Triassic Sherwood Sandstone Aquifer Research Articles

Permeability of rock discontinuities and faults in the Triassic Sherwood Sandstone Group (UK): insights for management of fluvio-aeolian aquifers worldwide

Fluvio-aeolian sedimentary successions host groundwater aquifers at shallow depths (<~0.15 km), which overlie geothermal and shale-gas reservoirs, and nuclear waste repositories at intermediate depths (~0.15–2.0 km). Additionally, such deposits represent petroleum reservoirs at greater depths (~2.0–4.0 km). The need to improve conceptual understanding of the hydraulic behaviour of fluvial-aeolian sandstone successions over a large depth interval (~0–4 km) is important for socio-economic reasons. Thus, the hydraulic properties of the Triassic Sherwood Sandstone aquifer in the UK have been reviewed and compared to similar fluvio-aeolian successions. The ratio between well-scale and core-plug-scale permeability (Kwell-test/Kcore-plug) acts as a proxy for the relative importance of fracture versus intergranular flow. This ratio (which typically varies from ~2 to 100) indicates significant contribution of fractures to flow at relatively shallow depths (<~0.15 km). Here, permeability development is controlled by dissolution of calcite-dolomite in correspondence of fractures. The observed ratio (Kwell-test/Kcore-plug) decreases with depth, approaching unity, indicating that intergranular flow dominates at ~1 km depth. At depths ≥ ~1 km, dissolution of carbonate cement by rock alteration due to groundwater flow is absent and fractures are closed. Aeolian and fluvial deposits behave differently in proximity to normal faults in the Sherwood Sandstone aquifer. Deformation bands in aeolian dune deposits strongly compartmentalize this aquifer. The hydro-structural properties of fluvio-aeolian deposits are also controlled by mineralogy in fault zones. A relative abundance of quartz vs. feldspar and clays in aeolian sandstones favours development of low-permeability deformation bands.

Age and quality stratification of groundwater in the Triassic Sherwood Sandstone aquifer of South Yorkshire and the East Midlands, UK

Groundwater from the Triassic Sherwood Sandstone aquifer of the English East Midlands shows a well-documented evolution in chemistry and residence time as it flows downgradient from shallow unconfined conditions to greater depths, confined by Mercia Mudstone. The flow path covers a lateral distance of some 40 km and depths approaching 800 m. The presence of fresh groundwater (Cl = 15 mg/L, Na = 17 mg/L, SO4 = 144 mg/L, TDS = 500 mg/L) of likely Pleistocene age can be demonstrated down to about 500 m depth in the aquifer, some 10 km downgradient of its confined/unconfined interface. This is amongst the deepest fresh groundwater identified in the UK. Beyond that zone, salinity increases in response to dissolution of gypsum or anhydrite, giving Ca-SO4 groundwater. Further downgradient still, saline groundwater (TDS = 10 g/L) is encountered in the deepest part of the confined aquifer. Downgradient geochemical evolution and increasing residence time are also observed in the South Yorkshire section of the aquifer, to the north of the East Midlands flow path. Here, analogous sequential geochemical changes are observed, controlled by carbonate and sulphate mineral reactions, redox changes and silicate hydrolysis, with evidence of increased inputs of e.g. Na, Cl and SO4 from modern pollution at shallow depths in the unconfined aquifer. Redox conditions are more variable in the western part of the South Yorkshire aquifer, due to local confinement or semi-confinement by fine-grained Quaternary deposits overlying the Sherwood Sandstone. Ratios of δ13C in this section of aquifer increase from −14‰ at outcrop to −11‰ in Mercia-Mudstone-confined conditions as a result of gradual equilibration with aquifer carbonate under closed-system conditions. Ratios of δ18O and δ2H become more depleted, reaching −9‰ and −61‰ respectively, suggesting recharge under climatic conditions significantly cooler than the modern era. Increasing residence time is also indicated by increasing concentrations of a number of trace elements (e.g. Li, Rb, Sr, Mo). These transitions are also observed vertically within investigated boreholes in unconfined and semi-confined sections of the aquifer. Deep groundwater (160–170 m depth) from a borehole within the unconfined aquifer of South Yorkshire has δ18O and δ2H ratios as depleted as −9.2‰ and −67‰ respectively. Such depth stratification indicates that good-quality palaeowater, likely of Pleistocene age, can exist at depth even in the unconfined Sherwood Sandstone aquifer, and that vertical hydraulic conductivity is much lower than lateral. A paucity of boreholes in the Mercia-Mudstone-confined section of the South Yorkshire aquifer makes estimation of the lateral extent of fresh groundwater there more problematic, though brackish groundwater (Cl = 1300 mg/L, TDS = 5.5 g/L) has been identified 12 km east of the aquifer's confined margin.

Isotopic composition of sulfate as a tracer of natural and anthropogenic influences on groundwater geochemistry in an urban sandstone aquifer, Birmingham, UK

Abstract Development and management of urban groundwater resources is limited in practice by a lack of knowledge of the nature, distribution and sources of groundwater contamination in an urban aquifer that may have had complex land use and abstraction history. Sulfate is associated with many of the solute sources to urban groundwater and in this study the use of dual isotope (δ18O and δ34S) “fingerprinting” of SO 4 2 - in groundwater to assess the contribution of different sources to urban groundwater is investigated. Groundwater (70 locations) and surface water samples from the city of Birmingham on the Triassic Sherwood sandstone aquifer have been analyzed for inorganic chemical and SO 4 2 - isotopic composition. Isotopic compositions of SO 4 2 - associated with various solute sources have also been determined. Sulfate derived from pyrite oxidation during recharge through Quaternary Drift deposits is characteristically depleted in 18O and 34S compared to other sources and is ubiquitous in unconfined zone groundwater, though the contribution from this source has increased markedly following draw down induced by abstraction from the aquifer. In unpolluted groundwaters other SO 4 2 - sources are dissolution of evaporite minerals (confined zone) and rainfall (unconfined zone). Unfortunately, SO 4 2 - isotopic composition cannot distinguish between made-ground (i.e. artificial man-made ground, usually demolition waste) and sewage sources of SO 4 2 - , which constitute the major contributions to SO 4 2 - in polluted urban groundwater at most sites. Contributions of SO 4 2 - from spilt industrial acids do have a distinctive isotopic composition seen at many metal-working and former metal-working sites. Estimates of solute contributions derived from these sources derived in this way provide a useful check on the “calibration” of aquifer and pollutant flux recharge models for urban groundwater.

Hydrochemical characterization of a coal mine plume detected by an airborne geophysical survey

AbstractA trial airborne electromagnetic (AEM) geophysical survey was carried out across a 13 × 9 km area of the northern Nottinghamshire (UK) coalfield. One of the objectives was to examine the influence of coal mines (collieries) and associated spoil heaps situated above the Triassic Sherwood Sandstone aquifer. The conductivity models obtained from the AEM survey revealed extensive zones of enhanced subsurface conductivity in the vicinity of all the collieries in the survey area. The present study provides information regarding subsequent investigations (ground geophysics and borehole) to confirm the AEM results and to investigate the geochemical nature of the conductive zone identified in the vicinity of one of the collieries.Following ground geophysical confirmation of the airborne results, three boreholes were drilled into one of the conductive zones to the east of a working colliery. Geophysical logs and analysis of pore fluid geochemistry demonstrate that there is a very strong correlation between the borehole induction logs (formation conductivity), total dissolved solids (TDS) and chloride in the pore waters. The three cored boreholes have confirmed that there is a groundwater plume with high concentrations of TDS, compared with background, moving east from the Thoresby Colliery on a hydraulic gradient of 0.023. Porewater geochemistry obtained from core has been compared with that of local aquifer waters and with samples obtained from the colliery. The results indicate that the plume chemistry could result from the mixing of a typical Sherwood Sandstone composition with colliery spoil leachate. The AEM conductivity models have been successfully correlated with ground geophysical models, borehole induction logs, TDS and chloride in the pore waters. By implication, the geophysical results obtained over a larger area indicate the wider extent of such impacts in relation to both former and current mining activities.

Pesticides in groundwater: some observations on temporal and spatial trends

AbstractFollowing a monitoring programme of private shallow boreholes a total of 22 different pesticides were detected in 14 shallow boreholes in the Triassic Sherwood Sandstone aquifer of south Yorkshire, UK between January 2002 and November 2003. Of the 294 positive detections made, 34% were detected in excess of the permitted concentration value (PCV) (drinking water standard) for individual pesticides. Pesticides were detected in 11 (78%) of the boreholes, and in eight (57%) at concentrations exceeding the PCV on at least one occasion. These findings were complemented by data from deeper boreholes in which concentrations exceeding the PCV were also detected. Pesticide occurrence could be related to landuse; mecoprop and isoproturon were frequently detected at several locations reflecting the arable landuse, dicamba at a golf course, and atrazine, propazine and terbutryn near a railway. Temporal variations have emerged at individual sites and across the study area as the result of extended and frequent monitoring.

Combined isotopic and modelling approach to determing the source of saline groundwaters in the Selby Triassic sandstone aquifer, UK

Abstract Groundwater abstraction from the Triassic Sherwood Sandstone aquifer in the Selby area, Yorkshire, UK, has caused decline of the water level in the aquifer and groundwater quality problems such as high chloride and sulphate concentrations. Geochemical and isotopic analysis of groundwaters and groundwater modelling of an approximately 25 × 30 km area around Selby was carried out to understand the groundwater flow conditions and identify the source of saline water. Isotopic compositions (δ 34 S and δ 18 O) of seawater sulphate, Coal Measures brine and Permian evaporite sources do not match that associated with salinity in the Selby wellfield. Rather, the source of saline groundwater in the Selby wellfield matches the isotopic composition of Triassic evaporites in the overlying Mercia Mudstones. Steady-state groundwater flow modelling demonstrates that that majority of water abstracted from the Selby wellfield is balanced by recharge from the west of Selby at Brayton Barff, and from the Escrick and York moraines to the north. A small proportion of the abstracted water originates from leakage from the River Ouse through the confining layer as a result of drawdown in the Selby area. The development of a cone of drawdown centred on Selby has created a new E-W hydraulic gradient to the east of Selby, allowing water ingress from the east. Capture-zone analysis indicates that the four abstraction boreholes contaminated by saline water collect groundwater from the north to NE (i.e. from the direction of the boundary between the Sherwood Sandstone and the Mercia Mudstone), confirming that the source of salinity is likely to be the Mercia Mudstone evaporites.

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.

Implications of flow patterns in the sandstone aquifer beneath the mature conurbation of Nottingham (UK) for source protection

The protection of groundwater sources in urban areas is difficult due to uncertainties in hydrogeology, future changes in pumping, and complexities of land use. This paper presents an improved understanding of the hydrogeology of the Permo–Triassic Sherwood Sandstone aquifer underNottingham, and considers protection policies for this example of a highly developed groundwater system. Groundwater flow models were constructed for the pre-industrial period (before 1850) and for the 1980 s, and confirmed the existing conceptual model of the hydrogeology. In the early period, spring flows were much more significant than they are now, while abstractions through wells account for virtually all the discharge in the recent period. In recent decades, abstraction has fallen, but there has been a significant reduction in recharge from mains leakage so that there has been no overall rise in groundwater levels. Particle tracking revealed that during recent times the regional aquifer has behaved as a series of local flow cells rather than a regional whole due to the numerous abstractions. Wells in the unconfined aquifer capture most of the urban (and potentiallypolluted) recharge with travel-times up to 200 years. Confined wells with travel-times over 10 4 years, will abstract pre-industrial waters for years to come, and are likely to be protected by natural attenuation of pollutants. Deterministic source protection zones will not be fully protective of public and industrial water supplies, due to the likely changes in pumping regimes in future and the resourcing implications of attempting to control polluting activities over the whole city. A risk-based approach to managing urban groundwater may be more practical and useful. It could predict the risks to existing water supplies, and help in the selection of low risk locations for new wells.

Seasonal variation of moisture content in unsaturated sandstone inferred from borehole radar and resistivity profiles

Understanding the processes controlling recharge to aquifers is critical if accurate predictions are to be made on the fate of contaminants in the subsurface environment. In order to understand fully the hydrochemical mechanisms in the vadose zone it is essential that the dynamics of the hydrology can be suitably characterised. The correlation between moisture content and both bulk dielectric and resistivity properties of porous media is well established. Using suitably placed sensors in boreholes detailed depth profiles of dielectric and resistivity behaviour have been monitored over a period of two years at a Triassic Sherwood Sandstone aquifer field site at Hatfield, England. The borehole–borehole transmission radar and borehole resistivity profiles show a significant correlation. Through appropriate petrophysical relationships, derived from core samples, seasonal dynamics of the vadose zone are seen to illustrate the migration of wetting and drying fronts over the monitoring period. At a second field site in Eggborough, located 17km from Hatfield, similar temporal changes in moisture content in the sandstone were observed using borehole radar profiles. Travel times of seasonal wetting fronts through the sandstone at both sites appear to be approximately 2m per month. The retardation of this front propagation in the top 3m is also common to both sites, suggesting that pollutant transport may be principally controlled by near surface sediments. The results have important consequences to existing groundwater modelling programmes that are being utilised to predict transfer of agricultural chemicals through the vadose zone.

Mine water rebound in South Nottinghamshire: risk evaluation using 3-D visualization and predictive modelling

Progressive abandonment of the South Nottinghamshire Coalfield raises concerns over the security of the Permo–Triassic Sherwood Sandstone aquifer which overlies the concealed part of the coalfield. A 3-D digital visualization package has been used to assemble and display the complex and diverse data-sets of relevance. Predictive scenarios have been run from these data using the University of Newcastle program GRAM (Groundwater Rebound in Abandoned Mineworkings). The work comprised three phases: (i) confirmation of the geological framework for the so-called ‘Pond 3’ area (southernmost part of the coalfield) and establishment of a water balance along with an outline groundwater flow path system for the Coal Measures and adjacent strata; (ii) the collation of detailed geometric information on the spatial distribution of discrete geological layers that are considered to have hydrogeological significance, the distribution of mineworkings within key horizons, and the locations of boreholes, shafts and pumping stations (both in the Coal Measures and within overlying strata). Possible flooding configurations have been assessed geometrically to identify ‘hot spots’ where mine water discharge to surface may occur, and areas where the piezometric level of the rising mine water might promote upward fluxes into the Permo–Triassic Sherwood Sandstone aquifer. In addition, critical areas where coal has been worked close to the base of the Permian and where hydraulic continuity may occur between the Sherwood Sandstone and Coal Measures have been identified; (iii) the GRAM model used data held in the 3-D visualization package VULCAN to define discrete ‘ponds’ within the coalfield. Recharge to the system allows each pond to fill until overflow pathways are reached, when the adjacent pond may start to fill. A variety of such scenarios have been completed and predictive data generated, which suggest that possible discharge to surface and into the Sherwood Sandstone might occur about 20 years after the end of dewatering.

The Triassic Sherwood Sandstone aquifer in Northern Ireland: constraint of a groundwater flow model for resource management

Abstract The most important aquifer in Northern Ireland is the Triassic Sherwood Sandstone formation that lies under the urban area of Belfast and the areas to the east (Newtownards) and south (Lagan Valley) of the city. The management of this groundwater resource is important in providing a sustainable supply for both public and industrial users. A groundwater flow model that collated relevant meteorological, hydrological and hydrogeological information for the Lagan Valley and Newtownards areas was developed. This conceptual model was coupled with geochemical and isotopic data (using inverse modelling) to constrain the groundwater flow parameters. The results of 14 C-dating suggest some of the groundwater in the aquifer may be up to 4000 years old, and that structural controls play a major role in both the flow rate and the spatial distribution of groundwater within the Sherwood Sandstone aquifer.

The potential for aquifer disposal of carbon dioxide in the UK

Natural analogues indicate that it is possible to dispose of CO 2 underground in closed structures on deep aquifers. Disposal into depleted or exhausted hydrocarbon fields has many advantages, e.g. proven seal, known storage capacity, no exploration costs. Unfortunately there are very few hydrocarbon fields in the UK onshore area, and their total CO 2 storage capacity is very low compared to annual UK CO 2 production from power generation. The best aquifers for CO 2 disposal onshore are the widespread Permo-Triassic sandstones. Further onshore potential exists in younger Mesozoic reservoirs. Offshore, disposal into depleted oil fields (where cost credits from enhanced oil recovery could be beneficial) or the Perno-Triassic gas fields of the southern North Sea, and nearby associated closures of the Triassic Sherwood Sandstone aquifer, appear to provide the best prospects.