Ecological and Environmental Impacts

of Large Volume Groundwater Abstraction on Ecosystems Linked

to the Table Mountain Group Aquifer

David Le Maitre, C Brown, C Colvin, C Hartnady, R Hay, K Reimann

TMG-related studies

• WRC funded• Research oriented• Experimental abstraction

• CCT funded• Feasibility study• Pilot abstraction +/-5 x 106 m3

• > 500 m depth

WRC Research Programme

Objectives:

1. Develop an understanding of TMG aquifer systems (recharge & flow mechanisms)

2. Develop an understanding of the environmental impacts of exploitation

3. Integration of groundwater into IWRM

Environmental Project Objectives

• Scope the full range and types of potential ecological impacts of large scale groundwater abstraction from the TMG aquifer;

• Identify geographical areas and ecosystems considered likely to be dependent on groundwater;

• Prioritise areas for future monitoring and research

Groundwater settings in the TMG

Regional to landscape scale systems

Landscape to habitat scale systems

+

f

f

f

+ +

+

++

+++

Cold “perched” spring

Cold spring (water table)

Cold spring

Hot spring (artesian)

Cold “perched” spring

Cold spring

Bokkeveld (shale)

Alluvium/colluvium

+

LEGEND

Nardouw (quartzite)

Cedarberg (shale)

Peninsula (quartzite)

Kaimans/Cango (metased)

TMG

Production boreholes

Spring

Valley floor

Piezometric surface

Potential impacts

Ecosystem components potentially affected

Ecosystem Processes

Biodiversity

Direct and Indirect ImpactsDrivers and Responses

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& R

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ifer

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Key effects/drivers

• Changes in groundwater discharge/levels:• Quantity & timing

• Quality & timing

• Temperature

• Knock-on effects on (for example)• Ecosystem water regime (e.g. mix of surface and

groundwater)

• Terrestrialisation (e.g changes in fire frequency)

• Downstream environments

Responses

• Ecosystem itself:• Structure & composition

• Groundwater dependent species/communities• Unique species (e.g. endemics)

• Habitats• Habitat specialists

• Function and processes (e.g nutrient cycling)• Functional linkages with associated

ecosystems (e.g.keystone species, key processes)

Discharge regime (structurally controlled)

Constant Highly variable

Lithology controlled (contact) springs & wetlands (type 3)

Fracture controlled springs & wetlands (type 2)

Perched springs & wetlands (type 1)

Terrestrial ecosystems

Cave ecosystems

Aquatic (riverine) ecosystems & estuaries

Species & ecosystemsSpecialist Generalist

Low HighResilience to natural climatic variation

System changes

• Continuous (e.g. proportional, linear, non-linear)• point and diffuse discharge, stream/river

systems

• Discontinuous (e.g. threshold)• water table within rooting zone

Resilience

• How has the system varied over long time scales – years to aeons?

• How easily are these: • Species • Communities• Linked systems

able to re-colonise and re-establish?

Provisional approach

Prioritisation

• Areas rich in endemic/special species

• Potential knock-on effects

• Potential GDEs already IDed by CAPE

• Rivers with priority estuaries

• Existing conservation areas

• Existing threats

Monitoring

• Depends on type of ecosystem• Abiotic

• Discharges/levels• Chemistry• Groundwater contribution

• Biotic• Indicator species populations• Unique species populations• Habitat extent and structure

Key challenges

• Development of predictive tools, innovative techniques & indicators of impacts

• Enhanced understanding of TMG-related GDEs & sensitivity to variations in groundwater regimes• Annual & seasonal• Low & drought flows

• Developing a statistically sound and innovative sampling design to distinguish abstraction from natural background variability and trends

Acknowledgements

• Water Research Commission

• City of Cape Town

• Co-authors

• Workshop participants

• CSIR