HDPE is a highly durable and lightweight polyethylene material, whose seams are permanently heat welded. HDPE stands for high density polyehtylene, a powerful polymer developed from petroleum. HDPE is extremely good at handling exposure to corrosive or abrasive chemicals. HDPE also won't leach chemicals into anything its containing, making it safe for food grade storage.
HDPE is certified by the NSF and FDA as safe for human consumption or use. It doesn't leach or offgas any toxins like some forms of plastic. While HDPE is most commonly found in landfills and for containing chemicals, it's also a highly durable option when constructing fish or reservoir ponds. A well installed HDPE liner can last for decades.
