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USGS2025

Characterization of DOC in riparian wetland pore water and its interference in stable water isotope analysis of δ2H and δ18O

Wetlands play an important role in carbon retention, which is greatly dependent on hydrological conditions. Therefore, the interest in wetland hydrology has increased over recent decades. In wetland hydrology, stable water isotopes ( δ 2 H, δ 18 O) have found increasing use, as laser-based instruments have become readily available and allow for faster analysis and lower costs. However, the use of laser-based determinations of stable hydrogen and oxygen isotopes in wetland pore waters might be challenging because of dissolved organic carbon (DOC), especially as wetland pore waters often contain elevated DOC concentrations. To study the potential interference of DOC on laser-based stable water isotopes determination, we determined concentration, fluorescence and absorbance characteristics of DOC in pore waters collected from six restored and six near-natural riparian wetlands. The water samples’ δ 2 H and δ 18 O values were determined by two laser absorption spectrometers (LAS) and, for reference, in parallel by dual-inlet isotope-ratio mass spectrometry (DI-IRMS). The two LAS methods showed significant deviations from the DI-IRMS-determinations. Variations in the specific UV absorbance index (SUVA 254 , the quotient of the absorbance at 254 nm and the DOC concentration), which indicates the aromaticity of the DOC, partially accounted for interference for δ 2 H measurements. Elevated SUVA 254 index values were especially linked to restored wetlands. Studies that investigate pore waters with high DOC aromaticity but where the DOC is not characterized, and which require a high accuracy of the δ 2 H-values for interpretation of the hydrological system, may be severely affected by DOC interference.

Journal of Hydrology↗

USGS2006

Tracer test with As(V) under variable redox conditions controlling arsenic transport in the presence of elevated ferrous iron concentrations

To study transport and reactions of arsenic under field conditions, a small-scale tracer test was performed in an anoxic, iron-reducing zone of a sandy aquifer at the USGS research site on Cape Cod, Massachusetts, USA. For four weeks, a stream of groundwater with added As(V) (6.7 μM) and bromide (1.6 mM), was injected in order to observe the reduction of As(V) to As(III). Breakthrough of bromide (Br − ), As(V), and As(III) as well as additional parameters characterizing the geochemical conditions was observed at various locations downstream of the injection well over a period of 104 days. After a short lag period, nitrate and dissolved oxygen from the injectate oxidized ferrous iron and As(V) became bound to the freshly formed hydrous iron oxides. Approximately one week after terminating the injection, anoxic conditions had been reestablished and increases in As(III) concentrations were observed within 1 m of the injection. During the observation period, As(III) and As(V) were transported to a distance of 4.5 m downgradient indicating significant retardation by sorption processes for both species. Sediment assays as well as elevated concentrations of hydrogen reflected the presence of As(V) reducing microorganisms. Thus, microbial As(V) reduction was thought to be one major process driving the release of As(III) during the tracer test in the Cape Cod aquifer.

Journal of Contaminant Hydrology↗