Hyporheic exchange of solutes and colloids with moving bed forms

Stream-subsurface exchange provides the opportunity for stream-borne substa nces to interact with streambed sediments in the subsurface hyporheic mixin g zone. The downstream transport of both solutes and colloids can be substa ntially affected by this exchange, with significant implications for contam inant transport and stream ecology. Several previous studies have demonstra ted that bed form-induced advective flows (pumping) and scour/deposition of bed sediments (turnover) will often be the dominant processes controlling local exchange with the streambed. A new model is presented for combined tu rnover and pumping exchange due to relatively fast-moving bed forms, i.e., when turnover dominates the exchange in the upper part of the bed where act ive bed sediment transport occurs. While turnover rapidly mixes the upper l ayer of the bed, advective pumping produces exchange with the deeper, unsco ured region of the subsurface. The net exchange due to these processes was analyzed using fundamental hydraulic principles: the initial exchange was c alculated using an existing geometric model for turnover, and then the late r exchange was determined by analyzing the advective flow induced under the moving bed form field. The exchange of colloidal particles due to moving b ed forms was also modeled by considering the further effects of particle se ttling and filtration in the subsurface. Experiments were conducted in a re circulating flume to evaluate solute (conservative Li+) and colloid (kaolin ite) exchange with a sand bed. The solute and colloid exchange models perfo rmed well for fast-moving bed forms, but underpredicted the colloid exchang es observed with lower rates of bed sediment transport. For very slowly mov ing bed forms it was found that turnover could be completely neglected, and observed colloid exchanges were represented well by a pure pumping model. In the intermediate case where turnover and pumping rates are similar, wate r carried into the bed by turnover is immediately released by pumping, and vice versa. Thus, while this work further elucidated the basic processes co ntrolling solute and colloid exchange with a bed covered by bed forms and p rovided a fundamental model for exchange due to fast-moving bed forms, exch ange in the intermediate case where turnover and pumping tend to compete ca n only be bounded by current models.