Plastic waste can either sit in landfills or make its way into rivers and ultimately the oceans. When plastic sits in landfills for years and with repeated rainfall, water absorbs the toxic water-soluble compounds in the plastic.

These toxic substances then leach into the soil, groundwater, and streams, poisoning the ecosystem.

https://www.bing.com/videos/search?q=what+really+happens+to+plastic+emma+bryce&view=detail&mid=9685D1D17D1A428F7F789685D1D17D1A428F7F78&FORM=VIRE

Other bulk pieces of plastic are literally dumped into streams and rivers which then find their way into our oceans