Hydromorphological control of phosphorus in a large free‐flowing gravel bed river: the Garonne River (France)

The objective of this paper is to relate phosphorus (P) transport dynamics and hydromorphological characteristics of a large human-influenced river, the River Garonne within a sector receiving the waste water of a sewage treatment plant for a population of 600000. Two studies were conducted in 1997 and 1999 during two different hydrological conditions at low-flow periods. The 1997 study was carried out on an 18-km stretch with discharges varying between 33 and 53 m 3 /s and with very small fluctuations. The 1999 study concerned a longer stretch of 47 km, divided into four smaller reaches, and with discharges fluctuating rapidly from 40 to 108 m 3 /s. Downstream of the sewage treatment plant, total phosphorus (TP) concentrations ranged from 0.19 to 0.27 mg/L and were mainly in the dissolved form: between 60 and 78% of TP was dissolved reactive phosphorus (DRP). P concentrations were significantly lower upstream of the sewage treatment plant. By a mass-balance approach, we estimated that the sewage treatment plant represents more than half the input (between 59 and 67%) of the studied sector. TP dynamic is linked to suspended solids for discharges above 60 m 3 /s. During established low-flow period in the 1997 study ( 60 m 3 /s, P retention is controlled as expected by suspended matter dynamics. We conclude that management of the hydrological regime can influence P retention during sensitive low-water periods.