Abstract
Stream water pH and composition are widely used to monitor ongoing recovery from the deposition of strong anthropogenic acids in many forested headwater catchments in the northeastern US. However, stream water composition is a function of highly complex and coupled processes, flowpaths, and variations in soil and bedrock composition. Spatial heterogeneity is especially pronounced in headwater catchments with steep topography, potentially limiting stream water composition as an indicator of changes in critical zone (CZ) dynamics during system recovery. To investigate the link between catchment characteristics, landscape position, and stream water composition we used long-term data (1991-2015) from the Sleepers River Research Watershed (SRRW) in northeastern Vermont. We investigated trends with time in stream water and trends with time, depth, and landscape position (upslope, midslope, and riparian zone) in groundwater (GW) and soil solution. We further determined soil elemental composition and mineralogy on archived (1996) and modern (2017) soil samples to assess changes in composition with time. SRRW is inherently well-buffered by calcite in bedrock and till, but soils had become acidified and are now recovering from acidification. Although base cations, especially Ca, decrease progressively with time in GW, riparian soils have become more enriched in Ca, due to a mixture of lateral and vertical transfers. At the same time stream water Ca fluxes increased over the past two decades, likely due to the leaching of (transient) legacy Ca from riparian zones and increased water fluxes. The stream water response therefore reflects the dynamic changes in soil chemistry, flow routing and water inputs.
Highlights
Streams indicate critical zone (CZ) function, because they represent an integrated signal of watershed processes (Frisbee et al, 2011) including internal stream processes (Mulholland and Hill, 1997; Dawson et al, 2001) and processes occurring at the interfaces (Bishop et al, 2004; Winterdahl et al, 2011)
Annual stream water sulfate flux decreased more than 30% since 1991 and Mann Kendall trend analysis on flow adjusted model residuals indicated a significant negative trend in sulfate concentration (τ = −0.442, Sens Slope = −0.007, p < 0.05)
Using the acid-impacted Sleepers River Research Watershed (SRRW) as a testbed, we investigated the potential and limitations of stream water composition as an indicator of watershed recovery from acidification and explored whether changes in soil composition contributed to a recovery signal
Summary
Streams indicate critical zone (CZ) function, because they represent an integrated signal of watershed processes (Frisbee et al, 2011) including internal stream processes (Mulholland and Hill, 1997; Dawson et al, 2001) and processes occurring at the interfaces (i.e., riparian or hyporheic zones) (Bishop et al, 2004; Winterdahl et al, 2011). Acid deposition forms when emission-derived sulfur dioxide and nitrogen oxides interact with precipitation, which impacts the entire CZ through a multitude of complex and coupled processes, including declining tree and soil quality due to low pH and base cation leaching (Matzner and Murach, 1995; Driscoll et al, 2001; Raddum et al, 2007). As such, weathering and soil development continuously change the composition of the solid CZ and the composition of percolating waters [e.g., soil solution and groundwater (GW)] Another important process is the leaching of base cations from acid-impacted soils that lead to declines in soil health. These changes can be clear in headwater catchments with pronounced topography, even within just a few decades of changes in precipitation chemistry due to the transfer of soluble materials to deeper soil layers, GW (vertical transfer), and/or via lateral transfer to low-lying landscape positions (Johnson et al, 2000; Nezat et al, 2004; Brantley et al, 2007; Bailey et al, 2014; Lawrence et al, 2015; Lybrand and Rasmussen, 2015)
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