Investigations into the Hyporheic Zone: Assessing the Influence of Groundwater-Surface Water Interactions on Metal Exposure and Effects to Aquatic Ecosystem

Metal contaminated sediments are a common stressor of biological communities in freshwater ecosystems. Sediments have the capacity to store metals (via various chemical binding ligands in sediments), rendering them unavailable for uptake by biological communities. Physical processes in streams, however, can influence the chemistry of sediments and ultimately control how sediments store metals. Groundwater-surface water interactions (hyporheic flows) are an example of a natural physical process that influences sediment chemistry and potentially the exposure of metals to aquatic biota. Hyporheic flows are generally characterized as either 'downwelling' (surface water entering streambed sediments) or 'upwelling' (from sediments into surface water). Flow direction can be related to the location in a stream riffle, as downwelling typically occurs at the head (upstream end) of a riffle and upwelling occurs on the tail (downstream end) of a riffle. Using multiple lines of evidence (field, mesocosm, and laboratory studies), this dissertation investigated the role of hyporheic flows on metal exposure and effects to aquatic organisms. Depending on the experiment, biological assessments included: test organism (Hyalella azteca) survival, benthic macroinvertebrate community composition, and biofilm structure and function. In the in situ experiments in Chapter 2, the heads of riffles were more oxidized and had greater benthic macroinvertebrate diversity and more sensitive species than the tails of riffles. Flow-through experiments in Chapter 2 also observed more oxidized sediments in downwelling conditions, compared to mesocosms without downwelling. In Chapter 3, oxygenated (oxic) hyporheic flows (downwelling) increased the bioavailability of metals, and subsequent declines in H. azteca survival were observed. In Chapter 4, the in situ experiments at field sites with upwelling hyporheic flows showed that sediments exposed to upwelling zones had less oxygen (were more reduced) and metals were less bioavailable than sediments not exposed to hyporheic upwelling. This research demonstrates the importance of hyporheic flows on redox-sensitive binding ligands and the subsequent effects on aquatic biological communities. It also suggests that inclusion of hyporheic flows in ecological risk assessments could more accurately characterize metal exposure pathways to aquatic biota.