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.     

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.     

Analysis of colloid and tracer breakthrough curves

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.     

210Po and 210Pb in zooplankton fecal pellets

²¹⁰Po and ²¹⁰Pb concentrations in fecal pellets from the zooplanktonic euphausiid Meganyctiphanes norvegica are reported. The ²¹⁰Po:²¹⁰Pb activity ratio is 2.2±0.3, a value in good agreement with that found in suspended particulate matter in surface seawater. Estimates of ²¹⁰Po and ²¹⁰Pb removal times from the mixed layer by fecal pellets alone yield values which are of the same order of magnitude as the removal times for these nuclides by all routes. It is suggested that there is a high probability that zooplanktonic fecal pellets play a significant role in the removal of both these nuclides from the surface layers of the ocean.     

The simultaneous modelling of metal ion and humic substance transport in column experiments

Pulsed column experiments using Co, fulvic acid and porous sediment packing, along with up/down-flooding experiments using Eu, humic acid and intact sandstone blocks have been performed. The elution of metal and humic and their distribution along the sandstone columns have been measured. A mixed equilibrium and kinetic coupled chemical transport model has been used to simulate the results. In both cases, one exchangeable and one non-exchangeable component have been used to simulate the interaction of metal and humic substance. For the pulsed experiments, a simple equilibrium approach was used to model humic sorption, while a two component, kinetic model was required for the sandstone columns.