Untangling hydrological pathways and nitrate diffusive sources by chemical appraisal in a stream network of a reservoir catchment
Authors: Yevenes-Burgos M. A. , Mannaerts C. M. 
Published: March 2011

Abstract. Stable water isotopes and water hydrochemistry of a catchment in the Alentejo region, south Portugal, were analysed to investigate source origins of water and nitrate flows towards a reservoir. The 353 km2 headwater catchment of Roxo river, is strongly influenced by agricultural impacts, and high variations in water and chemical inflows into an important drinking and irrigation water supply (108 m3) are observed. This leads to regular disputes on water quantity and quality amongst local authorities and population. Three sampling campaigns in different seasons were used to address the temporal and spatial variations in stream and groundwater hydrochemistry and water isotopic signatures. A total of 27 sampling points from the stream network, shallow groundwater and reservoir were used. Isotopic signatures and chemistry of precipitation were obtained from local data of the Global Network of Isotopes in Precipitation (GNIP) and the Global Atmosphere Watch (GAWSIS) network. Other meteorological, hydrological and environmental datasets were obtained from local authorities. The stable water isotopes deuterium (δ2H), oxygen-18 (δ18O) together with chloride (Cl–) and sulphate (SO42–) were used as environmental tracers in the hydrological pathways. Water pathways were then related with nitrate concentrations to elucidate potential relationships between the water and nutrient sources. Interpretation of isotope signatures showed a high degree of isotope enrichment in both surface (stream flow) and shallow groundwater. For the entire period, most of stream waters were located right of the global meteoric water line or GMWL and plotted along a local evaporation line (LEL) established for the study area. The LEL showed slopes similar to stream systems in other dry environments. Monthly stream flow and precipitation, seasonal isotope compositions and major ion chemistry data were used for an evaluation of the relative contribution of water sources using an end-member mixing analysis. An extensive PCA or principal component analysis preceded the mixing analysis. Contributions of the three water end-members in the catchment: groundwater, surface runoff and precipitation to stream flow could be identified based on their 2H, 18O and Cl– signatures. Also two hydro chemical data outliers for Cl– and NO3– from two sample points were identified by the analysis and could be related to local waste water outfalls, giving the method also diagnostic value for pollution source allocation. The shallow groundwater source could be related to stream nitrate concentrations during the wet seasons, indicating a linkage between hydrological flow paths, nitrate sources and season. Conversely, weak links between precipitation, and surprisingly also surface water runoff and nitrate levels were found. In this catchment, we found a consistent pattern of the particular groundwater end member, being main source of nitrate to the stream water and reservoir, based on conservative mixing of the different water sources.