Environ. Sci. Techno/. 1995, 29, 187-192
Impact of Sediment-Bmd Iron on Redox Buffering in a Landfill leachate Polluted Aquifer (Vejen, Denmark) GORM HERON* AND T H O M A S H . CHRISTENSEN Institute of Environmental Science a n d Engineering1 Groundwater Research Centre, Technical University of Denmark, Building 115, DK-2800 Lyngby, Denmark
Sediments sampled along a central flow line of the leachate pollution plume at the Vejen Landfill, Denmark, were characterized in detail with respect to the forms and pools of Fe(ll) and Fe(lll). After 15 yr of leaching, redox reactions had diminished the pool of iron(ll1) oxides and hydroxides in the reduced zones close to the landfill, and the aquifer oxidation capacity (OXC) related to iron oxides was depleted. Less than 2% of the total Fe(ll) was recovered as dissolved Fe(ll), whereas 1-20% was ion-exchangeable on the sediments. The majority of the Fe(ll) was in the solid state either as pyrite or in the ill-defined fraction extractable by 5 M HCI. The total reduction capacities (TRC) of these anaerobic sediments were significantly elevated relative t o the unpolluted sediments. Samples from the oxidized, weakly polluted part of the plume contained Fe(lll) minerals and insignificant amounts of Fe(ll). This study presents evidence of substantial iron reduction buffering the reducing power of landfill leachate entering a shallow aquifer. It is also proposed that reduced sedimentbound iron species form in the plume, thereby increasing the need for oxygen if the aquifer was to be remediated.
Introduction A possible environmentalimpact of landfills is the pollution of groundwater by landfill leachate. Dramatic changes in aquifer geochemistryand microbiology caused by leachate pollution have been reported in a number of cases as reviewed by Christensen et al. ( 1 ) . The introduction of organic matter and reduced inorganic species such as methane, ammonium, hydrogen sulfide, and dissolved iron into an aerobic aquifer leads to redox buffering reactions as discussed by Barcelona and Holm (2) and Heron et al. (3).A series of redox zones typically evolves in the affected aquifer ( 4 , s controlled by a complex set of geochemical reactions involving-in addition to the redoxprocesses-ionexchange, dissolution/precipitation, and sorption reactions. Lately, the important role of sediment associated iron species in these reactions has been pointed out (1-3). In shallow, uncontaminated aquifers, iron primarily occurs as iron(II1) oxides and hydroxides with varying crystallinity and structure (6'). Some Fe(II1) may also be present as part of the clay minerals, but at most relevant pH values no Fe(II1) is found in solution. Reduction of iron(II1)oxides in leachate-affected aquifers was suggested by refs 7-9 and demonstrated by refs 4 and 5. Thus, Fe(II1) associated with the aquifer sediment may be an important redox buffer against reduced organic and inorganic species found in leachate and may limit their migration and the extent of the anaerobic part of the contaminant plume. Once reduced to Fe(II), Fe(I1) may remain dissolved in the groundwater,may be ion-exchangedonto the sediment, or may precipitate, e.g., as ferrous sulfide or ferrous carbonate. The actual distribution of Fe(I1) will be controlled by the groundwater chemistry and the sediment type. If significant amounts of reduced species precipitate, this will limit the migration of the reduced iron species and hence limit the extent of the anaerobic part of the plume. The precipitates will also increase the reduction capacity of the sediment, and if remediation of the plume by the addition of oxygen is considered, this will dramatically increase the demand for oxygen (2). Characterization of the reactive pools of iron in soils and sediments has been addressed by various techniques (see reviews in refs 10-14). The applicability of the traditional methods for determiningFe(I1)and Fe(II1)forms and pools in unpolluted and polluted aquifer sediments were tested and discussed by Heron et al. (15). In this paper, some of these methods have been selected and used on 60 aquifer sediment samples from the leachate plume at the Vejen Landfill (Denmark) in order to demonstrate for the first time the importance of iron as a redox buffer in an actual landfill leachate plume. The presence of an iron-reducing zone in the Vejen Landfill leachate plume has previously been shown in terms of elevated concentrations of dissolved iron in the groundwater (4). Also the potential and presence of bacterially mediated Fe(II1) reduction in this plume has previously been demonstrated in a few selected sediment samples (16).
Methodology sediment Location and Sampling. Fresh sediment samples were collected in the leachate pollution plume downgradient of the municipalvejen Landfill in Denmark (4,17,
0013-936)(/95/0929-0187$09.00/0 0 1994 American Chemical Society
VOL. 29, NO. 1, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Distance from landfill [m] 100 200 300
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FIGURE 1. Redox zones identified from groundwater samples (from ref 4) and the amount of dissolved Fe from nearby driven wells and total Fe on the sediments extracted by 5 M HCI (excluding pyrite Fe)along the central flow line in the Vejen Landfill leachate plume.
18). The plume is located in a sandy glaciofluvial aquifer consisting of reddish-grey medium to coarse-grainedsand (19). Thewatertableislocated 1-3mbelowsurface. During approximately15yr,landfillleachaterichinorganicmatter and reduced inorganic species has entered the groundwater. Previously, a series of redox zones has been identified in the originally aerobic aquifer (Figure 1)based on analysis of water samples for a number of redox-sensitive species (4). For the current study, sediment sampleswere collected fromacentralflowline inasandylayerspanningtheentire redox sequence. Twenty sediment cores were sampled (anaerobically)from between 4 and 7 m below surface using a Waterloo piston sampler (20). The cores were sealed immediately in the field by stoppers and WC tape and storedattemperaturesbetweenlOand 15”Cforamaximum of 7 days. From each core (total length 1.5 m), three 20-cm sections were transferred to an anaerobic glovebox (Coy Laboratory Products Inc.). Inside the glovebox, the sediment subsampleswere transferred to gas-tight glass bottles 188 m ENVIRONMENTAL SCIENCE &TECHNOLOGY IVOL. 29. NO. 1.1995
and thenstored anaerobically at 10 “Cformaximally4 weeks prior to analysis. Sedimenthalysis. me methods described and tested by Heron et al. (15) were used. Briefly, all wet extraction techniques involved the mixing of sediment with the extractant followed by analysis of the extract. The extracts were centrifuged in order to remove particles greater than 0.25 pm (calculated from ref 21). The concentration of Fe(11) in extracts was determined using ferrozine in the presence of an acetate buffer at pH 5 (22). The total amount of Fe in the extract was determined by atomic absorption spectrophotometry(AAS). AU reportedvaluesare the means of five replicate extractions except the total reduction capacity (TRC; three replicates). Ion-EXchangeableFe(I0. The amount of Fe(I1)soluble in 1 M CaClz at pH 7.0 was determined by an anaerobic 24-h extraction at 20 “C followed by Fe(I1) quantification (15). Referring to a saturated aquifer volume, the contribution of Fe(1I) from the sediment-associated porewater was less than 10% of the Fe(l1) recovered by the CaClz extraction for all samples, assuming that water samples from nearby driven wells represent the porewater in the sediment sample. Only the ion-exchangeable Fe(I1) was extracted by this method, since no dissolution of other mineral phases was observed (15). Fe(I1) and Fe(II1) soluble in 0.5 M HC1 are operationally defined. mese were determined using a 24-h 0.5 M HCI extraction at 20 “C (15). Fe(lI1) was calculated as total Fe minus Fe(IO measured in the same extract. Note that 0.5 M HCI does not dissolve a specific mineral fraction. IonexchangeabieFe(II), ferrous monosulfides, and amorphous iron oxides are attacked by 0.5 M HCI. In addition, part of thesideriteandasmallfractionofthe crystallineironoxide minerals will dissolve (15). Since the dissolved Fe(I1) and Fe(II1) are stable at the low pH, this extraction gives an indication of the redox state of the noncrystalline iron in the sediment. Fe(I1) and Fe(lI1) Soluble in 5 M HCI. These were determined using a21-day 5 M HCI extraction at 20 “C (15). Fe(II1) was calculated as total Fe minus Fe(I1) measured in the same extract. After 21 days, the Fe concentration had stabilized, indicating that no more Fe would dissolve in 5 M HCI at this temperature. In addition to the species extracted by0.5 M HCi, this extraction completely dissolves crystalline iron oxides, siderite, and magnetite (15). Fe(I1) and Fe(II1) bound to clays and silicates will be partly dissolved (15). OddatJon Capacity and Ti(II1)-EDTA Emaetable Fe. The oxidation capacity (OXC, determined as pequiv/g) related to iron oxides and hydroxides in the sediment was determined by a 0.008 M Ti(III)-0.05 M EDTA extraction followed by redox titration with dichromate and determination of the extracted amount of Fe (determined as mg/g or pmol/g) by AAS (3). Total reduction capacity (TRC) corresponding to the chemical oxygen demand was determined by the method (Crz072-oxidation) proposed by Pedenen et al. (23). By varying the concentration of reactant and the sample sue, reduction capacities between 10 and 600 yequivlg could be determined. AU of the Fe(II),reduced sulfur forms, and organic matter are oxidized by dichromate (23). Groundwater Sampling and Analysis. Groundwater samples were collected from driven wells (10-cm screens) using the procedures previously published by Lyngkilde and Christensen (4). The groundwater migrated into the
well void and was then forced to the ground through a Teflon tube by a pressure of nitrogen. Dissolved iron was determined by AAS on filtered (0.1 pm) and acidified samples.
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Results and Discussion
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Redox Sequence and Bull< Analysis of Groundwater and Sediment. On the basis of the detailed analysis of groundwater samples from a vertical transect along the contaminant plume, Lynglulde and Christensen (4)mapped the governing redox environments. The assignment of the different zones is site-specific and based on a number of criteria. At the Vejen Landfill site, the zones of Fe(II1) reduction and Mn(IV) reduction could not be separated. Fe(II1)-reducing samples were defined by a minimum of 1.5 mg/L of dissolved Fe along with insignificant contents of oxygen ( < 1 mg/L), nitrate ('0.2 mg NIL), nitrite ( 1 mg/L) and less than 40 mg/L of sulfate. Sulfate-reducing samples contained elevated dissolved sulfide ('0.2 mg/L) and low concentrations of methane ( < 1 mg/L). By applying these criteria to groundwater samples from wells located with 40-m intervals, a large zone of FelMn reduction was found (Figure1). Alongthe selectedflowline, theFe/Mn-reducing zone covered the interval from approximately 75 to 300 m downgradient of the landfill. The concentration of dissolved Fe was elevated in this section relative to low concentrations (
