Water Table Dynamics and Biogeochemical Cycling in a Shallow, Variably-Saturated Floodplain.

Three-dimensional variably saturated flow and multicomponent biogeochemical reactive transport modeling, based on published and newly generated data, is used to better understand the interplay of hydrology, geochemistry, and biology controlling the cycling of carbon, nitrogen, oxygen, iron, sulfur, and uranium in a shallow floodplain. In this system, aerobic respiration generally maintains anoxic groundwater below an oxic vadose zone until seasonal snowmelt-driven water table peaking transports dissolved oxygen (DO) and nitrate from the vadose zone into the alluvial aquifer. The response to this perturbation is localized due to distinct physico-biogeochemical environments and relatively long time scales for transport through the floodplain aquifer and vadose zone. Naturally reduced zones (NRZs) containing sediments higher in organic matter, iron sulfides, and non-crystalline U(IV) rapidly consume DO and nitrate to maintain anoxic conditions, yielding Fe(II) from FeS oxidative dissolution, nitrite from denitrification, and U(VI) from nitrite-promoted U(IV) oxidation. Redox cycling is a key factor for sustaining the observed aquifer behaviors despite continuous oxygen influx and the annual hydrologically induced oxidation event. Depth-dependent activity of fermenters, aerobes, nitrate reducers, sulfate reducers, and chemolithoautotrophs (e.g., oxidizing Fe(II), S compounds, and ammonium) is linked to the presence of DO, which has higher concentrations near the water table.

Rethinking pump-and-treat remediation as maximizing contaminated groundwater.

For over half a century, the United States has developed water quality regulations (e.g., Safe Drinking Water Act), which has been accompanied by innumerable advances in contaminant transport and fate, site characterization, and remediation. Since the 1980s, "pump-and-treat" techniques have been the most widely used methods for groundwater contamination remediation. By 1982, pump-and-treat was included in 100 % of the U.S. Superfund groundwater remedy decisions, but applications decreased continuously after 1992. This was likely associated with the documented limitations of pump-and-treat for achieving complete remediation with site closure. Several factors can limit the effectiveness of pump-and-treat, a primary one being that contaminant mass residing in NAPL, sorbed, and low-permeability matrices is not removed in an effective or efficient manner. This ineffectiveness leads to extended cleanup times and the generation of enormous volumes of extracted groundwater, in effect creating conditions of maximizing the amount of contaminated groundwater needing treatment. We highlight a means by which to reassess our approach to remediation by recognizing that pump-and-treat, due to its well-documented limitations, often maximizes the generation of contaminated groundwater.

Impacts of Methane on Carbon Dioxide Storage in Brine Formations.

In the context of geological carbon sequestration (GCS), carbon dioxide (CO2 ) is often injected into deep formations saturated with a brine that may contain dissolved light hydrocarbons, such as methane (CH4 ). In this multicomponent multiphase displacement process, CO2 competes with CH4 in terms of dissolution, and CH4 tends to exsolve from the aqueous into a gaseous phase. Because CH4 has a lower viscosity than injected CO2 , CH4 is swept up into a 'bank' of CH4 -rich gas ahead of the CO2 displacement front. On the one hand, this may provide a useful tracer signal of an approaching CO2 front. On the other hand, the emergence of gaseous CH4 is undesirable because it poses a leakage risk of a far more potent greenhouse gas than CO2 if the cap rock is compromised. Open fractures or faults and wells could result in CH4 contamination of overlying groundwater aquifers as well as surface emissions. We investigate this process through detailed numerical simulations for a large-scale GCS pilot project (near Cranfield, Mississippi) for which a rich set of field data is available. An accurate cubic-plus-association equation-of-state is used to describe the non-linear phase behavior of multiphase brine-CH4 -CO2 mixtures, and breakthrough curves in two observation wells are used to constrain transport processes. Both field data and simulations indeed show the development of an extensive plume of CH4 -rich (up to 90 mol%) gas as a consequence of CO2 injection, with important implications for the risk assessment of future GCS projects.