Vadose Zone Hydrology (Land Surface to Phreatic Surface) Matthys Dippenaar Engineering Geology and...

Vadose Zone Hydrology(Land Surface to Phreatic Surface)

Matthys Dippenaar Engineering Geology and Hydrogeology

Department of GeologyUniversity of Pretoria

17 November 2014Water Research Commission Khuluma Sizwe Series:

Hydropedology in support of Hydrology and Eco-hydrology

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What happens (OFTEN) in Joburg

• Increased golf course irrigation on JDG– Waterlogged soils– Increased interflow & unsaturated seepage– Damage to infrastructure

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What happens (OFTEN) in Joburg

• Increased golf course irrigation on JDG– More common problem than expected as “recreational developments”

allow development of areas where large portions are zoned for no development

– Insurance company in court with developer as this is poor planning – Ca. 50 units affected in affluent golfing development

The South African Vadose Zone(Geological Perspective)

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Ca. 4

m d

epth

Lans

eria

tona

lite

gnei

ss, M

idra

nd

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The Vadose ZoneTransported Soils

ResiduumFerruginized/ ferricrete

Completely WeatheredErodible

Highly Weathered

Fresh Jointed Bedrock

Reg

olit

h

Sapr

olit

e

Pebble MarkerFerruginized/ ferricrete

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The Vadose Zone

• South African vadose zone is thick (easily tens of meters)

• Includes soil and rock, and often different lithologies with depth

• Given the age of our rocks and the intricate geomorphological past, geological profile development is often distinct throughout the depth

• Similarly, all hydrological parameters (porosity; conductivity; permeability; specific yield) vary with spatially and depth

• Remember: engineers call weak rock soil (UCS < 1 Mpa)

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Karoo dolerite sill at Gariepdam

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Colluvium

Ferruginized Horizon

Granite Saprolite

Porosity ca. 0.22Pore size smallConnectivity poorAdhesion dominates

Porosity ca. 0.15Pore size largeConnectivity goodCohesion dominatesPossibly periodically

saturated

Quartz

Feldspar

Clay/ Mica

Goethite

WaterLimited deep percolation

due to smaller pore sizes in saprolite

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Translocated downslope with shallow flowWeathers further into expansive claysDeposited at footslope as duplex soilOften waterlogged at surface

Smectite

Quartz

Feldspar

Clay/ Mica

Goethite

WaterLimited deep percolation

due to smaller pore sizes in saprolite

Kaolinite leached

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Expansive clay

Quartz

Feldspar

Clay/ Mica

Goethite

Water

Land Use Changes and the Impacts Thereof?

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Volume Change

• Variable and partial saturation affect and are affected by land use change

• Variable moisture contents triggering mechanism for– Heave (frequent swell-shrink cycles)– Collapse (threshold moisture content to weaken; saturated soils are more

self-supporting; permanent reduction in porosity post-development)

• Significant roleplayer in– Karst subsidence (ingress-scenarios >90% of all recorded)– Dispersivity and erodibility– Settlement

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Effects of Urbanisation

• Stormwater – reduced overall recharge, localised recharge, decreased stream runoff

• Irrigation and landscaping – changes in infiltration vs runoff

• Leaking underground services – increased subsurface water

• Subsurface drainage – interruption of natural hydrology

• Reductions in stream flow – loss of connectivity between channels and possibly between surface water and groundwater

• Aquifer vulnerability – variable and dense sources of contamination coupled with all of the above

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Effects of Urbanisation

• Compaction – reduced infiltration, permeability and porosity

• Surface sealing – reduced infiltration, increased runoff, interruption of connectivity of stream channels and wetlands

• Artificial ground – altered and highly variable hydraulic properties

• Manmade materials – altered properties of concrete, geotextiles, etc.

The Next Step?

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Some Ideas…

• Better investigation focused around all impacts of land use change– Consider the impacts of changing water budgets to the subsurface– Anticipate the long-term effects on runoff versus infiltration– Realize the impacts on surface ecology, surface drainage and groundwater– Design for impacts on infrastructure development

• Sort out the bedrock interface– (How) does water enter saprolite from soil?– Variably saturated fracture flow– Epikarst (dolomite vadose zone)

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Thank You!

Questions?

Bibliography and more information available in WRC report TT 584/13

madip@up.ac.za | www.up.ac.za/geology