Development of a mathematical and computer model to assess water quality impacts of hazardous compounds from minor gasoline spills in the inter-tidal zone of the Miami River, Florida

The objective of this study was to develop a model to predict transport and fate of gasoline components of environmental concern in the Miami River by mathematically simulating the movement of dissolved benzene, toluene, xylene (BTX), and methyl-tertiary-butyl ether (MTBE) occurring from minor gasoline spills in the inter-tidal zone of the river. Computer codes were based on mathematical algorithms that acknowledge the role of advective and dispersive physical phenomena along the river and prevailing phase transformations of BTX and MTBE. Phase transformations included volatilization and settling. ^ The model used a finite-difference scheme of steady-state conditions, with a set of numerical equations that was solved by two numerical methods: Gauss-Seidel and Jacobi iterations. A numerical validation process was conducted by comparing the results from both methods with analytical and numerical reference solutions. Since similar trends were achieved after the numerical validation process, it was concluded that the computer codes algorithmically were correct. The Gauss-Seidel iteration yielded at a faster convergence rate than the Jacobi iteration. Hence, the mathematical code was selected to further develop the computer program and software. The model was then analyzed for its sensitivity. It was found that the model was very sensitive to wind speed but not to sediment settling velocity. ^ A computer software was developed with the model code embedded. The software was provided with two major user-friendly visualized forms, one to interface with the database files and the other to execute and present the graphical and tabulated results. For all predicted concentrations of BTX and MTBE, the maximum concentrations were over an order of magnitude lower than current drinking water standards. It should be pointed out, however, that smaller concentrations than the latter reported standards and values, although not harmful to humans, may be very harmful to organisms of the trophic levels of the Miami River ecosystem and associated waters. This computer model can be used for the rapid assessment and management of the effects of minor gasoline spills on inter-tidal riverine water quality. ^

Hydrologic measurements and implications for tree island formation within Everglades National Park

Tree islands in the Shark River Slough of the Everglades National Park (ENP), in the southern state of Florida in the United States, are part of a wetland system of densely vegetated ridges interspersed within relatively open sloughs. Human alteration of this system has had dramatic negative effects on the landscape of the region and restoration efforts will require adjusting the hydrology of the region to assure the preservation of these important ecologic features. The primary objectives of this study were to document the hydrology in the vicinity of tree islands in ENP by measuring velocities in time and space and by characterizing suspended sediments. The results of such measurements were interpreted with respect to factors that may limit tree island growth. The measurements were conducted in the vicinity of three tree islands known as Black Hammock (BH), Gumbo Limbo (GL), and an unnamed island that was named for this study as Satin Leaf (SL). Acoustical Doppler Velocity (ADV) meters were used for measuring the low velocities of the Everglades water flow. Properties of suspended sediments were characterized through measurements of particle size distribution, turbidity, concentration and particle density. Mean velocities observed at each of the tree islands varied from 0.9 to 1.4 cm/s. Slightly higher mean velocities were observed during the wet season (1.2–1.6 cm/s) versus the dry season (0.8–1.3 cm/s). Maximum velocities of more than 4 cm/s were measured in areas of Cladium jamaicense die-off and at the hardwood hammock (head) of the islands. At the island’s head, water is channelized around obstructions such as tree trunks in relatively rapid flow, which may limit the lateral extent of tree island growth. Channelization is facilitated by shade from the tree canopy, which limits the growth of underwater vegetation thereby minimizing the resistance to flow and limiting sediment deposition. Suspended sediment concentrations were low (0.5–1.5 mg/L) at all study sites and were primarily of organic origin. The mean particle size of the suspended sediments was 3 μm with a distribution that was exponential. Critical velocities needed to cause re-suspension of these particles were estimated to be above the actual velocities observed. Sediment transport within the water column appears to be at a near steady state during the conditions evaluated with low rates of sediment loss balanced by presumably the release of equivalent quantities of particles of organic origin. Existing hydrologic conditions do not appear to transport sufficient suspended sediments to result in the formation of tree islands. Of interest would be to collect hydrologic and sediment transport data during extreme hydrologic events to determine if enough sediment is transported under these conditions to promote sufficient sediment accumulations.