Gradients controlling natural attenuation of ammonium
Introduction
Ammonium is a common pollutant at landfills and former gas work sites. This is due to the generally large amounts of N present at these sites (e.g., proteins) that are transformed to under reducing conditions and thus represent a strong potential of forming large scale groundwater pollution (contaminant plumes). Ammonium is fish toxic and may cause problems in drinking water wells. Several processes lead to attenuation of , such as sorption, dilution and biodegradation. Dilution, however may cause contaminant concentrations to drop below the legal limit, but does not lead to a decrease of contaminant fluxes in environmental compartments.
Sorption of is mainly controlled by ion exchange and leads to retardation but not to the destruction of contaminants. Retardation may increase the available time to perform remedial measures at a receptor of concern, and should be taken into account to decide whether an plume has reached its maximum extent. Biodegradation thus remains as the only process that is able to efficiently transfer to less hazardous (such as ) or harmless compounds (e.g., N2). Whether biodegradation takes place, is highly dependent on redox conditions.
Studies on attenuation processes at the field scale so far have mainly focussed on characterizing reaction processes and corresponding redox conditions (Buss et al., 2003, Bjerg et al., 1995, Erskine, 2000, Christensen et al., 2001). Rügner et al. (2004) determined degradation rates in the field based on measurements of mass flow rates at two consecutive control planes and conceptual (scenario specific) numerical modelling. Liedl et al., 2005, Maier and Grathwohl, 2006 have described analytical and numerical approaches to actually predict the steady state length for plumes or other aerobically degradable contaminants for simple scenarios. However, the assessment of degradation and the length of plumes under more complex boundary conditions is still challenging.
Biotransformation may take place as core controlled processes within the interior of the plume or as fringe controlled processes driven by external electron acceptors. Oxygen as a latter one is usually not present in landfill leachate. Core processes which allow for degradation without external mixing of electron acceptors are strongly dependent on the available electron acceptors, degradation kinetics and stoichiometry. Anaerobic degradation under reducing conditions using , Mn or Fe oxides as intrinsic electron acceptors (anammox) may contribute to degradation (Christensen et al., 2001, Jetten, 2001, Schink, 2002, Buss et al., 2003). However, field data indicate that these mechanisms, which are not considered in this study, are rather slow and may thus only contribute significantly to natural attenuation of in very long plumes i.e. of more than several hundred meters to 1 km length, which was complied as tolerable at the Osterhofen case study (Rügner et al., 2004). Thus, core processes are unlikely to contribute to natural attenuation of .
The most important degradation process is aerobic nitrification of to in the presence of O2 (Stumm and Morgan, 1996). As two different bacterial genuses are involved, the reaction proceeds in two steps. The intermediate product is instable, so that nitrification is commonly combined to the overall reaction:The kinetics of biodegradation (i.e. chemical reaction of two components A and B) are commonly expressed according to Michaelis and Menten (1931):In this case A represents and B O2. R [M L−3 T−1] denotes the reaction rate at a certain location, kmax [M L−3 T−1] is the Monod maximum utilization rate of the reaction, K1/2 [M L−3] is the Monod half utilization concentration of the reaction for each compound and Kthr [M L−3] is the threshold concentration of the specific compound that is just sufficient to maintain the reaction.
In many cases, processes at plume fringes and along gradients control natural attenuation. As these processes require the transport of external electron acceptors to the contaminant plume, the geometry of the aquifer and the contamination as well as the properties of the porous medium controlling mixing processes are very influential on the total amount of mass transformation (Liedl et al., 2005, Maier and Grathwohl, 2006). Werth et al., 2006, Cirpka and Kitanidis, 2000 identified zones where groundwater flow converges as major contribution to dispersive mass fluxes. In addition, Koussis et al., 2003, Chu et al., 2005, Maier and Grathwohl, 2006 justified that reaction kinetics are not limiting aerobic biodegradation in most cases under field conditions. When mixing of contaminant and O2 is the limiting process, steady state plume length is independent of groundwater flow velocity (Liedl et al., 2005, Maier and Grathwohl, 2006), whereas geometry and mixing characteristics were shown to be the crucial parameters as they play a decisive role in delivering the required electron acceptors to sustain biodegradation to the fringe of contaminant plumes. For a scenario with a homogeneous aquifer uniformly contaminated at the influx boundary and O2 supply from the water table, the steady state plume length L was predicted by Liedl et al., 2005, Maier and Grathwohl, 2006:where M is the aquifer thickness, αt the transverse vertical dispersivity and f(γ) a function of the stoichiometric ratio of contaminant to O2 demand, which generally appears as a weak non-linear relationship to plume length (logarithm or power ≈ 0.3). Furthermore, slow aqueous phase molecular diffusion or transverse dispersion may limit mass transfer across the capillary fringe (McCarthy and Johnson, 1993, Jellali et al., 2003), which is important for O2 supply to aquifers.
The aim of this study is to identify the main factors that control natural attenuation of for more complex boundary conditions which are frequently observed at the field scale. In particular, these are (i) partly contaminated aquifers with two reaction fronts and (ii) a spatially variable aquifer thickness, (iii) the influence of groundwater recharge and (iv) the influence of restricted supply of O2 to contaminated water by slow dispersion and diffusion processes across the capillary fringe. For this purpose, scenario specific numerical modelling was performed to assess the potential of natural attenuation at the aquifer scale by variation of environmental factors (Grathwohl et al., 2003). In contrast to narrow contaminant plumes caused by DNAPL infiltration it is frequently observed at landfills that the width of an plume usually corresponds to a major proportion to the width of the landfill itself (e.g. 110 m of 180 m at the “Osterhofen” field site; Rügner et al., 2004). Thus, an emphasis is given on gradients that control the vertical transport of reactants. Thus, numerical simulations were performed in 2-D vertical cross sections. This also reduces computation time.
Section snippets
Methodology – scenario specific modeling
The numerical code MIN3P (Mayer et al., 2002) was used, that allows for the simulation of a variable number of geochemical compounds and reactions. Biogeochemical reactions and transport processes are coupled by a global implicit solution method and solved using a finite volume algorithm. Physical–chemical properties of the geochemical species are defined in the external database of the geochemical equilibrium model MINTEQ (Allison et al., 1991). Advective-dispersive transport in the aqueous
Partly contaminated aquifer
Landfills may be entirely placed within the unsaturated zone, emitting contaminated seepage water into the aquifer, or comprise the unsaturated zone and the upper part of an aquifer. If contaminated water is underlain by oxygenated groundwater, a second reaction front will develop contributing to natural attenuation. Simulated concentration contours for a steady state plume of that scenario are given in Fig. 1 for and O2 and the reaction product . The NH4 plume is confined by two
Conclusions
The spatial zones and their relative importance for attenuation were evaluated with a focus on geometric factors affecting flow and transport in and into aquifers, depending on plume and aquifer geometry, groundwater recharge, and limited O2 supply.
A strong influence on the steady state plume length affected by biodegradation has to be expected from parameters such as the aquifer or plume thickness and geometry. Aquifer thickness M was reported by Liedl et al., 2005, Maier and Grathwohl,
Acknowledgement
The study was funded by the LfU (Environmental Agency) Baden-Württemberg which is gratefully acknowledged.
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