Sandstones of unexpectedly high diffusibility

Area of Science:

• Geochemistry
• Hydrogeology
• Sedimentary Petrology

Background:

• Understanding solute transport in porous media is crucial for environmental and geological applications.
• Previous studies on sandstone diffusibility have yielded varied results, necessitating further investigation.
• Red-bed sandstones are common geological formations with potential implications for groundwater resources.

Purpose of the Study:

• To measure diffusion coefficients and related parameters in Triassic red-bed sandstone.
• To compare the measured diffusibility and tortuosity with existing data.
• To assess the implications of these findings for solute transport in similar geological settings.

Main Methods:

• Steady-state diffusion measurements using a double reservoir method.
• Employing two tracers: calcium bromide (CaBr2) and amino-G-acid.
• Analysis of intact Triassic red-bed sandstone samples from northwest England.

Main Results:

• Average diffusibility (D') of 0.124, with a range of 0.075 to 0.215.
• Average tortuosity of 1.21, with a range of 1.06 to 1.47.
• Observed D' values up to 4 times greater and tortuosity lower than previously reported for red-bed sandstones.

Conclusions:

• Triassic red-bed sandstones exhibit higher diffusion rates and lower tortuosity than commonly assumed.
• The open fabric and high permeability of these shallow sandstones contribute to enhanced diffusion.
• Current literature may underestimate diffusion rates in similar continental sandstone systems.