External Sources

The conceptual model is driven by external sources of petroleum hydrocarbons as shown on Figure 4-1, including:

  • atmosphere

  • coastal runoff

  • rivers

  • spills

  • operational discharges

  • seeps

In Chapter 3 of this report, each of these sources is quantified by coastal zone within North America. To quantitatively describe the fate and effects of these external sources, they must be redefined as point sources or diffuse sources, and then further quantified in time and space. The division of the external sources into point and diffuse sources can be done as follows:

  1. Point sources:

    • Spills (vessels, platforms, pipelines, facilities)

    • Rivers

    • Produced water

    • Coastal refinery wastewater

  1. Diffuse sources:

    • Natural seeps

    • Atmosphere on open seas

    • Coastal urban runoff

    • Marine transport operations

    • Recreational boating

The time and space scales required to characterize these inputs depend on the degree of time and space definition desired for examining fate and effects. For example, to track the fate and effects of a medium-sized spill where the initial concern is toxic effects and physical adsorption on animals, the time scale would be on the order of days and the space scale meters. Each of the environmental modules (with the possible exception of bottom sediments) would also have to describe fates and effects within the compartment in similar time and space scales until the short-term effects of the spill were no longer discernible. Then the long-term effects of the spill might be calculated at seasonal time steps, with space scales in kilometers.

For minor spills that occur frequently in a geographic area, the effects of these discharges would most likely exhibit themselves as chronic effects. Thus, these minor spills could be categorized as diffuse sources, constant in time (or season) and space over a specified geographic area.

Most of the other inputs can be considered constant in time (or seasonal), and fixed in space. Time scales for the environmental compartments can thus be seasonal, with space scales in kilometers.

Transfer of Materials Within and Between Modules

Within each module, equations governing the processes that occur in the module can be developed to whatever extent is desirable, (i.e., simple models to complex models; single compound models to multiple-compound models). Most likely, a single physical oceanographic circulation model could be used as the basis for the transport calculations performed in the water column and the bottom sediments. The macro biological part of the biotic module is the ultimate system of interest in terms of effects, and the other two modules are required only to the extent necessary to accurately define the transfer paths between them and the biotic module.This model is probably good for all animals, including birds with respect to oil impacts due to ingestion, but it is not suitable for physical effects such as coating of the animals with oil, which has occurred during significant oil spills. The approach to quantitatively describing the fate and effect processes that occur in each environmental module is discussed further below.

The Water-Column Module

The water-column module does not describe any effects of oil in the ocean, but rather describes the fate of petroleum hydrocarbon compounds within the water column. The processes include interrelating transfers to and from the water column from external sources and the other two environmental modules and calculating internal biochemical transformations of petroleum hydrocarbon compounds (weathering). The water-column fate model can be expressed using a mass-balance model in the form of differential equations.

The Bottom-Sediment Module

The bottom sediments module shown on Figure 4-1 is also a fates module describing (1) chemical weathering of petroleum hydrocarbons within the sediment, (2) the transfer across the sediment-water interface of petroleum hydrocarbon compounds between the sediment module and the water column, and (3) transfers of petroleum hydrocarbon compounds between the sediment and biota modules by benthic organisms. It would be possible to include the benthic organisms in the sediment module, but it is conceptually easier to have the biological process confined to one module (the biota module). A simple two-dimensional (horizontal) mass-balance model can be written for the sediment module and also can be put in the form of a differential equation. More complex models can be envisioned involving, for example, aerobic and anaerobic processes that take place in the sediment and also, the water column.

Note that transport is included on Figure 4-1 in the bottom-sediment module. Transport of bottom sediment would occur in river deltas during times of high flow (such as seasonal high flows or flood flows) or anywhere in the intertidal

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