Susquehanna Shale Hills Critical Zone Observatory Stream water and Porewater Chemistry (2006-2010)

An intense field study was conducted at the Susquehanna/Shale Hills Critical Zone Observatory to use water chemistry to probe the subsurface hydrogeochemical conditions and water flow dynamics. Soil pore water sampled through tension lysimeters suggest that contemporary reaction rates depend on preferential flowpaths and solute residence times, and are limited by dissolution kinetics. Thus weathering processes are strongly coupled with hydrological features of shale rocks and soils. The chemistry of ground- and streamwater is being analyzed in relation to that of the soil porewaters to elucidate the mineral reactions occurring below the soils and to derive elemental fluxes over the whole catchment. [Link to datasets.]

 

 

 

Mg2+, Mg*, and Mg* (Cl-normalized) concentrations in the pore water as a function of depth at the SPRT (A, D, G), SPMS (B, E, H) and SPVF (C, F, I) sites. Shaded areas highlighted the variations of Mg concentrations in soil waters from different depths. The Mg2+ concentration is that measured in the soil water; Mg* is Mg concentration in soil water after correcting for atmospheric input; and Mg* (Cl-normalized) is Mg concentration corrected for atmospheric input and evapotranspiration. Notice changes in scale between Mg2+/Mg* and Mg* (Cl-normalized).

 

Soil water solutes are predominantly contributed by shale weathering, which is limited by clay dissolution kinetics. As such, solute concentrations are primarily controlled by the residence time of water in soil. Translocation and deposition of secondary clays created B-horizons that are more conductive to lateral flow right above or below it than vertical flow. Consistent with soil moisture monitoring data, water flows out of the steep-sloping hillslope predominantly laterally through preferred pathways that include soil horizon and soil-bedrock interfaces, and vertically via macropores. As a consequence, much higher concentrations of major cations are observed in porefluids within the B-horizon because water flows more slowly and mineral-water contact time is much longer. The amplitude of seasonal variations in O and H isotopes decreases in the order: precipitation>> soil water > stream > groundwater, suggesting water becomes progressively older as rainfall infiltrates the soil and eventually recharges to groundwater. Interestingly, Mg concentrations increase with the residence time of the water due to increasing contact time with minerals (clay and ankerite). Stream water is a mixture of groundwater and shallow soil water where the relative proportions change seasonally. The discharge of the first-order stream responds to precipitation closely, documenting the rapid release of both old groundwater and relatively young soil water during storms.

 

 

Datasets

  • Susquehanna Shale Hills Critical Zone Observatory Porewater Chemistry (2006, 2007, 2008, 2009, 2010)
  • Susquehanna Shale Hills Critical Zone Observatory Stream water Chemistry (2006, 2007, 2008, 2009, 2010)

 

Contributors 

Lead author
Brantley, Susan L.

 

Contributing authors
Andrews, Danielle; Bazilevskaya, Ekaterina; Bhatt, Maya; Duffy, Chris J.; Herndon, Elizabeth; Holleran, Molly; Holmes, George; Ketchum, Blake; Jin, Lixin; Kaiser, Nick; Nuester, Jochen; Ramesh, Ravela; Sullivan, Pamela L.; Sullivan, Tim; Thomas, Evan; Williams, Jennifer Z.; Yesavage, Tiffany

 

Related Publications

  • Jin, L., Andrews, D.M., Holmes, G.H., Duffy, C.J., Lin, H and Brantley, S.L. (2011) Opening the "Black Box": Water chemistry reveals hydrological controls on weathering in Susquehanna/Shale Hills Critical Zone Observatory (Central Pennsylvania, USA), Vadose Zone Journal 10:928-942, doi:10.2136/vzj2010.0133
  • Andrews, D.M., H. Lin, Q. Zhu, L. Jin, S.L. Brantley. Hot Spots and Hot Moments of Dissolved Organic Carbon Export and Soil Organic Carbon Storage in the Shale Hills Critical Zone Observatory. Vadose Zone Journal 10:943-954, doi:10.2136/vzj2010.0149.