Controls on solute concentration‐discharge relationships revealed by simultaneous hydrochemistry observations of hillslope runoff and stream flow: The importance of critical zone structure

Abstract

We investigated controls on concentration‐discharge relationships of a catchment underlain by argillite by monitoring both groundwater along a hillslope transect and stream chemistry. Samples were collected at 1–3 day intervals over 4 years (2009–2013) in Elder Creek in the Eel River Critical Zone Observatory in California. Runoff at our study hillslope is driven by vadose zone flux through deeply weathered argillite (5–25 m thick) to a perched, seasonally dynamic groundwater that then drains to Elder Creek. Low flow derives from the slowly draining deepest perched groundwater that reaches equilibrium between primary and secondary minerals and saturation with calcite under high subsurface pCO2. Arriving winter rains pass through the thick vadose zone, where they rapidly acquire solutes via cation exchange reactions (driven by high pCO2), and then recharge the groundwater that delivers runoff to the stream. These new waters displayed lower solute concentrations than the deep groundwater by less than a factor of 5 (except for Ca). Up to 74% of the total annual solute flux is derived from the vadose zone. The deep groundwater's Ca concentration decreased as it exfiltrates to the stream due to CO2 degassing and this Ca loss is equivalent of 30% of the total chemical weathering flux of Elder Creek. The thick vadose zone in weathered bedrock and the perched groundwater on underlying fresh bedrock result in two distinct processes that lead to the relatively invariant (chemostatic) concentration‐discharge behavior. The processes controlling solute chemistry are not evident from stream chemistry and runoff analysis alone.

Drainage from the Critical Zone: Lithologic Controls on the Persistence and Spatial Extent of Wetted Channels during the Summer Dry Season

“…By coupling our understanding of the subsurface hydrology of our sites (Dralle et. al, 2016; Druhan et al, ; Hahm et al, ; Kim et al, ; Link et al, ; Oshun et al, ; Rempe & Dietrich, ; Rempe et al, ; Salve et al, ) with wetted channel observations at the beginning and end of the summer dry season, our study demonstrates the connection between critical zone structure and the extent and duration of wetted channels.…”

Seasonal dynamics and exports of elements from a first‐order stream to a large inland lake in Michigan

“…Although other researchers have found that many of the analytes they measured displayed chemostatic behaviour and had flat C‐Q shapes with slopes below | b | < 0.2 (Basu et al, ; Godsey et al, ; Hunsaker & Johnson, ; Kim, Dietrich, Thurnhoffer, Bishop, & Fung, ; Thompson et al, ), we found that all positive and negative Honeysuckle Creek C‐Q relationships, with the exception of Mg 2+ at the upper flume, were strongly significant with slopes greater than | b | > 0.2. Our results did generally agree with the findings of these researchers that solute concentrations vary less than discharge.…”

Quantification of the seasonal hillslope water storage that does not drive streamflow

“…Given the distinctive roles of direct and indirect storages in the process of discharge generation, their physical arrangement and respective energetic statuses may have implications for hydraulic transport and hydrological model formulation. For example, at Elder Creek, Kim, Dietrich, Thurnhoffer, Bishop, and Fung () show that chemostasis in the stream can be partially explained by large volumes of tension‐held unsaturated moisture in soil and weathered rock, which is physically positioned above hillslope groundwater tables, buffering incoming rains and draining chemically reacted waters to the saturated zone during recharge events. In contrast, Dry Creek exhibits greater dilution of major cations at high flows, consistent with shorter residence times and lower fluid–solid interactions during high run‐off events dominated by shallow subsurface and saturation overland flow (Hahm, Druhan, Rempe, & Dietrich, ).…”