ABSTRACT: The present work deals with the main factors controlling groundwater quality in the northern part of the Nile Delta. Groundwa¬ter samples were collected and periodically analyzed as well as the groundwater heads in observation wells were measured within the pe¬riod .from 1988 till 1996. The inspections of chemical analyses and groundwater heads indicate the presence of different water types formed under different paleohydrogeological conditions. These water types are discussed as follows:-
i- Brackish-Saline water (2500-15000-25000 ppm) of Ca-Mg- sodium, CI- sulphate water type. This water type was detected at shallow depths ranging from 10, 18, 25 and 35 m, and represents old lagoonal, marches and swamp deposits formed during sea invasion in the Late Holocene time. Salinity and groundwater heads of this water type are highly influenced by irrigation, drainage systems and contamination from fertilizers, insecticides and sanitary waste.
ii- Hyper saline water ( 69000-120000 ppm) of Ca- Mg- sodium, chlo¬ride water type was detected in the northern margin of the study area at depths ranging between 38 and less than 80 m. This water type was formed under high rate of evaporation during arid climatic conditions which prevailed during Late Pleistocene and Early Holocene time. It acquires the shape of isolated bodies and it represents subenviron¬mental conditions which include deep lagoonal, marches and swamp deposits. This water type shows no change in water levels and salinity contents with time and is considered as old stagnant fossil water.
iii- Brackish water (less than 10000 ppm) of Mg- Ca sodium, HC03- S04- chloride water type was detected at depths more than 100 m. This water type represents the distributary mouth bars formed dur¬ing Early, Middle and Late Pleistocene. These accretion bars act as natural buried channels for the northward Nile fresh water flow or sea water encroachment. Low salinity of this water type reflects successive cycles of leaching and dilution of the inherited salinity by in¬filtrated rainfall and fresh Nile water flushing reported during MiddIe and Late Pleistocene. This water type shows fluctuation in water levels and salinity contents with time that reflects change in the magnitude of the fresh Nile water flow.
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