The Importance of Organic Content to Fractal Floc Properties in Estuarine Surface Waters: Insights from Video, LISST, and Pump Sampling

To better understand the nature of flocs of varying organic content in estuarine surface waters, LISST, video settling, and pump sampling were deployed in the York River estuary. A new in-situ method was developed to simultaneously solve for floc fractal dimension (F), primary particle size, primary particle size (d_p), and primary particle density (ρ_p) by fitting a simple fractal model to observations of effective floc density (Δρ) as a function of floc diameter (d_f) while ensuring the integrated particle size distribution was consistent with measurements of bulk apparent density (ρ_a). When fractal fits were statistically justified, application of the above methods showed the bulk fraction of organic matter (f_org) to be well correlated to multiple floc properties. As f_org increased, d_p and ρ_a also increased, while ρ_p, total suspended solids (TSS), and median floc size decreased. Notably for microflocs, neither F nor Δρ was significantly related to either f_org or TSS. This indicates that organic matter may partially displace water content within microflocs without fundamentally changing the flocs' inorganic structure. When pooling multiple samples, a marked decrease in F was seen at the transition to macroflocs, and most strongly for high f_org cases. This suggested that settling velocities ≥ ~ 1 mm/s may produce turbulent stresses that tend to tear macroflocs apart. This study also found that when fractal theory held, ρ_p had a near 1:1 correlation with the bulk dry density of filtered TSS, implying that primary particles are tightly bound aggregates of combined mineral and organic components.