Estimating subsurface fissure apertures in karst aquifers from equilibrium activities
Malcolm S. Field, and Douglas G. Mose
U. S. Environmental Protection Agency, National Center for Environmental Assessment, Washington, DC, United States
George Mason University, United States
222 Rn activities were determined for the karst aquifer underlying Walkersville, Maryland, in an area of ground-water discharge from a single geological unit during the summer and fall seasons. Radon-222 equilibrium activities in karst ground waters can be employed in mass-balance models to estimate microfissure, macrofissure, and conduit aperture dimensions. This approach defines 222 Rn generation and loss in karst aquifers as a function of fissure apertures and the 238 U content of the rock. High 222 Rn activities occur in tight fissures and low 222 Rn activities occur in conduits. In the vadose zone, 222 Rn activities are low as a result of degassing, especially if flow is turbulent and activities are decoupled from the phreatic zone. In the phreatic zone, if recharge to fissures causes a reduction of residence time below that required for equilibrium ( nearly equal 26 days), 222 Rn activities fall. At springs and in the vadose zone, after a rainfall event, 222 Rn activities increase as waters with long residence and with high 222 Rn activities are expelled from fissure and fracture storage. Field data and selected literature values were used to test the model. Models used to predict median microfissure apertures for this karst aquifer yield aperture estimates ranging from 2.8 mu m to 9.2 mu m. Median macrofissure apertures ranged from 5.53 cm to 5.88 cm. Median conduit apertures ranged from 1.16 m to 1.24 m. Comparison of the models results with published data on karst aquifers and observations at the field site suggest that the predicted apertures are reasonable.
This record provided courtesy of AGI/GeoRef.
