Investigation of Stratification in the Vaal Dam Using Electrical Conductivity Profiling

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Investigation of Stratification in the Vaal Dam using Electrical Conductivity Profiling Reveck Hariram*, Nicolene Van der Walt** Rand Water, Scientific Services Division, P.O. Box 1127, Johannesburg, 2000, South Africa E-mail: [email protected] ; [email protected]; Tel: +2711 682 0479/0735 ABSTRACT water to more than 11 million people in South Africa. Purifying it and selling it to various Municipalities, mines and industries. The Vaal Dam is continuously monitored to provide an early warning of changes to water quality so that the configuration of the water treatment plant can be adapted to ensure only the best quality water is supplied to our customers. er quality by sampling bi-monthly at four strategically located sampling sites. The two major tributaries entering the Vaal Dam are monitored, namely the Wilge and Vaal River respectively. Furthermore, the water quality at the confluence of these tributaries is also monitored together with the most essential It has been found that the electrical conductivity in the Vaal Dam has been relatively consistent over the years (2003 to 2014) with an average of 25 mS/m. The salinity of the Vaal Dam is greatly influenced by the water quality of the two major tributaries (Wilge and Vaal) and the source water from Lesotho (via Lesotho Highlands Water Scheme) that feeds into the Wilge tributary. The purpose of this study was to investigate and verify if any stratification occurs in the Vaal Dam as previous studies indicated no stratification. In 2014, a new monitoring tool was utilized called the CastAway- CTD. The instrument can be lowered into a river or dam capturing data such as GPS position, conductivity and depth along the vertical profile. Conductivity is an important variable to monitor since it is an early or quick indicator of change in water quality. This study investigated eight sampling points within the Vaal Dam from January 2014 to April 2015. Fluctuations were only identified in the vertical profile at the Sasol Secunda intake/abstraction point (SSI) which might be due to the impact of pumping on the water profile. The monitoring data also provided information that the Wilge River tributary dilutes the salinity of the Vaal River tributary entering the Vaal Dam. This is due to the high volumes of very low Electrical Conductivity (EC) water flowing down the Wilge River as compared to low volumes down the Vaal River with slightly elevated EC. The water quality at the four sampling points in the Vaal Dam is ideal to acceptable according to the Vaal Dam in-stream water quality guidelines. This study concluded that no stratification has been observed for this study period which spanned the different seasons. Keywords: CastAway-CTD, Electrical conductivity (EC), Stratification, Vaal Dam, Vaal River, Wilge River .

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