Caracterisation of Suspended and Sedimented Particulate Matter in SUDS Retention Ponds

SuDS retention ponds are designed to alleviate flood risk and provide water quality improvements among other multiple benefits. Characterisation of sedimented and suspended particulate matter (SPM) is indispensable for assessing SUDS functioning because of their role in biogeochemical cycling and pollutant adsorption. Here we report on the study of 9 ponds located around Edinburgh and Lothians (Scotland, UK). An important part of this project is analysis of particle size distribution (PSD). Size spectra have been derived for SPM and for a number of bottom sediments samples using a laser diffraction technique (MastersizerS and MastersizerX instruments for the SPM samples and XRF for sediment samples). A number of SPM samples have been examined using SEM EDX.

The results have shown that SPM in these ponds includes particles of both biological and abiotic origin. A considerable proportion of both suspended and sedimented particulates is smaller than 100 microns. There is normally a large fraction of small silt-sized particles, and often a considerable proportion of very fine particles (clay size). The abundance of fine particles has implications for adsorption of nutrients and pollutants, and is therefore relevant for biogeochemical cycling and for water quality issues. Although for some spectra unimodal distribution has been observed, in many cases the revealed PSD was bimodal, and in some instances more than two modes were revealed. This multimodality may have reflected contributions from different sources, both abiotic and biological. Furthermore, many of smaller particles appear to be interconnected by detrital matter. Among chemical elements routinely detected within the SPM in significant concentrations were C, O, Si, Al, Ca, Mg, Fe, K, Mn, P, Cl and S. In a number of cases, however, there were less expected elements such as Ti, Y, Mo, Cr and even Au; these may have reflected the effect of car park and road runoff and/or industrial pollution. Most of these elements (except Mo and Au) were also routinely detected in sediment samples whilst Cr and Y were below the detection limit in the majority of samples from the RBGE pond (used as a control in our study). However, such pollutants as Co, Cu, Ni, Zn and As were detected in bottom sediments of all ponds. Our presentation will discuss the differences observed among the ponds, and implications for their functioning and management.