Flushing of distal hillslopes as an alternative source of stream dissolved organic carbon in a headwater catchment

Abstract

We investigated potential source areas of dissolved organic carbon (DOC) in headwater streams by examining DOC concentrations in lysimeter, shallow well, and stream water samples from a reference catchment at the Hubbard Brook Experimental Forest. These observations were then compared to high-frequency temporal variations in fluorescent dissolved organic matter (FDOM) at the catchment outlet and the predicted spatial extent of shallow groundwater in soils throughout the catchment. While near-stream soils are generally considered a DOC source in forested catchments, DOC concentrations in near-stream groundwater were low (mean 5 2.4 mg/L, standard error 5 0.6 mg/L), less than hillslope groundwater farther from the channel (mean 5 5.7 mg/L, standard error 5 0.4 mg/L). Furthermore, water tables in near-stream soils did not rise into the carbon-rich upper B or O horizons even during events. In contrast, soils below bedrock outcrops near channel heads where lateral soil formation processes dominate had much higher DOC concentrations. Soils immediately downslope of bedrock areas had thick eluvial horizons indicative of leaching of organic materials, Fe, and Al and had similarly high DOC concentrations in groundwater (mean 5 14.5 mg/L, standard error 5 0.8 mg/L). Flow from bedrock outcrops partially covered by organic soil horizons produced the highest groundwater DOC concentrations (mean 5 20.0 mg/L, standard error 5 4.6 mg/L) measured in the catchment. Correspondingly, stream water in channel heads sourced in part by shallow soils and bedrock outcrops had the highest stream DOC concentrations measured in the catchment. Variation in FDOM concentrations at the catchment outlet followed water table fluctuations in shallow to bedrock soils near channel heads. We show that shallow hillslope soils receiving runoff from organic matter-covered bedrock outcrops may be a major source of DOC in headwater catchments in forested mountainous regions where catchments have exposed or shallow bedrock near channel heads.

Lateral water flux in the unsaturated zone: A mechanism for the formation of spatial soil heterogeneity in a headwater catchment

“…Similarly, in the E–Bhs podzol pair, lateral unsaturated fluxes may be responsible for mobilizing AOCs from the O horizon (including areas upslope of the E podzol with bedrock overlain by thin organic horizons) in solution as dissolved organic carbon (DOC) and translocating it downslope to the Bhs podzol (Bourgault et al, ). This is consistent with the results from Gannon, Bailey, McGuire, and Shanley (), who found the highest DOC concentrations in groundwater and soil water in E and Bhs podzols. In the bimodal‐Bh and typical‐Bh podzol pairs, lateral unsaturated flow from water draining from upslope typical or bimodal podzols may translocate soluble or solid phase AOCs to Bh podzols.…”

The new Landsat 8 potential for remote sensing of colored dissolved organic matter (CDOM)

“…Advances in field-deployable optical sensor technology since then have led to the routine use of compact and relatively inexpensive fluorometers in situ in coastal environments (Chen, 1999;Coble et al, 1998;Yamashita et al, 2015); and more recently in freshwaters. For example, fluorescence sensors have been used as high resolution proxies to better understand the transport of DOM from watersheds (Gannon et al, 2015;Pellerin et al, 2012;Saraceno et al, 2009;Spencer et al, 2007;Wilson et al, 2013), sources and cycling of DOM in lakes and reservoirs (Downing et al, 2008;Watras et al, 2015), the formation of disinfection by-products following drinking water treatment (Carpenter et al, 2013), and the transport and cycling of other constituents including methylmercury (Bergamaschi et al, 2011;Bergamaschi et al, 2012;Fichot et al, 2015).…”

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

“…This inference is congruent with the saturated variable source area model (Andrews, Lin, Zhu, Jin, & Brantley, ; Boyer, Hornberger, Bencala, & McKnight, ; Inamdar et al, ; Lambert et al, ). The variable source area model holds that rising water tables, and more extensive soil saturation allow for flushing of soil C to the stream from distal wetland areas that are disconnected from the stream at lower flows (Brown, McDonnell, Burns, & Kendall, ; Diamond & Cohen, ; Gannon, Bailey, McGuire, & Shanley, ; Grabs, Bishop, Laudon, Lyon, & Seibert, ; Hornberger, Bencala, & McKnight, ; Inamdar et al, ; Lottig et al, ). Importantly, because TSS also showed a positive (flushing) C‐Q relationship, it is likely that suspended solids account for an additional flux of C during high flows and peak events when saturated areas are most extensive and high‐energy stream flows can transport particulate organic matter down watershed.…”