Concluding Comments

Mounting evidence suggests that mNPs are toxic to a range of organisms, including subsurface bacteria, at concentrations from one to tens of ppm. In general, mNPs are likely to be fairly strongly attenuated in many intergranular flow aquifers, but attenuation may be easily reversed in some cases, and laboratory and field evidence suggests transport distances of concern for a small proportion of the initial pollutant loading. In fracture flow systems, mNP migration distances will be potentially considerable. mNPs may also have a role to play in modifying groundwater chemistry, and as an agent in dissolved contaminant remediation. Remediation of mNP pollution may involve the use of bacteria [7].

Acknowledgements I would Like to thank NERC for funding, and the students and staff, too numerous to list here, involved our 'colloid' work in recent years.

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