**6. Salinity**

86 Studies on Water Management Issues

is a gradual increase in rainfall and a reduction in temperatures moving from north to south. Mean annual rainfall (1950-2010) has a long-term average of 597 mm. There is a dry period with less than 20 mm rainfall per month from November to March (Figure 4a); the mean annual temperature is moderate, approximately 17 °C. In the Werribee Plains, rainfall variability throughout a typical year does not exhibit a clear seasonal bias like the Cape Flats but fairly distributed, with average monthly rainfall ranging from 36 mm/month (March) to

The results of the HARTT analysis for the selected bores are summarised into a Table in Appendix I. The groundwater trends determined in this analysis are comparable for both the Werribee Plain Delta aquifer and the Cape Flats aquifer. There is little or no delay in response to rainfall events. Although most of the bores showed a generally slight decline, few bores have a rising trend yet the rise is much less than 20 cm/yr. Most of the bores screened in the Werribee Delta aquifer have groundwater trend ranging -3 to 4 cm/yr (in exception of B112802 with more positive trend, 12 cm/yr). All of these bores showed no delayed response and a quick rise in response to the wet year 2010 (after lower than average rainfall from 2007-2009) (Appendix II a-d). The selected bores in the Werribee mostly showed the lowest groundwater levels (i.e. highest drawdown) in late 2003 and early 2004 except B59536 whose highest drawdown was in February 2007

The groundwater trend of bores within the Cape Flats aquifer ranges from -8 to 8 cm/yr (except BA232 with more positive trend, 14 cm/yr; which is within the Philippi allotment portion). The bores on the Cape Flats showed marked seasonal fluctuations and a more slightly downward trend in comparison to the Werribee bores (see Table in Appendix I). The Cape Flats bores are examples of good data records with missing gaps (Appendix II e-j). Most of the bores selected along the south coast on the Cape Flats also showed no delayed response except BA002 (Appendix II e). Although there are no lithologic logs for most of these bores, there are reports of occurrence of thick lenses of clay within the Cape Flats sand aquifer (Adelana, 2011; Gerber, 1976) that may contribute to delayed response of bores to

There are no significant negative trends (groundwater trend all < -9 cm/yr) in both study area, even though a few bores were also selected from the intensively irrigated Atlantis area of Western Cape. Examples of bores from the Cape Flats sand in north-western Cape (Atlantis) showing influence of pumping in the 1990s are presented in Appendix II (k-n) with summary table in Appendix I). The Cape Flats aquifer in the Atlantis area has been under the Managed Aquifer Recharge (MAR) program since the last 20 years. Although the extent to which this has influenced the response of the bores is not known since the data are not accessible, it is expected to contribute to a more positive trend. Irrigation is intense in the Werribee area but the conjunctive groundwater use (with surface water, recycle water) may

However, the Philippi-Mitchells Plain bores are still more negative relative to both Atlantis and Werribee Irrigation districts even though both have longer history of groundwater use for irrigation. This may be in response to groundwater usage. It was estimated that

have been responsible for no significant negative trend.

59 mm/month (October) (Figure 4b).

**5. Groundwater level response** 

(Appendix II d).

rainfall events.

The analysis of groundwater trends is critical in the study of salinity risk and the effectiveness of preventative measures. The majority of DPI bores were installed in response to reports of saline discharge outbreaks in the 1980s and 1990s (Clark & Harvey, 2008). Salinity has impacts on the social, economic and environmental values in any catchment. Therefore, groundwater monitoring co-ordinated by DPI and DSE provides an important tool in the understanding, measurement and management of salinity across the state of Victoria. Hence it is currently been reviewed and prioritised based on key assets in the state (Reid et al., 2011).

In both WID and the Cape Flats, salinity (as measured by electrical conductivity (EC) of groundwater or total dissolved solids (TDS)) revealed the varying quality of groundwater

Changes in Groundwater Level Dynamics in Aquifer Systems –

in place since the last 10 years (SRWA, 2004, 2006, 2009).

groundwater level observations.

changes in use.

management.

**7. Resource management implications** 

Implications for Resource Management in a Semi-Arid Climate 89

systems in the area. The spatial distribution of salinity (i.e. variation of EC in μS/cm) across the WID is shown in Figure 5. This distribution showed the northern and eastern parts of the districts as relatively higher in salinity than the central-southern part. The highest groundwater salinity was recorded in bore B145273, located closest to the coastline (although the bore is not screened in the Werribee Delta aquifer). This is most probably primary salinity and does not coincide with any of the state's key asset areas (Reid et al., 2011). Nevertheless, under the Southern Rural Water plan on the WID, the key driver of groundwater management is to avoid drawing down the aquifer to the point where seawater intrusion takes place. The source of the salinity in the Werribee Delta has been traced to more saline adjacent aquifers or seawater intrusion (although studies to confirm this are on-going). Several studies in line with the management strategy have therefore been

Total dissolved solids of the samples from bores screened in the Cape Flat sands are generally low compared to those of other aquifers in the area (Adelana, 2011). This salinity values varied from 67-4314 mg/L. The EC values of groundwater from the Cape Flats aquifer ranged from 9.2 to 4320 μS/cm. Field and monitoring data (Adelana, 2011) showed also that generally groundwater salinity increases following the groundwater flow direction, south-eastwards. Figure 6 illustrates the electrical conductivity areal distribution in the Cape Flats. The relations of chloride and EC with groundwater levels and well depths are not shown (in most cases) because the wells monitored for salinity are not necessarily used for

A more appropriate and adequate dataset is essential for the planning and management of aquifers. Monitoring is, therefore, closely linked to the aquifer management, since the results of monitoring may require changes or modifications in the management practice. For example, the higher than average rainfall in the 2010/2011 season across Victoria (all reflected in the hydrograph analysis) may have influence on water use decisions and water restrictions. However, sustainable groundwater management decisions would require long-term monitoring and projections. Such long-term data covering all key elements of the hydrological cycle including groundwater fluctuations and water-level trends are essential as a basis for management and for evaluating the implications of

Long-term monitoring using a number of observation bores has demonstrated that water levels have both declined and recovered over time and in the aquifer investigated. There has been full recovery of the aquifer over the past wet months (2010-2011), and this has been much more than what the recovery would have been over a normal wet year in Victoria. However, this is no cover against management measures except such higher than average rainfall becomes consistent over a longer period of time. Such monitoring data will be 'handy' information to support decision-making and demonstrate the impacts of climate on level changes in relation to resource management. Water level and quality data has been used a number of times in Victoria and (at least three occasions within the last 7 years) in Cape Town to change the extent of groundwater abstraction in order to support sustainable

by comparing historic data with recent measurements. The groundwater salinity monitoring in the Werribee Plains commenced in 2002 while in the Cape Flats regular monitoring began in 1979. Groundwater salinity in the Werribee Plains varies from 1000 to 6000 mg/L TDS, and this (according to Leonard, 1979; SKM, 2002) represents the best quality water in the aquifer

Fig. 5. The spatial distribution of salinity (i.e. variation of EC) across the WID (after SRWA, 2009)

by comparing historic data with recent measurements. The groundwater salinity monitoring in the Werribee Plains commenced in 2002 while in the Cape Flats regular monitoring began in 1979. Groundwater salinity in the Werribee Plains varies from 1000 to 6000 mg/L TDS, and this (according to Leonard, 1979; SKM, 2002) represents the best quality water in the aquifer

Fig. 5. The spatial distribution of salinity (i.e. variation of EC) across the WID (after SRWA,

2009)

systems in the area. The spatial distribution of salinity (i.e. variation of EC in μS/cm) across the WID is shown in Figure 5. This distribution showed the northern and eastern parts of the districts as relatively higher in salinity than the central-southern part. The highest groundwater salinity was recorded in bore B145273, located closest to the coastline (although the bore is not screened in the Werribee Delta aquifer). This is most probably primary salinity and does not coincide with any of the state's key asset areas (Reid et al., 2011). Nevertheless, under the Southern Rural Water plan on the WID, the key driver of groundwater management is to avoid drawing down the aquifer to the point where seawater intrusion takes place. The source of the salinity in the Werribee Delta has been traced to more saline adjacent aquifers or seawater intrusion (although studies to confirm this are on-going). Several studies in line with the management strategy have therefore been in place since the last 10 years (SRWA, 2004, 2006, 2009).

Total dissolved solids of the samples from bores screened in the Cape Flat sands are generally low compared to those of other aquifers in the area (Adelana, 2011). This salinity values varied from 67-4314 mg/L. The EC values of groundwater from the Cape Flats aquifer ranged from 9.2 to 4320 μS/cm. Field and monitoring data (Adelana, 2011) showed also that generally groundwater salinity increases following the groundwater flow direction, south-eastwards. Figure 6 illustrates the electrical conductivity areal distribution in the Cape Flats. The relations of chloride and EC with groundwater levels and well depths are not shown (in most cases) because the wells monitored for salinity are not necessarily used for groundwater level observations.
