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Tracer Studies and Contact Time

Contact time is the amount of time a pathogen may be in contact with a disinfectant. Tracer studies are used to determine a system’s effective contact time (T10) at a particular flow rate. T10 is the time it takes for 10% of the water flowing through a segment of the treatment plant to exit that segment. T10 is used to calculate a baffling factor for the total contact time at different flow rates. Tracer studies are performed to determine the T10. During tracer studies, a chemical tracer is injected at the same location as a system’s disinfectant and monitored downstream near a chlorine residual monitor or entry point chlorine monitor. Each segment for which a tracer study is conducted must have an injection point where the tracer enters the segment and a monitoring point where the tracer leaves the segment. The data collected from a tracer study allows a system to accurately determine the effective contact time pathogens have with disinfectant to determine log inactivation. Tracer studies should be performed using Appendix D of EPA’s Disinfection Benchmarking and Profiling Guidance Manual.

When a tracer study is not available, a theoretical baffling factor may be assigned; however, theoretical baffling factors are not as accurate as baffling factors determined through a tracer study that has been properly conducted and reviewed by a Professional Engineer. In the absence of a tracer study that is representative of the system, DEP uses the table below for theoretical baffling factors.

Baffling Factor Values


Baffling Factor


No Contact Time


  • Hydropneumatic bladder tanks
  • Standpipe with combined inlet/outlet
  • Storage tanks with combined inlet/outlet



  • Standpipes with separate inlets and outlets
  • Circular storage tanks with separate inlets and outlets
  • Unbaffled clearwells

Unbaffled with Diffuser Walls or Plates


  • Standpipe with diffuser walls or plates
  • Circular storage tanks with diffuser walls or plates
  • Unbaffled clearwells with diffuser walls or plates

Poorly Baffled


  • Clearwellls with one(1) or two (2) internal baffles
  • Sedimentation or flocculation basins with no or one internal baffle
  • Combined flocculators and sedimentation basins in a single basin with no intra-basin baffles in the sedimentation basin

Average Baffled


  • Clearwellls with three (3) or four (4) internal baffles
  • Sedimentation or flocculation basins with two (2) or more internal baffles
  • Pressurized vessels with inlet at the bottom and outlet at the top

Superior Baffled


  • Rectangular clearwells with at least five (5) baffles)
  • Filters



  • Pipeline equal to or greater than 5:1 length to diameter ratio



  • Pipeline equal to or greater than 40:1 length to diameter ratio

A PWS may have a special condition in their permit which requires the system to submit new tracer studies and baffling factors to DEP. A PWS using a baffling factor from a tracer study that has not been reviewed and approved by DEP may risk using incorrect information to determine log inactivation. Additionally, changes to disinfection segments may affect log inactivation, and therefore, these baffling factors are not valid in determining log Giardia inactivation for compliance with Pennsylvania Safe Drinking Water Regulations. For systems where the tracer study does not reflect changes in operations, a new tracer study must be performed, or the system may use theoretical baffling factors that are representative of current operation.

A PWS may submit a new tracer study using the instructions and form for Public Water System Tracer Study Demonstration (3940-FM-BSDW0023) found in DEP’s eLibrary. Systems should update their baffling factors by submitting the Request for Designation of Treatment Segments for Calculation 1.0-log Giardia Inactivation Form (3940-FM-BSDW0569).

Groundwater systems may also use a tracer study to determine baffling factors for use in demonstrating 4-log treatment for viruses. Groundwater systems must submit the Demonstration of 4-log Treatment for Viruses Form (3900-FM-BSDW0470) for any new groundwater system or for any modification made to disinfection.