Impact of urbanization on hydrochemical evolution of groundwater and on unsaturated-zone gas composition in the coastal city of Tel Aviv, Israel

The coastal city of Tel Aviv was founded at the beginning of the 20th centu ry. The number of its inhabitants and its water consumption increased rapid ly. This study analyses a 15-year record (1934-1948) of pre-industrial deve lopment of groundwater chemistry in the urban area. Archive data on concent rations of major ions, dissolved gases (CO2 and O-2), organic matter, and p H were available for each half-year during the period of 1934-1948. The maj or factors causing changes in the chemistry of groundwater flowing in three sandy sub-aquifers have been seawater encroachment due to overpumping, and infiltration of effluents from pit-latrine collectors. Influence of these factors decreases with depth. Landward-penetrating seawater passed through clayey coastal sediments, inte rbedded among sands and calcareous sandstones, and spread into the Kurkar G roup aquifer. This has led to exchange of sodium (dominant in seawater) wit h calcium adsorbed on clay particles, enriching groundwater with calcium. I ntensity of cation exchange decreases inland and with depth. Infiltration of pit-latrine effluents has introduced large amounts of ammon ium into the unsaturated zone. Its rapid oxidation in unsaturated sediments has caused massive nitrate production, accompanied by pore-water acidifica tion. This process induces dissolution of vadose carbonate, resulting in en richment of groundwater recharge in calcium. Anthropogenically induced diss olution of calcite in the unsaturated zone has been the major factor for th e increase of Ca2+ concentration in groundwater, accounting for about 80% o f this increase. In the interface zone, an additional 20% of calcium has be en supplied by cation exchange. Owing to pH increase caused by denitrificat ion in the aquifer, Ca2+-rich waters supersaturated with calcite could be f ormed, especially in the capillary fringe of the uppermost sub-aquifer, whi ch could induce calcite precipitation and ultimately lead to the cementatio n of sandy aquifers. Urban development has caused drastic changes in the gas content in the unsa turated zone and in groundwater. Carbon dioxide was intensively generated b y nitrification-denitrification processes, by hydration of urea, to a lesse r degree by oxidation of organic matter, and probably by anoxic biodegradat ion of organics. Between 1934 and 1948, concentrations of CO2 in unsaturate d sediment air rose from 3.2% to 7.6%. In the unsaturated zone, oxygen cons umption for oxidation of ammonium and organic matter lowered O-2 concentrat ions in sediment air to unusually low values of 3.9-12.9%. Nitrification in the urban unsaturated Zone Could thus serve as a pump, sucking in atmosphe ric oxygen at a rate of about 0.3-0.5 g m(-2) day(-1). The extreme concentr ations of CO2 and O-2 in unsaturated sediments have been preserved due to p roduction and consumption of gas under conditions of diminishing areas open to the atmosphere, uncovered by buildings and by roads. (C) 2001 Elsevier Science B.V. All rights reserved.