Study of heavy metal contamination in spreading Marrakech waste water (Morocco)
Molluscs are well known for their capacity to bioaccumulate heavy metals from water or sediments. This study involves an evaluation of the contamination level of the undergroud and surface waters from a specific irrigation system known as a « khetarra », located near the town of Marrakech (Morocco). Khettaras represent a traditional systern by means ofwhich the ground water is drained to the surface and then can be used as drinking water or for irrigation in some arid and semi-arid areas. The bioindicator chosen was the gasteropod Melanopsis praemorca L. Two stations (Kl and K2) were investigated in a khettara located in spreading Marrakech waste water. The contamination of soils and water by heavy metals, eipecially Zn, Cu and Cd was first described by EL MEZD1 (1985) and SEDKI(1990). The studied khettara (1500 m. in length) is contaminated in two different ways: first by infiltration of waste water from the Seguia system and secondly directly by over-flow and escape of a main common sewer which crosses over the khetarra between the Kl andK2 stations. At each station, sediment peripbyton, Spirogyra and M. praemorsa were sampled in different seasons. Each sample was prepared for analysis (S.A.A. varian 475.AA) according to the method described by COSSON (1987). Flame atomization was chosen for Zn and Cu, the grafite furnace for Cd. The main results concern the level of contamination, with a comparison between Kl and K2 stations, the variation in contamination according to the seasons, the dilferent routes of contamination, namely through water or by the trophic chaîn (periphfion and Spirogyra). The degree of contamination in M. praemona is signifïcatively higher at the Kl station, with the following ranking: Zn> Cu> Cd (fig. 2, 3 and 4). Compared with other literature data: FORSTNER et al., 1981; RADA, 1985; COSSON, 1987; PIP, 1992, the upper concentrations are the highest. The metal concentrations increase from winter to summer, with significant differences between winter and spring or summer values at Kl for Zn, Cu, Cd; the same distinction, but only for Zn and Cd at K2 station. The causes of variation in metal concentrations in the gasteropods from the two sites were investigated,by in situ cotransplantation experiences and by measuring metaflic concentrations in sediments, periphyton and Spirogyra. Results show that differences in gasteropod metal concentrations between sites appear to be related to differences in aqueous metal speciation. The data concerning the food chain show that metallic concentrations are higher for periphyton and, Spirogyra living at Kl station, this according to the data for M. praemorsa.In contrast, the level of contamination in sediments is higher at K2 station. The most probable hypothesis is that organic and suspended matter, present at a higher level in this station, bind the heary metals and reduce their bioavailability. The cotransplantation between Kl and K2 stations shows that the individuals transferred from Kl to K2 station for 15 days cease to bioaccumulate Zn and Cd. A detoxification process occurs for these two metals but not for Cu, which continues to increase; thus, bioavailability of different metals at the same station is not equivalent. On the other hand the individuals transferred from K2 to Kl station do not increase their Zn, Cu or Cd concentrations even after 30 days. Concerning this fact FRAZIER and GEORGE (1983) suggest the possibility of an adaptation to initial conditions. It seems that many other factors influence metal bioaccumulation and merit further study, e.g.: the size and the age of individuals, the biological cycle with the period of reproduction ; the interaction of abiotic factors: T°, salinity, dissolved orygen, turbidity, lighting.