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