Core Ideas
Studying the critical zone requires targeted research on water, energy, gas, solutes, and sediments.
The SSHCZO targets a 165‐km2 watershed on sedimentary rocks in the northeastern United ...States.
One SSHCZO subcatchment, Shale Hills, provides extraordinary data describing a shale CZ.
The Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) was established to investigate the form, function, and dynamics of the critical zone developed on sedimentary rocks in the Appalachian Mountains in central Pennsylvania. When first established, the SSHCZO encompassed only the Shale Hills catchment, a 0.08‐km2 subcatchment within Shaver's Creek watershed. The SSHCZO has now grown to include 120 km2 of the Shaver's Creek watershed. With that growth, the science team designed a strategy to measure a parsimonious set of data to characterize the critical zone in such a large watershed. This parsimonious design includes three targeted subcatchments (including the original Shale Hills), observations along the main stem of Shaver's Creek, and broad topographic and geophysical observations. Here we describe the goals, the implementation of measurements, and the major findings of the SSHCZO by emphasizing measurements of the main stem of Shaver's Creek as well as the original Shale Hills subcatchment.
Among the contributors to soil CO2 efflux, there remains uncertainty about the contribution of root activity to the overall soil efflux. Soil water and temperature frequently have been used to ...predict a large portion of the variation in soil CO2 efflux. We hypothesized that fine-root dynamics explain most of the remaining variability in soil CO2 efflux that cannot be explained by soil temperature and water content. We anticipated that seasonal increases in root production, mortality via decomposition, and standing crop would result in corresponding increases in soil CO2 efflux. We tested our hypotheses by collecting and analyzing two years of minirhizotron and soil chamber CO2 flux data from plots distributed throughout the Shale Hills Catchment of the Susquehanna-Shale Hills Critical Zone Observatory in Central Pennsylvania, USA. Here we showed that: (1) seasonal fluctuations in fine-root dynamics yielded only a very small increase in the predictability of soil CO2 efflux; (2) fine-root mortality effects on soil CO2 efflux were strongly tied to soil temperature; (3) fluctuations in fine-root presence or standing mass independent of temperature and moisture had little effect on soil CO2 efflux; and (4) new fine-root length and root length mortality had limited impacts on soil CO2 efflux rates. We conclude that, at least in temperate forests on rocky soils, characterizing fine-root dynamics may provide only limited improvement in the estimation of soil CO2 efflux.
Core Ideas
Two new subcatchments are used to test the importance of lithology and land use.
Differences in lithology and land use result in differences in soils and waters.
Despite differences, all ...catchments have a shallow and a deep water table.
The relative importance of flow paths controls distinct chemistry response to discharge.
Cross‐site comparison will ultimately enable upscaling from the catchment to large scale.
The footprint of the Susquehanna Shale Hills Critical Zone Observatory was expanded in 2013 from the forested Shale Hills subcatchment (0.08 km2) to most of Shavers Creek watershed (163 km2) in an effort to understand the interactions among water, energy, gas, solute, and sediment. The main stem of Shavers Creek is now monitored, and instrumentation has been installed in two new subcatchments: Garner Run and Cole Farm. Garner Run is a pristine forested site underlain by sandstone, whereas Cole Farm is a cultivated site on calcareous shale. We describe preliminary data and insights about how the critical zone has evolved on sites of different lithology, vegetation, and land use. A notable conceptual model that has emerged is the “two water table” concept. Despite differences in critical zone architecture, we found evidence in each catchment of a shallow and a deep water table, with the former defined by shallow interflow and the latter defined by deeper groundwater flow through weathered and fractured bedrock. We show that the shallow and deep waters have distinct chemical signatures. The proportion of contribution from each water type to stream discharge plays a key role in determining how concentrations, including nutrients, vary as a function of stream discharge. This illustrates the benefits of the critical zone observatory approach: having common sites to grapple with cross‐disciplinary research questions, to integrate diverse datasets, and to support model development that ultimately enables the development of powerful conceptual and numerical frameworks for large‐scale hindcasting and forecasting capabilities.
Understanding streamflow generation and its dependence on catchment characteristics requires large spatial data sets and is often limited by convoluted effects of multiple variables. Here we address ...this knowledge gap using data‐informed, physics‐based hydrologic modeling in two catchments with similar vegetation and climate but different lithology (Shale Hills SH, shale, 0.08 km2, and Garner Run GR, sandstone, 1.34 km2), which influences catchment topography and soil properties. The sandstone catchment, GR, is characterized by lower drainage density, extensive valley fill, and bouldery soils. We tested the hypothesis that the influence of topographic characteristics is more significant than that of soil properties and catchment size. Transferring calibration coefficients from the previously calibrated SH model to GR cannot reproduce monthly discharge until after incorporating measured boulder distribution at GR. Model calibration underscored the importance of soil properties (porosity, van Genuchten parameters, and boulder characteristics) in reproducing daily discharge. Virtual experiments were used to swap topography, soil properties, and catchment size one at a time to disentangle their influence. They showed that clayey SH soils led to high nonlinearity and threshold behavior. With the same soil and topography, changing from SH to GR size consistently increased dynamic water storage (Sd) from ~0.12 to ~0.17 m. All analyses accentuated the predominant control of soil properties, therefore rejecting the hypothesis. The results illustrate the use of physics‐based modeling for illuminating mechanisms and underscore the importance of subsurface characterization as we move toward hydrological prediction in ungauged basins.
Key Points
Soil and macropore properties predominantly control storage‐discharge relationships in shale and sandstone catchments
Dynamic water storage increases with catchment size because hillslope‐stream connectivity increases in larger catchments
Swap experiments can assess the impacts of topography and catchment size that a sensitivity analysis cannot
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.
Key Points
Cation exchange processes in the hyporheic zone influence concentration‐discharge (CQ) trends
The contribution of solutes to headwater streams from subsurface flow paths such as interflow and deep groundwater can vary with discharge
Differences in bedrock lithology (shale versus sandstone) lead to different concentration‐discharge trends in two headwater catchments
Although metal redox reactions in soils can strongly affect carbon mineralization and other important soil processes, little is known about temporal variations in this redox cycling. Recently, ...potentiostatically poised electrodes (fixed-potential electrodes) have shown promise for measuring the rate of oxidation and reduction at a specific reduction potential in situ in riparian soils. Here for the first time, we used these electrodes in unsaturated soils to explore the fine-scale temporal redox fluctuations of both iron and manganese in response to environmental conditions. We used three-electrode systems with working electrodes fixed at 100 mV (vs. SHE) and 400 mV at 50 cm and 70 cm in the valley floor soil of a headwater watershed. Electrodes fixed at 100 mV to mimic iron oxides and at 400 mV to mimic manganese oxides allowed real-time reduction and oxidation rates to be calculated from temporal variations in the electric current. Electrode measurements were compared to soil porewater chemistry, pCO
2
, pO
2
, groundwater level, resistivity measurements, and precipitation. The fixed-potential electrodes recorded fluctuations over timescales from minutes to weeks. A consistently negative current was observed at 100 mV (interpreted as oxidation of Fe), while the 400-mV electrode fluctuated between negative and positive currents (Mn oxidation and reduction). When the water table rose above the electrodes, reduction was promoted, but above the water table, rainfall only stimulated oxidation. Precipitation frequency thus drove the multi-day reduction or oxidation events (return interval of 5–10 days). These measurements represent the first direct detections of frequency, period, and amplitude of oxidation and reduction events in unsaturated soils. Fixed-potential electrodes hold promise for accurately exploring the fast-changing biogeochemical impacts of metal redox cycling in soils and represent a significant advance for reactions that have been difficult to quantify.
We used seismic refraction to image the P‐wave velocity structure of a shale watershed experiencing regional compression in the Valley and Ridge Province (USA). From estimates showing strong ...compressional stress, we expected the depth to unweathered bedrock to mirror the hill‐valley‐hill topography (“bowtie pattern”) by analogy to seismic velocity patterns in crystalline bedrock in the North American Piedmont that also experience compression. Previous researchers used failure potentials calculated for strong compression in the Piedmont to suggest fractures are open deeper under hills than valleys to explain the “bowtie” pattern. Seismic images of the shale watershed, however, show little evidence of such a “bowtie.” Instead, they are consistent with weak (not strong) compression. This contradiction could be explained by the greater importance of infiltration‐driven weathering than fracturing in determining seismic velocities in shale compared to crystalline bedrock, or to local perturbations of the regional stress field due to lithology or structures.
Plain Language Summary
Rock mechanic theory suggests that the depth to crystalline bedrock under hill‐valley‐hill landscapes mirrors the land surface when the landscape experiences strong compression. We tested for this in a region of compression for a watershed on shale and found the depth pattern was consistent only with weak compression. This observation may be because infiltration and chemical weathering are more important than mechanical fracturing in controlling density of near‐surface shale. Alternatively, local effects related to the last glacial advance or the differences in rock types might explain the observation. The depth of weathering (depth to bedrock) is apparently not only controlled by fracturing but rather is heavily influenced by hydrogeochemical processes on shale.
Key Points
The P‐wave velocity structure of a shale watershed under compression is imaged
Seismic images show little evidence of the expected bowtie structure
Results are explained by greater importance of chemical weathering than fracturing in determining seismic velocities in shale landscapes
We used seismic refraction to image the P-wave velocity structure of a shale watershed experiencing regional compression in the Valley and Ridge Province (USA). From estimates showing strong ...compressional stress, we expected the depth to unweathered bedrock to mirror the hill-valley-hill topography (“bowtie pattern”) by analogy to seismic velocity patterns in crystalline bedrock in the North American Piedmont that also experience compression. Previous researchers used failure potentials calculated for strong compression in the Piedmont to suggest fractures are open deeper under hills than valleys to explain the “bowtie” pattern. Seismic images of the shale watershed, however, show little evidence of such a “bowtie.” Instead, they are consistent with weak (not strong) compression. Here, this contradiction could be explained by the greater importance of infiltration-driven weathering than fracturing in determining seismic velocities in shale compared to crystalline bedrock, or to local perturbations of the regional stress field due to lithology or structures.
Oxidative weathering of pyrite plays an important role in the biogeochemical cycling of Fe and S in terrestrial environments. While the mechanism and occurrence of biologically accelerated pyrite ...oxidation under acidic conditions are well established, much less is known about microbially mediated pyrite oxidation at circumneutral pH. Recent work (Percak‐Dennett et al., 2017, Geobiology, 15, 690) has demonstrated the ability of aerobic chemolithotrophic microorganisms to accelerate pyrite oxidation at circumneutral pH and proposed two mechanistic models by which this phenomenon might occur. Here, we assess the potential relevance of aerobic microbially catalyzed circumneutral pH pyrite oxidation in relation to subsurface shale weathering at Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in Pennsylvania, USA. Specimen pyrite mixed with native shale was incubated in groundwater for 3 months at the inferred depth of in situ pyrite oxidation. The colonized materials were used as an inoculum for pyrite‐oxidizing enrichment cultures. Microbial activity accelerated the release of sulfate across all conditions. 16S rRNA gene sequencing and metagenomic analysis revealed the dominance of a putative chemolithoautotrophic sulfur‐oxidizing bacterium from the genus Thiobacillus in the enrichment cultures. Previously proposed models for aerobic microbial pyrite oxidation were assessed in terms of physical constraints, enrichment culture geochemistry, and metagenomic analysis. Although we conclude that subsurface pyrite oxidation at SSCHZO is largely abiotic, this work nonetheless yields new insight into the potential pathways by which aerobic microorganisms may accelerate pyrite oxidation at circumneutral pH. We propose a new “direct sulfur oxidation” pathway, whereby sulfhydryl‐bearing outer membrane proteins mediate oxidation of pyrite surfaces through a persulfide intermediate, analogous to previously proposed mechanisms for direct microbial oxidation of elemental sulfur. The action of this and other direct microbial pyrite oxidation pathways have major implications for controls on pyrite weathering rates in circumneutral pH sedimentary environments where pore throat sizes permit widespread access of microorganisms to pyrite surfaces.
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