Natural Attenuation in Fractured 

Rock Aquifers

USEPA-USGS Fractured Rock Workshop

EPA Region 2

14 January 2014

Allen M. Shapiro, USGS
Natural Attenuation in Fractured Rock 2

The Importance of Natural Attenuation in the 

Groundwater Management at Sites of 

Contamination in Fractured Rock Aquifers

 Monitored Natural Attenuation (MNA) is currently evaluated as a groundwater 

remediation strategy, like other groundwater remediation strategies (e.g., 

Pump-and-Treat, Thermal Treatments, In Situ Chemical Oxidation, etc.) 

 If the Remedial Action Objective is to restore groundwater, remediation 

strategies are evaluated on achieving ARARS (Applicable or Relevant and 

Appropriate Requirements) in a “Reasonable Time Frame” 

 MNA has been successfully applied at a large number of sites of groundwater 

contamination over the past 15 years, including some sites in fractured rock

 There are many (federal, industrial, and state) sites, where achieving ARARs in 

a “Reasonable Time Frame” is unlikely in fractured rock
 Difficulties in characterizing complex distribution of contaminants (e.g., source zone, flow paths, 

contaminant mass in flow-limited regions of the aquifer)

 Long-residence times of contaminants in flow-limited regions of the aquifer

 Challenges to remedial technologies in transforming/destroying contaminant mass in source 

zone and dissolved-phase plume in flow-limited regions of the aquifer

Naval Air Warfare Center, West Trenton, NJ

Weathered rock

Unweathered rock

Estimate of TCE Mass 

in Fractures and Rock Matrix

Natural Attenuation in Fractured Rock

Technical Challenges to Remediation in 

Fractured Rock in a Reasonable Time Frame
Program/Agency Number of 

Contaminated 

Facilities

Number of Sites Estimated Cost 

Complete ($B)

DoD 4,329 $12.8

CERCLA 1,364 $16 - $23

RCRA 2,844 $32.4

UST 87,983 $11

DOE 3,650 $17.3 - $20.9

Other Federal Sites >3,000 $15 - $22

State Sites >23,000 $5

TOTAL >126,000 $110 - $127

The Magnitude of the Problem

National Research Council, 2013

Natural Attenuation in Fractured Rock
Natural Attenuation in Fractured Rock

National Research Council, 2013

The Magnitude of the Problem

 >126,000 sites have residual groundwater contamination that prevents 

“closure” . . . likely an underestimate. . .e.g., counting of “facilities” and 

“sites” differ between programs, does not include DoD facilities with 

Remedy in Place (RIP) or Response Complete (RC), etc. 

 Cost of remediation $110 - $127 Billion. . .likely an underestimate given 

technical limitations of achieving Unlimited Use/Unrestricted Exposure 

(UU/UE). . . 

 Estimated ~10% of sites (~12,000 sites) are “complex” . . . restoration 

unlikely for decades or centuries. . . ~10% of all sites will account for ~70% 

of total cleanup costs [Ehlers and Kavanaugh, 2013] . . . 
Analysis of 80 Delisted NPL Groundwater Sites

14

4

19

14

2

4

12

6

5

MCL Characterization

MCLs Achieved: Active Remedy: 

No LTM

MCLs Achieved: Active 

Remedy: LTM

MCLs Not Achieved: LTM

MCLs Not Achieved: Deleted 

Based on Risk Assessment: No LTM

MCLs Not Achieved: Deleted 

Based on Risk Assessment: LTM

Remedial Objective 

Other Than Meeting 

MCLs

MCL Achievement Unknown

Not a Groundwater Site

MCLs Achieved: No Active 

Remedy: No LTM

Natural Attenuation in Fractured Rock

National Research Council, 2013

Are “Closed” Sites “Closed” ?
Schofield Barracks (U.S. Army), Oahu, Hawaii

17,000 acre facility

Land fill, sewage, industrial and vehicle waste, explosives

Water supply well impacted by TCE (100 ppb)

• Basaltic rock

• Thin, horizontal lava flows 

(hydraulic conductivity 100’s to 

1,000’s ft/day)

• Intrusive dikes compartmentalize 

groundwater

• Groundwater 500-600 ft below 

land surface

• U.S. Army applied for upfront TI Waiver as part of ROD, 1996

• Air strippers on drinking water wells, 1986

• Monitoring wells

• Delisted from NPL, 2000

• 5-year reviews

ARARs Waiver – Technical Impracticability

Natural Attenuation in Fractured Rock
Letterkenny Army Depot, Chambersburg, PA

• Testing, storage, overhaul of track vehicles

• Storage, transportation of industrial chemicals, petroleum

• Storage, modification of ammunition

• Groundwater contaminated with TCE, PCBs

• Soils contaminated with heavy metals, VOCs

• Facility divided into 7 Operable Units (OUs)

• In Situ Chemical Oxidation (ISCO) applied to VOC 

contamination in groundwater without success

• Facility overlies limestone and dolomite

• Structural faulting in the area

• Karstic features (sink holes, caverns, springs)

Letterkenny

Army Depot

• U.S. Army has been unable to obtain a TI 

waiver for selected OUs

ARARs Waiver – Technical Impracticability (?)

Natural Attenuation in Fractured Rock
77 TI waivers (up through November 2010) 

~3/4 of TI waivers attributed to hydrogeology 

or nature of contamination

Deeb et al., 2011

Hydrogeologic Setting

Type of Contaminant

ARARs Waiver – Technical Impracticability

Natural Attenuation in Fractured Rock
77 TI waivers (up through November 2010) 

Distribution of TI Waivers by EPA Regions

Approximately 1⁄2 of states have not had a TI waiver

Distribution of CERCLA sites is not evenly distributed over EPA regions

Hydrogeologic conditions differ from region to region

TI Waivers Granted Over Time

Natural Attenuation in Fractured Rock

ARARs Waiver – Technical Impracticability

Deeb et al., 2011
Natural Attenuation in Fractured Rock 11

The Importance of Natural Attenuation in the 

Groundwater Management at Sites of 

Contamination in Fractured Rock Aquifers

 Large number of sites are characterized as “complex”. . .ARARs unlikely to 

be achieved in decades to centuries. . . 

 Unlikely that stakeholders will accept wide spread application of ARARs 

Waivers (Technical Impracticability) at fractured rock sites

 Need to consider longer time frames of remediation and remedial 

strategies that may evolve over time, recognizing that some active 

remedies may reach a point of diminishing returns. . .Natural Attenuation 

will likely be a component in the management of a large number of 

fractured rock sites. . . 

 There is a need to document the existence of Natural Attenuation and 

understand the long-term prospects for continued Natural Attenuation
12

 MNA – encompasses all natural attenuation processes (not just 

biological) – preference for those processes that degrade or destroy 

contaminants

 Conditions at each site are unique, but common framework is applied 

in documenting natural attenuation

Natural Attenuation in Fractured Rock

Monitored Natural Attenuation

EPA Protocol (1999)

 Lines of evidence (1999)

 Historical chemical data indicating 

decrease in contaminants of concern 

along flowpaths

 Hydrogeological and geochemical data 

to demonstrate (indirectly) types of 

natural attenuation processes and rates

 Field or microcosm studies
Natural Attenuation in Fractured Rock 13

Monitored Natural Attenuation

Recent Advances

 Development of microbiological tools. . Polymerase Chain Reaction (PCR) . . 

.explicitly identify presence of Dehaloccoides (Dhc) species in groundwater 

known to carry out reductive dechlorination

 Compound Specific Isotope Analysis (CSIA) . . .ratio of carbon isotopes. . 

.dechlorination preferentially metabolizes 12C in comparison 13C, changing the 

isotope ratio of TCE, cis-DCE, VC, and ethene as reductive dechlorination

continues. . .clearly identifies that decreases in concentrations of chlorinated 

ethenes are a product of dechlorination rather dilution. . . 

 Statistical model correlating presence of Dhc with geochemical parameters . . 

.oxidation-reduction potential (ORP), methane, and nitrate + nitrite

 Recent advances provide quantitative (lines of) evidence for reductive 

dechlorination

Wilson 2010
Natural Attenuation in Fractured Rock 14

Monitored Natural Attenuation

Attributes that Lead to Success

 Detailed understanding of flowpaths from source to receptors – design of 

monitoring well network – a challenge in fractured rock

 Source zone control – to prevent further downgradient contamination –

remediation or containment

 Monitoring that demonstrates substantial reduction in contaminant 

concentration over a decade or more – reductions in concentrations by order 

of magnitude

 Monitoring includes geochemical and microbial parameters that document 

groundwater is an appropriate habitat for attenuation 

 Quantitative evaluation of spatial distribution of contaminants and their 

degradation products (usually through mathematical modeling tools)
Natural Attenuation in Fractured Rock 15

Monitored Natural Attenuation

Application to Fractured Rock

 Successful applications of MNA in dissolved phase plume in fractured rock 

document for selected areally extensive plumes . . .1000’s of feet. . . see, e.g., 

Twin Cities Army Ammunition Plant (MN) - sandstone, Bell Aerospace Textron 

Wheatfield Plant (NY) - dolomite

 Plumes over 1000’s of meters – monitoring wells 

interpreted as if along a single flow path

 Current struggle to interpret MNA in fractured

rock over dimensions where flowpaths are

convoluted (10’s -100’s of meters)

 “Flowpath-independent” interpretation of MNA

(see, e.g., Bradley et al., 2009) - an attempt

to address this issue at discrete monitoring

locations 
Monitored Natural Attenuation in Fractured Rock

Observations

16

 Natural attenuation (biotic and abiotic processes) will likely become an 

issue at some point in the life span of remedial activities at fractured rock 

sites

 Estimates of attenuation will to change over time. . .one cannot expect 

degradation rates to remain constant. . .for long time frames, one will 

need to document the processes and conditions that will maintain natural 

attenuation

 Over dimensions of 10’s – 100’s of meters, monitoring wells may not 

characterize representative flowpaths . . .under such conditions in 

fractured rock, it is difficult to infer if attenuation will reduce (spatially 

distributed) concentrations and contaminant mass 

Natural Attenuation in Fractured Rock
Natural Attenuation in Fractured Rock 17

Suggested References

Bradley, P. M., Lacombe, P. J., Imbrigiotta, T. E., Chapelle, F. H., and Goode, D. J., 2009, Flowpath independent monitoring of 

reductive dechlorination potential in a fractured rock aquifer, Ground Water Monitoring and Remediation, v. 29, no. 4. P. 46-

55.

Chapelle, F. H., Lacombe, P. J., and Bradley, P. M., 2012, Estimated trichloroethene transformation rates due to naturally 

occurring biodegradation in a fractured rock aquifer, Remediation, doi: 10.1002/rem.21307.

Chartrand, M. M. G., Morrill P. L., Lacrampe-Couloume, G., and Sherwood-Lollar, B., 2005, Stable isotope evidence of 

biodegradation of chlorinated ethenes at a fractured bedrock site, Environmental Science and Technology, v. 39, p. 4848-

4856.

Lacombe, P. J., 2011, Mass of chlorinated volatile organic compounds removed by pump-and-treat, naval Air Warfare Center, 

West Trenton, NJ, 1996-2010, U.S. Geological Survey Scientific Investigations Report 2011-5003, 48 p.

Wilson, J. T., 2010, Monitored natural attenuation of chlorinated solvent plumes, in Stroo, H. F., and Ward, C. H., eds., In Situ 

Remediation of Chlorinated Solvents, Springer, New York, p. 325-355.

Wilson, J. T., Kampbell, D. H., Ferrey, M., Estuesta, P., 2001, Evaluation of the protocol for natural attenuation of chlorinated 

solvents: Case study at the Twin Cities Army Ammunition Plant, EPA/600/R-01/025, U.S. Environmental Protection Agency, 

Washington, DC.

Yager, R. M., 2000, Simulated transport and biodegradation of chlorinated ethenes in a fractured dolomite aquifer near 

Niagara Falls, New York, U. S. Geological survey Water-Resources Investigations Report 00-4275, 55 p.

Yager, R. M., Bilotta, S. E., Mann, C. L., and Madsen, E. L., 1997, Metabolic adaptation and in situ attenuation of chlorinated 

ethenes by naturally occurring microogranisms in a fractured dolomite aquifer near Niagara Falls, New York, Environmental 

Science and Technology, v. 31, p. 3138-3147.