Tiernan Henry

Consequences of varied soil hydraulic and meteorological complexity on unsaturated zone time lag estimates.

The true efficacy of a programme of agricultural mitigation measures within a catchment to improve water quality can be determined only after a certain hydrologic time lag period (subsequent to implementation) has elapsed. As the biophysical response to policy is not synchronous, accurate estimates of total time lag (unsaturated and saturated) become critical to manage the expectations of policy makers. The estimation of the vertical unsaturated zone component of time lag is vital as it indicates early trends (initial breakthrough), bulk (centre of mass) and total (Exit) travel times. Typically, estimation of time lag through the unsaturated zone is poor, due to the lack of site specific soil physical data, or by assuming saturated conditions. Numerical models (e.g. Hydrus 1D) enable estimates of time lag with varied levels of input data. The current study examines the consequences of varied soil hydraulic and meteorological complexity on unsaturated zone time lag estimates using simulated and actual soil profiles. Results indicated that: greater temporal resolution (from daily to hourly) of meteorological data was more critical as the saturated hydraulic conductivity of the soil decreased; high clay content soils failed to converge reflecting prevalence of lateral component as a contaminant pathway; elucidation of soil hydraulic properties was influenced by the complexity of soil physical data employed (textural menu, ROSETTA, full and partial soil water characteristic curves), which consequently affected time lag ranges; as the importance of the unsaturated zone increases with respect to total travel times the requirements for high complexity/resolution input data become greater. The methodology presented herein demonstrates that decisions made regarding input data and landscape position will have consequences for the estimated range of vertical travel times. Insufficiencies or inaccuracies regarding such input data can therefore mislead policy makers regarding the achievability of water quality targets.

Arsenic contamination of drinking water in Ireland: A spatial analysis of occurrence and potential risk.

The presence of arsenic in groundwater has become a global concern due to the health risks from drinking water with elevated concentrations. The Water Framework Directive (WFD) of the European Union calls for drinking water risk assessment for member states. The present study amalgamates readily available national and sub-national scale datasets on arsenic in groundwater in the Republic of Ireland. However, due to the presence of high levels of left censoring (i.e. arsenic values below an analytical detection limit) and changes in detection limits over time, the application of conventional statistical methods would inhibit the generation of meaningful results. In order to handle these issues several arsenic databases were integrated and the data modelled using statistical methods appropriate for non-detect data. In addition, geostatistical methods were used to assess principal risk components of elevated arsenic related to lithology, aquifer type and groundwater vulnerability. Geographic statistical methods were used to overcome some of the geographical limitations of the Irish Environmental Protection Agency (EPA) sample database. Nearest-neighbour inverse distance weighting (IDW) and local indicator of spatial association (LISA) methods were used to estimate risk in non-sampled areas. Significant differences were also noted between different aquifer lithologies, indicating that Rhyolite, Sandstone and Shale (Greywackes), and Impure Limestone potentially presented a greater risk of elevated arsenic in groundwaters. Significant differences also occurred among aquifer types with poorly productive aquifers, locally important fractured bedrock aquifers and regionally important fissured bedrock aquifers presenting the highest potential risk of elevated arsenic. No significant differences were detected among different groundwater vulnerability groups as defined by the Geological Survey of Ireland. This research will assist management and future policy directions of groundwater resources at EU level and guide future research focused on understanding arsenic mobilisation processes to facilitate in guiding future development, testing and treatment requirements of groundwater resources.

Mapping arsenopyrite alteration in a quartz vein-hosted gold deposit using microbeam analytical techniques

Abstract An unworked quartz vein-hosted gold deposit occurs in the Clew bay area of County Mayo, western Ireland. The veins are late-Caledonian in age and transect greenschist-facies poly-deformed Silurian quartzites. The veins contain disseminated arsenopyrite thatmay be a primary mineral source for elevated levels of arsenic (As) found in groundwater samples recovered from wells related spatially to the gold deposit. Levels from 5 to 188 μg/L (significantly above the 7.5 μg/L threshold for safe drinking water) have been detected. A series of element distribution maps using a scanning electronmicroscope (Hitachimodel S-4700) linked to an energydispersive spectrometer (INCA® Oxford Instruments) and mineral distribution maps generated by QEMSCAN® (Quantitative Evaluation of Minerals by Scanning electron microscopy) were used to map the distribution of the primary arsenopyrite and related secondary As-bearing phases. Laser Raman microspectroscopy was used to identify the secondary As-bearing phases. ‘Island weathering’ of primary arsenopyrite together with hydrated pseudomorphs of arseniosiderite, pharmacosiderite and scorodite after arsenopyrite are recorded. Circulating groundwater hydrates the primary arsenopyrite, providing the release mechanism that forms the secondary As-bearing phases that occur as microfracture infills together with muscovite and biotite. The textural relationships between the primary and secondary As minerals indicate their potential as mineral sources of As that could enter transport pathways leading to its release into groundwater.

Terrestrial and marine electrical resistivity to identify groundwater pathways in coastal karst aquifers

Groundwater movement in karst aquifers is characterised by high-velocity fissure and conduit flow paths, and in coastal karst aquifers, these act as pathways for saline intrusion and freshwater discharge to the sea. This paper examines groundwater movement in two neighbouring catchments in the west of Ireland that represent canonical coastal karst aquifers dominated by discharges in the intertidal zone and at offshore submarine springs. Terrestrial and surface-towed marine electrical resistivity tomography, coupled with ancillary hydrogeological data, identifies the influence of faulting and conduits on groundwater egress/saltwater ingress. The on-shore and off-shore subsurface geometry of major fault zones is identified, and the tidal influence of seawater and groundwater flow is demonstrated in these zones and karst springs. Imaging of these sub-surface structures is a pre-requisite for numerical modelling of current and future climate-driven freshwater–seawater interactions in karst coastal aquifers.

A methodological framework to determine optimum durations for the construction of soil water characteristic curves using centrifugation

Abstract During laboratory assessment of the soil water characteristic curve (SWCC), determining equilibrium at various pressures is challenging. This study establishes a methodological framework to identify appropriate experimental duration at each pressure step for the construction of SWCCs via centrifugation. Three common temporal approaches to equilibrium – 24-, 48- and 72-h – are examined, for a grassland and arable soil. The framework highlights the differences in equilibrium duration between the two soils. For both soils, the 24-h treatment significantly overestimated saturation. For the arable site, no significant difference was observed between the 48- and 72-h treatments. Hence, a 48-h treatment was sufficient to determine ‘effective equilibrium’. For the grassland site, the 48- and 72-h treatments differed significantly. This highlights that a more prolonged duration is necessary for some soils to conclusively determine that effective equilibrium has been reached. This framework can be applied to other soils to determine the optimum centrifuge durations for SWCC construction.

Characterizing the Hydrogeology of Bell Harbour Catchment, a Coastal Karstic Aquifer Influenced by the Tide and Affected by a Saltwater Intrusion

A hydrogeological study focused on a karstic aquifer of a small catchment (~50 km2) located on the south coast of Galway Bay is described. The key aim of the study is to better understand the freshwater/seawater interaction in this coastal region west of Ireland. Discharge from the catchment is entirely through intertidal diffuse springs and submarine groundwater discharges (SGD). Logging of temperature, conductivity and water levels at coastal springs, turloughs and boreholes in the catchment is underway; water samples have been recovered for chemical analysis and water tracing has been undertaken. Initial results clearly show a tidal influence up to 2.5 km inland and an intrusion of seawater at least up to 1 km inland. This saltwater intrusion varies, depending on the balance between the tidal periods (spring/neap) and the groundwater level.