Association between purity of drug seizures and illicit drug loads measured in wastewater in a South East Queensland catchment over a six year period

Highlights: • Purity of seized illicit drugs were assessed against their loads measured in wastewater for 6 years. • Increase in purity of methamphetamine was associated with increase in its wastewater loads. • Similar phenomenon was not observed for MDMA and cocaine. • Information about purity could add value to the overall interpretation of WBE data. This study aimed to examine associations between the annual average purity of seized illicit drugs and their corresponding load measured in wastewater. Daily loads (averaging 81 samples/year) and purity of seized methamphetamine (average 287 samples/year), cocaine (50/year) and MDMA (70/year) were collected from a catchment that serviced approximately 220,000 persons in Queensland, Australia during 2010–2015. Using regression models for mass load and purity data, we found a strong linear increase in the mass load of methamphetamine detected across study years (363–1126 mg/1000 people/day, R² = 0.89). Strong linear increases in methamphetamine purity were also apparent (19–69%), and were closely correlated with detected mass load (r > 0.9). When differences in purity were controlled for, the linear trend in mass load over time was no longer significant (p > 0.27). For cocaine and MDMA there were no statistically significant trends in either mass load or drug purity over the study period. Our study demonstrates that purity changes may have accounted for a substantial proportion of increases of methamphetamine load measured in wastewater of the studied catchment. Wherever possible, when examining temporal trends in drug loads, or when making comparisons between geographic regions, purity trends should also be examined, as this can aid appropriate interpretation of findings by stakeholders and policy makers.