Groundwater modelling with BIOSCREEN
All Bioscreen input parameters were obtained from the research of Davis et al. (1999), Bioscreen User’s Manual Version 1.3 (Newell et al., 1996) and different bibliographic sources (see References at the bottom of the page).
Neither the duration nor the volume of the gasoline leakage were known at the beginning of field work by Davis et al. in April 1991; the tank had been out of operation since 1990.
Two scenarios were considered when modelling, in order to test the sensitivity of the parameters considered the most relevant to the spreading of the benzene plume:
Scenario 1: Seepage velocity: 100 m/yr (Davis et al., 1999); effective porosity: 0.26 (Davis et al., 1999); organic carbon partition coefficient KOC: 97 l/kg (Yang et al., 1995)
Scenario 2: Seepage velocity: 170 m/yr (Davis et al., 1999); effective porosity: 0.30 (Davis et al., 1999); organic carbon partition coefficient: 38 l/kg (ASTM, 1995)
Fig. 2. Groundwater benzene concentration along plume centreline after 1yr since beginning of field work. Upper: scenario 1. Lower: scenario 2.
Fig. 3. Groundwater benzene concentration variation along and across the modelled area, considering instantaneous reaction degradation has occurred during the elapsed modelled time (1yr). Upper: scenario 1. Lower: scenario 2.
Looking at Fig. 2, the three biodegradation modelling options yield very similar results; both scenarios show a significant discrepancy with field data from 800 ft downstream on.
Since scenario 1 represents worse spreading conditions (lower seepage velocity, higher KOC and lower effective porosity) than scenario 2 (higher groundwater velocities imply shorter times for the rate of removal of benzene from the NAPL (Geller and Hunt, 1993) and also account for larger volumes of water running through the source and carrying benzene downgradient more rapidly —plus the fact that lower KOC values enhance benzene aqueous availability) it seems plausible than the pollutant reaches higher concentrations at the same distance from the source for scenario 2.
The initial source soluble mass was estimated by means of field data provided after 1 yr of field work.
As to Fig. 3, in each of the plots the benzene plume spreads transversally a wider distance for scenario 2.
Despite seasonal fluctuations of the water table and flow direction, dispersion is small within the aquifer (Thierrin et al., 2005).
Little microbial-enhanced degradation of benzene is expected (Davis et al., 1999), though improved benzene biodegradation may take place at the zone of oxygenated rainwater recharge along the periphery of the plume and the area of NAPL exposure to the atmosphere. The improved natural attenuation that occurs at the edges of the plume could also be due to an increased contact of the contaminant with sulphate-rich groundwater and non-exhausted ferric minerals triggered by seasonal changes in groundwater flow (Prommer et al. 1998b).
Scenario 1 exhibits a lower plume mass (if no biodegradation occurs) and a higher mass of contaminant in the source (t= 1 yr) for all modelling options. The plume percentage removed by biodegradation is slightly greater for scenario 1 than for scenario 2 for the Instantaneous reaction model: 44% and 43%, respectively. It seems that the difference in actual plume mass does not affect the percentage of compound removal; for the First Order Decay model, the percentages are 45% for scenario 1 and 29% for scenario 2; in this case, the fact that more natural attenuation occurs for scenario 1 could be due to more sulphate availability (since the actual plume mass is smaller).
Potential errors in the calculations are the concentration values and widths assumed for each of the defined source zones as well as the source thickness in the saturated zone (here set equal to the maximum fluctuation of the water table, as suggested in the Bioscreen User’s Manual Version 1.3).
References
Appleyard, S. (2003): Groundwater Quality in the Perth Region. (4), 103-113.
Davis, G.B., Barber, C., Power, T.R., Thierrin, J., Patterson, J.M., Rayner, J.L. and Wu, Q. (1999): The variability and intrinsic remediation of a BTEX plume in anaerobic sulphate-rich groundwater. , 265-290.
Newell, C.J., McLeod, R.K. and Gonzales, J.R. (1996): BIOSCREEN. Natural Attenuation Decision Support System. User’s Manual Version 1.3. U.S. Environmental Protection Agency.
Yang, Y.J., Spencer, R.D., Mersmann M.A and Gates T.D. (1995): Ground-Water Contaminant Plume Differentiation and Source Determination Using BTEX Concentration Ratios. (6), 927-935.
Fig. 1. Bioscreen input values for scenario 1.