Groundwater flow, nutrient, and stable isotope dynamics in the parafluvial-hyporheic zone of the regulated Lower Colorado River (Texas, USA) over the course of a small flood

Abstract

Periodic releases from an upstream dam cause rapid stage fluctuations in the Lower Colorado River near Austin, Texas, USA. These daily pulses modulate fluid exchange and residence times in the hyporheic zone where biogeochemical reactions are typically pronounced. The effects of a small flood pulse under low-flow conditions on surface-water/groundwater exchange and biogeochemical processes were studied by monitoring and sampling from two dense transects of wells perpendicular to the river. The first transect recorded water levels and the second transect was used for water sample collection at three depths. Samples were collected from 12 wells every 2 h over a 24-h period which had a 16-cm flood pulse. Analyses included nutrients, carbon, major ions, and stable isotopes of water. The relatively small flood pulse did not cause significant mixing in the parafluvial zone. Under these conditions, the river and groundwater were decoupled, showed potentially minimal mixing at the interface, and did not exhibit any discernible denitrification of river-borne nitrate. The chemical patterns observed in the parafluvial zone can be explained by evaporation of groundwater with little mixing with river water. Thus, large pulses may be necessary in order for substantial hyporheic mixing and exchange to occur. The large regulated river under a low-flow and small flood pulse regime functioned mainly as a gaining river with little hydrologic connectivity beyond a narrow hyporheic zone.

Potential Drivers of the Level and Distribution of Nitrogen in the Hyporheic Zone of Lake Taihu, China

“…Sediment-water interactions driven by hydrodynamics, disturbance, or diffusion have been shown to play important roles in nutrient cycling dynamics [16][17][18][19]. Xu et al [10] estimated that the annual internal TN loading from sediment in Lake Taihu was about 5.52 × 10 6 to 8.93 × 10 6 kg year −1 during 2009-2012, equivalent to 13-20% of its external TN loading, suggesting that the release of nutrients from sediments also contributes significantly to nutrients in the surface water [17,20]. Generally, nutrient releases from sediment are subject to many environmental factors, including wind current flow and the exchange flow patterns between the hyporheic zone and surface water systems [17,21,22].…”

“…Xu et al [10] estimated that the annual internal TN loading from sediment in Lake Taihu was about 5.52 × 10 6 to 8.93 × 10 6 kg year −1 during 2009-2012, equivalent to 13-20% of its external TN loading, suggesting that the release of nutrients from sediments also contributes significantly to nutrients in the surface water [17,20]. Generally, nutrient releases from sediment are subject to many environmental factors, including wind current flow and the exchange flow patterns between the hyporheic zone and surface water systems [17,21,22]. Biogeochemical reactions are thought to occur at elevated rates within the hyporheic zone because this is where microbes and solutes are able to interact within a porous substrate [17,18,23].…”

“…Generally, nutrient releases from sediment are subject to many environmental factors, including wind current flow and the exchange flow patterns between the hyporheic zone and surface water systems [17,21,22]. Biogeochemical reactions are thought to occur at elevated rates within the hyporheic zone because this is where microbes and solutes are able to interact within a porous substrate [17,18,23]. Different flow patterns and heat transfer mechanisms in the two systems lead to complex interactions of diverse solute transport and transformations in the hyporheic zone.…”

Biogeochemical cycling at the aquatic–terrestrial interface is linked to parafluvial hyporheic zone inundation history

“…Previous studies of biogeochemical cycling in the parafluvial zone have sampled gravel bars to investigate hydrologic flow paths (Claret and Boulton, 2008;Deforet et al, 2009;Zarnetske et al, 2011) and have sampled during saturated conditions utilizing well/piezometer transects (Baker et al, 1999;Deforet et al, 2009;Briody et al, 2016;Graham et al, 2016;Stegen et al, 2016). Such studies have indicated that the parafluvial hyporheic zone is an active region of diverse biogeochemical transformations (Zarnetske et al, 2011;Briody et al, 2016) and microbial community dynamics (Baker et al, 1999;Graham et al, 2016Graham et al, , 2017Stegen et al, 2016), often influenced by residence times (Claret and Boulton, 2008;Deforet et al, 2009;Zarnetske et al, 2011).…”

“…Such studies have indicated that the parafluvial hyporheic zone is an active region of diverse biogeochemical transformations (Zarnetske et al, 2011;Briody et al, 2016) and microbial community dynamics (Baker et al, 1999;Graham et al, 2016Graham et al, , 2017Stegen et al, 2016), often influenced by residence times (Claret and Boulton, 2008;Deforet et al, 2009;Zarnetske et al, 2011). However, studies that have compared biogeochemical cycling in the parafluvial zone to other floodplain environments (e.g., forest, ponds, islands, wetlands) have found that stream and nearby gravel habitats have had lower respiration, bacterial abundance, enzyme activities, and sediment nitrogen nutrient leaching than adjacent pasture/grassland, vegetated islands, and/or mature forest habitats, but short flood pulses (24 h) have increased bacterial extracellular enzymes in sediment from both flooded riverbanks and wetlands (Burns and Ryder, 2001;Doering et al, 2011;Ostojić et al, 2013;Bodmer et al, 2016).…”