GEOGRAPHICAL TRENDS IN THE WATER CHEMISTRY OF WETLANDS IN THE SOUTH-WESTERN CAPE PROVINCE, SOUTH AFRICA

Abstract

Although it is becoming increasingly important for the aquatic chemistry of wetlands to be understood, little chemical information is available for the wetlands of South Africa. During a project aimed at beginning to redress this gap in our knowledge, conductivity and pH measurements were made at 73 wetlands in the south-western Cape Province. The concentrations of the major cations and anions were determined in samples from 42 of these wetlands. Total dissolved salts (TDS) concentrations ranged from below 20 mg l⁻¹ to more than 200 000 mg l⁻¹, and pH values as low as 4 and greater than 10 were recorded in some wetlands. Despite these extremes, the ionic ratios showed regional trends: Na⁺ and Cl are the dominant ions in wetlands near the coast, and HCO₃ is the dominant anion in inland wetlands. Since the biological components of wetlands are dependent on the water chemistry, it is essential that the side-effects of any developments in a wetland or its catchment be carefully considered in the light of that wetland's ability to absorb such changes. The wide diversity of wetland chemistries found in the study area implies that this careful analysis should be done for each wetland encountered.