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

• 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