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Using GIS to Assess Potential Abiotic Degradation of Chlorinated Ethenes

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

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Using GIS to Assess Potential Abiotic Degradation of Chlorinated Ethenes

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  1. Using GIS to Assess Potential Abiotic Degradation of Chlorinated Ethenes Tim Glover and Theodore Parks MACTEC Engineering and Consulting Kennesaw, GA USA

  2. Steps in the Evaluation • Partition Areas • Determine Scoring Method(s) • Compile Data • Generate Scores • Display Scores • Assess Potential(s)

  3. 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

  4. 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

  5. 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

  6. Thiessen Polygons Example Actual Soil sample locations

  7. 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.

  8. 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

  9. 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

  10. Some “Bad” Aspects • The opposite of any “good” aspect • Excessive sulfate • Excessive nitrate • Too cold

  11. 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

  12. 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

  13. 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)

  14. Some “Bad”Aspects • Lack of reduced iron oxide minerals • Presence of oxidized iron oxide minerals • General oxidizing chemical conditions

  15. Abiotic Scoring • No set scoring ranges (no protocol) • Positive scores suggest potential • Negative scores limit potential

  16. Compiling Data Five data sets • Soils – abiotic • Shallow groundwater – biotic • Shallow groundwater – abiotic • Deep groundwater – biotic • Deep groundwater - abiotic

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

  18. 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)

  19. 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

  20. Compile Data • Extract pertinent data and spatial coordinates from database • Consolidate data and quality check

  21. 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

  22. Soils – Abiotic

  23. Shallow Groundwater – Biotic

  24. Shallow Groundwater – Abiotic

  25. Deep Groundwater – Biotic

  26. Deep Groundwater – Abiotic

  27. Assess Potentials • Display scoring polygons • Overlay plume outline • Interpret potential for degradation

  28. Shallow Groundwater – Biotic

  29. Shallow Groundwater – Abiotic

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