Field-scale effective matrix diffusion coefficient for fractured rock: results from literature survey

Area of Science:

• Geosciences
• Hydrogeology
• Environmental Engineering

Background:

• Matrix diffusion is a critical solute transport mechanism in fractured rock formations.
• The effective matrix diffusion coefficient (D(m)(e)) is key for field-scale transport modeling.
• Understanding scale-dependent transport is crucial for environmental assessments.

Purpose of the Study:

• To survey and analyze field-scale effective matrix diffusion coefficients (D(m)(e)) from existing tracer tests.
• To investigate the relationship between D(m)(e) and observation scale in fractured rock.
• To assess the implications of scale-dependent transport for contaminant and radionuclide movement.

Main Methods:

• Literature survey of 40 field tracer tests at 15 fractured geologic sites.
• Calculation of field-scale D(m)(e) values from reported data or reanalysis.
• Application of analytic or semi-analytic solutions for tracer transport modeling.

Main Results:

• Field-scale D(m)(e) values are generally larger than lab-scale D(m) values (scale factor > 1).
• A trend of increasing scale factor with observation scale indicates statistical scale-dependency.
• Scale factor ranges from 0.5 to 884 across observation scales of 5 to 2000 m, with significant heterogeneity influence.

Conclusions:

• The effective matrix diffusion coefficient is statistically scale-dependent in fractured rock.
• Field-scale dispersivity also shows scale-dependency, consistent with prior research.
• Scale-dependent parameters are vital for accurate long-term transport predictions in nuclear waste disposal and remediation.