Geographical differences in the relationship between total dissolved solids and electrical conductivity in South African rivers

H Van Niekerk,Mj Silberbauer,M Maluleke

doi:10.4314/wsa.v40i1.16

Abstract

Electrical conductivity (EC) is a useful surrogate for total dissolved solids (TDS). EC is more rapidly and easily measurable with reasonably-priced equipment. However, as an indirect measure EC is subject to uncertainties that are not always apparent to the user. We set out to investigate the relationship between TDS and EC in 144 643 sample results available on the Department of Water Affairs water quality database. TDS is calculated as the sum of the major solutes determined by laboratory analysis and EC is a measurement in a flow cell. The median TDS:EC ratio for 332 high priority sites was 7 mg/l: 1 mS/m. Regional differences ranged from 4.8 to 8.6. Investigation of 38 of these sites using Maucha diagrams suggested that the differences are related to the dominant major ions, with sodium chloride waters having a lower TDS:EC conversion factor than calcium bicarbonate waters. The practical application of these findings is that users of EC meters should not simply apply a blanket conversion factor, but need to select an applicable factor for the river system in which they are measuring.

Highlights

Many water sector analysts routinely use total dissolved solids or salts (TDS) as a measure of water salinity
The South African Water Quality Guidelines assume a general conversion factor (CF) of 6.5 for TDS:electrical conductivity (EC), it is recommended by the Guidelines that site-specific CFs be used where more accurate TDS concentrations are required (DWAF, 1996)
The results for the 332 monitoring sites imply that using a standard instrument setting CF of 6.5 to convert EC to TDS can underestimate TDS by as much as 25% or overestimate it by up to 35%

Summary

Introduction

Many water sector analysts routinely use total dissolved solids or salts (TDS) as a measure of water salinity. Direct methods for determining TDS concentration are gravimetric, for example evaporation at 180°C (APHA, 1999), flow densitometry or determination of the major individual solutes by laboratory analysis and their algebraic summation (APHA, 1998). These methods are expensive and time consuming, and a much cheaper, easier and quicker method to infer TDS concentration is by measuring electrical conductivity (EC) using an EC meter and converting the value to TDS with a constant conversion factor. The South African Water Quality Guidelines assume a general conversion factor (CF) of 6.5 for TDS:EC, it is recommended by the Guidelines that site-specific CFs be used where more accurate TDS concentrations are required (DWAF, 1996)

Methods

Results

Conclusion