GIS-BASED ALGORITHMS FOR VULNERABILITY...

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GIS-BASED ALGORITHMS FOR VULNERABILITY ASSESSMENT Abraham Thomas 1 and Julian Conrad 2 1 University of the Western Cape Bellville 7535, Cape Town. 2 GEOSS (Pty) Ltd, Innovation Centre - TechnoPark, Stellenbosch 7600.

Transcript of GIS-BASED ALGORITHMS FOR VULNERABILITY...

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GIS-BASED ALGORITHMS FOR VULNERABILITY ASSESSMENT

Abraham Thomas1 and Julian Conrad2

1 University of the Western CapeBellville 7535, Cape Town.

2 GEOSS (Pty) Ltd, Innovation Centre -TechnoPark, Stellenbosch 7600.

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Objectives of GIS Component

To develop improved methods of aquifer vulnerability assessment using GIS. At least two independent GIS based methods will be developed. These methods will be the modified versions of the DRASTIC and UGIf models.

Incorporation of the results of the research being done in three areas of this project such as the roles of the soil, vadose zone and saturated zone in determining groundwater vulnerability.

Sensitivity analysis: The methods will be analyzed for their sensitivity to changes in all variables which they incorporate.

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Overview of UGIf Model

The UGIf model is built in ArcView GIS ver 3.x and has come out of a PhD research in UK (Thomas, 2001).

It is primarily meant for assessment/estimation of groundwater recharge and pollutant fluxes of specific pollutants viz. BTEX, nitrate ad chloride to an urban unconfined/primary aquifer covered by certain specified land use classes.

In its original stage, this model can give an estimation of pollutant transport velocity and travel time in vodose zone for specific pollutants such as BTEX compounds.

As it calculates travel time of pollutants, from the point of vulnerability assessments this model can be categorized as process based model based on analytical equations/empirical approaches.

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Recharge Pollutant Flux Model

Potential Pollutant Flux

Travel and ReactionThrough Vadose Zone

Actual Pollutant Flux Reaching Water Table

Chemical ConcentrationGroundwater Recharge

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Research Approach and Work Plan

Collection of available information, data etc. and their assessment for their suitability and use within the GIS based UGIf model;Restructuring of GIS based UGIf model for modeling recharge and pollutant fluxes to a primary aquifer in South Africa (Cape Flats aquifer) and vulnerability assessments;Preparation of input data sets which involve:

preparation of a land use / land cover map of a portion of Cape Flat aquifer area;Preparation of soil and geological data and assignment of hydrological parameters;Preparation of meteorological data for use within the UGIf model;Preparation of maps of elevation, slope, anisotropy, specific retention, vadose zone depth etc.Collation/preparation of Event Mean Concentration data;Collation of hydrogeological and geochemical parameter values;

Demonstration of the model for assessment of recharge pollutant fluxes and vulnerability assessment;Sensitivity analysis of the modified UGIf model.

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Interface of Recharge Pollutant Flux Model

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EMC Values of Selected Pollutants in Urban Runoff Water for NPS Pollution Modelling

Land Use / Land Cover Landuse

Code Tot. Nitrate As

N (mg/l) Chloride

(mg/l) Benzene

(µg/l) Toluene

(µg/l) Ethyl

benzene Xylene (µg/l)

Commercial/Business 1000 1.23 10.00 0.80 2.40 0.60 1.90 1Industrial 2000 1.89 20.00 0.60 1.80 1.50 3.70 1High Density Residential 3000 2.12 15.00 0.20 0.90 0.60 1.85 2Low Density Residential 5000 1.84 15.00 0.10 0.90 0.30 1.85 1Recreation Ground (Grass) 8000 1.83 15.00 0.00 0.00 0.00 0.00 1Agricultural/Horticultural/Farm 9000 4.06 0.91 0.00 0.00 0.00 0.00 4Open Ground/Grassland 12000 1.83 0.91 0.00 0.00 0.00 0.00 1Freeway/Motorway 16000 0.83 0.91 0.20 0.00 0.60 3.70 0Minor Road 19000 0.50 0.91 0.20 0.20 0.60 3.70 0Recreation Ground (Hard Surface) 24000 0.90 0.91 0.00 0.00 0.00 0.00 0Nature Conservation 31000 0.80 15.00 0.00 0.00 0.00 0.00 0Canal/Open Channel (Lined) 32000 0.59 0.91 0.00 0.00 0.00 0.00 0Canal/Open Channel (Unlined) 33000 0.59 0.91 0.00 0.00 0.00 0.00 0Dam/Reservoir (Lined) 34000 0.59 0.91 0.00 0.00 0.00 0.00 0Lake/Pond/Vlei/Wetland 36000 0.59 0.91 0.00 0.00 0.00 0.00 0River/Drain/Open Channel (Gaining) 37000 0.59 0.91 0.00 0.00 0.00 0.00 0Effluent Pond 39000 8.00 50.00 0.00 0.00 0.00 0.00 8Landfill/Waste Dumping 40000 6.00 50.00 0.80 2.40 0.00 0.00 6Major Road/Arterial 41000 0.50 0.91 0.20 0.20 0.60 3.70 0

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Vadose Zone Transport of Aqueous Phase BTEX

Velocity of aqueous phase contaminant migration

The velocity of the pollutant in the vadose zone Vp = Va / Rf where Va = aqueous or pore water velocity; Rf = retardation factor.

Rf = 1 + (ρbKd + (θs - θ) KH) / θ where ρb is the bulk density of the soil; θ is the volumetric water content of the soil; θs is the saturated water content of the soil on a volume basis; Kd = Koc foc which is the partition coefficient for the pollutant in the soil; KH is the dimensionless value of Henry’s law constant.

Final Pollutant Concentration:C1 = initial aqueous phases concentrationC2 = conc. exiting the vadose zone.T1/2 = the half life period; λ = 0.693/T1/2 = first order degradation rate of the chemical.

Final Recharge Pollutant Flux = Net Recharge Rate x Concentration of Pollutant reaching water table or Flux = qC2 where q is the net recharge rate.

The model also calculates travel time (Ttime) of BTEX (useful for vulnerability assessments).

θ soil, theofcontent water Volumetricqor V rate, Recharge V d

a =f

d

fp

RθV

Rθq v ==

⎥⎥⎥

⎢⎢⎢

⎡−

=−= 1/2

f

1

qTz

expC)Tλexp(CCθR 0.693

12

qRθ z T f

time =

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Predicted Vadose Zone Travel Times for Benzene in Recharge Waters around Coastal Park Area

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Predicted Final Benzene Conc. for a 4 Microgram Initial Conc. in Recharge around Coastal Park Area

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Modifications of UGIF for Vulnerability

Development of Screening Level Vulnerability Assessment Models:

Screening level models are relatively simple, easy-to-use, and provide management decision support in areas such as:

a. management of water resources (regional planning as related to groundwater control);

b. formulation and implementation of regulatory policies (zoning, land use alterations and practices that protect groundwater quality);

c. identification of “hot-spots” and selection of pollution abatement strategies; and

d. design and management of groundwater monitoring programs.

Three models have been chosen for development in ArcView GIS for organic compounds:

1. The Attenuation Factor Model of Rao et al. (1985),2. Leaching Potential Index Model of Meaks and Dean (1990) and 3. Ranking Index Model of Britt et al. (1992).

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The Attenuation Factor ModelThe Attenuation factor (AF) index denotes mass emission of a chemical from the unsaturated zone to groundwater as:

⎥⎥⎥

⎢⎢⎢

⎡−

== 1/2

f

qTz

exp MM AF

θR 0.693

1

2 ]θ

εθK ρ[1 R db

fΗΚ

++=

M1 = initial mass of chemical applied at the ground surfaceM2 = mass of chemical exiting the vadose zone.T1/2 = the half life period; λ = first order degradation rate coefficient for the chemical. Rf = Retardation factor. Rf = 1 + (ρbKd + (θs - θ) KH) / θρb is the bulk density of the soil; ε = air-filled porosity, θ is the volumetric water content of the soil; θs is the saturated water content of the soil on a volume basis; Kd = Koc foc which is the partition coefficient for the pollutant in the soil; KH is the dimensionless value of Henry’s law constant. Here Rf includes the effects of soluble-vapour phase distribution.

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Attenuation Factor Values for Benzene in Recharge Waters at Coastal Park Area

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Leaching Potential Index (LPI) Model

LPI is a methodology for ranking sites on the basis of their susceptibility to groundwater contamination. This method is a simplification of one dimensional mass balance equation for the convective transport-dispersion-reaction process of solutes in a homogeneous porous medium.

Assuming steady state conditions and negligible dispersion, Meeks and Dean (1990) simplified the mass balance equation as:

]z R)

T0.693(

θq

[ 1000 LPI f

1/2

=⎥⎥⎥

⎢⎢⎢

⎡−

== 1/2

f

qTz

exp MM

CC

θR 0.693

1

2

1

2

1000 is a constant that converts the LPI into a practical range. The term within the parenthesis is an indication of the vulnerability of a site. High values indicate greater susceptibility to contamination.

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Predicted Leaching Potential for Benzene in Recharge Waters around Coastal Park Area

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Ranking Index ModelRanking Index model is a methodology developed by Britt et al. (1992) for streamlining the pesticide registration and approval program of the Florida Department of Agricultural and Consumer Service, USA.The ranking index (RI) for a chemical denotes the vulnerability to groundwater contamination by that compound.RI is expressed as:

]T)

θq(

z R 0.693[ RI 2/1

f =

This model requires setting up of a threshold value for RI (e.g. 500); thus a chemical with an RI of 500 or higher value for a particular site was considered for registration. If the RI is less than 500, then a complete analysis involving studies on leaching, adsorption/desorption, hydrolysis, soil dissipation, and groundwater monitoring is required for registration.

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Predicted Ranking Indices Values for Benzene in Recharge Waters around Coastal Park Area

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Vulnerability of Conservative Contaminants

Assessment of intrinsic vulnerability of conservative contaminants can be done based on the evaluation of vertical travel time from the land surface to the aquifer.

Travel time through the vadose zone can be calculated using the simple formula:

d time

V θ z T =

Where Ttime is travel time in years,z = vadose zone depth (m),θ = average moisture content or volumetric water content and Vd is average recharge rate (m/day). A similar approach has been undertaken in Poland by Witkowski and Kowalczyp, 2004.

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Predicted Vadose Zone Travel Times for Conservative Solutes in Recharge Waters around

Coastal Park Area

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The Way Ahead

1. Preparation of other input data such as evapotranspiration, soil moisture deficits for a year and collation of chemical data (EMC data) for Cape Flats area

2. Demonstration of the UGIf model for assessment of recharge pollutant fluxes and vulnerability for the Cape Flats aquifer

3. Incorporation of suitable model for vulnerability assessment in fractured aquifers

4. Collation of information/data for Secunda site, preparation of model inputs and demonstration of the model

5. Sensitivity analysis in order to find out the effect of change of one parameter on the model output values

6. Preparation of Guidelines for vulnerability assessments

7. Preparation of vulnerability maps for the case study areas

8. Organising a short course on GIS based vulnerability assessments

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References

Britt, J.K., Swinell, S.E. and McDowel, T.C. 1992. Matrix decision procedure to assess new pesticides based on relative groundwater leaching potential and chronic toxicity. Environmental Toxicology and Chemistry. Vol. 11, pp. 721-728.

Meeks, Y.J. and Dean, J.D. 1990. Evaluating ground water vulnerability to pesticides. Journal of Water Resources Planning and Management. Vol. 116. No. 5, pp. 693-707.

Rao, P.S.C., Hornsby, A.G. and, R.E. Jessup. 1985. Indices for ranking the potential for pesticide contamination of groundwater. Proceedings of the Soil and Crop Science Society of Florida, Vol. 44, pp.1-8.

Thomas, Abraham, 2001. A Geographic Information System Methodology For Modelling Urban Groundwater Recharge And Pollution. Ph. D. Thesis. The School of Earth Sciences, The University of Birmingham, Birmingham, United Kingdom.

Witkowski, A.J. and Kowalczyk, A. 2004. A simplified method of regional groundwater vulnerability assessment. Proceedings of Groundwater Vulnerability Assessment and Mapping- Ustron, Poland -2004.

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THANK YOUVERY MUCH

&ANY QUESTIONS ?