Despite its status of master variable, there have been relatively few attempts to quantitatively predict the distributions of pH in biogeochemical reactive transport systems. Here, we propose a theoretical approach for calculating the vertical pore water profiles of pH and the rates of proton production and consumption in aquatic sediments. In this approach, the stoichiometric coefficients of species that participate in acid-base equilibrium reactions are treated as unknown variables in the biogeochemical reaction network. The mixed kinetic-equilibrium reaction system results in a set of coupled differential and algebraic equations and is solved using a new numerical solver. The diagnostic capabilities of the model are illustrated for depositional conditions representative of those encountered on the continental shelf. The early diagenetic reaction network includes the major microbial degradation pathways of organic matter and associated secondary redox reactions, mineral precipitation and dissolution processes, and homogeneous acid-base reactions. The resulting pH profile in this baseline simulation exhibits a sharp decrease below the sedimentwater interface, followed by an increase with depth and again a decrease. The features of the pH profile are explained in terms of the production and consumption of protons by the various biogeochemical processes. Secondary oxygenation reactions are the principal proton producers within the oxic zone, while reduction of iron and manganese oxyhydroxides are primarily responsible for the reversal in the pH gradient in the suboxic zone. Proton production in the zone of sulfate reduction outweighs alkalinity production, maintaining the undersaturation of the pore waters with respect to calcite. Integrated over the entire depth of early diagenesis, dissolution of CaCO 3 is the main sink for protons. Variations in the reaction rate order and rate constant for CaCO 3 dissolution do not fundamentally alter the shape of the pH profile. An entirely different shape is obtained, however, when the pore waters are assumed to remain in thermodynamic equilibrium with calcite at all depths. Pore water (bio)irrigation decreases the amplitude of pH changes in the sediment and may modify the shape of the pH profile.

Solid phase and pore water chemical data collected in a sediment of the Haringvliet Lake are interpreted using a multi-component reactive transport model. This freshwater lake, which was formed as the result of a river impoundment along the southwestern coast of the Netherlands, is currently targeted for restoration of estuarine conditions. The model is used to assess the present-day biogeochemical dynamics in the sediment, and to forecast possible changes in organic carbon mineralization pathways and associated redox reactions upon salinization of the bottom waters. Model results indicate that oxic degradation (55%), denitrification (21%), and sulfate reduction (17%) are currently the main organic carbon degradation pathways in the upper 30 cm of sediment. Unlike in many other freshwater sediments, methanogenesis is a relatively minor carbon mineralization pathway (5%), because of significant supply of soluble electron acceptors from the well-mixed bottom waters. Although ascorbate-reducible Fe(III) mineral phases are present throughout the upper 30 cm of sediment, the contribution of dissimilatory iron reduction to overall sediment metabolism is negligible. Sensitivity analyses show that bioirrigation and bioturbation are important processes controlling the distribution of organic carbon degradation over the different pathways. Model simulations indicate that sulfate reduction would rapidly suppress methanogenesis upon seawater intrusion in the Haringvliet, and could lead to significant changes in the sediment's solid-state iron speciation. The changes in Fe speciation would take place on time-scales of 20-100 years.

[1] A Knowledge-Based Reactive Transport Model (KB-RTM) for simulation of coupled transport and biogeochemical transformations in surface and subsurface flow environments is presented (http:// www.geo.uu.nl/$kbrtm). The scalable Web-distributed Knowledge Base (KB), which combines Information Technology (IT), an automatic code generator, and database management, facilitates the automated construction of complex reaction networks from comprehensive information stored at the level of biogeochemical processes. The reaction-centric approach of the KB-RTM system offers full flexibility in the choice of model components and biogeochemical reactions. The procedure coupling the reaction networks to a generalized transport module into RTMs is also presented. The workings of our KB-RTM simulation environment are illustrated by means of two examples of redox and acid-base chemistry in a typical shelf sediment and an aquifer contaminated by landfill plumes.

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Vapor intrusion (VI) involves migration of volatile contaminants from subsurface through unsaturated soil into overlying buildings. In 2015, the US EPA recommended an approach for screening VI risks associated with gasoline releases from underground storage tank (UST) sites. Additional assessment of the VI risk from petroleum hydrocarbons was deemed unnecessary for buildings separated from vapor sources by more than recommended vertical screening distances. However, these vertical screening distances did not apply to potential VI risks associated with releases of former leaded gasoline containing 1,2‐dichloroethane (1,2‐DCA), because of a lack of empirical data on the attenuation of 1,2‐DCA in soil gas. This study empirically evaluated 144 paired measurements of 1,2‐DCA concentrations in soil gas and groundwater collected at 47 petroleum UST sites combined with BioVapor modeling. This included (1) assessing the frequency of 1,2‐DCA detections in soil gas below 10−6 risk‐based screening levels at different vertical separation distances and (2) comparing the US EPA recommended vertical screening distances with those predicted by BioVapor modeling. Vertical screening distances were predicted for different soil types using aerobic biodegradation rate constants estimated from the measured soil‐gas data combined with conservative estimates of source concentrations. The modeling indicates that the vertical screening distance of 6 feet (1.8 m) recommended for dissolved‐phase sources is applicable for 1,2‐DCA below certain threshold concentrations in groundwater, while 15 feet (4.6 m) recommended for light nonaqueous phase liquid (LNAPL) sources is applicable for sites with clay and loam soils in the vadose zone, but not sand, if 1,2‐DCA concentrations in groundwater exceed 150 μg/L. This dependence of the predicted vertical screening distances on soil type places added emphasis on proper soil characterization for VI screening at sites with 1,2‐DCA sources. The soil‐gas data suggests that a vertical screening distance of 15 feet (4.6 m) is necessary for both dissolved‐phase and LNAPL sources.