An assessment of the impact of different land use activities on water quality in the upper Olifants River catchment

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Abstract

Routine sampling of water quality was conducted at sites along a longitudinal gradient from upstream to downstream in the Olifants River to determine spatial trends in nutrient and metal concentrations and to relate these trends to changes in land use activities in the catchment. In addition, once-off sampling was conducted at a number of sites located downstream of current mining, abandoned mining, agriculture, wastewater treatment works (WWTWs) and industry. Nutrient concentrations were relatively high and a number of sites within the catchment had average N:P ratios that were indicative of eutrophic to hypertrophic conditions. Routine and once-off sampling indicated that wastewater treatment works contribute high nutrient loads to the system. Trend analysis of Department of Water Affairs (DWA) data indicated significant positive trends in ortho-phosphate at 12 of 14 stations in the catchment. An increase in sulphate concentrations from upstream to downstream indicates that mining activities have a progressively greater impact on water quality with increasing distance downstream. While dissolved metal concentrations frequently exceeded chronic and acute effect aquatic ecosystem health guidelines (particularly aluminium, copper and zinc), there was no observable trend from upstream to downstream. Once-off sampling showed high variability in water quality parameters downstream of current mining activities, and some sites showed higher metal concentrations in comparison to other land use activities. However, the contribution of current mining activities to metals is low in comparison to the contribution from abandoned mines. Hydrological data showed that acidic rivers contribute proportionally higher flow volumes in comparison to neutral rivers during the drier winter months, which may significantly impact on the lower stretches of the upper Olifants River and into Loskop Dam. A prolonged drought period will most likely result in severe impacts to the lower reaches of the Olifants River and to Loskop Dam. Improved management and maintenance of wastewater treatment works and rehabilitation and/or treatment of abandoned mines and associated acid mine drainage are crucial. Proper rehabilitation of current mining activities is essential to avoid or minimise acid mine drainage related impacts in the future.

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Phosphorus in rivers — ecology and management

William Parr, Chris Mainstone (2002)

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Sewage-effluent phosphorus: a greater risk to river eutrophication than agricultural phosphorus?

Zane Neal, P. J. Withers, H JARVIE (2006)

Phosphorus (P) concentrations from water quality monitoring at 54 UK river sites across seven major lowland catchment systems are examined in relation to eutrophication risk and to the relative importance of point and diffuse sources. The over-riding evidence indicates that point (effluent) rather than diffuse (agricultural) sources of phosphorus provide the most significant risk for river eutrophication, even in rural areas with high agricultural phosphorus losses. Traditionally, the relative importance of point and diffuse sources has been assessed from annual P flux budgets, which are often dominated by diffuse inputs in storm runoff from intensively managed agricultural land. However, the ecological risk associated with nuisance algal growth in rivers is largely linked to soluble reactive phosphorus (SRP) concentrations during times of ecological sensitivity (spring/summer low-flow periods), when biological activity is at its highest. The relationships between SRP and total phosphorus (TP; total dissolved P+suspended particulate P) concentrations within UK rivers are evaluated in relation to flow and boron (B; a tracer of sewage effluent). SRP is the dominant P fraction (average 67% of TP) in all of the rivers monitored, with higher percentages at low flows. In most of the rivers the highest SRP concentrations occur under low-flow conditions and SRP concentrations are diluted as flows increase, which is indicative of point, rather than diffuse, sources. Strong positive correlations between SRP and B (also TP and B) across all the 54 river monitoring sites also confirm the primary importance of point source controls of phosphorus concentrations in these rivers, particularly during spring and summer low flows, which are times of greatest eutrophication risk. Particulate phosphorus (PP) may form a significant proportion of the phosphorus load to rivers, particularly during winter storm events, but this is of questionable relevance for river eutrophication. Although some of the agriculturally derived PP is retained as sediment on the river bed, in most cases this bed sediment showed potential for removal of SRP from the overlying river water during spring and summer low flows. Thus, bed sediments may well be helping to reduce SRP concentrations within the river at times of eutrophication risk. These findings have important implications for targeting environmental management controls for phosphorus more efficiently, in relation to the European Union Water Framework Directive requirements to maintain/improve the ecological quality of impacted lowland rivers. For the UK rivers examined here, our results demonstrate that an important starting point for reducing phosphorus concentrations to the levels approaching those required for ecological improvement, is to obtain better control over point source inputs, particularly small point sources discharging to ecologically sensitive rural/agricultural tributaries.