PHOSPHORUS RETENTION AND RELEASE CHARACTERISTICS OFSEWAGE-IMPACTED WETLAND SEDIMENTS

LONG M. NGUYEN�, JAMES G. COOKE and GRAHAM B. McBRIDENational Institute of Water and Atmospheric Research Ltd. (NIWA), P.O. Box 11-115, Hamilton,

New Zealand; � Author for all correspondence.

(Received 3 August 1994; accepted 15 August 1996)

Abstract. Sediment deposited in traps positioned along a sewage-impacted wetland receiving phos-phorus (P)-retaining reactants from natural wetland water was fractionated into different particle sizes,and the amount of P retained in these particle sizes was investigated. Subsamples of the sedimentscollected from different sites along the wetland system were also equilibrated with water at differentwater:sediment ratios and equilibration periods to investigate the extent of P released from thesesediments under aerobic and anaerobic conditions. Results obtained showed that most of P depositedin sediments is in fine fractions (<16 �m), particularly in sediments collected from confluence siteswhere water inflow from the natural wetland provides P-retaining reactants and from sites imme-diately below these confluence sites (postconfluence sites). The extent of P release from sedimentsdepended on the aerobic-anaerobic conditions of the sediments, equilibration period, water:sedimentratio and the position of sites within the wetland. The rate of P released from sediments associatedwith an increase in equilibration period tended to be higher under aerobic than anaerobic conditions.Water:sediment ratio was found to be a more important factor in controlling the release of P fromsediments under anaerobic than aerobic conditions. The amount of P released from the confluenceand postconfluence sites was higher than that from other sites over a range of equilibration periodsand water:sediment ratios under aerobic and anaerobic conditions.

Key words: iron, natural wetlands, phosphorus, retention, sediments, sewage, sorption-desorption,wetlands

1. Introduction

Sediment deposition has been shown to be the most important phosphorus (P) sinkin a sewage-impacted wetland system, where inflow from natural wetlands providesa renewable source of reactants for P removal by adsorption and precipitationprocesses (Cooke, 1992; Cooke et al., 1992). At sites where water merges fromthese two wetland systems (termed confluence sites) and at sites immediately belowthese confluences (termed postconfluence sites), the amount of P removed fromwastewater and deposited in sediments was found to be greater than that at othersites along the sewage-impacted wetland, where there was no inflow of naturalwetland water (Cooke et al., 1992).

Although Cooke et al. (1992) have investigated the mechanism of P deposi-tion in this sewage-impacted wetland, no information is available on the extentof P retention by different particle size fractions of wetland sediments and therelease characteristics of these sediments under aerobic and anaerobic conditions.Such information is essential in understanding the sustainability of this wetland inremoving P from wastewater. The potential release of deposited P from sediments

Water, Air, and Soil Pollution 100: 163–179, 1997.c 1997 Kluwer Academic Publishers. Printed in the Netherlands.

164 L. M. NGUYEN ET AL.

to wetland waters is unknown, but numerous studies have found that this P is morelikely to be released under anaerobic (reducing) than aerobic (oxidising) conditions,as a result of the reduction of P-retaining iron compounds from crystalline ferric(Fe3+) forms to more soluble ferrous (Fe2+) forms (Nichols, 1983; Masscheleynet al., 1992; Richardson and Craft, 1993). However, others have reported a morecomplex situation in which anaerobic sediments release more P to solution low insoluble P and adsorb more P from solution high in soluble P (1.8–5.3 mg P L�1)than aerobic sediments (Patrick and Khalid, 1974; Furumai and Ohgaki, 1989; Red-dy and Reddy, 1993). The higher adsorption maxima under anaerobic conditionsfor sediments with a high soluble P content is attributed to the conversion of ferricoxyhydroxide to amorphous and poorly crystalline ferrous forms (ferrous oxide orferrous hydroxide), which have a greater surface area for P adsorption and hence agreater P adsorption capacity than ferric oxyhydroxide (Patrick and Khalid, 1974;Khalid et al., 1977; Furumai and Ohgaki, 1989; Paludan and Jensen, 1995). Thusthe amount of P released from sediments to solution is expected to be lower underanaerobic than aerobic conditions. In wetland systems with a configuration where-by the water from a relatively small area of sewage-impacted wetland is mixed withthat of a much larger area of natural wetland, reaeration is likely to be an importantprocess at the sediment-water interface (Cooke et al., 1992). Thus there is a needto investigate P release from sediments under aerobic and anaerobic conditions.

The objective of this study is to investigate which sediment fractions are amajor contributor to P retention, and the influence of aerobic-anaerobic conditionson P released from sediments obtained at different sites along the wetland system.Phosphorus release at the confluence and postconfluence sites may differ from thatat other sites since the highest P deposition was found to occur at the confluenceand postconfluence sites because of the supply of P-retention reactant from naturalwetland water at these sites (Cooke, 1992; Cooke et al., 1992).

As demonstrated by various workers (Nichols, 1983; Reddy and Reddy, 1993),the sustainability of wetlands in removing P from wastewater is dependent not onlyon the sorption characteristics of the sediments but also on their release (desorp-tion) characteristics, and other factors such as the interstitial P concentration. Thelatter will probably be influenced by the wetland water:sediment ratio, which mayvary spatially, due to differences in burial rates and sediment characteristics andtemporally due to seasonal fluctuations in factors such as amounts of rainfall andeffluent discharge (Nichols, 1983; Cooke et al., 1992; Masscheleyn et al., 1992). Itis therefore important to investigate the effect of changes in this ratio on the releaseof P from sediments.

PHOSPHORUS RETENTION AND RELEASE IN WETLAND SEDIMENTS 165

Figure 1. Map of the wetland system showing sampling sites of sediments in relation to the point ofeffluent discharge.

2. Materials and Methods

2.1. EXPERIMENTAL SITE

The wetland system is located within the Waitangi forest, Northland, New Zealand(Figure 1). It receives treated sewage effluent (oxidation pond) from the resort townof Paihia and the water from this sewage-impacted wetland is merged with waterfrom the first and second arms of natural wetland at sites C and F (confluence sites).The sites below these confluences are sites D and G respectively (postconfluencesites).

The sewage-impacted wetland water has a marked gradient in terms of pHand dissolved reactive P (DRP) along the wetland flowpath (pH of 8.0 at site A,6.8–7.1 at site D and 6.0–6.5 at site G and DRP of 4–8 mg L�1 at the point ofeffluent discharge, 2–5 mg L�1 at site D and 0.2–1.5 mg L�1 at site G). Overall,the sewage-impacted wetland water is more alkaline and has a higher DRP contentthan the natural wetland water (pH and DRP in the natural wetland water were5.5–6.0 and 0.02 mg L�1 respectively). However,the concentration of P-retaining(Fe) reactants in natural wetland water (total Fe of 395 g m�3) was higher (2–4orders of magnitude) than that in sewage-impacted wetland water (0.48 g m�3) orin oxidation pond effluent (0.02 g m�3).

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2.2. COLLECTION OF WETLAND SEDIMENT AND EXPERIMENTAL PROCEDURE

Sediment samples were collected from triplicate sediment traps at Sites A toG in summer 1991. Each trap consisted of an outer PVC cylinder (100 cm by10 cm diam.) which housed an inner cylinder (8 cm diam.). The rationale behindthe sediment trap design and installation has been described by Cooke (1992).Collected sediments were filtered (glass fibre GF/A) to remove excess water. Theywere then dispersed in distilled water containing 0.67% sodium oxalate using apipette analysis technique (Day, 1965) to separate the sediments into differentparticle size fractions of 32–64, 16–32, 8–16, 4–8, 2–4 and <2 �m. Subsamplesof these particle sizes were analysed for total P using the method of Twine andWilliams (1971).

The release of P from collected sediments that were bulked on a site basiswas investigated by equilibrating duplicate samples with distilled water containingCaSO4.2H20, NaCl and KCl at 0.0859, 0.1271 and 0.0229 g L�1 respectively at awater:sediment ratio of 40:1, 100:1, 400:1 and 1000:1 in polypropylene tubes on anend-over-end shaker at 25 �C for 5, 30, 60 and 180 min under aerobic and anaerobicconditions. The aerobic equilibration experiment was conducted for sedimentstaken from all sites (Sites A to G), while the anaerobic experiment was carriedout only for sites A, C and F, which were selected to represent the non-confluencesite, the confluence at the first natural wetland and the confluence at the secondnatural wetland, respectively. Aerobic and anaerobic conditions were maintainedby continuous purging with compressed air or oxygen-free N2 respectively, thelatter in a gas glove box. After equilibration, both aerobic and anaerobic sediment-water samples were centrifuged at 3000 rpm for 10 min. The supernatant was thenfiltered through a 0.45 �m membrane and analysed for soluble P using the Murphyand Riley method (1962). All sample preparation, shaking and filtering of theanaerobic samples was performed under a N2 atmosphere to avoid the oxidation ofFe2+ to Fe3+, which could subsequently alter the solubility of sediment P (Rapinet al., 1986).

An estimate of the maximum P available for desorption was made by shakingsediment samples in 0.5 M NaHCO3 (pH = 8.5) at a solution: sediment ratio of50:1 for 30 min (Olsen and Sommers, 1982). The amount of potentially-desorbedP in filtered NaHCO3 extracts was determined by the molybdenum blue method ofMurphy and Riley (1962).

2.3. STATISTICAL ANALYSES

Log-log relationships between P released and equilibration period and between Preleased and water:sediment ratios were carried out to assess whether P released(dependent variable) has a power function relationship with equilibration period andwater:sediment ratios (i.e., independent variables). If a linear log-log relationshipexists between these dependent and independent variables, P released is considered

PHOSPHORUS RETENTION AND RELEASE IN WETLAND SEDIMENTS 167

to be a power function (i.e., proportional to some power) of equilibration periodand water:sediment ratios.

Analysis of covariance between the logarithm of P released (dependent vari-able) and factors (independent variables) affecting this release (e.g., the site fromwhich sediment was collected, the aerobic-anaerobic condition of sediments andlogarithms of water:sediment ratio and of equilibrium period) was conducted toestablish the relative importance and interactions of these independent variables onP released using the F-test and probability (p) values.

3. Results and Discussion

3.1. PARTICLE SIZE DISTRIBUTION AND PHOSPHORUS RETENTION IN SEDIMENTS

Approximately 60–90% of the total sediment collected at each site, particularly atthe confluence(Sites C and F) and postconfluence sites (Sites D and G) is in 32–64 �m fraction (Figure 2; Data for sites E and G are not presented since they aresimilar to those obtained for sites A and D respectively). The higher accumulationof this large fraction at both the confluence and postconfluence sites comparedwith other sites (e.g. Sites A and B) is attributed to a greater deposition rate ofparticulate material arising from reactions between the sewage-impacted wetlandwater and the natural wetland water at the confluence and postconfluence sites.

Although sediments with particle sizes of 32–64 �m occupy a major proportionof the total sediment mass (Figure 2), most of the P deposited in sediments is infiner fractions (particularly those <16 �m; Figure 3). This indicates that the fineparticle sizes have a higher surface area for P retention than the larger fraction (32–64 �m). Other studies (Hwang et al., 1976; Sharpley, 1980; Cooke, 1988) reportedsimilar findings in which the surface area of a constant mass of sediment increaseswith decreasing particle sizes and the extent of P retention is directly proportionalto the surface area.

Since fine particles are likely to be subject to resuspension and preferentiallytransported along the wetland flowpath (Syers et al., 1973; Richardson and Craft,1993), any change in wetland management or environmental factor (e.g., removalof wetland vegetation or changes in rainfall amount and intensity), that affects waterflow and hence the transport or burial rate of fine particles, is likely to influenceP release and retention characteristics of sediments at different sites along thewetland system.

Total P concentration in sediments, particularly in particle size fractions of<16 �m tends to be higher at sites that are further away from the effluent outfall(Figure 3; Data obtained for site E are not presented because this site has similardata to site B). Over the studied range of particle sizes, sediments from confluenceand postconfluence sites at the second arm of the natural wetland (Sites F and G)had a higher P concentration than those at the first arm of natural wetland (Sites Cand D) or those from sites (Sites A, B and E) along the wetland flowpath.

168 L. M. NGUYEN ET AL.

Figure 2. Cumulative percentage of particle size distribution of sediments collected from differentsites. Bars indicate S.E. (n = 3) for comparing differences between sites.

Figure 3. Phosphorus concentration in various particle size fractions of sediments collected fromdifferent sites. Bars indicate S.E. (n = 3) for comparing differences between sites.

PHOSPHORUS RETENTION AND RELEASE IN WETLAND SEDIMENTS 169

We attribute this increased P enrichment of sediments at sites F and G (secondconfluence) to a greater mass flow of P-adsorbing (Fe) reactants from the largersecond arm of natural wetland to these sites. Under the prevailing summer droughtconditions, when these sediments were collected, Cooke et al. (1992) showed thatthe first natural wetland did not contribute a significant water input (and henceFe supply) to the sewage-impacted wetland, whereas the larger second arm ofnatural wetland continued to flow. As Fe has been known to play a major role in Psorption/retention in sediments through the adsorption-precipitation reactions of Fewith soluble P from wastewater (Nichols, 1983; Cooke, 1992; Reddy and Reddy,1993), the Fe inputs from the natural wetland would provide loci for P deposition.The extent of this deposition would therefore be dependent on the supply of Fefrom the natural wetland, which in turn is influenced by the hydrologic conditionsthat affect water flow and water input to the sewage-impacted wetland.

3.2. PHOSPHORUS RELEASE CHARACTERISTIC OF SEDIMENTS

3.2.1. Site EffectsPhosphorus released from most of the studied sites was a power function of theequilibration period, since a linear log–log relationship exists between P releasedand equilibration period at any given water:sediment ratio (e.g., at a water:sedimentratio of 1000:1; Figure 4). The amount of P released from the confluence (SitesC and F) and postconfluence sites (Sites D and G) was found to be higher thanthat from other sites (Sites A, B, and E) over a range of equilibration periodsand water:sediment ratios under an aerobic condition (representative data obtainedat 1000:1 water:sediment ratio are shown in Figure 4). Similar findings (e.g.,maximum P released from anaerobic A, C and F sediment suspensions of 15, 459and 150 �g g�1 respectively; Table I) were also observed for sediments underanaerobic conditions (only sediments from sites A, C and F were investigated).

We attribute the higher amount of P released from the confluence (Sites C andF) and postconfluence sites (Sites D and G) under aerobic conditions to a higheramount of P deposition in sediments collected from these sites (Figure 3). Similarly,the higher P release at the confluence sites (Sites C and F) compared with that atsite A (Table I) under anaerobic conditions is consistent with the pattern of Pdeposition.

Phosphorus release characteristics of sediments may be influenced not only bythe amount of P deposition in these sediments as suggested above, but also by theirP retention capacity. Sediments rich in Fe have been found to have a high P retentioncapacity and they function as an effective sink for P from overlying water (Reddyand Reddy, 1993). Thus, the lower amounts of P released from the postconfluencesites (Sites D and G), compared with those from the confluence sites (Sites Cand F) over most of the range of equilibration period (Figure 4) are probably dueto the stronger P retention capacity of sediments at these postconfluence sites,which contain higher amounts of Fe. Furthermore, a higher proportion of Fe–P

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Figure 4. Relationship (normal and log scales) between phosphorus released and equilibration periodunder aerobic conditions at a water:sediment ratio of 1000:1 for sediments collected from differentsites. Straight lines on the log scale graphs are power curve fits.

PHOSPHORUS RETENTION AND RELEASE IN WETLAND SEDIMENTS 171

Table IMaximum amount of phosphorus released to water from aerobic and anaerobic sedi-ments expressed as concentration and as proportion of NaHCO3-extractable phospho-rus for sediments collected from different sites

Maximum P released at 1000:1Aerobic/ water:sediment ratio and after NaHCO3-anaerobic Site 180 min of equilibration extractable P

(�g g�1) (% NaHCO3-extractable P) (�g g�1)

Aerobic A 22 8.4 266B 28 3.1 909C 579 36.5 1585D 197 13.4 1463E 80 7.4 1079F 340 20.4 1667G 218 14.6 1492

Anaerobic A 15 5.6 266C 459 29.0 1585F 150 9.0 1667

in sediments at the postconfluence sites was found in the occluded (ferric) form(Cooke et al., 1992). According to Patrick and Khalid (1974), ferric (Fe3+) formof Fe–P is capable of binding soluble P more firmly than ferrous (Fe2+) form.Thus sediments with a higher proportion of Fe3+ form are likely to release loweramounts of P than those with a higher proportion of Fe2+ form (assuming that thesesediments have the same total P concentration). Results obtained from this studyare consistent with those reported in the literature in indicating that not only theamount of Fe but also the Fe chemical form play a major role in P retention-releasecharacteristics (Masscheleyn et al., 1992; Reddy and Reddy, 1993; Richardson andCraft, 1993).

The amount of P released from sediments at all sites was found to reflect theamount of potentially-desorbable P in these sediments as extracted by NaHCO3

solution (NaHCO3-extractable P). However, the amount of P released even after180 min at a water:sediment ratio of 1000:1 accounts for less than 37% ofpotentially-desorbable P (20.4–36.5%, 13.4–14.6% and 3.1–7.4% for confluences,postconfluences and other sites (Sites A, B and C respectively; Table I). It isunlikely that P release of this magnitude would occur under the relatively stablehydrological regime present in this wetland system since events that generate thesame degree of turbulence at the sediment-water interface as that occurring underlaboratory conditions, are rare (Cooke et al., 1990).

172 L. M. NGUYEN ET AL.

Figure 5. Relationship (normal and log) between phosphorus released and equilibration period underaerobic (open symbol) and anaerobic (solid symbol) conditions over a wide range of water:sedimentratios (# , ; �,�; �,� and 4,N for 40:1, 100:1; 400:1 and 1000:1 respectively) as a function oftime for sediments collected from sites A, C and F. Straight lines on the log scale graphs are powercurve fits.

PHOSPHORUS RETENTION AND RELEASE IN WETLAND SEDIMENTS 173

3.2.2. Aerobic and Anaerobic EffectsPhosphorus released from sediments tended to be higher under aerobic than anaer-obic conditions over a range of equilibration periods and water:sediment ratios(only sediments from sites A, C, and F were equilibrated under both aerobic andanaerobic conditions; Figure 5). This is consistent with that observed in otherstudies (Patrick and Khalid, 1974; Krairapanond et al., 1993; Reddy and Reddy,1993) in which aerobic sediment-water suspensions released much more P intothe solution containing relatively high P concentrations (1.8–5.3 mg L�1) than didthe anaerobic suspensions. Since such high concentration (1.5–8 mg L�1) existsat the sites collected for this laboratory experiment, it is not surprising that initialP release from the sediments of these sites is greater under aerobic than anaero-bic conditions. The higher P release under aerobic conditions is attributed to theincrease in crystalline Fe3+ that has lower surface areas for P adsorption thanamorphous Fe2+ (e.g., Pattrick and Khalid, 1974; Reddy and Reddy, 1993; Palu-dan and Jensen, 1995). Furthermore, it is known that when a reduced sediment(freshly-collected sediment) is exposed to aerobic conditions, Fe2+ is oxidisedto Fe3+ (Mitsch and Gosselink, 1986). Thus, P that has been retained by Fe2+

in ferrous hydroxide (Fe(OH)2)-phosphate compounds is likely to be released intosuspension and hence aerobic sediment may initially release more P than anaerobicsediment (Patrick and Khalid, 1974; Reddy and Reddy, 1993).

For freshly-collected sediments that remained in an anaerobic condition aftercollection from the field, P in Fe2+ form (ferrous oxide) which is water-soluble,may be readily released into solution with nil or low added P. However, this releasemay be negated to some extent by the retention of soluble P into Fe2+ compounds,particularly when the water:sediment suspension contains a high soluble P con-centration (Patrick and Khalid, 1974; Reddy and Reddy, 1993). Recently, De Best(1992) and Paludan and Jensen (1995) have shown that reduction of iron fromFe3+ (ferric oxyhydroxide) to Fe2+ (ferrous oxide) is not necessarily complete inanaerobic sediments because this reduction process is impaired by the adsorptionof P on organic matter and Fe3+ (especially at high P:Fe ratios). It is thereforelikely that during the course of equilibrating anaerobic sediment with distilledwater containing no added P, P in the form of ferrous oxide could be continuouslyreleased into the sediment-water suspension.

The above complexity of different processes that may exist in the release of Pfrom sediments containing high concentrations of P reactants (e.g., Fe compounds)serves to emphasise the need for subsequent study with isotopic tracers (32P) toinvestigate the transformation of different P fractions in sediments, and the releaseof native P from sediments with and without added P under aerobic and anaerobicconditions.

3.2.3. Effects of Equilibration PeriodPhosphorus released from aerobic and anaerobic sediments was found to increasewith an increase in equilibration period from 5 min to 180 min over a range of

174 L. M. NGUYEN ET AL.

Table IISlope values for the regression of logarithm of phosphorus released under aerobic andanaerobic conditions against logarithm of equilibration period at different water:sedimentratios for sediments collected from different sites

Aerobic/ Site Water sediment ratio

anaerobic 40:1 100:1 400:1 1000:1

Aerobic A 0.175 (0.05)a 0.185 (0.02) 0.172 (0.13) 0.333 (0.01)B 0.054 (0.74) 0.157 (0.23) 0.176 (0.05) 0.002 (0.96)C 0.237 (0.02) 0.195 (0.05) 0.259 (0.01) 0.274 (0.0001)D 0.255 (0.02) 0.464 (0.02) 0.306 (0.16) 0.239 (0.03)E 0.381 (0.03) 0.305 (0.03) 0.135 (0.37) 0.145 (0.05)F 0.367 (0.02) 0.385 (0.004) 0.329 (0.007) 0.226 (0.007)G 0.132 (0.54) 0.139 (0.12) 0.085 (0.49) 0.345 (0.13)

Anaerobic A 0.064 (0.25) 0.160 (0.19) 0.125 (0.04) 0.183 (0.03)C 0.360 (0.07) 0.164 (0.12) 0.230 (0.10) 0.249 (0.0007)F 0.214 (0.07) 0.127 (0.16) 0.184 (0.05) 0.279 (0.02)

a Number in parentheses is p-value of t test on slope, where the null hypothesis is that theslope is zero.

water:sediment ratios (Figures 4 and 5). We consider that the generally signif-icant log–log relationship between P released from sediments and equilibrationperiod at a given value of water:sediment ratio (Table II) reflects an increase in Pexchange/diffusion from sediments to water as a result of an increase in the amountof time that the water is in contact with sediments (Sharpley et al., 1981; Nichols,1983; Gale et al., 1994). However, the variability in the slope values of this log–logrelationship (Table II) indicates that there is no consistent pattern between differentsites in the increase in P released from sediments associated with an increase inequilibration period. As discussed earlier (Sections 3.2.1 and 3.2.2), we attributethis inconsistent pattern to the variability in the amounts and chemical forms ofFe in sediments at these sites, which in turn govern their P retention and releasecharacteristics.

The amount of P release at any given equilibrium period tends to be low-er in anaerobic than aerobic sediment suspensions over most of the range ofwater:sediment ratios (Figure 5). Although, this trend may not occur under somestudied experimental conditions (e.g., at a water : sediment ratio of 400:1 for siteC with an increase in equilibration period from 60 to 180 min; Figure 5), the totalamount of P released from all sediments, even after 180 min of equilibration, isstill lower under anaerobic conditions. This is due to the generally lower amountsof initial P release (after 5 min of equilibration) under anaerobic compared withaerobic conditions (Figure 5).

PHOSPHORUS RETENTION AND RELEASE IN WETLAND SEDIMENTS 175

Figure 6. Relationship (normal and log) between phosphorus released and water:sediment ratiosunder aerobic conditions after 180 minute of equilibration for sediments collected from differentsites. Straight lines on the log scale graphs are power curve fits.

176 L. M. NGUYEN ET AL.

Table IIISlope values for the regression of logarithm phosphorus released under aerobic and anaer-obic conditions against logarithm water:sediment ratios over a range of equilibrium periodfor sediments collected from different sites

Aerobic/ Site Equilibration period (minute)

anaerobic 5 30 60 180

Aerobic A 0.300 (0.07)a 0.298 (0.01) 0.430 (0.007) 0.425 (0.004)B 0.337 (0.08) 0.252 (0.04) 0.427 (0.004) 0.245 (0.02)C 0.310 (0.03) 0.303 (0.07) 0.267 (0.10) 0.409 (0.02)D 0.516 (0.05) 0.402 (0.07) 0.407 (0.07) 0.446 (0.02)E 0.548 (0.02) 0.224 (0.28) 0.365 (0.06) 0.233 (0.24)F 0.323 (0.34) 0.189 (0.07) 0.140 (0.03) 0.186 (0.003)G 0.374 (0.03) 0.504 (0.04) 0.647 (0.06) 0.509 (0.02)

Anaerobic A 0.606 (0.001) 0.648 (0.01) 0.697 (0.008) 0.597 (0.04)C 0.474 (0.04) 0.540 (0.009) 0.452 (0.02) 0.397 (0.01)F 0.527 (0.007) 0.606 (0.0001) 0.681 (0.01) 0.532 (0.003)

a Number in parentheses is p-value of t test on slope, where the null hypothesis is that theslope is zero.

3.2.4. Effects of Water Sediment RatioPhosphorus released from sediments tended to increase with an increase in thewater:sediment ratio (Figure 6) and in most cases this P release is a power functionof the water:sediment ratio (Table III). Only representative data on P released fromaerobic sediments are presented in Figure 6, since similar data from anaerobicsediments can be derived from Figure 5.

We consider that the increase in P release with increasing water:sediment ratiois due to an increase in P diffusion/desorption from sediment to overlying water(Masscheleyn et al., 1992; Gale et al., 1994; Reddy et al., 1995). This is in responseto a reduction (by dilution) in soluble P concentration in water:sediment suspensionas a result of an increase in the amount of water in this suspension (Nichols, 1983;Reddy et al., 1995).

The variability in the slope values of the log–log relationship between P releasedand water:sediment ratio (Table III) suggests that no consistent pattern existsbetween different sites in the increase in P release as a result of an increase inwater:sediment ratio. The effect of water:sediment ratio on P released from sedi-ments must therefore be accounted for in modelling net P removal in this wetlandsystem, where different sites along the wetland flowpath may behave different-ly to seasonal changes in water flow. It is however important to note that theresults obtained from this fully mixed and shaken experiment may not resem-ble those occurring in the field, where a similar magnitude of turbulence at thesediment-water interface is unlikely to occur, and a change in water velocity overa sediment surface may cause a change in the water:sediment ratio (Masscheleyn

PHOSPHORUS RETENTION AND RELEASE IN WETLAND SEDIMENTS 177

Table IVAnalysis of covariance between logarithm phosphorus releasedand different independent variables and their interactions

Source F-ratio p-value

Site (S) 3770 �0.0001Logarithm of water:sediment ratio (R) 973 �0.0001Aerobic/anaerobic (O) 529 �0.0001Logarithm of equilibrium period (P) 280 �0.0001Interactions:

S�O 103 �0.0001R�O 92 �0.0001P�O 9.5 0.0029S�R 6.3 0.0029S�P 5.7 0.0047S�R�O 6.0 0.0038

et al., 1992; Mitsch et al., 1995). Under these field conditions, diffusion across thesediment-overlying water interface may become an important process in controllingP released from sediments (Masscheleyn et al., 1992; Gale et al., 1994).

Higher slope values (between log of P released and log of water:sediment ratio)under anaerobic than under aerobic conditions over a whole range of equilibra-tion periods (except at 180 min for site C; Table III), suggest that changes inwater:sediment ratios are more likely to affect P released from sediments underanaerobic than under aerobic conditions. The increase in water:sediment ratio,which results in a subsequent decrease in soluble P and ferrous (Fe2+) concentra-tions in suspension, may enhance the solubility of ferrous-phosphate compoundsand hence the release of P from sediments. In contrast, in aerobic sediments thepredominant ferric (Fe3+)-phosphate compound is not water-soluble (e.g., Mass-cheleyn et al., 1992; Richardson and Craft, 1993) and so does not readily release Pinto suspensions, even when the concentration of P in suspensions is decreased byan increase in the water:sediment ratio.

3.2.5. Relative Significance of Studied Effects on Phosphorus ReleaseThe amount of P released from sediments is known to be governed by factors suchas P concentration in overlying water and the oxidation-reduction status in wetlandsoils (Nichols, 1983; Masscheleyn et al., 1992; Reddy and Reddy, 1993; Gale etal., 1994; Mitsch et al., 1995). However, the relative importance of these factors, islikely to vary from one wetland to another (e.g., Masscheleyn et al., 1992; Mitschet al., 1995). In this New Zealand wetland system, results obtained (Table IV) indi-cate that P released is most affected by the site from which sediment is collected.We believe that the importance of this variable (site) is due to the wetland configu-ration, which influences the supply and chemical form of Fe reactants from natural

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wetlands to different sites along the sewage-impacted wetland. At any given site,water:sediment ratio influences P released from sediments more significantly thanaerobic-anaerobic conditions of water:sediment suspensions (Table IV). Signifi-cant interaction effects exist between different factors (Table IV) on the amount ofP released from wetland sediments (particularly between aerobic-anaerobic condi-tions of wetland sediments and the position of sites within the wetland, or betweenaerobic-anaerobic conditions and water:sediment ratio). These interactions sug-gest that the sustainability of this wetland in removing P from wastewater dependsupon inter-related factors that influence the P adsorption-desorption capacity of thesediment and P diffusion from sediment to overlying water. These factors includethe supply of P-retaining reactants (e.g., Fe compounds), the concentration of P insolution, the amount of time that the overlying water is in contact with sedimentsand the reduction-oxidation processes of Fe compounds in response to changes inthe redox potential (aerobic-anaerobic conditions).

4. Conclusions

Wetland configuration and factors (e.g., hydrologic conditions) that affect the sup-ply of P-retaining Fe reactant from the natural wetlands to the sewage-impactedwetland, and the chemical form of this reactant determined the spatial variation inP deposition and the extent of P retention and release characteristics of wetlandsediments.

Phosphorus released from sediments tended to be a power function of thewater:sediment ratio and equilibration period and was most affected by the posi-tion of sites within the wetland. Water:sediment ratio influenced P released fromsediments more significantly than aerobic-anaerobic conditions of water:sedimentsuspensions. Anaerobic conditions may lead to a lower P release than aerobic con-ditions because of a possible increase in amorphous and poorly crystalline Fe2+

compounds which have high active P-sorbing surface areas. Further research withan isotopic label (e.g. 32P) is required to improve understanding of the release of Pfrom sediments under the influence of added P and variations in redox conditionsor water:sediment ratios in this wetland system.

Acknowledgements

Kerry Costley for both field and laboratory assistance and two anonymous refereesfor comments that greatly improved the quality of this manuscript.

PHOSPHORUS RETENTION AND RELEASE IN WETLAND SEDIMENTS 179

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