Environmental geophysics: Conceptual models, challenges, and the way forward MAX MEJU, University of Leicester, U.K.
Environmental geophysics deals with issues ranging from
local-scale fluid-rock changes to large-scale climatic changes caused by anthropogenic activities and natural processes. It is enjoying rapid growth for two reasons: (1) the present sociopolitical climate of increasing awareness of the effect of man’s past activities on the environment; and (2) new technological and multidimensional modeling advances that may have brought geophysical methods close to their theoretical resolving power. However, there are some outstanding problems: Realistic whole-site multidimensional imaging of contaminated land, understanding fluid-rock interactions, optimal data integration, and remote prediction of flow and subsoil compositions are currently the most pressing scientific issues. This article stresses that due to increasingly stringent statutory requirements, the road ahead will call for more than purely physical models of the subsurface. Integrated investigative approaches need to become routine, especially when there may be postsurvey legal connotations. Thus, more work needs to be done on integration of disparate data types, prediction of hydrochemistry from noninvasive profiling, and time-lapse characterization for monitoring natural attenuation or remediation processes (Table 1). Land and groundwater contamination. The decline in heavy industrial activities, past waste disposal practices
and accidental spills, military decommissioning activities, and past legislative inadequacies have left a legacy of closed and/or abandoned mines and quarries, military bases, oil and gas fields, petroleum refineries, etc. Many derelict sites are contaminated by petroleum liquids—such as light nonaqueous phase liquids (gasoline-based benzene, toluene, or xylene) or dense nonaqueous phase liquids (cleaning solvents such as trichloroethylene or heavy oils such as crankcase oils), mine spoils, and other inorganic pollutants. Accidental spills or poor disposal practice at these sites would result in significant concentrations of numerous types of both organic and inorganic contamination and would pose a severe threat to groundwater aquifers. Petroleum liquids occur in the subsurface as purephase organic liquids, vapor phase (in the vadose zone), and in very low concentrations in the dissolved phase. The presence of dissolved organic phases in drinking water, even at a low parts per billion level, is hazardous to human health. A network of boreholes is often used to determine the spatial distribution of such contaminants before remediation operations. Boreholes are expensive, furnish poor constraints on the distribution of contaminants (and hydraulic parameters), and risk liberating the organic compounds or triggering further migration of pure-phase organic contaminants. The ever-increasing urban population and the concurrent need for urban regeneration mean that some contaminated sites are increasingly being redeveloped.
Table 1. Envisioned multidisciplinary approaches to problem solving Topical issues Air quality/gas emission monitoring Noise/vibration monitoring, induced seismicity Urban geologic mapping Forensic/crime scene investigations Lithology and geological structure, depth to bedrock /water table. Cavities, mine entrances, subsidence Soil/rock physical properties Roads/bridges, railways, tunnels, canals, pipelines, cable detection and condition Groundwater distribution, quality, and usage Groundwater pollution Landfill leachate (formation, migration, and chemistry/reactivity) Fate of organic pollutants (light and dense petroleum liquids) Public health: geochemical factors, medical factors Safe repositories for toxic wastes Landslide, volcanic eruption, earthquake prediction, and amelioration planning Sea-level variations, global warming, catastrophic floods
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Multidisciplinary collaboration Pollution chemistry, fluid dynamics, radiometry/gas emanometry Seismic monitoring, engineering signal processing Geophysical imaging, remote sensing, geologic mapping Geophysical imaging, medical/engineering imaging, toxicology Geotechnics, geophysical imaging Geophysical imaging, civil engineering, archaeology Sedimentology, geotechnics, materials engineering, soil/rock physics Geophysical imaging, engineering, archaeology, history Geology, geophysics, water engineering, aqueous chemistry Geophysics, geotechnics, engineering, aqueous chemistry, microbiology, toxicology Hydrogeology, aqueous chemistry, engineering, toxicology, microbiology, fluid transport modeling, environmental law Geophysical imaging, chemistry, fluid flow modelling, reactivity modelling, environmental law Toxicology, medical statistics, geochemistry, geophysics Geophysics, engineering, toxicology Volcanology, seismology, mathematical modeling, geophysical imaging, engineering, geography-GIS, fluid dynamics Stratigraphy, paleobiology, volcanology, mathematical modeling, fluid dynamics, geophysics, geography-GIS, atmospheric physics
Current statutory regulations impose tight constraints on such developments and there is an urgent need for reliable noninvasive methods for subsurface characterization. Other sources of heavy chemical loading of the nearsurface and underlying groundwater aquifers are intensive agricultural development, unlined landfill sites over aquifers, and other activities. Because aquifers are not easily replenished and may be regarded as nonrenewable resources, it is vital to monitor the quality and quantity of water they contain. The determination of water quantity requires accurate permeability estimates, traditionally furnished by pump test analysis or more recently by geophysical (resistivity, neutron-neutron porosity, density, and sonic velocity) borehole logging. Cost-effective, noninvasive prediction of petrophysical properties of aquifers is the current research focus. The quality or economic value of a groundwater resource is dependent on the concentration of dissolved solids in the water, and this concentration can vary laterally and vertically in a given aquifer. Traditional water-quality assessments require hydrochemical measurements on samples from sparse monitoring wells. It will be more cost-effective if the water quality of an aquifer can be reliably determined without drilling. Understanding the habitat of groundwater and its vulnerability to contamination is vital for resource protection, modern town planning, and predictive transport modeling studies. Challenges for environmental geophysics. Geophysical methods are well established in groundwater and contaminated land investigations; their utility is underpinned by three factors. (1) Groundwater distribution is controlled by mapable geologic factors; (2) groundwater quality is controlled by geochemical factors; (3) rock resistivity is
inherently related to porosity, fluid content, and chemistry. The electrical conductivity of the subsurface is highly influenced by dissolved solids in groundwater, making electrical and electromagnetic (including ground-probing radar, GPR) methods indispensable in groundwater quality studies. Geophysics plays a role in engineering, mining, and natural disaster assessment/amelioration planning (Table 1) and several of its methods enable rapid, noninvasive evaluation of the lateral and vertical extents of the impacted volume in contaminated land. Technological and multidimensional modeling advances have brought geophysical methods close to their theoretical resolving power. Advances in digital technology and concurrent developments in numerical modeling and instrumentation have improved data acquisition and interpretation techniques, especially for bulk data. In particular, novel technical developments in GPR, shallow seismic reflection, spectral induced polarization, dc resistivity, and electromagnetics have enhanced the resolution of typical near-surface targets in model experiments. It is now possible to collect large volumes of field data of the highest achievable quality, push the methods to their limits, and quantify their utility and resolving power in environmental (particularly groundwater and derelict or contaminated land) investigations. However, some problems remain: Sophisticated integrative mathematical models and controlled field experimental studies. Many recent papers highlight the prospects and problems of 3D geophysical modeling and inversion (see “Suggested reading”) but for idealized structures. What is now needed is a demonstration of the resolving capability of 3D geophysical modeling/imaging of complex natural hydrogeological systems and urban waste sites. The ultimate aim will be to fully
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Table 2. Some industrial wastes buried in landfill sites Sources Food products: additives, grain mills, meat/fish Paper and paper products Soaps, detergents Textiles: silk, cotton, wool, synthetics Leather products Wood products Paints, varnishes Energy and petroleum: coal, nuclear, petroleum refining Metals, fabricated/scrap metals Mining/mineral processing Chemicals, fertilizers
understand the prospects and problems of 3D geophysical surveying and data interpretation in realistic environmental sites with forward models that incorporate contributions from pipes, fences, and other common infrastructure. There is also an urgent need for improved joint multidimensional inversion of bulk data. The capability for joint modeling of multiple physicochemical systems in complex media holds the key to cost-effective and improved geoenvironmental predictions. Thus, integrating disparate hydrochemical, sedimentological, and geophysical “soft” data in hydraulic permeability determinations and inverse hydrofacies modeling requires further work. Also, the impact of local heterogeneity in the unsaturated zone on
Approximate composition Organics and inorganic acids Sulfates, organics, soaps, mercaptans Surfactants, polyphosphates, aluminium-copper-oxides Acids, alkalis, metallic salts, solvents Chrome salts, oils, dyes Solvents, preservatives Metallic salts, toxic liquids Hydrocarbons, acids, metallic salts, radioactive materials Sulfur, chlorides, phenols, PCBs, oils, grease, chrome, alkalis, acids, metallic salts Acids, cyanide, metallic salts Sulfuric acids, organo-phosphates, copper sulfate, mercury arsenates solute transport to the water table is not precisely understood. Realistic controlled experiments (that upscale from laboratory to field size) can furnish vital constraints on hydrological models of flow in the near-surface. Current effort is directed at using diffuse source tracers and subsequent 3D monitoring measurements to accurately characterize solute transport processes in the vadose zone. Integrated decimeter-scale surface and borehole geophysical (EM, resistivity, radar, and seismic) monitoring of the unsaturated zone combined with core characterization may unravel this problem and provide insights into the temporal and spatial dynamics of moisture migration in the vadose zone. Development of robust surface-process models and corre-
spondence principles for geophysical anomalies. Understanding the interplay between physical, chemical, and biological processes in the near surface might lead to the development of robust integrated process models and correspondence principles for geophysical anomalies. This will in turn lead to improved success and consistency of approach in environmental geophysical investigations. In reviewing the biogeomorphic, geochemical, and physical processes operative in landfill sites, I developed a model for geoelectrical soundings that is consistent with these processes. A correspondence between geochemical and geoelectrical anomalies is explored and used to propose a novel scheme for estimating the age of fill (and associated leachate) analogous to current geochemical practice (see “Suggested reading”). Data from borehole geoelectrical probes at several sites in the U.S. Michigan Basin and constraints provided by biogeochemical observations elsewhere (Sauck) provided a model for the electrical response of light nonaqueous phase liquids in contaminated land. A comparable model needs to be developed for dense nonaqueous phase liquids. Improved integration of field techniques and time-lapse characterizations. It is highly desirable to adopt integrated geophysical, geochemical, and microbial methods in routine high-resolution temporal and spatial characterization of near-surface environments. For example, coworkers and I successfully combined dc resistivity, TEM, and AMT methods for regional aquifer mapping in Brazil. Shtivelman and Goldman (see “Suggested reading”) demonstrated the improved resolution obtained by integrating shallow seismic reflection and TEM methods in the study of a coastal aquifer in Israel. Interestingly, however, despite the large number of recent publications and symposia pre-
sentations of environmental geophysical results for contaminated land and groundwater environments, it is hard to find unequivocal models describing the variations in the relevant physical properties in the near surface. Many field studies employed individual surface techniques without adequate attention to temporal-spatial property variations, especially the geochemical-hydraulicbiologic characteristics of the respective sites and environments. For improved problem solution, there is a need for an integrated approach to contaminated land studies. For example, it may be instructive to investigate temporal and spatial variations in dielectric permittivity, electrical resistivity/phase and biogeochemical characteristics associated with plumes originating from free product LNAPL sources in granular sediments so as to understand better the behavior and migration pattern of low-density hydrocarbons in the subsurface and also resolve some existing discrepancies between field and controlled experiments. The study of the diverse properties of materials deposited in landfill sites (Table 2) requires an integrated approach. We need a better understanding of the anatomy and physiology of old buried landfills and attenuation processes in the host aquifers using noninvasive geophysical imaging. Integrated geophysical, hydrogeological, microbial, and geochemical time-lapse studies of contaminated sites and/or groundwater aquifers can help push geophysical methods to their limits to establish whether they can furnish diagnostic data to resolve the characteristics and properties of subsurface targets and to establish whether there is a relationship between hydrogeochemical patterns and 3D physical signatures across the investigated sites. Tracking or monitoring remediation processes. The effectiveness of new remediation techniques depends on timely
detection and delineation of causative bodies; expensive pilot wells only provide point samples (i.e., an incomplete picture) of the subsurface. Geophysical methods can image the region inaccessible to sparse monitoring wells and add valuable stratigraphic information to any model of contaminant transport for the sites. In combination with biogeochemical data, there is excellent potential for optimizing exploratory drilling and positioning of monitoring wells with attendant savings in expensive saturation drilling in conventional remediation operations. Geophysical, geotechnical engineering, and microbial methods are now becoming integral parts of remediation programs; understanding the temporal-spatial changes in contaminant behavior from concurrent geophysical imaging or time-lapse characterizations could furnish novel techniques for noninvasive monitoring of the onset and progress of natural attenuation of contaminants in the subsurface. Remote prediction of petrophysical, hydrochemical, and forensic attributes of near-surface targets. Reliable porosity/permeability estimation from surface measurements is unresolved and requires further research. The problem is even more difficult in fractured crystalline terrains. There are numerical schemes for predicting the transport properties of fractured rock from seismic data (Boadu), and multicomponent seismic surveying holds good promise for the future (Bates and Phillips). An integrated approach involving sedimentological, hydrogeologic, petrophysical, complex resistivity, electromagnetic, surface nuclear magnetic resonance (SNMR), and seismic/GPR input may be desirable. Because hydraulic conductivity or permeability is related to fluid transport and electrical conductivity is related to ionic transport in fluids, there must be some link between
hydraulic and electrical conductivities in aquifers. In my opinion, coupling these two transport processes is the way forward and the ultimate challenge. There is also some possibility of predicting leachate composition and age in old covered landfill sites using resistivity data. The suggested scheme is simple: (1) Determine bulk conductivity (σb in mS/m) by inversion of observed apparent resistivity data; (2) estimate total dissolved solids (TDS) content, fluid conductivity (σw), and chloride content using the empirical relations given in equations 1, 2, and 4 of my paper “Geoelectrical investigation of old/abandoned landfill sites in urban areas: Model development with a genetic diagnosis approach”; (3) estimate age of fill. Several sites need to be investigated to ascertain the validity of this potentially useful forensic concept. Conclusion. Much current work in environmental geophysics and geology covers a broad spectrum of quantitative investigation of fundamental fluid-rock processes in the earth. Numerical methods are used extensively to simulate earth system processes, and nonlinear optimization techniques then match hypothetical models to experimental data. Therefore, there are challenges ahead for better interaction between some research programs in geophysics, microbiology, chemistry, physics, mathematical modeling, and medical imaging. The road to effective integration might be rough, but this path would enable us to go from the purely physical models in environmental geophysics to becoming a recognized technology that can be accepted even in legal proceedings. Suggested reading. From the Journal of Applied Geophysics, 2000: “Multicomponent seismic surveying for near-surface investigations: Examples from central Wyoming and southern England” by Bates and Phillips; “Predicting the transport properties of fractured rock from seismic information: numerical experiments” by Boadu; “Environmental geophysics: the tasks ahead” and “Geoelectrical investigation of old/abandoned landfill sites in urban areas: Model development with a genetic diagnosis approach,” both by Meju; “A model for the resistivity structure of LNAPL plumes and their environs in sandy environments” by Sauck; “Integration of shallow reflection seismics and time-domain electromagnetics for detailed study of the coastal aquifer in the Nitzanim area of Israel” by Shtivelman and Goldman; “Cross-hole electrical imaging of a controlled saline tracer injection” by Slater et al.; and “Three-dimensional inversion of induced polarization data from simulated waste” by Weller et al. Also in J. of App. Geophysics, 1996: “Development of noncontact data acquisition techniques in electrical and electromagnetic explorations” by Shima et al. “Regional aquifer mapping using combined VES-TEM-AMT/EMAP methods in the semiarid eastern margin of Parnaiba Basin, Brazil” by Meju et al. (GEOPHYSICS, 1999). “3D resistivity forward modelling and inversion using conjugate gradients” by Zhang et al. (GEOPHYSICS, 1995). “The pole-pole 3D dc-resistivity inverse problem: A conjugate gradient approach” by Ellis and Oldenburg (Geophys. Jour. Int., 1994).”Engineering behaviour of urban refuse, compaction control and slope stability analysis of landfill” by Fang, in Waste disposal by landfill-GREEN’93, (A A Balkema, Rotterdam, 1995). “Leachate: Production and characterisation” by Farquhar (Can. J. Civ. Eng., 1989). “ERT monitoring of environmental remediation processes” by LaBrecque et al. (Measurement Science and Technology, 1996). “Pulled array continuous electric profiling” (First Break, 1996) and “The pulled array TEM method” (Proceedings of 3rd meeting of Engineering & Environmental Geophysical Society-European Section, Aarhus, Denmark), both E by Sorensen. L Corresponding author: M. Meju,
[email protected]
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