The Hidden Cost of Illicit Dumping: Construction Materials in Our Sewer Mains

Illicit dumping of construction materials into the sewer system is a growing concern for municipalities, wastewater divisions, and environmental agencies across the country. While the act may seem insignificant or even “out of sight, out of mind” to those responsible, the impacts are anything but invisible, and the consequences can be severe.

What Is Illicit Dumping?

Illicit dumping refers to the unauthorized disposal of waste materials into public infrastructure — in this case, the sanitary sewer system. For the construction industry, this might include concrete slurry, grout, mortar, plaster, joint compound, insulation, or even paint and adhesives. Some contractors or workers may see the nearest manhole or storm drain as a convenient way to get rid of leftover materials. However, this kind of shortcut is dangerous, illegal, and costly.

Why It Matters

1. Blockages and Overflows - Construction materials like concrete or grout are designed to harden quickly and form a permanent seal, which is the last thing you want inside a sewer main. These materials can reduce flow capacity, cause obstructions, and lead to backups or sanitary sewer overflows (SSOs). Once hardened inside the pipe, removal is labor-intensive, expensive, and sometimes requires full pipe replacement.
2. Environmental Damage - When sewer systems are compromised, wastewater may be discharged into local waterways, damaging ecosystems and contaminating natural habitats. The City of Thousand Oaks is part of the Calleguas Creek Watershed, where Arroyo Conejo Creek flows into the Pacific Ocean at Mugu Lagoon in Oxnard, California. Keeping our local watershed free of contamination is essential to protecting water quality and the surrounding environment.
3. Health & Safety Risks - Sewer backups don’t just damage property; they can also create serious public health concerns. Pathogens in untreated sewage can lead to illness. For field crews, cleaning hardened obstructions also poses increased safety hazards due to confined space entry and exposure to hazardous atmosphere conditions.
4. Financial Burden - The costs of investigating, cleaning, and repairing damage from illicit dumping are passed along to ratepayers. Municipalities may need to divert resources from scheduled maintenance to respond to emergencies caused by preventable acts of negligence. A recent incident occurred on August 20, 2025, when the City had to divert resources to address blue pool mortar that had been dumped into the sewer main. The response required four staff members and two service vehicles over a three-hour period, at a total cost exceeding - $1,400. An investigation is underway, and if the responsible party is identified, the City may file an action for civil damages in court of competent jurisdiction seeking recovery of all costs incurred.

Real-World Example

Crews may discover unusual buildup during routine CCTV inspections — a section of sewer main that suddenly shows partial blockage from a ue hardened substance. Further investigation reveals concrete or grout that was likely dumped into a nearby manhole. In some cases, identifying the responsible party is impossible, and the cleanup falls entirely on the City.

What Can Be Done?

1. Public Education & Outreach: Contractors and construction crews should be regularly reminded that the sewer system is not a dumping ground.
2. Enforcement & Penalties: The City’s Municipal Code prohibits the unauthorized discharge of pollutants, including the dumping of construction materials, into the sanitary sewer system. Violations of the sewer ordinance are subject to enforcement and may result in penalties, including reimbursement for the cost of repairs and removal of any discharged materials.

Sec. 10-1.501. General prohibitions on wastewater discharges.

Protecting Our Infrastructure Starts With Accountability

The sewer system is a shared resource — one that serves every household, business, and construction site in the community. Illicit dumping not only undermines this system’s integrity, but it also puts public health, the environment, and infrastructure at risk. Whether you're in the field, behind a desk, or working on a construction crew, awareness and responsibility go a long way toward keeping our systems flowing safely and smoothly. If you see any illicit dumping into the sewer system, please don’t hesitate to contact the City at toaks.gov/MSC or (805) 449-2499.