Hello everyone!
This will be my final post for my Groundwater Remediation series! Today I would like to share about Permeable Reactive Barriers (PRB).
PRB is a technology for in-situ groundwater remediation. A PRB is an “emplacement of reactive media in the sub-surface designed to intercept a contaminated plume, provide a flow path through the reactive media and transform the contaminant(s) into environmentally acceptable forms to attain remediation concentration goals downgradient of the barrier” (Speight, 2020).
The concept of PRBs is relatively easy to understand. A PRB material of permanent, semi-permanent, or replaceable reactive media is placed in the subsurface across the flow of the path of a plume of contaminated groundwater. Under natural gradient, the plume will flow through the barrier creating a passive treatment system. As the plume moves through the material, reactions occur that transform the contaminants into less harmful species. The PRB is not a barrier to groundwater but a barrier to contaminants. PRBs are designed to be more permeable than the surrounding aquifer materials so that groundwater will readily flow and contaminants can be treated (Thiruvenkatachari et al., 2008). This can be more easily understood through the illustration below (Moore et al., 2016).
Typical reactant media in the barriers includes media designed for degrading volatile organics, chelators for immobilizing metals, or nutrients and porous material, such as sand, to enhance groundwater flow through the barrier. Certain types of permeable reactive barriers also utilize biological organisms in order to remediate groundwater. Contaminants from groundwater are thus removed by degrading, transforming, precipitating, adsorbing, or adsorbing as the water flows through the barriers (Speight, 2020).
Two configurations are more frequently used known as Continuous and the Funnel-and-Gate PRB. The continuous PRB consists of a single reactive zone installed across the plume, while the funnel-and-grate system consists of a permeable gate (reactive zone) placed between two impermeable walls that direct the plume towards the reactive zone (Thiruvenkatachari et al., 2008). This can be better illustrated in this video by Brasfond.
Since PRBs is an in-situ treatment method, there is no need for the pumping of groundwater to bring them to the surface. There would be no need for expensive above-ground facilities like storage, treatment, or disposal thus also reducing human exposure risk. They also do not require a continuous input of energy since the natural gradient is used to carry contaminants through the reactive zone. Only periodic replacement or rejuvenation of the reaction medium might be required. However, the drastically reduced operating costs offset the high construction cost that is typical for PRBs, which could result in an overall decrease in the life cycle cost of PRB (Thiruvenkatachari et al., 2008). Unfortunately, so far, limited data are available on the performances of reactive barriers with different materials due to limited long-term field testing data available and field monitoring is in its infancy (Roehl et al., 2005).
Once again, the configuration of PRB and methods of construction would differ greatly from one location to another as it is highly dependent on the hydrogeological characteristics of the site and the reactive material cost. This seems to be the common flaw in all the different technologies I have introduced in this series. I think this further proves the point of the importance of continuous study and research and most importantly, learning how to stop groundwater contamination from its source.
Hopefully, this series was enriching for you and that brings me to the end of this post!
See you guys soon!
References
Moore, R., Szecsody, J., Rigali, M., Vermuel, V., & Luellen, J. (2016). Assessment of a Hydroxyapatite Permeable Reactive Barrier to Remediate Uranium at the Old Rifle Site, Colorado. Assessment of a Hydroxyapatite Permeable Reactive Barrier to Remediate Uranium at the Old Rifle Site, Colorado. Retrieved September 17, 2020, from https://www.researchgate.net/publication/308903561_Assessment_of_a_Hydroxyapatite_Permeable_Reactive_Barrier_to_Remediate_Uranium_at_the_Old_Rifle_Site_Colorado_-_16193
Roehl, K. E., Meggyes, T., Simon, F. G., & Stewart, D. I. (2005). Long-term performance of permeable reactive barriers. Amsterdam: Elsevier.
Speight, J. G. (2020). Remediation technologies. Natural Water Remediation, 263-303. doi:10.1016/b978-0-12-803810-9.00008-5
Thiruvenkatachari, R., Vigneswaran, S., & Naidu, R. (2008). Permeable reactive barrier for groundwater remediation. Journal of Industrial and Engineering Chemistry, 14(2), 145-156. doi:10.1016/j.jiec.2007.10.001
