Isotopic effects on inorganic carbon in a tropical river caused by caustic discharges from bauxite processing

Stable C isotope compositions of dissolved inorganic C (DIC) and carbonate sediment in a Jamaican river (Rio Cobre), are used as natural tracers of accidental spillage of bauxite processing liquor and waste water. Bauxite processing produces highly caustic (OH- and CO2-3) liquor and wash waters. These hydroxide-rich waters absorb atmospheric CO2 that is isotopically fractionated resulting in very negative carbonate d13C and d18O values. Accidental spillage of these liquors into rivers causes rapid precipitation of CaCO3 as a fine-grained suspension (‘whiting’) and subsequent deposition as calcite sediment. At the time of DIC sampling ‘whiting’ was not evident; however, d13CDIC values at sites with a history of contamination were about 2‰ more negative than ambient values. The history of bauxite processing spillages is recorded in the d13C values of carbonate riverbed sediments. At sites known to be impacted, particulate carbonate samples have d13C values between -11.2 and -14.2‰; values that are between 1 and 4‰ more negative than the predicted ambient d13C value. Similarly, d18O values of carbonate sediments at impacted sites are on average 2‰ more negative than those from sites above and below them, supporting the interpretation that the ‘whiting events’ form precipitates with isotopically negative values. Contamination is quite localized because carbonate sediments downstream of impacted sites show no evidence of anomalous isotope values. This suggests that the particulate carbonate is either flushed or re-dissolves, and is diluted downstream. The carbonate ‘whitings’ are thus highly visual but relatively benign, although the associated pH and dissolved Al3+ and Na+ flushes might have more serious impacts on the river environment.