Surface water–groundwater interaction at restored streams and associated reference reaches

Abstract

Stream restoration goals include improving habitat and water quality through reconstruction of morphological features found at analogous, pristine stream reaches. Enhancing hyporheic exchange may facilitate achieving these goals. Although hyporheic exchange at restoration sites has been explored in a few previous studies, comparative studies of restored versus reference or control streams are largely absent. We hypothesized that restoration cross-vanes enhance hyporheic exchange, resulting in biogeochemical alteration of stream water chemistry in the streambed. Two streams restored using cross-vanes to control erosion and improve habitat were compared with their associated reference reaches, which provided the basis for the restoration design. Thirteen temperature profile rods with vertically stacked sensors were installed at each site for 2 weeks. Heat tracing was used to quantify vertical flux in the streambed from the diurnal temperature fluctuations in the subsurface. Stream water and bed pore waters from mini-piezometers were analysed for ion and nutrient chemistry. In general, mean vertical flux rates through the streambed were small throughout reference sites (À0.3 to 0.3 m/day) and at most locations at restored sites. Immediately adjacent to cross-vanes, vertical flux rates were larger (up to 3.5 m/day). Geochemistry of pore waters shows distinct differences in the sources for the reference and restored sites. Strong downwelling zones adjacent to cross-vanes showed high dissolved oxygen (10.75 mg/l) and geochemistry in the streambed similar to surface water. Reference sites had lower dissolved oxygen in the streambed (0.66-5.14 mg/l), and geochemical patterns suggest a mixture of discharging groundwater and surface water in the hyporheic zone. Restored sites also clearly show sulfate and nitrate reduction occurring in the streambed, which is not observed at the reference sites. The stream restoration sites studied here enhance rapid hyporheic exchange, but upwelling of groundwater has a stronger influence on streambed geochemistry at reference sites. Figure 4. Mapped bed temperature ( C) at a depth of 7 cm at each field site, derived from field measurements with a hand-held temperature probe at the points indicated 3736 T. L. DANILUK ET AL

Pre- and postrestoration assessment of stream water–groundwater interactions: effects on hydrological and chemical heterogeneity in the hyporheic zone

“…Our results suggest that restoration shifted the groundwater discharge zone and increased its spatial extent. One explanation for the difference in findings is that Daniluk et al (2013) studied sites 6 to 8 y after restoration, whereas we studied a newly restored site. Clogging of the top layer of channel sediments can occur over time following restoration, thereby reducing hydraulic conductivity of bed sediments and shifting patterns of surfacewater-groundwater interaction through time (Kasahara and Hill 2006).…”

“…A distinct layer of higher permeability materials overlaying lower permeability materials could produce an effective anisotropy that favors flow parallel to the layers and hinders flow and mixing perpendicular to the layers (Marion et al 2008, Sawyer and Cardenas 2009, Marklund and Wörman 2011. Daniluk et al (2013) observed stronger influence of groundwater upwelling on porewater geochemistry in reference reaches than in restored reaches, and concluded that restoration did not enhance the groundwater connection to the stream bed. Our results suggest that restoration shifted the groundwater discharge zone and increased its spatial extent.…”

“…Thus, the distribution of groundwater upwelling may change over time as the restoration structure evolves. An alternative explanation is that Daniluk et al (2013) used a space-for-time substitution and compared restored and reference sites on different stream reaches. Thus, spatial patterns of groundwater upwelling might have been inherently different between sites.…”

“…Daniluk et al (2013) and Gordon et al (2013) used methods similar to ours to calculate vertical exchange rates (the VFLUX method) at reference and restored reaches in predominantly agricultural watersheds in central New York where the goal of the restoration was to reduce erosion and channel migration. In both studies, upwelling or downwelling at the bed interface was minimal (<40 cm/d) at reference reaches or areas distant from restoration structures, and downwelling into the stream bed was strong (50-350 cm/d) adjacent to restoration structures.…”

“…Cross-vanes are large boulder dams typically installed at the heads of riffles or in glides to reduce bank erosion, establish grade control, improve ecological habitat, and focus stream energy to the thalweg (Rosgen 2001). Cross-vanes create a head differential from upstream to downstream of the cross-vane and focus flow to the channel center, so they typically deepen an excavated plunge pool immediately downstream of the structure (e.g., Daniluk et al 2013). However, when crossvanes are paired with engineered rock-riffles, the plunge pool is filled with boulders and cobbles, which increase turbulence and simulate a high-gradient riffle (Hester and Gooseff 2011).…”