Trevor Elliot

Ten Year Performance Evaluation of a Field-Scale Zero-Valent Iron Permeable Reactive Barrier Installed to Remediate Trichloroethene Contaminated Groundwater

The Monkstown zero-valent iron permeable reactive barrier (ZVI PRB), Europes oldest commercially-installed ZVI PRB, had been treating trichloroethene (TCE) contaminated groundwater for about 10 years on the Nortel Network site in Northern Ireland when cores from the reactive zone were collected in December, 2006. Groundwater data from 2001-2006 indicated that TCE is still being remediated to below detection limits as the contaminated groundwater flows through the PRB. Ca and Fe carbonates, crystalline and amorphous Fe sulfides, and Fe (hydr)oxides have precipitated in the granular ZVI material in the PRB. The greatest variety of minerals is associated with a approximately 1-2 cm thick, slightly cemented crust on top (up-gradient influent entrance) of the ZVI section of the PRB and also with the discontinuous cemented ZVI material ( approximately 23 cm thick) directly below it. The greatest presence of microbial communities also occurred in the up-gradient influent portion of the PRB compared to its down-gradient effluent section, with the latter possibly due to less favorable conditions (i.e., high pH, low oxygen) for microbial growth. The ZVI filings in the down-gradient effluent section of the PRB have a projected life span of >10 years compared with ZVI filings from the continuous to discontinuous cemented up-gradient ZVI section (upper approximately 25 cm) of the PRB, which may have a life span of only approximately 2-5 more years. Supporting Information from applied, multi-tracer testing indicated that restricted groundwater flow is occurring in the upper approximately 25 cm of the ZVI section and preferential pathways have also formed in this PRB over its 10 years of operation.

Hydrochemical trends, palaeorecharge and groundwater ages in the fissured Chalk aquifer of the London and Berkshire Basins, UK

The hydrochemical, radiochemical, stable isotope, 14C and dissolved noble gas composition of groundwaters has been determined along two profiles across the confined, fissured Chalk aquifer of the London Basin of southern England, and for selected sites in the adjacent Berkshire Basin. During downgradient flow in the London Basin aquifer, the groundwater chemistry is modified by water–rock interactions: congruent and incongruent reaction of the carbonate lithology resulting in enhanced Mg/Ca and Sr/Ca ratios and 13C contents with increased residence times; redox and ion exchange reactions; and towards the centre of the Basin, mixing with a residual saline connate water stored in the Chalk matrix. There is evidence from anomalous water chemistries for a component of vertical leakage from overlying Tertiary beds into the confined aquifer as a result of historical dewatering of the aquifer. Dissolved noble gas contents indicate the climate was up to 4.5°C cooler than at present during recharge of the waters now found in the centres of both Basins; stable isotope (2H and 18O) depletions correspond to this recharge temperature change. For evolved waters having δ13C > −8‰ PDB a negative linear correlation is demonstrated between derived recharge temperatures and δ13C values, which is interpreted as mixing between relatively warm, light isotopic, fracture-borne waters and cooler stored waters of the matrix having a 13C signature more or less equilibrated with the Chalk. From geochemical (14C, 4He) age estimates, the abstracted water is interpreted as being either of wholly Holocene/post-Devensian glacial origin, or an admixture of Holocene and Late Pleistocene pre-glacial (cold stage interstadial) recharge. Devensian pleniglacial stage waters of the Last Glacial Maximum are not represented.

Carbon, helium, neon and argon isotopes in a Po Basin (Northern Italy) Natural Gas Field.

The abundance and isotopic composition of He, Ne and Ar have been determined in 19 gas samples taken from the Dosso degli Angeli hydrocarbon gas field, between 3000- and 3750-m depth in the Po Basin. Carbon isotope ratios in these samples vary between δ13C(CH4) = −67.5 and −73.4%, indicating a predominantly biogenic source for the methane. C2+ [=Σ(C2H6 + C3H8 + …)] concentrations increase systematically from 0.04% to 0.17% by volume with depth, suggesting that there is a small additional thermogenic or diagenetic hydrocarbon contribution to these gases. CH4/36Ar ratios in the Dosso degli Angeli gases are in the range predicted for a groundwater which has derived its rare gas by equilibration with the atmosphere, and is CH4-saturated under reservoir conditions. There is also up to 10% excess of CH4 which is not associated with the groundwater. These results are consistent with > 90% of the CH4 in the Dosso degli Angeli reservoir originating from CH4-saturated groundwater. Measured 20Ne/36Ar ratios depart from the predicted ratio in air-equilibrated groundwater. This fractionation is consistent with water/gas phase equilibration under reservoir conditions, and is similar to that observed in gas reservoirs in the Vienna and Pannonian Basins, Austria and Hungary, respectively. The He in the Dosso degli Angeli gases is almost entirely radiogenic and derived from the crust. The concentration of 4He in the reservoir gases requires that He is mostly sourced from outside the reservoir volume. Radiogenic 4He/40Ar ratios input into the reservoir gas must be much higher than averaged crustal production ratios, as no shift from the atmospheric 40Ar/36Ar ratio is observed. Preferential release of 4He over 40Ar from the sites of their production is the preferred explanation. This observation contrasts with previous studies from active extensional basins, in which radiogenic 4He and 40Ar appear to have been stored, released and transported at near production ratio on a regional scale. The most obvious difference between these studies is the comparatively low thermal gradient of the Po Basin.

Assessments of the sensitivity to climate change of flow and natural water quality in four major carbonate aquifers of Europe

Abstract A numerical modelling approach has been developed to predict the vulnerability of aquifers to future climate change. This approach encompasses changes in recharge regime, dynamics of flow and storage patterns within aquifers, and natural hydrochemical changes. An application of the approach has been made to four hypothetical spring catchments representative of major carbonate aquifers in three European climatic zones. Since prolific carbonate aquifers typically combine a high transmissivity with a low specific yield, they can be expected to be more sensitive than clastic aquifers to changes in recharge patterns. Simulations of the study systems to the middle of the 21st century predict different outcomes in the three different climate zones: (1) in the northern maritime zone (UK) recharge (and therefore discharge) is predicted to increase by as much as 21 0n response to anticipated increases in precipitation; (2) in the continental zone (Germany) recharge in winter is predicted to remain approximately the same as at present, but summer recharge will decline dramatically (by as much as 32%), so that a net decrease in aquifer discharge is predicted; and (3) in the Mediterranean zone (Spain) recharge is predicted to decrease by as much as 160f the present-day values. For all three systems, increases in water hardness in response to rising CO2 are predicted, but are expected to be negligible in water resources terms.

Chalk fracture system characteristics: implications for flow and solute transport

Abstract Bulk groundwater flow and solute advection occur in the fractures of the UK Chalk, and a knowledge of the frequency and aperture of these fractures is crucial to understanding these two key processes in the Chalk. Fracture frequencies in the Chalk of southeast England have been assessed regionally by means of outcrop scanline measurements. Measurements of radon dissolved in Chalk groundwaters allow the estimation of fracture apertures in the saturated zone. These data provide valuable controls on the conceptualization and estimation of hydraulic properties for the Chalk aquifer. In particular, standard hydraulic models of fractured rock permeability applied on the basis of these data are shown to be insufficient to explain the higher permeabilities measured in the unconfined Chalk beneath valley axes. In these settings, conduit flow mechanisms must be invoked; this in turn implies that modifications to standard matrix diffusion models are required to describe solute transport accurately.

Characterizing a heterogeneous hydrogeological system using groundwater flow and geochemical modelling

Characterization of heterogeneous hydrogeological systems plays an important role in groundwater protection and remediation of contaminated sites. However, sparse field observations and/or lack of relevant in situ test results hinder regional characterization process. In such cases, advanced modelling techniques can improve characterization of such complex hydrogeological systems. A sequential approach using groundwater flow modelling with nonlinear inverse calibration, advective transport and geochemical modelling and isotope study to characterize heterogeneous systems has been developed in this case study. It was used to better understand the regional hydrogeology and groundwater system of the Triassic Sherwood Sandstone aquifer underlying the Belfast area, Northern Ireland. Based on the inversely calibrated flow model and advective transport modelling by particle tracking, geochemical interpretation of groundwater quality data and isotopic and geochemical inverse modelling at well-distributed spatial locations in the sandstone were carried out to verify the flow pathways and residence times and to identify the geochemical evolution in this heterogeneous hydrogeological system. These approaches provide multiple lines of evidence for characterization of the heterogeneous aquifer system. The flowpaths and residence times from the flow and advective transport modelling agreed well with the geochemical modelling and isotopic radiocarbon dating. This demonstrates the importance of incorporating both flow and geochemical analysis techniques in a hydrogeological study.

Water quality impacts and palaeohydrogeology in the Yorkshire Chalk aquifer, UK

A large hydrochemical data-set for the East Yorkshire Chalk aquifer has been assessed. Controls on the distribution of water qualities reflect: water-rock interactions (affecting especially the carbonate system and associated geochemistry); effects of land-use change (especially where the aquifer is unconfined); saline intrusion and aquifer refreshening (including ion exchange effects); and overexploitation (in the semi-confined and confined zones of the aquifer). Both Sr and I prove useful indicators of groundwater esidence times, and I/Cl ratios characterize two sources of saline waters. The hydrochemical evidence clearly reveals the importance both of recent management decisions and palaeohydrogeology in determining the evolution and distribution of groundwater salinity within the artesian and confined zones of the aquifer. Waters encountered in the aquifer are identified as complex (and potentially dynamic) mixtures between recent recharge waters, modern seawater, and ancient seawater which entered the aquifer many millennia ago.

Detection of mixing dynamics during pumping of a flooded coal mine

In complex hydrogeological environments the effective management of groundwater quality problems by pump-and-treat operations can be most confidently achieved if the mixing dynamics induced within the aquifer by pumping are well understood. The utility of isotopic environmental tracers (C-, H-, O-, S-stable isotopic analyses and age indicators-(14) C, (3) H) for this purpose is illustrated by the analysis of a pumping test in an abstraction borehole drilled into flooded, abandoned coal mineworkings at Deerplay (Lancashire, UK). Interpretation of the isotope data was undertaken conjunctively with that of major ion hydrochemistry, and interpreted in the context of the particular hydraulic setting of flooded mineworkings to identify the sources and mixing of water qualities in the groundwater system. Initial pumping showed breakdown of initial water quality stratification in the borehole, and gave evidence for distinctive isotopic signatures (δ(34) S(SO4) ≅ -1.6‰, δ(18) O(SO4 ) ≅ +15‰) associated with primary oxidation of pyrite in the zone of water table fluctuation-the first time this phenomenon has been successfully characterized by these isotopes in a flooded mine system. The overall aim of the test pumping-to replace an uncontrolled outflow from a mine entrance in an inconvenient location with a pumped discharge on a site where treatment could be provided-was swiftly achieved. Environmental tracing data illustrated the benefits of pumping as little as possible to attain this aim, as higher rates of pumping induced in-mixing of poorer quality waters from more distant old workings, and/or renewed pyrite oxidation in the shallow subsurface.

Carbon isotopic fractionation of CFCs during abiotic and biotic degradation

Carbon stable isotope ((13)C) fractionation in chlorofluorocarbon (CFC) compounds arising from abiotic (chemical) degradation using zero-valent iron (ZVI) and biotic (landfill gas attenuation) processes is investigated. Batch tests (at 25 °C) for CFC-113 and CFC-11 using ZVI show quantitative degradation of CFC-113 to HCFC-123a and CFC-1113 following pseudo-first-order kinetics corresponding to a half-life (τ(1/2)) of 20.5 h, and a ZVI surface-area normalized rate constant (k(SA)) of -(9.8 ± 0.5) × 10(-5) L m(-2) h(-1). CFC-11 degraded to trace HCFC-21 and HCFC-31 following pseudo-first-order kinetics corresponding to τ(1/2) = 17.3 h and k(SA) = -(1.2 ± 0.5) × 10(-4) L m(-2) h(-1). Significant kinetic isotope effects of ε(‰) = -5.0 ± 0.3 (CFC-113) and -17.8 ± 4.8 (CFC-11) were observed. Compound-specific carbon isotope analyses also have been used here to characterize source signatures of CFC gases (HCFC-22, CFC-12, HFC-134a, HCFC-142b, CFC-114, CFC-11, CFC-113) for urban (UAA), rural/remote (RAA), and landfill (LAA) ambient air samples, as well as in situ surface flux chamber (FLUX; NO FLUX) and landfill gas (LFG) samples at the Dargan Road site, Northern Ireland. The latter values reflect biotic degradation and isotopic fractionation in LFG production, and local atmospheric impact of landfill emissions through the cover. Isotopic fractionations of Δ(13)C ∼ -13‰ (HCFC-22), Δ(13)C ∼ -35‰ (CFC-12) and Δ(13)C ∼ -15‰ (CFC-11) were observed for LFG in comparison to characteristic solvent source signatures, with the magnitude of the isotopic effect for CFC-11 apparently similar to the kinetic isotope effect for (abiotic) ZVI degradation.

Nitrate Levels and the Age of Groundwater from the Upper Devonian Sandstone Aquifer in Fife, Scotland

The tritium concentrations in 13 groundwater samples from boreholes throughout the Upper Devonian sandstone aquifer of Fife have been measured. Due to atmospheric variations in tritium concentrations over the last century, this radioactive tracer can be used as a groundwater age indicator. In this study, the groundwater tritium concentrations have allowed for the area to be divided into three zones, and the variable chemistry of the groundwater samples, including the problem of recent elevated nitrate levels in the Fife Aquifer, has been interpreted in terms of their relative ages.