Interpretation of measured concentration profiles in sediment pore water

A robust numerical procedure for biogeochemical interpretation and analysis of measured concentration profiles of solutes in sediment pore water has been developed. Assuming that the concentration-depth profile represents a steady state, the rate of net production or consumption as a function of depth can be calculated, together with the flux across the sediment-water interface. Three kinds of vertical transport can be included in the analysis: molecular diffusion, bioturbation, and irrigation. The procedure involves finding a series of least square fits to the measured concentration profile, followed by comparisons of these fits through statistical F-testing. This approach leads to an objective selection of the simplest production-consumption profile that reproduces the concentration profile. Because the numerical procedure is optimized with respect to speed, one prediction can typically be done in a few minutes or less on a personal computer. The technique has been tested successfully against analytical solutions describing the transport and consumption of 0, in sediment pore water. In other tests, measured concentration profiles of O,, NO;, , NH:, and ZCO, have been interpreted using the new procedure. Concentration profiles in sediment pore water are used widely in studies of biogeochemical processes. Profiles on a macroscale are measured using a variety of methods, including in situ samplers (Sayles et al. 1976; Kuivila et al. 1989), dialysis cells, so-called peepers (Hesslein 1976; Brand1 and Hanselmann 1991), slicing techniques (Reeburgh 1967; Emerson 1976; Thamdrup et al. 1994a), and gel samplers (Krom et al. 1994). High-resolution techniques that employ microelectrodes (Revsbech et al. 1980) are used to measure profiles on a submillimeter scale. Although a fair amount of information and understanding can be achieved by simple visual examination of these measured profiles, their full value is realized by applying modeling techniques to their interpretation. Such applications make it possible to achieve a much more precise prediction of the location of zones of production or consumption, the extent of these ’ Corresponding author. Acknowledgments We thank K. Jensen for construction of NO; and NH; sensors and L. B. Petersen for construction of 0, microsensors, L. l? Nielsen for valuable discussions, K. McGlathery and D. Canficld for reading the manuscript, and F! Van Cappellen, an anonymous reviewer, and B. P. Boudrcau for their constructive reviews. This study was supported by grants from the Danish National Science Research Councils contract 9501025 (S. Rysgaard). This work is a contribution to the European Union ELOISE program (ELOISE no. 040) in the framework of the NICE project carried out under contract MASSCT-96-0048 (N. Risgaard-Petersen). zones, and the resulting fluxes across the sediment-water interface. In many studies, the movement of solutes in sediments has been attributed to a vertical diffusion phenomenon (Blackburn et al. 1994; Glud et al. 1994; Thamdrup et al. 1994b; Rysgaard and Berg, 1996). Vertical diffusion can be separated into two contributions: molecular diffusion and bioturbation (i.e., the diffusion-like transport caused by random movements of meiofauna). Other studies have shown clearly that irrigation (i.e., the pumping activity of tubedwelling animals) can significantly influence the transport of solutes in sediments (Aller 1983; Pelegri et al. 1994; Wang and Van Cappellen, 1996). Although irrigation is obviously a three-dimensional problem (Aller 1980), it is possible to include irrigation in one-dimensional formulations using the nonlocal source-sink function suggested by Boudreau (1984). Assuming steady state conditions and neglecting the effect of pore water movements due to burial, compaction, groundwater flow, and wave action, the one-dimensional mass conservation equation that accounts for the effects of molecular diffusion, bioturbation, and irrigation is i +$D, + D,,$ + pcx(C, C> + R = 0, (1) where C is the pore water concentration, C, is the bottom water concentration, x is the depth, cp is the porosity, D, is the molecular diffusivity corrected for tortuosity, D, is the biodiffusivity, cx is the irrigation coefficient, and R is the net rate of production (or consumption if R is negative) per unit