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Mapping Groundwater Vulnerability to
Contamination in Texas
Mapping Groundwater Vulnerability to
Contamination in Texas
Shannon Nicole StokesGIS for Water Resources
CE 394K.3Term Project Presentation
University of Texas at AustinNovember 19, 2001
Presentation OutlinePresentation Outline Problem Development – Why do we need to model
groundwater vulnerability? Objectives Specific Goals DRASTIC
What is it?Summary of each DRASTIC ParametersDownfalls of DRASTIC
What’s left for me to do Conclusions
Legend
seymour polygon
ogallala polygon
hueco polygon
gulfcoast polygon
edwards polygon
cenozoic polygon
carrizo polygon
trinity polygon
Public Water Supplies - Groundwater
Overall ObjectiveOverall 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??How Do We Do This??
Protect Water Supply from ContaminationRemediate Contaminated Soils and Aquifers if spills do occurLimited Financial Resources… If we cannot remediate every contaminated site immediately, which sites should we address first?
Specific GoalsSpecific 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 parametersIn EXCEL calculate DRASTIC INDEXES for groundwater PWSShow the DRASTIC INDEXES graphically in GIS
What is DRASTIC ??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 PARAMETERSDRASTIC PARAMETERS
D- Depth to Water R- Recharge A- Aquifer Media S- Soils T- Topography I- Impact of Vadose Zone C- Hydraulic Conductivity
DRASTIC INDEXDRASTIC INDEX
Higher the Value, greater vulnerabilityDrastic Index =DrDw+RrRw+ArAw+SrSw+TrTw+IrIw+CrCw
Where w = weight r = rank
Stacking of Drastic Layers to Produce a Vulnerability MapStacking 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 WaterDepth 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 ft100-300 ft300-600 ftGreater than 600 ft
Net RechargeNet 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 MediaSoil MediaRange Rating
Thin or Absent 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.
SOILS
Soil Media – Raster Map from USGSSoil Media – Raster Map from USGS
TopographyTopography
Low Slope higher DRASTIC ratingContaminant released is less likely to become run-off and therefore more likely to infiltrate to the aquifer
Slope data is available from DEM
Hydraulic ConductivityHydraulic 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) andaquifer thickness (m)
Aquifer MediaAquifer 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 ZoneImpact 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 ratingNot clear where I can find this data. May have to make assumptions based on well log data.
Major Assumptions of DRASTICMajor Assumptions of DRASTIC
Contaminant is introduced at ground surface Contaminant is flushed into the groundwater by
precipitation Contaminant has the mobility of water
Next StepsNext 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!!
CONCLUSIONSCONCLUSIONS 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