The Dissolved Unreactive Phosphorus Fraction as a Tracer of Urban Sewage in Environmental Water Samples: Application to an Estuarine Basin

Eutrophication in the lower Guadalquivir River estuary is often attributed to runoff from riverside rice paddies. We tested whether dissolved unreactive phosphorus (DUP)—the difference between total dissolved phosphorous (TDP) and dissolved reactive phosphorous (DRP)—can discriminate urban wastewater from agricultural P inputs. Because DUP is derived from two fractions measured with the standard molybdenum-blue method, it requires no specialized equipment and adds little cost to routine monitoring. In total 288 surface-water samples from 31 reference, agricultural, urban and mixed estuarine stations were analyzed, plus effluents from the five wastewater treatment plants (WWTPs) serving Seville. DUP showed a sharp contrast: it was present in all WWPT effluents (0.03–0.25 mg P L−1) and in 86% of urban samples (median 0.09 mg P L−1), but remained below 16 µg P L−1 in reference waters and rice-paddy drainage outlets, and rare (<10%) in other agricultural channels, confirmed at station level. Regarding agricultural water resources, the monitored rice-paddy circuit did not enrich irrigation water in dissolved P: TDP decreased by 10–52% between intake and drainage outlets on 9 of 10 dates. Urban effluents therefore appear to be the main source of dissolved P in the estuary. DUP is a robust, low-cost, context-dependent indicator that helps managers target urban P sources and avoid misattributing them to agricultural water users.