The oxygen isotope composition of dissolved anthropogenic phosphates: a new tool for eutrophication research?

High-precision oxygen isotope analyses were carried out on dissolved phosphate extracted from discharge waters from three wastewater treatment plants (WTP) located in western France, as well as on the different phosphate-based fertilizers applied by farmers in the same region. Measured delta(18)O values of phosphate from chemical fertilizers range from 19.6 to 23.1parts per thousand, while those of phosphate from WTP discharge waters are more tightly grouped between 17.7 and 18.1parts per thousand. The variability in delta(18)O values of phosphate fertilizers is attributed to oxygen isotope variations of the phosphorite deposits front which France's fertilizers are manufactured. The significance of the delta(18)O values of phosphate from WTP discharge waters is less straightforward. At present. it is trot clear whether these values are primary isotopic compositions corresponding, e.g.. to the oxygen isotope composition of phosphate builders included in detergents (delta(18)O(P) = 17.9parts per thousand), or represent secondary values reflecting biological recycling of the phosphate in equilibrium with ambient WTP water The restricted difference in isotopic composition obtained between phosphate front fertilizers and phosphate front WTP discharge waters (< 2parts per thousand), as well as the fairly large internal isotopic variability observed in both end-members (greater than or equal to 1.5parts per thousand), cast doubt about the possibility that the oxygen isotope composition could serve as a tracer for the source of anthropogenic phosphates in waters. (C) 2004 Elsevier Ltd. All rights reserved.