J.J.W. Higgo

A study of the degradation of phenoxyacid herbicides at different sites in a limestone aquifer

The biodegradation of three phenoxyalkanoic acid herbicides, viz. MCPA, dichlorprop and mecoprop, under aerobic and anaerobic conditions was investigated using microcosm techniques. The field studies were conducted in a limestone aquifer that had suffered contamination from leaking landfill sites in which phenoxyalkanoic acid herbicides (mostly mecoprop) had undergone disposal. The results from in situ field and laboratory microcosms indicated that under microbially active aerobic conditions the biotransformation of all three herbicides was rapid and that lag phases were short. Under fully aerobic conditions the concentration of each of the three herbicides was reduced from 2,000 micrograms/l to below the detection limit (approx. 10 micrograms/l) of the HPLC system, used for their analysis, within 14 days. However, under microbially active anaerobic conditions no degradation of the herbicides could be discerned over the 100-200 day duration of the experiments. This finding has significant implications for the disposal of phenoxyalkanoic acid herbicides particularly in situations where any resulting leachate may find its way into underlying water resources.

Complexation of Co2+, Ni2+ , UO2+ and Ca2+ by Humic Substances in Groundwaters

Humic and fulvic acids were extracted from large volumes of three different groundwaters. After purification they were characterized in terms of their proton-binding properties, molecular weight, UV spectra and trace impurities. The Schubert ionexchange method was then used to measure the proportions of free and humic-bound metal in solution over a range of ionic strengths and pH. The experiments were carried out at trace metal concentrations with humic concentrations ranging from zero to >100 ppm. Intrinsic binding constants were calculated using Humic Ion-Binding Model V which was found to describe the dependence of binding strength on pH and ionic strength reasonably well. Species distributions in the native groundwaters were calculated and the effects of competition with the alkaline earth metals are discussed.

Colloid transport in a glacial sand aquifer. Laboratory and field studies

Abstract The migration of colloids through a well-characterised shallow sandy aquifer has been studied under controlled conditions of groundwater flow. Before carrying out field experiments a range of colloids of differing charge, size and shape were studied in laboratory columns packed with the aquifer sediment. The results were used to predict the outcome of the field experiments. For the field experiment, four colloids with different characteristics were selected. These included two polymeric colloids, namely fulvic acid extracted from groundwater (taken from the field site) and then radio-labelled with 125I (molecular weight approx. 1000, diameter approx. 1 nm), and sodium polystyrene-p-sulphonate radio-labelled with 125I (molecular weight 500 000, diameter approx. 20 nm). The two “particulate” colloids were silica particles (25 nm in diameter), detected by photon-correlation spectroscopy, and polystyrene fluorescent beads (100 nm in diameter), detected spectrofluorimetrically. The behaviour of these colloids differed markedly from one another, but in general, scaling up from column experiments to the field experiment was successful. The fact that colloids are mobile, and that it has been possible to predict their behaviour in the field from simple column experiments, has important implications in the field of contaminant transport.

Analysis of colloid and tracer breakthrough curves

Abstract We consider the dispersion and elution of colloids and dissolved nonsorbing tracers within saturated heterogeneous porous media. Since flow path geometry in natural systems is often ill-characterized macroscopic (mean) flow rates and dispersion tensors are utilized in order to account for the sub-model scale microscopic fluctuations in media structure (and the consequent hydrodynamic profile). Even for tracer migration and dispersal this issue is far from settled. Here we consider how colloid and tracer migration phenomena can be treated consistently. Theoretical calculations for model flow geometries yield two quantitative predictions for the transport of free (not yet captured) colloids with reference to a non-sorbing dissolved tracer within the same medium: the average migration velocity of the free colloids is higher than that of the tracer; and that the ratio of the equivalent hydrodynamic dispersion rates of colloids and tracer is dependent only upon properties of the colloids and the porous medium, it is independent of pathlengths and fluid flux, once length scales are large enough. The first of these is well known, since even in simple flow paths free colloids must stay more centre stream. The second, if validated suggests how solute and colloid dispersion may be dealt with consistently in macroscopic migration models. This is crucial since dispersion is usually ill-characterized and unaddressed by the experimental literature. In this paper we present evidence based upon an existing Drigg field injection test for the validity of these predictions. We show that starting from experimental data the fitted dispersion rates of both colloids and non-sorbing tracers increase with the measured elution rates (obeying slightly different rules for tracers and colloids); and that the ratio of colloid and nonsorbing tracer elution rates, and the ratio of colloid and nonsorbing tracer dispersion rates may be dependent upon properties of the colloids and the medium (not the flow regime). It is important to realize that even for unretarded species, an earlier peak in the breakthrough curve does not necessarily correspond to a faster mean elution rate, or vice versa. But rather that a colloid may elute faster but disperse less than an equivalent tracer. Hence its peak may be retarded compared to that of the tracer, even assuming no retardation. Hence one must consider a combination of mean elution rate and mean dispersion rate, and not rely on “peak times” to corroborate chromatographic effects. The importance of this lies in the fact that these processes are not independent and yet upscale differently. Thus realistic estimates of effective colloid dispersion rates should be upscaled in a way consistent with that adopted for tracers within the same system.

Determination of organic pollutants in small samples of groundwaters by liquid-liquid extraction and capillary gas chromatography

Abstract A method is presented for the determination of 22 organic compounds in polluted groundwaters. The method includes liquid-liquid extraction of the base/neutral organics from small, alkaline groundwater samples, followed by derivatisation and liquid-liquid extraction of phenolic compounds after neutralisation. The extracts were analysed by capillary gas chromatography. Dual detection by flame Ionisation and electron capture was used to reduce analysis time.

Denitrification and phenol degradation in a contaminated aquifer.

A natural groundwater system modified by pollutant phenols and agricultural nitrate has been modelled in the laboratory by a series of sacrificial microcosm experiments. Samples of aquifer sediment and groundwater from the margin of the phenol plume were used to inoculate anaerobic microcosms enriched in nitrate and pollutant phenols. Rapid degradation of phenol and p-cresol was observed over a 35-day period leading to the generation of inorganic carbon and a number of transient intermediates. O-cresol proved to be recalcitrant on the experimental time-scale. A mass balance calculation shows that, during degradation, carbon was conserved in the aqueous phase. Groundwater-sediment interactions were monitored using carbon stable isotope data. A mass balance for solution TIC indicates thatp-cresol degradation stimulated the dissolution of sedimentary carbonate phases due to the formation of carbonic acid. Compound-specific carbon isotope analysis (GC-IRMS) was used to search for 13C enrichment in residual p-cresol. A slight enrichment trend (epsilon = -2.5/1000) was tentatively identified. The potential of this fractionation effect for obtaining in situ degradation rates is discussed. Results from the microcosm experiments help to explain the observed distribution of nitrate and phenols within the polluted aquifer.

The mobility and stability of iodine-humic and iodine-fulvic complexes through sand

Abstract Natural organic carbon is believed to enhance the transport of associated contaminants through the geosphere and it is important to understand the migration behaviour of natural organic compounds. The stability and mobility of 125 I labelled humic and fulvic material in a glacial sand-groundwater system was, therefore, studied in columns of glacial sand using natural groundwater from the same aquifer as eluent. Aldrich humic acid and fulvic acid extracted from Drigg groundwater were labelled with 125 I in the presence of an oxidising agent. The complexed organics were separated from free I and from oxidising agent using size-exclusion gel chromatography. About 55% of a pulse of 125 I-Fu complex which had been prepared from fulvic acid which had been extracted from Drigg groundwater emerged from a 55 cm column unchanged. Such material could conceivably be used as a tracer in field tests but the difficulty of extracting a sufficient quantity of fulvic acid makes this impractical. Aldrich humic was much more strongly sorbed in the columns of sand and would be of little use as an analogue for the natural material. It is well known that dissolved organic carbons in groundwaters can be sorbed onto geological materials by a variety of mechanisms and it appears from this work that the mobility of a humic or fulvic material depends on the extent to which the available sites are already occupied.

The determination of the molecular mass of humic substances from natural waters by analytical ultracentrifugation

Abstract UV scanning analytical ultracentrifugation has been applied to the determination of the weight-average molecular masses ( M w) for a range of fulvic and humic acids extracted from natural waters and soils. It is demonstrated that the technique of approach-to-equilibrium (Archibald method) gives a relatively rapid means of determining M w. which for low molecular mass fulvic acids is in good agreement with the conventional sedimentation equilibrium method. For higher molecular mass humic substances it is found that the sedimentation equilibrium method underestimates M w due to the loss of material from the concentration gradient and the approach-to-equilibrium method is the only means of estimating the molecular mass for the whole sample. For very high molecular mass humic substances ( M w of 30 000 or more) a double extrapolation procedure is described for obtaining the true weight-average molecular mass at zero time and zero force field. The application of the approach-to-equilibrium method to samples taken directly from natural water without purification and to ion binding of fulvic acids is described.

Microcosm studies of microbial degradation in a coal tar distillate plume

Investigation of a groundwater plume containing up to 24 g l(-1) phenolic compounds suggested that over a period of nearly 50 years, little degradation had occurred despite the presence of a microbial community and electron acceptors within the core of the plume. In order to study the effect of contaminant concentration on degradation behaviour, laboratory microcosm experiments were performed under aerobic and anaerobic conditions at four different concentrations obtained by diluting contaminated with uncontaminated groundwater. The microcosms contained groundwater with total phenols at ca. 200, 250, 660 and 5000 mg l(-1), and aquifer sediment that had been acclimatised within the plume for several months. The microcosms were operated for a period of 390-400 days along with sterile controls to ascertain whether degradation was microbially mediated or abiotic. Under aerobic conditions, degradation only occurred at concentrations up to 660 mg l(-1) total phenols. At phenol concentrations below 250 mg l(-1) a benzoquinone intermediate, thought to originate from the degradation of 2,5-dimethylphenol, was isolated and identified. This suggested an unusual degradative pathway for this compound; its aerobic degradation more commonly proceeding via catecholic intermediates. Under anaerobic conditions, degradation only occurred in the most dilute microcosm (total phenols 195 mg l(-1)) with a loss of p-cresol accompanied by a nonstoichiometric decrease in nitrate and sulphate. By inference, iron(III) from the sediment may also have been used as a terminal electron acceptor, in which case the amount of biologically available iron released was calculated as 1.07 mg Fe(III)/g of sediment. The study shows that natural attenuation is likely to be stimulated by dilution of the plume.

The Influence of Organics in Field Migration Experiments

Within the European Commissions MIRAGE II research programme, attention has been focused on the potential for natural and anthropogenic organic compounds to complex and mobilize radionuclides. As part of this study, an array of boreholes has been constructed in a shallow confined glacial sand aquifer at Drigg in Cumbria, so that in situ radio-tracer tests can be performed in the presence of organics, under controlled conditions of groundwater flow. The effect of organics on radionuclide mobility was also studied in the laboratory using batch sorption and column experiments, and the results used to predict the outcome of a field test with the Co-EDTA complex.