Water quality of the upper Orange River

[摘要] English: The Upper Orange River is defined as the region between the source in the Drakensberg (Lesotho)and the Orange-Vaal confluence. Information on the water quality, phytoplankton compositionand the influence of the dams on the Upper Orange River is limited. This study was conducted inorder to determine seasonal and spatial patterns in the river system., as well as to determine theinfluence of the tributaries and the two major dams on the river system. Physical, chemical andbiological factors were taken into account.Hypolimnetic water released from the Gariep and Vanderkloof Dams caused a significant decreasein the temperature downstream of the dams. The dams acted as sediment traps and caused the TSSand turbidity in the river system to decrease. A decrease in turbidity resulted in an increase in thelight penetration. This created more favourable conditions for primary production and led to algalblooms in the dams.The dams are used for hydro-electric power generation and almost completely diminished seasonalflow patterns. This changed the Upper Orange River into a highly regulated system. Theinterference with seasonal flow patterns in the river is probably one of the main reasons foroutbreaks of the pest blackfly, Simulium chutteri downstream of the dams.The oxygen-depth profile in the Gariep Dam during summer stratification indicated relatively highdissolved oxygen concentrations in the epilimnion (8.2 mg.l-1) and anoxic, near anaerobic,conditions (0.1 mg.l-1) in the hypolimnion. The oxygen decline in the hypolimnion was indirectlythe cause of high algal production in the epilimnion during algal blooms.The average pH of the Upper Orange River system was mainly alkaline at 8.1, with an averagetotal alkalinity of 95.2 mg CaC03.rl. This suggested that the Upper Orange River is a wellbufferedsystem.The average conductivity in the Upper Orange River was relatively low at 19.6 mS.m-1, butincreased significantly since 1980. Major ions at Aliwal North occurred in the followingproportions, i.e. Ca> Na > Mg > K for cations and CO3 > Cl> SO4 for anions. In the Gariep Dam and at Marksdrift the proportions were: Na > Ca > Mg > K for cations and CO3 > Cl > SO4 foranions.The average P04-P concentration was relatively low at 28.3 µg.l-1 and the average TPconcentration was 107 µg.I-1. The average N03-N concentration was high at 312 µg.l-1 andincreased in the dams and downstream probably due to agricultural fertilizers and nitrogen fixationby algae.The average chlorophyll a concentration ranged between 0.4 and 1084 µg.I-1, with and averageconcentration of 10.8 µg.l-1. The very high chlorophyll a concentration was due to a bloom ofMicrocystis sp. during February in the Gariep Dam. Phytoplankton dynamics manifestedthemselves in seasonal cycles. Cyanobacteria and Bacillariophyceae dominated during warmermonths. The Chlorophyceae replaced cyanobacteria during cooler months.Light penetration in the Gariep Dam and the Vaal River had a major effect on the primaryproduction. The maximum primary production in the Gariep Dam was 14.6 mg C. mg Chla-1 h-1and in the Vaal River the maximum primary production was 76 mg C. mg Chla-l. h-1. Accordingto the primary production in the Gariep Dam it can be classified as mesotrophic, while the highprimary production in the Vaal River indicated eutrophic conditions.The dams exert major influences on the physical, chemical and biological parameters in the river.Low SASS4 scores were obtained and are an indication of poor water quality. This could beascribed to disturbances by the dams. The serial discontinuity concept (SDC) is applicable in theUpper Orange River. It is likely that agricultural runoff, as well as urban and industrial pollutionfrom the Vaal River catchment would mask the downstream recovery after the Orange- Vaalconfluence.

[发布机构] University of the Free State