Ensuring Safe Drinking Water
Total Trihalomethanes (TTHMs) are byproducts that can form when chlorine used to disinfect drinking water reacts with naturally occurring organic matter. Learn how TTHMs form and how DPW monitors and manages them to protect drinking water quality. The Baltimore City Department of Public Works (DPW) routinely monitors drinking water to ensure it meets all federal and state safety standards. This page provides general information about Total Trihalomethanes (TTHMs), how they form, and how DPW manages them as part of normal water treatment operations.
In March 2026, customers in the Sparks area of Baltimore County were notified that TTHM levels at one sampling location measured above the drinking water standard during a past monitoring period. These notifications are required by state and federal regulations when the annual running average exceeds the standard at a monitoring location.
For additional details about the March 2026 notification and monitoring results, please review the TTHM Frequently Asked Questions (FAQ).
What Are Total Trihalomethanes (TTHMs)?
Total Trihalomethanes (TTHMs) are compounds that can form when disinfectants such as chlorine react with naturally occurring organic matter in water.
Chlorine disinfection is an essential step in the water treatment process because it kills harmful bacteria and protects public health. Because TTHMs can form during this process, federal and state regulations set limits to ensure long-term exposure remains within safe levels.
What are TTHMs?
Total Trihalomethanes (TTHMs) are a group of chemical compounds that can form in drinking water.
They are created when chlorine, used to disinfect water, reacts with naturally occurring organic matter found in source water.
Chlorine disinfection is essential for public health it eliminates harmful bacteria and viruses that can cause serious illness.
While TTHMs are a byproduct of this necessary process, water systems work to keep levels as low as possible
Drinking Water Standards
Compliance with this standard is determined using a Locational Running Annual Average (LRAA), calculated from four consecutive quarters of testing at each monitoring location.
The regulatory limit for TTHMs is strictly enforced by state and federal environmental agencies to ensure long-term public health.
Why TTHM Levels Can Change
Several environmental and seasonal factors can influence TTHM levels in drinking water:
Higher Water Temperatures
Warmer water temperatures, especially during summer months, can increase the rate of TTHM formation.
Natural Organic Material
Seasonal changes in vegetation and runoff can increase organic matter levels in source water.
Natural Organic Material
Seasonal changes in vegetation and runoff can increase organic matter levels in source water.
How DPW Manages TTHMs
The Department of Public Works employs multiple strategies to manage and reduce TTHM levels:
- Adjusting Reservoir Withdrawal Depths
Drawing water from optimal depths to minimize organic material intake. - Optimizing Treatment Processes
Fine-tuning coagulation, filtration, and disinfection to reduce TTHM formation. - Hydrant Flushing Programs
Systematically flushing water mains to improve water circulation and reduce stagnation. - Routine Monitoring and Testing
Conducting regular water quality testing at multiple locations throughout the distribution system.
TTHM Frequently Asked Questions (FAQ)
Download our comprehensive FAQ document for detailed information about TTHMs and drinking water safety.
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