Dipankar Dwivedi

Identifying geochemical hot moments and their controls on a contaminated river floodplain system using wavelet and entropy approaches

Geochemical hot moments are defined here as short periods of time that are associated with disproportionally high levels of concentrations (biogeochemically-driven or transport-related) relative to longer intervening time periods. We used entropy and wavelet techniques to identify temporal variability in geochemical constituents and their controls along three transects within a contaminated floodplain system near Rifle CO. Results indicated that transport-dominated hot moments drove overall geochemical processing in the contaminated groundwater and seep zones. These hot moments were associated with seasonal hydrologic variability (~4 months) in the contaminated aquifer and with annual hydrologic cycle and residence times in the seep zone. Hot moments associated with a naturally reduced zone within the aquifer were found to be biogeochemically-driven, with a different dominant frequency (~3 months) and no correlation to hydrologic or weather variations, in contrast to what is observed in other regions of the floodplain. We interrogated complex, multivariate geochemical datasets to identify hot moments in a floodplain environment.We introduced a novel wavelet-entropy approach to classify hot moments and their distribution.Geochemical hot moments were found to be primarily transport-related/hydrologically-driven at the site.Within a naturally reduced zone, hot moments were dominated by lithologic characteristics (biogeochemically-driven).Water quality managers can use this wavelet-entropy tool for identifying contaminant hot moments in other floodplain settings.

Hot Spots and Hot Moments of Nitrogen in a Riparian Corridor

Author(s): Dwivedi, D; Arora, B; Steefel, CI; Dafflon, B; Versteeg, R | Abstract:

Evaluating temporal controls on greenhouse gas (GHG) fluxes in an Arctic tundra environment: An entropy-based approach

There is significant spatial and temporal variability associated with greenhouse gas (GHG) fluxes in high-latitude Arctic tundra environments. The objectives of this study are to investigate temporal variability in CO2 and CH4 fluxes at Barrow, AK and to determine the factors causing this variability using a novel entropy-based classification scheme. In particular, we analyzed which geomorphic, soil, vegetation and climatic properties most explained the variability in GHG fluxes (opaque chamber measurements) during the growing season over three successive years. Results indicate that multi-year variability in CO2 fluxes was primarily associated with soil temperature variability as well as vegetation dynamics during the early and late growing season. Temporal variability in CH4 fluxes was primarily associated with changes in vegetation during the growing season and its interactions with primary controls like seasonal thaw. Polygonal ground features, which are common to Arctic regions, also demonstrated significant multi-year variability in GHG fluxes. Our results can be used to prioritize field sampling strategies, with an emphasis on measurements collected at locations and times that explain the most variability in GHG fluxes. For example, we found that sampling primary environmental controls at the centers of high centered polygons in the month of September (when freeze-back period begins) can provide significant constraints on GHG flux variability - a requirement for accurately predicting future changes to GHG fluxes. Overall, entropy results document the impact of changing environmental conditions (e.g., warming, growing season length) on GHG fluxes, thus providing clues concerning the manner in which ecosystem properties may be shifted regionally in a future climate.