1. Vadose Zone Hydrology (Land Surface to Phreatic Surface)
Matthys Dippenaar
Engineering Geology and Hydrogeology
Department of Geology
University of Pretoria
17 November 2014
Water Research Commission Khuluma Sizwe Series:
Hydropedology in support of Hydrology and Eco-hydrology

2. What happens (OFTEN) in Joburg
Increased golf course irrigation on JDG
Waterlogged soils
Increased interflow & unsaturated seepage
Damage to infrastructure
A case in point is water damage resulting from increased golf course irrigation in Fourways. The site, underlain by Lanseria Gneiss of the Johannesburg Dome Granite, is intrinsically prone to perched water tables and ephemeral hillslope seeps.
This was further exacerbated by increased infiltration and subsequently an insurance lawsuit ensued.

3. 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

4. The South African Vadose Zone (Geological Perspective)

5. Lanseria tonalite gneiss, Midrand
Ca. 4 m depth

6. The Vadose Zone Transported Soils Pebble Marker Regolith Residuum
Ferruginized/ ferricrete
Completely Weathered
Erodible
Highly Weathered
Fresh Jointed Bedrock
Regolith
Saprolite
Pebble Marker

7. 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)

8. Karoo dolerite sill at Gariepdam
Thick, variable and often fractured vadose zone.

9. Limited deep percolation due to smaller pore sizes in saprolite
Porosity ca. 0.22
Pore size small
Connectivity poor
Adhesion dominates
Colluvium
Porosity ca. 0.15
Pore size large
Connectivity good
Cohesion dominates
Possibly periodically saturated
Ferruginized Horizon
Quartz
Feldspar
Clay/ Mica
Goethite
Water
Granite Saprolite
The case of Randjesfontein in Midrand illustrates this where honeycomb ferricrete has low overall porosity, but pore spaces are large and well connected.

10. Limited deep percolation due to smaller pore sizes in saprolite
Kaolinite leached
Smectite
Quartz
Feldspar
Clay/ Mica
Goethite
Water
Translocated downslope with shallow flow
Weathers further into expansive clays
Deposited at footslope as duplex soil
Often waterlogged at surface
Kaolinite resulting from weathered orthoclase are mobilised with interflow water and deposited downslope, resulting in waterlogging on surface.
Although not at all the purpose of the paper, this emphasises the importance of understanding interflow in, for instance, the reasons behind the occurrence of wetlands rather than depending solely on biotic markers and shallow soil markers.

11. Quartz Feldspar Clay/ Mica Goethite Water Expansive clay
Kaolinite resulting from weathered orthoclase are mobilised with interflow water and deposited downslope, resulting in waterlogging on surface.
Although not at all the purpose of the paper, this emphasises the importance of understanding interflow in, for instance, the reasons behind the occurrence of wetlands rather than depending solely on biotic markers and shallow soil markers.

12. Land Use Changes and the Impacts Thereof?

13. 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
Without elaboration, another clear concern is the likelihood of water triggering compression or expansion of soils with associated damage to infrastructure.

14. 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
So why the focus on urban environments? I’ve given it some thought, and very pronounced effects result in areas prone to rapid, variable land use change. Although it is impossible to address all, some examples include, for instance:
The impacts of compaction of soils or the sealing of land surfaces. Both reduce infiltration and subsequently alter the amount of water entering the subsurface.
Stormwater removal lowers recharge or promotes localised recharge at some discharge point and is also associated with decreased stream runoff which in turn results in isolation of sections of drainage features
Irrigation and landscaping commonly have the opposite effect where a water surplus environment is created sometimes without proper design for handling this induced interflow.
The use of made ground which is essentially disturbed recompacted soil, or mandmade materials such as geosynthetics and concretes have varying influences but also result in altered flow paths.
Increased underground water from leaking pipelines and dewatering and drainage practices should also be incorporated.

15. 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.
So why the focus on urban environments? I’ve given it some thought, and very pronounced effects result in areas prone to rapid, variable land use change. Although it is impossible to address all, some examples include, for instance:
The impacts of compaction of soils or the sealing of land surfaces. Both reduce infiltration and subsequently alter the amount of water entering the subsurface.
Stormwater removal lowers recharge or promotes localised recharge at some discharge point and is also associated with decreased stream runoff which in turn results in isolation of sections of drainage features
Irrigation and landscaping commonly have the opposite effect where a water surplus environment is created sometimes without proper design for handling this induced interflow.
The use of made ground which is essentially disturbed recompacted soil, or mandmade materials such as geosynthetics and concretes have varying influences but also result in altered flow paths.
Increased underground water from leaking pipelines and dewatering and drainage practices should also be incorporated.

16. The Next Step?

17. 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)

18. Thank You!
Questions? Bibliography and more information available in WRC report TT 584/13 |