A variety of special groundwater wells have been developed, which have two common features. They cause groundwater circulation and stripping within the well, resulting in an intensive throughput of groundwater and, therefore, an efficient supply of nutrients and electron acceptors (air oxygen or others, e.g., H2O2). Furthermore, volatile compounds are stripped within the well. The waste air is extracted and cleaned on-site.
The individual techniques are called groundwater circulation wells, in-well stripping, and BioAirliftT. The wells consist of a combined system of groundwater removal and infiltration within the wells. To allow circulation of groundwater the well is screened at the bottom and at the groundwater table. Both areas are separated by a cover pipe and bentonite sealing. A pipe is used to inject atmospheric air at the bottom of the well. This causes upstreaming of the water according to the principle of a mammoth pump and simultaneously a stripping of volatile compounds. The elevation of the water table within the well leads to infiltration of the oxygen-enriched groundwater at the top of the groundwater level. After circulating within the aquifer, the water enters the well again at the bottom.
Additional elements can include nutrient infiltration pipes within the well. An electric water pump may be installed instead of the mammoth pump. If so, no stripping and no oxygen enrichment of the groundwater occur. An electric pump may also be installed in addition to a mammoth pump. Furthermore, a permeable bioreac-tor containing immobilized contaminant-degrading bacteria can be installed between the points of water input and output. However, the water flow velocity is usually too high to allow significant degradation of the contaminants within the residence time in the bioreactor. Therefore, the reactor material also contains activated carbon. The contaminants are sorbed onto the carbon, which is reactivated by biodegradation of the contaminants within the reactor. The reactor may also contain other materials such as an ion exchanger (to remove heavy metals) or only activated carbon, if the contaminants are not sufficiently biodegradable (e.g., PAH).
If there is additional contamination of the unsaturated soil, the outlet of the groundwater may be installed above the groundwater table. With this type of operation the unsaturated zone (at least the groundwater fluctuation zone) can be flushed with nutrient-enriched water. Alternatively, the well can be combined with a vapor-extraction system. Here, bioventing of the unsaturated soil may also be induced. These special wells have been used to remediate sites polluted with petroleum hydrocarbons, aromatic hydrocarbons, or volatile chlorinated hydrocarbons.
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