L. Jared West

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 17 km 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 2 m per month. The retardation of this front propagation in the top 3 m 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.

A critical review of hydrological data collection for assessing preservation risk for urban waterlogged archaeology: A case study from the City of York, UK

Environmental change caused by urban development, possibly augmented by climate change, may result in accelerated decay of in situ archaeological resources. Damage may be related to changes in hydrological processes. Such archaeological resources have to be considered in environmental planning. In this paper we highlight the need for improved hydrological data from urban archaeological sites using the case study of the City of York, UK, arguably one of the most well studied and well preserved urban archaeological environments globally. We suggest that the quality of hydrological data collected during routine surveys and experimental work must be improved and standardised in order for us to produce reliable archaeological risk models for urban sites.

In search of experimental evidence for the biogeobattery

Recent work has suggested that the electrical self-potential (SP) geophysical technique may be used to noninvasively map redox conditions associated with contaminant plumes or bioremediation schemes. The proposed mechanism linking SP response and redox involves the generation of a current source and sink in the subsurface whereby electrons are transferred between anoxic and oxic environments via a conductive biofilm and/or biominerals, creating a biogeobattery. To investigate the conditions required for biogeobattery formation, we successfully created contrasting redox zones in a flow-through column setup. In this setup, an oxic section, containing clean sand, transitioned into an Fe(III)-reducing section. Fe(III) reduction was mediated by either a natural microbial community or a pure culture of the model organism Shewanella oneidensis MR-1 in two different column experiments. Visual observations and electron microscopy showed that ferrihydrite was sequentially transformed to goethite and magnetite; despite this change, no SP signal was generated in either column. Electron microscopy suggested that in the pure culture column, S. oneidensis MR-1 cells did not form a continuous, interconnected biofilm but rather interacted with the iron (oxyhydr)oxide surfaces as individual cells. In our experiments we therefore did not form the conductor of the biogeobattery. We thus conclude that generation of a biogeobattery is nontrivial and requires specific geochemical and microbiological conditions that will not occur at every contaminated site undergoing microbially mediated redox processes. This conclusion suggests that SP cannot be used in isolation to monitor subsurface biogeochemical conditions. Copyright 2011 by the American Geophysical Union.

A new framework for estimating englacial water content and pore geometry using combined radar and seismic wave velocities

[1] Ice mechanical properties, and hence the response of glaciers to climate change, depend strongly on the presence of liquid water at ice-grain boundaries. The propagation velocities of radar and seismic waves are also highly sensitive to this water. Mixing laws, typically the Looyenga and Riznichenko formulae, have traditionally been used to quantify liquid water content within glaciers from such velocity data; however, it has become apparent that these mixing laws are geometrically inconsistent. We present an inclusion-based effective medium approximation in which we model water inclusions within solid ice. Two types of inclusions are used: spherical inclusions to represent water in the grain junction nodes, and high-aspect ratio spheroidal inclusions to represent water in the grain boundary veins. We apply this model to radar and seismic data from a polythermal glacier in Svalbard to quantify both inclusion geometry and the unfrozen water content within the warm ice.

Dielectric Permittivity Measurements on Ice Cores: Implications for Interpretation of Radar to Yield Glacial Unfrozen Water Content

The dielectric permittivity of glaciers and ice caps can be measured in field or airborne surveys using ice-penetrating radar. Permittivity contrasts in polar ice caps indicate ice stratigraphy and age, whereas those in temperate glaciers have been interpreted as changes in unfrozen water content, which is an important control on glacier mechanics. Many previous workers have assumed simple relationships between permittivity (inferred from radar velocity) and unfrozen water content, but these relationships have never been verified in the laboratory. Here, we present measurements of the dielectric properties of ice cores from a temperate glacier in Switzerland, using the Time Domain Reflectometry (TDR) technique, which has a measurement frequency close to that of radar. The objectives of the measurement were to quantify the effects of intercrystalline unfrozen water and air content on ice dielectric permittivity. TDR probes were specially designed and built for ice core measurement to allow them to be pressed onto the ice core surface, and to maximize the signal travel time.

A Forward Modeling Approach for Interpreting Impeller Flow Logs

A rigorous and practical approach for interpretation of impeller flow log data to determine vertical variations in hydraulic conductivity is presented and applied to two well logs from a Chalk aquifer in England. Impeller flow logging involves measuring vertical flow speed in a pumped well and using changes in flow with depth to infer the locations and magnitudes of inflows into the well. However, the measured flow logs are typically noisy, which leads to spurious hydraulic conductivity values where simplistic interpretation approaches are applied. In this study, a new method for interpretation is presented, which first defines a series of physical models for hydraulic conductivity variation with depth and then fits the models to the data, using a regression technique. Some of the models will be rejected as they are physically unrealistic. The best model is then selected from the remaining models using a maximum likelihood approach. This balances model complexity against fit, for example, using Akaikes Information Criterion.

Toxic metal mobility and retention at industrially contaminated sites

Abstract This paper is concerned with industrial sites contaminated with heavy metals, specifically cadmium (Cd), chromium (Cr), arsenic (As), mercury (Hg), lead (Pb), copper (Cu), nickel (Ni) and zinc (Zn), as these are the heavy metal contaminants that cause most concern. It is intended as a source of information to aid engineers and geologists concerned with risk assessment and remediation at such sites. The risks from heavy metals depend strongly on their chemical form, and therefore literature on the chemical forms of metals released at industrial sites is reviewed. Changes in the chemical forms of heavy metals resulting from environmental interactions are discussed, and methods for determining metal chemistry (leaching tests, chemical extractions, direct methods) are outlined. The paper concludes with a case study where the chemical forms of heavy metals at a former metal smelting and vehicle scrapyard site are investigated using a scanning electron microscope with energy dispersive X-ray spectroscopy and selective sequential chemical extractions. The case study showed that Zn is currently mobile at the site whereas Pb, Cu, or Ni are not, and this highlights the importance of adopting a multidisciplinary approach to investigating toxic metal mobility and retention.

Well flow and dilution measurements for characterisation of vertical hydraulic conductivity structure of a carbonate aquifer

The paper aims to characterize vertical variations in horizontal hydraulic properties in a fractured carbonate aquifer, the Cretaceous Chalk in East Yorkshire, UK. Two approaches are used: an inverse model of well flow applied to flow logs of pumped open wells, and open well dilution testing. In this case study, transmissivity in the unconfined part of the aquifer is dominated by the highly permeable zone of water table fluctuation, where carbonate dissolution has occurred enhancing fracture aperture; a similar enhanced permeability zone is present at the top of the aquifer where it is confined beneath glacial deposits, although periglacial physical weathering during Quaternary cold periods, rather than carbonate dissolution, is responsible. The aquifer is also shown to contain deeper permeable horizons of stratigraphic origin, which are better developed in the unconfined section.

Electrokinetic transport in natural soil cores

Abstract Electrokinetic transport in natural soils has been investigated by applying a constant voltage across 500mm long by approximately 200mm diameter natural soil cores for periods of up to 8 weeks. Contaminant ions were circulated through a fluid filled reservoir between the anode and the soil, and distilled water was circulated through a similar reservoir adjacent to the cathode. During the experiments electrical current, voltage along the core, water flow rate, and anolyte and catholyte pH were monitored at regular intervals. Periodically, the electrical supply to the power electrodes was switched off and detailed electrical measurements were made using 68 monitoring electrodes positioned around the soil core, in order to produce three dimensional electrical resistivity images of the cores. After testing the cores were dissected and analysed for contaminant content, pore fluid composition, and pH. Data are reported that show that zinc tracer transport is initially strongly retarded, with the zinc predominately sorbed to the soil. Initially zinc enters the anode region mainly by electromigration. This cannot change the pore fluid ionic strength due to charge balance constraints, and hence zinc influx is limited by initial ionic strength. However, after 8 weeks of testing, the ionic strength of the pore fluid in the anode half of the cores were significantly elevated by co-diffusion and electroosmotic advection of the anolyte into the core.