Nitrate concentration in groundwater is generally restricted by law. The European laws pose limit with regard to the classification of the aquifer, in view of the specific use of water. In Italyconcentrations less than 25 mg L 1 are allowed for good chemical quality of groundwater. Although nitrogen contained in olive mill wastes was mainly in the organic form (Table 2), nitrate may accumulate in the soil after organic nitrogen mineralization by soil microorganisms (Gómez-Muñoz et al., 2011; Sierra et al., 2007), and may be potentially released to groundwater.