Land Use and COVID-19 Lockdowns Influence Debris Composition and Abundance in Stormwater Drains

Stormwater drains act as a pathway for anthropogenic debris from land to sea, particularly in urbanised estuaries where impervious surfaces expedite the process. Debris type and abundance in stormwater drains may vary due to land use and human activity, and knowledge of this variation is necessary to manage the growing threat of debris. Surveys of stormwater debris can inform targeted reduction and remediation efforts by intercepting and identifying pollutants near their source. We surveyed replicate stormwater gross pollutant traps (GPTs) across four land use zones (city centre, shopping centre, transportation hub, industrial precinct) and across changing patterns of human activity before and during COVID-19 lockdown measures. GPTs were installed in 120 drains in Greater Melbourne, Australia, and citizen scientists trained by Tangaroa Blue Foundation weighed and classified debris at 6-week intervals between October 2019 and October 2020. Four survey cycles were conducted before lockdowns were implemented, then another four during lockdowns. Cigarette butts were the most abundant macro debris item (>5 mm) in every survey cycle, regardless of lockdowns. Industrial land use zones had the lowest macro debris counts but contained over 90% of the micro debris (1-5 mm). The amount of total macro debris decreased during lockdowns, however the most abundant and problematic debris items such as cigarettes and single-use plastics did not decrease similarly to concomitant reductions in human activity. OH&S items, (e.g. masks and gloves), increased (144%) during COVID-19 lockdowns. Micro debris counts did not change in industrial zones during lockdowns, suggesting that workplace interventions may be necessary to reduce this debris leakage. COVID-19 lockdowns revealed how changes in human activity might impact the flow of debris, and when combined with the patterns revealed across land uses, we show that tracing the pathway of debris from source to sea can inform reduction and long-term management strategies.