Groundwater Quality Management (Proceedings of the GQM 93 Conference held at Tallinn, September 1993). IAHS Publ. no. 220, 1994. 161 The fate of ammonium and phosphate in lake sediments M.A.A. PAALMAN Department of Geochemistry, Institute of Earth Sciences, University of Utrecht, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands S.M. HASSANIZADEH National Institute of Public Health and Environmental Protection (RIVM), P.O. Box 1, 3720 BA Bilthoven, The Netherlands Abstract The fresh-water lake Ketelmeer, in the central part of The Netherlands, forms an important sedimentation area at the mouth of the river IJssel. The bed sediments form an important interface between the surface water and the groundwater and there is a strong relationship between the quality of these waters. Accumulation of pollutants in the sediments is an important threat to groundwater. In this study, results of measurement of minerals in sediments of Ketelmeer are presented. These results are used in the simulation of the distribution of ammonium and phosphate in the pore water. The simulations are based on an analytical solution of the equations describing one-dimensional transport of solutes in a porous medium. Processes which are taken into account are: advection, diffusion, decomposition of organic matter, adsorption, precipitation, and accumulation of sediments. Closed-form analytic expressions are obtained which give the distribution of ammonium and phosphate in the sediments. These expressions are used to calculate fluxes of nutrients toward the lake water and groundwater. INTRODUCTION An important aspect of water quality studies in The Netherlands is the interaction between surface water and groundwater. Many surface water bodies and lakes in The Netherlands are surrounded by low-lying polders. It is known that water infiltrates through the lake sediments into the shallow groundwater and eventually ends up in the surface water of polders. The lake sediments are often enriched with nutrients released by agricultural activities. In addition to this external source, as a result of diagenesis of organic matter in sediments, organic N- and P-compounds are mineralized into ammonium and phosphate, which dissolve in the pore water. Thus, infiltrating water is an important source of contamination of the polders groundwater and surface water. To quantify the extent of this contamination, it is necessary to identify all processes which contribute to the production and consumption of nutrients and to calculate the distribution of nutrients in lake sediments. In this paper, results of a case study carried out in relation with the fresh­ water lake Ketelmeer is presented. The study consisted of taking sediments cores at two locations, measurement of ammonium, nitrate, phosphate, and metals such as iron and aluminium in pore water, and measurement of major elements and inorganic carbon in the solid phase. Then, the distribution of nitrogen and phosphate in the sediment profile is modelled by means of a one- dimensional transport model. 162 M. A. A. Paalman & S. M. Hassanizadeh STUDY AREA, MEASUREMENTS, AND ANALYSIS OF DATA In this section a brief description of the study area and the measurements is given; more details are provided in Paalman et al. (1993). Lake Ketelmeer is a shallow fresh-water lake in the central part of The Netherlands, forming an important sedimentation area for the river Rhine. In the sediments underlying the Ketelmeer various geological formations can be distinguished. At the base are the Pleistocene sands (formation of Twente). In the western part of the lake these sands are covered by a base-peat layer. On top of these layers there are Holocene clay deposits (formations of Calais and Dunkirk), with intercalated peat deposits. The formation of Dunkirk consists largely of a marine deposit of the former Zuiderzee (Zu), covered by the recent fresh-water Usselmeer deposits (Urn) (Winkels & Van Diem, 1991). As a result of land reclamation activities a few decades ago, two polders were created, which now surround the lake. The hydrology of the Ketelmeer shows that water infiltrates through the sediment into the groundwater of the polders. The infiltration rate is largely controlled by the presence of the base-peat layer. At locations where there is a base-peat layer, the infiltration rate is significantly lower than where the peat layer is missing (Bruinsma, 1989). For the analysis of the Ketelmeer sediments, cores of 30 cm long were taken in triplicate, at two different locations (Kl and K2 in Fig. 1) on 22 April 1991. The cores were taken from locations with fine-grained sediments that consisted only of IJm-deposits. They have been sliced into many sections, 0.5 to 1.5 cm in thickness. The pore water is analyzed for determining phosphate, ammonium, and nitrate. Metal in pore water and major and trace elements in solid phase were measured with a multi-element Inductive Coupled Plasma Emission Spectrometry (ICPES) analyzer. Also, organic carbon and authigenic phosphate minerals in the solid phase were measured. The sedimentation rate was estimated on the basis of 134Cs and 137Cs measurements (Beurskens etal, 1993). At location Kl, the sedimentation rate was estimated to be 1.0 ± 0.1 cm year1. Location K2 is situated within a former sand pit area, where the sedimentation rate was estimated to be 10 ± 2 cm year"1. Fig. 1 Overview of the River Ussel and the sampling locations Kl and K2. The fate of ammonium and phosphate in lake sediments 163 The collected cores had a dark-greyish colour except for the top few millimetres which had a brownish colour. The brownish colour of the thin top layer could be caused by coatings of ferric hydroxides, indicating an oxygenated environment, whereas the greyish colour indicates the presence of reduced iron species under anoxic conditions (Lyle, 1983). Results of solid-phase analysis show that the distribution of major elements in both cores is very similar. In both cores the organic carbon content is between 3.06 and 5.93% and shows no remarkable gradient with depth. The total P content is between 0.28 and 0.50% and the pattern of P is similar to that of organic C. For both cores it appears that the ammonium and phosphate concentrations are higher in the pore water than in the lake water (Figs 2a and 2b). For phosphate there is a large gradient in the top few centimetres of the sediment. Deeper in the sediment the phosphate concentration reaches a maximum, and even shows a slight decrease with depth (Fig. 2b). The nitrate profiles show a different pattern (Fig. 2c). In both cores, directly beneath the sediment-water interface a reduction of nitrate is observed. At a depth of approximately 2 cm beneath the interface, all the nitrate has been depleted from the pore-water solution as a result of denitrification processes. Figure 2d shows that, for both cores, the iron concentration increases with depth. The increase is NH4 (mM) u P04 (mM) 0.00 0.07 0.14 0.21 '-10 N03 (mM) Fe (mM) 0.00 0.15 0.30 0.45 0.0 0.2 0.4 0.6 5- Fig. 2 Pore water concentration profiles of ammonium (2a), phosphate (2b), nitrate (2c), and ferrous ions (2d) at locations Kl and K2 (indicated with * and Q, respectively). The solid lines on Figs 2a and 2b represent the model calculations fitted to the measurements. 164 M.A.A. Paalman & S.M. Hassanizadeh not observed directly beneath the interface, but below a depth of approximately 0.5-1 cm. The increase in the concentration of ammonium and phosphate in the pore water is due to the decompositions of organic matter under anoxic conditions (see e.g. Berner, 1977, and Martens et al, 1978). In addition to the mineralization of organic P, phosphate can be released by desorption due to the reductive dissolution of ferric hdroxides under sub-oxic conditions (Krom & Berner, 1981). The increase in the concentration of iron with depth in the pore water (Fig. 2d) indicates the instability of ferric hydroxides. Some of the dissolved ferrous ions will react with sulphides to form various iron sulphide precipitates (Berner, 1984). Moreover, under anoxic conditions ferrous ions can react with liberated phosphate to form ferrous phosphate precipitates, such as vivianite (Nriagu, + 1972; Emerson, 1976). In Fig. 3, the amount of ammonium (NH4 ) produced in the pore waters is plotted against the amount of phosphate (PO43") produced. At low concentrations, just as in the top layers of the sediment, there seems to be a fairly constant ratio between the change in concentrations of ammonium and phosphate (denoted by ACN/ACP). The ratio (ACN/ACP) is approximately 3.0 for core Kl and 4.7 for core K2. At higher concentrations (greater depths), the ratio ACN/ACP starts to deviate from linearity, indicating a process of phosphate removal under anoxic conditions, possibly as a result of authigenic phosphate mineral formation. 0.21 / / 0.14 o it °-0.07 - <3 0.00 0 1 2 3 A NH4 (mM) Fig. 3 The change in concentration of ammonium plotted against the change in phosphate concentration at locations Kl and K2 (indicated with * and D, respectively). The dashed lines represent the approximation to the curve for the top layer of the sediments. SIMULATION OF AMMONIUM AND PHOSPHATE DISTRIBUTION In this section, a mathematical model of ammonium and phosphate distribution in the sediment is presented. The model is based on the one-dimensional advection-dispersion equation. Processes which are taken into account are: advection, diffusion, decomposition of organic matter, adsorption, precipitation of phosphate, and accumulation of sediments. Oxidation of ammonium is neglected. First, major assumptions underlying our model are listed and discussed. These assumptions can be divided into two categories. Some assumptions are The fate of ammonium and phosphate in lake sediments 165 made in order to simplify the model so that the analytical solution of the governing equations can be obtained.