Water for a food-secure world

Interaction between canal water and groundwater

in the Uda Walawe irrigation system:hydrochemistry and isotopes

Mauro Prado, Kevin Hiscock, Lorraine Rajasooriyar, Eline Boelee

Universities of East Anglia , Jaffna&

International Water Management Institute

Presented at Session C “Innovation and Technologies” of International Symposium 2010, Ruhuna University, Sri Lanka

November 16, 2010

Water for a food-secure worldWater for a food-secure world

Outline

• Approach• Study area: geology and sampling points• Sampling results and analysis• Discussion, conclusions and

recommendations

Water for a food-secure worldWater for a food-secure world

Approach• Isotope study; part of a larger research project• Objectives

– Understand determining factors of groundwater quality– Interactions between surface waters and groundwater– Consequences for management

• Mapping– Water infrastructure, geology, land use– Results

• Sampling• Analysis

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Study area: Uda Walawe irrigation system

• Average Temp 28°C• Humidity 77%• ETpot 1988 mm• Rain 1979 mm

Source: IWMI, 2002 Etpot :Potential Evapotranspiration

Water for a food-secure worldWater for a food-secure world

N

Uda Walawe

Habaraluwewa

Kirribanwewa

Suriyawewa

Ridiyagama

Hambantota

Ambalantota

Left Bank Canal

Right Bank Canal

The study areaWalawe river

StreamsMajor StremasMinor StreamsCanals

ReservoirsSea

Liyangastota

Uda Walawe irrigation system < 400 km2

Source: L. Rajasooriyar, 2003

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Sub-basins and sampling points• Two sub-basins

– Uda Walawe– Ridiyagama

• Three sampling rounds– Tube wells / boreholes– Shallow dug wells– Canals

• Chemical analysis– Ion specific electrodes, ion

chromatography, titration, spectrophotometry

• Isotope analysisSource: MCCM .Prado, 2002

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Results - I• Small sample size for isotope analysis• Combining with data from hydrochemical study (fluoride)• Variation between surface and groundwater in terms of

physical and chemical featuressurface water groundwater

dissolved oxygen 4.6-6.7 mg/l 1.1-5.4 mg/l

pH 7.6-8.5 6.4-7.9

Eh 101-228 mV -26-180 mV

electrical conductivity <450 μS/cm <3910 μS/cm

chloride <0.1-24.8 mg/l 4.4-921.4 mg/l

fluoride 0.15-0.56 mg/l 0.25-4.66 mg/lSource: Created by Eline Boelee for the purpose of this presentation

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Results - II• Contrast between surface and groundwater• More pronounced for deeper groundwater

0

2

4

6

8

-100 0 100 200 300

Eh (mV)

D.O

. (m

g/l

)

Surface water

Shallow groundwater

Deep groundwater

Source: Created by Eline Boelee for the purpose of this presentation

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Results - III• Trend towards enrichment in heavier isotopes

towards coast

-7

-6

-5

-4

-3

-2

-1

0

0 10 20 30 40

Distance (km)

Oxy

gen

-18

(‰)

Surface water

Deep groundwater

Shallow gw - unlined areas

Shallow gw - lined areas

Source: Created by Eline Boelee for the purpose of this presentation

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Results - IV

• Trend towards enrichment – possible explanations– Surface water: increasing evaporation– Groundwater:

• Enrichment in discharge areas close to the sea• Lower rainfall in south• Influence sea spray

• Mapping various data to understand better

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N

Landuse PaddyCoconutOther PlantationGardenMarshScrubForestGrasslandChenaReservoirsPondsWater HolesRock OutcropsSand or BeachInland Lagoons

Uda Walawe land use

Source: L.Rajasooriyar, 2003

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hjhgjkhsjkhg

Uda Walawe geology

N

1 0 1 Km

Not classifiedGranite GneissCharnockitic Gneiss and Charnockitic Biotite GneissCharnockiteMarble (Dolomite)Biotite GneissGametiferous Quartzofeldspathic GneissPegmatiteHornblende-biotite Gneiss & hornblende biotite migmatiteHornblende Gneiss or AmphiboliteGarnet-sillimanite-biotite GneissBiotite-hornblende Gneiss and Biotite-hornblende MigmatiteQuartziteMetagabbro

Walawe riverReservoirs

Shear ZonesFault, Fracture or Major joint

A

B Geosutur

Source: L.Rajasooriyar, 2003

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N

Walawe riverReservoirs

10 m interval contours

Uda Walawe groundwater flow

Source: L.Rajasooriyar, 2003

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Uda Walawe fluoride

Source: L.Rajasooriyar, 2003

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Uda Walawe fluoride related to canal recharge

Source: L.Rajasooriyar, 2003

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Discussion• Groundwater is recharged from paddy fields,

unlined canals and tanks• Isotope analysis limited in sample size• Results in line with those from hydrochemical

analysis • Important implications for groundwater quality and

drinking water supply• No quantification of surface – groundwater

interactions• Actual situation may have evolved…

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Conclusion• Groundwater recharge dilutes contaminants, e.g.

fluoride• Shallow wells are preferred sources of drinking water• Recharge = seepage = leakage?• Lower irrigation efficiency?• Integrated approach required for efficient and

equitable management of surface and groundwater in Uda Walawe

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Related Publications

• Rajasooriyar L (2003). A study of the hydrochemistry of the Uda Walawe Basin, Sri Lanka, and the factors that influence groundwater quality. Ph.D. thesis, University of East Anglia, UK

• Prado MCCM (2002).Contribution of irrigation canal water to aquifers in

South Sri Lanka. M.Sc. Thesis, University of East Anglia, UK

Water for a food-secure worldWater for a food-secure world

Thank you!

IWMI - www.iwmi.org

University of Jaffna - www.jfn.ac.lk/

University of East Anglia - www.uea.ac.uk

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