NATO/CCMS Pilot Study Evaluation of Demonstrated and ... - CLU-IN

NATO/CCMS Pilot Project on Contaminated Land and Groundwater (Phase III) January 2002
3. DESCRIPTION OF THE RESEARCH ACTIVITY
PHC contain benzene, toluene, ethylbenzene, and xylenes (BTEX) and polycyclic aromatic hydrocarbons
(PAH), which are regulated hazardous compounds. These substances potentially dissolve into
groundwater in relevant concentrations at petroleum release sites, posing risks to drinking water supplies.
Understanding this process is important, because it provides the basis to perform initial remedial actions
and plan a long term remedial strategy for contaminated sites. Fortunately the dissolved BTEX and PAH
compounds are degradable under various conditions in aquifers. The biodegradation process leads to a
reduction of total mass of PHCs. Therefore the evaluation of the effectiveness of the biodegradation
processes is another key step in applying in situ remediation techniques to reduce risks. These processes
were studied in a laboratory system consisting of the following sequence (Figure1): dissolution of PHCs
into the aqueous phase (section A), anaerobic (section B) and aerobic biodegradation (section C) of the
dissolved compounds.
Figure 1: Experimental setup of the laboratory study on dissolution of diesel fuel compounds into sterile
groundwater (section A) and biodegradation in two laboratory aquifer columns under denitrifying (section
B) and aerobic (section C) conditions
4. RESULTS AND EVALUATION
section A section B
section C
diesel fuel sand sand
flowthrough
reactor denitrifying column aerobic column
4.1 Dissolution of NAPL Compounds in a Batch System
The purpose of the first study was to develop a modeling approach for the quantification of mechanisms
affecting the dissolution of NAPLs in the aqueous phase using the slow stirring method (SSM) and thus to
provide a tool for the interpretation of experimental data regarding the interaction between NAPLs and
water [4]. Generally, mass transfer from the NAPL to the aqueous phase increases with the stirring rate.
This can be interpreted as a decrease of the thickness of the aqueous stagnant layer at the water/NAPL
interface across which diffusion occurs. Therefore, the time to reach saturation depends on the mechanical
agitation and the aqueous diffusion coefficient of the chemical. This is only true as long as transport
within the NAPL does not control the overall mass transfer of the different NAPL components. It is
known that NAPL viscosity can influence the dissolution kinetics of PAHs. The phenomenon was
attributed to transport limitation within the NAPL of constituents with high viscosity. Thus, the existence
of a depletion zone in the NAPL phase (which in the SSM is not directly stirred) was postulated.
An analytical model was developed to provide a qualitative understanding for the different processes that
determine the temporal evolution of the combined NAPL/aqueous phase system. For situations were the
employed quantitative approximations are no longer valid a short recipe how the equations can be solved
numerically and without restrictions regarding the relative size of certain terms was presented. The
theoretical framework was validated with experimental data. The experiment was performed by running
section A of the laboratory setup (Figure 8) in batch mode.
With focus on the applicability of the preparation of water soluble fractions in slow stirring batch system
the results can be summarized as follows: Once equilibrium is reached in the system a fraction of a
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