HSSM Model - hydrocarbon spill screening model for light nonaqueous phase liquids (LNAPLs)
HSSM Model Category: multiphase flow and transport
HSSM (Hydrocarbon Spill Screening Model) is an EPA model which simulates subsurface releases of light nonaqueous phase liquids (LNAPLs). The HSSM model includes separate modules for LNAPL flow through the vadose zone, spreading in the capillary fringe, and transport of chemical constituents of the LNAPL in a water-table aquifer. These modules are based on simplified conceptualizations of the flow and transport phenomena which were used so that the resulting model would be a practical, though approximate tool. Both DOS and Windows interfaces are provided to create input data sets, run the model, and graph the results. HSSM includes the executable, source code, and technical support.
HSSM, the hydrocarbon spill screening model, simulates the impacts of water-immiscible contaminants (LNAPLs: Light NonAqueous Phase Liquids) on water-table aquifers. The HSSM model is based on approximate treatments of flow through the vadose zone, LNAPL spreading along the water table, and miscible transport of a single chemical constituent of the LNAPL through a water-table aquifer to various receptor points. Emergency response, initial phases of site investigation, facilities siting, and underground storage tank programs are potential areas for use of HSSM.
In HSSM, the LNAPL (or hydrocarbon) is assumed to be composed of two components. The first component is the LNAPL itself which is a liquid that is separate from and does not mix with the subsurface water. HSSM contains a set of equations for tracking the motion of the LNAPL phase. Several of the results and graphs produced by HSSM depict the distribution of the LNAPL phase. The second component is referred to as a chemical constituent of the LNAPL because typical LNAPLs are composed of many individual chemicals. HSSM tracks the transport of one of these chemicals. Since the chemical constituent may dissolve into the subsurface water, it can be transported by the groundwater and contaminate down-gradient receptor points. For example. HSSM may be used to simulate a gasoline release. Benzene could be the chemical constituent of interest. All of the rest of the chemicals composing the gasoline would be treated as being part of the LNAPL. When the impact of another constituent of gasoline, say toluene, needed to be determined, the chemical constituent would be the toluene. In this way, HSSM could be run for several of the important chemical constituents of the LNAPL. The model user could develop a feel for the behavior of the different chemicals by comparing the results.
HSSM is designed for LNAPLs. It is not suitable for denser-than-water NAPLs (DNAPLs) as the NAPL is assumed to "float" on the water table. The vadose zone module of HSSM could, however, be used for a DNAPL as the qualitative behavior of that module is not affected by fluid density.
HSSM is a screening model; it includes a number of chemical and hydrologic phenomena, assumes subsurface homogeneity, executes rapidly, and excludes some phenomena. For example, if gasoline is spilled, HSSM may be used to give a rough estimate of groundwater concentrations of constituents of the gasoline The HSSM model is intended only to give order-of-magnitude results, because a number of potentially important processes are treated in the model in an approximate manner or are ignored entirely. Also, one would not expect to calibrate HSSM by adjusting the spatial distributions of the parameters as heterogeneity is not included in the model.
If simulation of complex heterogeneous sites is needed or other approximations made in HSSM are unacceptable, then a more inclusive model such as the MOFAT code should be used instead of, or in addition to, HSSM. Both DOS and Windows versions of HSSM are available.
A typical release of an LNAPL pollutant at the ground surface is imagined. The LNAPL flows downward through the vadose zone due to gravity and capillary forces. The LNAPL is deflected from its downward path by geologic heterogeneities it encounters on its way toward the water table. Infiltrating rainwater may push the LNAPL down faster than it would move on its own. Once in the vicinity of the water table, the LNAPL floats in the capillary fringe since it is a nonwetting phase that is less dense than water. Fluctuation of the water table due to natural causes or wells may create a smear zone containing trapped LNAPL. Contact with the groundwater or infiltrating recharge water causes the chemical constituents of the LNAPL to dissolve, resulting in aquifer contamination. The constituents may be leached at different rates due to their diverse properties. Depending on their volatility, the constituents also partition into the vadose zone air.
Once in the aquifer, limited mixing leaves the constituents in a relatively narrow band near the top of the aquifer. These constituents are transported by advection and dispersion through the aquifer. The aquifer, like the vadose zone, is heterogeneous and flow may follow preferential pathways.
HSSM is based on a simple conceptualization of an LNAPL release. Within HSSM, the LNAPL follows a one-dimensional path from the surface to the water table. Properties of the subsurface are taken as being uniform. The LNAPL is composed of two components: one is the LNAPL phase and the other is the chemical constituent of interest. At the water table, the LNAPL spreads radially, which implies that the regional gradient has no effect on the flow of the LNAPL. Dissolution of the chemical constituent obeys local equilibrium partitioning, but is driven by the flowing groundwater and recharge water reaching the water table. The chemical constituent is transported by advection and dispersion to multiple receptor points in the uniform aquifer.
The model is composed of three modules, based on the simplified conceptualization presented above. All of the modules are in the form of semi-analytical solutions of the governing equations so the modules of HSSM do not use discretization of the flow domain nor iterative solution techniques. These approximations are designed to execute rapidly. The conceptual basis of the modules is discussed in the following paragraphs. The mathematical details of the modules are presented in the HSSM documentation.
The HSSM model is intended to address the problem of LNAPL flow and transport from the ground surface to a water-table aquifer. Assuming that the principle interest lies with water quality, an emphasis of the model is the determination of the NAPL lens size and the mass flux of contaminants into the aquifer. These quantities define the source condition for aquifer contamination and must be based upon multiphase flow phenomena in the vadose zone. The first two modules of HSSM address the vadose zone flow and transport of the LNAPL. These two are the Kinematic Oily Pollutant Transport (KOPT) and OILENS modules. KOPT and OILENS are combined into one computer code, HSSM-KO, which provides a time- variable source condition for the aquifer model.
A chemical constituent dissolved in both the LNAPL and water phases is tracked by KOPT and OILENS. Once that chemical constituent reaches the water table, it contaminates the aquifer by contact with the recharge water and by dissolution from the LNAPL lens. Thus, the third part of the HSSM model is transported through the aquifer of one chemical constituent of the LNAPL. Notably, the mass flux from OILENS is time-varying so that the aquifer model must be capable of simulating a time-varying source condition. In keeping with the level of approximation used in KOPT and OILENS, one suitable choice is the Transient Source Gaussian Plume (TSGPLUME) model which uses an analytical solution of the advection-dispersion equation. TSGPLUME uses different numerical techniques than KOPT and OILENS so it is not incorporated within HSSM-KO, but rather is implemented in the computer code HSSM-T. The TSGPLUME model takes the dissolution mass flux from the OILENS module of HSSM-KO and calculates the expected concentrations at a number of down gradient receptor points.
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