Fines settle out where the current is slowest, with successively coarser material settling out in the faster sections. Such gravimetric separation may result in deposits of zircon or monazite sands. Airborne particles exhibit similar behavior, with the coarser material settling closest to the source. Early mornings are typically calm, so radon escaping from the ground stays near the surface causing radon levels to rise. Click Here for Figure 3.
Dose rates rise gradually as soil dries out.
Water shields the radiation coming from the NORM in the ground, and dilutes the concentrations of NORM in the soil. After rainfall, the background values drop. Rainfall scours radon and its progeny from the atmosphere, causing radiation levels to rise at ground level. Some larger storms may double the gamma exposure rate for a short period of time. Shielding is dependent on the water equivalent of the snow because a heavy wet snow is more effective at shielding than a dry snow.