Light-scattering features of turbidity-causing particles in interconnected reservoir basins and a connecting stream

Light-scattering features of minerogenic particles in interconnected reservoir basins and a connecting stream in the watershed of New York City's water supply system, where these particles dominate scattering, were characterized by scanning electron microscopy interfaced with automated X-ray microanalysis and image analysis (SAX). SAX provided information on composition (in terms of elemental X-rays), shapes, number concentration, size distribution, and projected area concentration (PAVm) of particle populations. Mie theory calculations based on SAX results were used to estimate the scattering coefficient and the mean scattering efficiency at a wavelength of 660 nm [bm(660) and ]. Throughout the study system, nonspherical clay mineral particles in the 1–10 μm size range dominated PAVm, light scattering and its surrogate, nephelometric turbidity (Tn). Patterns of particle size contributions to bm(660) (and Tn) remained relatively invariant over a wide range of Tn (more than 200-fold difference). The median size for these contributions was most often ∼2.5 μm. The credibility of the SAX characterizations of the light-scattering features of the minerogenic particles and the calculations based on Mie theory for the study system was supported by (1) the strength of the Tn–PAVm relationship, (2) the reasonable closure between Tn measurements and calculated values of bm(660), and (3) the closeness of to the limiting value of 2 for polydispersed particle populations. Upstream sources of turbidity-causing particles within the study system were demonstrated to have highly similar light-scattering features. This indicates similar potencies for the particle populations from these sources for turbidity impacts in downstream waters and supports the direct incorporation of Tn measurements into loading calculations to evaluate relative contributions of these inputs with respect to such impacts.