Stream pollution concentration in riffle geomorphic units (Yzeron basin, France)

In urbanized areas, small streams can be greatly damaged by urban inflows and combined sewer overflows. These polluted inputs can be several times higher than the natural stream flow over short time periods. Sound knowledge of the spatial distribution of the discharged pollutants in sediments is therefore crucial for designing monitoring strategies and suitable remediation operations. This field study combines geomorphic characterization, hydraulic conductivity measurement and pollutant assays in sediments of a small suburban river. The study site was divided up into geomorphic units: riffles, pools and runs. The last two were grouped into one class named "pool-runs" owing to their closely similar open channel flow hydraulics. Benthic and hyporheic sediments were sampled at 2m intervals. Conventional particulate pollutants (Cr, Pb, N(org), P(tot) & C(org)) were assayed in samples. The main result was: pollutants were not randomly distributed in the stream sediments, but their location showed clear concentration differences by geomorphic units, with preferential accumulation in the hyporheic zones of riffle units and a lesser one in the hyporheic zones of pools. A decrease in hydraulic conductivity was significantly correlated with an increase in pollutant concentration. This occurred mainly at the transition between riffles and pool units. The down-welling water fluxes in the sediment calculated using Darcy's formula reflect this slowdown. Our findings highlight the need to take into account the geomorphological and hydrological functioning of a stream to accurately locate the biogeochemical hotspots to be treated and thereby develop more relevant monitoring and remediation methodologies.