Tracing Nitrate-Nitrogen Sources and Modifications in a Stream Impacted by Various Land Uses, South Portugal

The identification of nitrate-nitrogen (NO3-N) origin is important in the control of surface and ground water quality. These are the main sources of available drinking water. Stable isotopes (N-15 and O-18) for NO3-N and along with a 1-D reactive transport model were used to study the origin and processes that lead to nitrogen transformation and loss in a major stream that flows into a reservoir within an intensively cultivated catchment area (352 km(2)) in Alentejo-Portugal. Seasonal water samples (October-November 2008, March 2009 and September 2009) of stream surface water, wells and sediment pore water were collected. The results showed consistently increasing isotope values and decreasing NO3-N concentrations downstream. During winter (wet period, November 2008 and March 2009) slightly higher NO3-N concentrations were found in comparison to early fall (dry period: October 2008) and summer (dry period: September 2009). Isotopic composition of N-15 and O-18 values in surface water samples from the stream and wells indicated that the dominant NO3-N sources were derived mainly from the soil and fertilizers. There was also significant nitrification in surface water at the head of the stream. Sediment pore waters showed high NO3-N values near the sediment-water interface (reaching 25 mgNL(-1)) and NO3-N concentrations sharply decreasing with sediment depth, suggesting significant NO3-N consumption. Denitrification was also detected using the N-15 signature in upstream waters, but not downstream where very low NO3-N levels were measured. In the stream, the calculated isotopic enrichment factor for NO3-N was -2.9 parts per thousand for N-15 and -1.78 for O-18, this indicates that denitrification accounts for 7.8% to 48% of nitrate removal.