Comparing and improving different methods to quantify silt and clay abundance within estuarine mud

Abstract

Estuarine mud (also referred to as “fine-material”) is comprised of clay and silt particles, whose settling velocity differs by up to four orders of magnitude. Hence, quantifying their relative abundance within the mud fraction is key to understand and predict estuarine mud dynamics. Here we improve this quantification, using mud from two differing estuaries, one with a low clay-mud ratio (Massachusetts, northeast coast of USA) and one with a high clay-mud ratio (South Carolina, southeast coast of USA). First, we compare four different laboratory methods (XRD, laser diffraction, pipette method, Owen tube) to estimate the clay-mud ratio. Estimates based on laser diffraction are confounded by the fact that particles in the 2–30 μm size class could be either silt or “hyper-stable clay aggregates”, but this can be resolved by using the particle size distribution of pure clay as a reference. With this correction, laser diffraction agrees with the other three methods. Second, we show that optical backscatter (turbidity) and acoustic backscatter provide – at least under laboratory settings – a good estimate of suspended clay and silt, respectively. These two measurements correctly estimate the clay-mud ratio of the two samples, and also provide an estimate of silt and clay settling velocity in still water. These velocities are consistent with those from standard settling tests (Owen tube). Turbidity and acoustic backscatter should be able to monitor suspended clay and silt concentration in the field, provided that sand is negligible.