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