The Hydrocarbon Spill Screening Model

Input data from file C:\hssm150\testm.hdt

The Hydrocarbon Spill Screening Model (HSSM) simulates releases of fuels and other petroleum hydrocarbons using a suite of semi-analytical models. HSSM includes modules for

• Flow and transport through the vadose zone,
• Formation and decay of a LNAPL lens in the capillary fringe,
• Dissolution of a contaminant into the ground water, and
• Contaminant transport in a water table aquifer

Contents

• Input Narrative. A narrative description of the problem contained in C:\hssm150\testm.hdt based on the values of the input flags and parameters.
• Tabular Input Data. Tables containing all the HSSM input parameter values, their units, sources of data, and comments. Further information on parameter values can be optained from the U.S. EPA Internet Modeling Course
• Model Results. Text and graphics showing selected model outputs. Under Development

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Run Identification

HSSM Input Data Narrative

Problem Definition

The Hydrocarbon Spill Screening Model (HSSM) Version 1.50 was run with input data taken from C:\hssm150\testm.hdt. This data set was intended to simulate the release of MTBE-Gasoline, at or near the ground surface, and its flow to the water table. If a sufficient amount of MTBE-Gasoline was released, formation of a lens in the capillary fringe would be simulated. Because of its water solubility, MTBE, a component of MTBE-Gasoline would dissolve into the aquifer and be transported with the flowing ground water.

Simulation Timing

Output Selection

Aquifer

The aquifer is 50 ft thick. Its conductivity is assumed represented by a uniform value of 10 ft/d, and its porosity by 0.3. The representative gradient in the aquifer is 0.001, at an angle of 0 degrees from north. The longitudinal, transverse and vertical dispersivities of the aquifer are taken to be 17.92799 ft, 1.792799 ft, and 0.1792799 ft, respectively. Aquifer recharge is represented by an average flux of 10 in/yr. Recharge moving through the vadose zone fills some fraction of the pore space. This fraction of the pore space is calculated in HSSM and is not available for the MTBE-Gasoline and air phases.

Vadose Zone

The thickness of the vadose zone is 15 ft. It is composed of B-Sand soil, whose conductivity and porosity are 10 ft/d, and 0.3, respectively. The capillary pressure curve is approximated by the Brooks and Corey model, with pore size distribution index (lambda), 0.573, air entry head, 35.5 cm, and residual water saturation, 0.048.

Contaminants

The NAPL phase released is MTBE-Gasoline , which contains MTBE at mass fraction (mass MTBE/mass MTBE-Gasoline) of 0.14. Thus the MTBE composes 0.14 of the MTBE-Gasoline when measured by mass. The density of the MTBE-Gasoline is 0.88 g/ml. Since the NAPL phase is less dense than water, it "floats" on the water table. Its viscosity and surface tension are 0.45 cP, and 35 dyne/cm. The fraction of the pore space that remains filled by trapped droplets of MTBE-Gasoline in the vadose zone is 0.05, while that for the aquifer is 0.15. The amount of NAPL in the pore space varies across the NAPL lens at the water table. In HSSM, an average NAPL saturation is used to represent the mass and movement of the NAPL. For this simulation, the value is 0.3.

Because the contaminant, MTBE , has a certain solubility in water, it can contaminate the aquifer water. The relationship between the amount of MTBE in the MTBE-Gasoline and the the amount of MTBE dissolved in the water is characterized by an equilibrium partition coefficient between the MTBE-Gasoline and water for MTBE. This value is 15. Sorption is represented by a distribution coefficient that is the product of the fraction organic carbon in the aquifer, 0.001, and the organic carbon/water partition coefficient of the MTBE, 11 *. For this case, the distribution coefficient is thus 0.011 *. No biodegradation is assumed to occur.

Release

The source of contamination is assumed to be located at the coordinates ( 0 ft, 0 ft). The release occurred over an area of 200 ft^2. This value was estimated by assuming that the area of the tank pit is 400 ft^2 , and that 50 percent of it was contaminated. At this location MTBE-Gasoline was released from 1985 Sep 22 - 12:0:0 to 1990 Mar 13 - 12:0:0 . The volume of NAPL ( MTBE-Gasoline ) released was 1500 gal.