I've proposed a bioretention area for a project which is sized based on the runoff volume. This is in California and is based on the C3 Technical Guidance Manual. I used a void ratio for biotreatment soil and for Caltrans class 2 permeable material of 0.35. However, the County Planning Dept reviewer said the void ratio for both is 0.01 to 0.05, which is ridiculously low. I tried to find void ratios for both of these materials on line, but could find nothing. Does anyone know of a reliable data source for these two materials?