Permo-Triassic Sandstone Aquifer Research Articles

A personal perspective of 20 years of regional groundwater resource modelling of the Permo-Triassic Sandstone aquifers in England

In 1998, the Permo-Triassic Sandstone aquifer of the West Midlands–Worfe (WMW) area became the subject of one of the first major groundwater modelling projects initiated under the Environment Agency of England's national groundwater modelling framework. The 4R runoff–recharge code that allowed these models to apply a semi-integrated approach to simulating catchment hydrology was developed and trialled in parallel with the WMW project. This combined groundwater and surface water modelling approach has allowed the Environment Agency of England to make complex water resource management decisions in a wide range of aquifers and catchments based, in part, on outputs from such models. Since that time, the Environment Agency has used the framework to develop around a dozen further models of Permo-Triassic Sandstone aquifers, of varying levels of complexity. Many of the older models have subsequently been through one or more cycles of updating and/or recalibration. This paper provides a high-level review of these models based on the personal experience of the author and compares and contrasts their implementation in the US Geological Survey's groundwater modelling code MODFLOW. This includes a discussion of key conceptual issues that are common across the models, or unique to particular models, and the uses to which the models have been put. The paper concludes with a discussion of future opportunities and challenges for the programme. Thematic collection: This article is part of the Hydrogeology of Sandstone collection available at: https://www.lyellcollection.org/topic/collections/hydrogeology-of-sandstone

How groundwater time series and aquifer property data explain heterogeneity in the Permo-Triassic sandstone aquifers of the Eden Valley, Cumbria, UK

A novel investigation of the impact of meteorological and geological heterogeneity within the Permo-Triassic Sandstone aquifers of the River Eden catchment, Cumbria (UK), is described. Quantifying the impact of heterogeneity on the water cycle is increasingly important to sustainably manage water resources and minimise flood risk. Traditional investigations on heterogeneity at the catchment scale require a considerable amount of data, and this has led to the analysis of available time series to interpret the impact of heterogeneity. The current research integrated groundwater-level and meteorological time series in conjunction with aquifer property data at 11 borehole locations to quantify the impact of heterogeneity and inform the hydrogeological conceptual understanding. The study visually categorised and used seasonal trend decomposition by LOESS (STL) on 11 groundwater and meteorological time series. Decomposition components of the different time series were compared using variance ratios. Though the Eden catchment exhibits highly heterogeneous rainfall distribution, comparative analysis at borehole locations showed that (1) meteorological drivers at borehole locations are broadly homogeneous and (2) the meteorological drivers are not sufficient to generate the variation observed in the groundwater-level time series. Three distinct hydrogeological regimes were identified and shown to coincide with heterogeneous features in the southern Brockram facies, which is the northern silicified region of the Penrith Sandstone and the St Bees Sandstone. The use of STL analysis in combination with detailed aquifer property data is a low-impact insightful investigative tool that helps guide the development of hydrogeological conceptual models.

Use of seasonal trend decomposition to understand groundwater behaviour in the Permo-Triassic Sandstone aquifer, Eden Valley, UK

The daily groundwater level (GWL) response in the Permo-Triassic Sandstone aquifers in the Eden Valley, England (UK), has been studied using the seasonal trend decomposition by LOESS (STL) technique. The hydrographs from 18 boreholes in the Permo-Triassic Sandstone were decomposed into three components: seasonality, general trend and remainder. The decomposition was analysed first visually, then using tools involving a variance ratio, time-series hierarchical clustering and correlation analysis. Differences and similarities in decomposition pattern were explained using the physical and hydrogeological information associated with each borehole. The Penrith Sandstone exhibits vertical and horizontal heterogeneity, whereas the more homogeneous St Bees Sandstone groundwater hydrographs characterize a well-identified seasonality; however, exceptions can be identified. A stronger trend component is obtained in the silicified parts of the northern Penrith Sandstone, while the southern Penrith, containing Brockram (breccias) Formation, shows a greater relative variability of the seasonal component. Other boreholes drilled as shallow/deep pairs show differences in responses, revealing the potential vertical heterogeneities within the Penrith Sandstone. The differences in bedrock characteristics between and within the Penrith and St Bees Sandstone formations appear to influence the GWL response. The de-seasonalized and de-trended GWL time series were then used to characterize the response, for example in terms of memory effect (autocorrelation analysis). By applying the STL method, it is possible to analyse GWL hydrographs leading to better conceptual understanding of the groundwater flow. Thus, variation in groundwater response can be used to gain insight into the aquifer physical properties and understand differences in groundwater behaviour.

Nested monitoring approaches to delineate groundwater trichloroethene discharge to a UK lowland stream at multiple spatial scales

Integrated approaches for the identification of pollutant linkages between aquifers and streams are of crucial importance for evaluating the environmental risks posed by industrial contaminants like trichloroethene (TCE). This study presents a systematic, multi-scale approach to characterising groundwater TCE discharge to a 'gaining' UK lowland stream receiving baseflow from a major Permo-Triassic sandstone aquifer. Beginning with a limited number of initial monitoring points, we aim to provide a 'first pass' mechanistic understanding of the plume's fate at the aquifer/stream interface using a novel combination of streambed diffusion samplers, riparian monitoring wells and drive-point mini-piezometers in a spatially nested sampling configuration. Our results indicate the potential discharge zone of the plume to extend along a stream reach of 120 m in length, delineated by a network of 60 in-situ diffusion samplers. Within this section, a 40 m long sub-reach of higher concentration (>10 μg L(-1)) was identified; centred on a meander bend in the floodplain. 25 multi-level mini-piezometers installed to target this down-scaled reach revealed even higher TCE concentrations (20-40 μg L(-1)), significantly above alluvial groundwater samples (<6 μg L(-1)) from 15 riparian monitoring wells. Significant lateral and vertical spatial heterogeneity in TCE concentrations within the top 1m of the streambed was observed with the decimetre-scale vertical resolution provided by multi-level mini-piezometers. It appears that the distribution of fine-grained material in the Holocene deposits of the riparian floodplain and below the channel is exerting significant local-scale geological controls on the location and magnitude of the TCE discharge. Large-scale in-situ biodegradation of the plume was not evident during the monitoring campaigns. However, detections of cis-1,2-dichloroethene and vinyl chloride in discrete sections of the sediment profile indicate that shallow (e.g., <20 cm) TCE transformation may be significant at a local scale in the streambed deposits. Our findings highlight the need for efficient multi-scale monitoring strategies in geologically heterogeneous lowland stream/aquifer systems in order to more adequately quantify the risk to surface water ecological receptors posed by point-source groundwater contaminants like TCE.

Groundwater Resources Modeling: A Case Study from the U.K.

As populations grow, climates change, and water resources (and budgets) dwindle, well-documented, wellcoordinated, and well-maintained groundwater management tools are needed more than ever. This book (Shepley et al. 2012), a collection of papers written specifically for this volume, provides not only case studies of individual groundwater modeling efforts, but also an important case study of an overall effort to model a nation’s aquifers. I read this book with great personal interest because I have been involved with a similar modeling program in Texas. (Texas is not a nation, but it used to be!) This collection of papers documents a national program administered by the United Kingdom’s Environment Agency between 1998 and 2009 to develop groundwater flow models of England’s and Wales’ primary bedrock aquifers. While most nonnatives will be aware of the internationally famous Chalk Aquifer, the book also discusses the Permo-Triassic Sandstone Aquifer, the Carboniferous Limestone Aquifer, and the Scottish alluvial aquifers, among others. The editors were closely involved in the national effort; therefore, they were well-positioned to coordinate and deliver a thorough and well-documented product. Broadly, the papers in the book fall into one of three categories: (1) organization and execution of the national modeling effort; (2) descriptions of the modeling efforts; and (3) discussions of conceptual model issues, methodologies, and use of the models. After a foreword by Mary P. Anderson (who also served as a reviewer of the volume), the first five papers focus on the organization and execution of the modeling program, including the history of modeling in the United Kingdom, the background of the national modeling program, the management of the program and its standards, how stakeholders were included in the process, and how final models have been served to stakeholders and others. Eight of the remaining 17 papers are primarily focused on descriptions of specific regional modeling efforts, with the other nine papers focused on conceptual model issues, methodologies, and use of the models. Topics include estimating recharge, using modeling to better understand groundwater flow, and developing and

Simulation of a water transfer tunnel at catchment scale in the Permo-Triassic Sandstone aquifer, UK

Abstract The Lichfield Permo-Triassic Sandstone aquifer is a strategic groundwater resource for the UK West Midlands. However, the flows of a number of streams have been heavily affected by abstraction. The catchment of one of these, the Leamonsley Brook, is under-drained by the Hanch Tunnel, a 5 km-long water transfer tunnel that is more than 150 years old. The Hanch Tunnel was excavated below the water-table for its entire length and is only partly lined. Regional-scale simulation of the aquifer was undertaken using a version of the USGS MODBRNCH code, which had been previously adapted to enable simulation of both open channel and pressurized pipe flow systems (the ‘Adit Code’). This paper reports on the simulation of the Hanch Tunnel, with an emphasis on the representation of this tunnel, and its effects on the groundwater flow in the Leamonsley Brook catchment. Critical aspects to consider when implementing the Adit Code are identified and discussed. The value of the application of the Adit Code for options appraisal to resolve the low-flow conditions of the Leamonsley Brook is explored.

Comparison of rates of ureolysis between Sporosarcina pasteurii and an indigenous groundwater community under conditions required to precipitate large volumes of calcite

Ureolysis-driven calcite precipitation has potential to seal porosity and fracture networks in rocks thus preventing groundwater flow and contaminant transport. In this study urea hydrolysis and calcite precipitation rates for the model bacterium Sporosarcina pasteurii were compared with those of indigenous groundwater communities under conditions required to precipitate large volumes of calcite (up to 50 g L −1). We conducted microcosm experiments in oxic artificial and anoxic natural groundwaters (collected from the Permo-Triassic sandstone aquifer at Birmingham, UK) that were inoculated with aerobically grown S. pasteurii. The rate constants for urea hydrolysis, k urea, ranged between 0.06 and 3.29 d −1 and were only affected by inoculum density. Higher Ca 2+ concentration (50–500 mM Ca 2+) as well as differences in fO 2 did not inhibit the ureolytic activity of S. pasteurii and did not significantly impact k urea. These results demonstrate that S. pasteurii has potential to improve calcite precipitation in both oxic and anoxic groundwaters, especially if indigenous communities lack ureolytic activity. Urea hydrolysis by indigenous groundwater communities was investigated in anoxic, natural groundwaters amended with urea and CaCl 2. A notable increase in ureolysis rates was measured only when these communities were stimulated with dilute nutrients (with best results from blackstrap molasses). Furthermore, there was a considerable lag time (12–20 days) before ureolysis and calcite precipitation began. Calculated ureolysis rate constants, k urea, ranged between 0.03 and 0.05 d −1 and were similar to k urea values produced by S. pasteurii at low inoculum densities. Overall, this comparative study revealed that the growth of ureolytic microorganisms present within groundwaters can easily be stimulated to enhance rates of urea hydrolysis in the subsurface, and thus can be used to induce calcite precipitation in these environments. The time required for urea hydrolysis to begin is almost instantaneous if an inoculum of S. pasteurii is included, while it may take several weeks for ureolytic groundwater communities to grow and become ureolytically active.

Discussion of ‘The use of electrical resistivity tomography in deriving local-scale models of recharge through superficial deposits’, by M.O. Cuthbert, R. MacKay, J.H. Tellam &amp; R.D. Barker, Quarterly Journal for Engineering Geology and Hydrogeology , 42 , 199–209

Cuthbert et al . (2009) present an interesting and detailed study of the effects that superficial glacial deposits have on recharge to a bedrock aquifer, in this case the Permo-Triassic Sandstone of the Tern catchment in the West Midlands of England. In their discussion they suggest that the current recharge model used by the Environment Agency may not be adequate for the purposes of groundwater abstraction management because it oversimplifies recharge processes through the superficial deposits. In the present discussion paper this point is further explored with reference to the original management objectives of the Environment Agency project that produced a regional groundwater model of the East Shropshire Permo-Triassic Sandstone of the Tern catchment, of which the recharge model was an integral part. The objectives of the East Shropshire groundwater model have been outlined by Streetly & Shepley (2005), in the final report of the Environment Agency project. This report provides an extensive description of the conceptual and numerical model; the main details of the latter have been summarized by Shepley & Streetly (2007). The other reports of the Environment Agency project (Bridge et al . 2002; Streetly et al . 2002; Streetly & Young 2004) quoted by Cuthbert et al . (2009), with the exception of Bridge et al . (2002) are intermediary reports, and not further referred to in this discussion paper. This discussion paper focuses on three aspects: (1) the philosophy behind using models for groundwater management decision making; (2) some background on the East Shropshire groundwater model; (3) an example of how the East Shropshire groundwater model has been used for groundwater management. This provides a basis for a more detailed appraisal of Cuthbert et al .'s work in the context of the management of the East Shropshire Permo-Triassic Sandstone aquifer. …

The use of electrical resistivity tomography in deriving local-scale models of recharge through superficial deposits

Abstract The way in which superficial deposits affect groundwater recharge is often a significant source of uncertainty in groundwater resources and vulnerability assessments. A study of a small catchment in Shropshire, UK, shows how electrical resistivity tomography (ERT), with a degree of borehole control, can be an effective tool for defining the geometry of superficial deposits for purposes of inferring the hydraulic processes controlling groundwater recharge. Major lithological units were mapped to within c . 0.5 m vertically and 5 m horizontally using ERT surveys with a minimum electrode spacing of 2 m. Interpretation was aided by the strong contrast in resistivity between till and glaciolacustrine deposits (20–40 Ω m) and glaciofluvial deposits (generally >100 Ω m) that overlie the Permo-Triassic sandstone aquifer (saturated resistivity 60–145 Ω m) in the study area. A range of local-scale (tens to hundreds of metres) recharge models are presented, based on the findings of the field surveys, and it is shown how existing mapping misses key features of the superficial geology that may be very significant in enhancing or restricting aquifer recharge.

The impacts of coal mining subsidence on groundwater resources management of the East Midlands Permo-Triassic Sandstone aquifer, England

There are extensive concealed coal mines beneath the East Midlands Permo-Triassic Sandstone aquifer, most of which have used longwall mining techniques. Knowledge of the pumping regimes of the concealed mines shows that there is currently little vertical leakage from the Permo-Triassic Sandstone into these mines. Long-term plans to control the groundwater rebound should prevent any large-scale pollution of the Permo-Triassic Sandstone aquifer system from ingress of mine waters. Currently, the largest impacts on the Permo-Triassic Sandstone aquifer system stem from subsidence associated with the mine workings. This paper examines the impacts on groundwater–surface water interaction, in the context of improving the surface water environment to mitigate effects of long-term over-abstraction. Integral to the assessment of the groundwater–surface water interactions are (i) the change to the hydraulic characteristics of the aquifer system related to the fracturing in the Permo-Triassic Sandstone that the subsidence induces and (ii) the reliability of the installations used to monitor groundwater levels and flows across the aquifer system. These two temporal impacts can be difficult to assess from the available data. The former tends to increase aquifer transmissivity, whereas the latter leads to the underestimation of the impacts of groundwater abstraction caused by datum subsidence of several metres. Two example case histories of impacts on groundwater–surface water interactions are considered: streambed fissuring in the catchments of the River Maun and River Meden, and flow improvements caused by subsidence in the Dover Beck catchment. These two case histories show that there are both beneficial and detrimental outcomes for groundwater resources management caused by coal mining induced subsidence.

The estimation of ‘natural’ summer outflows from the Permo-Triassic Sandstone aquifer, UK

This paper examines some techniques that have been applied to estimate the ‘natural’ summer outflow for the Permo-Triassic aquifer of England, the second most important aquifer in the UK for public water supply. A method for naturalizing groundwater models is discussed and the naturalized flows predicted by the groundwater models of three Permo-Triassic Sandstone aquifers are compared against results of an analytical method to determine natural summer outflows for the equivalent aquifer properties. The results from the naturalization of the three groundwater models are broadly comparable and indicate that the simulated naturalized Q85 (i.e. the flow exceeded for 85% of the time) lies in the range of 70–90% of the naturalized long-term average recharge with a mean of 80% long-term average recharge. This provides the basis for a reasonably robust ‘first pass’ method to determine the natural summer outflows of an unconfined Permo-Triassic Sandstone aquifer unit. Some of this variability in the estimated natural summer outflows can be accounted for by the average distance from the ‘upstream’ edge of the aquifer unit to the discharge boundary (i.e. the perpendicular length). Comparison of the results of the naturalized simulations with the predictions of an analytical solution shows that the typical minimum summer baseflow of the analytical solution appears to be most consistent with the Q85 flow condition simulated by the groundwater models. However, it is clear that, at shorter perpendicular lengths (&lt;1.5 km), the simple idealization of the aquifer implicit in the analytical solution is unable to predict natural summer outflows for the Permo-Triassic Sandstone aquifer system. This is likely to be a result of the increased significance of vertical components of flow in smaller aquifer units.

Tracing sources of carbon in urban groundwater using δ13CTDIC ratios

Total dissolved inorganic carbon (TDIC) and its stable isotope ratio δ13CTDIC are used to trace the evolution of the carbon system of groundwater in three UK Permo-Triassic sandstone aquifers. Samples were collected from multilevel piezometers, open boreholes and sewer sampling points in the British Midlands (Nottingham, Birmingham and Doncaster) to evaluate both local and regional variations in δ13CTDIC. δ13C samples of matrix and pore water have also been analysed in each aquifer to further constrain the interpretations. Combining δ13CTDIC ratios with measurements of TDIC and pH clearly distinguishes the principal processes underlying the geochemical evolution of groundwater in Triassic sandstone aquifers, where processes can be both natural (e.g. carbonate dissolution) and anthropogenic (sewer-derived recharge). The paper shows that δ13CTDIC resolves ambiguities that arise from the interpretation of TDIC and pH measurements in isolation. Field measurements demonstrate that, under natural conditions, the carbonate system evolves similarly in each aquifer. An open-system evolution during recharge largely saturates the groundwater with carbonate depending upon its availability in the sandstone matrix. The contribution of sewer exfiltration to urban recharge is readily distinguished by lower pH and higher TDIC values without significant changes in δ13CTDIC.

Hydrochemical evidence of the depth of penetration of anthropogenic recharge in sandstone aquifers underlying two mature cities in the UK

Pollution of urban groundwater is routinely reported but the profile of contamination with depth in urban aquifers is rarely resolved. This limits understanding of the depth of penetration of urban recharge and contaminants, and use of urban groundwater. Penetration of anthropogenic solutes (major ions, trace metals) in Permo-Triassic sandstone aquifers underlying two mature conurbations in the UK was investigated through depth-specific, groundwater sampling of dedicated multilevel piezometers. Identification of solute origin and biogeochemical processes (e.g. denitrification, mineral dissolution) was aided by use of stable isotope ratios ( 34S/ 32S, 18O/ 16O, 15N/ 14N, 13C/ 12C) and chemical speciation modelling (PHREEQC). Depth profiles of aquifer hydrochemistry reveal penetration of anthropogenic solutes to depths of between 30 and 47 m below ground in the unconfined sandstone and confirm the contributions of faecal and industrial effluents to urban recharge. They also highlight the complexity of solute loading and difficulty resolving solute origin from the range of potential sources in urban groundwater. Faecally-derived NO 3 is the most pervasive contaminant exceeding drinking-water quality guidelines and is associated with elevated concentrations of B and SO 4. Elevated concentrations of Li, B, Cr and Co are observed at depth in groundwater contaminated by long-term industrial land use (metalworking). Observed penetration of anthropogenic solutes in the unconfined sandstone is consistent with post-development recharge of urban groundwater (residence times <230 a) indicated by flow modelling, and suggests tentatively that urban abstraction to depths of up to 50 m below ground in the unconfined Permo-Triassic sandstone is required to scavenge contaminated groundwater.

Investigation and management of a kerosene leakage into a Permo-Triassic sandstone aquifer in the UK

Abstract Leakage over a 35-year period, starting in the 1960s, of approximately 9 × 10 6 l of kerosene from an industrial complex located above fractured Permo-Triassic sandstone aquifer resulted in the potential for a significant dissolved phase. However, a phased, risk-based approach identified no significant risks in terms of dissolved-phase petroleum hydrocarbons, BTEX, or volatile or semi-volatile organic compounds. This is considered to be due to biodegradation of potential contaminants via metabolic pathways using electron acceptors primarily provided by the groundwater recharge (oxygen, nitrate and sulphate). Contaminant concentrations have remained below regulatory limits for protection of the receptor since monitoring was initiated in 2000. This case study demonstrates the potential for biodegradation within the Permo-Triassic sandstone aquifer of the UK: the risk-based approach described and the management plan developed could be applicable at other sites.

Characterization of permeability distributions in six lithofacies from the Helsby and Wilmslow sandstone formations of the Cheshire Basin, UK

Abstract The lithofacies and permeability structure of the Abbey Wood borehole, near Delamere, Cheshire, UK, have been investigated so that permeability distributions can be established for characteristic sandstone lithofacies in the Permo-Triassic sandstone aquifer. The borehole penetrates the Helsby Sandstone and Wilmslow Sandstone formations and six major facies (coarse- and fine-grained fluvial channel fill sandstones, sandy sabkha, sand sheet and dune aeolian sandstones, and a massive sandstone facies) and one minor lithofacies, a mudstone facies, have been identified. The permeability distributions for each lithofacies show subtle systematic differences. The two fluvial sandstone facies and the massive sandstone facies have systematically lower permeabilities than the aeolian sandstones, and within the aeolian sandstones the sand sheet and dune sandstones appear to be systematically more permeable than the sabkha facies. Statistical tests on the permeability distributions of each facies indicate that the distributions are statistically distinct and so support the lithofacies classification.

Calcrete as a source of heterogeneity in Triassic fluvial sandstone aquifers (Otter Sandstone Formation, SW England)

Abstract Carbonate is not generally considered as a potential source of large-scale heterogeneity in Permo-Triassic sandstone aquifers. This study shows that carbonate, in the form of early diagenetic calcrete, forms an abundant component of the Triassic Otter Sandstone Formation in south Devon. Three types of calcrete are described from the outcrop of this fluvial sandstone dominated aquifer: rhizocretions; calcrete sheets; and calcrete conglomerates. Data obtained from core plugs show that calcrete reduces sandstone permeability to less than 10 −15 m 2 and porosity to less than 12%. Calcrete can be sufficiently abundant to produce a significant (up to 30%) reduction in the total effective porosity of the aquifer. Moreover, calcrete conglomerates can form laterally extensive (up to 1 km) low-permeability sheets that will represent major baffles to vertical flow.

Structural controls on groundwater flow in the Permo-Triassic sandstones of NW England

Abstract There is increasing evidence that groundwater flow in many parts of the major Permo-Triassic sandstone aquifers of NW England is influenced strongly by predominantly N-S-trending faults. These structural controls on groundwater flow may only become apparent when the aquifers are subject to abstraction stress. A series of case examples are presented, from the Fylde Sandstone aquifer north of Preston, and from the sandstone aquifers of the Lower Mersey Basin, Manchester and Wirral areas. In these studies the ‘compartmentalization’ of the aquifers by faults has been recognized in field investigations and also in numerical modelling studies related to groundwater resources development on both local and aquifer-wide scales.

Prioritisation of abstraction boreholes at risk from chlorinated solvent contamination on the UK Permo-Triassic Sandstone aquifer using a GIS

A 250 km 2 area of the Permo-Triassic Sandstone aquifer in the West Midlands of England, UK, was selected as a test region for the development of a geographic information system (GIS)-based risk assessment methodology that incorporates contaminant source, groundwater vulnerability and groundwater abstraction catchment elements in order to prioritise areas and boreholes potentially at risk from chlorinated solvent pollution on a regional scale. Factors incorporated in the vulnerability assessment include the nature of soils, presence or absence of superficial or glacial deposits, fault density and depth to water table. arcview gis was employed with a simple ranking system from which the derived vulnerability assessment index was combined with current chlorinated solvent user industry data and source protection zone components. Results indicate the presence of high-risk areas in urban locations where locally dense distributions of chlorinated solvent user industries combine with high vulnerability aquifers within the catchment of supply boreholes. Ranking of catchment-specific risk reveals the abstraction points under greatest stress. The proposed methodology has applications as a regional-scale initial screening tool to guide site selection for regulatory inspections and assist in prioritising monitoring strategies for existing boreholes. Future developments will provide guidance for locating new urban boreholes in areas of lowest risk.

Assessing the natural attenuation of organic contaminants in aquifers using plume-scale electron and carbon balances: model development with analysis of uncertainty and parameter sensitivity

A quantitative methodology is described for the field-scale performance assessment of natural attenuation using plume-scale electron and carbon balances. This provides a practical framework for the calculation of global mass balances for contaminant plumes, using mass inputs from the plume source, background groundwater and plume residuals in a simplified box model. Biodegradation processes and reactions included in the analysis are identified from electron acceptors, electron donors and degradation products present in these inputs. Parameter values used in the model are obtained from data acquired during typical site investigation and groundwater monitoring studies for natural attenuation schemes. The approach is evaluated for a UK Permo-Triassic Sandstone aquifer contaminated with a plume of phenolic compounds. Uncertainty in the model predictions and sensitivity to parameter values was assessed by probabilistic modelling using Monte Carlo methods. Sensitivity analyses were compared for different input parameter probability distributions and a base case using fixed parameter values, using an identical conceptual model and data set. Results show that consumption of oxidants by biodegradation is approximately balanced by the production of CH 4 and total dissolved inorganic carbon (TDIC) which is conserved in the plume. Under this condition, either the plume electron or carbon balance can be used to determine contaminant mass loss, which is equivalent to only 4% of the estimated source term. This corresponds to a first order, plume-averaged, half-life of >800 years. The electron balance is particularly sensitive to uncertainty in the source term and dispersive inputs. Reliable historical information on contaminant spillages and detailed site investigation are necessary to accurately characterise the source term. The dispersive influx is sensitive to variability in the plume mixing zone width. Consumption of aqueous oxidants greatly exceeds that of mineral oxidants in the plume, but electron acceptor supply is insufficient to meet the electron donor demand and the plume will grow. The aquifer potential for degradation of these contaminants is limited by high contaminant concentrations and the supply of bioavailable electron acceptors. Natural attenuation will increase only after increased transport and dilution.

Sediment filled fractures in the Permo-Triassic sandstones of the Cheshire Basin: observations and implications for pollutant transport

Fracture mapping in a tunnel system and at nearby outcrop on the Runcorn Peninsula, UK, suggests the need for a review of the potential pathways for pollutant transport in Permo-Triassic sandstone aquifers. Sediment infilling is pervasive in the largest sub-vertical multi-layer fractures in the study area, both at the surface and to a depth of about 40 m below ground level. Sediment infill is inferred to have formed in situ. The conventional models of pollutant transport in fracture networks assume that they comprise open fractures, with pollutant mobility depending on fracture connectivity (a function of density, length, orientation and intersection) and aperture. The presence of extensive sediment fills in fractures will materially change their permeability, thereby reducing pollutant flux, and be of significance in the assessment of risks arising from chemical spillages. There has been little or no substantive evidence for such fills in Permo-Triassic sandstones in the UK, apart from observations at outcrop and anecdotes of sand being pumped from boreholes. Here, we report surface and rare, but complementary, subsurface observations of extensive fills in the Cheshire basin, and argue that they will only act as preferential pathways where they crosscut low-permeability horizons such as mudstones.