Keith D. Stolzenbach

Aggregation of fine particles at the sediment‐water interface

The presence of a bottom sediment layer agitated by mechanical stirring or by resident organisms (tubificid oligochaetes) significantly increases the rate at which fine (1 μm) cohesive particles are removed from suspension in laboratory columns. Measured rates of particle removal are equivalent to deposition velocities ranging from 0.23 m day−1 to 0.41 m day−1. These rates are an order of magnitude faster than deposition by gravitational settling or coagulation with larger particles in the water column as observed in experimental controls. It is hypothesized that the increased removal rate is the result of aggregation in a sediment layer at the bed-water interface characterized by loosely bound (fluffy), porous material hydrodynamically coupled to the water column. According to this hypothesis particle removal occurs when motion of the overlying water or organism activity causes suspended fine particles to collide with and stick to the interfacial sediment. This new hypothesis is supported by the mass and size distribution of tracer particles recovered in cores and sediment traps at a coastal site and by theoretical estimates of interfacial aggregation rates.

Scavenging of small particles by fast-sinking porous aggregates

Abstract Flow through a sinking porous aggregate theoretically can increase the rate at which it collides with and sticks to smaller suspended particles. Small particles follow the flow into the aggregate and stick efficiently to aggregated material of comparable size. The scavenging efficiency, including the probability of collision and sticking, is theoretically a function of the solid fraction of the porous aggregate and the particle-aggregate size ratio. Estimates of maximum scavenging efficiencies, based on the theory of flow through a porous sphere and assumed relationships for aggregate permeability and filtration rate, are between 10 −2 and 10 −1 , and thus greatly exceed theoretical collision efficiencies for impermeable spheres under similar conditions. Calculated scavenging efficiencies for diatom flocs and marine snow range from 1.8 × 10 −4 to 2.3 × 10 −2 . These values are sufficiently large to make scavenging of smal particles by these aggregates an important component of particle transport throughout the oceanic water column.

The effect of particle density on collisions between sinking particles: implications for particle aggregation in the ocean

A theory developed by Wacholder and Sather (1974, Journal of Fluid Mechanics, 65, 417–437) for impermeable spheres moving at low Reynolds numbers predicts that the likelihood of collision between two sinking particles of different size may be essentially zero if the larger one settles faster but is less dense than the smaller. When the ratios of the settling velocity and the excess density of the two particles exceed critical values determined by the theory, the trajectory of the small particle is closed in a region of finite size surrounding the large particle. Small particles overtaken by the large one as it sinks are deflected around this region and collisions are impossible. These findings are confirmed by laboratory experiments utilizing model particles sinking in a viscous fluid. A review of measured sinking rates and densities of marine particles indicates that a substantial fraction of the particles in the oceanic water column exceed the critical values of settling velocity and excess density with respect to other particles within a factor of 10 or so in size. Thus, aggregation by differential sedimentation is probably significant only between very small and very large particles, for which the effect of particle porosity must also be considered.

ATMOSPHERIC DRY DEPOSITION OF TRACE METALS IN THE COASTAL REGION OF LOS ANGELES, CALIFORNIA, USA

Emissions of trace metals to the atmosphere and subsequent deposition, either directly to a waterbody surface or indirectly to the watershed as washoff during rainfall, represents a potential source of contamination to surface waters near urban centers. The present study provides measurements of atmospheric concentrations of particle-bound trace metals, and it estimates the dry deposition mass loading of trace metals in coastal watersheds in the Los Angeles, California, USA, air basin. Coarse-particle atmospheric concentrations of metals were measured seasonally using a Noll Rotary Impactor at six urban sites and one nonurban site. Dry deposition fluxes were calculated by summing the product of air concentration and the theoretical deposition velocity for each particle size fraction. Mean fluxes at urban sites ranged from 3.2 to 9.1, 11 to 34, 3.8 to 8.8, 8.3 to 29, and 69 to 228 microg/m2/d for chromium, copper, nickel, lead, and zinc, respectively. Mean concentrations and fluxes were significantly higher at urban sites compared with the nonurban site, although differences between urban and nonurban sites were reduced when sampling took place within 5 d after rainfall. Dry deposition to watershed land surfaces was substantial, representing a potentially large source of trace metals based on comparisons with load estimates from stormwater runoff.

Dry deposition of airborne trace metals on the Los Angeles Basin and adjacent coastal waters

[1] We present an assessment of the deposition rates of airborne trace metals onto the Los Angeles Basin and adjacent coastal waters. For this purpose, the UCLA Surface Meteorology and Ozone Generation (SMOG) air pollution modeling system has been used to simulate the geographical distributions of trace metals and their deposition fluxes. Calculations were performed for average summer and winter conditions, as well as for extreme meteorological events, in particular, for Santa Ana winds. Thus, a series of simulations were carried out that define the range of meteorological conditions contributing to dry deposition in the region. These predictions have been calibrated and validated using measurements collected in the LA area. Significant spatial and temporal variability are found in trace metal concentrations and deposition rates. Large spatial gradients occur near the coast as well as at the mountainous boundaries of the airshed. Considerable diurnal and seasonal variations in trace metal deposition are also noted. For example, the development of a daytime sea breeze, particularly in the warmer months, leads to greater deposition in the northern and eastern basin as well as in the high desert. A nighttime land breeze, especially in the colder months, enhances deposition onto coastal ocean surfaces. Large particles dominate local trace metal deposition in central urban (and adjacent) areas, while fine particles export metals over regional scales through long-range advection. Since the majority of urban metal deposition occurs on particles larger than 10-mm diameter, routine measurements of PM10 or PM2.5 concentrations for air quality characterization may not be reliable indicators of local sources. Some 35–45% of all trace metal emissions are deposited locally within the Los Angeles Basin on an annual basis. Santa Monica Bay and its watersheds receive about 6% of this amount, which can have a significant impact on trace metal concentrations in the surface waters of the bay, primarily through land runoff following storms. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339, 4504)

General Spectral Computations of the Nonlinear Shallow Water Tidal Interactions within the Bight of Abaco

Abstract An iterative frequency–time domain finite element tidal circulation model is applied to the Bight of Abaco in the Bahamas to study the nonlinear interactions that occur between the various astronomical, overtide and compound-tide constituents. The nonlinear origin of the significant shallow water constituents is determined by suppressing the various nonlinear terms in the shallow water equations. Furthermore, the extend to which nonlinear constituents interact with and effect each other is studied in detail by suppressing the interaction of selected tides within the framework of the interative frequency-time domain formulation. It is found that second nonlinear interaction between the astronomical tides and the shallow water tides themselves can significantly affect overtide compound tides and even tides. Some important examples include (i) the M2 interaction with M6 which generates the M8 tide, (ii) the Significant reductions in M6, M8 and M10 responses (respectively by 12%, 15% and 25%) due to ...

First-order contaminant removal in the hyporheic zone of streams: physical insights from a simple analytical model.

A simple analytical model is presented for the removal of stream-borne contaminants by hyporheic exchange across duned or rippled streambeds. The model assumes a steady-state balance between contaminant supply from the stream and first-order reaction in the sediment. Hyporheic exchange occurs by bed form pumping, in which water and contaminants flow into bed forms in high-pressure regions (downwelling zones) and out of bed forms in low-pressure regions (upwelling zones). Model-predicted contaminant concentrations are higher in downwelling zones than upwelling zones, reflecting the strong coupling that exists between transport and reaction in these systems. When flow-averaged, the concentration difference across upwelling and downwelling zones drives a net contaminant flux into the sediment bed proportional to the average downwelling velocity. The downwelling velocity is functionally equivalent to a mass transfer coefficient, and can be estimated from stream state variables including stream velocity, bed form geometry, and the hydraulic conductivity and porosity of the sediment. Increasing the mass transfer coefficient increases the fraction of streamwater cycling through the hyporheic zone (per unit length of stream) but also decreases the time contaminants undergo first-order reaction in the sediment. As a consequence, small changes in stream state variables can significantly alter the performance of hyporheic zone treatment systems.

Tidal flushing of an estuarine embayment subject to recurrent dinoflagellate blooms

A rhodamine dye tracer study was conducted over eight tidal cycles to investigate mixing and tidal exchange processes in Perch Pond, a Cape Cod embayment subject to recurrent blooms of the toxic dinoflagellate, Gonyaulax tamarensis. Dye injected at the inlet to Perch Pond during flood tide became well-mixed within the pond in one day and was removed at an effective first order rate of 0.36 d−1, equivalent to a 70% utilization of the maximum possible tidal exchange. This relatively high flushing efficiency can be attributed to a density-driven circulation within the pond, consisting of a subsurface inflow of high salinity dense water on the flood tide followed by removal of lighter surface layers through the shallow inlet during ebb tide. The formation of a frontal convergence near the inlet on flood tide is consistent with the observed distribution of G. tamarensis cysts and shelifish toxicity. It is also clear that phytoplankton like G. tamarensis, whose maximum growth rates approximate the rate of tidal flushing, can only bloom within the embayment by avoiding the outflowing surface waters. Mixing within the pond is probably less efficient and population losses greater during dry periods when the pond salinity is higher and the stratification weaker.

The 2-D, Unsteady, Transport Equation Solved by the Combined Use of the Finite Element Method and the Method of Characteristics

The 2-D transport equation is split into separate convection and dispersion equations. The first is solved by a backwards method of characteristics, and the second by a quadratic Galerkin FEM. Accuracy is briefly discussed, both theoretically and through solution of test problems.

Exchange of polycyclic aromatic hydrocarbons among the atmosphere, water, and sediment in coastal embayments of southern California, USA

The present study investigated cross-media transport between both the sediment and the water column and between the water column and the atmosphere, to understand the role of each compartment as a source or a sink of polycyclic aromatic hydrocarbons (PAH) in southern California, USA, coastal waters. Concentrations of PAH were measured in the atmosphere, water column, and sediment at four water-quality-impaired sites in southern California: Ballona Creek Estuary, Los Angeles Harbor, Upper Newport Bay, and San Diego Bay. These concentrations were used to calculate site-specific sediment-water and atmosphere-water exchange fluxes. The net sediment-water exchange of total PAH (t-PAH) was positive, indicating that sediments were a source to the overlying water column. Furthermore, the net atmosphere-water exchange (gas exchange + dry particle deposition) of t-PAH was typically positive also, indicating the water column was a net source of PAH to the surrounding atmosphere through gas exchange. However, in all cases, the magnitude of the diffusive flux of PAH out of the sediments and into the water column far exceeded input or output of PAH through air/water exchange processes. These results demonstrate the potential importance of contaminated sediments as a source of PAH to the water column in coastal waters of southern California.