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

摘要

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 (PAV_m) 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 [b_m(660) and ]. Throughout the study system, nonspherical clay mineral particles in the 1-10 μm size range dominated PAV_m, light scattering and its surrogate, nephelometric turbidity (T_n). Patterns of particle size contributions to b_m(660) (and T_n) remained relatively invariant over a wide range of T_n (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 T_n-PAV_m relationship, (2) the reasonable closure between T_n measurements and calculated values of b_m(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 T_n measurements into loading calculations to evaluate relative contributions of these inputs with respect to such impacts.