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REMEDIATING AND MANAGING KARST SITES
workshop “Belastete Standorte über Karstgrundwasser”, June 28, 2018
Keith A. White, C.P.G.
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Outline• Overview• CERCLA Remedies at Karst
Sites• Remedial Techniques and
Strategies• Performance Monitoring
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OVERVIEW
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Remediation Challenges
Complex permeability
structure
Remediation “state of the science” less
mature
Limited number of
experienced karst
practitioners
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CERCLA REMEDIES AT KARST SITES
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Review of RODs for USEPA Karst Sites • Reviewed 161 Records-of Decision (RODs) for 71
karst sites contained in USEPA online database• Looked at types of remedial elements and
associated monitoring required• Key Findings:
1. Dominant components of remedies = soil remediation, “institutional controls”, and long-term monitoring
2. Groundwater remediation required at 58% of sites; most-common technology was “pump & treat”
3. Performance monitoring: only 16% of sites required spring monitoring
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REMEDIAL TECHNIQUES AND STRATEGIES
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Remedial Techniques and Strategies
1. Address Source Zones2. Manage Impacted
Groundwater3. Eliminate Exposure
Pathways4. Impose Institutional
Controls**Non-engineered items, such as administrative and legal controls, that help minimize the potential for exposure to contamination and/or protect the integrity of a remedy
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1. Address Source Zones• Excavation/capping• In-situ mass
destruction• Physical, hydraulic, or
“chemical” containment
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1. Address Source Zones• In-situ mass destruction – chemical
oxidation and enhanced reductive dechlorination (ERD)• Not aware of proven success stories in
epikarst (a.k.a. subcutaneous zone) and bedrock• One site: contaminant rebound in groundwater
observed after chemical oxidation• Currently designing ERD field-scale pilot study
• Difficulties include:• Locating complexly-distributed sources and
getting reagents in contact with them• Controlling injected fluids; can potentially move
fast and far
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1. Address Source Zones• In-situ mass
destruction – thermal treatment• Can be effective for
reducing mass• Decreased
concentrations in groundwater sustained; but drinking water standards not met
• Challenge:– Rapid flux of
groundwater in conduits could preclude sufficient heating
Site A BTarget Compound Chlorobenzene
(DNAPL)TCE (DNAPL & vadose
soil)Remedial Goals 80% reduction in
concentrations80% reduction in concentrations
Pre-ERH Average Concentrations
90,400 ug/L chlorobenzene
853,000 ug/L TCE
Post-ERH Average Concentrations
13,300 ug/L chlorobenzene (85%
reduction)*
43,100 ug/L TCE (94.9% reduction)
Approximate Treatment Volume (m3)
12,600 8,200
Mass of Contaminant Removed (kg)
1,336 1,270
Operation Period 08/17/13 to 02/10/14 10/24/13 to 01/22/14
Energy to Subsurface (kWh)
2,900,000 2,040,000
Cost per m3 $214 $318
*Two “deep” wells (30 m) did not meet Remedial GoalTRS Group, Inc. http://www.thermalrs.com
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1. Address Source Zones• Mass reduction via removal (continued)
“EVE” (Epikarst Vapor Extraction)• Air-filled, interconnected nature of many
epikarsts lends itself to this technology• Particularly applicable to poorly consolidated
limestones (“young” carbonates and chalk)• Limited to volatile compounds• Used successfully to address gasoline spill in
Bermuda
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1. Address Source Zones• Physical and chemical containment
(e.g. barrier walls, permeable-reactive barriers)
Often technically or cost prohibitive (only 6% of USEPA karst RODs)Main obstacles include:• Costs to excavate rock to the base of impacts
and collect/treat water generated often cost-prohibitive
• Excessive grout loss during pressure grouting• For PRBs, quantifying and addressing
sedimentation can be a significant issue
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1. Address Source Zones• Hydraulic containment
Pump & treat/hydraulic containment (26% of USEPA karst RODs)• System design is challenging; cannot rely on
standard methods and models• Low-likelihood of pumping wells intercepting
important elements of the permeability structure (e.g., conduits)
• Bedrock trenches that collect both nonaqueous-phase liquids (NAPLs) and groundwater have been installed and operated successfully
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2. Manage Impacted GroundwaterEnhanced bioremediation
• May be viable; though more research is needed
• Has been attempted; but peer-reviewed performance evaluations are lacking
• Delivery of nutrients to where they are needed, and preventing adverse impacts to springs, are key challenges
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2. Manage Impacted GroundwaterMonitored Natural Attenuation
• Regulators and consultants seem to like it– Included as a remedial component in 27% of
USEPA karst RODs• May be a valid approach; especially if the
contaminated groundwater does not pose a risk to human health or the environment
• Clearly, an appropriate monitoring approach and high-quality monitoring data are critical to evaluate effectiveness
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3. Eliminate Exposure Pathways“Treating at the Tap” – Perhaps the most useful active remedial component in karst1. Treating or Replacing Potable Water Supplies
• Commonly employed in karst sites• Expect long-term O&M costs – particularly
where source zone cannot be adequately addressed
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3. Eliminate Exposure Pathways2. Capturing and Treating Spring Water
• Has been successfully performed using caissons and other means to isolate and collect flow for treatment
• Is underutilized – selected as a remedial component at only 3 of 71 CERCLA karst sites
• Spring biota will likely be affected
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PERFORMANCE MONITORING
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Performance MonitoringRemedial components that are designed to affect groundwater movement or quality often require karst-specific methods
• For groundwater movement, tracer tests should be included to assess performance– Tracer monitoring should focus on
springs, extraction systems (if any), and monitoring wells previously shown by tracing to be relevant
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Performance MonitoringFor groundwater quality monitoring:
• Often the best places to monitor are springs and, in some cases, extraction systems
• Water-quality sampling often needs to be “event-based”
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• Consider including evaluation of biota• In some cases may be a better
means of assessing the quality of emerging groundwater than analysis of periodic water samples
– The science is well-developed for surface water; however, springs/cave streams have lower biodiversity. More research may be needed on using these biota for assessing water quality
Performance Monitoring
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CContact: [email protected]
Thank You!
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