Nitrate contamination of Sminja aquifer groundwater in Zaghouan, northeast Tunisia: WQI and GIS assessments

Abstract

Groundwater resources play a very important role in satisfying people’s everyday needs in terms of consumption, irrigation and tourism. In Tunisia, as in most countries in the Mediterranean, groundwaters are fragile, under an arid or semi-arid climate, vulnerable to pollution, and influenced by climate change. The Sminja aquifer in northeast Tunisia is one of the major sources of fresh water for the city of Zaghouan. Twenty-three wells and boreholes were sampled in January 2013 and subjected to salinity analysis of major elements and nitrates. To study the geochemical quality of the Sminja aquifer, we used a geographic information system to develop geographic location maps and spatial distribution maps of nitrates, salinity, and the water quality index (WQI) of the study area. Nitrate concentrations ranged between 8 and 137 mg/l. The highest values of this parameter in groundwater samples found in northern and eastern parts of the Sminja aquifer have a direct relation with the fertilization of agricultural land. The salinity of the water varies from 1,170 upstream to 9,570 mg/l downstream of the aquifer. The parameters that were considered in the calculation of the WQI were calcium, magnesium bicarbonate, sodium, chloride, nitrate, sulfate, potassium, and total dissolved salts. Based on the values of the WQI, the water samples of Sminja aquifer were classified into three groups: poor water quality, very poor water quality, and unfit for consumption (the FS1 well). The groundwaters of the Sminja aquifer, apart from the southern part, do not meet the drinking water standards set by the World Health Organization with respect to salinity and nitrate levels.