Subsurface Processes
Our work on microsphere mobility was the cover art of the issue.
Subsurface Environment
Subsurface soil not only plays a critical role in supporting plants and lives on earth but also filters pollutants from surface runoff, thereby preventing groundwater contamination. Thus, understanding of the physical, chemical, and biological processes in subsurface soil can help engineer nature-based treatment systems for remediation of contaminated soil or removal of pollutants from infiltrating water.
Our group examines the effect of changing weather patterns on these processes to predict performance of stormwater treatment systems and to design stormwater biofilters that can be more resilient under changing climates.
Green infrastructures such as rain garden, green roofs, and bioinfiltration systems are increasingly being used in urban areas to harvest stormwater for reuse or groundwater replenishment, but the fate of stormwater contaminants in these systems under changing climate is not well understood. The study under this category aims to improve the design of bioinfiltration system to achieve better bacterial removal.
Moist biochar lowers the negative impaction of compaction on infiltration rate of stormwater pollutant.
Compared to dry biochar under compaction, moist biochar under the same condition removed more pathogen and maintain relatively higher hydraulic conductivity. Thus, protocol for the application of biochar in roadside biofilters, where soil-amendment mixtures are typically compacted, should include addition of water before compaction.
Ghavanloughajar, M., Le, H., Rahman, M.D., Valenca, R., Borthakur, A., Ravi, S., Stenstrom, M., and Mohanty, S.K. (2020) Compaction conditions affect the capacity of biochar-amended sand filters to treat road runoff. Science of the Total Environment. 735, 139180. https://doi.org/10.1016/j.scitotenv.2020.139180 [ pdf ]
Intermittent infiltration of stormwater may make the stormwater biofilter a source, not sink of pathogen.
The study shows that intermittent flow can mobilize some of the attached bacteria from traditional bioinfiltration geomedia, thereby making the bioinfiltration system as a net source (instead of a sink) of bacteria. Increasing saturation and augmenting bioinfiltration geomedia with iron oxide coated sands increase removal and decrease detachment of bacteria during intermittent infiltration of stormwater, although iron oxide coated sands become ineffective in the presence of dissolved organic carbon.
Mohanty, S.K., Torkelson, A.A., Dodd, H., Nelson, K.L. and Boehm, A.B. (2013) Engineering solutions to improve the removal of fecal indicator bacteria by bioinfiltration systems during intermittent flow of stormwater. Environmental Science & Technology. 47(19), 10791-10798. https://doi.org/10.1021/es305136b [ pdf ]
Biochar can increase removal of pathogens in stormwater biofilters
The results show that addition of small quantity of biochar not only improved bacterial removal capacity of bioinfiltration systems but also decreased the remobilization of the attached bacteria during intermittent infiltraiton of stormwater. The study also revealed the mechanism of bacterial removal in biochar.
Mohanty, S.K., Cantrell, K.B., Nelson, K.L. and Boehm, A.B. (2014) Efficacy of biochar to remove Escherichia coli from stormwater under steady and intermittent flow. Water Research. 61, 288-296. https://doi.org/10.1016/j.watres.2014.05.026 [ pdf ]
Biochar capacity to remove bacterial pollutants decreases with increases in particle size of biochar and presence of compost.
This work shows that bacterial removal capacity of biochar-augmented bioinfiltration systems could remain high despite increases in infiltration rate (or rainfall intensity) and bacterial loading. However, a decrease in biochar particle size and presence of compost significantly reduced bacterial removal. Thus, for biochar to be effective adsorbent, biochar size should be small, and it should be applied without compost.
Mohanty, S.K. and Boehm, A.B. (2014) Escherichia coli removal in biochar-augmented biofilter: Effect of infiltration rate, initial bacterial concentration, biochar particle size and presence of compost. Environmental Science & Technology. 48(19), 11535-11542. https://doi.org/10.1021/es5033162 [ pdf ]
Physical weathering has a net positive effect on bacterial removal by biochar-augmented biofilters.
Biochar particles were mobilized during intermittent infiltration of stormwater, but the mobilization depended on temperature and antecedent conditions. Chemical weathering (or exposure to natural organic carbon in stormwater) decreased bacterial removal capacity of the bioinfiltration system partly due to exhaustion of attachment sites by NOM adsorption, but intermediate drying cycle helped replenish some of the attachment sites.
Mohanty, S.K. and Boehm, A.B. (2015) Effect of weathering on mobilization of biochar particles and bacterial removal in a stormwater biofilter. Water Research. 85, 208-215. https://doi.org/10.1016/j.watres.2015.08.026 [ pdf ]
