The conditions for the success of a remediation measure are the controllability of its bioprocesses and the technological skills of the operators. The reactions occurring within an aquifer during in situ microbiological remediation are manifold, including both abiotic and biotic processes. In situ bioremediation can be successful only if the transport of nutrients and electron acceptors and donors to the contaminants and the removal of metabolic end products (e.g., CH4, N2) is sufficient. Otherwise, gas bubbles can form, resulting in a change in the transport process. For this reason, hydrogeological parameters have to be included in the monitoring. Remediation can be successful only if it is accompanied by close monitoring. The tasks of monitoring are therefore to obtain information for the following purposes:
• controlling the addition of electron acceptors, donors, and nutrient salts
• controlling biogeochemical site conditions
• demonstrating that remediation processes are occurring
• indicating when the remediation target has been reached
In a continuous remediation process, the monitoring data are used to optimize the bioprocess and to correct malfunctions.
When unsaturated soil is treated, the most important monitoring instrument is the in situ respiration detector, which determines the consumption of oxygen and formation of carbon dioxide in the soil vapor without soil vapor extraction. Because CO2 formation can be influenced by abiotic CO2 fixation within the soil matrix (e.g., as carbonates), only O2 consumption allows good determination of the actual degradation rate with the help of stoichiometric factors. That is, we know that the mineralization of 1 g of mineral oil hydrocarbons requires the consumption of 3.5 g of O2 (neglecting biomass formation). The change in the degradation rate with time allows assessment of the quality of the installed remediation measures. A decrease in the degradation rate may indicate a lack of water, nutrients, or degradable contaminants.
Monitoring remediation measures in saturated soil is often more complex. Usually, groundwater samples are taken and the following parameters are determined:
• metabolites (specific or as dissolved organic carbon, DOC)
• degradation end products (CO2, CH4, ethene, ethane)
• electron acceptors (O2, NO-, Fe(III), Mn(IV), SO42-)
• reduced electron acceptor species (NO-, Fe(II), Mn(II), S2-)
• electron donors
• field parameters (pH, redox potential, electrical conductivity, temperature)
• optional bacterial counts (total counts, contaminant degraders, denitrifiers)
Because of the variety of possible metabolites, it is usually not possible to determine specific compounds separately; they are usually summarized as DOC. Sometimes (e.g., when water treatment plants are involved in the remediation process), more detailed information on the character of the DOC is required. This information can be supplied by LC-OCD (liquid chromatography with organic carbon detection) analysis, by which the DOC is divided into fractions of humic substances, building blocks, low molecular weight acids, amphiphilic substances, and polysaccharides. Monitoring the degradation end products is necessary if balancing of the remediation is required and complete biodegradation must be shown. However, until now no reliable methods for balancing technical-scale in situ processes are available. Usually a more pragmatically approach is chosen, i.e., only a limited number of parameters are determined. These parameters include monitoring the nutrients and electron acceptors or electron donors (in anaerobic processes) to ensure a sufficient supply of these supplements. The field parameters are monitored to ensure that the remediation measures have led to environmental conditions optimal for contaminant degradation. Here, an increase in the counts of specific contaminant-degrading bacteria is expected. However, investigations have shown that remediation meas ures strongly interfere with the indigenous microflora, changing the biodiversity in complex ways. The effects are not yet well understood.
Because many in situ technologies are based on controlled hydraulic conditions, it is evident that maintenance of the designed infiltration rate or groundwater flow rate is of utmost importance. Most pollutants exhibit low solubility and a high tendency to bind to the soil matrix, resulting in strong retardation of their transport. Elimination of the contaminants is enhanced by biodegradation at the place where the contaminants are located. Hence, transport of nutrients and electron acceptors and donors to the contaminants is necessary. Once the pollutants are solubilized, they are degraded prior to resorption. Thus, biodegradation increases the concentration gradient between sorbed/solid and dissolved substances, which accelerates desorption, dissolution, and diffusion of pollutants out of micropores (where they are not available for biodegradation). Changing the hydraulic conductivity of the aquifer strongly affects the progress of remediation. Hence, monitoring the hydraulic conductivity is necessary. A decrease in hydraulic conductivity can be caused by formation of gas bubbles (due to excess infiltration of H2O2 or high denitrification rates), precipitation of Fe oxide (during changing redox conditions), clogging with produced biomass, or shifting of fine soil particles caused by fast infiltration. By correlating an observed decrease in hydraulic conductivity with actual remediation measures, the influence of the remediation on the hydraulic conductivity can be minimized.
Generally, owing to the partitioning equilibrium of soluble gases between groundwater and soil vapor, monitoring of volatile contaminants, degradation end products (CO2, CH4), electron acceptor (O2), and natural tracer radon (222Rn) can be used to control the remediation. Degradation end products are monitored to balance the in situ processes. Monitoring O2 can prevent oversaturation with respect to this electron acceptor. By monitoring the natural tracer radon, transport from groundwater to soil vapor and from soil vapor to the atmosphere and also dilution of soil vapor with atmospheric air can be determined.
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