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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
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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
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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.
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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
[email protected] | www.up.ac.za/geology