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

Abstract

To better understand the nature of flocs of varying organic content in estuarine surface waters, Laser in situ Scattering and Transmissometry, video settling, and pump sampling were deployed in the York River estuary. A new in situ method was developed to simultaneously solve the floc fractal dimension (F), primary particle size (dp), and primary particle density (ρp) by fitting a simple fractal model to observations of effective floc density (Δρ) as a function of floc diameter (df), while ensuring that the integrated particle size distribution was consistent with measurements of bulk apparent density (ρa). When fractal fits were statistically justified, application of the above methods showed the bulk fraction of organic matter (forg) to be well correlated to multiple floc properties. As forg increased, dp and ρa also increased, while ρp, total suspended solids (TSS), and median floc size decreased. Notably for microflocs, neither F nor Δρ was significantly related to either forg 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 forg 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 the fractal theory held, ρp 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.