This work dealt with the physical and biogeochemical processes that favored the natural attenuation of chloroethene plumes of aged sources located in the hyporheic zone in the presence of co-contaminants, such as nitrate and sulfate. Four working hypotheses were stated:
i) Reductive dechlorination is increased in areas where the river–aquifer relationship results in the dilution of other electron acceptors, the reduction potential of which exceeds that of chloroethenes.
ii) Zones where silts predominate or where textural changes occur are zones in which preferentially biodegradation occurs.
iii) The reductive dechlorination in the hyporheic zone is total when there is a good coupling between reductive and oxidative dechlorinating microorganisms.
iv) The degradation rate of the source can be significantly increased when implementing combined on-site techniques
A field site on a Quaternary alluvial aquifer at Torelló, Catalonia (Spain) was selected to validate these hypotheses. This aquifer is adjacent to an influent river, and redox conditions of groundwater favor reductive dechlorination.
The main findings showed that the low concentrations of nitrate and sulfate due to dilution caused by the input of surface water diminished the competition for electrons between dechlorinating microorganisms and denitrifiers and sulfate-reducers.
Under these conditions, the most bioavailable electron acceptors were perchloroethylene (PCE) and metabolites, which favored their biodegradation. The hyporheic zone represents an important ecotone where reductive and oxidative dechlorinating microorganisms coexist. As isotopic results show, the dechlorination rate in the hyporheic zone is increased because of the low concentrations of co-contaminants, which result in an increase in the abundance of dechlorinating microorganisms.
In addition, the high availability of organic matter and the seasonal mobility of the oxic-anoxic interface of the hyporheic zone favor a good coupling between the dechlorinating communities, which favors the rapid degradation of the most toxic metabolites of PCE, such as cis-dichloroethylene and vinyl chloride detected in the source area.
Microcosm experiments performed at the laboratory scale using sediments and groundwater from the ecotone represented by the hyporheic zone have demonstrated that the combination of In-Situ Chemical Reduction methods with biostimulation of the dechlorinating microorganisms is able to efficiently enhance the rates of dissolution of the source and to increase the degradation rates of the plume.