Hydrologic Transport and Fate of Particulate Phosphorus

This dissertation investigates the transport of particulate phosphorus (PP) in streamflow, and its decadal-scale fate in reservoir sediment of the Southern Piedmont physiographic region. First, we show how a fine-resolution measurement of particle sizes of suspended sediment allows us to investigate the mechanisms behind the removal of PP in stormwater wetlands, since the diameter of particles influences both the settling velocity and the amount of sorbed P on a particle. Measuring and counting individual particles in stormwater and using the total surface area as a proxy for PP allows us to put forth a simple mechanistic model of PP removal via gravitational settling, which we test with laboratory experiments to show that a mechanistic model better predicts PP removal compared to traditional first-order removal models.

Next, we question the appropriateness of the widely-used operational definition for particulate phosphorus. We analyze public data from across the United States and demonstrate that “dissolved” Iron (Fe) and “dissolved” reactive phosphorus (DRP) have a strong relationship in many watersheds, despite Fe being largely insoluble in circumneutral oxic waters, suggesting that filtrate particles are inflating DRP values in watersheds across the United States. In a laboratory experiment we use different sizes and types of filters on the same grab samples from both an urban and a rural stream in the Southern Piedmont, which results in a strong linear relationship between DRP and “dissolved” Fe. Transmission electron microscopy (TEM) is used to analyze individual colloids and nanoparticles rich in Fe and P, and small enough to easily pass through the pores of 0.45μ filters.

We document the presence of authigenic nanoparticles, rich in Fe and P, in baseflow of urban and rural streams in the Southern Piedmont. Little is known about the prevalence of these authigenic nanoparticles in streams, their impact on biogeochemical fluxes, or the bioavailability of P associated with them. We use a simple centrifugation and ultrafiltration technique to separate amorphous authigenic nanoparticles from truly dissolved (<1 kDa) and crystalline mineral/coarse organic fractions in baseflow, employing three different quality control methods to verify a successful separation: X-ray diffraction, electron microscopy, and stoichiometry of Fe and aluminum. This allows us to quantify the amount of Fe and P in three different fractions of baseflow: truly dissolved, authigenic nanoparticles, and crystalline mineral/coarse organic particles. Throughout a year-long time series, on average, authigenic nanoparticles in baseflow transported 66% of Fe, with baseflow concentrations ranging from 80 μg/L to 650 μg/L. Authigenic nanoparticles also transported an average of 38% of reactive P, depending upon seasonality and time elapsed since the last storm event.

Using Western Lake Erie as a case study, we use numerous methods to demonstrate that colloids/nanoparticles have a hidden influence on “dissolved” reactive P loads. Using TEM, we document the presence of various Fe and P-rich particles small enough to pass through filtration. We centrifuge previously-filtered samples and show that additional Fe and P can be removed (40% of DRP and 75% of Fe). We use ultrafiltration on grab samples from the Maumee River to estimate the truly dissolved concentrations of Fe and reactive P; Fe in ultrafiltration is an order of magnitude lower than in samples using 0.45μ filters. Finally, we analyze public data to estimate that approximately half of the DRP flux in the Maumee River is not truly dissolved orthophosphate, rather it is instead particulate P which passed through 0.45μ filters.

We then explore the relationships between turbidity, suspended sediment, and phosphorus transport in stormflow of the Southern Piedmont. The majority of annual P flux is transported during storm events in the Southern Piedmont, and we demonstrate that turbidity has the potential to be used as a strong proxy for sediment and P flux. We analyze the mineralogy of grab samples of suspended sediment during different stages of storm hydrographs, which changes from quartz-dominated sediment during the rising limb to clay-dominated during the late falling limb/baseflow. We also demonstrate that both kaolinite and quartz are transported as discrete minerals of very different size classes, in contrast to existing literature which assumes that composite particles comprise the majority of suspended sediment in most fluvial systems.

Finally, we turn our attention to PP burial in reservoir sediments, which is often the least understood component of P budgets for lakes and reservoirs. We show that carbon and nitrogen decrease with depth in sediment as organic matter decomposes, yet P concentrations remain relatively constant with depth. We analyze porewater which shows high levels of Fe and Mn, indicating reduced conditions in the sediment; yet porewater P remains low, suggesting a P sink in reduced conditions. NaOH-extractable P increases with depth in sediment cores, supporting the hypothesis of an inorganic P sink that can immobilize porewater P in reduced sediments. By comparing incoming suspended sediment with that deposited in the reservoir, we estimate that over 90% of incoming PP is ultimately buried in the sediments on decadal timescales, due to the strong P binding capacity of Southern Piedmont sediments during both oxic and anoxic conditions.

My dissertation research contributes to the understanding of the types and sizes of particles which transport PP, and the fate of those particles upon reaching reservoir sediment. I challenge existing decades-old methods such as sand/silt/clay size fractionation and the use of 0.45μ filters, which tend to treat PP as an operationally-defined black box, limiting scientific understanding. I use methods to accurately characterize the colloidal/nanoparticle PP fraction, such as flow-imaging particle size analysis and 0.02μ XRD filters, which have not previously been used for PP research. The findings in this research have practical applications for improving water quality through improved stormwater BMP design, setting regulatory guidelines for watershed P fluxes, managing accumulating sediment in reservoirs, and modeling ecological responses to P loading.