EFFECTS OF URBANIZATION AND AGRICULTURAL ACTIVITIES ON THE CHEMICAL AND ISOTOPIC COMPOSITIONS OF PERCOLATING WATER

The Coastal Plain Aquifer of Israel, providing 25% of the country's annual water consumption, consists of Holocene sand dunes, calcareous sandstone, loam or red fossil soils, and clay interlayers. Over the past 15 years, the overlying Coastal Plain has undergone intensive urbanization. Many rural, cultivated and undeveloped areas have become urban residential, industrial and commercial. The potential damage to the quantity and quality of the water percolating to the underlying aquifer was the trigger for a 4-year study of the urban hydrology in the city of Ashdod, a model city whose population has doubled over the past 15 years alone. The assessment was based on a large chemical, isotopic and mineralogical dataset, including rainwater (64 rain samples from 46 rain events), sediment and pore water from the (8-32 m thick) vadose zone (129 samples from six boreholes) and groundwater (samples from 28 observation and groundwater-supply wells), collected under different land uses within the city, and in the surrounding cultivated and undeveloped sand dunes.

The vadose zone of the undeveloped sand dune, serving as a reference for the other, anthropologically altered land uses, and that below the residential areas had similar salt concentrations and distribution: the salts seemed to occupy primarily the upper part of the vadose zone, mainly as a result of calcite dissolution and near-surface evaporation of the percolating water. Below the cultivated areas, salt buildup was observed across the entire vadose zone down to the water table, following the application of fertilizers and the use of treated wastewater for irrigation. Calcium and bicarbonate were the dominant ions in the undeveloped, residential and industrial land uses, while sodium and sulfate prevailed below the agricultural areas. Whereas the urbanization of undeveloped areas appears to result in the deterioration of groundwater quality, the shift from cultivated to residential areas has the (surprising and unexpected) potential of improving groundwater quality due to the decreased salt load at land surface.