Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

This section shows the results of an analysis done by an ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry). The analysis was done of water samples that were collected from ground surface runoff in Paarden Island. Figure 5-9 shows the location of where the 3 ground surface water samples were taken.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 5‑9: Location of ground surface runoff water sample locations

 

Assumptions

They following assumptions were made concerning the assessment of the ICP-OES analysis:

• The results of the ICP-OES would not be representative of Paarden Island but give an indication of possible chemical element concentrations of selected element contamination that maybe present in Paarden Island. Only 3 samples were used that were all collected in the same day after a rain event had occurred in Paarden Island. The concentrations that are measured would most likely only be residual contamination that is left over after the majority of the chemical substances have already been washed away. 3 samples would also not be a representative size for Paarden Island.

• It was assumed that all readings from the ICP-OES that were negative and below 200 ppb would be inconclusive. It was recommended by the chemist who conducted the experiments that all readings below 200 ppb are below the ICP-OES limit of detection.

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Assessment of the ICP-OES analysis

The detection limit of the instrument is dependent on the element, wavelength used, equipment available (nebuliser, detector,etc.), matrix of the sample and standards and signal to noise ratios per calibration. The UCT Chemical Engineering Department standards methods used was the lowest calibration standard which was 200 ppb. Any sample that had a recorded value of lower than 200 ppb was recorded as < 200 ppb.

 

The results of the ICP-OES analysis is summarised in table 5-4 below. The results are compared to table 3-3 which shows the target water quality requirements of the Department of Water Affairs and Forestry.

 

The chemical elements of Arsenic, Cadmium, Chromium, Copper and Lead all had results that were inconclusive due to the concentration of these elements being too low for the ICP-OES to detect. It was also noted that the recommended level of Arsenic, Cadmium, Chromium and Lead according to table 3-3 were all below the detection limit of the ICP-OES and thus would actually require testing to be done by a more sensitive ICP-OES machine. More extensive testing should be carried out on the chemical elements that are listed to verify whether the chemicals are present in the ground surface runoff or not.

 

The concentration of Iron from 2 of the 3 samples indicated that the iron was above the recommended water quality requirement for domestic consumption and industrial categories 1, 2 and 3. This result would indicate that the presence of iron in the ground surface runoff should be of concern when considering that the samples were taken after a rainfall event had washed most of the chemical elements away. It is not clear where exactly the sources of iron came from but suggestions would be from old corrugated iron roofing that is present on some old buildings in Paarden Island as well as some erven containing scrap metal in their yards that have been degraded by corrosion.

 

The concentration of Magnesium, Sodium and Zinc were all below the target water quality requirements for all uses. It is recommended that further testing should be carried out on all of these chemical elements, since it is not clear based on the number of samples and the manner in which the samples were taken if the concentration of these elements would be an issue.

 

Further testing should also be carried out on other types of quality criteria such as biological and organic contaminants that could be present in the surface runoff. Separate quality tests should also be conducted on roof surface runoff to help distinguish the difference between roof and ground surface quality in Paarden Island.

 

Table 5‑4: Summary results from an ICP-OES analysis carried out on ground surface runoff in Paarden Island