Transcript Document 7591611

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
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
What’s left for me to do
 Conclusions

Public Water Supplies - Groundwater
Legend
seymour polygon
ogallala polygon
hueco polygon
gulfcoast polygon
edwards polygon
cenozoic polygon
carrizo polygon
trinity polygon
Overall Objective
To Adequately Protect Human Health, We Need to
Ensure that Potential Contaminants Do not Enter
the Public Water Supply
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?
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
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)
DRASTIC PARAMETERS
 D- Depth to Water
 R- Recharge
 A- Aquifer Media
 S- Soils
 T- Topography
I- Impact of Vadose Zone
 C- Hydraulic Conductivity

DRASTIC INDEX
 Higher the Value, greater vulnerability
Drastic Index =
DrDw+RrRw+ArAw+SrSw+TrTw+IrIw+CrCw
Where w = weight
r = rank
Stacking of Drastic Layers to Produce a
Vulnerability Map
Stenson, M.P. & Stachotta, C.P., Queensland’s Groundwater Vulnerability Mapping Project.
Queensland’s Department of Natural Resources.
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
0-100 ft
100-300 ft
300-600 ft
Greater than 600 ft
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
Soil Media
Range
Thin or Absent
Rating
10
Gravel
10
Sand
9
Peat
8
Shrinking and/or Aggregated Clay
7
Sandy Loam
4
Loam
5
Silty Loam
4
Clay Loam
3
Muck
2
Nonshrinking and Nonaggregated Clay
1
* Source: Aller et al., EPA, 1987.
Soil Media – Raster Map from USGS
SOILS
Topography
 Low Slope  higher DRASTIC rating
Contaminant released is less likely to become runoff and therefore more likely to infiltrate to the aquifer
 Slope data is available from DEM
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)

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.
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.
Major Assumptions of DRASTIC
 Contaminant is introduced at ground surface
 Contaminant is flushed into the groundwater by
precipitation
 Contaminant has the mobility of water
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!!
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
