Assessing the spatial, temporal and depth variation using electrical - - PowerPoint PPT Presentation

UNESCO Chair on Groundwater

Danica Carnow 3244225@myuwc.ac.za

Supervisors: Dr. T.O.B Kanyerere and Prof. Y. Xu By

Assessing the spatial, temporal and depth variation using electrical conductivity in Cape Flats Aquifer, Western Cape, South Africa

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PRESENTATION OUTLINE

• Study Area Map with Sampling Sites
• Conceptual Model Of Groundwater System For Cape Flats
• Focus Of The Study
• Data Collection and Analysis Methods
• Key Results
• Discussion Of Results
• Conclusion

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UNESCO Chair on Groundwater

Study Area Map with Sampling Sites

Figure 1. Location of sampling sites in Cape Flats study area

Tuesday, 15th September 2015

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Hydrogeological Conceptual Model of Groundwater Salinity System for Cape Aquifer

Figure 2. Conceptual model of the groundwater salinity system of the Cape Flats aquifer unit (based on information retrieved from literature and field observation)

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Focus of the Study

Main Aim:



The aim of this study is to provide spatial status of electrical conductivity profiling as an indicator of salinity levels in the Cape Flats aquifer which will improve knowledge and provide understanding of salinity variation in the Cape Flats aquifer. Objectives:



Describe the groundwater quality system using an initial hydrogeological conceptual model which will be refined during my MSc studies

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Determine salinity levels of groundwater in Cape Flats aquifer by using specific depth sampling technique



Assess the suitability of groundwater quality in the Cape Flats aquifer for irrigation use using a hydrogeochemical analysis method with focus on SAR

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Research Problem

 The extent to which groundwater salinity levels vary spatially, temporally and with depth in the Cape Flats aquifer is little understood.  The main concern regarding high salinity in groundwater is the degradation and impairment of water quality for irrigation use.

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Research Question

To what extent does salinity levels vary in groundwater of Cape Flat Aquifer with respect to space, time and depth?

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Data Collection and Analysis Methods

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

Data collection entailed depth- specific depth sampling and Low-Flow Purging in five sampling sites in Cape Flats



Graphical methods were used to display data and descriptive statistics were used to describe trends

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Key Results

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500 1000 1500 2000 2500 5 10 15 20 25 30 Electrical Conductivity (µS/cm) Distance away from coast (Km)

Figure 3. Spatial variation of electrical conductivity (µS/cm) for study sites in the Cape Flats aquifer Figure 4. Electrical conductivity (µS/cm) versus distance away from False Coastline (Km)

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Key Results

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Table 1. Temporal variation of electrical conductivity (µS/cm) for August, September and October sampling period Figure 4. Boxplots of temporal variation of electrical conductivity (µS/cm) for the Cape Flats aquifer during August, September and October sampling

Sampling points Electrical Conductivity (µS/cm) August September October Average Standard Deviation Lentegeur 1665 2353 2350 2122,67 ± 396,35 Westridge 694 873 789 785,33 ± 89,56 Philippi 1207 1130 1257 1198 ± 63,98 University

• f the

Western Cape 325 345 356 342 ± 15,72 Bellville 892 1 690 1082 1221,33 ± 416,85 Average for all sampling points 956.6 1278.2 1166.8 1133.86 ± 163,31

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Key Results

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Figure 5: Schematic diagram of borehole depth variation at study sites and electrical conductivity (µS/cm) in the Cape Flats aquifer

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Key Results

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Table 1. Average electrical conductivity (µS/cm)

• f depth-specific sample for study sites in Cape

Flats aquifer

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BG00139 G32691 BG00153 UWC 4

G46051

5 10 15 20 25 30 35 40 45 500 1000 1500 2000 2500 Depth ( m) Electrical Conductivity (µS/cm)

Study site Sampling point Depth- specific sample (m) Average EC (µS/cm)

Lentegeur BG00139 9 2122.67 Westridge G32691 15 785.33 Philippi BG00153 16 1198.00 University

• f the

Western Cape UWC 4 19 342.00 Bellville G46051 39 1221.33 Figure 6. Electrical conductivity (µS/cm) variation with depth for sampling sites in the Cape Flats aquifer

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Discussion of results

• Main findings suggest that: -salinity decreases with increasing distance

from the shoreline/ coast agreeing with theory

• temporal variation displays a general trend

in which salinity increases with time which agrees with theory

• depth variation of salinity has revealed

a trend whereby salinity increases with decreasing depth disagreeing with theory

• Relation with previous studies -spatial and temporal variation agrees with

studies conducted by Kortasi and Jorgensen (2001) and Aza-Gnandji et al (2013) (high salinity levels maybe be attributed to seawater mixing and residence time causing mineralisation)

• Depth Variation agrees with a study

conducted by Mitra et al (2007) as increasing salinity was attributed to anthropogenic activities

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Discussion of results

 Strengths – ability to constrain salinity level stratification as the aquifer is heterogeneous (geology varies with respect to depth and space)

• little is known about the variation of quality with depth in

the Cape Flats aquifer thus the study is able to provide

• insight into this
• Weaknesses - sampling time and sample size
• require multiple boreholes, screened at multiple depths
• the study could not constrain the source causing the

variation

• Implications - High salinity levels may have negative implications for

irrigation activities and agricultural practices as

• Negative implications for serving as a supplementary

resource for drinking water supply

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Conclusion

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• It may be deduced that salinity levels in groundwater of the

Cape Flats aquifer varies with respect space, time and depth which concomitantly influences quality.

• This means groundwater quality (in terms of salinity)

deteriorates with decreasing depth (anthropogony), with decreasing distance from the shore (seawater intrusion) and with time (mineral dissolution).

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THANK YOU

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