M

Nort-rcac.. no disp

Advective Mass Flow

200 Distance <m)

FIGURE 4.8 First-moment, vertically integrated mass and second moment for advective, nonreactive transport (a), advective, dispersive, nonreactive transport (b), and advective, dispersive transport with linear equilibrium sorption (retardation factors R of 1.45 and 1.88, respectively) (c and d). The solution for steady-state flow (thick lines) and solutions from the transient simulations (thin lines) are indicated. Reprinted from reference 50 with permission from Elsevier.

200 Distance <m)

FIGURE 4.8 First-moment, vertically integrated mass and second moment for advective, nonreactive transport (a), advective, dispersive, nonreactive transport (b), and advective, dispersive transport with linear equilibrium sorption (retardation factors R of 1.45 and 1.88, respectively) (c and d). The solution for steady-state flow (thick lines) and solutions from the transient simulations (thin lines) are indicated. Reprinted from reference 50 with permission from Elsevier.

vertically integrated concentrations could be interpreted as biodégradation.

The effect of sorption reactions on the discussed plume characteristics was investigated by comparing "no-sorption" simulations with two scenarios that included linear equilibrium sorption reactions. In the first scenario, the sorption corresponds to a retardation factor of R = 1.45 and, in the second, to R = 1.88. The simulation results shown in Fig. 4.8 demonstrate that the inclusion of the sorption reactions exceeds a "buffering" effect on the vertical movement of the plume that results from seasonal hydraulic changes. This is indicated by the smaller variability of the computed first spatial moment (vertical position of plume center) and the computed second spatial moment (plume spreading) . The results also show that for the transverse dispersivity used in these simulations (0.4 mm), the accelerating flow has a converging effect for the plume that is even stronger than the spreading caused by hydrodynamic dispersion. More details are given elsewhere (50).

Reactive Multicomponent Transport

Based on the previous simulations, in which degradation reactions were ignored, the influence of the transient flow field on reactive processes was studied in a second step. As discussed earlier in this chapter, biodégradation of oxidiz-able contaminants (in this case, BTEX compounds) might occur mainly at contaminant plume fringes where organic substrates and electron acceptors (in the present case sulfate) are simultaneously available. Thus, it might be intuitively expected that the vertical movement of the plume caused by the seasonal water table fluctuations would facilitate increased mixing of reactants and that in response the rates of total mass removal by biodégradation would increase. This increased mixing of reactants might be expected when electron donor(s) and electron acceptor(s) have different sorption characteristics. The reactive multicomponent transport model PHT3D (49) was used to investigate the magnitude of this effect by comparing the results from reactive transport simulations that use a transient flow field with the corresponding results from simulations based on a steady-state flow field.

Previous studies (19, 48) identified sulfate reduction as the dominant mechanism for the attenuation of BTEX compounds. Oxygen was observed only in low concentrations in the proximity of the water table during times of recharge. There it is consumed rapidly, either directly by organic matter or by groundwater constituents and/or minerals that have been reduced previously. Nitrate was generally not found in this portion of the aquifer. The water composition used for the numerical study, which was assumed to be representative for the uncontaminated portion of the aquifer, was based on a water sample taken upstream of the contaminated area. The recharge water was assumed to have an identical composition, except that the solution had been equilibrated with an additional 3.2 mg of oxygen liter"1. Two minerals (pyrite and siderite) that had been identified as reaction products in a soil core taken within the contaminated zone (48) were also included in the simulation. SRB and toluene (other organic compounds were excluded) were treated as kinetic species for which rate expressions were formulated on the basis of the concepts discussed earlier in this chapter.

Contamination Source

The investigation of core material from the contamination source zone showed that residual NAPLs were distributed over a 1- to 1.5-m-thick zone below the water table (19). The measurements of dissolved concentrations from a nearby multilevel sampling device indicated that the location and vertical extent of the zone from which elevated concentrations of BTEX compounds dissolve would largely follow the

Toluene x 10

Toluene x 10

ISO 200 IM

Distance (m)

FIGURE 4.9 Contour plots for toluene, sulfate, and SRB for a snapshot from the transient simulations (concentrations are given in moles per liter) (after the work of Prommer et al. [50]).

ISO 200 IM

Distance (m)

FIGURE 4.9 Contour plots for toluene, sulfate, and SRB for a snapshot from the transient simulations (concentrations are given in moles per liter) (after the work of Prommer et al. [50]).

Was this article helpful?

0 0
Organic Gardeners Composting

Organic Gardeners Composting

Have you always wanted to grow your own vegetables but didn't know what to do? Here are the best tips on how to become a true and envied organic gardner.

Get My Free Ebook


Post a comment