Using GIS to Assess Potential Abiotic Degradation of Chlorinated Ethenes

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Using GIS to Assess Potential Abiotic Degradation
of Chlorinated Ethenes

• Tim Glover and Theodore Parks
• MACTEC Engineering and Consulting
• Kennesaw, GA USA

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Steps in the Evaluation

• Partition Areas
• Determine Scoring Method(s)
• Compile Data
• Generate Scores
• Display Scores
• Assess Potential(s)

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Partition Areas

• Site broken into layers
• Soils
• Shallow groundwater
• Deep groundwater
• Then
• Existing sample points for each identified
• Representative areas assigned to each data point

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Partition Areas

• Thiessen Polygons First used by Descartes in
  1644 in astronomy
• Described by Thiessen in 1911 for weather
  observations
• Known by many other names
• Region of Influence method
• Includes all areas closer to a data point than to
  any other data point

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Thiessen Polygons Example

• Data points plotted
• Lines equidistant from closest points drawn
• Polygons generated from these line segments
• Value of data point is a best estimate for
  value anywhere in polygon

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Thiessen Polygons Example

• Actual Soil sample locations

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Determine Scoring Method(s)

• For chlorinated ethenes, at least two degradation
  modes biotic and abiotic
• Biotic is caused or greatly facilitated by
  biological activity
• Abiotic does not need direct biological
  activity to proceed.

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Biotic Scoring Method

• Industry Standard Weidermeier Protocol
• Weight of evidence method
• Varying positive points for good aspects
• Varying negative points for bad aspects
• The sum of points (positive and negative) is used
  to assess the potential

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Some Good aspects

• Low dissolved oxygen
• Reducing conditions (negative ORP)
• Elevated bicarbonate and chloride
• Sufficient soil organic carbon
• Near-neutral pH
• Evidence of breakdown products

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Some Bad Aspects

• The opposite of any good aspect
• Excessive sulfate
• Excessive nitrate
• Too cold

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Biotic Scoring Ranges

• Evidence for anaerobic biodegradation of
  chlorinated organics
• 0 to 5 - Inadequate evidence
• 6 to 14 - Limited evidence
• 15 to 20 - Adequate evidence
• More than 20 - Strong evidence

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Abiotic Scoring Method

• No existing industry standard protocol
• Developed one modeled after biotic protocol
• Also weight of evidence
• Single point (1) for good aspect
• Single point (-1) for bad aspect
• Final score is sum of points

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Some Good Aspects

• Different aspects from biotic
• Mineral evidence of reduced iron oxides
• Chemical evidence of reducing conditions
• Breakdown products (different than biotic
  breakdown products)

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Some BadAspects

• Lack of reduced iron oxide minerals
• Presence of oxidized iron oxide minerals
• General oxidizing chemical conditions

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Abiotic Scoring

• No set scoring ranges (no protocol)
• Positive scores suggest potential
• Negative scores limit potential

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Compiling Data

• Five data sets
• Soils abiotic
• Shallow groundwater biotic
• Shallow groundwater abiotic
• Deep groundwater biotic
• Deep groundwater - abiotic

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Data Sources

• Classic MNA (monitored natural attenuation) data
  for biotic
• AMIBA (Aqueous and Mineralogical Intrinsic
  Bioremediation Assessment ) data for abiotic

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MNA - Biotic

• From Weidermeier Protocol standardized
• General measures of redox conditions (H 2 and DO)
• Biotic breakdown products (DCE, VC)
• Inorganic breakdown products (HCO3-, Cl-)
• Competing redox reactions (SO4, NO3)

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AMIBA - Abiotic

• No standardized protocol innovate!
• Designed for fuel hydrocarbons not chlorinated
  solvents
• Assesses oxidative capacity for fuel spills
• Can be used backwards to assess reductive
  capacity for solvents

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Compile Data

• Extract pertinent data and spatial coordinates
  from database
• Consolidate data and quality check

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Generate Scores

• Run queries to assign points for each scoring
  method
• Sum assigned points for each layer and scoring
  method
• Generate Thiessen shape files (5) one for each
  scoring method and layer

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Soils Abiotic
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Shallow Groundwater Biotic
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Shallow Groundwater Abiotic
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Deep Groundwater Biotic
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Deep Groundwater Abiotic
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Assess Potentials

• Display scoring polygons
• Overlay plume outline
• Interpret potential for degradation

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Shallow Groundwater Biotic
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Shallow Groundwater Abiotic