Sediment‐water interaction in a small stream: Adsorption of 137Cs by bed load sediments

In this paper we describe the interaction between dissolved solutes and stationary bed load sediments in White Oak Creek, east Tennessee. Irreversible adsorption of 137Cs by bed load sediments of White Oak Creek, a small gravel‐bottomed riffle‐and‐pool stream, was intensively studied to characterize sediment‐water interaction. Adsorption and desorption measurements in stream water show that adsorption is essentially irreversible, with no measurable desorption taking place during 6 years of exposure to stream water. The irreversible adsorption rate constant for 137Cs for 2‐ to 4‐mm gravel is 2 × 10−3 s−1. The adsorption rate constant is directly proportional to surface area and is related to mineralogy so that the adsorption rate constant for shale is much higher than for chert or limestone (<1 × 10−3 s−1). Newly deposited, uncontaminated sediment quickly becomes contaminated by 137Cs in White Oak Creek. Even though 137Cs is strongly adsorbed, measurable contamination to 20 cm depth in 64 days indicates that significant underflow takes place and results in the exchange of about 40 pore volumes of water per day. The measured adsorption rate constant is used to estimate a vertical dispersivity for underflow of about 10−1 cm2 s−1. Vertical dispersivity is probably induced by horizontal flow and is not directly related to net vertical flow. Such a high amount of sediment‐water interaction results in much greater contamination of sediments by highly adsorptive ions than is possible without significant underflow and exchange of pore fluids.