Biomedical subjects

Michael O Rivett

Publications and source records attributed to Michael O Rivett.

6 recordsLinked to original sources

PubMed2006

Quantifying urban river-aquifer fluid exchange processes: a multi-scale problem.

Groundwater-river exchanges in an urban setting have been investigated through long term field monitoring and detailed modelling of a 7 km reach of the Tame river as it traverses the unconfined Triassic Sandstone aquifer that lies beneath the City of Birmingham, UK. Field investigations and numerical modelling have been completed at a range of spatial and temporal scales from the metre to the kilometre scale and from event (hourly) to multi-annual time scales. The objective has been to quantify the spatial and temporal flow distributions governing mixing processes at the aquifer-river interface that can affect the chemical activity in the hyporheic zone of this urbanised river. The hyporheic zone is defined to be the zone of physical mixing of river and aquifer water. The results highlight the multi-scale controls that govern the fluid exchange distributions that influence the thickness of the mixing zone between urban rivers and groundwater and the patterns of groundwater flow through the bed of the river. The morphologies of the urban river bed and the adjacent river bank sediments are found to be particularly influential in developing the mixing zone at the interface between river and groundwater. Pressure transients in the river are also found to exert an influence on velocity distribution in the bed material. Areas of significant mixing do not appear to be related to the areas of greatest groundwater discharge and therefore this relationship requires further investigation to quantify the actual remedial capacity of the physical hyporheic zone.

Cities↗

PubMed2006

Assessing the impact of VOC-contaminated groundwater on surface water at the city scale.

This study is believed to be one of the first to assess the impact of urban VOC-(volatile organic compound) contaminated groundwater on river-water quality at the city scale. A network of riverbed piezometers was used to study the 7.4-km urbanised reach of the River Tame that flows across the groundwater-effluent unconfined Triassic sandstone aquifer underlying the city of Birmingham (UK). Aquifer groundwater contained significant chlorinated VOC contamination due to the city's industrial heritage. Chlorinated VOC-contaminated baseflow was widespread along the reach with trichloroethene (TCE) dominant. VOC concentrations in riverbed piezometers were in the range 0.1-100 microg/l with typical regulatory limits occasionally exceeded by an order of magnitude. Although anaerobic biodegradation products such as cis-dichloroethene were widespread, they were unlikely to have formed in the generally aerobic riverbed. The lack of anaerobic conditions was ascribed to insufficient accumulation of low-permeability, organic-carbon rich riverbed sediments in this medium-high energy river. Assumptions a priori that natural attenuation of chlorinated VOCs will occur via reductive dechlorination in urban riverbeds are likely in error, particularly where deposits of medium-high permeability exist transmitting much of the baseflow. Surface-water quality impacts were nevertheless still low with in-river TCE increasing by just 2 microg/l over the 7.4-km reach. Agreement of baseflow contaminant flux estimates based on five flow-concentration product methods was achieved to within an order of magnitude with 22-200 kg/yr of TCE estimated to discharge to the 7.4-km reach (equivalent to 0.8-7.5 mg/d/m2 of riverbed). Such uncertainty was not regarded as unreasonable when the large measurement scale and geological and chemical heterogeneities are considered. Improved flux estimation methods and greater monitoring densities are nevertheless warranted. Considering Birmingham's long industrial history and known incidence of VOC-contaminated groundwater, the city-scale impact of VOC-contaminated groundwater upon surface-water quality was judged to be relatively modest.

Aerobiosis↗

PubMed2006

Pump-and-treat remediation of chlorinated solvent contamination at a controlled field-experiment site.

Pump-and-treat (P&T) remediation and associated concentration tailing are investigated at the field scale in a mildly heterogeneous sandy aquifer through the extraction of dissolved chlorinated solvent plumes that had developed over 475 d from a multicomponent dense nonaqueous-phase liquid (DNAPL) source intentionally emplaced in the aquifer at the Borden (ON) research site. Extraction was accomplished via a source-containment well located 25 m from the source and two further downgradient plume-centerline wells to remove the advancing high-concentration dissolved plumes. The 550 days of detailed P&T field data demonstrated the following: remediation, albeit slowly, of the leading 25-60 m plume section to around typical drinking water standard concentrations; concentration tailing (reduction) over 4 orders of magnitude in the plume; a steady-state concentration "plateau" in the source-containment well capturing the steadily dissolving DNAPL source; influences of extraction rate changes (concentration rebounds); and, lengthy tailing from inter-well stagnation-zone areas. Much of the contaminant behavior during the P&T appeared to be "ideal" in the sense that with appropriate specification of the source term and pumping regime, it was reasonably predicted by 3-dimensional numerical model (HydroGeoSphere) simulations that assumed ideal (macrodispersion, linear sorption, etc.) transport. Supporting lab studies confirmed nonideal sorption was, however, important at the point sample scale with enhanced PCE (tetrachloroethene) sorption to low- and high-permeability strata and moderate nonlinear and competitive sorption influences. Although there was limited evidence of nonideal tailing contributions to the field data (underprediction of some tailing curve gradients), such contributions to P&T tailing were not easily discerned and appeared to play a relatively minor role within the mildly heterogeneous aquifer studied.

Chlorine↗

PubMed2005

Integrated understanding of urban land, groundwater, baseflow and surface-water quality--the City of Birmingham, UK.

Integrated understanding of urban land, groundwater (shallow and deep), baseflow and surface-water quality relationships is required for effective urban water-quality management. Chemical quality data from across these media have been collected for the Birmingham (UK) aquifer--River Tame conurbation to assess chemical transport from contaminated land to groundwater to baseflow to surface water. Although metals concentrations were high in soils, low leachability and attenuation caused concentrations in groundwaters and baseflow discharging to surface water to be generally low with only sporadic elevated concentrations attributed to localised point sources. Hydrocarbon VOCs (volatile organic compounds) were similarly absent or at low concentration attributable to their ready natural attenuation. Chlorinated VOCs, however, were widely encountered in groundwater, discharging as baseflow to surface water and impacting surface-water quality. This is attributed to their DNAPL (dense nonaqueous-phase liquid) properties and relative recalcitrance although there was some evidence of biodegradation, albeit insufficient to protect surface water and groundwater abstraction receptors. Some inorganic trends were evident across the various media; nitrate was the most significant quality concern. Generic conclusions are drawn on urban water-quality management and the need for risk-based management strategies to optimise use of urban, sporadically contaminated groundwater in conjunction with surface water highlighted.

Anions↗

PubMed2005

Dissolution of an emplaced source of DNAPL in a natural aquifer setting.

Field-scale dissolution of a multicomponent DNAPL (dense nonaqueous-phase liquid) source intentionally emplaced below the water table is evaluated in a well-characterized natural aquifer setting. The block-shaped source contained 23 kg of a trichloromethane, trichloroethene, and perchloroethene mixture homogeneously distributed at 5% saturation of pore space. Dissolution was monitored for 3 yr via down-gradient samplers (1-m fence) and occasional intra-source sampling. Although intra-source equilibrium dissolution was shown and endorsed by supporting modeling and literature lab data, less than equilibrium concentrations were predominantly monitored in the 1-m fence. This was ascribed to significant by-passing of the source by groundwater flow due to its low permeability relative to the aquifer and associated dilution of concentrations emitted from the source. Heterogeneous source dissolution occurred despite the relative homogeneity of the source and aquifer and was ascribed to dissolution fingering, which has not been previously field-demonstrated. Bulk bypass of groundwater flow around the source zone caused slow dissolution rates, with 77% of the source remaining after 3 yr and a projected longevity of approximately 25 yr. Observed dissolution fingering would have significantly increased longevity as it increasingly caused intra-source bypass of remaining DNAPL. Our dissolution interpretations were endorsed by additional data collected after 6 yr during source remediation via permanganate oxidation.

Chloroform↗

PubMed2003

A controlled field experiment on groundwater contamination by a multicomponent DNAPL: dissolved-plume retardation.

A natural gradient emplaced-source (ES) controlled field experiment was conducted at the Borden aquifer research site, Ontario, to study the transport of dissolved plumes emanating from residual dense nonaqueous-phase liquid (DNAPL) source zones. The specific objective of the work presented here is to determine the effects of solute and co-solute concentrations on sorption and retardation of dissolved chlorinated solvent-contaminant plumes. The ES field experiment comprised a controlled emplacement of a residual multicomponent DNAPL below the groundwater table and intensive monitoring of dissolved-phase plumes of trichloromethane (TCM), trichloroethylene (TCE), and perchloroethylene (PCE) plumes continuously generated in the aquifer down gradient from gradual source dissolution. Estimates of plume retardation (and dispersion) were obtained from 3-D numerical simulations that incorporated transient source input and flow regimes monitored during the test. PCE, the most retarded solute, surprisingly exhibited a retardation factor approximately 3 times lower than observed in a previous Borden tracer test by Mackay et al. [Water Resour. Res. 22 (1986) 2017] conducted approximately 150 m away. Also, an absence of temporal trend in PCE retardation contrasted with the previous Borden test. Supporting laboratory studies on ES site core indicated that sorption was nonlinear and competitive, i.e. reduced sorption of PCE was observed in the presence of TCE. Consideration of the effects of relatively high co-solute (TCE) concentration (competitive sorption) in addition to PCE concentration effects (nonlinear sorption) was necessary to yield laboratory-based PCE retardation estimates consistent with the field plume values. Concentration- and co-solute-based sorption and retardation analysis was also applied to the previous low-concentration pulse injection test of Mackay et al. [Water Resour. Res. 22 (1986) 2017] and was able to successfully predict the temporal field retardation trends observed in that test. While it is acknowledged that other "nonideal transport" effects may contribute, our analysis predicts differences in the PCE retardation magnitude and trend between the two experiments that are consistent with field observations based on the marked solute concentration differences that resulted from contrasting source conditions. Solute and co-solute concentration effects have heretofore received little attention, but may have wide significance in aquifers contaminated by point-source pollutants because many plumes contain mixed solutes over wide concentration ranges in strata that are likely subject to nonlinear sorption.

Humans↗