Environmental isotopes and hydrochemistry approach to evaluate the source of recharge and pollution load in Manzala and Bardawil Lakes, Egypt

The present study has been conducted to evaluate the pollution load that has been exerted on two of the major lakes in Egypt; Bardawil and Manzala lakes. Bardawil lake is mainly sea water subjected to evaporation while Manzala lake is a mixture of saline water and fresh water and receives drainage water and effluents of different wastes. The study involves both environmental isotopes (18O and D), major ions, nitrate and phosphate analysis of about 75 water samples collected through horizontal and vertical profiles in the two lakes. Bardawil lake water (the average of TDS equals to 46521 ppm) is dominated by Cl, Na and Mg salts. The distribution of the ions is controlled by the inflow from sea water, the concentration by evaporation, the precipitation due to supersaturation and the specific gravity variations. The hydrochemical composition of Manzala lake water (average of TDS equals to 2554 ppm) is less homogenous than that of Bardawil lake water and reflects the effects of several factors and recharge sources; irrigation water / drainage water discharge, waste water load, sea water or groundwater inflow, bathometry and evaporation intensity. The concentrations of nitrate and phosphate acquire considerably high values in Manzala lake which indicate a high degree of pollution and eutrophication. The isotopic content of Bardawil lake water has average values of 3.69 per mille and 28.59 per mille for δ 18O and δ D, respectively. It has a relatively wide range of variation reflecting the effect of the different recharge sources (seawater, precipitation and seepage of groundwater) and the influence of the different hydrogeochemical conditions. The distribution of Bardawil lake water on δ 18O vs δ D conventional diagram, follows a linear pattern. δ D = 4.70 δ 18O +10.31 of lower slope (4.70) than the meteoric line (slope = 8). This reflects the effect of evaporation on the occurring surface water in the lake. The starting points of the samples line (evaporation line) lie very close to Mediterranean Sea water which represent the original sources of lake water. The relative position of the water samples on the evaporation line and their deviation from the original points is a function of evaporation intensity. The effect of evaporation on the isotopic composition of Bardawil lake water is also shown by the direct relationship between 18O and TDS. This figure shows that the correlation coefficient is low particularly in the deeper zones (bottom water) than that of surface zones (surface water). This can be related to salt precipitation after stages of supersaturation in the surface water due to excessive evaporation processes which reaches about 1.6 m/y. The isotopic content of Manzala lake water is characterized by average values of 5.07 per mille and 29.79 per mille for δ 18O and δ D, respectively. The frequency distribution of the isotopic values reflects the multiplicity of the recharge sources (precipitation, seawater, drainage, irrigation and groundwater). The relationship between 18O and deuterium for Manzala lake water is direct and follows a linear pattern of evaporation trend δ D = 4.76 δ 18O +7. The isotopic contents of Nile water, seawater and drainage water (the recharging sources of the lake) lie close to the starting points of the evaporation line. The scatter of the points around the regression line is relatively high due to the variant proportion of the recharge sources and the different evaporation intensity. The relationship between 18O and TDS, is very weak and the pattern is not easily defined. This can highlight on the great difference of salinity between the recharging sources (due to leaching, evaporation, dissolution etc.), while the isotopic content is not so variant