Reverse Linkage

70 REVERSE LINKAGE have been identified during the rainy season and one during the low-flow period. It was then possible to assess the quality of the raw water using the results of the analyses. It was found that the phospho- rus values ranged from36 to 56mg/l, characteristic of eutrophic lakes. Nitrate values ranged from 0.5 mg/l to 1.6 mg/l, indicat- ing that nitrate-mineralised nitrogen is strongly present in the lake waters, exceeding the tolerable threshold of 0.3mg/l.These concentrations are indicative of lake water pollution. Following this operation, it was agreed that samples should be taken at Ziga every two weeks at 17 collection points and every week at four collection points – the water surface, the first and second water intakes and the bottom of the dam. Samples would be taken every month at the other sites. To date, 2,032 samples have been collected and analysed. The bottom valves at the Ziga and Loumbila dams have now been opened, allowing the collection of accumulated sediments during the rainy season. This has reduced the level of sediment by 40cm, averting the loss of intake. At Ziga, both catches are used alternately depending on the quality of the raw water. These actions contributed to a considerable reduction in the algal load in Ziga water – from 5 million cells per millilitre of water to 15,600 cells per millilitre during peak algal periods. In addition, two people have been trained in eutrophication, one on the identification of algal toxins; the other on algal categorisation, and the ability to conduct activities related to the monitoring of raw water quality. It is now clear that improvements to the ONEA’s standards can be achieved through combating eutrophication. The search for solutions has now ventured upstream from the dams, as it is known that there are organisms in the aquatic ecosystem that consume the algal biomass responsible for the degradation of water quality. Among other phytopha- gous creatures, the silver carp – a predator of harmful algae – has been used extensively in Morocco for this purpose, so it was proposed to carry out a control test with the introduc- tion these fish to restore the equilibrium of the ecosystems. But the researchers and the ministry in charge of fisheries resources objected to the proposal, despite an IsDB-funded mission to share Morocco’s experience in the use of silver carp to fight eutrophication. Optimisation of water treatment units Following investigations into the raw water, the project’s second phase focused on the optimisation of processing units at the Ziga and Paspanga stations. Treatment tests of raw water were carried out in the laboratory to identify malfunctions in the processing and determine appropriate remedial action. The resulting activities managed a reduc- tion in turbidity from 3 or 4 NTU to 1.5 NTU at the station outflow. Also, the pre-chlorination system has been put back into service at the Paspanga and Ziga treatment units and the calc-carbonic equilibrium was corrected through adjusting remineralisation by adding lime to water. New complementary controls have been introduced at treatment plant laboratories including the measurement of The Ziga dam, Burkina Faso Image: ONEA pH 0 2 Ortho Cha Nitrates 7.9 7.6 270 0.03 <0.3 7.5 6.7 460 0.01 <0.3 7.4 6.6 560 0.06 <0.3 pH 0 2 Ortho Cha Nitrates 7.6 7.7 410 0.001 2.7 7.6 7.7 260 0.006 2.3 7.6 7.7 310 0.001 2.4 0.2m 1m 2m 5m 7.3 7.8 400 0.01 <0.3 6.9 4.7 360 0.000 2.3 Ziga dam Loumbila dam Surface Surface Depth 10.5m Depth 5.5m Orthophosphate: hypereutrophic Chlorophyll: ultra oligotrophic Orthophosphate: hypereutrophic Chlorophyll: ultra oligotrophic Source: Bouloud, 2013 Evaluation of physico-chemical constituents of the Ziga and Loumbila dams