Hydrologic dynamics and geochemical responses within a floodplain aquifer and hyporheic zone during Hurricane Sandy

Abstract

Storms dominate solute export budgets from catchments and drive hydrogeochemical changes in the near-stream environment. We captured near-stream hydrogeochemical dynamics during an intense storm (Hurricane Sandy, October 2012), by instrumenting a riparian-hyporheic zone transect of White Clay Creek in the Christina River Basin Critical Zone Observatory with pressure transducers, redox probes, and pore water samplers. In the floodplain aquifer, preferential vertical flow paths such as macropores facilitated rapid infiltration early in the storm. Water table rose quickly and promoted continuous groundwater discharge to the stream. Floodplain-hillslope topography controlled poststorm aquifer drainage rates, as the broad, western floodplain aquifer drained more slowly than the narrow, eastern floodplain aquifer adjacent to a steep hillslope. These changes in groundwater flow drove heterogeneous geochemical responses in the floodplain aquifer and hyporheic zone. Vertical infiltration in the floodplain and hyporheic exchange in the streambed increased DOC and oxygen delivery to microbially active sediments, which may have enhanced respiration. Resulting geochemical perturbations persisted from days to weeks after the storm. Our observations suggest that groundwater-borne solute delivery to streams during storms depends on unique interactions of vertical infiltration along preferential pathways, perturbations to groundwater geochemistry, and topographically controlled drainage rates.

Concentration–discharge relationships describe solute and sediment mobilization, reaction, and transport at event and longer timescales

“…A. Vaughan, Belmont, et al, ). This overall pattern is consistent with observed event‐based clockwise hysteresis (Table ), regional prevalence of streambank erosional sources (Dhillon & Inamdar, ), and observations of subsurface storm flow dominating hydrologic pathways (McLaughlin, ; McLaughlin & Kaplan, ; Sawyer et al, ). Further, piecewise regression analysis indicates that although the overall trend increases, at flows above 0.97 m 3 s −1 , the slope of the C‐Q relationship remains positive but decreases relative to smaller event flows (Figure , Table ).…”

“…Previous work in White Clay Creek has demonstrated that during baseflow and early stormflow conditions, discharge is generated primarily by deeper groundwater flowpaths including spring seeps and, to a lesser extent, shallow riparian aquifers; this results in a mixture of primarily deep groundwater and lesser contributions of shallower riparian groundwater to the stream on the rising limb (McLaughlin, ; McLaughlin & Kaplan, ; Sawyer et al, ). In contrast, discharge peaks reflect a mixture of groundwater, overland flow, and direct precipitation inputs, whereas discharge on the falling limb is dominated by prolonged drainage from riparian floodplain aquifers (McLaughlin & Kaplan, ; Sawyer et al, ; Figure ). At both event and longer timescales, the patterns of clockwise hysteresis and solute dilution observed for geogenic and exogenous solutes are consistent with this sequence of hydrologic connections between various biogeochemical source areas and the stream.…”

Spectral analysis of continuous redox data reveals geochemical dynamics near the stream–aquifer interface

“…The existence of high nitrate concentrations above this layer is consistent with potential nitrification under oxidizing conditions, whereas the nonconservative decline in nitrate concentrations below is consistent with denitrification under anaerobic conditions (Table S1). These observations collectively highlight the importance of lithology on subsurface geochemistry in the banks, similar to observations by Sawyer et al () in the nontidal portion of the same river. There, E h in permeable gravel layers varied strongly over time during a major storm but remained moderately stable in less permeable soils.…”

“…These drivers can interact and tend to alternately dominate in different settings. Vorenhout, van der Geest, van Harum, Wattel, and Eijsackers () illustrated how lithology controlled the magnitude of redox response to tides, whereas in other settings, E h gradients were surprisingly stable due to the lithology (Sawyer, Kaplan, Lazareva, & Michael, ). Other studies suggest hydrologic variability as the dominant factor, as it is often a direct control of oxygen transport.…”

Scaling down hyporheic exchange flows: from catchments to reaches

“…In fact, precipitations cause changes in the spatial structure of the HZ by variating river and groundwater levels, and HEF is expected to change accordingly with the oscillation of hydraulic heads and larger-scale patterns of groundwater recharge and discharge (Section 4.1) (Wondzell, 2006). At catchment scale, quick increases of the groundwater table during a precipitation event promote continuous groundwater discharge 20 (Sawyer et al, 2014) and in turn changes hyporheic bank storage (Brunke and Gonser, 1997). After precipitation, stream levels normally rise faster than the groundwater table thus leading to dynamic storage of water in stream banks and floodplains (Krause, 2007;Gulley et al, 2011).…”