Stream chemical dynamic and metal accumulation in a temperate watershed affected by agricultural practices (Penzé, NW France).

Abstract

Understanding the fate of metals in agricultural land is an important issue for agronomic sustainability. This study aimed at quantifying the export/retention of metals in a temperate watershed subject to important manuring activities. The chemical composition of the Penzé stream was examined at high resolution during a 1-year study in 2012. After immediate on-site filtration, here demonstrated as necessary to avoid modification of the dissolved-particulate partition, the concentrations of 21 elements were determined using inductively coupled plasma (ICP) optical emission spectrometry and ICP mass spectrometry. This dataset was extended with the local atmospheric deposition of several metals (Cd, Cr, Cu, Pb, Ni and Zn) monitored on a monthly basis. Two groups were distinguished according to the evolution of the concentrations during floods. Some major cations (Na, Ca, Mg, Sr, K, Ba) and nitrate followed counter-clockwise hysteresis patterns originating from the dilution of the enriched groundwaters by surface waters. Conversely, Al, Fe, Mn, Ti, V, Cr, Co, Ni, Cu, Zn, Cd, Pb and U displayed high dissolved concentration increases at the early stage of floods due to washing out of the enriched soils. The comparison of stream output fluxes for the two main inputs for the watershed, i.e. atmospheric deposition and manure spreading, indicates that the vast majority of the Cu and Zn (>99 and 96%, respectively), mainly originating from pig manure, is accumulated in the watershed. The accumulation rates for other metals were >60% for Ni and Cr, >75% for As and >90% for Pb and Cd.