Solute-particle interactions and the enhanced dissolved barium flux from the Ganga River estuary
Dissolved and particulate barium (Ba) were investigated in samples that were collected in six periods of contrasting water discharge over two years (2012 and 2013) by Saumik and Dalai (2016, see reference below) in the Ganga (Hooghly) River estuary. The authors thoroughly documented anthropogenic sources and submarine groundwater discharges, which account for less than 2% and 5%, respectively, of the total dissolved Ba discharged annually by this estuary to the oceans. A dominant fraction of dissolved Ba results from desorption of Ba from clay minerals and/or iron-manganese hydroxides in the particulate matter.
The estimates of Ba flux show that annually (1.5–1.9) x 107 moles of Ba is transported by the Hooghly River. Additionally, about (3.6–4.3) x 107 moles of Ba is generated annually in the estuary through ion-exchange and desorption. This means that in the Ganga River estuary, the solute-particle interactions enhance the riverine Ba flux by >300%.
Figure: Variation of dissolved Ba, particulate magnesium (Mg) / aluminium (Al), exchangeable Mg and potassium (K) as a function of salinity in the Hooghly estuary. Similar variation patterns of particulate Mg/Al and dissolved Ba (with a few exceptions) are as a result of desorption of Ba in the low- to mid-salinity regions in response to adsorption of Mg. The distribution patterns of dissolved Ba in the estuary are inferred to be a direct consequence of adsorption of Mg and K in the particulate phases as evident from the variation of exchangeable Mg and K concentrations. Click here to view the figure larger.
Samanta, S., & Dalai, T. K. (2016). Dissolved and particulateBarium in the Ganga (Hooghly) River estuary, India: Solute-particle interactions and the enhanceddissolved flux to the oceans. Geochimica et Cosmochimica Acta, 195, 1–28. doi: 10.1016/j.gca.2016.09.005
Testament of the efficiency of environmental policies
Human activities, such as the combustion of leaded petrol, emissions from non-ferrous metal smelting, coal combustion and waste incineration constitute major environmental lead (Pb) sources during the past century. This resulted in a considerable increase of anthropogenic Pb in the surface and deep waters of the North Atlantic, large enough to mask the natural lead signal.
Increased usage and then phasing-out of leaded-petrol since the mid-70's yielded a decrease of this contamination. By measuring lead concentrations and isotopes (excellent tracers of the different sources of lead) along the GEOTRACES sections GA02 and GA06, Bridgestock and his co-workers (2016, see reference below) reveal for the first time that natural lead can be detected again in the surface water of the North Atlantic. Indeed, significant proportions of up to 30–50% of natural Pb, derived from mineral dust, are observed in Atlantic surface waters off the Sahara. This clearly reflects the success of the global effort to reduce anthropogenic Pb emissions.
Figure: Locations of the surface seawater samples analyzed in this study (left). The brown shaded box shows the area found to contain the highest amounts of naturally sourced lead (Pb) resulting from the deposition of North African mineral dust. Significant inputs of natural Pb can be identified by higher Pb isotope ratio values (206Pb/207Pb and 208Pb/207Pb; right).
Bridgestock, L., van de Flierdt, T., Rehkämper, M., Paul, M., Middag, R., Milne, A., Lohan, M.C., Baker, A.R., Chance, R.,, Khondoker, R., Strekopytov, S., Humphreys-Williams, E., Achterberg, E.P., Rijkenberg, M.J.A., Gerringa, L. J.A., de Baar, H. J. W. (2016). Return of naturally sourced Pb to Atlantic surface waters. Nature Communications, 7, 12921. doi:10.1038/ncomms12921