Philip Morgan

Nitrate attenuation in groundwater: A review of biogeochemical controlling processes

Biogeochemical processes controlling nitrate attenuation in aquifers are critically reviewed. An understanding of the fate of nitrate in groundwater is vital for managing risks associated with nitrate pollution, and to safeguard groundwater supplies and groundwater-dependent surface waters. Denitrification is focused upon as the dominant nitrate attenuation process in groundwater. As denitrifying bacteria are essentially ubiquitous in the subsurface, the critical limiting factors are oxygen and electron donor concentration and availability. Variability in other environmental conditions such as nitrate concentration, nutrient availability, pH, temperature, presence of toxins and microbial acclimation appears to be less important, exerting only secondary influences on denitrification rates. Other nitrate depletion mechanisms such as dissimilatory nitrate reduction to ammonium and assimilation of nitrate into microbial biomass are unlikely to be important in most subsurface settings relative to denitrification. Further research is recommended to improve current understanding on the influence of organic carbon, sulphur and iron electron donors, physical restrictions on microbial activity in dual porosity aquifers, influences of environmental condition (e.g. pH in poorly buffered environments and salinity in coastal or salinized soil settings), co-contaminant influences (particularly the contrasting inhibitory and electron donor influences of pesticides) and improved quantification of denitrification rates in the laboratory and field.

Cultivation-Dependent and -Independent Approaches for Determining Bacterial Diversity in Heavy-Metal-Contaminated Soil

ABSTRACT In recent years, culture-independent methods have been used in preference to traditional isolation techniques for microbial community analysis. However, it is questionable whether uncultured organisms from a given sample are important for determining the impact of anthropogenic stress on indigenous communities. To investigate this, soil samples were taken from a site with patchy metal contamination, and the bacterial community structure was assessed with a variety of approaches. There were small differences in microscopic epifluorescence bacterial counts. Denaturing gradient gel electrophoresis (DGGE) profiles of 16S rRNA gene fragments (16S-DGGE) amplified directly from soil samples were highly similar. A clone library generated from the most contaminated sample revealed a diverse bacterial community, which showed similarities to pristine soil communities from other studies. However, the proportion of bacteria from the soil samples that were culturable on standard plate-counting media varied between 0.08 and 2.2%, and these values correlated negatively with metal concentrations. The culturable communities from each sample were compared by 16S-DGGE of plate washes and by fatty acid profiling of individual isolates. Each approach indicated that there were considerable differences between the compositions of the culturable communities from each sample. DGGE bands from both culture-based and culture-independent approaches were sequenced and compared. These data indicated that metal contamination did not have a significant effect on the total genetic diversity present but affected physiological status, so that the number of bacteria capable of responding to laboratory culture and their taxonomic distribution were altered. Thus, it appears that plate counts may be a more appropriate method for determining the effect of heavy metals on soil bacteria than culture-independent approaches.

Hydrocarbon Degradation in Soils and Methods for Soil Biotreatment

The cleanup of soils and groundwater contaminated with hydrocarbons is of particular importance in minimizing the environmental impact of petroleum and petroleum products and in preventing contamination of potable water supplies. Consequently, there is a growing industry involved in the treatment of contaminated topsoils, subsoils, and groundwater. The biotreatment methodologies employed for decontamination are designed to enhance in situ degradation by the supply of oxygen, inorganic nutrients, and/or microbial inocula to the contaminated zone. This review considers the fate and effects of hydrocarbon contaminants in terrestrial environments, with particular reference to the factors that limit biodegradation rates. The potential efficiencies, advantages, and disadvantages of biotreatment techniques are discussed and the future research directions necessary for process development are considered.

Physiology of aliphatic hydrocarbon-degrading microorganisms.

This paper reviews aspects of the physiology and biochemistry of the microbial biodegradation of alkanes larger than methane, alkenes and alkynes with particular emphasis upon recent developments. Subject areas discussed include: substrate uptake; metabolic pathways for alkenes and straight and branched-chain alkanes; the genetics and regulation of pathways; co-oxidation of aliphatic hydrocarbons; the potential for anaerobic aliphatic hydrocarbon degradation; the potential deployment of aliphatic hydrocarbon-degrading microorganisms in biotechnology.

A review of ammonium attenuation in soil and groundwater

Ammonium attenuation in subsoils and groundwater is predominantly due to cation exchange and/or nitrification (biological oxidation) processes. These processes have been little studied in UK formations and this relative lack of information can result in reduced consistency and robustness in the assessment of risks posed by ammonium contamination arising from landfills, effluent soakaways, contaminated sites and other sources. A review of ammonium fate and transport in the subsurface has been completed and guidance developed on the key processes that contribute to attenuation. The amount of relevant literature is small but sufficient to provide indicative ranges of partition coefficients and biological nitrification rates for ammonium in UK subsoils and aquifers. Ammonium attenuation was found to be highly sensitive to the clay mineralogy and pore size of the strata, the availability of oxygen and the chemical composition of the contaminated fluid. The values derived may have application in the initial (screening) phases of risk assessment where the conceptual model for the site under consideration matches that from which the presented data originate.

Comparison of microbial and meiofaunal community analyses for determining impact of heavy metal contamination

The impact of long-term heavy metal contamination on soil communities was assessed by a number of methods. These included plate counts of culturable bacteria, community level physiological profiling (CLPP) by analysis of the utilization of multiple carbon sources in BIOLOG plates, community fatty acid methyl ester (C-FAME) profiling and dehydrogenase enzyme activity measurements. These approaches were complemented with microscopic assessments of the diversity of the nematode community. Samples from two sites with different histories of heavy-metal input were assessed. Major differences in microbial and meiofaunal parameters were observed both between and within the sites. There was a large degree of congruence between each of the microbiological approaches. In particular, one sample appeared to be distinguished by a reduction in culturable bacteria (especially pseudomonads), limited response to carbon sources in CLPP, and major differences in extracted fatty acid profiles. The use of multivariate analysis to examine the relationship between microbial and physicochemical measurements revealed that CLPP and plate counts were useful for determining the gross effect of metals on soil microbial communities, whereas proportions of metal-resistant bacteria and dehydrogenase activity differentiated between the two sites. Copper and zinc concentrations and pH all showed significant correlation with the microbial parameters. Nematode community structure was affected to a greater extent by soil pH than by metal content, but the within-site rankings were the same as those achieved for microbiological analyses. The use of these methods for field evaluation of the impact of industrial pollution may be possible provided care is taken when interpreting the data.

Comparison of abilities of white-rot fungi to mineralize selected xenobiotic compounds

SummaryThe abilities of the white-rot fungi Chrysosporium lignorum, Trametes versicolor, Phanerochaete chrysosporium and Stereum hirsutum to mineralize 3,4-dichloroaniline, dieldrin and phenanthrene were investigated. S. hirsutum did not mineralize any of the test compounds but the other strains partly mineralized them all to varying degrees. The relative degradation rates per unit biomass were T. versicolor > C. lignorum > P. chrysosporium. Evidence was obtained for the production of water-soluble metabolic intermediates but no attempt was made to characterize these. It was found that mineral salts-glucose medium supplemented with trace mineral nutrients, vitamins and 1.5 mm 3,4-dimethoxybenzyl alcohol (veratryl alcohol) resulted in the highest mineralization rate. At no time in these experiments was there detectable extracellular ligninase (lignin peroxidase) activity.

Factors limiting the supply and efficiency of nutrient and oxygen supplements for the in situ biotreatment of contaminated soil and groundwater

Abstract In situ biotreatment of contaminated soil and groundwater requires the provision of optimal conditions for biodegradation in the subsurface. The supply of inorganic nutrient solutions and oxygen in the form of dilute H 2 O 2 was investigated using a number of soils in order to determine limitations of injection and infiltration technologies. It was found that migration of phosphate was limited by the precipitation of insoluble salts and that this reduced soil permeability. Sodium tripolyphosphate was found to reduce partially the amount of precipitation but disrupted soil structure. The addition of inorganic nitrogen to an oil-contaminated soil was found to inhibit mineralization of glucose and phenanthrene. The use of H 2 O 2 as an oxygen source at concentrations above approx. 100 mg H 2 O 2 l −1 was restricted by decomposition reactions. Precipitation of oxidation products and bubble formation owing to degassing resulted in significant reductions in soil permeability. Sodium tripolyphosphate also reduced chemically-catalysed cleavage of H 2 O 2 but extensive biologically-mediated breakdown still occurred. The results demonstrate that significant difficulties may be encountered when using inorganic nutrient and H 2 O 2 solutions for site bioremediation but the effects are site-specific. Detailed assessments of individual sites are a necessary pre-requisite to any in situ biotreatment programme.

Growth and biodegradation by white-rot fungi inoculated into soil

Abstract The colonization of sandy loam soil following inoculation with spore suspensions of the white-rot fungi Phanerochaete chrysosporium ATCC 24725 and Chrysosporium lignorum CL1 was confirmed by an epifluorescence microscopy-image analysis method. These fungi and Trametes versicolor PV1 mineralized 3,4-dichloroaniline and benzo(a)pyrene in soil at concentrations up to 250 μg g −1 . Successful inoculation and biodegradation required supplementary carbon sources. Addition of inorganic nutrients had no stimulatory effect. Glucose, hay, wood chips, pine bark, loam and peat all promoted growth and degradation but chopped wheat straw was the best substrate. Increasing the content of straw in the soil led to increased biomass and mineralization. The optimum ratio of straw: soil for mineralization was 1:4. Both strains sporulated within 7 days of inoculation before a further increase in hyphal growth but this had no effect on the mineralization rate. These results indicate that use of white-rot fungi in biotechnological soil treatment may be feasible.

Nitrate occurrence and attenuation in the major aquifers of England and Wales

The current occurrence of nitrate in the major aquifers of England and Wales is presented and the evidence for denitrification is critically reviewed. Denitrification is the principal nitrate attenuation process in the subsurface and its potential for mitigating the widespread nitrate inputs is considered. The study focuses on the three most important major aquifers: the Cretaceous Chalk, Permo-Triassic Sandstone and Jurassic Limestone. Elevated ground-water nitrate concentrations are shown to be widespread and continue to rise, with leaching from soils predicted to remain significant. Some 60% of groundwater bodies in England may fail to meet the Water Framework Directive requirement of ‘good status’ by 2015. Acquiring reliable evidence of denitrification is non-trivial and typically comprises integrated assessment of electron donor conditions, nitrogen oxide products, stable isotope ratios, nitrogen/argon ratios, microbiology and comparative velocity estimates. The available field studies confirm that denitrification in unconfined aquifers is relatively limited. Detailed unsaturated zone studies of both the Chalk and Sherwood Sandstone have demonstrated only minor decreases in nitrate concentrations, estimated at just 1–2% of the nitrate load within infiltrating water. Such decreases are unlikely to significantly influence regional groundwater quality. Within the saturated zones of the Chalk, Sherwood Sandstone and Jurassic Limestone aquifers, denitrification was significant only once these aquifers became confined and dissolved oxygen depleted. However, evidence for denitrification is typically weak at the regional aquifer scale and low nitrate concentrations may sometimes be simply ascribed to dilution or non-arrival of plumes. Denitrification within the Chalk or Jurassic Limestone matrix, although geochemically possible, may not occur, as bacteria are potentially excluded by the narrow pore throats. Although it is concluded that denitrification is unlikely to lead to very significant mitigation of the nitrate problems manifest in these aquifers, further research is still warranted to better understand its role. Relatively little denitrification research has been conducted in the major aquifers over the last decade. Organic carbon controls, in-fracture v. in-matrix denitrification and recent trends in nitrate and denitrification at historically monitored sites should all be further investigated.