Mapping Groundwater Vulnerability to Contamination in Texas

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Mapping Groundwater Vulnerability to
Contamination in Texas

• Shannon Nicole Stokes
• GIS for Water Resources
• CE 394K.3
• Term Project Presentation
• University of Texas at Austin
• November 19, 2001

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Presentation Outline

• Problem Development Why do we need to model
  groundwater vulnerability?
• Objectives
• Specific Goals
• DRASTIC
• What is it?
• Summary of each DRASTIC Parameters
• Downfalls of DRASTIC
• Whats left for me to do
• Conclusions

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Overall Objective

• To Adequately Protect Human Health, We Need to
  Ensure that Potential Contaminants Do not Enter
  the Public Water Supply

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How Do We Do This??

• Protect Water Supply from Contamination
• Remediate Contaminated Soils and Aquifers if
  spills do occur
• Limited Financial Resources If we cannot
  remediate every contaminated site immediately,
  which sites should we address first?

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Specific Goals

• Use GIS and DRASTIC to determine what PSWs are
  most vulnerable to contamination
• Specifically
• Use GIS, ACCESS and EXCEL files to get DRASTIC
  input parameters
• In EXCEL calculate DRASTIC INDEXES for
  groundwater PWS
• Show the DRASTIC INDEXES graphically in GIS

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What is DRASTIC ??

• A method developed by the EPA to provide a
  systematic evaluation of the potential for
  groundwater contamination that is consistent on a
  national basis
• (Aller, L et. al. NWWA/EPA Series. 1987)

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DRASTIC PARAMETERS

• D- Depth to Water
• R- Recharge
• A- Aquifer Media
• S- Soils
• T- Topography
• I- Impact of Vadose Zone
• C- Hydraulic Conductivity

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DRASTIC INDEX

• Higher the Value, greater vulnerability
• Drastic Index
• DrDwRrRwArAwSrSwTrTwIrIwCrCw
• Where w weight
• r rank

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Stacking of Drastic Layers to Produce a
Vulnerability Map
Stenson, M.P. Stachotta, C.P., Queenslands
Groundwater Vulnerability Mapping Project.
Queenslands Department of Natural Resources.
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Depth to Water

• Depth to Water affects the Time available for a
  contaminant to undergo chemical and biological
  reactions
• (Dispersion, Oxidation, Natural Attenuation,
  Sorption, etc.)
• Greater Depth ? Lower Vulnerability Rating

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0-100 ft 100-300 ft 300-600 ft Greater than 600 ft
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Net Recharge

• Using data from Climate Rasters available from
  USGS Datasets
• Apply a mass balance on the water
• Net Recharge Precipitation Evaporation
  Runoff
• Higher Recharge ? Greater vulnerability

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Soil Media
Source Aller et al., EPA, 1987.
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Soil Media Raster Map from USGS
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Topography

• Low Slope ? higher DRASTIC rating
• Contaminant released is less likely to become
  run-off and therefore more likely to infiltrate
  to the aquifer
• Slope data is available from DEM

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Hydraulic Conductivity

• Relates the factures, bedding planes and
  intergranular voids which become pathways for
  fluid movement
• High Hydraulic Conductivity ? high movement once
  contaminant has entered aquifer ? high DRASTIC
  rating
• Requires transmissivity (m2/day) and
• aquifer thickness (m)

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Aquifer Media

• Ratings are based on the permeability of each
  layer of media
• High Permeability ? high DRASTIC rating
• Some of this data is available in the well logs
  for the public water supplies. I have not
  determined how much more information I need yet.

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Impact of Vadose Zone

• Zone below the typical soil horizon and above the
  water table
• Unsaturated or discontinuously saturated
• High Permeability of vadose zone ? high DRASTIC
  rating
• Not clear where I can find this data. May have to
  make assumptions based on well log data.

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Major Assumptions of DRASTIC

• Contaminant is introduced at ground surface
• Contaminant is flushed into the groundwater by
  precipitation
• Contaminant has the mobility of water

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Next Steps

• Finish collecting data for DRASTIC layers
• Export DRASTIC parameter ratings to EXCEL to
  calculate DRASTIC Indexes
• Prepare GIS map of DRASTIC Indexes
• Overlay DRASTIC map with PWS to get a better
  understanding for what water supplies are
  vulnerable
• Get everything done by Dec. 7!!

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CONCLUSIONS

• DRASTIC can be used to model groundwater
  vulnerability
• Results of applying DRASTIC model must be used
  carefully. This applies a framework but does not
  account for all the particulars of the chemicals
  released.
• A detailed study of a particular spill must
  incorporate the chemical properties of the
  contaminant
• GIS can help make the results of a complicated
  model more clear through visual representation