干旱区生态环境知识资源中心

Egypt planned to reclaim large areas in both the Eastern and Western desert fringes of the Nile Delta with an increasing use of sewage effluent from Greater Cairo. The Gabal el Asfar farm which has been irrigated with raw to primary treated sewage effluent for a period of seventy five years was selected as a pilot area to investigate the groundwater quality. This farm covering 3000 feddan (1250 ha), is located in the sandy area northeast of Cairo.

The main objective of the study was to assess the impact of agricultural application of sewage effluent on groundwater quality. Observation wells were constructed and field and laboratory measurements were carried out in and around the Gabal el Asfar area. The water balance and mass balances have been assessed. The study yielded useful recommendations to decrease the impact on groundwater quality and possible risks in Gabal el Asfar, concerning 1) type and amount of pollution from sewage effluent, 1) design criteria to control possible spreading of pollutants, and finally 3) the use of groundwater in the area. The following comprises the main conclusions: Irrigation with sewage effluent had a positive impact on salinity of the initially brackish groundwater. On the contrary, groundwater quality is negatively affected from sewage effluent with regard to nitrogen contents (ammonium and nitrate), phosphate, heavy metals and faecal coliforms. The drinking water limit for nitrate is sometimes exceeded. Moreover, faecal coliforms are found in water from all shallow handpumps in the area. The general suitability of the groundwater is therefore limited while the use of handpumps for drinking water purposes should be avoided. High boron contents of sewage effluent indicate a risk for crops sensitive to boron. The accumulation of heavy metals and phosphorus in the soil adversely affects the soil system. Pretreatment of sewage effluent in Gabal el Asfar should be improved to limit the pollution of the aquifer. Industrial discharge in sewage effluent should be avoided. Groundwater recharged by sewage effluent is mainly collected by drains in or at the borders of the area, so spreading of pollutants out of the area is largely controlled.