The potential impact of an inter-basin water transfer on the Modder and Caledon River systems

[摘要] English: The Modder River is a relatively small river which drains an area of 7 960 km2 in the central region of the Free State Province, South Africa and has a mean annual run-off of 184 x 106 m3. Water demand in the Modder River catchment has exceeded supply in the past which necessitated the development of two transfer schemes: 1) From the Caledon to the Modder (Caledon-Bloemfontein pipeline), and 2) The Caledon to Modder also known as the Novo transfer scheme. This study focussed on the ecological effects of the Novo transfer scheme on both the Modder and Caledon Rivers from a limnological point of view, thus excluding the human component and land-based investigations. The transfer of water from the turbid Caledon River caused an increase in turbidity in Knellpoort Dam. However, this turbidity decreased when transfer of water was terminated, and conditions returned to those prior to the transfer of the water. This increase in turbidity brought about certain effects, like limitation of underwater light penetration, and a consequent decrease of the euphotic depth. The turbidity did, however, not extend to the inflow of the dam where the Novo pump station is located and is thus not expected to increase turbidity in the Modder River. The largest fraction of sediment particles in the Caledon River, consists of very fine sand. This has implications in terms of the bioavialability of nutrients and consequently, the maximum bioavailability of nutrients in the sediment load of the Caledon River was found to 339 mg/kg N and 106 mg/kg P. The transfer of water (and nutrient-rich sediments) from the Caledon River, via Knellpoort Dam, to the Modder River caused an increase in the concentrations of TP, PO4-P, NO3-N and SiO2-Si in Knellpoort Dam. Since the effect of the inflowing water seems to be localised to the area at the inflow channel, the water quality at the Novo pump station (on the opposite side of the impoundment) was little influenced. However, if transfer takes place over a long period of time, it can have a significant impact on the nutrient concentrations of the impoundment. Similarly, if the ionic composition changes over a long period of water transfer, the algal communities in the different systems can change. In the different waters of the Novo Transfer Scheme, there are large variations in the algal composition. In the Caledon River, the Bacillariophyceae (specifically Cyclotella and Stephanodiscus spp.) dominated the phytoplankton throughout the study period. Turbulence in the river, together with high turbidity, probably limited the development of cyanobacterial blooms. In Knellpoort Dam, the algal community was dominated by Bacillariophyceae and Chlorophyceae, and Cyanophyceae numbers increased only after the transfer of nutrient-rich water from the Caledon River. This could have serious implications for the uses of the impoundment, which include water purification for drinking water and recreation, as cyanobacterial blooms can be harmful to humans and animals. In the Modder River, the algal community was dominated by Bacillariophyceae, Chlorophyceae and Cyanophyceae, and it was the same in Rustfontein Dam, an impoundment in this river used for domestic water supply and recreation. As water was not transferred to the Modder River during the study period, the effects on the invertebrate community in this river could not be determined, and possible impacts are predicted on observations made from other studies. From these, it is not expected that there will be a transfer of undesirable invertebrate species from the Caledon River. Species diversity at the transfer site is generally low, and the high flow conditions during transfer periods will limit the species diversity even more. The main concern in terms of invertebrate diversity is the potential impact of increased flow in the Modder River during periods of transfer. Based on this study, the following recommendations are made: �?Abstraction from the Caledon River must be done at the end of the rainy season, when turbidity levels are lower, but stream flow is still high enough. �?The sediment channels into the Knellpoort Dam are not very effective and alternative measures should be investigated to remove the sediment. �?The nutrient and turbidity status of the Knellpoort Dam must be carefully monitored to ensure that the impoundment does not become eutrophic or too turbid. �?When discharging the water from Knellpoort Dam into the Modder River, cognisance must be taken of the natural seasonal flow of the river, and care must be taken not to exceed the normal flow with too big a margin. Erosion measures must be put in place at the point of discharge and carefully monitored.

[发布机构] University of the Free State