Local inhomogenity ter scale covers local aquifer systems. From scale to scale, different constraints limit the processes. In general, the constraints of the largest scale, i.e., hydraulic site conditions, might be the overall limiting processes. Hydraulically favorable conditions are offered by course sediments with a hydraulic conductivity of >10-4 m s-1.
Some of the main features of in situ technologies are to transport supplements (nutrient salts, electron acceptors or donors) to the contaminants and to remove reaction products, as well as to induce proper environmental conditions for contaminant degradation. This may, e.g., be implemented by pumping of reaction-product-enriched groundwater, water cleaning, and reinfiltration of water enriched with nutrients.
The efficiency of in situ biodegradation depends importantly on the distribution of the nutrients in suitable concentrations within the aquifer. Biodegradation processes that require aerobic conditions may be limited by the lack of oxygen. The maximum O2 concentration dissolved in water that can be infiltrated is given by its solubility of about 50 mg L-1. The oxygen supply can be increased by using hydrogen peroxide (H2O2), which is decomposed to O2 and water enzymatically or by geogen-ic components. However, the concentrations of H2O2 in the infiltration water are limited by its toxicity to bacteria at concentrations above 1000 mg L-1 and the possibility of O2 bubble formation, which would lead to a decrease in hydraulic conductivity and thus to decreased nutrient and oxygen supply.
The demand for oxygen or other electron acceptors decreases during the course of the remediation. In the beginning, electron acceptor consumption is usually limited by the supply rate. In a later phase, when all readily available compounds have been degraded, the electron acceptor consumption is limited by the rate of contaminant dissolution or diffusion out of micropores. Especially in this phase is it important, if O2 is added, to use proper O2 infiltration concentrations to avoid bubble formation.
The use of denitrifying conditions for contaminant degradation by the infiltration of nitrate may also lead to the formation of bubbles, because the end product of the denitrification is nitrogen (N2), which has a comparably low solubility. If nitrate is overdosed or the exfiltration rate is too slow, this may result in N2 degassing and bubble formation.
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