Alternative Cleanup Methods for Chlorinated VOCs

1
Alternative Cleanup Methods for Chlorinated VOCs

• Getting beyond pump and treat

2
Soil Vapor Extraction

• Vacuum is applied through extraction wells
• Creates a pressure gradient that induces
  gas-phase volatiles to be removed from soil
• Also is known as
• in situ soil venting
• in situ volatilization
• enhanced volatilization
• soil vacuum extraction

3
Soil Vapor Extraction
4
Soil Vapor Extraction

• Works only in the vadose (unsaturated) zone
• Typically used with shallow extraction wells
  (5-10 ft)
• Has been used as deep as 300 ft
• Extraction wells can be either vertical or
  horizontal

5
SVE Applicability

• Target contaminant groups
• Volatile compounds (chlorinated or not)
• Fuels (especially lighter fractions)
• Will not remove heavy oils, metals, PCBs, or
  dioxins
• Can promote in-situ biodegradation of
  low-volatility organic compounds

6
SVE Limitations

• Low permeability soil or high degree of
  saturation requires higher vacuums (increasing
  costs)
• Heterogeneous subsoils may require large screened
  intervals to get even flows of vapor
• Reduced removal rates when soil is highly
  sorptive (high organic content)
• Off-gases may require treatment

7
SVE Possible Improvements

• Impermeable cap on soil surface can improve
  removal rates (but not always that effective)
• Horizontal wells may be efficiently laid in
  trenches can improve removal
• De-watering by pump drawdown can expose more
  unsaturated zone (especially with floating LNAPLs)

8
SVE Performance

• Has worked well at many sites, but often find
  lower removal rates, higher costs than expected
• Site-specific pilot study needed to establish
  feasibility and fine tune the design
• Intermittent (pulsed) extraction can improve
  efficiency be allowing vapor levels to build up
  between pulses

9
SVE Pulsed Operation
10
Air Sparging

• Air is injected through wells into a contaminated
  aquifer
• Air traverses horizontally and vertically through
  the soil column
• Creates an in-situ air stripper
• Usually used in conjunction with SVE to capture
  contaminant-rich vapors

11
Air Sparging
12
Air Sparging Applicability

• As with any stripping system, limited to volatile
  compunds (VOCs) and light components of fuels
• Can double as a source of oxygen to stimulate
  biodegradation of hydrocarbons

13
Air Sparging Limitations

• Physics of air-flow in saturated zone poorly
  understood
• Preferential channels can short circuit much of
  the air, by-passing much of the contaminated zone
• Contaminated air may escape the capture zone of
  SVE system
• In heterogeneous aquifer only the porous zones
  will get much air flow little removal from less
  permeable layers

14
Air Sparging Performance

• Has been used successfully at many sites
• But still very hard to generalize from that
  experience
• Hard to say why it is working in some cases
• Not very effective if there is extensive DNAPL
  free-product below the sparging zone

15
Enhanced Biodegradation

• Microbes can degrade most pollutants
• But rate can be VERY slow if they lack proper
  conditions
• Groundwater often lacks what they need
• electron acceptors (like oxygen)
• nutrients (N, P, K, trace elements)
• co-metabolites (for chlorinated cmpds)

16
Enhanced Biodegradation

• SOLUTION (?) Inject materials that microbes need
  to degrade contaminants

17
Examples

• Add oxygen via sparging
• Add oxygen via hydrogen peroxide
• Add alternate electron acceptor (nitrate that
  substitutes for oxygen)
• Micro nutrients
• Hydrogen-releasing compounds (for reductive
  dehalogenation)

18
Enhanced Biodegradation
19
E.B. Limitations

• If heterogeneous, very difficult to deliver the
  nitrate or hydrogen peroxide evenly
• Safety precautions when handling hydrogen
  peroxide
• Concentrations of H2O2 gt 100 to 200 ppm is
  inhibiting to microorganisms
• A groundwater circulation system must be created
  so contaminants dont escape from zones of active
  biodegradation
• Many states prohibit nitrate injection

20
Regenesis Corp.

• Mfr of proprietary solid-phase products for
  enhancing biodegradation
• ORC Oxygen Release Compound (patented Mg
  peroxide)
• Stimulates aerobic breakdown)
• HRC Hydrogen Release Compound (poly-lactate gel)
• Stimulates reductive dechlorination of
  chlorinated solvents

21
Regenesis ORC Case Study

• Service station in Wisconsin, underground storage
  tank (UST) leakage
• Contaminants Gasoline, BTEX and MTBE
• Treatment ORC Slurry Injection
• Soil Type Loose to medium to course grain sand
• Project Cost 16,150 (ORC Only)

22
Regenesis ORC Case Study

• UST was removed along with some of the
  contaminated soils
• Residual soil and groundwater contamination
  remained in source area.
• Continuing groundwater plume contained MTBE up to
  800 ppb and BTEX concentrations ranging up to
  14,000 ppb

23

• ORC slurry was applied into the source area via
  Geoprobe injection
• A total of 1,700 pounds of ORC powder were
  injected in a slurry

24
ORC Slurry Injection Method
25
ORC Injection Scheme
26
Regenesis ORC Results
27
Regenesis ORC Results

• Both BTEX and MTBE were apparently degraded by gt
  99.9 within 10 months of ORC application
• Post-treatment monitoring throughout a complete
  hydrogeologic cycle, showed no significant
  rebound in contaminant concentrations

28
ORC Reputed Savings

• Compared with Air Sparging plus Vapor Containment
• All values were derived independently by the
  sites consultants. The costs are full systems
  costs with the objective of site closure.
  Regenesis

29
Permeable Reactive Barriers

• A permeable barrier zone is placed across front
  of contaminant plume
• Contaminant can passively flow into barrier
• Chemical or biological reactions in barrier
  destroy or otherwise remove contaminants from
  water

30
Permeable Reactive Barriers
31
Permeable Reactive Barriers

• Most common material used are zero-valent
  (metallic) iron (ZVI)
• ZVI removes chlorines from chlorinated solvents
• Chemistry not completely understood but it
  certainly works
• Also interest in ion-exchange barriers (for
  metals, etc.) and biological barriers (zones of
  enhanced bacteria)

32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
(No Transcript)
37
(No Transcript)
38
(No Transcript)
39
(No Transcript)
40
(No Transcript)
41
(No Transcript)
42
(No Transcript)
43
PRBs Limitations

• Passive treatment walls may lose their reactive
  capacity, requiring replacement of the reactive
  medium.
• Passive treatment wall permeability may decrease
  due to precipitation of metal salts
• Depth and width of barrier.
• Limited to a subsurface lithology that has a
  continous aquitard at a depth that is within the
  vertical limits of trenching equipment.

44
Natural Attenuation

• Not an action but a methodology for closing out
  a site safely with no further action
• Well discuss this more in Wednesdays lecture