Determination of the residence time of suspended particles in the turbidity maximum of the Loire estuary by 7Be analysis

Abstract

The aim of the present work was to evaluate the half life of suspended particles in the Loire estuarine turbidity maximum by analysis of 7Be budgets. The methodology was based on in situ sampling and further measurements aiming at quantifying 7Be sources (atmospheric deposition and river inputs) and 7Be stock in the water column of the turbidity maximum. 7Be river inputs were determined by monthly 7Be measurements performed upstream of the estuary. 7Be atmospheric deposition was estimated by using an empirical relation between 7Be deposition and rainfall. 7Be in particles of the estuarine turbidity maximum was measured at eight different dates corresponding to different tidal and hydrological conditions. 7Be sources and stocks thus determined have been compared to a mathematical model. Results allow to quantify the ‘standard half life’ of suspended particles in the Loire estuarine turbidity maximum and show that it depends on the season (6–10 months in summer and about 0.7 month during flood periods). Furthermore, a rather good linear correlation was observed between the standard half life of particles and the sum of flow rates in the Loire river during 60 days before each sampling date. The kinetic evolution of the mass of particles within the turbidity maximum could be estimated by this method and appeared to be consistent with previous studies. Moreover, the method proposed in this study could presumably be used for estimating 60Co concentrations in the estuarine turbidity maximum.