Observations on soil and groundwater contamination with nitrate: A case study from Lebanon-East Mediterranean

► Intensive agriculture and bad practices are the main source of nitrate pollution of the soil–groundwater system in Lebanon East Mediterranean. ► Farmers rely on wells to pump groundwater for irrigation. Attention is paid to groundwater quality in view of environmental and health risks. ► Four cropping systems: vegetable, field crops, succession vegetables and field crops and table grape were studied between 2007 and 2009. ► Heavy application of N fertilizers and the method of application and irrigation techniques enhanced nitrate leaching to groundwater. ► The seasonal N load to the soil and groundwater was by ascending order: table grape, wheat–spring potato, lettuce–wheat–potato and mint. The impact of agricultural practices on soil–groundwater quality in the sub-humid Bekaa plain of Lebanon-East Mediterranean was monitored in four fields (F) between July 2007 and July 2009. These were occupied by continuous mint (F1), summer potato/wheat/potato (F2), lettuce/lettuce/potato/wheat/summer potato (F3) and table grapes (F4). N input calculated on a two-year basis, was in the following ascending order F4, F2, F3 and F1. Soil samples, analyzed down to 200 cm depth, showed high nitrate and chloride concentrations at the end of the 2007 and 2008 seasons. Soil chloride and nitrate peaks recorded in October 2007 and 2008 disappeared below 200 cm overwinter. The calculated N biannual discharge ranged from 130 (F4), to 516 (F2), to 778 (F1), to 879 kg ha −1 (F3). Groundwater quality was studied in 21 wells distributed along a sequence stretching from the Litani River to the eastern water dividing line. Based on the nitrate concentrations, the well located at the top of the water dividing line was the only one suitable for drinking purposes. Eight wells were mildly contaminated, therefore suitable for irrigation purposes except for sensitive crops. Twelve wells, positioned in the plain, showed a nitrate level exceeding 200 mg L −1. Protecting the soil and groundwater quality is a top priority to maintain the ecological and agricultural functions of water.