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 (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 microm 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 approximately 2.5 microm. 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.