Fate and transport of volatile organic compounds in glacial till and groundwater at an industrial site in Northern Ireland

Volatile organic compound (VOC) contamination of subsurface geological material and groundwater was discovered on the Nortel Monkstown industrial site, Belfast, Northern Ireland. The objectives of this study were to (1) investigate the characteristics of the geological material and its influences on contaminated groundwater flow across the site using borehole logs and hydrological evaluations, and (2) identify the contaminants and examine their distribution in the subsurface geological material and groundwater using chemical analysis. This report focuses on the eastern car park (ECP) which was a former storage area associated with trichloroethene (TCE) degreasing operations. This is where the greatest amount of volatile organic compounds (VOCs), particularly TCE, were detected. The study site is on a complex deposit of clayey glacial till with discontinuous coarser grained lenses, mainly silts, sands and gravel, which occur at 0.45–7.82 m below ground level (bgl). The lenses overall form an elongated formation that acts as a small unconfined shallow aquifer. There is a continuous low permeable stiff clayey till layer beneath the lenses that performs as an aquitard to the groundwater. Highest concentrations of VOCs, mainly TCE, in the geological material and groundwater are in these coarser lenses at ~4.5–7 m bgl. Highest TCE measurements at 390,000 µg L-1 for groundwater and at 39,000 µg kg-1 at 5.7 m for geological material were in borehole GA19 in the coarse lens zone. It is assumed that TCE gained entrance to the subsurface near this borehole where the clayey till was thin to absent above coarse lenses which provided little retardation to the vertical migration of this dense non-aqueous phase liquid (DNAPL) into the groundwater. However, TCE is present in low concentrations in the geological material overlying the coarse lens zone. Additionally, VOCs appear to be associated with poorly drained layers and in peat

Anatomy and facies association of a drumlin in Co. Down, Northern Ireland, from seismic and electrical resistivity surveys

Seismic refraction and electrical resistivity geophysical techniques were used to reconstruct the internal architecture of a drumlin in Co. Down, Northern Ireland. Geophysical results were both validated and complemented by borehole drilling, ground water flow modelling, and geologic mapping. The geophysical anatomy of the drumlin consists of five successive layers with depth including; topsoil, partially saturated and saturated glacial tills, and weathered and more competent greywacke bedrock. There are numerous, often extensive inclusions of clay, sand, gravel, cobbles, and boulders within the topsoil and the till units. Together geophysical and geotechnical findings imply that the drumlin is part of the subglacial lodgement, melt-out, debris flow, sheet flow facies described by previous authors, and formed by re-sedimentation and streamlining of pre-existing sediments during deglaciation of the Late Devensian ice sheet. Seismic refraction imaging is particularly well suited to delineating layering within the drumlin, and is able to reconstruct depths to interfaces to within ± 0.5 m accuracy. Refraction imaging ascertained that the weathered bedrock layer is continuous and of substantial thickness, so that it acts as a basal aquifer which underdrains the bulk of the drumlin. Electrical resistivity imaging was found to be capable of delineating relative spatial changes in the moisture content of the till units, as well as mapping sedimentary inclusions within the till. The moisture content appeared to be elevated near the margins of the drumlin, which may infer a weakening of the drumlin slopes. Our findings advocate the use of seismic refraction and electrical resistivity methods in future sedimentological and geotechnical studies of internal drumlin architecture and drumlin formation, owing particularly to the superior, 3- D spatial coverage of these methods.

Comparative tracing experiments to investigate epikarst structural and compositional heterogeneity

Comparative tracer testing may be used to evaluate the vulnerability of groundwater to specific contaminants by comparing reactive tracer response to that of a simultaneously injected non-reactive “conservative” substance. Conversely, knowledge of tracer reaction with specific materials permits information about subsurface heterogeneity to be inferred. A series of tests completed in the vadose zone overlying a limestone aquifer employed a cocktail of particles along with reactive and non-reactive solute tracers to investigate transport rates between the ground surface and monitoring points approximately 10 m below ground. Short pulse tests revealed both solutes and particulate contaminants could travel at rates of over 10 m/h. Comparison of particle (microorganisms) and non-reactive solute tracer breakthrough revealed that particle tracers experience pore exclusion resulting in higher peak relative concentrations which arrive earlier than those of the solute. Prolonged tracer injection during subsequent experiments confirmed the response observed and illustrated that over 40 % of flow paths between injection and monitoring points were inaccessible to particles, but could allow solutes to pass through them. Similarly, the difference in response between various reactive tracers demonstrated tracers reached monitoring points via multiple flow paths and suggests geochemical heterogeneity plays an important role in influencing tracer behaviour. The results of this investigation highlight the complexity of water flow through the epikarst and the vulnerability of groundwater in karst aquifers to contamination when soil cover is thin to absent.

The typology of Irish hard rock aquifers based on an integrated hydrogeological and geophysical approach

Groundwater flow in hard-rock aquifers is strongly controlled by the characteristics and distribution of structural heterogeneity. A methodology for catchment-scale characterisation is presented, based on the integration of complementary, multi-scale hydrogeological, geophysical and geological approaches. This was applied to three contrasting catchments underlain by metamorphic rocks in the northern parts of Ireland (Republic of Ireland and Northern Ireland, UK). Cross-validated surface and borehole geophysical investigations confirm the discontinuous overburden, lithological compartmentalisation of the bedrock and important spatial variations of the weathered bedrock profiles at macro-scale. Fracture analysis suggests that the recent (Alpine) tectonic fabric exerts strong control on the internal aquifer structure at meso-scale, which is likely to impact on the anisotropy of aquifer properties. The combination of the interpretation of depth-specific hydraulic-test data with the structural information provided by geophysical tests allows characterisation of the hydrodynamic properties of the identified aquifer units. Regionally, the distribution of hydraulic conductivities can be described by inverse power laws specific to the aquifer litho-type. Observed groundwater flow directions reflect this multi-scale structure. The proposed integrated approach applies widely available investigative tools to identify key dominant structures controlling groundwater flow, characterising the aquifer type for each catchment and resolving the spatial distribution of relevant aquifer units and associated hydrodynamic parameters.

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—14C, 3H) 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 (d34S(SO4) ~= -1.6‰, d18O(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.

Arsenic mobilization from iron oxyhydroxides is regulated by organic matter carbon to nitrogen (C:N) ratio

Arsenic (As) is mobilized from delta and floodplain aquifer sediments throughout S.E. Asia via reductive dissolution of As bound to iron (Fe) oxyhydroxides. The reductive driving force is organic carbon, but its source and constitution is uncertain. Here batch incubation experiments were conducted to investigate the role of organic matter (OM) carbon:nitrogen (C:N) ratio on the mobilization of arsenic, Fe and N from As dosed, Fe oxyhydroxide coated sands. As mobilization into pore waters from the sand was strongly regulated by the C:N ratio of the OM, and also the concentration of OM present. The lower the C:N, the more As released. Fe and ammonium release were similarly dependent on the quality and quantity of OM, but Fe mobilization was more rapid and ammonium release slower than As suggesting that the mobilization of these 3 moieties although interdependent, were not directly linked. It was concluded that low C:N ratios for OM responsible for reducing aquifers were As in groundwater is observed were likely.

Codeposition of organic carbon and arsenic in Bengal Delta aquifers

We present data showing that arsenic (As) was codeposited with organic carbon (OC) in Bengal Delta sediments as As and OC concentrations are highly (p <0.001) positively correlated in core profiles collected from widely dispersed geographical sites with different sedimentary depositional histories. Analysis of modern day depositional environments revealed that the As-OC correlations observed in cores are due to As retention and high OC inputs in vegetated zones of the deltaic environment. We hypothesize that elevated concentrations of As occur in vegetated wetland sediments due to concentration and retention of arsenate in aerated root zones and animal burrows where copious iron(III) oxides are deposited. On burial of the sediment, degradation of organic carbon from plant and animal biomass detritus provides the reducing conditions to dissolve iron(III) oxides and release arsenite into the porewater. As tubewell abstracted aquifer water is an invaluable resource on which much of Southeast Asia is now dependent, this increased understanding of the processes responsible for As buildup and release will identify, through knowledge of the palaeosedimentary environment, which sediments are at most risk of having high arsenic concentrations in porewater. Our data allow the development of a new unifying hypothesis of how As is mobilized into groundwaters in river flood plains and deltas of Southeast Asia, namely that in these highly biologically productive environments, As and OC are codeposited, and the codeposited OC drives As release from the sediments.

Co-deposition of organic carbon and arsenic in Bengal Delta aquifers.

We present data showing that arsenic (As) was codeposited with organic carbon (OC) in Bengal Delta sediments as As and OC concentrations are highly (p < 0.001) positively correlated in core profiles collected from widely dispersed geographical sites with different sedimentary depositional histories. Analysis of modern day depositional environments revealed that the As/OC correlations observed in cores are due to As retention and high OC inputs in vegetated zones of the deltaic environment. We hypothesize that elevated concentrations of As occur in vegetated wetland sediments due to concentration and retention of arsenate in aerated root zones and animal burrows where copious iron(III) oxides are deposited. On burial of the sediment, degradation of organic carbon from plant and animal biomass detritus provides the reducing conditions to dissolve iron(III) oxides and release arsenite into the porewater. As tubewell abstracted aquifer water is an invaluable resource on which much of Southeast Asia is now dependent, this increased understanding of the processes responsible for As buildup and release will identify, through knowledge of the palaeosedimentary environment, which sediments are at most risk of having high arsenic concentrations in porewater. Our data allow the development of a new unifying hypothesis of how As is mobilized into groundwaters in river flood plains and deltas of Southeast Asia, namely that in these highly biologically productive environments, As and OC are codeposited, and the codeposited OC drives As release from the sediments. We present data showing that arsenic (As) was codeposited with organic carbon (OC) in Bengal Delta sediments as As and OC concentrations are highly (p < 0.001) positively correlated in core profiles collected from widely dispersed geographical sites with different sedimentary depositional histories. Analysis of modern day depositional environments revealed that the As?OC correlations observed in cores are due to As retention and high OC inputs in vegetated zones of the deltaic environment. We hypothesize that elevated concentrations of As occur in vegetated wetland sediments due to concentration and retention of arsenate in aerated root zones and animal burrows where copious iron(III) oxides are deposited. On burial of the sediment, degradation of organic carbon from plant and animal biomass detritus provides the reducing conditions to dissolve iron(III) oxides and release arsenite into the porewater. As tubewell abstracted aquifer water is an invaluable resource on which much of Southeast Asia is now dependent, this increased understanding of the processes responsible for As buildup and release will identify, through knowledge of the palaeosedimentary environment, which sediments are at most risk of having high arsenic concentrations in porewater. Our data allow the development of a new unifying hypothesis of how As is mobilized into groundwaters in river flood plains and deltas of Southeast Asia, namely that in these highly biologically productive environments, As and OC are codeposited, and the codeposited OC drives As release from the sediments.

Alternate wetting and drying irrigation for rice in Bangladesh: Is it sustainable and has plant breeding something to offer?

The crop management practice of alternate wetting and drying (AWD) is being promoted by IRRI and the national research and extension program in Bangladesh and other parts of the world as a water-saving irrigation practice that reduces the environmental impact of dry season rice production through decreased water usage, and potentially increases yield. Evidence is growing that AWD will dramatically reduce the concentration of arsenic in harvested rice grains conferring a third major advantage over permanently flooded dry season rice production. AWD may also increase the concentration of essential dietary micronutrients in the grain. However, three crucial aspects of AWD irrigation require further investigation. First, why is yield generally altered in AWD? Second, is AWD sustainable economically (viability of farmers' livelihoods) and environmentally (aquifer water table heights) over long-term use? Third, are current cultivars optimized for this irrigation system? This paper describes a multidisciplinary research project that could be conceived which would answer these questions by combining advanced soil biogeochemistry with crop physiology, genomics, and systems biology. The description attempts to show how the breakthroughs in next generation sequencing could be exploited to better utilize local collections of germplasm and identify the molecular mechanisms underlying biological adaptation to the environment within the context of soil chemistry and plant physiology.

Numerical assessment of potential impacts of hydraulically fractured Bowland Shale on overlying aquifers

Natural gas extracted from hydraulically fractured shale formations potentially has a big impact on the global energy landscape. However, there are concerns of potential environmental impacts of hydraulic fracturing of the shale formations, particularly those related to water quality. To evaluate the potential impact of hydraulically fractured shale on overlying aquifers, we conduct realizations of numerical modeling simulations to assess fluid flow and chloride transport from a synthetic Bowland Shale over a period of 11,000 years. The synthetic fractured shale was represented by a three-dimensional discrete fracture model that was developed by using the data from a Bowland Shale gas exploration in Lancashire, UK. Chloride mass exchange between fractures and the rock matrix was fully accounted for in the model. The assessment was carried out to investigate fluid and chloride mass fluxes before, during, and after hydraulic fracturing of the Bowland Shale. Impacts of the upward fracture height and aperture, as well as hydraulic conductivity of the multilayered bedrock system, are also included this assessment. This modeling revealed that the hydraulically fractured Bowland Shale is unlikely to pose a risk to its overlying groundwater quality when the induced fracture aperture is ≤200 µm. With the fracture aperture ≥1000 µm, the upward chloride flux becomes very sensitive to the upward fracture height growth and hydraulic conductivity of the multilayered bedrock system. In the extremely unlikely event of the upward fracture growth directly connecting the shale formation to the overlying Sherwood Sandstone aquifer with the fracture aperture ≥1000 µm, the upward chloride mass flux could potentially pose risks to the overlying aquifer in 100 years. The model study also revealed that the upward mass flux is significantly intercepted by the horizontal mass flux within a high permeable layer between the Bowland Shale and its overlying aquifers, reducing further upward flux toward the overlying aquifers.

Geophysical and hydrogeological characterisation of the impacts of on-site wastewater treatment discharge to groundwater in a poorly productive bedrock acquifer

Contaminants discharging from on-site wastewater treatment systems (OSWTSs) can impact groundwater quality, threatening human health and surface water ecosystems. Risk of negative impacts becomes elevated in areas of extreme vulnerability with high water tables, where thin unsaturated intervals limit vadose zone attenuation. A combined geophysical/hydrogeological investigation into the effects of an OSWTS, located over a poorly productive aquifer (PPA) with thin subsoil cover, aimed to characterise effluent impacts on groundwater. Groundwater, sampled from piezometers down-gradient of the OSWTS percolation area displayed spatially erratic, yet temporally consistent, contaminant distributions. Electrical resistivity tomography identified an area of gross groundwater contamination close to the percolation area and, when combined with seismic refraction and water quality data, indicated that infiltrating effluent reaching the water table discharged to a deeper more permeable zone of weathered shale resting on more competent bedrock. Subsurface structure, defined by geophysics, indicated that elevated chemical and microbiological contaminant levels encountered in groundwater samples collected from piezometers, down-gradient of sampling points with lower contaminant levels, corresponded to those locations where piezometers were screened close to the weathered shale/competent rock interface; those immediately up-gradient were too shallow to intercept this interval, and thus the more impacted zone of the contaminant plume. Intermittent occurrence of faecal indicator bacteria more than 100 m down gradient of the percolation area suggested relatively short travel times. Study findings highlight the utility of geophysics as part of multidisciplinary investigations for OSWTS contaminant plume characterisation, while also demonstrating the capacity of effluent discharging to PPAs to impact groundwater quality at distance. Comparable geophysical responses observed in similar settings across Ireland suggest the phenomena observed in this study are more widespread than previously suspected.

Integrating geophysical data and upscaling techniques to support regional groundwater flow modelling: A practical example of the Lagan Valley, Northern Ireland

When studying heterogeneous aquifer systems, especially at regional scale, a degree of generalization is anticipated. This can be due to sparse sampling regimes, complex depositional environments or lack of accessibility to measure the subsurface. This can lead to an inaccurate conceptualization which can be detrimental when applied to groundwater flow models. It is important that numerical models are based on observed and accurate geological information and do not rely on the distribution of artificial aquifer properties. This can still be problematic as data will be modelled at a different scale to which it was collected. It is proposed here that integrating geophysics and upscaling techniques can assist in a more realistic and deterministic groundwater flow model. In this study, the sedimentary aquifer of the Lagan Valley in Northern Ireland is chosen due to intruding sub-vertical dolerite dykes. These dykes are of a lower permeability than the sandstone aquifer. The use of airborne magnetics allows the delineation of heterogeneities, confirmed by field analysis. Permeability measured at the field scale is then upscaled to different levels using a correlation with the geophysical data, creating equivalent parameters that can be directly imported into numerical groundwater flow models. These parameters include directional equivalent permeabilities and anisotropy. Several stages of upscaling are modelled in finite element. Initial modelling is providing promising results, especially at the intermediate scale, suggesting an accurate distribution of aquifer properties. This deterministic based methodology is being expanded to include stochastic methods of obtaining heterogeneity location based on airborne geophysical data. This is through the Direct Sample method of Multiple-Point Statistics (MPS). This method uses the magnetics as a training image to computationally determine a probabilistic occurrence of heterogeneity. There is also a need to apply the method to alternate geological contexts where the heterogeneity is of a higher permeability than the host rock.

Integrating Multiple Point Statistics with Aerial Geophysical Data to assist Groundwater Flow Models

The process of accounting for heterogeneity has made significant advances in statistical research, primarily in the framework of stochastic analysis and the development of multiple-point statistics (MPS). Among MPS techniques, the direct sampling (DS) method is tested to determine its ability to delineate heterogeneity from aerial magnetics data in a regional sandstone aquifer intruded by low-permeability volcanic dykes in Northern Ireland, UK. The use of two two-dimensional bivariate training images aids in creating spatial probability distributions of heterogeneities of hydrogeological interest, despite relatively ‘noisy’ magnetics data (i.e. including hydrogeologically irrelevant urban noise and regional geologic effects). These distributions are incorporated into a hierarchy system where previously published density function and upscaling methods are applied to derive regional distributions of equivalent hydraulic conductivity tensor K. Several K models, as determined by several stochastic realisations of MPS dyke locations, are computed within groundwater flow models and evaluated by comparing modelled heads with field observations. Results show a significant improvement in model calibration when compared to a simplistic homogeneous and isotropic aquifer model that does not account for the dyke occurrence evidenced by airborne magnetic data. The best model is obtained when normal and reverse polarity dykes are computed separately within MPS simulations and when a probability threshold of 0.7 is applied. The presented stochastic approach also provides improvement when compared to a previously published deterministic anisotropic model based on the unprocessed (i.e. noisy) airborne magnetics. This demonstrates the potential of coupling MPS to airborne geophysical data for regional groundwater modelling.

Hydrogeochemistry and Arsenic Contamination of Groundwater in the Haor Basins of Bangladesh

Hydrogeochemical relationships and the level of arsenic (As) contamination of groundwater in the Haor Basin, a low-lying, semi-natural, region of remnant wetland environs to the northeast of Bangladesh, were studied to assess the As biogeochemical cycling. Most of the shallow and deep tubewells in the study area are contaminated with As (2-331 mu g/l). The relatively higher proportions of Na+ (8-156 mg/l) in groundwater suggest a mixing of connate marine water with freshwater aquifer. Non-significant association between As and PO43- has been found. Highly significant (P <0.001) relationship of As with DOC in groundwater indicates biodegradation of organic matter, creating an overall reducing environment in the aquifer sediments, which facilitates the release of As in the groundwater. The inverse As-Fe, As-Mn, As-Ca and As-Mg relationships in groundwater could be related to the precipitation of Fe-, Mn-, Ca-and Mg-minerals.

Pathways Project Final Report Volume 1: Field Investigation and Catchment Conceptual Models

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Executive Summary
The Pathways Project field studies were targeted at improving the understanding of contaminant transport along different hydrological pathways in Irish catchments, including their associated impacts on water quality and river ecology. The contaminants of interest were phosphorus, nitrogen and sediment. The working Pathways conceptual model included overland flow, interflow, shallow groundwater flow, and deep groundwater flow. This research informed the development of a set of Catchment Management Support Tools (CMSTs) comprising an Exploratory Tool, Catchment Characterization Tool (CCT) and Catchment Modelling Tool (CMT) as outlined in Pathways Project Final Reports Volumes 3 and 4.
In order to inform the CMST, four suitable study catchments were selected following an extensive selection process, namely the Mattock catchment, Co. Louth/Meath; Gortinlieve catchment, Co. Donegal; Nuenna catchment, Co. Kilkenny and the Glen Burn catchment, Co. Down. The Nuenna catchment is well drained as it is underlain by a regionally important karstified limestone aquifer with permeable limestone tills and gravels, while the other three catchments are underlain by poorly productive aquifers and low permeability clayey tills, and are poorly drained.
All catchments were instrumented, and groundwater, surface and near-surface water and aquatic ecology were monitored for a period of two years. Intensive water quality sampling during rainfall events was used to investigate the pathways delivering nutrients. The proportion of flow along each pathway was determined using chemical and physical hydrograph separation techniques, supported by numerical modelling.
The outcome of the field studies broadly supported the use of the initial four-pathway conceptual model used in the Pathways CMT (time-variant model). The artificial drainage network was found to be a significant contributing pathway in the poorly drained catchments, at low flows and during peak flows in wet antecedent conditions. The transition zone (TZ), i.e. the broken up weathered zone at the top of the bedrock, was also found to be an important pathway. It was observed to operate in two contrasting hydrogeological scenarios: in groundwater discharge zones the TZ can be regarded as being part of the shallow groundwater pathway, whereas in groundwater recharge zones it behaves more like interflow.
In the catchments overlying poorly productive aquifers, only a few fractures or fracture zones were found to be hydraulically active and the TZ, where present, was the main groundwater pathway. In the karstified Nuenna catchment, the springs, which are linked to conduits as well as to a diffuse fracture network, delivered the majority of the flow. These findings confirm the two-component groundwater contribution from bedrock but suggest that the size and nature of the hydraulically active fractures and the nature of the TZ are the dominant factors at the scale of a stream flow event.
Diffuse sources of nitrate were found to be typically delivered via the subsurface pathways, especially in the TZ and land drains in the poorly productive aquifer catchments, and via the bedrock groundwater in the Nuenna. Phosphorus was primarily transported via overland flow in both particulate and soluble forms. Where preferential flow paths existed in the soil and subsoil, soluble P, and to a lesser extent particulate P, were also transported via the TZ and in drains and ditches. Arable land was found to be the most important land use for
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the delivery of sediment, although channel bank and in-stream sources were the most significant in the Glen Burn catchment. Overland flow was found to be the predominant transport sediment pathway in the poorly productive catchments. These findings informed the development of the transport and attenuation equations used in the CCT and CMT. From an assessment of the relationship between physico-chemical and biological conditions, it is suggested that in the Nuenna, Glen Burn and Gortinlieve catchments, a relationship may exist between biological water quality and nitrogen concentrations, as well as with P. In the Nuenna, there was also a relationship between macroinvertebrate status and alkalinity.
Further research is recommended on the transport and delivery of phosphorus in groundwater, the transport and attenuation dynamics in the TZ in different hydrogeological settings and the relationship between macroinvertebrates and co-limiting factors. High resolution temporal and spatial sampling was found to be important for constraining the conceptual understanding of nutrient and sediment dynamics which should also be considered in future studies.