The Waiahole Tunnel System runs through residential areas, where pesticides applied to lawns easily run into the ditch because there are no effective barriers around the ditch. The system intersects other waterways, which will be affected by any pollutants that have contaminated the tunnels and ditches in the system. Mililani is built on a former plantation owned by Castle and Cooke, and after the plantation system became less prevalent, the land was used for residential areas. The first Mililani homes were sold in 1968, and still had the ditch running through it from the plantation era. Waiahole Ditch is made of concrete, with dirt and patches of grass surrounding it. The ditch runs through Mililani Golf Club, beside Mililani High School, and beside neighborhood houses, all of which can release pollutants. The Waikele watershed, where Mililani and parts of the Waiahole Ditch and tunnel system are located, has been evaluated for environmental safety multiple times, and overall the watershed is rated at a level that is not optimal, but not in need of an immediate remedy. The rating was based on environmental standards, land development, native species, etc.