Abstract
This study evaluates the hypothesis that increased organic-contaminant sorption and hence residence time in urban riverbed deposits may occur due to the presence of highly sorbing sediment components of anthropogenic origin that contain thermally altered carbonaceous materials (TACM). The hypothesis was confirmed through laboratory-batch studies examining tetrachloroethene (PCE) sorption on riverbed sediment samples from the highly urbanised River Tame headwaters catchment (Birmingham, UK) and the River Tern (Shropshire) providing a nearby rural benchmark. Urban-riverbed sorption was elevated with observed organic-carbon partition coefficient (Koc) values over an order of magnitude greater than rural values. The latter compare to the widely used US EPA geometric-mean Koc and attributed to absorption-dominated partitioning into (recent) natural organic matter. Use of that mean is inappropriate for the urban riverbed, potentially underestimating retardation by an order of magnitude. Organic petrography confirmed the urban riverbed contained a high proportion of TACM-rich organic-matter particles to which significant adsorption might be expected; furthermore, elevated urban Koc values were consistent with TACM-rich sediment literature estimates. The TACM source appears to be riverine transport of particulate matter from historical industrial/urban or coal-mining/processing activities prevalent upstream since the Industrial Revolution. Consequences of the increased urban riverbed residence time on attenuation and bioavailability assumption sensitivities were modelled. Under the standard assumption of sorbed contaminant being non-bioavailable, rapid rates of aqueous-phase biodegradation are necessary for complete attenuation within a riverbed. Conversely, if adsorbed contaminant is bioavailable, then complete attenuation is probable where highly adsorbing (Anthropocene urban) riverbed sediments are present, even for long half-lives. Therefore, Anthropocene-sediment occurrence may significantly influence modern-day contaminant fate within the urban riverbed; the principal implication being greatly increased sorption and residence times may occur in urban riverbeds than conventionally assumed. Further research should focus upon whether adsorbed contaminants have any bioavailability causing increased contaminant flux attenuation and surface-water quality protection.
Highlights
The residence time of groundwater organic contaminants in riverbed sediments prior to discharge to surface-waters may be perceived as short compared with the time that the plume has spent in the aquifer prior
The models illustrate quantitatively that even quite a long half life may result in complete attenuation of breakthrough if a sorbed phase has bioavailability, but even very short half-lives may have very limited effects if Greater PCE sorption, and increased residence time, occurred in the study’s urban riverbed deposits than in the study’s rural riverbed deposits. This was shown to be due to the presence of highly-sorbing sediments containing organic matter rich in thermally altered carbonaceous material (TACM) of anthropogenic origin
The evidence for this was obtained from PCE batch sorption experiments and organic matter (OM) characterisation of riverbed sediments from urban and rural sites
Summary
The residence time of groundwater organic contaminants in riverbed sediments prior to discharge to surface-waters may be perceived as short compared with the time that the plume has spent in the aquifer prior. ⇑ Corresponding author at: GroundH2O Plus Ltd, Quinton, Birmingham B32 1DY, inants occurs leading to retarded contaminant transport, extended riverbed-residence times and delayed breakthrough. If riverbed conditions favour bio(geochemical) degradation of contaminants, the combination of sorption and degradation may lead to an enhanced natural attenuation (NA) of contaminants and reduced, delayed or even completely attenuated contaminant discharge to a surface-water receptor. Assessment of riverbed reactivity and its influence upon contaminant residence time and fate at the groundwater – surface water interface (Krause et al, 2017) represents a key research focal point within the greater subsurface hydrological system residence (de Dreuzy and Ginn, 2016).
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