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