Quantitative interpretation of pH distributions in aquatic sediments: A reaction-transport modeling approach

Abstract

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.

pH modelling in aquatic systems with time-variable acid-base dissociation constants applied to the turbid, tidal Scheldt estuary

“…The second aspect, however, has received far less attention. Improving and extending previous work by Jourabchi et al (2005) and Soetaert et al (2007), we have recently presented an extension of the conventional pH modelling approach, developing a procedure that allows to quantify how different biogeochemical processes contribute to the overall proton cycling in an aquatic ecosystem (Hofmann et al, 2008a). This method basically splits the total rate of change of protons into individual contributions of the processes that are driving the pH change (details are given 1540 A. F. Hofmann et al: pH modelling with time variable acid-base constants below).…”

“…This explicit approach was originally pioneered by Jourabchi et al (2005) and Soetaert et al (2007), though these treatments were partially incomplete. The approach proposed in Jourabchi et al (2005) is only applicable to steady state conditions, and it also treats the effect of advective-diffusive transport on proton cycling incompletely (the proton transport term is omitted).…”

“…Extending and improving the work of Jourabchi et al (2005) and Soetaert et al (2007), we recently presented a new approach to pH calculations in reactive transport models (Hofmann et al, 2008a). This method, called explicit here, allows us to describe the pH evolution over time (just like the implicit method), but in addition, it derives the contribution of separate biogeochemical processes to the overall rate of change of the proton concentration.…”

Geochemistry of trace metals in a fresh water sediment: Field results and diagenetic modeling

“…Reaction-transport model calculations were carried out with the Biogeochemical Reaction Network Simulator (BRNS; Aguilera et al, 2005;Jourabchi et al, 2005). The development and calibration of the model used to describe the sediment in Haringvliet Lake is presented in Canavan et al (2006).…”

Modelling the vertical profiles of O2and pH in saturated freshwater sediments

“…Their relevance is evaluated by comparing our values to those found in the literature (Table 4). pH modelling pH determination requires proton concentrations [H + ] which depend on a complex set of reactions which control the acid-base properties of pore water (Jourabchi et al 2005). In our model, pH is a calculated function and not a forced function as in other studies (Wijsman et al 2001).…”