Dennis, Massachusetts, zip code 02638, currently holds a Critical hazard designation regarding environmental safety due to the proximity of active contamination zones. The area contains two hazardous waste sites within a ten mile radius, with the most significant threat stemming from the Harwich Sanitary Landfill. Located just 1.9 miles from the 02638 boundary, this facility is currently designated as an active site on the EPA National Priorities List. Being in an active cleanup stage means the Environmental Protection Agency is actively assessing or managing the site, though permanent remediation remains incomplete. A secondary active facility, Cape Cod Septic Services Inc, sits approximately 7.8 miles away, compounding regional environmental burdens.

The primary pollutants associated with these facilities are classified by the EPA as unspecified hazardous substances. This regulatory category typically encompasses volatile organic compounds, industrial chemicals, and heavy metals common in landfill leachate. If these substances migrate into the local aquifer, they can severely compromise municipal and private drinking water supplies. Prolonged environmental exposure to these complex chemical mixtures via ingestion or inhalation can increase the risk of neurological damage, organ toxicity, and chronic cellular mutations.

Given the active status of nearby sites, residents should immediately install certified reverse osmosis water filtration systems to protect household drinking water. Additionally, homeowners relying on private wells should hire certified laboratories to conduct comprehensive water testing for volatile organic compounds and heavy metals twice a year. Finally, residents should seal basement foundations to prevent subsurface chemical vapors from entering indoor air spaces.