**4. Description of the study area**

80 Studies on Water Management Issues

Cape Flats aquifer were selected. These bores were investigated by evaluating the groundwater levels and salinity (specifically the electrical conductivity) within shallow aquifers. The criteria for selection were continuous groundwater level record (minimum of 10 years record, with minimal interruptions or errors) and screened within the respective

The time-series groundwater data at selected locations within Cape Town area were compared with those of bore network data in the Werribee Plain. The analysis of this data was undertaken using Hydrograph Analysis: Rainfall and Time Trends (HARTT), a statistical tool that analyses groundwater data using the effect of long-term rainfall patterns, determined by accumulative residual techniques (Ferdowsian et al., 2001). This method can differentiate between the effect of rainfall fluctuations and the underlying trend of groundwater level over time. Rainfall is represented as an accumulation of deviations from average rainfall, and the lag between rainfall and its impact on groundwater is explicitly represented. HARTT produces a fitted curve through the groundwater level readings.

According to Ferdowsian et al. (2001), two variables are used to produce this line:

Rainfall variable (X1); accumulative monthly residual rainfall (AMRR, mm), or accumulative

The R2 value (the coefficient of determination) is the degree of fit of the calculated curve compared to the recorded water levels (a value of 1 is a perfect fit; the degree of fit becomes less with decreasing values below 1). The p-value indicates the level of significance of each variable. If the p-value is less than 0.05, then the variable is significant. If it is less than 0.01, then it is highly significant. If the trend is not significant (as determined by R2) then the rate of rise or fall is not reliable. And if the rainfall variable is not significant then the reliability of the effect and the delay period (in the hydrograph response to effective rainfall) is low

The method improves the estimation of time trends and allows for better interpretation of treatment effects on groundwater levels. The advantage and limitation of this method over other techniques of hydrograph analyses have been highlighted in Cheng et al. (2011).

Y = c + aX1(rainfall) + bX2 (time trend) (1)

Y' = aX1(rainfall) (2)

Y'' = c + bX2 (time trend) (3)

aquifers under this study.

annual residual rainfall (AARR, mm).

Where:

c is the intercept.

(Ferdowsian et al., 2001).

Time trend (X2) (1,2,3 days…from first reading)

At any point along the fitted curve, the following equation holds:

a and b are coefficients calculated in the multiple regression analysis.

So, to calculate the effect of rainfall, the following equation is used:

And to calculate the underlying trend, the following equation is used:

Y is the water level depth at a point along the fitted curve.

The vegetable growing Werribee Irrigation District (WID) lies on Melbourne's rapidlydeveloping western urban fringe underlain by shallow Delta aquifer. The name of the management area for the Werribee Delta aquifer is the Deutgam Water Supply Protection Area (WSPA). The aquifer is linked to both Port Phillip Bay and the tidal extent of the Werribee River (SRWA, 2009). Deutgam WSPA is located around the Werribee South irrigation area (Figure 1). On the other hand, the fresh fruits and vegetable farm area in Cape Town is located on the Cape Flats, especially the Phillipi-Mitchells Plain Irrigation area. A large portion of the area around Cape Town is the sand-covered coastal plain (Cape Flats) shown in figure 2. The City of Cape Town Management Area (CMA) is largely surrounded by the Atlantic Ocean to the west and south with the most prominent landmass being the Cape Peninsula, attached to the mainland by the sandy plain of the Cape Flats (Schalke, 1973; Theron et al., 1992). The greater portion of the entire sand cover of the Western Cape are been considered in this study, particularly the south-western part of the City of Cape Town and the north-western end (Atlantis), where basic data and bore information are available.

Fig. 1. Location of the Werribee Plains and Deutgam WSPA, western fringe of Melbourne. Inset: Deutgam WSPA (red spot) in Victoria (grey shade) within map of Australia

Changes in Groundwater Level Dynamics in Aquifer Systems –

semi-confined conditions in places (Adelana et al., 2010).

these exceed 120 m in thickness (Holdgate et al., 2001).

**4.1.2 The Werribee Plains** 

(Rodda & Kent, 2004).

Implications for Resource Management in a Semi-Arid Climate 83

According to Meyer (2001) bore yield (from about 497 boreholes in the Sandveld Group) indicates that 41% of boreholes yield 0.5L/s and less while 30% yields 2L/s and more. Transmissivity values range from 32.5-619m2/d (from recent pumping test data in Adelana, 2011), but typical values between 200 and 350 m2/d were recorded in Gerber (1981). A detailed description of the hydrogeology of the different geological units is documented in Meyer (2001). The net groundwater recharge to the Cape Flats aquifer in the south-western Cape varies between 15% and 47% of mean annual precipitation (Adelana, 2011). The general aquifer configuration and flow direction in the Cape Flats has been presented as indicating flow from western and south-eastern to the coast. A conceptual model of the aquifer has been developed to indicate all flow is regionally unconfined and two-dimensional with negligible vertical components, although inter-bedded clay and peat layers produce

The Deutgam WSPA includes all geological units to 40m below the natural surface, encompassing the shallow Werribee Delta sediments (DWSPACC, 2002). An alluvial deposit up to 20 m thick has accumulated in the gorge of the Werribee River. This gorge is the major terrain feature of the Werribee Plains with its alluvial deposit known to be gravely at the base and fumes upwards to become clayey at the surface (Condon, 1951). According to this work and more recent studies (Holdgate et al., 2001, 2002; Holdgate & Gallagher, 2003), the alluvial terraces on the valley floor provide evidence of Pleistocene and Holocene sea level changes. This alluvium, eroded by rejuvenated streams, was deposited (in Late Quaternary times) along the base of the Werribee River. There are prominent intra-volcanic sands within the Newer Volcanics (along the Werribee Plains) while the Older Volcanics were picked in few bores between coal-bearing sediments of the Werribee Formation (Holdgate et al., 2001). In general, the Werribee Formation is disconformably overlain by marine sandstone and mudstone/marlstone (Taylor, 1963 as cited in Holdgate et al., 2002; Holdgate & Gallagher, 2003). Across the Werribee Plains

The groundwater system used in the Werribee South is called the Werribee Delta aquifer. The Werribee Delta sediments consist of sand and gravel lenses situated within clays and silts. The variable nature of the deltaic sediments resulted in a wide variation in aquifer parameters (SKM, 2002). According to SKM (1998), within the coarser sand horizons the hydraulic conductivity ranges from 10 to 15m/day, with a specific yield of 0.01 to 0.2 but the overall hydraulic conductivity of the aquifer is less than 5m/day with representative specific yield in order of 0.04. Typical bore yields for the Werribee Delta aquifer system are generally less than 5L/s, however yields up to 15L/s have been recorded (SKM, 2002). The selected bores for this study were screened in the Werribee Delta aquifer system, which are mostly sandy or silty clay material at shallow depths but with significant sand and gravel seams at a relatively deeper depth. The Werribee Delta aquifer system is unconfined to semi-confined and groundwater depth varied between 4-7m below ground surface. Recharge to the aquifer system is predominantly from direct rainfall infiltration and surplus irrigation water (SKM, 2002) as well as leakage from the ageing concrete-lined channels

#### **4.1 Geology and hydrogeology**

#### **4.1.1 The Cape Flats**

A study of the geological units show the oldest rock in Cape Town and suburbs are the meta-sediments of the pre-Cambrian Malmesbury Group, which occupy the coastal plain between Saldanha and False Bay in the west, to the first mountain ranges in the east (Meyer, 2001). Several erosional windows to this Group are exposed in mainly fault-controlled valleys further to the east and south. Natural features are varied and include narrow flats, kloofs and gorges, cliffs, rocky shores, wave-cut platforms, small bays and sandy and gravel beaches. On the Cape Flats sand dunes are frequent with a prevalent southeasterly orientation; and the highest dunes are only 65 m above sea level (Schalke, 1973; Theron et al., 1992). The sand is derived from two main sources: (i) weathering followed by deposition, under marine conditions, of the quartzite and sandstone of the Malmesbury Formation and the Table Mountain Series; (ii) the beaches in the area, from where Aeolian sand was deposited as dunes on top of the marine sands.

Fig. 2. Location of the Cape Flats sand in the Western Cape, South Africa (Adelana et al., 2010)

According to Meyer (2001) bore yield (from about 497 boreholes in the Sandveld Group) indicates that 41% of boreholes yield 0.5L/s and less while 30% yields 2L/s and more. Transmissivity values range from 32.5-619m2/d (from recent pumping test data in Adelana, 2011), but typical values between 200 and 350 m2/d were recorded in Gerber (1981). A detailed description of the hydrogeology of the different geological units is documented in Meyer (2001). The net groundwater recharge to the Cape Flats aquifer in the south-western Cape varies between 15% and 47% of mean annual precipitation (Adelana, 2011). The general aquifer configuration and flow direction in the Cape Flats has been presented as indicating flow from western and south-eastern to the coast. A conceptual model of the aquifer has been developed to indicate all flow is regionally unconfined and two-dimensional with negligible vertical components, although inter-bedded clay and peat layers produce semi-confined conditions in places (Adelana et al., 2010).
