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Modeling Drinking Water 

Related Human Health 

Risks from Hydraulic 

Fracturing (HF) Additives

Manu Sharma, M.S., P.E.

Gradient, Cambridge, MA

EPA HF Workshop

March 29, 2011
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Analysis Objectives

• Understand potential drinking water related human 

health risks due to HF activities

• Use simplified models and worst-case release scenarios 

consistent with a screening level analysis

• Focus on New York State's Marcellus shale region

• Evaluate drinking water risks due to:

 Surface releases to groundwater and surface water 

 Hypothetical migration from Marcellus to drinking water aquifers

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Conceptual Site Model

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Model HF Fluid

• Used model fluid composition presented in NYSDEC's SGEIS (NYSDEC, 2009) 

Purpose Compound HF Concentration (mg/L)

Surfactant/Cross Linker/Scale Inhibitor Methanol 984

Biocide Glutaraldehyde 90

Corrosion Inhibitor Propargyl alcohol 1.5

Crosslinker Ethylene glycol 180

Breaker, Surfactant Propylene glycol 500

Breaker Diammonium peroxidisulphate 100

Crosslinker Monoethanolamine 18

Friction Reducer Acrylamide 10

Gelling Agent Naphtha (petroleum), hydrotreated heavy 275

Scale Inhibitor Formaldehyde 0.5

Solvent Xylene 3

Notes: 1) HF additive concentrations in model fluid, except naphtha, are less than their aqueous solubility limits. 

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Flowback Characterization 

Flowback data from samples collected from Marcellus shale in PA and WV (NYSDEC, 2009)

Notes: 1) Concentrations for all organics compounds in flowback listed above are less than their aqueous solubility limits.

2) A constituent, 4-nitroquinoline -1- oxide (4-NQO), was not evaluated since the reported concentrations appear to be anomalous.

Parameter No. Samples No. Detected Units Maximum Median

Inorganics

Barium 34 34 mg/L 15700 661.5

Boron 26 9 mg/L 26.8 2.06

Bromide 6 6 mg/L 3070 616

Cadmium 29 5 mg/L 1.2 0.032

Cobalt 25 4 mg/L 0.58 0.3975

Copper 29 4 mg/L 0.157 0.035

Lithium 25 4 mg/L 161 55.75

Manganese 29 15 mg/L 14.5 2.18

Nickel 29 6 mg/l 0.137 0.0465

Strontium 30 27 mg/L 5841 821

Sulfate 58 45 mg/L 1270 3

Zinc 29 6 mg/l 0.09 0.048

Organics

Benzene 29 14 ug/L 1950 479.5

Toluene 29 15 ug/L 3190 833

Ethyl Benzene 29 14 ug/L 164 53.6

Xylenes 22 14 ug/L 2670 487

Phenols 25 5 ug/L 440 191

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Surface Release Scenarios

• Two types of accidental releases simulated for HF 

additives and flowback

• Acute spill or release during down hole pumping

 High release rate, short duration 

• Diffuse or chronic spill

 Occasional drips and leaks during handling

 Small volumes, but possibly over longer time periods 

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Diffuse Spill Release Scenario

Diffuse Surface Release Scenario

HF Fluid Used (gal/well) 4,030,000

Flowback Volume @ 20% (gal/well) 806,000

Number of Wells (per pad per year) 8

Total Flowback Volume (gal) 6,448,000

Spill Volume @ 1% of Flowback Volume (gal) 64,480*

Assumed spill area (acres) ~ 4

** (126m x 126m)

50% to Surface Water (gal) 32,240

50% to Soil/Groundwater Leaching (gal) 32,240

Notes:

1) * Represents 0.2% of HF used to frac 8 wells

2) ** Average pad size envisioned in New York (NYSDEC, 2009)

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Acute Spill Release Scenario

Pipe/Pump Leak Scenario

HF Injection Rate (gpm) 3,400

Rupture Duration (min) 5

HF Release (gal) 17,000

Assumed Spill Area (acres) 0.25 acre (33m x 33m)

50% to Surface Water (gal) 8,500

50% to Soil/Groundwater Leaching (gal) 8,500

•Some spill data available in State databases

•Data difficult to access and analyze

•Largest reported spills – 8,000 to 13,000 gals

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Modeling Approach

• Groundwater release:

 Partitioning in soil between aqueous and adsorbed 

phases (DAFpw)

 Downward migration through vadose zone (DAFL)

 Mixing into groundwater flowing to a well (DAFgw)

• Surface water release:

 Mass-balance mixing

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Groundwater 

Pathway

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HF Concentration in Drinking Water Well

pw L gw

HF

DAF x DAF x DAF

C

Cwell

Cwell = Concentration at well (μg/L)

CHF = Concentration of HF additive released to the surface (μg/L)

DAFpw = Dilution factor for initial pore water concentration 

(unitless)

DAFL = Dilution factor from pore water to leachate at the water 

table (unitless)

DAFgw = Dilution factor of leachate into groundwater (unitless)

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Surface Spill to Surface Water

• Mass-balance mixing of spill directly into stream flow:

Csw = HF concentration in surface water (μg/L)

QHF = Discharge of HF or flowback fluid

Qsw = Stream flow

• Lowest annual mean daily discharge (LAMDD) at 10th percentile stream in the 

region of New York overlying the Marcellus (8.1 cfs)

• Based on data from 147 USGS stations overlying Southern New York State

HF sw

HF

sw HF Q Q

Q

C C

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Model Input Parameters

Parameter Value Source

Net infiltration (cm/yr) 36.8 (cm/yr) USGS (1998)

Water content ( ) 0.3 cm3/cm3 US EPA, 1996

Porosity ( ) 0.43 cm3/cm3 US EPA, 1996

Bulk density ( ) 1.5 g/cm3 US EPA, 1996

Organic carbon (foc) 0.006 g-oc/g-soil US EPA, 1996

Chemical OC partition coefficient (Koc) Chemical-specific (L/kg) Compound-specific

Dispersivity 0.25, 0.5, 2.5 cm Typical range of values

Initial soil contamination depth (xo) 2 cm, 7 cm

Calculated based on spill 

characteristics

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Model Input Parameters

Parameter Diffuse Spill Acute Spill 

Spill Area (A) ~ 4 acres 0.25 acres

Spill Width (W) 126 m 33 m

Depth water table 5 m, 10 m 5 m, 10 m

Total Percolation (q = I × A) 5,613 m3

/yr 395 m3/yr

Groundwater Specific Discharge** (QD)

10-4 cm3/cm2-sec

31.5 m3/m2-yr

10-4 cm3/cm2-sec

31.5 m3/m2-yr

Groundwater Flow Volume (Q)

Well Depth (20 m) 77,900 (m3/yr) 20,655 (m3/yr)

Well Depth (50 m) 194,743 (m3/yr) 51,637 (m3/yr)

Notes: 1) ** Assumed K = 10-2 cm/sec and a horizontal gradient of 1% - typical values for glacial outwash (sand) deposits

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Summary of DAFs

Diffuse Spill Acute Spill

DAFpw

Compound – specific Compound - specific

DAFL 

Vadose Zone Depth

5 m 57, 78, 164 18, 25, 52

10 m 78, 108, 222 25, 35, 73

DAFgw 

Well Depth

20 m 15 53

50 m 36 132

US EPA (1996) 90th Percentile DAF ~ 15 ~ 600

Combined DAF (DAFL x DAFgw)**

1,170 – 3,888 1,325 – 4,620

Surface Water DAF* 59,000 n/a

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*10th percentile LAMDD

** Median DAFL used in combined DAF calculation
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Risks from Diffuse HF Fluid Spills

Hazard Quotient

Parameter Groundwater

Upperbound 

(ug/l) 

Surface Water 10th

Percentile LAMDD 

(ug/l)

Drinking Water RBC 

(ug/L)

RBC Basis Groundwater

Upperbound

Surface Water

10th Percentile LAMDD

Methanol 811.2 17 18,000 EPA RSL n 0.05 0.00093

Glutaraldehyde 57.9 1.53 5,600 OPP RED RfD 0.01 0.00027

Propargyl alcohol 1.2 0.03 73 EPA RSL n 0.02 0.00035

Ethylene glycol 152.8 3.06 73,000 EPA RSL n 0.002 0.00004

Propylene glycol 422.4 8.49 730,000 EPA RSL n 0.001 0.00001

Diammonium

peroxidisulphate

85.5 1.70 479.5 Derived** 0.18 0.00354

Monoethanolamine 15.1 0.31 1,400 Derived** 0.01 0.0022

Acrylamide* NA NA 0.015 EPA RSL c <1 <1

Naphtha 9.4 4.67 1,300 TPHCWG, 1997 0.007 0.00359

Formaldehyde 0.3 0.01 7,300 EPA RSL n 0.00005 0.000001

Xylenes 0.2 0.05 10,000 Drinking water MCL 0.00002 0.00001

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* Given its extremely short half-life, estimated to be less than 36 hours in soil, and less than 12 days in water (US EPA, 1994), acrylamide is expected to 

biodegrade rapidly to levels below the RBC.

** Derived using reported no observable adverse effects level and an uncertainty factor.
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Risks from Diffuse Flowback Spills

Hazard Quotient

Parameter Groundwater

Upperbound (ug/l) 

Surface Water

10th Percentile LAMDD 

(ug/l)

Drinking Water 

RBC (ug/L)

RBC Basis Leaching to 

Groundwater

Upperbound

Surface Water

10th Percentile 

LAMDD

Barium 65 267 2,000 Drinking water MCL 0.03 0.13

Boron 1 0.46 7,300 EPA RSL n 0.0002 0.0001

Bromide 2,624 52 6,000 WHO 0.4 0.01

Cadmium 0.0027 0.02 5 Drinking water MCL 0.0005 0.004

Cobalt 0.0022 0.01 11 EPA RSL n 0.0002 0.0009

Copper 0.0008 0.003 1,300 Drinking water MCL 0.0000006 0.000002

Iron 5.5 14 26,000 EPA RSL n 0.0002 0.0005

Lithium 0.09 3 70 Derived (see Section 5) 0.001 0.04

Manganese 0.04 0.25 880 EPA RSL n 0.00004 0.0003

Nickel 0.0004 0.002 730 EPA RSL n 0.0000005 0.000003

Strontium 28 99 22,000 EPA RSL n 0.001 0.005

Sulfate 1,085 22 500,000 EPA Health-based 

level/MCL Canada (sulfate) 0.002 0.00004

Zinc 0.0002 0.002 11,000 EPA RSL n 0.00000002 0.0000001

Organics

Benzene 1 0.03 5 Drinking water MCL 0.1 0.007

Ethyl Benzene 0.01 0.003 700 Drinking water MCL 0.00001 0.000004

Toluene 0.30 0.054 1,000 Drinking water MCL 0.0003 0.0001

Xylenes 0.14 0.045 10,000 Drinking water MCL 0.00001 0.000005

17 Phenols 0.11 0.007 11,000 EPA RSL n (phenol) 0.00001 0.000001
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Assumptions/Limitations

• Surface release on unconsolidated deposits 

• Mixing in drinking water aquifer assumes porous media 

system

• Complete mixing at a drinking water well – a reasonable 

assumption

• Releases of dissolved organics (except naphtha) –

consistent with injected fluids

• Potential releases from impoundments not simulated 

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Surface Release Findings

• Human health risks via drinking water affected by 

model HF fluid/flowback surface spills are expected 

to be insignificant

• Number of conservative assumptions used

– Release scenarios

– Limited attenuation mechanisms 

– Expected to result in overestimated exposure 

concentrations

• EPA case studies would be useful for validating that 

modeled DAFs are conservative

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Marcellus Migration

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Transport via Fractures

• Highly unlikely in Marcellus shale

• Aquifers separated by thousands of feet of low 

conductivity rock

– Multiple rock units

– Isolate high TDS water from drinking water aquifers

– Have resulted in trapped methane for geologic time scales

• HF induced fractures are expected to be constrained 

– Short duration of HF treatment (1 to 2 days)

– Layered rock system

• Many stress contrasts limit fracture growth

• Embedded concretions stop hydraulic fractures

– Field data show that actual fracture growth is limited and remains 

thousands of feet below drinking water aquifers (Fisher, 2010)

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Source: Fisher (2010) 22
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Source: Fisher (2010)
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Marcellus Cross-Section

Source: USGS, 2005
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Transport via Pore Spaces

• Migration during HF process infeasible

– HF takes 1 to 2 days

– ICF (2009) concluded that HF pressure would have to remain 

applied for years to thousands of years for transport to drinking 

water aquifers

• Migration of “trapped fluids” post-HF also not expected

– Low hydraulic conductivity of shale and overlying rocks

– Multiple phases (water, oil, and gas) further reduce ability to 

transmit water

– High capillary pressures

– Unclear if there is a driving head, especially considering brine 

density

– Extreme-case migration analysis undertaken 

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Extreme-Case Migration Analysis

• Darcy’s law based analysis with simplifying 

assumptions

– Ignore brine density and multi-phase effects

– Ignore retardation and biodegradation

• Only considers mixing with porewater in bedrock 

and in drinking water aquifer 

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Predicted Dilution-Attenuation Factors 

for Upward Migration

Surface Aquifer 

Thickness

DAFBR

(Mixing within 

Bedrock )*

DAFaq

(Mixing in Shallow 

Aquifer Flow)

Combined DAF

DAFBR × DAFaq

100 ft >300 5,500 >1.65 x 106

1,000 ft >300 29,000 >8.7 x 106

Note: DAFaq based on migration from 4,500-foot length fracture zone upward to aquifer of specified 

thicknesses.

* From ICF (2009).

Extreme-case analysis indicate that additive concentrations would be 

significantly attenuated if they were to reach drinking water aquifers

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Conclusions

• Generalized scenarios provide a framework to 

understand potential human health risks to drinking 

water from HF activities 

– Upward migration from Shale implausible exposure pathway

– Risks from surface spills and vertical migration from 

Marcellus Shale expected to be insignificant for model HF 

fluid and flowback

– Additional model sensitivity analyses being undertaken to 

define effect of input parameter variability and uncertainty

• EPA case studies and other data could help validate 

modeling results 

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Equations and References

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Porewater DAF

w

H F

w

p

p

C

C

DAF ( d )

p

K

DAF w

or

Where: DAFpw = Pore Water DAF

Cpw = Concentration in pore water

= Soil water content (cm3 – water/cm3 – soil)

Kd = Soil water partition coefficient (L/Kg); and

S = Soil dry bulk density (g/cm3)

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Leaching DAF 

• Downward transport through the vadose zone 

(Enfield et al.,1982)

Where: Co = Initial concentration in leachate

Zo = Initial contaminant depth

D = Dispersion coefficient

CL = Leachate concentration at water table

v = Percolation rate

• DAFL = 1/(CL/Co) – averaged over the time 

period that leachate enters the water table

1/ 2 1/ 2

2 2 2

1

Dt

z t

erf

Dt

z z t

C C erf o

o

L

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Groundwater DAF

• Transport of chemicals to a drinking water well:

 Cgw = Concentration at a well

 QI = Volumetric rate of downward seepage

 Qgw = Groundwater flow rate through aquifer cross section 

to drinking water well screen bottom

I gw

I

gw Q Q

Q

C CL

I

I gw

gw Q

Q Q

DAF

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References

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Enfield, CG; Carsel, RF; Cohen, SZ; Phan, T; Walters, DM. 1982. "Approximating pollutant transport to ground 

water." Ground Water 20(6):711-722.

Fisher, K. 2010. "Data Confirm Safety of Well Fracturing." The American Oil & Gas Reporter, July.

Gradient. 2009. "Human Health Risk Evaluation For Hydraulic Fracturing Fluid Additives Marcellus Shale 

Formation, New York.” 127p., December 31.

ICF International. 2009. "Technical assistance for the draft supplemental generic EIS: oil, gas and solution mining 

regulatory program—Well permit issuance for horizontal drilling and high-volume hydraulic fracturing to 

develop the Marcellus shale and other low-permeability gas reservoirs." August 7.

New York State Dept. of Environmental Conservation (NYSDEC). 2009. "Supplemental Generic Environmental 

Impact Statement on the Oil, Gas and Solution Mining Regulatory Program: Well Permit Issuance for 

Horizontal Drilling and High-Volume Hydraulic Fracturing to Develop the Marcellus Shale and other LowPermeability Gas Reservoirs (Draft)." Division of Mineral Resources, Bureau of Oil & Gas Regulation, 804p., 

September.

US EPA. 1996. "Soil Screening Guidance: Technical Background Document." Office of Solid Waste and Emergency 

Response. NTIS PB96-963502; EPA-540/R-95/128; OSWER Publication 9355.4-17A. Accessed on January 29, 

2001 at www.epa.gov/oerrpage/superfund/resources/soil/toc.htm, May.

United States Geological Survey (USGS). 1998. "Water Resources of the Batavia Kill Basin at Windham, Greene 

County, New York." USGS Water-Resources Investigations Report 98-4036. 105p.

United States Geological Survey (USGS). 2005. “Assessment of Undiscovered Natural Gas Resources in Devonian 

Black Shales, Appalachian Basin, Eastern U.S.A.” U.S. Geological Survey Open File Report 2005-1268.