Abstract:
Rn-222 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 Rn-222 generation and loss in karst aquifers as a function of fissure apertures and the U-238 content of the rock, High Rn-222 activities occur in tight fissures and low Rn-222 activities occur in conduits, In the vadose zone, Rn-222 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 (approximate to 26 days), Rn-222 activities fall, At springs and in the vadose zone, after a rainfall event, Rn-222 activities increase as waters with long residence and with high Rn-222 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