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Hydrologic Tracer Tests with the 10AU for Risk Assessment Studies

Parameters: Fluorescent Dye Tracing

The National Center for Environmental Assessment (NCEA) of the Office of Research and Development (ORD) of the United States Environmental Protection Agency (EPA) conducts hydrologic tracer tests as part of risk assessment conducted for local communities, state environmental programs and EPA regional offices. Hydrological tracer testing is the most reliable diagnostic technique available for establishing flow trajectories and hydrologic connections and for determining basic hydraulic and geometric parameters necessary for establishing operative solute-transport processes all of which as essential to the development of sound risk assessments.

Constant rate injection of the dye

Malcolm Field is the principle scientist conducting the tracer tests. He uses Model 10-AU-005 fluorometers to obtain real-time fluorescence data at an appropriately calculated frequency such that the tracer-breakthrough curve (BTC) is sufficiently defined so that data aliasing is minimized. The mass of tracer to release and the sampling frequency are calculated by the recently developed computer program, EHTD, and the resulting BTC is processed using the program, QTRACER2, both of which were developed at NCEA. To facilitate data processing of data automatically logged by a Model-10-AU-005 by QTRACER2, a new program, FLOWTHRU, is currently under development at NCEA. FLOWTHRU quickly and efficiently converts logged data to decimal time units, allows for time breaks in the data, and automatically plots the data so that data trends may be observed. The converted data may then be copied to a QTRACER2 station data file for BTC analysis.

Adding two dyes as a slug injection

Most recently, two Model 10-AU-005 fluorometers were deployed at a major production well in Poolesville, Md. where coliform contamination has become a problem in recent years. Preliminary tracing by the Maryland Department of the Environment (MDE) suggested that nearby solid waste sewers might be leaking which requires well abandonment. However, more rigorous tracer testing by EPA and MDE provided evidence that the sewers are intact and that the source of the coliform contamination is not the sewers. The source of the coliform contamination remains an unknown and will perhaps require more comprehensive tracer testing from other potential source areas. Once the source area(s) is(are) established the tracing data can be used to facilitate a detailed risk assessment for the well head and appropriate actions taken. Similar efforts are being played out in a number of communities. NCEA continues to provide as timely and inexpensive testing as possible to local communities so that limited resources can be dedicated to resolving the problems uncovered.

Author: Dr. Malcolm Field Institution: The National Center for Environmental Assessment, Poolesville, Maryland, USA

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