Effects of old landfills on groundwater quality. Phase 2, investigation of the Thriplow landfill 1996–1997

Disused sand and gravel excavations overlying the major Chalk aquifer at Thriplow in Cambridgeshire have been filled with domestic waste in two phases. One area (Phase 1) was filled between 1957–77 with little compaction of the refuse and was left uncapped, while Phase 2 was deposited between 1981–87 and capped with clay. Aerial photography and surface resistivity surveys indicate that the site geometry is complex, with several phases of landfilling into excavations of differing depths. Drilling through the waste indicates that leachate production and waste stabilisation proceed at different rates in capped and uncapped landfills. Analysis of leachate obtained by centrifugation or squeezing appears to give more insight into the pollution potential than do leach tests with distilled water. The Biological Methane Potential (BMP) of the waste appears to be related to the quantity of decomposable material but the chemical oxygen demand (COD) values are distorted by the presence of reduced metals. Too few boreholes have been drilled to define the leachate source in terms of its spatial distribution and little is known of how its composition has changed with time. However, hydraulic conductivity measurements on the landfill caps suggest that it is sufficiently permeable for all rainfall to potentially infiltrate the waste. Boreholes outside the landfill penetrate the Upper and Lower Chalk, and identify the Melbourn Rock and underlying Plenus Marls at the junction of the two formations about 20 m below ground level (bgl). Surface resistivity surveys using the BGS RESCAN system, confirm aerial photographs of the extent of the landfill and also suggest that leachate has migrated beyond the base of the landfill. Evidence of leachate migration in pre-existing screened boreholes completed above and below the Plenus Marls suggests that leachate is flowing above the Plenus Marls. Hydraulic head measurements whilst drilling a borehole to the base of the lower Chalk approx. 70 m bgl revealed the potential for upward groundwater flow through the Plenus Marls. Thus, previously-drilled boreholes penetrating the Plenus Marls are expected to recharge upwards into the shallow aquifer above the Plenus Marls diluting any leachate in the upper aquifer and distorting the flow regime. Several of these boreholes have subsequently been modified to stem the flow across the Plenus Marls. One borehole down-gradient to the west of the site revealed a large thickness of drift composed of both sand and clay rich material. This suggests the existence of a buried channel, the hydrogeological significance of which has yet to be assessed. Groundwater chemistry appears to be influenced by three major factors. (a) the landfill leachate (b) the composition of shallow groundwater in the top 10 m of the Chalk, and (c) the composition of water from the Lower Chalk. Limited groundwater monitoring data appear to display a cyclic variation in chloride concentration. The origin for this is not clear but it may correlate with cyclic variations in groundwater levels when the water table rises into the waste. Cyclic flushing of the landfill may release leachate into the aquifer giving rise to pulses of chloride. Alternatively changes in chloride may arise by the changing direction of groundwater flow which as yet has not been assessed. A conceptual hydrogeological model in which flow is limited to above the Plenus Marls has been used to develop a more appropriate groundwater flow and solute transport model. However, the model lacks data on aquifer properties, on contaminant inputs concentrations, fluxes and spatial variations, and there is a paucity of monitoring data for calibration. Nonetheless preliminary transport modelling using an equivalent porous medium approach shows that an effective porosity of about 5% best fits the regional data. Since this is much less than the total porosity of about 40% for the Chalk, it would appear that only part of the Chalk is available for flow but that matrix diffusion could play an important role in leachate attenuation. Discrete fracture modelling using the FRACTRAN code has allowed some scoping to be made of the hydraulic properties of the aquifer by comparison with chloride hydrographs, but these again need to be better conditioned by in-situ measurement of fracture distributions and transmissivities. A number of additional activities are required to improve the understanding of flow and contaminant transport at the site. These include better spatial definition of the waste distribution, improved data on the hydraulic properties of the Chalk aquifer, and the use of automatic monitoring to record temporal changes in groundwater chemistry and groundwater levels

A review of transport and attenuation of dissolved-phase volatile organic compounds (VOCs) in the unsaturated zone

This report is concerned with transport and attenuation of dissolved-phase volatile organic compound (VOC) plumes in the unsaturated zone. The report reviews the international literature in this subject area, but emphasis is placed upon UK context and relevance. The review makes reference to, but does not critically evaluate how current understanding of VOC migration in the unsaturated zone is translated into practical risk-based assessment frameworks. It does, however, make observations as to the critical migration processes that should be considered in such frameworks to allow more effective knowledge-based decision making for the development of conceptual site models and the implementation of remedial actions appropriate to contaminated sites. The key drivers for this review are the cost-effective management of VOC sources and associated contaminant plumes and the protection of groundwater. This requires an understanding and quantification of the processes by which the unsaturated zone offers protection to underlying groundwater resources from chemical releases at ground surface. Times taken for water to infiltrate and recharge the underlying water table may be long, possibly several decades where zones are thick. Furthermore, dissolved-phase chemical contaminants within infiltrating water may undergo attenuation and potentially be prevented from reaching the water table or delayed in arrival with much reduced concentrations present. A combination of processes including dispersion, sorption, volatilisation, chemical reaction and biodegradation may contribute to contaminant attenuation and protection of underlying aquifers. The review is structured in eight sections, which comprise: Section 1, where the context, aims and objectives of the review study are described. In assessing VOC plumes in the unsaturated zone leached from shallow source zones it is important that: (i) understanding of transport and attenuation of chemical contaminants in the unsaturated zone is sufficient to both recognise and quantify the critical controlling processes; and, (ii) that understanding is translated into practical risk-based assessment frameworks to allow contaminated land problem holders, investigators and regulators alike to make effective knowledge-based decisions on remedial actions appropriate to contaminated sites

Rapid site characterisation using MIP integrated with a GC-MS-field trial

This report describes the integration of a portable Gas Chromatograph-Mass Spectrometer (GCMS) with a Membrane Interface Probe (MIP) site survey system in order to provide on-site qualitative and quantitative Volatile Organic Compound (VOC) data at the SABRE research cell in Derbyshire. The first part of the report gives an introduction to both technologies and a background to the site conditions. Methodologies and limitations are discussed. The second part of the report discusses and compares the results obtained from the MIP pushes, GC-MS analysis of the gas stream and soil samples collected from boreholes drilled adjacent to the MIP wells. An attempt is made to calibrate the MIP voltage responses using data from the GC-MS, results are presented both in the text and in the appendices. A comparison of on-site and off-site soil analysis data is made, highlighting the potential for loss of volatiles when samples are sent away for analysis. Soil data is also compared to the on-site down hole gas data

Sediment-filled fractures in Triassic sandstones : pathways or barriers to contaminant migration?

Observations of sediments infilling fractures in Triassic Sandstones, an important aquifer in the United Kingdom may explain sand production in some water supply boreholes in the UK and why the aquifer transmissivity increases with time, which would occur if sediments were being washed from the fractures. The fracture infills are variable from sands to complex interlaminated sand, silt and clay. It is clear that, depending on the fill material, fractures may form either pathways or barriers to contaminant migration, questioning current concepts for flow in fractured sandstones, and throwing doubt on our ability to predict or remediate contaminant migration in this nationally important aquifer. Mineralogical differences between the fracture fill material and the host sandstone may also affect sorption and precipitation reactions, which, if not considered, may significantly alter the prediction of contaminant migration through either the fracture fill or host sandstone matrix. At present, there is little knowledge of the extent of fracture fills, how they form, whether they occur above and below the water table and whether in vertical joints or bedding plane fractures. There are few measurements of their hydraulic properties to be able to assess their hydrogeological significance and how they should be represented in contaminant transport models. For instance, if the fill has a similar permeability to the rock matrix, a porous medium model may suffice. If the fractures are filled with clay with lower permeability, or if preferential flow along fractures exists, a different modelling approach involving a porous medium with planar barriers may be required. It is hoped the improved representation of fractures in solute or NAPL migration models will lead to better assessment of contaminant migration and risk, focused remediation techniques, and more precise evaluation and management of groundwater resources. This ongoing study aims to understand the nature and geological controls on how fracture fills develop and to improve the representation of fractures in solute or NAPL migration models. The first stage in this is to establish the distribution and occurrence of these features at outcrop, to establish formation mechanisms and provide data such as textural information and permeability measurements that will contribute to a better understanding of their influence on hydrogeology. This will lead to a better assessment of contaminant migration and risk, more focused remediation techniques, and more precise evaluation and management of groundwater resources. A key problem is that sediment infills may not be preserved at outcrop and may also be destroyed during standard borehole sampling procedures. This requires a careful approach to sampling, both at outcrop and in the borehole

Monitoring hydraulic processes with automated time-lapse electrical resistivity tomography (ALERT)

Hydraulic processes in porous media can be monitored in a minimally invasive fashion by time-lapse electrical resistivity tomography (ERT). The permanent installation of specifically designed ERT instrumentation, telemetry and information technology (IT) infrastructure enables automation of data collection, transfer, processing, management and interpretation. Such an approach gives rise to a dramatic increase in temporal resolution, thus providing new insight into rapidly occurring subsurface processes. In this paper, we discuss a practical implementation of automated time-lapse ERT. We present the results of a recent study in which we used controlled hydraulic experiments in two test cells at reduced field scale to explore the limiting conditions for process monitoring with cross-borehole ERT measurements. The first experiment used three adjacent boreholes to monitor rapidly rising and falling water levels. For the second experiment we injected a saline tracer into a homogeneous flow field in freshwater-saturated sand; the dynamics of the plume were then monitored with 2D measurements across a 9-borehole fence and 3D measurements across a 3x3 grid of boreholes. We investigated different strategies for practical data acquisition and show that simple re-ordering of ERT measurement schemes can help harmonise data collection with the nature of the monitored process. The methodology of automated time-lapse ERT was found to perform well in different monitoring scenarios (2D/3D plus time) at time scales associated with realistic subsurface processes. The limiting factor is the finite amount of time needed for the acquisition of sufficiently comprehensive datasets. We found that, given the complexity of our monitoring scenarios, typical frame rates of at least 1.5–3 images per hour were possible without compromising image quality

Assessing Risk from DNAPLs in Fractured Aquifers

Chlorinated solvents are among the most widespread pollutants of groundwater. As DNAPLs (dense nonaqueous phase liquids), they can move rapidly and in complex patterns through fractures to reach and contaminate large volumes of aquifer, and then dissolve to cause significant pollution of groundwater. However, clean-up of DNAPLs in fractured rocks is virtually impossible and certainly expensive. Risk assessment should be used to decide whether the pollution is serious enough to justify major expenditure on clean-up or containment. A key aspect of risk assessment for DNAPLs in fractured aquifers is to understand how deep they are likely to have penetrated through the fracture network. This paper addresses two aspects of such predictions: measuring fracture apertures in situ and the connectivity of fracture networks with respect to DNAPLs. Fracture aperture is an in-situ field technique that has been developed and implemented to measure aperture variability and NAPL entry pressure in an undisturbed, water-saturated rock fracture. The field experiment also provided the opportunity to measure the wetting phase relative permeability at residual non-wetting phase saturation. The RADIO (Radial Aperture Determination by the Injection of Oil) method employs a constant rate injection of a non-toxic NAPL into a fracture isolated by a double packer array. The method was applied at the field site in Scotland, and measured apertures out to ~5m from the borehole. It showed that hydraulic aperture (from packer tests) was a poor estimator of the controlling aperture for DNAPL movement. This is the first time such large-scale aperture measurements have been made, and the technique is the first which can provide useful aperture estimates for risk analysis of DNAPL movement.Network connectivity is a fundamental property of the fracture system. DNAPL connectivity extends the concept to take account of the fluid properties

An illustrated handbook of DNAPL transport and fate in the subsurface

Executive summary Dense non-aqueous phase liquids (DNAPLs) such as creosote, coal tar, chlorinated solvents and polychlorinated biphenyl oils represent a particular class of soil and groundwater contaminant that exist as a separate liquid phase in the presence of water. DNAPLs come to rest in the subsurface as disconnected blobs and ganglia of liquid referred to as residual DNAPL, and in potentially mobile distributions referred to as pools. The region of the subsurface containing residual and pooled DNAPL is referred to as the source zone. Groundwater flowing through the source zone slowly dissolves the DNAPL, giving rise to aqueous phase plumes of contamination hydraulically down-gradient of the source zone. Some DNAPL compounds are resistant to biodegradation and sorb little; they can therefore give rise to substantial aqueous phase plumes. Other DNAPL compounds are relatively immobile in groundwater and, therefore, are highly retarded relative to the rate of groundwater flow. In unsaturated media, volatile DNAPLs give rise to vapour phase contamination. Because DNAPLs are only slightly soluble in water, DNAPL source zones can persist for many decades and, in some cases, even hundreds of years. Some DNAPLs are highly toxic and even very low concentrations in groundwater or the atmosphere can pose an unacceptable risk to human health or the environment. The fact that DNAPLs are denser than water allows them to migrate to substantial depths below the water table in both unconsolidated deposits and fractured bedrock. Delineating the spatial extent of the DNAPL source zone at a site can be a substantial undertaking, requiring at times several years of investigation and significant financial resources. Remediation strategies are site-specific, with separate approaches often warranted for the DNAPL source zone and its associated aqueous phase plume. There has been limited success in removing all DNAPL from below the water table at sites, particularly in a fractured rock environment. Remediation strategies are therefore often directed towards source zone containment or stabilisation, partial mass removal, plume management or plume interception, within the framework of appropriate risk-management objectives. The purpose of this handbook is to provide a user-friendly overview of the nature of DNAPL contamination in a UK context. It is intended to assist site investigators, site owners and regulators in conducting site investigations, conducting risk assessments and selecting remediation approaches. While this handbook reflects the state-of-the-art at the time of publication, it should be noted that the discipline of groundwater and soil contamination by hazardous organic liquids is evolving continuously and is relatively ‘young’ compared with many other areas of science and engineering. Readers are therefore advised to keep abreast of the new advances in understanding and approaches expected in the foreseeable future

Release of contaminants from a heterogeneously fractured low permeability unit underlying a DNAPL source zone

The invasion of DNAPL into fractured low permeability deposits results in the formation of secondary source zones that represent a long-term source of VOCs to adjacent aquifers. We present data from a site underlain by a fractured mudstone contaminated with TCE DNAPL that was poised for release following remediation of the overlying aquifer. Observations of contaminant distributions and fracture networks from the site and a nearby mudstone exposure respectively, enabled prediction of the imminent aquifer recontamination. The fractures, likely formed by gypsum dissolution, were characterised by fracture apertures and spacings that ranged from 0.01 to 49 mm and 0.047 to 3.37 m (10th and 90th percentile values) respectively. Numerical model results show that prediction of outward mass flux in the first year was highly variable (8 to 32 g/m2/d for an initial constant concentration with depth profile) and dependent on both the fracture spacing and aperture and the contaminant distribution. However after 1 year, assuming a heterogeneous distribution of fractures, mass flux was predictable within a narrow range of values (at 20 years; 0.04-0.08 g/m2/d). Similar results were obtained from more typical fracture networks with spacings of 0.1 to 0.5 m and apertures of 10 to 100 µm. These results suggest that when considering potential recontamination in a bounding aquifer, fracture characterisation may not be necessary and instead the focus should be on determining the surface area contributing contaminant mass to an aquifer, the contaminant concentration depth profiles, the hydraulic properties of the receiving aquifer and the elapsed time since aquifer remediation

Hydrogeophysical imaging of deposit heterogeneity and groundwater chemistry changes during DNAPL source zone bioremediation

Robust characterization and monitoring of dense nonaqueous phase liquid (DNAPL) source zones is essential for designing effective remediation strategies, and for assessing the efficacy of treatment. In this study high-resolution cross-hole electrical resistivity tomography (ERT) was evaluated as a means of monitoring a field-scale in-situ bioremediation experiment, in which emulsified vegetable oil (EVO) electron donor was injected into a trichloroethene source zone. Baseline ERT scans delineated the geometry of the interface between the contaminated alluvial aquifer and the underlying mudstone bedrock, and also the extent of drilling-induced physical heterogeneity. Time-lapse ERT images revealed major preferential flow pathways in the source and plume zones, which were corroborated by multiple lines of evidence, including geochemical monitoring and hydraulic testing using high density multilevel sampler arrays within the geophysical imaging planes. These pathways were shown to control the spatial distribution of the injected EVO, and a bicarbonate buffer introduced into the cell for pH control. Resistivity signatures were observed within the preferential flow pathways that were consistent with elevated chloride levels, providing tentative evidence from ERT of the biodegradation of chlorinated solvents