The Rocky Mountain Arsenal (RMA), located north of Denver, CO, is a large (27 sq mi.) Superfund site composed of many habitats. To meet specific sampling needs, investigators selected several smaller reference areas representing specific habitats at RMA. The Wellington Wildlife Refuge, for example, more than 50 miles north of RMA, was selected for collecting mallard ducks, mallard eggs, cottontail rabbits, mule deer, and prairie dogs for chemical analyses. And investigators selected land five miles away as a reference site for soils because it was composed of the same soil type as that found at RMA. Investigators should try to match soil characteristics--such as particle size distribution, organic matter content, hydrologic regime, and pH--as closely as possible. For example, organic matter content, pH, and texture determine a soil's cation exchange capacity, or its ability to adsorb positively charged ions such as H(+), Mg(+2), or Cd(+2). These cations may include pollutants as well as nutrients.