Use of a Bayesian isotope mixing model to estimate proportional contributions of multiple nitrate sources in surface water

To identify different NO3− sources in surface water and to estimate their proportional contribution to the nitrate mixture in surface water, a dual isotope and a Bayesian isotope mixing model have been applied for six different surface waters affected by agriculture, greenhouses in an agricultural area, and households. Annual mean δ15N–NO3− were between 8.0 and 19.4‰, while annual mean δ18O–NO3− were given by 4.5–30.7‰. SIAR was used to estimate the proportional contribution of five potential NO3− sources (NO3− in precipitation, NO3− fertilizer, NH4+ in fertilizer and rain, soil N, and manure and sewage). SIAR showed that “manure and sewage” contributed highest, “soil N”, “NO3− fertilizer” and “NH4+ in fertilizer and rain” contributed middle, and “NO3− in precipitation” contributed least. The SIAR output can be considered as a “fingerprint” for the NO3− source contributions. However, the wide range of isotope values observed in surface water and of the NO3− sources limit its applicability.

► The dual isotope approach (δ15N- and δ18O–NO3−) identify dominant nitrate sources in 6 surface waters.
► The SIAR model estimate proportional contributions for 5 nitrate sources.
► SIAR is a reliable approach to assess temporal and spatial variations of different NO3− sources.
► The wide range of isotope values observed in surface water and of the nitrate sources limit its applicability.