Richard B. Greswell

Cost-effective mini drive-point piezometers and multilevel samplers for monitoring the hyporheic zone

The development of cost-effective approaches to monitor groundwater–surface-water exchange processes and contaminant fate within the hyporheic zone fundamentally underpins implementation of legislation such as the European Community Water Framework Directive, which requires integrated management of groundwater and surface water. Cost-effective mini drive-point piezometers (MDPs) and multilevel samplers (MLSs) are presented that use cheap construction materials, involve simple fabrication and installation procedures, and have a proven durability with low vulnerability to flood events and vandalism. They have been used across a range of hydro(geo)logical settings in the UK and proven to be effective in discerning flow exchange, geochemical trends, and contaminant transport and attenuation over monitored depths of 0.25 to 2 m at a resolution as low as 0.05 m. Example depth profiles, cross-river transects and river-reach longitudinal profiles from the River Tame catchment (West Midlands, UK) illustrate the value of MDP–MLS approaches in establishing surface-water–groundwater mixing zone depths, contaminant natural attenuation as a result of biotic activity within the hyporheic zone, and estimates of contaminant flux exchanges between groundwater and surface water. The MDP–MLS approaches allow discernment of contaminated groundwater plume discharges that may go undetected, or at best poorly resolved, if reliance was solely placed on conventional riverside monitoring wells and/or surface-water sampling. The MDP–MLS approaches described also have potential to be used in the investigation of shallow sediment aquifers, lake shorelines and wetland features.

THE MICRO-SCALE HYDROGEOLOGICAL PROPERTIES OF THE LINCOLNSHIRE LIMESTONE, UK

Abstract As part of an integrated study of the ‘dual porosity’ hypothesis, a detailed micro-scale hydrogeological study was conducted of the rock material from a quarry excavated in the Lincolnshire Limestone. Permeability and porosity determinations allow comparisons to be made with the lithostratigraphy and show that the matrix hydraulic conductivities are inconsequential in terms of groundwater flow. The relatively high porosities typical of these limestones, combined with large surface areas associated with the major bedding plane fissures, provide a mechanism in which diffusive transport can become a significant factor in contaminant migration. The diffusion coefficients (D*) of three tracers used in field investigations (bromide,fluorescein and amino g acid) were measured in each of the main rock materials from the site using both double reservoir and reservoir depletion techniques. In addition, an assessment was made of D* estimation using an electrical resistivity method. Alteration of the matrix due to calcite dissolution of the fissure faces creates a zone of enhanced permeability and porosity. The presence of a fissure fill results in a ‘multiporosity’ interface between the fissure and the matrix that may significantly increase the mass of contaminant entering the pore fluid by diffusion in comparison with a simplistic conceptualization of this zone.

An integrated study of controls on solute transport in the Lincolnshire limestone

Abstract A study site located in a quarry on the Lincolnshire Limestone has been selected to examine the validity of the ‘dual porosity’ hypothesis and its application in models for the prediction of solute migration. An integrated approach has been adopted for the experimental programme, combining data obtained from both field and laboratory-scale investigations with mathematical modelling. The results of the first to stages of the experimental developments at the site are reported and the results are discussed in relation to he development of a full tracer test programme. Fracture mapping has been conducted on the face and the floor of the quarry where the vertical fractures are seen to be discontinuous over depth. However, extensive horizontal bedding plane fissures are readily identifiable in the boreholes and represent the main paths for lateral flows. An extensive data collection programme has been conducted to characterize the hydraulic properties of these fissures, as well as the adjacent altered and unaltered matrix. Important results for future tracer testing were obtained from the hydraulic tests which suggest the strong possibility of non-Darcian flow in the fissures under pumping and tracer test conditions and the probability of strong spreading of solutes at injection wells under forced inflows. Preliminary tracer tests confirm the difficulty of employing laboratory defined diffusion coefficients as input data to ‘dual porosity’ models of local-scale transport.

Converging flow tracer tests in fissured limestone

As part of an investigation into the dual porosity behaviour of fractured limestone aquifers, a sequence of converging flow tracer tests was devised and conducted at an extensively investigated experimental site in a major aquifer in the UK. The tests were designed specifically to produce detailed, high-resolution information about the tails of the breakthrough curves typically observed in this kind of aquifer and test. A set of mutually compatible, low detection limit tracers was identified through laboratory investigations. Two tests were carried out over distances of 20 m and 40 m along each of the two radii towards an abstraction borehole where tracer concentration was monitored. Simple dual porosity models were calibrated using the data from one test on each radius. Blind validations of these models were undertaken by attempting to predict the outcome of the second test on each radius, producing one success and one significant failure.

A Method for Conducting Simultaneous Convergent Tracer Tests in Multilayered Aquifers

Forced gradient tracer tests between two boreholes can be used to study contaminant transport processes at the small field scale or investigate the transport properties of an aquifer. Full depth tests, in which tracer samples are collected just from the discharge of the abstraction borehole, often give rise to breakthrough curves with multiple peaks that are usually attributed to different flow paths through the aquifer that can rarely be identified from the test results alone. Tests in selected levels of the aquifer, such as those between packer-isolated sections of the boreholes, are time consuming, expensive; and the identification of major transport pathways is not guaranteed. We present a method for simultaneously conducting multiple tracer tests covering the full depth of the boreholes, in which tracer sampling and monitoring is carried out by a novel multilevel sampling system allowing high frequency and cumulative sampling options. The method is applied to a tracer test using fluorescein conducted in the multilayered sandstone aquifer beneath the city of Birmingham, UK, producing six well-defined tracer breakthrough curves.