Particle characterization and settling velocities for a water supply reservoir during a turbidity event

Characterization of the particle population for a location in a water supply reservoir, Kensico Reservoir, N.Y, is documented for a high turbidity event, from its onset, through alum treatment and its waning. Supporting in situ measurements included the beam attenuation coefficient at 670 nm (C-670) and 660 nm (C-660) [surrogates of turbidity (T-n)], particle concentrations (N) and size distributions (PSDs), and size class specific settling velocities (SVs). Laboratory measurements included chemical and morphometric analyses of individual particles, and routine measurements of T,,. The turbidity is shown to be primarily derived from clay minerals, mostly in the size range of 1.5-6 mu m. An initial high C-670 level (40 m(-1); T-n similar to 100 NTU) decreased sevenfold in less than 1 week in response to alum treatment that largely eliminated the particle size classes responsible for the elevated turbidity. Successful SV experiments, made using a laser in situ scattering and transmissometry (LISST) instrument, for seven particle size classes in the range of 1.25-129 mu m yielded SV values of 0.17-69.4 m day(-1). Size classes larger than similar to 5 mu m settled much slower than Stokes law predictions, before alum treatment, indicating that these classes existed as porous floes or aggregates. Decreases in SVs following treatment suggest changes in floc character consistent with increased porosity. In situ measurements of C-670, N, PSDs, and SVs can contribute to the development and testing of a multiple particle size class model to simulate fate, transport, and impacts of suspended particles.