Hydraulic circuits comprise groundwater pumping, cleaning, addition of nutrients, and reinfiltration. The contaminants are degraded in the subsurface (in situ) or are removed with the groundwater and eliminated in the groundwater treatment plant. This technology is the first to be applied in situ and thus is the most understood. The technique has mainly been used to treat mineral oil contamination. Mineral oil hydrocarbons (MHC) are extractable to only a very small extent. During complete remediation less than 1% of the MHC can be removed with the exfiltrated groundwater; the rest has to be biodegraded in situ. However, monoaromatic hydrocarbons, a cocontaminant of mineral oil products, can be washed out to a greater extent. The rate of in situ degradation and exfiltration depends on the solubility of the contaminants, the kinetics of biodegradation, and the process technology. Because pumping and treatment of groundwater are expensive, the process may be designed to minimize the amount of groundwater to be pumped. Both the process and the complete in situ infrastructure, including the positioning and size of pumps and infiltration wells, can be chosen on the basis of process modeling.
Below, a specific hydraulic circuit design is described. At the site a gravel filter (0.5 m) was installed at the level of the groundwater table after and before refilling of the excavation on-site treated of unsaturated soil. The gravel filles contained drainage pipes that collected and transported the groundwater to the pumping wells. The pumped groundwater was sent to a water treatment plant, where Fe, Mn (to prevent clogging of the plant by hydroxides), and the contaminants were removed. The cleaned water was supplemented with nutrients (urea, phosphoric acid) and the electron acceptors hydrogen peroxide (H2O2) and nitrate (NO3-). The supplemented water was infiltrated at the bottom of the aquifer and pumped from the groundwa-ter table. The resulting vertical groundwater flow direction improved uniform distribution of the nutrients. The locations of the pumping wells have to be chosen in so that no contaminants can escape from the site along with the natural groundwa-ter flow. This may also be achieved by enclosing the site within a slurry wall reaching down to the aquiclude (groundwater-impermeable layer).
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