United States

Department of 

Agriculture

Forest Service

Northern 

Research Station

Research Paper-NRS-14

Chloride Concentration 

Gradients in Tank-Stored 

Hydraulic Fracturing 

Fluids Following Flowback

Pamela J. Edwards

Linda L. Tracy

William K. Wilson
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Abstract

A natural gas well in West Virginia was hydraulically fractured and the fl owback was 

recovered and stored in an 18-foot-deep tank. Both in situ fi eld test kit and laboratory 

measurements of electrical conductivity and chloride concentrations increased substantially 

with depth, although the laboratory measurements showed a greater increase. The fi eld 

test kit also underestimated chloride concentrations in prepared standards when they 

exceeded 8,000 mg L-1, indicating that laboratory analyses or other more accurate methods 

of detection should be used to determine chloride concentrations in fl owback when they may 

be approaching West Virginia regulatory levels (12,500 mg L-1) that disallow disposal by land 

application. The gradation of chloride with depth also has implications for procedures used 

to collect fl owback samples from reserve pits or tanks before disposal to ensure the resulting 

composite chloride concentration is representative of the total volume.

Cover Photo

William Wilson uses a fi eld kit to determine the chloride concentrations of a fl owback sample. 

Photo by Pamela Edwards, U.S. Forest Service.

The use of trade, fi rm, or corporation names in this paper is for the information and 

convenience of the reader. Such use does not constitute an offi cial endorsement or 

approval by the U.S. Department of Agriculture or the Forest Service of any product or 

service to the exclusion of others that may be suitable.

Manuscript received for publication 16 November 2010
1

INTRODUCTION

Production of natural gas generally involves a process known as 

hydraulic fracturing, which is more commonly called hydrofracing or 

fracing (sometimes spelled fracking). As its name implies, hydraulic 

fracturing creates fractures that extend from the well bore hole out into 

adjacent geology; these fractures then act as conduits through which 

the natural gas held in the geologic formation can fl ow back to the well. 

Fracing typically is accomplished by injecting a mixture of acids, water, 

gases, and additives under high pressure through the bore hole (Agbaji 

et al. 2009). Nitrogen and carbon dioxide, typically in their liquid 

phase (Agbaji et al. 2009), have become a relatively recent component 

of fracing fl uids (Reidenbach et al. 1986) and are used to reduce the 

volume of needed water (Agbaji et al. 2009), which can range from 

several tens of thousands to several millions of gallons (Weston 2008).

Th e exact composition of fracing fl uids depends upon the geologic 

layer to be fractured and the availability of the individual acids, water, 

gases, and additives (Agbaji et al. 2009); however, fracing additives 

fall into a variety of applications, and each application has a specifi c 

purpose during the fracturing process (Table 1). Additives may not 

be needed for every application and therefore would not be included 

in all fracing fl uid formulations (URS 2009). Typically, most of the 

chemicals involved are in low concentrations because large volumes of 

water are used in the fracing process (URS 2009). Proppants, which 

are used to “prop” open fractures and voids to allow gas to fl ow to the 

well, are often present in substantially higher concentrations than the 

other additives in fracing fl uids (Arthur et al. 2009). Sand is the most 

common proppant (GWPC and ALL Consulting 2009).

Once fracing is completed, a portion of the fracing fl uid is recovered 

through a process called fl owback (Sullivan et al. 2004, URS 2009, 

Woodroof et al. 2003a). Th e recovered fl uid (also called fl owback) 

typically is stored temporarily in open pits near the well called reserve 

pits (Deuel et al. 1999, Leuterman et al. 1988) or sumps (French 

1980). Th ese pits also may hold drill cuttings, drilling fl uids, lubricants 

and soaps, rig wash, wastes from casing cement operations, and 

precipitation (Th urber 1992, Veil 2002). In some situations, fl owback 

is stored in tanks (Agbaji et al. 2009), but this is more typical in wells 

that undergo refracing (after the pit has been emptied and reclaimed) 

or in locations that have restrictions on the use of open pits for storing 

drilling-related fl uids.

Th e Authors

PAMELA J. EDWARDS, research 

hydrologist, currently is engaged in research 

on the eff ects of forest management and energy 

development on hydrology and water quality, 

and on the development of new techniques 

and tools for assessing the implementation 

and eff ectiveness of best management practices 

used within forested landscapes. She works at 

the Northern Research Station’s Timber and 

Watershed Laboratory in Parsons, WV.

LINDA L. TRACY, geologist, has managed 

the minerals and geology program on the 

Mongahela National Forest for 33 years. Her 

offi ce is located in Elkins, WV.

WILLIAM K. WILSON, mineral assistant, 

has 19 years of minerals administration 

experience. He currently oversees and 

administers the mineral operations on the 

Monongahela National Forest and works out 

of the Greenbrier Ranger District Offi ce in 

Bartow, WV.
2

Fluids held within the reserve pit are disposed of 

in several ways. Th e more common techniques are 

recycling, onsite treatment, treatment at a publically 

owned treatment plant, treatment at an industrial 

treatment plant, injection into depleted wells or deep safe 

formations through permitted Underground Injection 

Control wells, or land disposal (Nesbit and Sanders 

1981, URS 2009). Recycling is done only on a limited 

basis (GWPC and ALL Consulting 2009) because 

the chemistry of fl owback is not entirely suitable for 

fracing, so fresh additives and water need to be added 

to the mixture before it can be reused (URS 2009). A 

primary reason for recycling is to reduce the amount of 

freshwater needed (GWPC and ALL Consulting 2009, 

URS 2009, Weston 2008). Onsite or off site treatment is 

expensive so both are uncommon, and onsite treatment 

typically occurs only at the largest well sites (Al-Harthy 

and Al-Ajmi 2009, URS 2009). Flowback is generally 

considered industrial wastewater (Gaudlip et al. 2008, 

URS 2009), so treatment is regulated and permitted as 

such. Only a small subset of privately or publically owned 

facilities is equipped to treat fl owback fl uids (URS 2009). 

Underground injection has been the most common 

method of fl owback disposal (Burnett 2008, GWPC 

and ALL Consulting 2009), and it is sometimes used 

in combination with treatment for disposing of residual 

chemicals (URS 2009). Injection has the negative 

consequence of removing the fracing water from the 

hydrologic cycle (Agbaji et al. 2009). Land disposal can 

occur by either burial (French 1980, McFarland et al. 

1994, Nesbit and Sanders 1981) or surface application 

(Bauder et al. 2005, Nelson et al. 1984). Land disposal 

is allowed only in some states. Where allowed, however, 

land disposal may be common because it is economical.

Th ere are environmental implications associated with the 

chemical composition of fracing fl uids and their disposal, 

but research on these issues has been outpaced by 

research and development of extraction techniques. Th e 

proprietary nature of newer fracing chemicals can make it 

diffi cult to fi nd out their exact chemical content (Soeder 

and Kappel 2009). But even when the chemical content 

can be determined from material safety data sheets 

(MSDS), the diff erences in concentrations between the 

raw chemicals and the much more diluted concentrations 

present in fracing fl uids (and thus fl owback) limit the 

Table 1.—Applications and purposes of additives used in hydraulic fracturing for natural gas production 

(Adapted from URS 2009)

Application Purpose

Proppants “Props” open fractures to allow produced gas to fl ow to the well 

Acids Helps create fractures 

Breakers Reduces viscosity of fracing fl uid to release proppant into fractures and to 

improve recovery of fracing fl uids in fl owback

Bactericides Prevents the growth of organisms that could generate gases that would 

contaminate produced natural gas. Also prevents growth of organisms that 

could reduce proppant fl ow into fractures

Clay stabilizers Prevents clay swelling and release and movement of clays associated with 

the formation that could clog pores and fractures

Corrosion inhibitors Used with fracing fl uids that include acids, to reduce oxidation of steel 

tubing, casings, tools, and storage tanks

Iron controls Inhibits the metal oxide precipitation in the formation to keep fractures open

Crosslinking agents Increases the viscosity of the fracing fl uid to allow greater amounts of 

proppants to fl ow into the fractures

Gelling agents Increases the viscosity of the fracing fl uid to allow greater amounts of 

proppants to fl ow into the fractures

Friction reducers Reduces friction to allow more optimal injection of fracing fl uid

Scale inhibitors Prevents precipitation of carbonate and sulfate compounds that could clog 

pores and fractures

Surfactants Reduces the surface tension of fracing fl uids to improve recovery of fl owback 
3

usefulness of the MSDS information for projecting 

biological eff ects (Bishop 2009). Available studies show 

fl uids can impact plant growth negatively because of high 

concentrations of salts or toxic metals (e.g., McFarland 

et al. 1992, 1994; Miller and Pesaran 1980; Miller et al. 

1980; Nelson et al. 1984; Younken and Johnson 1980), 

and there is a broader concern about how reserve pit 

chemicals may aff ect ground water and surface water 

quality (Harrison 1983, Hudak and Blanchard 1997, 

Soeder and Kappel 2009). More information about the 

chemical characteristics and behavior of fracing fl uids 

and fl owback would be valuable for better deciding 

upon treatment or disposal options or for developing 

more thorough science-based recommendations for land 

applications, such as establishing maximum allowable 

loadings for the most concentrated and/or toxic 

chemicals present.

Background of this Study

In 2008, a natural gas well (well B-800) was drilled by 

a private oil and gas lessee on the Fernow Experimental 

Forest in Tucker County, West Virginia. Th is 

experimental forest is part of the Monongahela National 

Forest (MNF) and is administered by the U. S. Forest 

Service, Northern Research Station. Th e well is 7,882 

feet deep and was drilled into the Oriskany Sandstone 

and Huntersville Chert geologic formations. After 

drilling was completed, the bedrock at that depth was 

prepared for release of gas by fracing with a 70,350-

gallon mixture of fracing fl uids (Table 2) injected into 

the well bore hole under high pressure. When fracing was 

completed, fl owback was stored temporarily in an open 

pit just downslope of the well pad that also contained 

drilling fl uids and drilling waste. Pit fl uid was treated 

with activated carbon to bring fl uids into compliance 

with the general water pollution control permit (GPWV-1-88) requirements for land application, and it was 

mixed by circulating fl uid within the pit using pumps. 

Land application of the pit fl uid began after 2 days of 

settling time.

In accordance with West Virginia minerals laws, fracing 

fl uids may be land applied if the Cl concentration of 

the pit-stored fl uid is less than 12,500 mg L-1 (http://

www.wvdep.org/Docs/16150_General%20Water%20

Pollution%20Control%20Permit%20.pdf). Th e Cl 

concentration of the pit fl uid on the Fernow was 

determined using the procedure required by state 

Table 2.—Additives contained in the initial fracing fl uid mixture used May 23, 2008, for gas well B-800a

Product trade name/Additives contained in product Application Concentration

Sand – premium/Crystal silica, quartz (60-100%) Proppant 1,300 lb/1,000 gal

SP Breaker/Sodium persulfate (60-100%) Breaker 0.25 lb/1,000 gal

WLC-6 Fluid Loss Additive/Tallow soap (60-100%) Surfactant 150 lb/100,000 gal

HC-2/Sodium chloride (5-10%) + inner salt of alkyl amines 

(10-30%)

Scale inhibitor 2 gal/1,000 gal

Clayfi x-II Material/Alkylated quaternary chloride (30-60%) Clay stabilizer 1 gal/1,000 gal

GBW-30 Breaker/Hemicellulase enzymes (5-15%) + 

carbohydrates (85%)

Breaker 1 lb/1,000 gal 

BC-140/Monoethanolamine + ethylene glycol + boric acid 

solution (10-30% for each)

Crosslinker 2.5 gal/1,000 gal

OPTIFLO-HTE/Walnut hulls (60-100%) + crystalline silica 

(quartz) (10-30%)

Breaker 0.25 lb/1,000 gal

LGC-35 CBM/Paraffi nic solvent (30-60%) + polysaccharide 

(30-60%) 

Gelling agent 5.75 gal/1,000 gal

BE-3S Bactericide/2-mononbromo-3-nitrilopropionamide 

(1-5%) + 2, 2 dibromo-nitrilopropionamide (60-100%)

Biocide 1 lb/10,000 gal

aFour thousand (4,000) gallons of hydrochloric acid were introduced into the well bore under pressure; mixture of 

approximately 70,350 gallons of water obtained from a local stream, proppant, and fracing chemicals was forced into the 

well bore following the hydrochloric acid.
4

regulations: the well owner’s contractor obtained a 

single composite sample collected by mixing equal 

volumes of six equally distributed grab samples taken 

from various locations in the pit and then determined 

the chloride (Cl) concentration in the fi eld using a 

color-indicator fi eld test kit on the day of, but prior 

to, land application. Th ere is no formal explanation 

in the state regulations of what is meant by six equally 

distributed samples from various locations in the pit, 

but the individual who collected the samples stated they 

were collected from around the edge of the pit, just 

below the layer of fl oating scum (pers. comm., Charles 

Krushansky, Action Environmental, Jan. 26, 2010). Th e 

fi eld test kit Cl concentration of that composite sample 

was approximately 7,500 mg L-1, so land application 

proceeded.

Th e fl uids were pumped through a hose and hand 

sprayed onto two previously approved areas. During 

the 12-day application period in June 2008, additional 

fracing fl uid samples were taken at the hose nozzle by 

Forest Service personnel. Several of these samples were 

analyzed for Cl by ion chromatography at the Forest 

Service’s Timber and Watershed Laboratory, using EPAapproved procedures (Edwards and Wood 1993).

Th e results from these samples indicated that Cl 

concentrations applied to the land varied temporally, 

ranging from 10,200 to 14,200 mg L-1. Consequently, 

there was speculation that the Cl concentrations in the 

pit fl uids may have become spatially nonhomogeneous 

between treatment and mixing and the start of land 

application 3 days later. Th erefore, well refracing in 

October 2009 to improve gas production was seen as an 

opportunity to examine Cl characteristics of fl owback 

during short-term storage. Well refracing was done in 

the Sycamore Grit Formation in the hopes of reaching 

more gas with less associated water. Th is formation lies 

above the Oriskany Sandstone and Huntersville Chert 

Formations that were originally fraced, so the well was 

plugged beneath the Sycamore Grit to better confi ne 

fracing within that formation.

Th is paper describes the Cl results associated with 

fl owback obtained from the Sycamore Grit Formation.

METHODS

Refracing and Recovery Processes

Th e 2009 fracing process diff ered from the original 

operation in 2008. First, 500 gallons of hydrochloric acid 

(30-60%) were injected into the well under pressure on 

September 27, 2009, several days before the other fracing 

fl uids were injected. An unknown percentage of this acid 

was recovered into the well’s production tank (not the 

tank where fl owback was collected) and then transported 

off site.

On October 7, 2009, refracing was accomplished using 

a chemical formulation that involved a relatively high 

percentage of foam to reduce the volume of water needed 

(pers. comm., David Berry, October 7, 2009). Th e 

refracing process began by introducing approximately 

840 gallons of fresh water into the well bore. Th is was 

followed by a foam mixture composed of approximately 

1,000 gallons of water and 1,197,110 standard ft3 of 

liquid nitrogen. Th e remaining fracturing additives were 

then combined using the following mixture and injected 

under pressure into the well bore:

• 14,406 gal of water, 242 lb WG-35 gelling 

agent, and 28.8 gal BC-140 crosslinker

• 6,429 gal of water and 108 lb WG-35 gelling 

agent

• 750,000 lb of premium sand

• 20 lb of GBW-30 breaker

• 3 lb of BE-3S bactericide

• 7 gal of LOSURF-300M surfactant

• 31 gal of Clayfi x-II material organic salt additive

• 12 gal of GasPerm 1000 surfactant

• 3 gal of HC-2 salt additive

Another approximately 4,200 gallons of fresh water with 

no chemical additives were introduced into the well bore 

to push the fracing fl uids into the formation for hydraulic 

fracturing. Based on the total volume of water used 

(~26,875 gallons), the fi nal concentration of each of the 

fracturing additives is given in Table 3.
5

Approximately 11,000 gallons of fracing fl uid were 

recovered during fl owback, which were stored in two 

18-foot-tall metal tanks brought to the site for short-term 

storage (Fig. 1). One tank was fi lled and the second held 

the remaining ~500 gallons. Initially, the well owner’s 

contractor intended to land apply this fl uid (excluding 

the hydrochloric acid) at a site mutually agreed upon 

with the Forest Service. However, because the volume of 

fl uid was relatively small, the contractor transported all of 

the fracing fl uid off site for disposal.

Table 3.—Additives contained in the fracing fl uid mixture used for the October 7, 2009, refracturing of well B-800

Product trade name/Additives contained in product Application Concentration

Sand – premium/Crystalline silica (quartz) (60-100%) Proppant 2,787 lb/1,000 gal

BC-140/Monoethanolamine + boric acid + ethylene glycol (10-30% for each) Crosslinker 1.70 gal/1,000 gal

BE-3S Bactericide/2-mononbromo-3-nitrilopropionamide (1-5%) + 2, 2 

dibromo-nitrilopropionamide (60-100%)

Biocide 1.12 lb/10,000 gal

Clayfi x-II Material/Alkylated quaternary chloride (30-60%) Clay stabilizer 1.15 gal/1,000 gal

GBW-30 Breaker/Hemicellulase enzymes (5-15%) + carbohydrates (85%) Breaker 0.744 lb/1,000 gal

HC-2/Sodium chloride (5-10%) + inner salt of alkyl amines (10-30%) Scale inhibitor 0.112 gal/1,000 gal

WG-35 Gelling Agent/Guar gum (60-100%) Gelling agent 13.02 lb/1,000 gal

LOSURF -300M/1,2,4 Trimethylbenzene (0-1%) + Naphthalene (0-1%) + 

Poly(oxy-1,2-ethanediyl), alpha-(4-nonylphenyl)-omega-hydroxy-, branched 

(5-10%) + heavy aromatic petroleum (10-30%) + ethanol (30-60%)

Surfactant 0.26 gal/1,000 gal

GasPerm 1000/Isopropanol (10-30%) + terpenes and terpenoids of sweet 

orange-oil (10-30%)

Surfactant 0.558 gal/1,000 gal

Figure 1.—The tank on the right is 

the one from which the fl owback was 

sampled following refracing of well 

B-800 on the Fernow Experimental 

Forest. The access inlet is on the top 

of the tank between the top of the 

ladder and the person preparing for 

sampling. The valves for emptying 

or sampling fl uid are located near 

the base of the ladder; there are two 

valves on each of these tanks and 

they are visible on the second tank 

from the right, just above the buckets 

on the ground. Photo by William 

Wilson, U.S. Forest Service.
6

Had land application occurred, the fl uid would have 

been mixed by recirculating it among tanks using a 

pump. Following state regulations, a 10-day waiting 

period would have been required to allow solids to 

settle before the Cl concentration was tested. However, 

in the current situation, the contents of the tanks were 

not mixed because the operator decided against land 

application. Th erefore, the measurements described 

in this report were made on fl uids that received no 

additional mechanical mixing other than what occurred 

during chemical preparation, high pressure injection, and 

fl owback.

Field Methods

Th e current investigation of the Cl concentrations took 

place on October 14, 7 days after the fl uid was pumped 

into the storage tanks. Only the single tank that held the 

~10,500 gallons of fl uid was included in the study.

Access to the tank was through a covered inlet on top 

of the tank (Fig. 1). To obtain some initial immediate 

information about the tank contents, a YSI EcoSense®

EC300 conductivity cell was lowered into the tank and 

conductivity measurements were taken every 2 feet, 

beginning at 0 feet (i.e., just below the fl uid surface). 

Intervals of 2 feet were chosen somewhat arbitrarily, 

but the authors believed this spacing would be suffi cient 

to defi ne the general characteristics and variability of 

the tank’s contents. A weight constructed of 0.75-

inch-diameter x 11.5-inch-long PVC pipe fi lled with 

water was attached to the conductivity cell cable just 

above the cell (Fig. 2) to reduce the buoyancy of the 

conductivity cell in the fracing fl uid. Two-foot intervals 

were delineated on the cable (measured from the 

contact points of the cell) using permanent marker. Th e 

conductivity cell provides temperature-adjusted readings 

and can shift automatically between μS cm-1 and mS 

cm-1 units as needed, depending upon the conductivity 

of the solution.

After the conductivity measurements were taken and 

the cell was removed from the tank, fracing fl uid 

samples were collected from 4-, 8-, 12-, and 16-foot 

depths (measured from the top of the fl uid level) using 

a model 3700 ISCO® automatic water sampler with 25 

feet of uptake tubing with 1-foot intervals marked on 

the tubing. Th e pump motor was disconnected from 

the case for use (Fig. 3). Th e strainer was retained on 

the end of the tubing to provide weight to reduce the 

Figure 2.—The capped PVC pipe 

attached to the conductivity cell cable 

contains water that was used to provide 

extra weight to the cell to keep it from 

becoming buoyant when lowered into 

the fl owback fl uid in the tank. Photo by 

Frederica Wood, U.S. Forest Service.
7

Figure 3.−An ISCO® 3700 sampler removed 

from its housing was used to obtain fl owback 

fl uid samples from the tank at 4-, 8-, 12-, and 

16-foot depths. The strainer (i.e., intake) of the 

sampler is the perforated tube on the left front 

of the photograph. Photo by Frederica Wood, 

U.S. Forest Service.

buoyancy of the tubing (Fig. 3). An approximately 1-L 

sample was collected at each of the four depths using the 

manual setting on the ISCO® sampler. Only one sample 

was collected from each of the four depths because the 

small tank opening and fl exibility of the uptake tubing 

made it diffi cult to take multiple samples from diff erent 

locations at those depths. To avoid carryover between 

samples, after the tubing was lowered to the 8-, 12-, and 

16-foot depths, a volume of sample that exceeded the bore 

volume of the 25 feet of tubing was pumped into a refuse 

container. After all sampling was completed, the fl uid in 

the refuse container was poured back into the storage tank.

A subsample of each of the four samples was analyzed 

in the fi eld for Cl concentration using a model CD-51 

Hach® High Range Chloride Test Kit. Th e analysis is 

colorimetric and uses silver nitrate as the titrant, and 

chloride measurements are estimated in 500-mg L-1

increments for concentrations up to 10,000 mg L-1. Th e 

fi eld measurements were made by the MNF geologist, 

using the same techniques and equipment normally 

used on the MNF for assessing Cl concentrations in 

fracing fl uids before land application. Th is brand of kit 

also is used commonly in West Virginia to make fi eld 

assessments of Cl concentrations in reserve pits.

Laboratory Methods

Th e fracing fl uid samples were returned to the Timber 

and Watershed Laboratory for laboratory determinations 

of electrical conductivity and Cl concentrations. 

Electrical conductivity was measured because there 

was some concern that the fi eld measurements made 

at 0-, 2-, 4-, and 6-foot depths were incorrect because 

the conductivity values were extremely low and the 

readings were unstable (described later). Laboratory 

measurements of electrical conductivity were determined 

using a Radiometer® CDM 83 conductivity meter with 

a platinum cell. Th e conductivity cell was calibrated at a 

single value, 995 μS cm-1.
8

Chloride concentrations were determined in the 

laboratory using a Dionex® DX500 ion chromatograph to 

obtain more accurate readings than the fi eld tests because 

those tests estimate Cl concentrations only to the nearest 

500 mg L-1. Each sample was vacuum fi ltered through 

0.45-μm glass microfi ber fi lters. Two to three hours were 

required to fi lter approximately 50 ml because of the high 

viscosity of the fracing fl uid. Th ree replicate dilutions 

were prepared for each sample using 1:1,000 dilutions 

for the 4- and 8-foot samples and 1:5,000 dilutions 

for the 12- and 16-foot samples. Th e required dilution 

levels needed to obtain results within the instrument’s 

calibration range were determined from the fi eldmeasured results (described later).

Because of the limited amount of data collected in this 

study, analyses are limited to descriptive statistics of 

means, standard deviations, and coeffi cients of variation 

(Sokal and Rohlf 1994). Th ose statistics were calculated 

using Microsoft® Offi ce Excel® 2007 software.

RESULTS

Field conductivities increased markedly with depth 

(Table 4). At depths ≤6 feet, they were very low and 

seemed suspect because of the continuous drift of 

the readings and general inability to obtain a stable 

measurement. Such drift had not occurred during 

previous use with this meter in low ionic strength 

stream water. Th ese shallow readings became even more 

suspect when the deeper values showed much higher 

conductivities (Table 4) that did not drift. Indeed, the 

laboratory-measured conductivity values (Table 4) for 

the 4- and 8-foot depth samples were much greater than 

those obtained in the fi eld. We believe the ≤6-foot-deep 

fi eld-measured values were in error because there was 

poor contact between the fl uid and conductivity cell, 

perhaps due to particles that partially blocked the sensor 

contacts or gas bubbles in the cell that became dislodged 

as the head increased with depth.

Th e fi eld-measured conductivity readings below 6 feet in 

combination with the four laboratory measurements of 

conductivity suggest that the Cl concentrations increased 

to about a depth of 14 feet (Table 4). Deeper than 14 

feet, conductivity varied much less, suggesting that Cl 

concentrations also stabilized. Diff erences in the physical 

appearances of the shallow (4- and 8-foot) and deeper 

(12- and 16-foot) samples also suggested the overall 

chemistry of these samples was quite diff erent. Th e two 

shallower samples were a whitish opaque color, while the 

deeper samples were a darker orange or rust (Fig. 4).

Field and laboratory determinations of Cl confi rm the 

gradient suggested by the conductivity results. Th e 

fi eld-measured Cl concentrations in the 12- and 16-

foot samples were 7,500 and 8,500 mg L-1, respectively, 

which were about 2 to 4 times greater than the 

concentrations at the shallower depth (Table 5). Th e 

concentration gradient was even more striking for the 

laboratory-measured values. Th e average laboratorymeasured Cl concentrations at 12- and 16-foot depths 

were between 11,000 and 13,000 mg L-1 and were 4 to 

10 times greater than the 927 and 3,463 mg L-1 averages 

at 4- and 8-foot depths, respectively. Th e largest increases 

in concentrations for both types of Cl measurements 

occurred between 8 and 12 feet; the concentrations from 

4 to 8 feet increased by about 1,000 to 2,000 mg L-1, 

depending upon whether the analysis was in the fi eld or 

in the laboratory. Th e concentrations from 12- to 16-

foot depths increased by 1,000 mg L-1 for fi eld testing 

while the average decreased by about 1,000 mg L-1 for 

laboratory measurements (Table 5).

Table 4.—Field-measured and laboratory-measured 

electrical conductivities of fracing fl uids

Distance below fl uid 

surface in tank

Electrical conductivity

Field Laboratory

(ft)

0 4.1 μS cm-1 --

2 7.0 μS cm-1 --

4 11.9 μS cm-1 3.490 mS cm-1

6 26.3 μS cm-1 --

8 1.375 mS cm-1 8.770 mS cm-1

10 4.330 mS cm-1 --

12 21.00 mS cm-1 23.20 mS cm-1

14 23.09 mS cm-1 --

16 23.26 mS cm-1 23.70 mS cm-1

18 23.30 mS cm-1 --
9

Not surprisingly, the largest standard deviations for the 

laboratory measurements of Cl were associated with the 

two samples containing the greatest Cl concentrations 

and involving the 1:5,000 dilutions (Table 5). However, 

the largest coeffi cient of variation, 24.8 percent, 

corresponds to the 4-foot-deep sample that had the 

lowest replicate and average Cl concentrations. Th e 

coeffi cient of variation for the 12- and 16-foot-deep 

samples ranged from about 11 to 14 percent and was 

only 2 percent for the 8-foot-deep sample.

Field and laboratory testing yielded similar 

concentrations of Cl for the two shallowest samples. Th e 

4- and 8-foot samples had lower laboratory-measured 

concentrations than obtained by fi eld testing, but the 

8-foot fi eld measurement was essentially equal to all 

three laboratory replicates and the average. By contrast, 

as the Cl concentrations increased, the deviation 

between the fi eld and laboratory measurements increased 

substantially. For 12- and 16-foot-deep samples, the 

laboratory averages were approximately 3,000 to 5,000 

mg L-1 greater than the fi eld values, and the individual 

replicate values were 2,000 to 6,650 mg L-1 greater than 

the fi eld-measured values.

DISCUSSION

Before sampling, we were uncertain whether the fl owback 

fl uid would have elevated Cl concentrations because 

the primary source of Cl was the hydrochloric acid in 

the fracing fl uids, which had been recovered separately 

Figure 4.—Four fl owback samples collected for laboratory analyses, showing the color 

differences between the shallow (4- and 8-foot) and deep (12- and 16-foot) samples. Photo by 

Pamela Edwards, U.S. Forest Service.

Table 5.−Field-measured Cl concentrations determined using model CD-51 Hach® High Range 

Chloride Test Kit and laboratory-measured Cl concentrations determined using ion chromatography

Depth Field measurement Laboratory measurement

Replicate Mean ±

1 std. dev.

Coeffi cient of 

variation

123

(ft) -------------------------------------------- mg L-1 --------------------------------------------- (%)

4 2,000 1,160 700 920 927 ± 230 24.8

8 3,000 3,530 3,470 3,390 3,463 ± 70 2.0

12 7,500 11,900 11,700 14,150 12,583 ± 1,360 10.8

16 8,500 11,300 13,200 10,050 11,517 ± 1,586 13.8
10

before the other additives were introduced. Th ere were 

Cl-containing compounds in the fl uids, but they were 

generally in relatively low concentrations (Table 3) so 

their infl uence on fl owback chemistry was not expected 

to be substantial.

Both the fi eld and laboratory data show that Cl 

concentrations present in the fl owback were substantial. 

Carryover of chemicals from the storage tank from use 

at other sites is not believed to be an important source of 

the Cl because tanks are emptied and the inside surfaces 

are hosed out to remove residual material before removal 

from a site. While reactions between the entire suite of 

fracing chemicals and the geology may have released 

some Cl, it is likely that much of it was attributable to 

unrecovered hydrochloric acid. We do not have data to 

estimate the percent of recovery of the hydrochloric acid, 

but fl owback recoveries are never complete (Sullivan 

et al. 2004; Woodroof et al. 2003a,b). Recovery data 

are largely unavailable on a broad scale (Sullivan et al. 

2004), but recovery from wells drilled in the Marcellus 

Shale in Pennsylvania averaged 35 to 40 percent, with a 

maximum recovery for a single well of 62 percent (URS 

2009). Th is average approximates the post-acidized 

recovery of fl uid found in the non-Marcellus Shale in 

this investigation; if it is representative of the percentage 

of hydrochloric acid recovered in the earlier, separate 

acid fl owback, about half or more of the acid would 

have remained in the geologic formation. Th us, residual 

hydrochloric acid was a likely source of much of the Cl 

recovered in the post-acidized fl owback.

Samples from the tank showed that the Cl concentrations 

increased with depth, and diff erences in fl uid 

characteristics with depth also were evident visually. 

But it is impossible to know whether this gradation 

occurred after fl owback was complete or if the fl uid 

delivered to the tank had variable chloride concentrations 

during the delivery period that resulted in the increasing 

concentrations with depth. Th e composition of fl owback 

has been documented to change during fl owback 

recovery (URS 2009); limited data from drilling in the 

Marcellus Shale in Pennsylvania showed increases in total 

dissolved solids, chloride, and barium concentrations 

over time through fl owback processes (URS 2009). 

In the current study, the tank was fi lled from the top, 

so decreasing Cl concentrations through the fl owback 

process would have been required for the gradation we 

observed. Even if some Cl diff erences existed during 

fl owback, substantial mixing within the tank would be 

expected due to the 18-foot-deep tank being fi lled from 

the top and the resulting turbulence from fl uid falling 

into the tank. Th ese pieces of evidence suggest that Cl 

gradation occurred following fl owback and not as the 

result of temporal diff erences in fl owback chemistry.

Th e occurrence of Cl gradation has several potential 

implications with respect to land application of fracing 

fl uids. For tank-stored fl uids, depending on how the 

composite sample is taken before land application, very 

diff erent Cl concentrations could result. If the composite 

sample is taken from the valves at the bottom of the tank 

(Fig. 1), based on our fi ndings this sample would be 

expected to approximate the highest Cl concentrations 

in the tank. By contrast, a composite sample taken from 

the top of the tank could grossly underestimate both the 

maximum and the average Cl concentrations in the tank.

Because a tank is physically diff erent from an open pit, 

and pits are used to hold both drilling fl uids and fracing 

fl uids, there is some uncertainty about how these results 

apply to pit-stored fl uids. However, Cl concentrations 

in the samples collected during 2008 land application 

following the original well fracing support either that 

gradation occurred or that Cl chemistry in the pit was 

spatially nonhomogeneous even after mixing. Because the 

fi eld conductivity data and the Cl results showed that the 

greatest change in Cl concentrations occurred between 

10- and 12-foot-depths, concentration diff erences may 

be most evident in pits that are at least that deep. If Cl 

diff erences can occur through time, the samples taken 

from near the surface and edges of reserve pits may 

not adequately represent the Cl chemistry present, and 

further study on reserve pit chemistry is warranted.

Th e comparisons of the fi eld and laboratory results 

currently have only limited implications because only 

one test kit was used and only four samples were tested. 

However, we also compared known Cl concentrations 

of standards prepared from sodium chloride mixed in 

deionized water to measurements of those standards 

made with the Hach® kit. Using the manufacturer’s 
11

instructions, the Hach® High Range Cl Test method was 

used for determining concentrations up to 10,000 mg 

L-1, and the Hach® Expanded High Range Cl Test (also 

a titration with silver nitrate) was used for concentrations 

of 10,000 to 25,000 mg L-1. In the expanded range, the 

Hach® test estimates concentrations only to the nearest 

5,000 mg L-1.

Th e Hach® methods routinely overestimated Cl 

concentrations in the range of 500 to 25,000 mg L-1

(Table 6). Th at the Cl concentrations in the fl owback 

samples were underestimated in only the highest 

concentration samples (Table 5) suggests chemicals were 

present in those fl owback samples that interfered with 

the titration results. Th erefore, simple calibration of this 

type of fi eld kit to conventional “clean” standards may be 

insuffi cient to understand how their results will compare 

to more rigorous laboratory measurements.

Because it would be diffi cult and expensive to develop 

calibration standards that contain similar chemical 

constituencies to those found in fl owback, it may 

be easier to use laboratory analyses to determine Cl 

concentrations in fl owback for assessing allowable 

disposal methods. It would obviously take longer to 

obtain results from laboratory procedures than fi eld test 

kits, but the results would be more accurate. Th e fi eld test 

kit is useful as a fi lter, so if Cl concentrations are reasonably 

low subsequent laboratory testing would not be necessary; 

for this particular kit, concentrations of approximately 

8,000 mg L-1 appear to be in the range that would not 

require additional laboratory testing and could be assumed 

to be less than 12,500 mg L-1. Because analysis with the 

fi eld test kit requires use of the Expanded High Range Cl 

Test for concentrations above 10,000 mg L-1, even if this 

brand of fi eld kit had a 1:1 relationship with laboratory 

results, it would be necessary to default to laboratory 

procedures or some other type of more sensitive fi eld 

analysis to determine actual concentrations (since results 

for the Expanded High Range Test are reported only to 

the nearest 5,000 mg L-1).

Laboratory results also have some level of error associated 

with them, and preparation of dilutions can add to that 

variability. Because only small amounts of fl owback fl uid 

were pipetted for the dilutions, very small diff erences in 

delivered volumes can lead to relatively large diff erences 

in calculated concentrations, especially for the 1:5,000 

dilutions. Th e high viscosity of the fracing fl uid also may 

aff ect the effi cacy and repeatability of pipette delivery.

Th e variability among replicates of an individual sample 

ranged from tens to thousands of milligrams per liter, 

with the greatest absolute variability associated with 

concentrations that were above 10,000 mg L-1 (Table 

5). In both the 12- and 16-foot-deep samples, one of 

the replicates exceeded the 12,500-mg L-1 maximum 

concentration for land application (had land application 

been considered), while the other two replicate values 

and the average of all three values were below that 

concentration (Table 5). Th erefore, even if laboratory 

procedures are used to determine Cl concentrations, it 

may be prudent to analyze 2 or 3 replicates of a sample 

when the Cl concentrations are within 2,000 to 3,000 

mg L-1 of the threshold value (12,500 mg L-1) and use 

the average concentration to determine whether land 

application is appropriate.

It is important to note that the results presented here 

are from a single tank at a single point in time, and the 

data result from fracing a specifi c geologic layer using a 

specifi c fracing technique and additive mixture. However, 

these limitations do not make the data inconsequential 

Table 6. —Chloride concentrations of known standards 

determined by CD-51 Hach® test kit

Hach® method Cl standard 

concentration

Hach® test-kit

concentration

------------------mg L-1------------------

High Range Cl Test 500 1,000

1,000 1,500

2,000 2,500

5,000 5,000

8,000 8,500

10,000 10,000

Expanded High Range 10,000 15,000

Cl Test 12,000 15,000

15,000 15,000

20,000 25,000

25,000 30,000
12

because there are few data of this type in the literature. 

Diff erent Cl concentrations or diff erences in their spatial 

relationships may result if diff erent fracing additives, 

fracing techniques, and/or geologic layers are involved 

or if fl owback effi ciencies are substantially diff erent. 

Consequently, much more research is needed to build 

relevant databases from which the physical and chemical 

conditions associated with fl owback can be characterized 

and the associated environmental impacts of holding and 

disposing of those fl uids can be better understood.

CONCLUSION

Following refracing of a natural gas well in West 

Virginia, fl owback was recovered and stored in a 

10,500-gallon tank. In situ electrical conductivity and 

laboratory analyses of electrical conductivity and Cl 

concentrations made after 7 days of storage showed that 

the Cl concentrations in fl owback increased substantially 

with depth. Th is fi nding suggests that Cl associated 

with fl owback stored in reserve pits also has the ability 

to become graded with depth. In West Virginia, land 

application of fl owback is permitted only if the Cl 

concentration of a representative sample is less than 

12,500 mg L-1. Th us, if samples collected to make that 

assessment are not representative of the pit or the tank 

in which they are stored, the selected disposal method 

may be inappropriate or unnecessary relative to state 

regulations.

Chloride concentrations in the tank-stored fl owback 

were underestimated by a common type of colorimetric 

fi eld test kit at concentrations above 3,000 mg L-1. 

For fi eld measurements of approximately 8,000 mg 

L-1 or greater, reanalysis of the sample using laboratory 

procedures is recommended to ensure that more accurate 

results are obtained on which to base decisions about 

fl uid disposal. Calibration of the fi eld unit to typical 

“clean Cl” standards will not be suffi cient to compensate 

for diff erences between laboratory and fi eld kit test 

results, because fl owback fl uid may have chemicals 

that interfere with colorimetric tests. Additionally, we 

recommend that replicate laboratory analyses be used so 

an average of those results is employed to represent the 

Cl concentration of fl owback, whether stored in pits or 

tanks.

Th is study examined the contents of only a single 

tank of fl owback. However, there is surprisingly little 

information in the literature on fl owback chemistry, 

and substantial additional research is needed to fully 

understand the character and environmental eff ects of 

fracing fl uids and fl owback. Th is information becomes 

increasingly important as the Nation turns to its own 

natural gas reserves for energy independence.

ACKNOWLEDGMENTS

Th e authors thank David Berry, owner of Berry Energy, 

Inc., for his cooperation in providing access to the tank 

and information on fracing additives and procedures. We 

also thank Charles Krushansky of Action Environmental, 

Inc., for providing information about the procedures 

used to collect fl owback samples from the B-800 gas well 

reserve pit. Finally, we thank John Pearce and Frederica 

Wood for their assistance in preparing and analyzing Cl 

samples in the laboratory.

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Edwards, Pamela J.; Tracy, Linda L.; Wilson, William K. 2011. Chloride 

concentration gradients in tank-stored hydraulic fracturing fl uids following 

fl owback. Res. Pap. NRS-14. Newtown Square, PA: U.S. Department of 

Agriculture, Forest Service, Northern Research Station. 14 p.

A natural gas well in West Virginia was hydraulically fractured and the fl owback was 

recovered and stored in an 18-foot-deep tank. Both in situ fi eld test kit and laboratory 

measurements of electrical conductivity and chloride concentrations increased 

substantially with depth, although the laboratory measurements showed a greater 

increase. The fi eld test kit also underestimated chloride concentrations in prepared 

standards when they exceeded 8,000 mg L-1, indicating that laboratory analyses 

or other more accurate methods of detection should be used to determine chloride 

concentrations in fl owback when they may be approaching West Virginia regulatory 

levels (12,500 mg L-1) that disallow disposal by land application. The gradation of 

chloride with depth also has implications for procedures used to collect fl owback 

samples from reserve pits or tanks before disposal to ensure the resulting composite 

chloride concentration is representative of the total volume.

KEY WORDS: natural gas, unconventional well development, hydraulic fracturing, 

fl owback recovery, chloride concentrations, specifi c conductivity
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