Gorm Heron

Characterization of redox conditions in groundwater contaminant plumes.

Evaluation of redox conditions in groundwater pollution plumes is often a prerequisite for understanding the behaviour of the pollutants in the plume and for selecting remediation approaches. Measuring of redox conditions in pollution plumes is, however, a fairly recent issue and yet relative few cases have been reported. No standardised or generally accepted approach exists. Slow electrode kinetics and the common lack of internal equilibrium of redox processes in pollution plumes make, with a few exceptions, direct electrochemical measurement and rigorous interpretation of redox potentials dubious, if not erroneous. Several other approaches have been used in addressing redox conditions in pollution plumes: redox-sensitive compounds in groundwater samples, hydrogen concentrations in groundwater, concentrations of volatile fatty acids in groundwater, sediment characteristics and microbial tools, such as MPN counts, PLFA biomarkers and redox bioassays. This paper reviews the principles behind the different approaches, summarizes methods used and evaluates the approaches based on the experience from the reported applications.

Attenuation of landfill leachate pollutants in aquifers

Abstract Landfill leachate contains a variety of pollutants that may potentially contaminate the ground water and affect the quality of surface waters and well waters. The literature has been critically reviewed in order to assess the attenuation processes governing the contaminants in leachate‐affected aquifers. After an introductory section on leachate composition, the physical and chemical frameworks for the attenuation processes are discussed in terms of dilution/dispersion and redox zones in the plume, respectively. A separate section focuses on the microbiology in terms of the occurrence of bacteria in plumes, the fate of pathogens, and microbial mediation of redox processes. In individual sections, the attenuation of dissolved organic matter, anthropogenic‐specific organic compounds, inorganic macrocomponents as anions and cations, and heavy metals are discussed. The focus is on laboratory experiences and field investigations. The review shows that most leachate contamination plumes are relatively ...

Speciation of Fe(II) and Fe(III) in contaminated aquifer sediments using chemical extraction techniques

The iron mineralogy of aquifer sediments was described by chemical extraction techniques. Single-step extractions including 1 M CaCl 2 , NaAc, oxalate, dithionite, Ti(III)-EDTA, 0.5 M HCl, 5 M HCl, hot 6 M HCl, and a sequential extraction by HI and Cr II HCl were tested on standard iron minerals and nine aquifer sediments from different redox environments sampled in a landfill leachate plume. Ion-exchangeable Fe(II) is easily quantified by anaerobic CaCl 2 extraction. A rapid indication of the redox status of a sediment sample can be achieved by a 0.5 M HCl extraction. This extraction gives an indication of the content of amorphous Fe(III) and reduced Fe(II) species such as FeS and FeCO 3 , though the fractions are not quantified

Impact of Sediment-Bound Iron on Redox Buffering in a Landfill Leachate Polluted Aquifer (Vejen, 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(II) and Fe(III). After 15 yr of leaching, redox reactions had diminished the pool of iron(III) 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(II) was recovered as dissolved Fe(II), whereas 1-20% was ion-exchangeable on the sediments. The majority of the Fe(II) was in the solid state either as pyrite or in the ill-defined fraction extractable by 5 M HCl

Oxidation capacity of aquifer sediments

A laboratory extraction method (Ti 3+ -EDTA extraction) for the determination of the oxidation (electron accepting) capacity related to oxides and hydroxides of aquifer sediments was developed. At room temperature, titanous ions (0.008 M Ti 3+ ) in a solution of 0.05 M ethylenediaminetetraacetic acid (EDTA) reduced oxidized aquifer species. This operationally defined oxidation capacity (ORC) was determined as μmoles of electrons accepted per gram of sediment sample. Well-described oxidized iron and manganese minerals were reduced (ferrihydrite 98%, akageneite 100%, goethite and hematite 93%,magnetite 9%, pyrolmite 99%) whereas organic matter in the sediments was not reduced significantly

Anaerobic microbial redox processes in a landfill leachate contaminated aquifer (Grindsted, Denmark)

The distribution of anaerobic microbial redox processes was investigated along a 305 m long transect of a shallow landfill-leachate polluted aquifer. By unamended bioassays containing sediment and groundwater, 37 samples were investigated with respect to methane production, sulfate, iron, and manganese reduction, and denitrification. Methane production was restricted to the most reduced part of the plume with rates of 0.003–0.055 nmol CH4/g dry weight/day. Sulfate reduction was observed at rates of maximum 1.8 nmol SO42−/g dry weight/day along with methane production in the plume, but sulfate reduction was also observed further downgradient of the landfill. Iron reduction at rates of 5–19 nmol Fe(II)/g dry weight/day was observed in only a few samples, but this may be related to a high detection limit for the iron reducing bioassay. Manganese reduction at rates of maximum 2.4 nmol Mn(II)/g dry weight/day and denitrification at rates of 0.2–37 nmol N2O–N/g dry weight/day were observed in the less reduced part of the plume. All the redox processes were microbial processes. In many cases, several redox processes took place simultaneously, but in all samples one process dominated accounting for more than 70% of the equivalent carbon conversion. The bioassays showed that the redox zones in the plume identified from the groundwater composition (e.g. as methanogenic and sulfate reducing) locally hosted also other redox processes (e.g. iron reduction). This may have implications for the potential of the redox zone to degrade trace amounts of organic chemicals and suggests that unamended bioassays may be an important supplement to other approaches in characterizing the redox processes in an anaerobic plume.

In situ and laboratory studies on the fate of specific organic compounds in an anaerobic landfill leachate plume, 1. Experimental conditions and fate of phenolic compounds

The transformation of specific organic compounds was investigated by in situ and laboratory experiments in an anaerobic landfill leachate pollution plume at four different distances from the landfill. This paper presents the experimental conditions in the in situ microcosm and laboratory batch microcosm experiments performed and the results on the fate of 7 phenolic compounds. Part 2 of this series of papers, also published in this issue, presents the results on the fate of 8 aromatic compounds and 4 chlorinated aliphatic compounds. The redox conditions in the plume were characterized as methanogenic, Fe(III)-reducing and NO3−-reducing by the redox sensitive species present in groundwater and sediment and by bioassays. With a few exceptions the aquifer redox conditions were maintained throughout the experiments as monitored by redox sensitive species present in groundwater during the experiments, by redox sensitive species present in the sediment after the experiments and by bioassays performed after the experiments. Transformation of nitrophenol was very fast close to the landfill in strongly reducing conditions, while transformation was slower in the more oxidized part of the plume. Lag phases for the nitrophenols were short (maximum 10 days). Phenol was only transformed in the more distant part of the plume in experiments where NO3−, Fe(III) and Mn(IV) reduction was dominant. Lag phases for phenol were either absent or lasted up to 2 months. Dichlorophenols were only transformed in experiments representing strongly reducing, presumably methanogenic, redox conditions close to the landfill after lag phases of up to 3 months. Transformation of o-cresol was not observed in any of the experiments throughout the plume. Generally, there was good accordance between the results obtained by in situ and laboratory experiments, both concerning redox conditions and the fate of the phenolic compounds. However, for phenol and 2,4-dichlorophenol, transformation was observed in some in situ experiments but not in the corresponding laboratory experiments. In some experiments, this coul be explained by differences in the redox conditions developing during the experiments. Nitrophenols were apparently transformed abiotically in the most reduced part of the plume, at 2 m from the landfill.

Geology and sediment geochemistry of a landfill leachate contaminated aquifer (Grindsted, Denmark)

Abstract A landfill leachate affected aquifer was investigated with respect to the geology and sediment geochemistry (solid organic carbon, cation exchange capacity, oxidation capacity, reduced iron and sulfur species) involving 185 sediment samples taken along a 305-m-long and 10–12-m-deep transect downgradient from the landfill. The geology showed two distinct sand layers (upper Quaternary, Weichselian and a lower Tertiary, Miocene) sandwiching thin layers of silt/clay deposits, peat and brown coal. The organic carbon content (TOC) and the cation exchange capacity (CEC) of the sandy sediments were low (TOC, 100–300 μg C (g DW) −1 ; CEC, 0.1–0.5 meq per 100 g DW) and correlated fairly well with the geology. Processes in the contaminant plume caused depletion of oxidation capacity and precipitation of reduced iron and sulfur species. However, some of these parameters were also affected by the geology, e.g. the oxidation capacity (OXC) was significantly higher in the Quaternary layer (OXC, 14–35 μeq g DW −1 ) than in the Tertiary sand layer (OXC, −1 ). The intermediate layers (silt/clay and brown coal) have significantly higher values of most of the parameters investigated. This work demonstrates the need for a small scale geological model and a detailed mapping of the geochemistry of the sediments in order to distinguish impacts caused by the contaminant plumes from natural variations in the aquifer geochemistry.

Degradation of the herbicide mecoprop in an aerobic aquifer determined by laboratory batch studies

Abstract The potential of a shallow aerobic aquifer to degrade the herbicide Mecoprop (2-(2-methyl-4-chlorophenoxy)propionic acid) was evaluated in the laboratory using groundwater and sediment suspension batches. Mecoprop was added to the batches to obtain concentrations of 65, 140, 400 and 1400 μg/l. Mecoprop was degraded at 10°C at all concentration levels during the 200 days experimental period. Acclimation periods varied from 20 to 110 days. Initial degradation of approximately 50 % of the added Mecoprop was followed by a second lag period. In most batches, complete degradation was observed after the second lag period. The natural degradation potential for mecoprop varied among locations within a distance of few metres and was affected by different mecoprop concentrations. The observed stepwise degradation was supposedly due to different populations or different microbial mechanisms involved in the degradation of the two stereo-chemical forms of Mecoprop.

In-pile thermal desorption of PAHs, PCBs and dioxins/furans in soil and sediment

Current overall treatment costs for soil and sediment heavily contaminated with Semi-Volatile Organic Compounds (SVOCs) such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polychlorinated dibenzodioxins and furans (PCDD/Fs) can be as high as