Groundwater at Rocky Flats moves only a few feet to 100 feet a year. Because groundwater moves slowly, contaminants do not mix or spread quickly. Instead they remain concentrated in slow moving plumes that may persist for many years. Groundwater is located at different depths beneath the Rocky Flats site. Below the shallow groundwater is several hundred feet of claystone composed of tightly fitting clay particles which serve as a natural barrier.
Water can move downward through claystone at the rate of only inches per year.
Below the claystone is a deeper layer of groundwater found in the bedrock Laramie Fox Hills formation. Groundwater flowing horizontally on top of the claystone layer can emerge downslope as springs during some periods of the year at the edges of valleys created by Walnut Creek and Woman Creek, which flow through the plant site. Thus, the contaminated groundwater can become surface water, but it is captured in detention ponds built on the two creeks in 1979 to prevent most of the uncontrolled flows of storm water off the plant site. These ponds are sampled regularly. Walnut Creek, which drains about half of the Rocky Flats site, used to flow eastward into Great Western Reservoir, a former drinking water supply for the City of Broomfield.