| Description |
In this study, breakthrough-elution curves were developed for three sizes (0.2 um, 1 um, and 5 um) of carboxylate-modified polystyrene latex microspheres (CML) in two environmentally-equilibrated concrete-lined streambeds containing well-sorted pea gravel and sand porous media. Microspheres were introduced at the streambed inlets and samples were collected at distances increasing geometrically (factor of two) spaced downgradient. Attachment rate coefficients, kf's, were calculated for each sampling location using the observed breakthrough plateau concentrations for each microsphere size. Trends of decreasing kf versus distance downgradient from the source were observed for all colloid sizes studied, deviating from the constant kf that is assumed when upscaling pore-scale colloid retention (collector efficiency, n) using the advection-dispersion-filtration equation. Observed retention was lower for 1 um CML relative to the other sizes in both media. This relatively low retention was attributed to decreased diffusive and settling properties for this size. Observed retention for all CML sizes was greater in sand relative to gravel media, consistent with expectations from filtration theory, which predicts greater retention with decreased grain size and decreased porosity. These retention profiles have previously been observed at the laboratory scale utilizing clean uniform media, whereas this field-scale experiment shows similar retention characteristics even when utilizing environmentally-equilibrated media, confirming the applicability of laboratory findings to the field scale. |
