Stability of cocaine and its metabolites in municipal wastewater – the case for using metabolite consolidation to monitor cocaine utilization

Transformations of cocaine and eleven of its metabolites were investigated in untreated municipal sewage at pH?≈?7 and 9, 23, and 31 °C. Results indicated that hydrolysis—possibly bacterially mediated—was the principal transformation pathway. Residues possessing alkyl esters were particularly susceptible to hydrolysis, with pseudo-first-order rate constants varying from 0.54 to 1.7 day^?1 at 23 °C. Metabolites lacking esters or possessing only a benzoyl ester appeared stable. Residues lacking alkyl esters did accumulate through hydrolysis of precursors, however. As noted previously, this may positively bias cocaine utilization estimates based on benzoylecgonine alone. Reported variability in metabolic excretion was used in conjunction with transformation data to evaluate different approaches for estimating cocaine loading. Results indicate that estimates derived from measurands that capture all major cocaine metabolites, such as COC_tot (the sum of all measurable metabolites) and EC_hyd (the sum of all metabolites that can be hydrolyzed to ecgonine), may reduce uncertainty arising from variability in metabolite transformation and excretion, possibly to?≈?10 % RSD. This is more than a two-fold reduction relative to estimates derived from benzoylecgonine (>26 % RSD), and roughly equivalent to reported uncertainties from sources that are not metabolite-specific (e.g., sampling frequency, flow variability). They and other composite measurands merit consideration from the sewage epidemiology community, beginning with efforts to evaluate the stability of the total cocaine load under realistic sewer conditions.