Estimating Effective Porosity in Bedrock Aquifers

Area of Science:

• Hydrogeology
• Environmental Science

Background:

• Bedrock aquifer flow predominantly occurs through fracture networks.
• Effective porosity is a critical parameter for understanding groundwater movement and contaminant transport.
• Tracer tests offer a direct method for measuring groundwater travel times and effective porosity.

Purpose of the Study:

• To highlight the importance of tracer tests for determining effective porosity in bedrock aquifers.
• To compare tracer test results with other commonly used methods like specific yield.
• To discuss the implications of low effective porosity on groundwater velocity and contamination risk.

Main Methods:

• Point-to-point tracer tests using applied tracers.
• Analysis of calculated effective porosity values from tracer tests.
• Review of common methods for estimating effective porosity (e.g., centrifuge, water table fluctuations, pumping tests, packer tests).

Main Results:

• Tracer tests yield effective porosity values typically between 10⁻⁴ and 10⁻² (0.01% to 1%).
• These low values confirm preferential flow through fracture and channel networks.
• Specific yield, often used as a proxy, varies significantly with testing method and is less reliable than tracer tests for effective porosity.

Conclusions:

• Tracer testing is the most reliable method for estimating effective porosity in bedrock aquifers.
• Low effective porosity leads to high groundwater velocities (often >100 m/d) and increased microbial contamination risk.
• Accurate determination of effective porosity is essential for assessing groundwater travel times and managing risks in bedrock aquifers.