1. 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

2. 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

9. Potential Sources of Contamination are very close to PWS

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

11. 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?

12. 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

13. (Aller, L et. al. NWWA/EPA Series. 1987)
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)

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

15. DRASTIC INDEX Higher the Value, greater vulnerability Drastic Index =
DrDw+RrRw+ArAw+SrSw+TrTw+IrIw+CrCw
Where w = weight
r = rank

16. 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.

17. 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

18. 0-100 ft 100-300 ft 300-600 ft Greater than 600 ft
Explain – Depth of wells not depth of aquifers. Using this as a surrogate measure for the depth of aquifer at a particular location. Not the best estimate. If anyone knows where one can find the actual depth of aquifer info, I’d love to know it.
0-100 ft ft ft
Greater than 600 ft

19. 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
Higher recharge – more dilution, but also recharge is a major transportation mechanism for pollutants to reach the aquifer

20. Soil Media Range Rating Thin or Absent 10 Gravel Sand 9 Peat 8
Shrinking and/or Aggregated Clay 7
Sandy Loam 4
Loam 5
Silty Loam
Clay Loam 3
Muck 2
Nonshrinking and Nonaggregated Clay 1
* Source: Aller et al., EPA, 1987.

21. Soil Media – Raster Map from USGS
Raster Data of Soils. Need to classify into Drastic Index values

22. 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

23. 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)

24. 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.

25. 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.

26. Major Assumptions of DRASTIC
Contaminant is introduced at ground surface
Contaminant is flushed into the groundwater by precipitation
Contaminant has the mobility of water

27. 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!!

28. 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