Geologic, Hydraulic, and Geochemical Controls on Fate, Transport, and Remediation of VOCs USEPA-USGS Fractured Rock Workshop EPA Region 2 14 January 2014 Allen M. Shapiro, USGS

Controls on Fate, Transport, and Remediation of VOCs2 Diversity of Fractured Rock Aquifers Granite and schist Mirror Lake, NH Madison Limestone, Rapid City, SD Biscayne Limestone, Ft. Lauderdale, FL Lockatong Mudstone, West Trenton, NJ Tonalite, Washington, DC Silurian Dolomite Argonne, 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 Controls on Fate, Transport, and Remediation of VOCs3

Sand and gravel, glacial outwash Cape 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 flow Chemical transport Controls on Fate, Transport, and Remediation of VOCs4 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 Controls on Fate, Transport, and Remediation of VOCs5 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 Watershed Grafton 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 bedding showing fault zone location Lockatong Mudstone, Newark Basin West Trenton, New Jersey

Controls on Fate, Transport, and Remediation of VOCs6 Expectations of fractured rock: Large variability in capacity to transmit groundwater

Borehole H1 Mirror 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... Controls on Fate, Transport, and Remediation of VOCs7 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 Controls on Fate, Transport, and Remediation of VOCs8 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 Watershed Grafton County, New Hampshire Lockatong Mudstone, Newark Basin West 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 Controls on Fate, Transport, and Remediation of VOCs9 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* Controls on Fate, Transport, and Remediation of VOCs10 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 Controls on Fate, Transport, and Remediation of VOCs11 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 Controls on Fate, Transport, and Remediation of VOCs12 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... Controls on Fate, Transport, and Remediation of VOCs13 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  g/L TCE concentration (36BR interval A): 102 – 112 ft below land surface = 19,000  g/L Controls on Fate, Transport, and Remediation of VOCs14 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 m 2 /s Transmissivity of 36BR – interval B (112 – 125 ft below land surface) – 1.0 x m 2 /s Flux averaged concentration: C A x (1.00/1.01) + C B x (0.01/1.01) = C openhole C A = 19,000  g/L C openhole = 89,000  g/L C B > 1,000,000  g/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 30 o 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 Controls on Fate, Transport, and Remediation of VOCs15 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 Controls on Fate, Transport, and Remediation of VOCs16 Consequences of Complexity in Fractured Rock: Complex spatial distribution of contaminants

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 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 Controls on Fate, Transport, and Remediation of VOCs17 Consequences of Complexity in Fractured Rock: Complex spatial distribution of contaminants

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... Controls on Fate, Transport, and Remediation of VOCs18 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 Controls on Fate, Transport, and Remediation of VOCs19 Consequences of Complexity in Fractured Rock: Significance of “flow limited” regions of the aquifer

Lockatong Mudstone Naval Air Warfare Center, West Trenton, NJ Depth where DNAPL detected during coring Detection limit for TCE in groundwater samples is 1  g/L Controls on Fate, Transport, and Remediation of VOCs20 Consequences of Complexity in Fractured Rock: Significance of “flow limited” regions of the aquifer Depth (feet below land surface) Sandstone Simi Hills, Ventura County, California Monitoring interval #6 Monitoring interval #3 Sterling et al., 2005 Multilevel monitoring Jan 1998

Matrix Diffusion in Fractured Rock 100 meters 25 meters Vertical exaggeration x 4 Log 10 (C/C o ) Fracture (v = 100 m/yr) Rock matrix C = Co (0 < t < 10 years) C = 0 (t > 10 years)

Matrix Diffusion in Fractured Rock 100 meters 25 meters Vertical exaggeration x 4 Log 10 (C/C o ) Fracture (v = 100 m/yr) Rock matrix C = Co (0 < t < 10 years) C = 0 (t > 10 years) elapsed time = 10 years

Matrix Diffusion in Fractured Rock 100 meters 25 meters Vertical exaggeration x 4 Log 10 (C/C o ) Fracture (v = 100 m/yr) Rock matrix C = Co (0 < t < 10 years) C = 0 (t > 10 years) elapsed time = 20 years

Matrix Diffusion in Fractured Rock 100 meters 25 meters Vertical exaggeration x 4 Log 10 (C/C o ) Fracture (v = 100 m/yr) Rock matrix C = Co (0 < t < 10 years) C = 0 (t > 10 years) elapsed time = 30 years

Matrix Diffusion in Fractured Rock 100 meters 25 meters Vertical exaggeration x 4 Log 10 (C/C o ) C = Co (0 < t < 10 years) C = 0 (t > 10 years) elapsed 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.

Controls on Fate, Transport, and Remediation of VOCs26 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 Mudstone Naval Air Warfare Center, West Trenton, NJ Controls on Fate, Transport, and Remediation of VOCs27 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 Fromation on 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... Controls on Fate, Transport, and Remediation of VOCs28 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 Controls on Fate, Transport, and Remediation of VOCs29 What’s important ? Expectations: Complex spatial distribution of contaminants in fractures and the rock matrix...

Final Thoughts Controls on Fate, Transport, and Remediation of VOCs30 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...