Protecting public groundwater supplies - ACS Publications - American

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Protecting public groundwater supplies Tough new rules are in place, but a better understanding of contaminant sources, fate, and transport is needed for compliance and for containment and mitigative measures Among the major threats to groundwater from which public drinking water and industrial water supplies are obtained are leachates from improperly constructed wastewater impoundments and waste disposal sites. For example, a recent EPA study estimated that there are more than 80 000 wastewater impoundments nationwide, of which 36 200 are municipal, 25 700 are industrial, and 19 200 are agricultural. About 70% of the industrial impoundments have no impermeable liner, and only 5% are presently being properly monitored, noted Jeffrey Sgambat, associate of the geohydrological consulting firm of Geraghty & Miller, Inc. (G&M, Annapolis, Md.). He added that about one-third of the sites have hazardous wastes and about one-third are one mile or less from a water supply well. Experts in the field believe that 1% or less of the total volume of groundwater in the U.S. is severely contaminated. Unfortunately, this " 1 % or less" comprises "valuable aquifers"—solesource water supplies, water sources in areas of dense population, and the like. This is one reason why scientists and engineers are calling for more understanding of how contaminants reach groundwater—as an important first step toward preventing the " 1 % or less" number from increasing sharply (indeed, some experts believe that the number is now 2%). Other emerging goals seem to be to find ways to mitigate the spread of contaminants and, in some cases, to recover and treat contaminated groundwater, as an alternative to abandonment of wells. Deleterious effects of the migration of contaminants from disposal sites have already been experienced. For example, 35 public supply wells sampled in Delaware were found to have water containing organic contaminants; water from seven of those wells had trichloroethylene, a suspected carcinogen. In New Jersey, 111 public water supply wells were found to contain organic chemicals at concentra502A

Environ. Sci. Technol., Vol. 16, No. 9, 1982

tions above those allowable for drinking water. Among those chemicals were carbon tetrachloride, chloroform, and other substances classified as trihalomethanes. Such contaminants, as well as inorganic ones, are said to migrate from improperly constructed impoundments and waste disposal sites. Long-awaited regulations With the primary aim of preventing contaminants from leaching out of waste disposal sites, EPA issued its long-awaited regulations governing the placement of hazardous wastes in landfills, surface impoundments, and other repositories. Issued in July, the regulations go into effect next January. Groundwater contamination: other culprits While waste disposal sites may be a major contamination threat to groundwater, other threats must also be taken into account. For example, municipal sludge from an industrialized city, disposed to land, could release various metals and organics to the soil. These materials might eventually find their way down to groundwater. Pesticides sprayed on farms and in orchards could also percolate to the water table. So can animal and agricultural wastes, which could raise nitrate concentrations in groundwater. Salt used to melt snow and ice on highways might contribute chloride ions to aquifers, and it may be instructive to determine how much lead from gasoline sources has been, and is being, carried down. Or take the case of trichloroethylene contamination. While it may be traceable to chemical waste sites, trichloroethylene (and similar compounds) is also used to remove grease from septic tanks. Such material finds its way to groundwater; a case in point involves trichloroethylene contamination of wells on Long Island, N.Y.

This set of final rules (Fed. Regist., 47, No. 143, p. 32274) specifies, for instance, that liners must be used to prevent chemicals from leaking from such facilities. It also sets forth how groundwater zones surrounding a site must be monitored for possible contamination and mandates that corrective action must be taken if contamination shows up—even up to 30 years after a site ceases operations. EPA estimates that compliance could cost affected industries up to $500 million, or even more, if it turns out that many of the existing hazardous waste sites need extensive cleanup programs. The major regulatory provision requires liners of plastic or other synthetic impermeable material at landfills. Another provision would call for a backup leachate collection system to catch any hazardous liquids before they can reach groundwater. If, despite such precautions, groundwater contamination is still found, EPA may require a site owner or operator to initiate corrective action, which would be costly. In order to avoid problems that could be caused by the excavation of existing sites, the agency is not mandating installation of liners and leachate catchments at active sites. However, even these sites will have to be capped after closure, in a way that prevents rainfall, snow melt, and other water from leaching hazardous substances into the ground. Moreover, operators of such existing facilities could be required to undertake extensive compliance monitoring, especially if any groundwater contamination is detected. Also, they will have to comply with the latest liner and leachate collection regulations for any future expansions of existing sites, as well as for the construction of new facilities. Rita Lavelle, EPA's assistant administrator in charge of hazardous wastes, expects that as many as 40 states will take over control of hazardous waste landfills, impoundments,

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© 1982 American Chemical Society

and other such repositories by 1985. She also believes that, despite the stringency of the regulations, fed­ eral/state permits for new and existing land disposal sites can be issued in four years. Charles Johnson, technical director of the National Solid Wastes Man­ agement Association ( N S W M A , Washington, D.C.), characterized the regulations as "tougher than anyone expected, but technically achievable.

Johnson: standards are tough, "hut technically achievable" We agree with EPA's insistence on the integrity of the liner," he added. But Johnson said that the regulations are "unnecessarily restrictive and inflexi­ ble" in at least one respect: He feels that EPA "should accept any liner material, synthetic or natural, which performs as an impermeable bar­ rier." David Anderson, formerly of Texas A & M University and now with K. W. Brown & Associates, Inc. (College Station, Tex.), points out that the regulations do not mandate synthetic liner material in so many words. He says that the regulations are so written, however, that a nonsynthetic liner simply would not allow a site to meet other applicable standards. Liner caveats When a new site is planned, the op­ erator may be able to use flexible lin­ ers, some of which have a permeability as low as 1 0 - 1 2 c m / s . Water falling upon such material would eventually build up some head on top of the liner. Depending upon the specific design conditions, it theoretically could take decades to centuries for any water to move through a typical liner thickness. However, there is no data base to ver­ ify that this would indeed be the case for membrane liners actually exposed to full-strength hazardous waste leachates, especially primary leachates, which are liquids that can mi­ grate out of the wastes. Such liners are needed for their ca­

pability of keeping hazardous con­ taminants from reaching groundwater. But there are certain caveats. The structural integrity of the liner is crit­ ical; if incorrectly installed, it could be punctured or its seams could break. And, speaking to the Seminar on Ef­ fects of Groundwater Contamination and Public and Industrial Water Supplies, which Geraghty & Miller held at Annapolis, Md., in June, G & M principal James Geraghty observed that "everything is permeable, more or less." Permeability of a liner seems to in­ crease with the weight of overburden above that liner. However, what ac­ tually happens is that the interstitial velocity of fluid flow through the liner increases because of the pressure head brought about by the overburden, Aaron Jennings of the University of Notre Dame (Indiana) reminds ES& T. Moreover, any leakage accel­ erates under such conditions, Geraghty pointed out. He also listed other physical reasons why one might not always be able to rely fully on numer­ ical ratings of permeability of liners (ES&T, Vol. 14, No. 9, p. 1032). Some clay liner tests There might also be chemical problems; an EPA staff member said that synthetic liner material could possibly react with components of hazardous waste in a landfill and de­ compose, or fail in other ways. To ad­ dress this possibility, the new regula­ tions require waste/liner compatibility tests. In any event, EPA's regulatory decisions show that the agency believes synthetic liners to be superior to clay liners, even though many clays have permeabilities of 1 0 - 7 c m / s or less, which represents a very low perme­ ability. A chemical study of the perme­ ability of clay soil liners may provide food for thought as to whether EPA's decision to require synthetic liners was well considered. Kirk Brown and his associates conducted tests of certain organic chemicals on four clays at Texas A & M University. The clay soils had permeabilities of less than 1 0 - 7 cm/s—they were in the range of 1 0 - 8 - 1 0 - 9 cm/s—which, from an engineering standpoint, should, in principle, have qualified them as haz­ ardous waste site liner material. Clays chosen were mixed-cation illite, mixed-cation kaolinite, and calcareous and noncalcareous smectite (montmorillonite). Baseline permeabilities were evaluated with a "standard lea­ chate" composed of 0.01 Ν calcium sulfate ( C a S 0 4 ) solution at constant pressures of 10 and 60 psi.

Concentrated organic liquids re­ placed the C a S 0 4 "leachate" for testing, and consisted of glacial acetic acid, acetone, aniline, ethylene glycol, heptane, methanol (20% water), and xylene. Each was chosen because it represented a class such as acidic, basic, neutral polar, and neutral nonpolar organic liquids. These com­ pounds are found among organic chemical wastes. Any changes in this liner perme­ ability were mediated by the type of clay soil, the chemical introduced, and the amount of each chemical added, reckoned in terms of multiples of the liner's pore volume. Overall (though with some exceptions) kaolinitic and calcareous smectitic clay soils showed greater resistance to permeability in­ creases upon exposure to organic liq­ uids. The permeability increases that occurred were ascribed to chemical reactions between the organic com­ pound and the material in the clay soil liner.

Geraghty: "everything is permeable, more or less" With acetic acid, permeabilities were less than 10~ 7 c m / s . However, acetic acid does react with the clays, so these figures might not remain firm, David Anderson explained. Aniline and ethylene glycol—an aromatic weak base and a neutral polar com­ pound, respectively—raised permea­ bilities of some clays to as much as 1 0 - 5 c m / s . The addition of acetone, heptane, methanol, and xylene resulted in dramatic increases in permeability, and generally rendered the liners highly unsuitable for hazardous waste containment. Subterranean interactions Suppose a site is unlined or that even the best liner proves faulty. Will an aquifer-dependent public water supply receive the contaminants leached out? One must bear in mind that contami­ nants, as well as the groundwater itself, move in slow, tortuous three-dimen­ sional paths through openings in subEnviron. Sci. Technol., Vol. 16. No. 9, 1982

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terranean formations of rock or sediments. Because movement is slow, there is ample time for contaminants to interact with formation materials. For example, in the vadose (unsaturated) zone, on the way to an aquifer, acidic contaminants might encounter limestone or other basic materials and be neutralized. Metals could undergo oxidation-reduction (redox), ion exchange, complexing, or precipitation reactions with the soil, rock, or mineral formations that could immobilize the metals. Organic materials could be volatilized, adsorbed, or biodegraded; an example can be found in the adsorption of various organic compounds by certain types of clay or by humic substances in the soil. These reactions generally reduce the amounts of contaminants threatening groundwater. An aquifer and contaminants dissolved in it are in intimate association with the underground formation and its component materials; the water flow is laminar. Dissolved contaminants flow with the groundwater, but their concentrations may be lowered by dispersion; acid-base, redox, and ion exchange reactions; volatilization; adsorption; and biodégradation (although necessary organisms populate aquifers much more sparsely than they do upper regions of soils and surface waters). A major consequence of these attenuation mechanisms is that the contaminants move out from a source at different effective velocities, depending upon the chemical properties of the substances involved. A con-

tamination plume may be composed of several overlapping plumes of different lengths that vary with the contaminants comprising them, said Robert Saar, a senior geochemist with Geraghty & Miller. An ion such as chloride moves almost as fast as groundwater does. Examples of more slowly moving constituents, arranged from relatively fast to relatively slow, are alkali metal ions (Na + , K + ); alkaline earth metal ions (Ca 2+ , Mg 2 + ); and trivalent metals, such as chromium. For organic compounds, it is generally the case that the more water soluble the compound, the more rapidly it will move in the aquifer. Digital computer model

Numerous physical, chemical, and even some biological factors, then, must be ascertained in order to determine whether all, some, or none of the contaminants leached from a waste site will reach public or industrial water supply wells. Obtaining this knowledge involves a thorough acquaintance with the size, shape, and physical characteristics of the aquifer. It also could require monitoring networks designed to avoid the many pitfalls encountered in sampling (ES&T, Vol. 14, No. 1, pp. 41-42, and Vol. 15, No. 9, pp. 993-96). One approach involves digital computer models for predicting the migration of contaminants from their sources into groundwater. Transport models being developed by Notre Dame's Jennings and his colleagues are designed to forecast the fate of

Relative mobility of certain contaminants in groundwater Potassium

Herbicides

Phosphates Heavy metals {trivalent chromium) Radionuclides

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Arsenic (hexavalent chromium) Chlorides

DDT

PBB

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Synthetic organic chemicals ι 1 h

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Source: Seminar lecture by G&M scientist Robert Saar

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multicomponent solutions released to the subsurface. They will take into account the complex system of solidphase and solution-phase reactions that accompany many contamination episodes. Ultimately, these models should be able to predict the simultaneous movement of large numbers of organic and inorganic contaminants. Another technique for tracing underground transport of contaminants in groundwater uses bacteriophagetype viruses. Bruce Keswick of the University of Texas Medical School at Houston, and Charles Gerba of the University of Arizona explained that the small size, ease of assay, and— most important—nonpathogenicity of bacteriophages make them eminently suitable as tracers of groundwater movement and contaminant transport. Many disease-causing microorganisms are also groundwater contaminants—as much of a health threat as chemical contaminants. They note that chemical tracers "do not always reflect the movement of microorganisms in groundwater." Contamination assessment

Knowledge of how both chemical and microbial contaminants reach, and travel in, groundwater would be an invaluable aid to assessing contamination in an aquifer. The entire aquifer must be thoroughly mapped, described, sampled, and monitored. All potential sources of contamination must be cataloged and compared with locations, yield, and construction details of public and private water supply wells; cones of depression (water table drawdown) caused by those wells; sewer systems in the area; chemical constituents of surface waters; and many other parameters. G&M senior scientist William Thompson pointed out that after all this has been done, a permanent sampling/monitoring network may be needed. This permanent monitoring program would be one long-term groundwater protection objective, Thompson added. For contaminated aquifers, other aims are to determine the safest, most cost-effective alternatives for mitigative actions and to determine liabilities. But how would liabilities be determined? Thompson described a situation in a Middle Atlantic state in which water in 30 residential water supply wells was found to be contaminated with low levels of trichloroethylene. An industrial site in the area was immediately blamed. However, upon further study, it turned out that a flowing stream between the industrial site and the wells presented a natural geohy-

A contaminant plume9.

Plume boundary

.Groundwater ftow direction

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Monitoring well location

— —

50

Elevation of water table in feet

100

(ft)

Plume boundary

. . . and a treatment system for groundwater renovation"b Vapor recovery

Well field

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Air

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stripper

ί ρ ^ metal I removal

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Biological

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Recharge trench •At Sylvester Site, N.H. "Conceptual Source: Seminar lecture by G&M senior scientist Don Lundy

drologic barrier that no contaminants from the site could cross. Thompson reminded the seminar that all possible contamination sources in a ground­ water area must be thoroughly studied before liability can be assigned. Nevertheless, one can expect that groundwater contamination and ef­ fects thereof will be stimuli for a growing volume of litigation nation­ wide. An indicator of this trend is a $2.5-billion class action suit brought in a federal court in Jackson, Tenn., by residents of Hardeman County, Tenn. (about 70 miles east of Memphis), in June. In the suit, plaintiffs' attorneys al­ leged that increases in incidences of cancer and other adverse health effects were caused by contamination of the plaintiffs' water supply wells, origi­ nating from a chemical waste disposal

site established by a pesticide manu­ facturer in 1964 and operated until the early 1970s. Claims for damages are being made, even though the company allegedly violated no existing laws or regulations during the time that the site was established and operated. Remedial options Certain options are available when groundwater used as public/industrial supplies turns out to be contaminated. G&M senior scientist Don Lundy said that the most common, quickest, and cheapest approach is to abandon the well(s) involved. That was done in Anne Arundel County, Md., for ex­ ample, when chromium from a wood treatment facility, disposed of on the land, showed up in supply wells. In other cases, abandonment is not possible. Then other—usually more

costly—options must be exercised. Sometimes the contaminated water can be recovered and used for nonpotable purposes. This approach allows contaminants to be pumped from the aquifer; also, the physical influence of pumpage could cause cleaner groundwater to flow slowly into the well area, and reduce contaminant concentration to a certain extent by modifying the contamination plume. This approach has been used at many sites in the U.S. Another option is recovery of well water with treatment. In a case in New Jersey, water was found to contain 1,1,1-trichloroethane, at concentra­ tions exceeding 103 ppb. In that case, treatment with granular activated carbon was the answer. Other treat­ ment possibilities might include ozone, reverse osmosis, or a combination of methods. Still another technique involves fluid isolation, by which physical barriers to fluid movement, and therefore con­ taminant movement, are installed in the aquifer. This fairly expensive op­ tion is being used at the Sylvester Site near the Nashua River, in New Hampshire. Groundwater contami­ nants consist of various organics and metal ions. The contained fluid is pumped up and treated. Lundy said that capital costs could range from several hundred thousand to several million dollars. Operation and main­ tenance costs for wells and treatment facilities might be as high as $1 million a year, if pumpage rates are as great as several thousand gallons per minute. Each remedial option has its tangi­ ble and intangible costs. For instance, the abandonment of a well or wells normally means the permanent loss of a water resource. Who can place an exact dollar value on that loss? And the loss is permanent if one considers that given the nature of groundwater, the contamination that led to well abandonment may be present for many years, perhaps for millennia. Other techniques would usually in­ volve difficult engineering work and high (yet more calculable) costs. An optimum approach might be found in emphasizing waste management methods—particularly for hazardous chemical and biological wastes—that do not call for their disposal to land. On the other hand, the technology for recovering contaminated groundwater is becoming better known and more widely used; so while such contami­ nation is still best avoided wherever possible, in many cases countermeasures are available if it does occur. —Julian Josephson Environ. Sci. Technol., Vol. 16, No. 9, 1982

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