Impact of Man?s Activities on a Closed Fishing-Lake, Lake Maryout in Egypt, as a Case Study

Abstract

Lake Maryout is an important fishing lake lining Alexandria City, Egypt at its southern side. It has no direct connection with the Mediterranean Sea. Most of the landed fish come from its Main Basin (one of the lake’s four sub-basins). This basin, area 6000 acres, since the sixties of the last century is suffering from pollution as it receives untreated sewage and industrial effluents from Alexandria City at four hot spots, distributed along the coasts of its eastern half. This has led to a noticeable decline with time in the quantity and quality of the fish catch and to subsequent social and economic problems to the fishermen. In a solution for these problems the government has erected two treatment plants in 1993 to primary treat these waste effluents before discharging into this Main Basin at only two sites. One at a (an old) site lies at its southeast side where one of the plants discharges is indirectly reaching the lake via an agricultural drain while the other plant discharge is directly flowing at a new site lies at the lake northwest side. Obviously the other three old hot spots are completely blocked. Water (on monthly basis) and sediment samples were collected from the Main Basin as well as from the other basins of the lake to study the state of the water quality and level of some priority metal pollutants in addition to organic carbon (org-C) in the sediments of the lake after the erection of the two treatment plants. The results reveal that the water at the (north) eastern side of the Main Basin gets improvement while that in front of the new site at its northwest side becomes polluted and its sediments become enriched with the studied metals (Ag, As, Sb, Cr, Hg, Ni, (Pb), Cd, Zn and Cu). The enrichment was more noticeable for the (spoiled) sediments in front of all the old hot spots in addition to those off the new source. Also, this basin as a whole still shows remarkable high levels of sewage indicating elements including nutrients, organic carbon, fecal coliform, biochemical oxygen demand (BOD), chemical oxygen demand (COD) and hydrogen sulfide (H2S) simultaneously with noticeable low levels of total dissolved solids and dissolved oxygen relative to the other lake basins. The last basins are in healthy conditions as they only fed with agricultural drainage waters free from the treatment plants effluents. The elevated concentrations particularly for the last six studied metals in the sediments are found to be at levels comparable to their corresponding of the median effect-range of Long and Morgan recommended by the U.S. Environmental Protection Agency (EPA), that may have possible effect on bottom fauna and other organisms (including edible fish) live in this polluted basin. This situation makes this important lake basin as a dangerous source for health-hazard fish. Suggested solutions are presented here for rehabilitation of the lake and its sediments. The most accepted ones by the government include diversion of the sources simultaneously with dredging of the spoiled sediments and advanced treatments of the effluents to reuse them for irrigation of reclaimed neighboring desert lands.