Assessing the impacts of agriculture on groundwater quality using nitrogen isotopes : preliminary results on the Gabbros of Beja Aquifer System, South Portugal

Abstract

This paper intends to present preliminary results of nitrogen isotope assessment and source identification and characterization of nitrate non-point pollution in the context of official publication of ''Gabbros of Beja'' Vulnerable Region (Portaria 1100/2004, 3 of September). Research is based on funding project ''Assessing the impacts of agriculture on groundwater quality using nitrogen isotopes'' in progress in the rural region of Beja (south Portugal), under semiarid conditions in the period 2004-2007. Supported by previous hydrogeological characterization of ''Gabbros of Beja'' hard rock aquifer, seasonal groundwater chemistry and nitrate content evolution under different land use scenarios is possible to use nitrate isotopes and oxygen isotopes to trace nitrogen origin of nitrates in groundwater and nitrogen cycle. Stable nitrogen isotopes (15N/14N ratios) can offer a direct means of source identification of the pollutants in groundwater systems, In the research area the two major sources of nitrate were identified in agricultural areas, fertilizer and manure, which present different isotopic d15N signatures. The relative contributions of these two sources to groundwater or surface water can be estimated by mass balance. The analysis of d18O of nitrate in conjunction with d15N improves the ability to trace nitrate sources and cycling. Preliminary results are not conclusive that major source of nitrate-N in groundwater comes from agriculture. Further work is necessary under regular hydrological conditions. It is intended to give scientific proves to support political decisions, taking into account the sustainable development of the region and the appropriate use of nitrogen fertilizers on the basis of several European and national directives, namely vulnerable regions and EC Water Framework Directive.