Exposed Streambed Sediments: A Source of Pathogen, Sediment, Nitrogen, DOC, and Phosphorus Loading to Surface Water.

Stream and river bed sediments that become exposed to the atmosphere when surface water levels drop may be a source for surface water contamination. In this study, rainfall simulations designed to simulate a 15 minute duration 50 year return period rainfall were conducted over exposed bed sediments at four distinct study sites in eastern Ontario, Canada, in order to quantify the potential for contaminant loading from these often overlooked source areas.  At each study site, simulations were performed at three replicate locations. Runoff from the simulations was examined for: total suspended solids (TSS), nitrate, total N (TN), DOC, reactive P (RP), total P (TP), fecal indicator bacteria (FIBs), selected bacterial pathogens, and Bacteroidales microbial source tracking (MST) markers. Runoff was consistently generated soon after rainfall initiation. For TSS, TN, RP, TP, and DOC, the maximum observed total runoff loads were ~1000, ~1.7, ~0.14, ~1.5, and ~3.0 kg ha^-1, respectively; for FIBs, maximum loads were ~10¹⁰ CFU ha^-1; and for Clostridium perfringens and Aeromonas, maximum loads were ~10¹⁰ and ~10¹² CFU ha^-1, respectively. Salmonella spp. and/or Campylobacter spp. were present in runoff from all but one sampling site.  Ruminant MST markers were present in the runoff water at two sites, including a site that was a cattle pasture. Ruminant, as well as bovine MST markers were also present in sediment at the cattle pasture.  This study showed that runoff from exposed stream and river bed sediments can be an important source of surface water contamination that should not be overlooked.