RESTORATION OF POLLUTED GROUNDWATER: IS IT POSSIBLE?
BY L.C. GOLDENBERG * AND A.L. MELLOUL
Geological Survey of Israel, Ministry of Energy and Infrastructure
Hydrological Service of Israel, Water Commissioner, Ministry of Agriculture
Fresh groundwater is becoming a scarce resource in the world. A main reason for this scarcity is the influx of chemicals and biological agents into the aquifers which adversely affects the water quality. The detrimental factors derive from man-produced point- and diffuse-sources. Main factors of concern are harmful chemical compounds such as salts of heavy metals, organic substances, and harmful bacteria and viruses. These materials, generally addressed as "contaminants" or "pollutants" may remain present in subsurface waters even after passing through natural filters like calcareous vadose (unsaturated) zones.
If the natural filters are not effective enough, man-aided and man-induced rehabilitation schemes have to be applied. So far, no definite strategies for groundwater rehabilitation exist. This paper presents some of the future possible trends of decontamination tools and rehabilitation protocols for polluted aquifers.
Some Existing Techniques in Groundwater Remediation
Certain success was reported in the treatment of limited contamination (Lund, 1971; Bouwer, 1984; R.S.C., 1990; Harmsen et al., 1991). However, in general, conservative treatment methods of polluted aquifers are inefficient in terms of economy and time consumption (Holgate, 1983; Konikow and Thompson, 1984), and may leave the aquifer still contaminated even after a prolonged process of remediation. A hydrological problem may appear as well, if large scale pumping is required to limit pollutant migration and to extract polluted water. A difficulty may appear also if injection of imported fresh water is involved (e.g., influence on the aquifer properties as two chemically-different types of water meet or pollution of the imported water itself). In addition, the cost of the imported water may be significantly higher.
If the pollution is "severe" (an estimation relevant to the situation in each particular aquifer), it is technically impossible to restore aquifers to sound conditions, as attempted by Superfund* and other state agencies, even when no economical constraints are involved (USEPA, 1988; Perry et al., 1989; USEPA, 1989; Travis and Doty, 1990; MacDonnell and Guy, 1991). For instance, cleaning sites where water contains insoluble constituents, such as jet-fuel, requires thousands of years. Attenuation of groundwater containing dense, non-aqueous-phase liquids at the bottom of an aquifer, to meet drinking water standards, is unachievable at all (Freeze and Cherry, 1989).
Some Perspectives of Groundwater Rehabilitation
Groundwater rehabilitation requires new interdisciplinary approaches. A new conceptual approach is suggested by the New Jersey Department of Environmental Protection (N.J.D.E.P., 1989).
It maintains that restoration of contaminated groundwater to its natural state is not always necessary and upgrading rehabilitation programs of water quality should be in accordance to future use (Travis and Doty, 1990). The use of water for agricultural or industrial purposes, for example, may not require drinking water standards. However, whatever the approach is, some rehabilitation is or will be needed for almost all polluted aquifers in the world. One possible rehabilitation scheme appears in Figure 1.
The first step required for groundwater rehabilitation is gaining insight into the processes of groundwater contamination and possible decontamination processes. This is obtained by computer probablistic-stochastic, stochastic-analytic, and/or analytic- numeric methodologies. These methodologies may help in a quick identification of the parameters of the contaminated groundwater system. They may also be useful in the prediction of the movement and the fate of water and pollutants, and may suggest possible management alternatives (Mercado, 1984; Black and Freyeberg, 1987; Massmann and Freeze, 1987; Vomvoris and Gelhar, 1990; Harmsen et al., 1991; Hirsch et al., 1991; Wallach et al., 1991).
The practical efficacy of the models increases as they incorporate complex intradisciplinary results (laboratory experiments and field surveys). The laboratory and field parts of the complex (which, in this context may be referred to as a remediation tool) supply data required for model construction and validation. The hydrological data required may be supplied by monitoring programs based on monitoring networks of boreholes, or on data obtained from surface geophysical methods, such as ground penetrating radar and electromagnetic seismic reflection (Barr, 1973; Benson, 1991; Benson et al., 1991; Shahai et al., 1991).
Crystallization of the approach to remediation is the second step of this possible rehabilitation scheme. It is based on the results supplied by the programs aimed at gaining insight into the pollutant-groundwater system. Crystallization of the approach may refer, for instance, to the possible use of reclaimed effluents for irrigation (Ashboren, 1975). The treated effluents may attain a very low level of toxic pollutants (James, 1983). Thus, they may be beneficial from the remedial point of view since they substitute pumpage responsible for the formation of hydrological depressions. However, there are also negative aspects, such as the possible presence of high T.D.S., organic material or suspended material in the recharging water.
The crystallized approach is used to establish an adequate policy and to enforce activity. These two elements build the legal framework of decontamination protocols (e.g. Thayer, 1983; Toft et. al., 1987). At present, legislation regarding pollution control is becoming one of the most far reaching acts affecting many fields of life, most notably industry (C & E.N, 1991). Positive results due to legal activity are, however, not guaranteed (see, for example, the partial success of the U.S. Federal Water Pollution Control Act of 1956, of the Water Quality Act of 1965, of the Federal Water Pollution Control Act Amendments of 1972, the Clean Water Act of 1977, Groundwater Protection Strategy, 1984, etc.). (MacDonnell and Guy, 1991). A step in groundwater rehabilitation is understanding the reasons for this partial success in various circumstances.
The success of the remedial activity increases with increased environmental education, in general (Whitman, 1988; Zoller, 1991). Thus, environmental education may be incorporated in aquifer remediation schemes since it leads to a deeper understanding of the man-environment interaction. This understanding, in turn, brings about a positive change in public opinion (Whitman, 1988).
The conceptual, analytical and educational steps of remediation mentioned previously form the basis for the practical steps of rehabilitation. Some future possible techniques of decontamination are:
1. Plume containment and pollutant mass reduction in groundwater by "barriers" of micro (pore) or macro (geological layer, aquifer) scale are a primary remediation goal (Travis and Doty, 1990). These actions are carried out in heterogenic porous systems of different flow domains and transport properties, which may contain old and recent pollutants (Hirner et al., 1991; Vengosh et al., 1991). The heterogeneity leads to significant uncertainty when estimating the evolution of the contaminated plume, thus reducing the chances of aquifer remediation (Rubin, 1991; Vasak et al., 1981; Sudicky and Huyakorn, 1991; Tsakiris et al., 1991). Under such conditions, a "self-controlled" system which encircles the pollution plume and neutralizes the contaminant adverse action is of benefit. This sort of treatment activity was not applied in the field yet. However, its components were checked independently in various situations, and are described in the following.
Reduction of the hydraulic conductivity may be obtained if alterations of the pore and pore throats configuration of an aquifer occur as a reaction to an ongoing process of contamination by organic compounds. In this case, concentration of clay colloid particles in the throats of the pores and/or the desaturation of a certain portion of the porous space at the plume borders (Goldenberg, 1987) may occur (see in the following).
The required gases for the desaturation of the pores in the border of the contaminated plume may appear due to bacterial activity. Bacteria multiply when placed in appropriate conditions. This is true also for artificially injected genetically engineered or pollutant-adapted microorganisms (Scholl et al, 1990). Addition of proper nutrients at the plume border stimulates the development of indigenous bacteria colonies. Higher environmental temperature, if desired to enhance bacterial activity, can be achieved by injecting warm waste water (Baehr et al, 1989; Pop and Gorla, 1991). The bacteria degrade the pollutants, and produce significant volumes of gas (Moore and Knowles, 1989). For example, domestic sewage gas produces about 1 litre/gram of volatile solids destroyed (Andrews and Graef, 1971). In suitable conditions the gas forms bubbles which:
a. desaturate the porous formation, thus reducing the hydraulic conductivity and limiting contaminant spreading, and,
b. accumulate fines (clay minerals, etc.) at the gas-liquid interface. The fine agglomerations further retard saturated flow, if saturated conditions will appear again.
2. A hydrological-biological scheme may be employed to disinfect contaminated groundwater. This program was not yet applied in the field. It consists of induced gradients which cause a leachate containing microorganisms to reach a part of the aquifer which serves as a natural filter. In this environment, the ambient conditions are not suitable for microbiological growth. The filter may consist of a geological layer having an adverse effect on the microorganism cell membranes, and/or on the toxic compounds secreted by the microorganisms (Carlini and Guimaraes, 1991).
3. Treatment of organic pollutants, and especially hydrocarbons, on their way through the vadose zone by microorganisms and/or application of ventilation is a step in the remediation of groundwaters. Its components were mentioned by several researchers (Yaron et al., 1989; Baehr et al., 1989; Ostendorf and Kampbell, 1991). Jet fuel, for instance, may be efficiently degraded in the vadose zone by microorganisms and removed by ventilation (Staps, 1989). Techniques for the removal of volatile organic materials such as TCE, DBCP, ammonia, hydrogen sulfide and phenol are mentioned as useful methodologies in close and/or similar areas, like in the stripping of wastewater treatment (Tchobanoglous and Schroeder, 1985). Induced temporal changes across the unsaturated space may supply additional time to neutralize the pollutants by microorganisms (Kachnoski and de Jong, 1988).
4. Injection of compounds such as styrenic-supports (the so called gel-type) and macroporous resins into plume borders is a technique which was not used yet in large scale aquifers. However, this technique has large potential to both reduce rock porosity and partly immobilize and remove different organic solvents in groundwater (Guyot, 1988). This refers to the injection of chelating resins which swell and/or bind undesired chemical species like Cd, Ce, Co, Cr, Cu, Sc, U, V, Zn, Hg, and Pb in parts of polluted aquifers (Warshawsky, 1988; Tiravanti et al., 1989). Inorganic gel may also be produced in the pollutant environment (Goldenberg and Arad, 1991). Encouraging results were obtained in continuous recovery of uranium by its adsorption on resins and fibers of cross-linked acry-lonitrite-ethyle acrylate copolymers. These techniques may be considered to be used either for in situ treatment or for "pump-and-treat" techniques. Research has still to be conducted in order to find materials that will remain stable in the aggressive leachate environments. They are expected to be both chemically inert and biostable (Anderson and Zho, 1991).
5. The technique of introducing surfactants while practicing vacuum pumping techniques, through preferred flow pathways may enable the pumping of forming emulsions from the aquifer.
6a. In-situ catalytic combustion may be used to remove floating and water-insoluble contaminants like jet fuels and the resulting gas vapors from groundwater (Musialik-Petrowska and Syczewska, 1989). The base of this method appears in practices of oil recovery.
6b. Heating by radio or ultrasound sources may be used to deform pollutants in either the vadose and the saturated zones. Difficulties may arise from the fact that the energy cannot be directed exactly and efficiently to the places of pollutants concentrations or to the flow paths. The lack of efficiency is due to the large variability of heat transfer in the subsurface even when fluid properties are constant (Pop and Gorla, 1991). The various levels of heating have variable influence on the different pollutants. Low temperature up to 100 C may be used to treat high-molecular oil compounds. Under such conditions, their viscosity decreases, and they can flow to pumping wells. Higher temperatures, 100 C – 900 C, may be used to volatilize and burn most organic pollutants. In temperatures over 1400 C, all organic compounds are burned or vitrified (Urba, 1991).
6c. Heating of carefully-chosen portions of the aquifer may reduce permeability due to enhanced precipitation of minerals, thus encirculating the contaminants plume.
7. Injection of saline water into groundwater along the border of a contaminant plume can be employed to "activate" existing in- situ clay minerals, thus reducing hydraulic conductivity. Here, clay particles form gel-packets that impair flow. The technique was not used for this purpose on a field scale. However, indications exist that these types of reactions occur in real aquifers, such as the coastal dunes aquifer of Coos Bay, Oregon, or in the Tel-Aviv, Israel aquifer (Goldenberg, 1987). In the Coos
Bay sandy aquifer, there is partial disconnection between fresh and saline waters due to reactions of clay particles (Margaritz and Luzier, 1985). Introduction of a certain amount of sea water may also be used in certain conditions as a trap for pollutants which absorb on the surface of clay platelets, and for contaminated clay particles. The colloid particles become immobilized in gel-voluminous packets which in turn are immobile.
A representative example is the fate of radionuclides, that once released into effluent water were captured and adsorbed by particles. (Nightingale and Bianchi, 1977; Buddemeir and Hunt, 1988; Wanty et al., 1991). This phenomenon was characterized in laboratory as well (Torok et al., 1990).
In conclusion, the perspectives of groundwater protection and rehabilitation were summarized by the environment ministers of the world’s 24 industrialized nations (accounting for about 72% of the global industrial wealth), in a meeting convened in 1991. The ministers stated that environmental challenges require more than the "identify and repair" approach of the 1970s and the "anticipate and prevent" strategy of the 1980s. What is now needed is an array of environmental management and environmental education methods, based on long-term strategies (O’Sullivan, 1991).
Time is of the essence in tackling the challenge of groundwater rehabilitation. In the race against time, gaining insight into the aquifer system constitutes an essential means of rehabilitation since it enables the interception and the application of proper steps in every stage of the ongoing deterioration process. Increased environmental education and awareness will lead to the incorporation of environmental principles in every sector of life for the well-being of present and future generations.
* Superfund or CERCLA: the Comprehensive Environmental Responses, Compensation and Liability Act of 1980, requires EPA to identify the most hazardous waste sites in the U.S. and to assure the cooperation of responsible parties in their cleanup or if necessary to use federal funds for cleanup.
The authors are grateful to Dr. A. Arad for his useful remarks.
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