CHEMICAL MODIFICATIONS OF GROUNDWATER CONTAMINATED BY RECHARGE OF TREATED SEWAGE EFFLUENT

Long-term monitoring of the chemical composition of recharge sewage ef fluent and associated contaminated groundwater from the Dan Region Sew age Reclamation Project shows, after 16 years of recharge operation, t he presence of a distinct saline plume (up to 400 mg/l Cl), extending 1600 m downgradient in the Coastal Plain aquifer of Israel. The record ed electrolyte composition of groundwater in the vicinity of the recha rge area reflects the variations in the compositions of the sewage eff luents, as well as water-rock interactions induced by the recharge of treated sewage effluents. The original sewage composition was modified , particularly during early stages of effluent migration in the unsatu rated zone, by cation-exchange and adsorption reactions. Since the soi l sorption capacity is finite these reactions caused only limited modi fications, and once the system reached a steady state the inorganic co mposition of the contaminated groundwater became similar to that of th e recharge water. Decomposition of organic matter in the unsaturated z one resulted in CO, generation and dissolution of CaCO3 minerals in th e aquifer. It was shown that chemical and/or bio-degradation of organi c matter takes place mainly in the unsaturated (vadose) zone. Hence, m onitoring the efficiency of the vadose zone to retain contaminants is essential for evaluating the quality of groundwater since it was shown that organic compounds behave almost conservatively once the effluent s enter and flow within the saturated zone.