, Broome, andChemung) with the explicit goal of collecting one sample withineach grid cell to optimize uniform spatial coverage. The NYS waterwell contractor program database, containing data on all water

Fig. 3. Sampling locations for stream waters and drinking water wells. NYS formationinterval. Marcellus shale rock samples were collected from a nearby core in Yates Coun

wells drilled since April 2000 in a GIS format, was combined withGIS-based county cadastral data to identify potential sampling tar-gets. We targeted deep and maximum penetrating wells withineach grid cell. Wells sampled for this study range in depth from20 m to 122 m. In addition to shallow groundwater samples, sur-face water samples were collected from headwater streams inthe counties of interest with upstream drainage areas between50 and 100 km2.

Standard quality control procedures were followed for fieldmeasurements and sampling. All well waters were collectedupstream of water treatment equipment at either outdoor spigotsor at the water pressure tank. Before sampling, water was purgedfrom the well until the temperature stabilized. Field blanks werealso taken daily. Water samples were collected in prewashed HDPEbottles that were triple-rinsed with sample and filled until therewas no headspace. Water samples were filtered with a 0.45-lm

water wells from Osborn et al. (2012). D60 is the deep well drilled into Marcellusty. The symbols for Type I to III waters are assigned based on Br/I ratios in Fig. 5.

32 Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36

nylon filter and split into aliquots for various analyses. All sampleswere stored at 4 �C before analyses.

2.2. Analytical methods

Chloride concentrations were measured at Syracuse Universityvia ion chromatography (IC) using a Dionex ICS-2000 with columnAS18 for anions. The IC was calibrated with five internal laboratorystandards and the calibration quality was validated using threeexternal United States Geological Survey Standard Reference Sam-ples (N103, M178, and P50, http://bqs.usgs.gov/srs/). Percent errorfor chloride is less than 3%. Total dissolved iodine and bromineconcentrations were measured on a Bruker Aurora M90 ICP-MSat Syracuse University. Fresh iodine calibration standards wereprepared before the measurements. Ultrapure potassium iodidepowders were weighed on a microgram balance and diluted in atertiary amine matrix with cesium as internal standards. Blankswere monitored every three samples and calibration standardswere run every six samples. Instrumental error for iodine and bro-mine concentrations are typically below one percent and are notreported individually.

To better constrain the source rock halogen composition, twoshale samples were collected from the Marcellus interval in theMorton Salt core drilled near our study area (Fig. 3). Iodine andbromine associated with the organic matter were extracted withan alkaline solution from well-homogenized shale powder. Thesolutions were then diluted for concentration measurements onICP-MS.

Spatial patterns in iodine concentrations and ratios were com-pared to the geologic variability of the source rocks using GIS.Available GIS resources included the Finger Lakes sheet of the1:250,000 geologic map of NYS (NYS Museum, 1999). Contour lineswere derived from 10 m resolution National Elevation Dataset data(http://ned.usgs.gov) and isopach maps for the Marcellus shale.Contour lines for total organic carbon (TOC), depth, thickness,and thermal maturity of Marcellus shale were obtained from theNYS Museum and overlaid over the study area. The values TOC,depth, thickness, and thermal maturity at water wells were esti-mated by measuring the distance from well to nearest contour line.

3. Results

Histograms for iodine and bromine in all of the samples areshown in Fig. 4. Samples with concentrations <1 lM for bromineand <0.1 lM of iodine are considered to represent relatively pris-tine surface water and shallow groundwater in the region. 35 out

0.0 0.1 0.2 0.3 0.4 0.50

5

10

15

20

Coun

ts

I (µM)

a

Fig. 4. Histograms for iodine and bromine concentrations in all samples of this

of 56 drinking water wells fall in this category and are groupedas Type I water regardless of their geographic locations (Fig. 5a).Rest of the wells have elevated concentrations and are groupedinto Type II and III waters with distinct Br/I ratios. All streamwaters have bromine concentrations below 1 lM, except for twosamples (Fig. 5a). Iodine concentrations in stream waters rangebetween 0.01 and 0.04 lM, a range slightly higher than iodine con-centrations reported in an earlier study in western NY (Rao andFehn, 1999).

From our groundwater analyses, two separate linear relation-ships in bromine versus iodine are apparent in the samples withelevated iodine and bromine concentrations (Fig. 5a). These twolinear trends broadly correspond to two geographic regions. Nineof the 10 wells located in Steuben County fall on a line definedby higher Br/I ratios (17.5–26.9), which we classify as Type IIwaters. Nine of 11 wells located throughout the remaining threecounties, Chemung, Tioga, and Broome, form the other linear rela-tionship, which is defined by lower Br/I ratios of 1.7 to 6.9 whichwe classify as Type III waters. In three wells, Br/I ratios differ fromthat found in their geographic grouping (Figs. 3 and 5a). Linearregressions defined Type II and Type III water average Br/I ratios(Fig. 5a) as 19.3 and 5.6, respectively.

4. Discussion

4.1. Mixing endmembers and trends

To set up the context for interpreting the significance of Type IIand III water signatures, it is necessary to examine the potentialendmembers in different mixing scenarios. The sources/endmem-bers for halogens include halite, formation waters and organic richshale (Fig. 5a). Due to water–rock interaction, the halogen compo-sitions in formation waters can be affected by halite dissolutionand leaching from shale, thus the compositions may vary verticallyand horizontally within a large basin.

Only a few deep wells in the New York State have been reportedfor their bromine and iodine concentrations. The Br/I ratios arebetween 17.5 and 49, in formation waters sampled from Middleto Upper Devonian formations, including the Marcellus shale(Osborn et al., 2012). Halogen extractions done on Marcellus shalein this study (Fig. 5a) had an average Br/I ratio of 0.59, much lowerthan the ratio in formation waters. Salt deposits are common in theAppalachian Basin (i.e. Salina Formation; Fig. 2) due to evaporationof seawater (Hanor, 1994). Halite has low iodine concentrations(0.1–10 mg/kg) compared to bromine (12.6–1584.9 mg/kg), and ahigh Br/I ratio (Schijf, 2007). Water chemistry data from the Salina

Coun

ts

0

5

10

15

Br (µM)

b

0 1 2 3 4 5 6

study. Majority of the values are below 1 lM for Br and below 0.1 lM for I.

Cl (µ

M)

Cl (µ

M)

Br (µM)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 0.1 0.2 0.3 0.4 0.5 0.6

Br (µ

M)

I (µM)

Type I

Streams

Hal

ite

y = 1

9.32

x -0.

08

R² =

0.8

9

Upp

er D

evon

ian

(D14

)

Mar

cellu

s form

atio

n(D60

)

Known NY Formation Waters

0.05%

0.01%

0.02%

0.03%

y = 5.57x - 0.28

R² = 0.88

Marcellus Shale extractions

Type II

Type III

Stronger halite

signature

Stronger organic

signature

0

2000

4000

6000

8000

10000

12000

14000

I (µM)

0

2000

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0 0.1 0.2 0.3 0.4 0.5 0.6

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Upper Devonian (D14)

Marcellus water (D60)

Hal

iteH

alite

Upper and Middle

Devonian

a b

c

Fig. 5. Bivariate plots for halogen concentrations. All of the samples are grouped into stream waters, Type I, II and III drinking waters, based on the Br, I concentrations andratios. To compared with these groups, calculated mixing trends are shown for halite dissolution, Appalachian Basin formation waters in NYS (shaded region) (Osborn et al.,2012), Marcellus flowback water from PA, and halogen extractions from Marcellus shale rock sample. Two NYS formation waters wells with highest and lowest Br/I ratios aremarked D14 and D60 (Osborn et al., 2012). D60 represents the Marcellus formation water and the ticks on this mixing trend mark the percentages of formation water in thetwo-endmember mixing scenario.

Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36 33

Formation, as major halite bed of Silurian age (Dresel and Rose,2010), was used to derive the halite dissolution mixing line. Allof the formation waters have Br/I ratios between the halite andshale extractions, indicating that the halogens in the formationwaters were ultimately derived from halite, organic matters andalso the seawater. Stronger organic signature would result in lowerBr/I ratios in formation waters, because iodine is highly enriched inmarine organic matters (Fig. 5a). Stronger halite signature wouldincrease the Br/I ratios since halite is enriched in bromine rela-tively to iodine.

Among the four groups of water samples (Type I, II and III wellwaters and streams), the stream waters are more influenced by thehalite end-member, relative to pristine shallow groundwaters,Type I, (Fig. 5a), but the stream waters do not show perfect halitedissolution mixing. The stream water halogen system is likelyinfluenced by residual road salt applied during winter. Both TypeII and III water groups have Br/I ratios between that of halite andshale extractions (Fig. 5a). Type III waters have a stronger organicsignature compared to Type II, and Type II waters have Br/I ratiossimilar to the NYS formation waters (Fig. 5a).

The I/Cl ratios in Type II and Type III water also indicate thathalite dissolution cannot be the sole source of elevated iodine leveland are more consistent with the I/Cl ratios in formation waters(Fig. 5b). Compared to Type I groundwater samples, stream watershave I/Cl ratios more similar to halite dissolution, rather thanformation waters. A plot of Cl versus Br does not contradict theobservations made in the Br–Cl and I–Cl plots (Fig. 5c).

4.2. Formation waters

To potentially pinpoint the source of excess halogens in Type IIand III shallow groundwater, it is important to know the formationwater compositions at various depths and also the spatial hetero-geneity of groundwater chemistry in the same geologic formationacross the study area. Iodine historically has not been used as anatural tracer in deep groundwater in NY State. Only one studyreported iodine and bromine concentrations in formation waters

from several deep wells in New York State (Osborn et al., 2012).Mixing between these formation waters and freshwater wouldresult in mixing relationships bracketing the Type II group, warrantmore detailed discussions. Type III group has a significantly lowerBr/I ratios and cannot be explained by these known formationwater compositions (Fig. 5a).

One of the formation water wells (Osborn et al., 2012), D60 wasdrilled into the Marcellus shale and is located �15–45 km from theType II group samples in Steuben County (Fig. 3). The calculatedmixing line between D60 and freshwater is nearly identical tothe linear relationship shown for Type II group (Fig. 4a). It is possi-ble that the elevated bromine and iodine in the Type II waters areinfluenced by Marcellus formation waters. Assuming Marcelluswater as the source for this trend, a two-component mixing modelindicates that only a trace amount of formation water (<0.05%) hasentered the shallow groundwater aquifer in this area. However, itis not clear whether formation waters associated with theRhinestreet, Middlesex and Utica shale (Fig. 2) may have Br/I ratiossimilar to that of the Marcellus.

There are noticeable depth and spatial patterns of halogenratios in the formation waters sampled within the New York State(Osborn et al., 2012). Formation waters appear to have decreasingBr/I and Br/Cl and increasing I/Cl with depth (Fig. 6). Type II shal-low groundwater has average Br/I and Br/Cl ratios similar to D60,the deep well sampled Marcellus formation water. The Type IIIgroup has lower Br/I (4.0 ± 1.7) and Br/Cl ratios (1.2 ± 1.7), buthigher I/Cl (0.50 ± 0.89) ratios than Type II and any of the knownNYS formation waters. These differences suggest Type III samplesmay have solute contributions from formation waters deeper thanthe Marcellus shale (Fig. 6).

On the other hand, the Br/I ratios in formation water decreasefrom west to east across NY State (Fig. 7); D60 is the deepest andeasternmost sample of NY formation waters with reported iodinevalues. It is located geographically close to the Type II group ofsamples (Steuben County) and west of the Type III group. So thecurrent knowledge about formation water halogen compositionsdo not allow us to argue whether the Type III group is affected

0

100

200

300

400

500

600

700

800

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1000

)m(

htpeD

I/Cl (x 103 μM/μM)

Type III0.50±0.89Type II

0

100

200

300

400

500

600

700

800

900

1000

)m(

htpeD

Br/Cl (x 103 μM/μM)

1.2± 1.7Type II

D10

D14

D6

D17

D60

D10

D14

D6

D17

D60

Type III

Deeper source

lower B

r/I?

Deeper source

higher I/Cl?

Deepe

r sou

rce

low

er B

r/Cl

?

0

100

200

300

400

500

600

700

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1000

0.05 0.1 0.15 0.2 0.25 0.3

2.0 3.0 4.0 5.0

10.0 20.0 30.0 40.0 50.0 60.0

)m(

htpeD

Br/I (μM/μM)

4.0±1.7Type III

D10

D14

D6

D17

D60

Type II a

b

c

Fig. 6. Depth profiles of halogen ratios in NYS formation waters (Osborn et al.,2012). Type II groundwaters are plotted with the average well depths versus theaverage and standard deviation of its Br/I. Type III waters, marked by arrows andnumbers, have ratios beyond the range of know NYS formation waters.

10

15

20

25

30

35

40

45

50

55

-79.5 -79 -78.5 -78 -77.5 -77 -76.5

Br/I

(µM

/µM

)

Longitude

Type II

Eastern source

lower Br/I?

Type III

Fig. 7. West-east trend in Br/I in formation waters. Symbols as Fig. 6.

34 Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36

by a deeper source or an eastern source, compared to the Type IIgroup.

4.3. Potential geological controls

The Br/I ratios indicate stronger organic signature in Type III,relative to Type II (Fig. 5a). Such a hypothesis can be tested byexamining the spatial difference in potential source formations,like thickness of source rock, total organic carbon (TOC) and ther-mal maturity of the source rock (Fig. 8). Here we use the Marcellusas an example to demonstrate the approach, although we fullyacknowledge the possibility that shale other than the Marcellusmay be the source of excess halogens in Type II and III waters.

The thickness of the Marcellus shale increases from approxi-mately 50 feet in the west to greater than 550 feet in the east(Fig. 8a), although organic rich part of the Marcellus shale in theeast only thickens to about 150 ft. Nine of the ten wells in the TypeII group are located where the thickness ranges between 50 and100 feet in Steuben County. The thickness of the Marcellus shaleunderlying the Type III water group varies from 50 to over 550 feet.Nine of the eleven wells identified as Type III water are locatedwhere the Marcellus thickness is greater than 100 feet in Chemung,Tioga, and Broome Counties.

The TOC of the Marcellus is highest in western Chemung/East-ern Steuben counties and decreases to the east (Fig. 8b). Wemultiplied the thickness of the shale by TOC as a rough index forthe size of sedimentary organic halogen pool which potentiallycan influence the Type II and III waters. Type III waters with lowerBr/I ratios appear to be associated with a larger organic halogenpool (Fig. 9). There is a greater chance for a larger organic sourceto release more iodine into the overlaying groundwaters, asobserved in the Type III waters (Fig. 5a). In addition, the thermalmaturity of the Marcellus shale is highest in the eastern sectionof the study area (Fig. 8c).

It is plausible that other organic-rich shale formations may havesimilar spatial patterns as the Marcellus. To further test whetherMarcellus formation water may be responsible for the Type III sig-nature, we obtained Marcellus flow-back waters (Chapman et al.,2012) from Bradford County, PA, which borders Broome County,NY. These flow-back waters have Br/I ratios (23–27), similar to thatof the Marcellus formation water in NYS (Fig. 5a), much higherthan the ratios in Type III group. If the Br/I ratios in these PAflow-back waters are representative for the Marcellus formationwater in Broom County, then the Type III signature would probablyindicate a source of halogen other than the Marcellus. However it isunclear whether the Br/I ratios in flow-back water were affected bythe initial compositions in hydraulic fracturing fluids and by thefracturing process. Currently, the Type III signature cannot be con-clusively tied to any specific shale formation.

4.4. Pros/cons for iodine as a groundwater tracer and future work

This study demonstrates the potential of iodine being an impor-tant natural tracer in groundwater for the influence of organic richformations, particularly relevant for the debate about hydraulicfracturing. Iodine and bromine concentrations can be rapidlyanalyzed on ICP-MS at very reasonable cost, suitable for large scalehydro-geochemical studies. Because iodine is the most biophilichalogen, Br/I ratios have unique advantage tracking the influenceof marine source rocks, compared to Br/Cl ratios. For example,the Br/I ratios in Type II group has a standard deviation

Fig. 8. Sampling locations of Type II and Type III waters are shown on contour mapsfor Marcellus shale (a) Thickness, (b) total organic carbon, and (c) thermal maturity.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 50 100 150 200 250 300

Br/I

(µM

/µM

)

TOCxThickness

Type II

Type III

Fig. 9. Br/I plotted against TOC � shale thickness for Type II and Type III waters.TOC and thickness values were estimated from Fig. 8.

Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36 35

significantly smaller than those of Br/Cl and I/Cl ratios, whichmakes Br/I ratio a better forensic tool to potentially pinpoint thesource (Fig. 6). The large scatters in Br/Cl and I/Cl ratios are prob-ably due to additional sources of salinity in shallow groundwaters.Iodine, as a tracer of salinity, may be powerful when the sourcesalso carry different organic signatures. However, such a power

needs to be further tested when the salinity derives from differentinorganic sources (e.g. halite formations of various ages).

Correct interpretations of iodine data in shallow groundwatercannot be achieved without comprehensive knowledge about thedeep formation waters. The available iodine measurements in for-mation waters do not cover multiple shale units in the New YorkState and do not allow any estimate of spatial variability in thesame unit. Once such a data set becomes available, the Br/I ratiosmay be a key tracer to definitively pinpoint to a specific sourceformation.

Currently, there is not sufficient information to discuss thehydrological mechanisms for the migration of excess halogens intoshallow drinking water wells. Possible scenarios include, but notlimit to: (1) slow diffusion of halogen (possibly methane) fromdeep formations; (2) preferential conduits such as naturallyexisting faults and fracture zones introducing trace amount of deepformation waters into shallow aquifers; (3) groundwater rechargepicking up halogens from uplifted shale units. Denser samplingacross the southern tier in future work will allow us furtherevaluate these hypothesis.

5. Conclussions

Distinct iodine and bromine concentrations and ratios occur inshallow groundwater wells located in southern NYS. According tothe available information about formation waters, elevated iodineand bromine concentrations in Steuben County may be related totrace quantity of solutes derived from Marcellus formation water(Type II water). In Chemung, Tioga, and Broome Counties (TypeIII water), the source for excess halogen probably is not the Marcel-lus shale. Varying shale thickness, TOC, and thermal maturity ofthe source rock across southern NYS may cause the differences inBr/I ratios between Type II and III groups. Further study of moreformation waters from greater depth range and spatial coveragemay allow iodine to be used as a sensitive tracer for quantifyingdegrees and sources of groundwater contamination associate withorganic-rich shale.

Acknowledgements

Funding for this project was provided by a seed grant from Syr-acuse University and from the National Science Foundation (EAR1313522). We thank all the volunteer homeowners for allowingus to sample their well water. Egan Waggoner was instrumentalin organizing the sampling campaign and Natalie Teale and MaxGade helped collect water samples during the summer of 2012.

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