Effective RWH Systems – Significance of Geology and Hydrogeology

J.SaravananConsulting Hydrogeologist

Thrust Geoconsultants P Ltd, Chennai

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Presentation Structure

•What is effective RWH System?

•What is the Geology & Hydrogeology of

Chennai city?

•How do I know the hydrogeology of my

locality?

•Why effective RWH?

•Towards effective water management in

Chennai city

Effectiveness wrt RWH Coverage…

Effective wrt RWH designDesigning a RWH system

(particularly in the case of ) groundwater recharge based on the hydrogeology of an area.

• Better functioning of the system• Maximum realisation of benefits• Cost benefit• Case of Besant Nagar, Anna

Nagar, Tambaram

Besant Nagar•Coastal Alluvium•Sandy strata (aquifer)•Good Porosity and Permeability•Water table : Shallow

       

   

   

Coastal Alluvium(Sandy)

   

   

   

Anna Nagar•Gondwana - Clay /Shale mainly with intermittent silt / fine sand •Highly Porous but poor in permeability•Water Table Shallow to moderate

  Clay  

 

   

   

Fine Sand / Silt

Clay

Shale

Tambaram•Hard rock area•Weathered / fractured rock (aquifer)•Moderate to Poor in Porosity and Permeability•Water Table : Shallow? To Deep

 

Top Soil / Weathered rock

 

Partly Weathered / Fractured rock

Fr. Rock

Hard rock w minor fr.

Geology of ChennaiEra Formation Hydro Type area

Recent and Sub-Recent

Alluvium,laterite, Conglomerate & Black clay.

Good (exl. Clay)

Coastal, central chennai

Tertiary(2.6 M yrs)

Sand and sandstone mainly

Good to very good

North Chennai, Ambattur..

Sriperumbudur beds (150 M yrs)

Clay, Shales, Felspathic Sandstone.

Poor

East Chennai, Porur, AnnanagarLower

Gondwanas Green Shales and boulders.

Poor

Archaean (4500 M yrs)

Dolerite, Charnockite and Gneiss.

Moderate - Poor

South Chennai

Chennai’s Aquifer capacity

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The total storage capacity of chennai city aquifer works out to 392 mcm. (1 mcm is 1000 MLt; 2.74 MLD)

The storage capacity reduces drastically with falling water table.

The capacity at 10m water level is 259 mcm ie., 66% of the total storage capacity

The storage capacity at 20m water table is 126 mcm which is about 32% of the total storage capacity.

This may be due to the fact that the major yielding zones are restricted to the top 30m bgl which is mainly alluvium.

Beyond 30m depth the porosity and specific yield of the formations such as shale and hard rock is very poor and hence cannot store nor yield sufficient quantity of groundwater.

This also corroborates with the reduction of yield in borewells during summer when the water table depletes drastically.

Hydrogeology of my locality

Geotechnical Report for foundation designing

Hydrogeology of my locality…

Hydrogeological Report for groundwater surveysObserving BW drilling in neighbourhoods (hand auger, rotary, DTH)

Government Departments – CGWB, CMWSSB, TWAD, PWD Groundwater.http://cgwb.gov.in/District_Profile.htmlhttp://www.mdws.gov.in/node/2623

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Hydrogeology of my locality…

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Alternate Days Supply

Alternate Days Supply

GW Reg. Act

Telugu Ganga Opted

Telugu Ganga Realised

RWH Mandatory for new buildings

Piped water supply stopped

RWH Mandatory for all buildings Veeranam tapped

100MLD Desal Unit

Public Supply Vs Water Policy Vs RF

Source Dependance by Chennai ite’s

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Source % of dependance to meet the daily water requirement

Metro Water 35 %Groundwater 62 %

Status of Public Supply Present Public System is highly subsidised

and hence lacks economic sustenance

Cost varies from Rs. 10 to Rs. 45 per KL

At present tanker supply costs Rs. 100 per KL

Sources :

AK Basin Surface Water T G Veeranam

Neyveli Aquifer Kollidam?? RO Lowest Per capita supply in the country

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Towards Effective Water Management System

Water Requirement Estimation Percapita requirement (70.…

135….150lpcd)????

Development should be based on resource availability or FSI or Guideline Value?

Case of OMR and other High value projects Aquifer carrying / bearing capacity….

Development based on public water supply system??? Environment Consideration….

Importance to sea facing or sustained fresh water availability?

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Effective management…

Need for a comprehensive study on Chennai’s groundwater resourcesMapping of Aquifers – Aquifer Potential – Recharge sources – Safe Yield estimation – Conservation and Management

Conducting Groundwater Studies prior to construction

Geology, Hydrogeology, Geophysical Surveys Part of EIA –(Safe, Critical, Semi Critical, Over

Exploited)

Identifying potential groundwater sources within the siteSelection of suitable drilling techniqueBorehole diameter

Effective management ….

Record on Wells, drilling, well designing and construction – to record the lithology and decide slot position

Well development and yield measurement through V notch plateWater level measurement after well drilling / developmentSelection of suitable pump based on the total working head, yield of the borewell and water levelWell completion report containing all the above details

Effective management ….

Provision of space, atleast to drill a borewell in future in the rest of identified potential pockets.Dug wells still a viable option at localities – case of Selvaraj Nagar, Urapakkam; Ramapuram

Tapping shallow zone during post monsoon by provision of slots in borewell casingShallow water table zones with clayey Soil Safe disposal of waste water during constructionSafe disposal of RO reject and STP treated water

Do’s and Do’nts….

Constant depletion of water table is an indication of

Over exploitation of GW sourcesReduced recharge

Sustaining the water table fluctuation through RWH and GWR&R to avoid dwindling of water tablePeriodic water quality analysis