Concentrations and fluxes of dissolved organic carbon in runoff from a forested catchment: insights from high frequency measurements

Abstract

Concentrations of dissolved organic carbon (DOC) in runoff from catchments are often subject to substantial short term variations. The aim of this study was to identify the spatial sources of DOC and the causes for short term variations in runoff from a forested catchment. Furthermore, we investigated the implication of short term variations for the calculation of annual runoff fluxes. High frequency measurements (30 min intervals) of DOC in runoff, of discharge and groundwater table were conducted for one year in the 4.2 km<sup>2</sup> forested Lehstenbach catchment, Germany. Riparian wetland soils represent about 30% of the catchment area. The quality of DOC was investigated by three dimensional fluorescence excitation-emission matrices in samples taken from runoff, deep groundwater and shallow groundwater from the riparian wetland soils. The concentrations of DOC in runoff were highly variable at an hourly to daily time scale, ranging from 2.6 mg l<sup>−1</sup> to 34 mg l<sup>−1</sup> with an annual average of 9.2 mg l<sup>−1</sup>. The concentrations were positively related to discharge, with a pronounced, counter clockwise hysteresis. Relations of DOC to discharge were steeper in the summer/fall than in the winter/spring season. Dynamics of groundwater table, discharge, DOC concentrations and DOC quality parameters indicated that DOC in runoff originated mainly from the riparian wetland soils, both under low and high flow conditions. The annual export of DOC from the catchment was 84 kg C ha<sup>−1</sup> yr<sup>−1</sup> when calculated from the high frequency measurements. If the annual export was calculated by simulated random fortnightly samplings, the range was 47 to 124 kg C ha<sup>−1</sup> yr<sup>−1</sup>. Calculations of DOC export fluxes might result in significant errors when based on infrequent (e.g. fortnightly) sampling intervals. Future changes in the precipitation and discharge patterns will influence the DOC dynamics in this catchment, with largest effects in the summer season

Colloid-associated export of arsenic in stream water during stormflow events

“…The high DOC concentrations during stormflow are related to near-surface runoff in the organic-rich upper soil layers (Lischeid et al, 2007;Strohmeier et al, 2012). In addition, some of the DOC may also be released from the deeper, anoxic layers due to reductive dissolution of iron (oxy)hydroxides and the release of associated DOC (Zak and Gelbrecht, 2007;Knorr, 2012), and due to desorption of DOC from soil minerals due to the pH increase during reducing conditions (Grybos et al, 2009).…”

“…Furthermore, previous investigations have shown that NOM mobilization from riparian wetland soils into stream water is affected by season and by hydrological conditions (Strohmeier et al, 2012;Jirsa et al, 2013). Temporal variations in NOM export are a direct consequence of the composition of the hydraulically active layers in the soil profile and their respective discharge patterns (Knorr, 2012;Strohmeier et al, 2012). It has been suggested that future changes in the hydrological regime will influence the dynamics of NOM export (Strohmeier et al, 2012), which might in turn influence the dynamics of iron and arsenic export.…”

“…It has thus been suggested that the correlation between arsenic and NOM is a result of mobilization of arsenic-NOM colloids (Rothwell et al, 2009), but detailed studies are lacking. Furthermore, previous investigations have shown that NOM mobilization from riparian wetland soils into stream water is affected by season and by hydrological conditions (Strohmeier et al, 2012;Jirsa et al, 2013). Temporal variations in NOM export are a direct consequence of the composition of the hydraulically active layers in the soil profile and their respective discharge patterns (Knorr, 2012;Strohmeier et al, 2012).…”

“…Temporal variations in NOM export are a direct consequence of the composition of the hydraulically active layers in the soil profile and their respective discharge patterns (Knorr, 2012;Strohmeier et al, 2012). It has been suggested that future changes in the hydrological regime will influence the dynamics of NOM export (Strohmeier et al, 2012), which might in turn influence the dynamics of iron and arsenic export.…”

Exposure times rather than residence times control redox transformation efficiencies in riparian wetlands

“…high concentrations of dissolved oxygen or nitrate in deep and very old groundwater (e.g. Strohmeier et al, 2013 or Bartsch et al, 2014) clearly proof, that transformation efficiencies for nitrate can't be very high.…”