Distribution of Dissolved Organic Carbon and Bacteria at the Interface between the Rhône River and Its Alluvial Aquifer

Abstract

To understand the efficiency of interstitial habitats in the elimination of organic matter as it moves from surface water to groundwater (bank filtration), we studied spatial and temporal variations of sediment organic matter concentration, biodegradable (BDOC) and refractory (RDOC) fractions of the dissolved organic carbon, bacterial abundances, and microbial enzymatic activity in the first metre of sediment of the Rhône River immediately downstream of a large city. The study area was fed most of the year by the surface water inflow (downwelling area), because of groundwater pumping wells located ∼ 80 m from the river. Decreasing gradients from surface water to deep sediments and from the river to the shore were observed in most of the cases for the four variables. The decrease in RDOC concentrations did not vary seasonally (this decrease is probably due to physical process, such as adsorption on fine mineral particles), whereas decreases in BDOC concentrations only occurred when microbial enzymatic activities were high; BDOC is rapidly assimilated by microbial communities. Physical and biological processes together make this first metre of sediment an efficient filter for organic matter.