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Journal of Applied Geophysics 44 2000 63–65
www.elsevier.nlrlocaterjappgeo
Preface
Environmental geophysics: the tasks ahead
Groundwater is a very valuable natural re-source and it is important for us to be able to monitor its quality and quantity in subsurface reservoirs. Since mineralisation, hydrocarbon accumulations and contaminant dispersal pro-cesses in the subsurface are all genetically con-nected by water and heat, it is vital to continue to conduct fundamental research on groundwa-ter and its role in geological processes. Also, the decline in heavy industrial activities, past waste disposal practice and accidental spills, military decommisioning activities and past legislative inadequacies have left us a legacy of derelict
Ž
sites disused minesrquarries, military bases, oil fieldsrrefineries and petrol stations, and car
.
wrecker sites , many of which are contaminated Ž
with light non-aqueous phase liquids LNAPLs, such as gasoline-based benzene, toluene or
xy-. Ž
lene , dense non-aqueous phase liquids DNA-PLs, such as cleaning solvents like
trichloroeth-. ylene or heavy oils like crank-case oils , mine
Ž
spoils and other inorganic pollutants such as Pb .
or Ni added to petrol . Accidental spills or poor disposal practice at these sites would result in significant concentrations of numerous different types of both organic and inorganic contamina-tion posing a severe threat to groundwater
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aquifers. LNAPLs and DNAPLs petroleum liq-.
uids for short occur in the subsurface as pure-Ž
phase organic liquids, vapour phase in the va-.
dose zone , and in very low concentrations in the dissolved phase. The presence of dissolved organic phases in drinking water, even at the low parts per billion level, is hazardous to
hu-man health. A network of boreholes is often used to determine the spatial distribution of such contaminants before remediation opera-tions. Boreholes are expensive, furnish poor constraints on the distribution of contaminants Žand hydraulic parameters , and run the risk of. liberating the organic compounds or triggering further migration of pure-phase organic contam-inants. The ever increasing population and the concurrent need for urban regeneration mean that some contaminated sites are increasingly being re-developed to provide for new domes-tic, retail, administrative and light industrial in-frastructure. Current statutory regulations im-pose tight constraints on such developments and there is an urgent need for reliable non-intrusive methods for subsurface characterisation.
Geophysical methods are established in groundwater resource evaluations and can be used to find solutions to other problems related to our natural environment such as engineering, mining, hydrogeology and natural hazards as-sessment and amelioration planning. They have a tremendous potential for rapid, non-intrusive evaluation of the lateral and vertical extents of the impacted volume in contaminated land as well as for mapping the geological structure of the site. Environmental geophysics is enjoying rapid growth in the present socio-political climate of increasing awareness of the effect of man’s past activities on the environment. Interestingly, new technological and multi-di-mensional modelling advances have brought geophysical methods close to their theoretical
0926-9851r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved.
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resolving power. New advances in digital tech-nology and concurrent developments in
numeri-Ž
cal modelling e.g., Ellis and Oldenburg, 1994;
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Zhang et al., 1995 and instrumentation e.g., . Shima et al., 1996 and references therein have led to improved data acquisition and interpreta-tion techniques. In particular, novel technical developments in ground radar, shallow seismic reflection, spectral induced polarisation, dc re-sistivity, electromagnetic and other geophysical methods have enhanced the resolution of typical near-surface targets in model experiments as well as brought these popular methods within range of their theoretical resolving capability. It is now possible to collect field data sets of the highest achievable quality so as to push the methods to their limits and hence quantify their utility and resolving power in groundwater and derelict or contaminated land investigations.
However, despite the large number of recent publications and symposia presentations of en-vironmental geophysical results, it is hard to find unequivocal models describing the varia-tions in the relevant physical properties in the near-surface. Many of the field studies em-ployed individual surface techniques and with-out adequate attention to the temporal–spatial property variations, especially the geoche-mical–hydraulic–biologic characteristics of the respective sites and environs. For improved problem solution, there is a need for an inte-grated approach to contaminated land studies. For example, it may be instructive to investigate the temporal and spatial variations in dielectric permittivity, electrical resistivityrphase and
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geochemical plus biological? characteristics associated with plumes originating from free product LNAPL sources in granular sediments so as to understand better the behaviour and migration pattern of low-density hydrocarbons in the subsurface and also resolve some of the existing discrepancies between field and con-trolled experiments. Integrated geophysical,
hy-Ž drogeological and geochemical time-lapse
tem-.
poral–spatial variation studies of contaminated sites or groundwater aquifers can help push the
geophysical methods to their limits to establish whether they can furnish diagnostic data to resolve the characteristicsrproperties of subsur-face targets as well as establish whether there is a relation between hydrogeochemical patterns, say and 3-D physical signatures across the in-vestigated sites.
New remediation techniques are emerging but apparently, their effectiveness depends on timely detection and delineation of causative bodies; the usual expensive pilot wells only
Ž .
provide point samples i.e., incomplete picture of the subsurface. Geophysical methods can im-age the region inaccessible to typical sparse monitoring wells and add valuable stratigraphic information to any model of contaminant trans-port for the sites. In combination with geo-chemical data, there is excellent potential for optimising exploratory drilling and positioning of monitoring-wells with attendant savings in expensive saturation drilling on a grid in reme-diation operations. Geophysical methods are now becoming integral parts of remediation pro-grammes; understanding the temporal–spatial changes in contaminant behaviour from geo-physical concurrent imaging or time-lapse char-acterisations could furnish a technique for non-intrusive monitoring of the onset and progress of natural attenuation of contaminants in the subsurface.
The contributions to this Special Issue ad-dress some of the problems highlighted above and may be grouped loosely under three head-ings.
( )a Mathematical models and controlled ex-perimental studies.
properties of fractured rock from seismic data and illustrative synthetic examples.
( )b DeÕelopment of surface-process models and correspondence principles for geophysical anomalies.
Understanding the interplay between physi-cal, chemical and biological processes in the near-surface holds the key to improving the success rate of geophysical investigations of contaminated land. Meju reviews the biogeo-morphic, geochemical and physical processes operative in landfill sites and develops a model for geoelectrical soundings over landfill sites that is consistent with these processes. A corre-spondence between geochemical and geoelectri-cal anomalies is explored and used to propose a novel scheme for estimating the age of fill in analogy to current geochemical practice. Using data from borehole geoelectrical probes at
sev-Ž .
eral sites in the Michigan Basin USA and constraints provided by biogeochemical obser-vations elsewhere, Sauck provides a model for the electrical response of LNAPL-contaminated land. Atekwana et al. provide a practical evalua-tion of integrated surface and borehole geoelec-trical methods for mapping
hydrocarbon-con-Ž .
taminated sites in the Michigan Basin USA and also provide an important discussion of the issue of spatial–temporal biogeochemical ef-fects on geoelectrical signatures.
( )c ImproÕed field techniques and integrated case histories.
Yaramanci presents detailed examples of application of high-resolution time-lapse resis-tivity technique to safety assessment of waste disposal in underground mines in rocksalt in northern Germany. The paper by Shtivelman and Goldman is of particular relevance to effec-tive management of groundwater resources and demonstrated the improved resolution obtained by integrating shallow seismic reflection and
TEM methods in the study of a coastal aquifer in Israel. Meju et al. evaluate the usefulness of small-loop TEM sounding and inversion for locating an important regional aquifer under variable glacial cover in an intensively farmed region in northern England where it may not be possible to deploy large transmitter loops. Tezkan et al. present the results of 2-D inver-sion of radiomagnetotelluric data from waste sites in Germany which demonstrate that useful information can be obtained from a limited bandwidth of observational frequencies. Bates provides an introduction to and illustrative ex-amples of environmental geophysical applica-tions of multi-component seismic surveying. Aristodemou and Thomas-Betts present the re-sult of integrated geoelectrical studies at a waste disposal site in England and attempt to estimate some hydraulic properties from the inversion results.
References
Ellis, R.G., Oldenburg, D.W., 1994. The pole–pole 3-D dc-resistivity inverse problem: a conjugate gradient ap-proach. Geophys. J. Int. 119, 187–194.
Shima, H., Sakashita, S., Kobayashi, T., 1996. Develop-ment of non-contact data acquisition techniques in elec-trical and electromagnetic explorations. J. Appl. Geo-phys. 35, 167–173.
Zhang, J., Mackie, R.L., Madden, T.R., 1995. 3-D resistiv-ity forward modelling and inversion using conjugate gradients. Geophysics 60, 1313–1325.
Max Meju)
Department of Geology, UniÕersity of Leicester, UniÕersity Road, Leicester LE1 7RH, UK E-mail address: mxw@le.ac.uk
