Antecedent moisture conditions control mercury and dissolved organic carbon concentration dynamics in a boreal headwater catchment

Abstract

The fate and transport of mercury (Hg) deposited on forested upland soils depends on the biogeochemical and hydrological processes occurring in the soil landscape. In this study, total Hg (THg) and dissolved organic carbon (DOC) concentrations were measured in streamwater from a 7.75 ha upland subcatchment of the METAALICUS watershed in northwestern Ontario, Canada. THg and DOC concentration-discharge relationships were examined at the seasonal-scale and event-scale to assess the role of antecedent moisture conditions on the mobilization of these solutes to receiving waters. At the seasonal-scale, subcatchment discharge poorly explained THg and DOC concentration dynamics; however, the inclusion of antecedent water storage and precipitation metrics in a multiple regression model improved the prediction of THg and DOC concentrations significantly. At the event-scale, a comparison of THg and DOC concentrations for two small summer storms with similar total discharge showed that the storm following the wet snowmelt period had a significantly lower total flux of THg and DOC than the storm following warm and dry conditions in late summer due to a distinct shift in the concentration-discharge relationship. Measurements of soil water and groundwater THg and DOC concentrations, as well as a three-component mixing analysis, suggest that there was an accumulation of potentially-mobile DOC-bound THg in the wellhumified organic soil layer in the catchment during the warm and dry summer period and that as the catchment became wetter in the autumn, there was an increase in soil water THg and DOC concentrations and these solutes were subsequently flushed during the autumn storm.

Association of dissolved mercury with dissolved organic carbon in U.S. rivers and streams: The role of watershed soil organic carbon

“…The observed behavior of the stream water Hg D :DOC ratio scaling with SOC content can be further explored by considering soil‐level processes. For individual watersheds, models might account for spatial heterogeneity (both vertical and horizontal) in soil Hg and carbon pools [ Demers et al ., ; Burns et al ., ] and/or depict transport along distinct flow pathways under variable hydrological conditions [ Demers et al ., ; Oswald and Branfireun , ; Haynes and Mitchell , ]. Modeling the coupled transport of Hg D and DOC across a diverse array of watersheds necessarily requires simplifications, with a focus on first‐order processes and bulk properties of the watersheds.…”

“…We note that the relationship derived between Hg D :DOC and SOC based on our meta‐analysis may gloss over some potentially important factors that influence Hg D :DOC at individual watersheds. Among other factors, disturbance history [ Amirbahman et al ., ], vertical variability of Hg:C within soil horizons [ Demers et al ., ], variable source area [ Demers et al ., ; Oswald and Branfireun , ], the spatial distribution of SOC within watersheds, and soil texture may play roles in governing stream Hg D :DOC ratios. Also, the relationship derived here is based on relatively undisturbed sites and may not apply in more disturbed settings, such as urban/suburban or agricultural land uses.…”

“…In watersheds without wetlands, the more complex flow paths involved in runoff generation lead to greater variability in stream water Hg D and DOC. However, statistically significant relationships between Hg D and DOC in these watersheds are nonetheless observed [ Riscassi and Scanlon , ; Schelker et al ., ; Oswald and Branfireun , ].…”

Precipitation input and antecedent soil moisture effects on mercury mobility in soil—laboratory experiments with an enriched stable isotope tracer

“…However, few studies consider the potential influence of antecedent soil moisture conditions in controlling Hg fluxes through terrestrial environments. A recent study by Oswald and Branfireun () illustrated that antecedent soil moisture conditions account for some of the differences observed in the Hg concentration–discharge relationships that occur following the wet snowmelt period as compared to the dry late summer period. Higher THg and soil water dissolved organic carbon (DOC) concentrations were observed following warmer, drier periods in the boreal Shield watershed (Oswald and Branfireun ).…”

“…A recent study by Oswald and Branfireun () illustrated that antecedent soil moisture conditions account for some of the differences observed in the Hg concentration–discharge relationships that occur following the wet snowmelt period as compared to the dry late summer period. Higher THg and soil water dissolved organic carbon (DOC) concentrations were observed following warmer, drier periods in the boreal Shield watershed (Oswald and Branfireun ). These studies suggest that a systematic and controlled testing of the relative controls of antecedent soil moisture and the magnitude of precipitation inputs on Hg mobility is required.…”