The plastic problem: Stormwater's role in polluting Lake Rotorua

Abstract

With the ever-increasing rate of plastic production, pollution of the aquatic environment is worsening. While microplastics (<5 mm), and marine plastic pollution are well studied, less is known about large plastic and its fate in freshwater systems. Large plastics include mesoplastics (5 mm – 2.5 cm) and macroplastics (>2.5 cm). This study aims to provide a baseline quantification of large plastics entering Lake Rotorua, focusing on the Rotorua city urban stormwater streams which are a known source of plastic entering the lake. A five-month sampling campaign collected large plastics from seven 50 m stream reaches across Rotorua city, including four sites along the longitudinal gradient of the Utuhina Stream (Headwater, Devon, Blomfield, and Depot Street sites) and three stream mouth sites (Waingaehe, Owhata Point, and Waiowhiro Stream). Sampled plastics were visually classified by polymer type to determine what plastics have been entering the lake, this was done using the New Zealand recycling and polymer guidelines. Results of the field campaign and plastic polymer classification were coupled with a systematic literature review of in situ plastic capturing devices and community initiatives to inform potential plastic reduction management options for Rotorua city. Devices were assessed against tailored suitability criteria (including Rotorua stormwater stream specific variables like channel depth and narrowness). The monthly sampling captured a total of nearly 6 kg of plastic, 30.2% (51 of 169 collected samples) of which was recyclable plastics (polyethylene terephthalate (PET), high-density polyethylene (HDPE), and polypropylene (PP)). From the literature review, booms and boom-receptacles were the best suited devices for Rotorua streams; however, they are best paired with stormwater traps to increase effectiveness. Additionally, education on recycling procedures and the plastic lifecycle, increasing awareness, and implementation of community-led initiatives are management strategies that are best coupled with in-situ devices. The findings from this study provide a foundational understanding of large plastic in the Rotorua urban stormwater system.