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“Improved methods for aquifer vulnerability assessments and protocols (AVAP) for producing vulnerability maps, taking into

account information on soils”

WRC Project K5/1432

PROGRESS REPORT 2004/2005

Deliverable: Unsaturated Zone with emphasis on the regolith

Department of Earth Sciences University of the Western Cape

February 2005

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1. Introduction This report contributes to a larger WRC funded project entitled: “Improved methods for aquifer vulnerability assessments and protocols for producing vulnerability maps, taking into account information on soils. This report specifically deals with the 2004-2005 work programme (Testing and documenting suitable vulnerability assessment methods in key study areas) where the following is discussed:

��Progress and activities to date; ��Potential problems/bottlenecks.

2. Progress and activities Data are currently being collated mainly for the Cape Flats aquifer (CFA). Mr Alfred Majola is currently creating a database of all the relevant information on a larger scale on the CFA. The data will be used to create a vulnerability map of the CFA, using mainly existing data and knowledge. The data will then be screened to see what parameters of the CFA are well known and where additional data need to be collected. In the unsaturated zone, different flow and transport models were reevaluated by Dr Nebo Jovanovic. The models identified during the literature review phase were VLEACH, MIKE SHE and Urban Groundwater Pollutant Flux (UGPF). The difference between this review/reevaluation and the review for the first deliverable is that we had to find models that are freeware. Another difference is that, at this point in time, we have tried and tested the models downloaded from the web sites by trial simulations in order to familiarize ourselves with the operation, assumptions, required inputs and type of outputs. The models identified are:

��SWAT 2D; ��SWMS 2D; and ��SWMS 3D.

The main objective of numerical modelling will be to compare the results to those obtained from the improvement of the estimation of the I rating in the DRASTIC index method. However, the modelling results should also be used by the other team members dealing with soil and groundwater. SWMS_2D SWMS_2D (Simulating Water and solute Movement in 2D variably Saturated media) is the model code used in the Windows-based modeling package HYDRUS2D (see HYDRUS2D model). SWMS_3D SWMS_3D (Simulating Water and solute Movement in 3D variably Saturated media) is the 3D version of the SWMS_2D model. Required inputs - Data source Runoff – WR90 data Recharge – To be estimated by research team / Available information Preferential flow – To be estimated by research team / Data from similar environments. Chemistry – Sampling and measurements

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SWAT SWAT (Soil and Water Assessment Tool) is a model that predicts the effects of climate and vegetative changes, reservoir management, groundwater withdrawals and water transfer on hydrology, pesticide and nutrient cycling, erosion and sediment transport in large, complex, rural river basins. SWAT can analyse watersheds and river basins of 100 square miles by subdividing the area into homogenous parts. It uses daily time step for continuous periods from 1 to 100 years. The hydrology is based on the water balance. Soil profiles can be subdivided into 10 layers. SWAT also provides for runoff, by-pass (preferential) flow, lateral water flux, sediment yield and size, interactions between surface and groundwater, whilst the SWAT-GIS linkage incorporates advanced visualization tools capable of statistical analysis of output data. Inputs include information from databases and information from a GIS interface. A soil database includes information on soil type, texture, depth and hydrologic classification. Spatially distributed parameters of elevation, land use, soil types and groundwater table are used in the model. More specific information can be entered singly, for each area or for the watershed as a whole. Main outputs are sub-basin attributes (coordinates and boundaries), topographic attributes (stream length, stream slope and geometrical dimensions, accumulation area, sediment loss), groundwater attributes (time lag of groundwater flow for each sub-basin), routing structure for sub-basins, based on the elevation map. Also, it defines the channel width and depth using a neural network that is embedded in the interface, based on the drainage area and average elevation of a sub-basin. Required inputs - Data source Runoff – WR90 data Soil characteristics - Sampling and measurements / Existing information Recharge – To be estimated by research team / Available information Preferential flow – To be estimated by research team / Data from similar environments. Chemistry – Sampling and measurements To run the specific models, we need (Zhu and Anderson, 2002):

1. Specific information describing the geological system (stratigraphy, thickness, hydraulic properties etc.);

2. Type of chemicals and concentrations (conceptualization of what chemical reactions are occurring and what chemical reactions are important); and

3. Thermodynamic, kinetic and phase-partitioning properties for the specific chemical system.

4. Data collection The data collection to date has been focussed on acquiring data/information needed to run several models.

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Geological system Students are currently being trained to collect soil and unsaturated zone samples and analyses. Porosity and pore size distribution are currently being determined on several samples from the UWC Campus Aquifer Test Site, which is similar to the sands of the iThemba lab case study area. The equipment used is the Eijkelkamp soil moisture retention systems (plate 1). Plate 1: Equipment used to determine porosity and pore-size distribution. The water retention data for the UWC Campus site are shown in Appendix A. We are currently in contact with the Council for Geoscience for the use of their portable soil-coring rig. The coring rig will enable us to sample in the unsaturated zone for detailed descriptions, soil moisture extractions, etc. Soil moisture extractions will allow us to construct chloride profiles as well as soil moisture profiles, which may give us an indication of the relative contributions of preferential and matrix flow. Funds for deep core sampling at iThemba labs are available from another project funded by the NRF and the results of the measurements will be used in this project for modelling purposes. The coring exercise will take place in the next few weeks. Conceptualization of what chemical reactions are occurring and what chemical reactions are important The chemical reactions occurring as the pollutant flux moves through the unsaturated zone will be evaluated. This will require additional modeling and the use of the database of properties of chemicals developed by Usher et al., 2004). Thermodynamic, kinetic and phase-partitioning properties for the specific chemical system. Existing data will be used (Usher et al., 2004). 5. Problems/Bottlenecks Problems/bottlenecks encountered to date:

1. The student working on the project has taken an internship with the Department of Water Affairs and Forestry.

2. The costs of “suitable” commercial software packages.

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These problems are currently minor and can probably be accommodated by: 1. Finding a suitable student to take over. 2. Using freely available software.

References Usher BH, Pretorius JA, Dennis I, Jovanovic N, Clarke S, Cave L, Titus R and Xu Y (2004). Identification and prioritisation of groundwater contaminants and sources in South Africa’s urban catchments. WRC Report No. 1326/1/04. Zhu C and Anderson G (2002). Environmental Applications of Geochemical Modeling. Cambridge University Press.

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Appendix A Water retention properties of vadose zone samples collected at the UWC Campus Aquifer Test site.

Bulk density: 1,50 g cm-3 Porosity: 40.4%

Depth 100 cm

0.1

1

10

100

0 0.1 0.2 0.3 0.4 0.5

Volumetric Water Content (m3m-3)

Pre

ssur

e (k

Pa)

6

Bulk density: 1.31g cm-3 Porosity: 40.0%

Depth: 180 cm

0.1

1

10

100

0 0.1 0.2 0.3 0.4 0.5

Volumetric Water Content (m3m-3)

Pre

ssur

e (k

Pa)

7

Bulk density: 1.36g cm-3 Porosity: 46.1%

Depth: 250 cm

0.1

1

10

100

0 0.1 0.2 0.3 0.4 0.5 0.6

Volumetric Water Content (m3m-3)

Pre

ssur

e (k

Pa)

8

Bulk density: 1.28g cm-3 Porosity: 47.0%

Depth: 320 cm

0.1

1

10

100

0 0.1 0.2 0.3 0.4 0.5

Volumetric Water Content (m3m-3)

Pre

ssur

e (k

Pa)