geologic, hydraulic, and geochemical controls on fate, transport, and remediation of vocs usepa-usgs...

Geologic, Hydraulic, and Geochemical Controls

on Fate, Transport, and Remediation of VOCs

USEPA-USGS Fractured Rock WorkshopEPA Region 2

14 January 2014

Allen M. Shapiro, USGS

Controls on Fate, Transport, and Remediation of VOCs 2

Diversity of Fractured Rock Aquifers

Granite and schistMirror Lake, NH

Madison Limestone, Rapid City, SD Biscayne Limestone,

Ft. Lauderdale, FL

Lockatong Mudstone,West Trenton, NJ

Tonalite, Washington, DC

Silurian DolomiteArgonne, IL

Sykesville Gneiss,Washington, DC


Fractured rock aquifers are highly diverse . . .however. . .all fractured rock aquifers share

similar physical attributes Similar attributes provide for. . .

Generic discussion of physical and chemical transport processes

Standardized approaches to characterization and monitoring

Design and application of diagnostic and modeling tools Expect site specific complexities

Diversity of Fractured Rock Aquifers


Sand and gravel, glacial outwashCape Cod, Massachusetts

Schematic of intergranular void space in an unconsolidated sand

Length-to-width ratio of void space ~1

Heterogeneous nature of geologic materials is limited. . .direction of groundwater flow can be readily identified from the hydraulic gradient. . .

“Pore” scale variability in fluid velocity (magnitude and direction) results in spreading of a chemical constituent. . .

Groundwater flowChemical transport

Expectations for unconsolidated porous media

Similar interpretations of groundwater flow and chemical transport cannot be applied to fractured rock aquifers because of unique physical attributes of fractured rock. . .

Dimensions of the void space are small relative to scale of the problem of interest (groundwater flow, chemical transport)


Expectations of fractured rock: Hierarchy of void space

Iron-hydroxide precipitate staining the rock matrix (primary/intrinsic rock porosity)

Fractures exposed on a road cut (fracture porosity)

Fault zone exposed on a road cut

Granite and schist, Mirror Lake WatershedGrafton County, New Hampshire

Residual wetting of rock core (primary/intrinsic rock porosity)

Fractures parallel and perpendicular to bedding (fracture porosity)

Schematic cross section perpendicular to beddingshowing fault zone location

Lockatong Mudstone, Newark BasinWest Trenton, New Jersey


Expectations of fractured rock: Large variability in capacity to transmit groundwater


Borehole H1Mirror Lake Watershed, NH

Granite and Schist

Packer apparatus used for testing

individual or closely spaced

fractures

Shapiro and Hsieh, 1998; Shapiro et al., 2007

Expect both vertical and horizontal variability. . .

Expectations of fractured rock:

K of the intrinsic rock (matrix) porosity is orders of magnitude less than that of fractures

Abrupt spatial changes in hydraulic properties


Local and regional stress distribution, lithology, and weathering lead to complex connectivity of fractures and their hydraulic properties. . .

Boundary

Iron staining No staining

Schist: fewer fractures; longer, undulating fracture surfaces

Granite: higher fracture density; shorter, more planar fractures

Combination of large variation in K coupled with complex fracture connectivity = convoluted groundwater flow paths

Expectations of fractured rock: Complex fracture connectivity

Bedding plane parting along black, carbon-rich section of mudstone

Joints perpendicular to bedding (parallel and perpendicular to rock face)

Fracture density perpendicular to

bedding varies with proximity to fault

Granite and schist, Mirror Lake WatershedGrafton County, New Hampshire

Lockatong Mudstone, Newark BasinWest Trenton, New Jersey


Fracture surfaces have complex topology. . .fracture aperture varies due to points of contact and asperities between fracture walls

. . .similar to the large variability in hydraulic properties that is anticipated from one fracture to the next, there is large variability in hydraulic properties within an individual fracture. . .

Neretnieks et al., 1982; Tsang and Neretnieks 1998

Convoluted groundwater flow paths within individual fractures

Expectations of fractured rock: Fracture surfaces have complex topology


What’s important ?How do we approach this level of complexity for site- and regional-scale investigations? • Identify lithologic and geomechanical controls• Identify most permeable features and barriers to

groundwater flow over relevant dimensions• Spatial connectivity of permeable features• Mapping and characterization of every fracture is not

warranted• Variability within an individual fracture is below our

resolution capacity*

Expectations: Convoluted groundwater flow paths over dimensions from meters to kilometers. . .


Characterizing fluid advection and the migration of aqueous phase contaminants. . .monitoring hydraulic head in discrete intervals of boreholes

Consequences of Complexity in Fractured Rock:

Characterizing hydraulic head is a 3-D concept. The direction of groundwater flow must be inferred in concert with the characterization of permeable features and flow barriers.

Characterizing the direction of groundwater flow


Characterizing fluid advection and the migration of aqueous phase contaminants. . .monitoring hydraulic head in discrete intervals of boreholes

Consequences of Complexity in Fractured Rock:Characterizing the direction of groundwater flow

Maintaining the integrity of multilevel monitoring equipment. . . proper monitoring of hydraulic head is critical to inferring directions of groundwater flow.


Borehole H1, Granite and Schist, Mirror Lake Watershed, NH

Monitoring geochemical conditions in fractured rock. . .boreholes open to multiple fractures. . .

Pumping. . .mixing contributions from multiple fractures . . .

Pumping. . .groundwater drawn preferentially from most transmissive fractures. . .

Consequences of Complexity in Fractured Rock:Characterizing the distribution of contaminants


Naval Air Warfare Center, West Trenton, NJ, Lockatong Mudstone, Newark Basin

TCE concentration (36BR open interval): 102 – 125 ft below land surface = 89,000 mg/L

TCE concentration (36BR interval A): 102 – 112 ft below land surface = 19,000 mg/L

Consequences of Complexity in Fractured Rock:Characterizing the distribution of contaminants

Transmissivity of 36BR – interval A (102 – 112 ft below land surface) – 1.0 x 10 -5 m2/s Transmissivity of 36BR – interval B (112 – 125 ft below land surface) – 1.0 x 10 -7 m2/s

Flux averaged concentration: CA x (1.00/1.01) + CB x (0.01/1.01) = Copenhole

CA = 19,000 mg/LCopenhole = 89,000 mg/L

CB > 1,000,000 mg/L


DNAPLs in geologic media

DNAPL pooling at a boundary between larger beads [0.85 – 1.23 mm] (upper region) and smaller beads [0.49 – 0.70 mm] (lower region)

Schwille 1988

Complex DNAPL migration in unsaturated sands. DNAPL shown in red (Sudan IV dye). Bedding dips 30o below horizontal

15 cm

Poulsen and Kueper, 1992

• Capillary forces define the distribution of DNAPLs• Complex spatial distribution of DNAPLs (both vertically and

laterally) from minor variations in pore space geometry• DNAPLs at great depths - density > groundwater• DNAPL “pool” heights force DNAPL into small pore throats;

hydraulic conditions may not be capable of removing DNAPL from small pore throats

• Pumping and drilling may re-mobilize “pools” of DNAPL• DNAPLs dissolve into groundwater• Dissolved-phase DNAPLs diffuse into lower-permeability

geologic materials • VOCs sorb to geologic materials with organic content

Consequences of Complexity in Fractured Rock:Complex spatial distribution of contaminants


Complex topology of fractures affects contaminant distribution. . .

Entry of DNAPLs into fractures depends on physical properties of fractures and the DNAPL,and capillary forces. . .

Kueper and McWhorter, 1992; Kueper et al., 2003



Fracture aperture affects contaminant distribution. . .

• For a given “pool height” of DNAPL, fractures to the right of these curves would allow entry of DNAPL

• 9 micron (9 x 10-6 meters) fracture aperture needed to stop 1 meter “pool” height of TCE

• Diameter of human hair ~50 microns

Kueper and McWhorter, 1992; Kueper et al., 2003



Retention and slow release of contaminants in “flow limited” regions of the aquifer. . .a significant impediment to achieving remedial objectives in a reasonable time frame. . .

from Doner and Sale, Colorado State University Low-permeability material embedded in a permeable sand. . .

Dye injection. . .

Consequences of Complexity in Fractured Rock:Significance of “flow limited” regions of the aquifer

Flushing. . .

• Low permeability material may not be significant with respect to volumetric groundwater flow. . .

• During contaminant “loading”, dye diffuses from permeable pathways to low-permeability materials due to concentration gradient

• During “flushing”, dye diffuses from low-permeability materials to permeable pathways due to concentration gradient


The primary/intrinsic porosity of the rock (rock matrix) offers a fluid-filled void space available to chemical constituents . . .

Wood et al., 1996

Frequency histogram of porosity in rock types of the Mirror Lake Watershed, New Hampshire

Iron hydroxide staining on fracture surfaces and in the rock matrix. . .oxygen diffusing into the rock matrix and reacting

with dissolved iron



Lockatong MudstoneNaval Air Warfare Center, West Trenton, NJ

Depth where DNAPL detected during coring

Detection limit for TCE in

groundwater samples is 1

mg/L


Dep

th (f

eet b

elow

land

sur

face

)SandstoneSimi Hills, Ventura County, California

Monitoring interval #6

Monitoring interval #3

Sterling et al., 2005

MultilevelmonitoringJan 1998

Matrix Diffusion in Fractured Rock

100 meters

25 m

eter

s

Vertical exaggeration x 4Log10 (C/Co)

-25 -20 -15 -10 -5 0

Fracture(v = 100 m/yr)

Rock matrix

C = Co (0 < t < 10 years)

C = 0 (t > 10 years)


100 meters

25 m

eter

s


-25 -20 -15 -10 -5 0


Rock matrix

C = Co (0 < t < 10 years)

C = 0 (t > 10 years)

-1-3

-10

-15

-5

-20

elapsed time = 10 years


100 meters

25 m

eter

s


-25 -20 -15 -10 -5 0


Rock matrix

C = Co (0 < t < 10 years)

C = 0 (t > 10 years)

-3 -1-1

-3

-10

-15

-5



100 meters

25 m

eter

s


-25 -20 -15 -10 -5 0


Rock matrix

C = Co (0 < t < 10 years)

C = 0 (t > 10 years)

-5

-3

-3

-10

-1

-1



100 meters

25 m

eter

s


-25 -20 -15 -10 -5 0

C = Co (0 < t < 10 years)

C = 0 (t > 10 years)

-5

-3

-3elapsed time = 50 years

Concentration gradient driving contaminant mass toward the fracture. . .

Concentration gradient driving contaminant mass away from the fracture. .

Over time, concentration gradient toward fracture decreases. . . reducing mass flux to the fracture.


Consequences of Complexity in Fractured Rock:Matrix diffusion. . .a curse or a blessing ?

the curse. . .retention of contaminants in flow limited regions of the aquifer. . .limiting access to remediation amendments. . .slow release of contaminants to permeable pathways yields a long-term contaminant source. . .

An example: Pulse injection and monitoring 50 m downgradient

the blessing. . .retention of contaminants in flow limited regions of the aquifer. . .delaying downgradient migration of contaminants. . .attenuating the downgradient concentrations. . .

. . .matrix diffusion is the rationale for the licensing of selected geologic environments as sites for waste isolation (e.g., WIPP site, New Mexico, USA)


Lockatong MudstoneNaval Air Warfare Center, West Trenton, NJ

Consequences of Complexity in Fractured Rock:Other processes in “flow limited” regions of the aquifer

Sharp concentration gradients in the rock matrix adjacent to fractures. . .

Over time, diffusion tends to diminish sharp concentration gradients. . .

Retention and release of contaminants also controlled by surface processes (sorption/desorption) on fracture surfaces and in the rock matrix. . .

Organic contaminants (e.g., TCE) have a surface affinity for organic materials (e.g., organic carbon)

Sorption/desorption changes dynamics for retention and release of contaminants


Naval Air Warfare Center, West Trenton, NJ

Isocontours of TCE concentration at 100 ft below land surface

Mudstone units of the Lockatong Fromationon Cross Section G – G’

G

G’

F

F’100 ft below land surface

Interpreting contaminant distribution based on water samples collected from short (~20 ft) intervals open in a fractured mudstone. . .

Consequences of Complexity in Fractured Rock:Interpreting the spatial distribution of contaminants

Lacombe 2011

Interpretation of concentrations in mobile groundwater (fractures). . .does not necessarily reflect total in situ contaminant mass. . .


How do we approach this level of complexity for site- and regional-scale investigations? • “Classical” (Gaussian-shaped) plumes are unlikely• Differentiate between contaminants in mobile and

immobile (flow-limited) regions of the aquifer• Quantify contaminant mass in flow-limited regions of the

aquifer. . . the rock matrix may retain significant contaminant mass (aqueous chemical diffusion and surface processes)

• Grasp the significance of residence times and mass exchange between fractures and the rock matrix

What’s important ?

Expectations: Complex spatial distribution of contaminants in fractures and the rock matrix. . .


Final Thoughts

Keep in mind. . .fractured rock aquifers have similar physical attributes. . .as well as site specific complexities. . .

These (similar) attributes are the starting point for the development of Conceptual Site Models. . .a conceptual understanding of the hydrogeologic and biogeochemical controls on groundwater flow and contaminant fate and transport. . .site specific complexities are needed to fill in the details. . .

These (similar) attributes need to be recognized in characterization, monitoring, and modeling at fractured rock sites. . .

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