GROUNDWATER CONTRIBUTION TO STREAM SALINITY IN A SHALE CATCHMENT, RSA

The salinity in the Breede River in the Robertson area (S.W. Cape, South Africa) has increased at an alarming rate over the past two decades. Water analyses have shown that a few tributary streams which drain Bokkeveld Shale catchments are responsible for the high salinity in this agriculturally important river. Research carried out to investigate the sources of salinity in one such catchment, the Poesjesnels River Valley, found that heavy salt loads were contributed by return flow seepages after irrigation of deeply ripped, thin soils which overlie decomposed shale. A smaller contribution was coming from ground water which had leached out some salt from these shales while moving through bedrock fractures under pressure from the head in the encircling mountains, and eventually seeped into the river; evidence for this was provided by comparison of the oxygen isotope ratios of the various water bodies in the valley with that of the river water. A salt balance model drawn up to differentiate between the sources of salinity gave an indication of the systems which need management in future hydrological and agricultural development in this and similar valleys.