Hyporheic zone influences on concentration‐discharge relationships in a headwater sandstone stream

Abstract

Complex subsurface flow dynamics impact the storage, routing, and transport of water and solutes to streams in headwater catchments. Many of these hydrogeologic processes are indirectly reflected in observations of stream chemistry responses to rain events, also known as concentration-discharge (CQ) relations. Identifying the relative importance of subsurface flows to stream CQ relationships is often challenging in headwater environments due to spatial and temporal variability. Therefore, this study combines a diverse set of methods, including tracer injection tests, cation exchange experiments, geochemical analyses, and numerical modeling, to map groundwater-surface water interactions along a first-order, sandstone stream (Garner Run) in the Appalachian Mountains of central Pennsylvania. The primary flow paths to the stream include preferential flow through the unsaturated zone (''interflow''), flow discharging from a spring, and groundwater discharge. Garner Run stream inherits geochemical signatures from geochemical reactions occurring along each of these flow paths. In addition to end-member mixing effects on CQ, we find that the exchange of solutes, nutrients, and water between the hyporheic zone and the main stream channel is a relevant control on the chemistry of Garner Run. CQ relationships for Garner Run were compared to prior results from a nearby headwater catchment overlying shale bedrock (Shale Hills). At the sandstone site, solutes associated with organo-mineral associations in the hyporheic zone influence CQ, while CQ trends in the shale catchment are affected by preferential flow through hillslope swales. The difference in CQ trends document how the lithology and catchment hydrology control CQ relationships. Plain Language Summary Stream chemistry serves as a fingerprint for the processes that occur in the critical zone, which extends from unweathered bedrock to the top of the tree canopy. The critical zone thus includes all resources critical to life. This paper evaluates chemical and physical processes in the critical zone, specifically in soils, streams, and groundwater. Our work suggests the important influence of groundwater-surface water interactions on mountain streams with an emphasis on the transport of water and solutes through the streambed. By comparing these transport processes on sandstone and shale, we hypothesize that the type of rock underlying a watershed can influence the relative importance of groundwater-surface water interactions on stream chemistry.

Soil CO2 and O2 Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations

“…Our data do not indicate significant CO 2 dissolution in rainwater that others have reported (Ma et al, 2013; Sánchez‐Cañete et al, 2018). Although CO 2 is more soluble than O 2 in water, at a pH between 4 and 5 (Hoagland et al, 2017; Li et al, 2018) CO 2 dissolution is limited, and storage of CO 2 in dissolved species would not be favored in the soils of Garner Run (Angert et al, 2015). Instead, we observe that soil O 2 was more responsive than CO 2 to a large rainfall event in June 2017 (Fig.…”

“…Garner Run is a 134‐ha first‐order catchment underlain by the Silurian Tuscarora Sandstone Formation. The watershed consists of undissected planar slopes that are 10 times longer and less steep than those in Shale Hills (Brantley et al, 2016; Hoagland et al, 2017; Li et al, 2018). Garner Run is the perennial stream that drains the watershed.…”

“…Therefore, the Garner Run sample locations are named Leading Ridge Valley Floor (LRVF), Leading Ridge Midslope (LRMS), Leading Ridge Ridgetop (LRRT), and Tussey Mountain Midslope (TMMS). The LRVF soils are Fine‐loamy, mixed, active, mesic Typic Fragiaquults of the Andover Series; the LRMS and TMMS soils are Loamy‐skeletal, siliceous, active, mesic Typic Dystrudepts of the Hazleton series; and LRRT are Loamy‐skeletal, siliceous, active, mesic Typic Dystrudepts of the Dekalb series (Hoagland et al, 2017).…”

Distinct Source Water Chemistry Shapes Contrasting Concentration‐Discharge Patterns

“…Flushing behaviors have been attributed to rising water tables tapping shallow soils with enriched organic carbon and nutrients, increased hydrological connectivity between uplands and streams under high flow conditions, and control of source element distribution and varying temperature (Herndon et al, ; Pacific et al, ; Seibert et al, ; Winterdahl et al, ). Dilution behaviors have been attributed to high groundwater concentration that is diluted by rainwater at high discharge (Li, Bao, et al, ; Miller et al, ), short residence time for dissolution, and rapid depletion of solute stores (Herndon et al, ; Hoagland et al, ). Musolff et al () attribute distinct C‐Q patterns to spatial structure heterogeneity of source waters and travel times for different extent of transformation reactions in a watershed.…”

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

“…In contrast to the studies of larger watersheds where chemostatic C‐Q trends dominated for many analytes, studies that focused on smaller headwater catchments (80–162 ha) observed significant positive or negative C‐Q relationships for most analytes measured. Herndon et al (), Hoagland et al (), and Hunsaker and Johnson () did not necessarily observe the same C‐Q shape for all analytes, but they did observe significant C‐Q relationships for Mn, Ca 2+ , K + , Cl − , Fe, Al, DOC, and NO 3 − . Na + , Mg 2+ , SO 4 2− , and Si relationships that varied depending on study location.…”