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 (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. Most of these assumptions can be relaxed if one decides to construct a numerical solution. A second group of assumptions are introduced because of the lack of data and/or information on thermodynamic parameters: 1) Percolation of water through the sediment is taken into account. At locations Kl and K2, the water percolation velocity is low because of the presence of a base-peat layer. It is estimated to be v=6.4 cm year-1 (Bruinsma, 1989), assumed constant in our model. 2) The rate of sedimentation is assumed to be constant. A value of (0=1 cm year"1 at location Kl and «=10 cm year-1 at location K2 is employed. 3) The transport process is assumed to be steady-state in the frame of reference fixed to sediment-water interface. 4) The bulk sediment diffusion coefficient is obtained by correcting the corresponding free ion diffusion coefficient in the pore water for the effect of the sediment structure. The diffusion coefficients DN (for ammonium) and Dp (for phosphate) are estimated to be 296 and 125 cm2 year1, respectively. These values agree well with the results obtained by Krom& Berner (1980). 5) The dispersion process is assumed to be diffusion dominated. In one dimension, the additional dispersion as a result of advection is given by av, where a is the longitudinal dispersivity. Assuming a value of 1 cm for a (which is an overestimation for fine sediments of limited thickness), one obtains an equivalent dispersion coefficient of 6.4 cm2 year1, which is negligible compared to the bulk sediment diffusion coefficients given above. 6) Decomposition of nitrogen-bearing organic matter is described by first- order kinetics (Berner, 1980; Middelburg, 1990). In a fixed frame of reference, this is described by dN

where N is the mass of nitrogen per unit mass of decomposable organic -1 matter of sediments (mol kg ) and k& is the rate of decomposition of organic matter (year-1). 7) Decomposition of organic matter produces phosphate as well. The rate of phosphate production is given by a relationship analogous to (1): w~k°p (2)

where it is assumed that the value of the decomposition constant, kz, is the same for ammonium and phosphate. It is likely that with regard to the sediments of the Ketelmeer, phosphate is also produced by the reduction of ferric hydroxides. In principle, one must model the two phosphate- producing processes separately. This, however, requires information on the amount of ferric hydroxides available and their rate of reduction. At present such information is not available. Therefore, we assume that the rate of production of phosphate resulting from the reduction of ferric hydroxides is the same as that from the decomposition of organic matter. In the equation of mass conservation, therefore, one term represents both processes of phosphate production. 166 M.A.A. Paalman & S.M. Hassanizadeh

8) Adsorption of ammonium occurs through the exchange with calcium. This is probably the most significant adsorption mechanism in fresh water sediments (Davison & Woof, 1990). It is assumed that this process obeys a linear equilibrium isotherm. The adsorption coefficient, KN, is estimated to vary between 0.5 and 2.0 (Bruggenwert & Kamphorst, 1982; Middelburg, 1990). 9) Adsorption of phosphate is also described by a linear equilibrium isotherm. The adsorption coefficient, Kp, is estimated to be 1.8 under anoxic conditions (Krom & Berner, 1980). It should be pointed out that the adsorption behaviour of phosphate in fresh-water sediments under anoxic conditions is not precisely known. On the one hand, the prevailing anoxic conditions cause ferric hydroxides to be unstable and thus decrease the adsorption of phosphate. On the other hand, the pH of sediments of the Ketelmeer decreases with depth, which will enhance phosphate adsorption (Brinkman & Van Raaphorst, 1986). It is assumed that these two effects cancel. 10) Precipitation of vivianite occurs. It is assumed that the rate of precipitation is given by a first-order kinetic formula:

•¥-=-km(C-CP) (3) dt m e

where z' is the distance from a point fixed in space and F is a concentration conversion factor (kg nr3). These equations, together with (1) and (2) must be solved for CN and Cp. However, because of assumptions (1) and (2), if one writes these equations in a frame of reference attached to the sediment-water interface (thus moving with the velocity co), the time derivative terms may be set to zero and the resulting equations will be: d2CN dCN N D = (CD (l+KN) + v) +F ka N = 0 (6a) dz1 dz dN ka (6b) = _ _JL. N dz co for ammonium, and The fate of ammonium and phosphate in lake sediments 167

SCp dCp DP —^ -/co (l+KP) + v] ~1- +FkaP - km(CP-CPeq) = 0 (7a)

dP k a (7b) dz co for phosphate. In these equations, z=z'+ cot is the depth below the sediment- water interface. These equations are subject to the following boundary conditions:

N N P P at z=0; C =C0 , C =C0 , N=N0, P=P0 as z..>oo ; C"~>CJ, CP->Cecf N P where C0 and C0 are the concentration of dissolved ammonium and phosphate, respectively, at the sediment-water interface (z=0) and NQ and P0 are the amount of decomposable organic N and P, respectively, in fresh sediments. Solution of equation (6b) for N, substitution in (6a), and integration of the resulting equation yields the following expression for CN:

CN=CN+(CN-C") [l-exp(^z)] (8) where,

C »= C»+ ^^ (9) N 2 D ka + v co + (1 + KN) co A similar procedure yields the following expression for Cp:

Cp= CP + (Cp-Cp) [1- exp(- a z)]+K[exp(- az)- exp ( ^ z)] (10) where, //co (1 + K ) + vf + 4 k DP]112 - /co (1 + K )+ v] a= P m P (lla) 2 DP and co2 F P K= 0 2 k 2 llb DPka + vu + (l+KP) co - -^ co < ) "a Most parameters in these equations are known and/or can be estimated from measurements indirectly. As explained in the previous section, v, co, CQN, P N p CQ , and CM are estimated from measurements, whereas D^, D , KN, KP, F and Cp are taken from the literature. The values employed here are listed in upper half of Table 1. Four parameters remain to be determined using the data of Figs 2a and 2b. These are ka, km, N0 and P0. The following procedure is followed. First, the decomposition rate ka is calculated by fitting expression (8) to the measured ammonium profile, for each core separately (the solid lines in Fig. 2a). Next, N0 is computed from equation (9). Then, P0 is determined making use of 3 the ratio of concentrations of NH4+ and P04 " in the top layer of the sediment. CN and Cp seem to be linearly correlated, as seen in Fig. 3. We will assume that there is a similar correlation between N0 and P0. However, corrections for the 168 M.A.A. Paalman & S.M. Hassanizadeh

Table 1 Model parameters used for fitting pore-water profiles of ammonium and phosphate in the sediments of the Ketelmeer.

Parameter CoreKl Core K2

C N 0.033 mM 0.033mM c p 0.012mM 0.012 mM c p 0.13 mM 0.15 mM 2 1 2 1 DN 296 cm year 296 cm year Dp 125 cm2 year1 125 cm2 year1 1 1 CO 1 cm year 10 cm year 1 1 V 6.4 cm year 6.4 cm year F 0.66 g cm-3 0.66 g cm-3

1 No 72.0 umol g" 49.5 umol g-' 1 1 Po 33.8 umol g- 12.7 umol g- K 0.038 year1 0.446 year1 1 1 ^m 15 year 20 year differences in concentration of ammonium and phosphate due to diffusion, advection and adsorption have to be made. The following expression suggested by Berner (1977) is employed:

N p 2 N0_ AC (D ka + v co + ( 1 + KP) co ; P N 2 (12) P0 ~ AC (D ka + v w + ( 1 + KN) co ) where ACN/AC^ is the slope of the curve in Fig. 3 at low concentrations. Finally, km is obtained by fitting equation (10) to the data for phosphate in Fig. 2b (the solid line). The fitted parameter values for both curves are given in the lower half of Table 1.

DISCUSSION OF RESULTS AND CONCLUSIONS

The analysis provided here is simplified so that an analytical solution of the problem can be obtained. The assumptions of a constant rate of sedimentation (to) and a constant supply of nutrients (N0 and P0) do not account for seasonal variations. In practice, seasonal variations of ammonium and phosphate in lake sediments have been observed (Boers & Van Hese, 1988). Furthermore, the effect of temperature fluctuations, which may affect decomposition and precipitation rates and diffusion coefficients, have been neglected here. Therefore, the analysis must be seen as providing average values over a long period of time (many years). The assumption of homogeneity of the sediment profile disregards the variation of chemical and physical conditions with depth. For example, in the top few millimetres of the sediments the conditions are oxic and, as a result, the value of ka, will be higher than in deeper parts. Also, it is known that in the top layer of the sediment, the effective diffusion coefficient may be altered by processes such as bioturbation and wave currents which stir the top layer of the sediment The fate of ammonium and phosphate in lake sediments 169

(Berner, 1980; Toet & Blom, 1990). Another effect that may be important is the wind. A strong wind could result in a resuspension of the sediment top layer (Toet & Blom, 1990). Due to this resuspension process, the anoxic sediment could interact with the oxygenated surface water, resulting in the dissolution of vivianite precipitates and thereby increasing phosphate concentrations in the surface water. But, given the smooth and gradual variation of the concentration of various species in pore waters of both cores in the top layer (see Fig. 2), these processes may be assumed to be negligible in this situation. Another parameter which may be depth-dependent is the rate of precipitation of vivianite, km. Vivianite precipitation occurs at a depth where the pore water is saturated in vivianite. The top layer of the sediment shows undersaturation (Fig. 3) and thus for phosphate modelling, a multilayer model would be desirable. Supersaturation of vivianite deeper in the sediment would suggest that precipitation kinetics other than a first-order reaction rate is applicable. As mentioned earlier, the rate of decomposition of organic matter, ka, is assumed to be the same for ammonium and phosphate. In doing so, however, one must realize that, whereas N0 is the amount of decomposable organic nitrogen, P0 includes the amount of phosphate resulting from the reduction of ferric hdroxides, as well as that produced by the decomposition of organic matter. This fact probably explains the relatively low value of the ratio NQIPQ as calculated from equation (13) (about 2.2 for core Kl and 4.0 for core K2, see

Table 1). Values reported in the literature are NQ/P0=16 for decomposable organic matter (Redfield, 1958), and NQ/P0=13 according to the molar ratio as used by Boers & De Blés (1991) for the fresh-water lake Loosdrecht. Another explanation is simply that there are differences in the composition of different organic matter. From Table 1, it is evident that the value of ka, for core K2 is about 11.5 times larger than that for core Kl. This is probably due to the fact that, because of the high rate of sedimentation at location K2, the organic matter is fresher and thus more rapidly decomposable. Indeed, the decomposition rate of organic matter for marine depositional sediments is known to increase with the 2 sediment accumulation rate. The corresponding relationship is ka= A co , where A is an empirical constant estimated to be 0.04 year cm-2 (Toth & Lerman, 1977). The decomposition rate of core Kl compares well with the value obtained from this formula, whereas ka for core K2 it is much smaller. This is probably due to the range of validity of the formula which is confined to low settling rates typical of marine deposits.

Table 2 Calculated fluxes of ammonium and phosphate from the sediment towards the lake water.

Depth Parameter CoreKl Core K2

2 2 1 z=0 NH4+ -10.4 mg N nr day-' -42.2 mgNm- day 2 2 P043- -6.9mgPm' day-' -9.5 mg Prrr day-'

2 2 1 z=l m NH4+ +5 mg N nr day-' +20 mg N nr day- 2 1 2 1 P043- +0.75 mg P m- day +0.8 mg P nr day- 170 M.A.A. Paalman & S.M. Hassanizadeh

The expressions obtained here for the distribution of ammonium and phosphate in the Lake Ketelmeer sediments can be used to calculate the total mass flux of nutrients into the lake water at the sediment-water interface, and into the polder groundwater below the base peat layer. In Table 2, the flux of ammonium and phosphate towards the surface water and at a depth of one meter are given. Despite the high burial and infiltration rates for core K2, the fluxes are still determined mainly by diffusion processes. Because of the many simplifications made in this analysis, these fluxes, and specially those at the sediment-water interface, must be considered only as indicative values. For ammonium, the flux can be affected by the nitrification in the top few millimetres of the sediment, and for phosphate the much stronger adsorption on ferric hydroxides at the oxic interface layer may affect the flux towards the surface water. Also, it is difficult to extrapolate the fluxes to the entire Ketelmeer surface water, because the sediment types vary greatly (Winkels & Van Diem, 1991) and concentration profiles may vary substantially. Nevertheless, it is obvious that the amount of nutrients entering the lake and reaching the polders groundwater are high. This results in elevated concentrations in both surface water and groundwater. This is obviously a threat to the water quality and appropriate measures must be taken to alleviate the problem.

Acknowledgements The authors gratefully acknowledge the assistance of H. Minderhout, H. Winkels, and J. Vink of the Ministry of Transport, Public Works and Water Management (Directorate ) in collecting samples and D. van de Meent-Olieman in analysing the samples. This work was supported by The Netherlands Integrated Soil Research Programme under contract number PCBB 8960.

REFERENCES

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