Using GIS to Assess Potential Abiotic Degradation of Chlorinated
Ethenes
Tim Glover and Theodore Parks
MACTEC Engineering and Consulting
Kennesaw, GA USA
Steps in the Evaluation
• Partition Areas
• Determine Scoring Method(s)
• Compile Data
• Generate Scores
• Display Scores
• Assess Potential(s)
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
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
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
Thiessen Polygons Example
Actual Soil sample locations
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.
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
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
Some “Bad” Aspects
• The opposite of any “good” aspect
• Excessive sulfate
• Excessive nitrate
• Too cold
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
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
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)
Some “Bad”Aspects
• Lack of reduced iron oxide minerals
• Presence of oxidized iron oxide minerals
• General oxidizing chemical conditions
Abiotic Scoring
• No set scoring ranges (no protocol)
• Positive scores suggest potential
• Negative scores limit potential
Compiling Data
Five data sets
• Soils – abiotic
• Shallow groundwater – biotic
• Shallow groundwater – abiotic
• Deep groundwater – biotic
• Deep groundwater - abiotic
Data Sources
• Classic MNA (monitored natural attenuation) data for biotic
• AMIBA (Aqueous and Mineralogical Intrinsic Bioremediation Assessment ) data for abiotic
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)
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
Compile Data
• Extract pertinent data and spatial coordinates from database
• Consolidate data and quality check
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
Soils – Abiotic
Shallow Groundwater – Biotic
Shallow Groundwater – Abiotic
Deep Groundwater – Biotic
Deep Groundwater – Abiotic
Assess Potentials
• Display scoring polygons
• Overlay plume outline
• Interpret potential for degradation
Shallow Groundwater – Biotic
Shallow Groundwater – Abiotic