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Shallow Lanct Burial of Municipal Wastes 4 INTRODUCTION KEROS CARTWRIGHT lllinois State Geological Survey AB STRACT Environmental laws and regulations promulgated since the early 1960s have increased the use of the sanitary landfill and changed its character. Total isolation of wastes from the environment is not possible; some migration of leachate from wastes buried in the ground will always occur. Disposal sites should be judged on a site-by-site basis rather than by rigid criteria for site selection and design. The performance should take into account several factors: (1) the nature of the wastes; (2) the site hydrogeology, including a complete water balance; (3) the attenuation of contaminants by geologic materials; and (4) the release rate of unattenuated contaminants to surface waters or groundwaters. In recent years there has been a trend away from numerous, small disposal sites toward fewer and larger sites. Large disposal sites increase the environmental stress, since the attenuation capacity of the geologic material has a finite, though generally not well-defined, limit. Fine-grained geologic materials, which have low hydraulic conductivities and attenuating characteristics considered favorable for waste disposal, are geologic conditions generally considered suitable for disposal of wastes. Landfill covers are, perhaps, the least regulated engineered part of sanitary landfills, yet they are critical in controlling leachate migration and the water balance. Also, the role of migration in the unsaturated zone is often overlooked, especially in humid areas. The environmental effects of land disposal are difficult to de- termine because the subsurface is complex and we do not understand it well enough to be able to describe it completely and monitor it properly. Studies of the hydrologic systems in fine-grained geologic materials into which current disposal practices direct most wastes were almost nonexistent 20 years ago. However, during the past two decades, studies of fine- grained materials have developed methods to provide the data required to study waste-disposal sites (e. g., Farquhar and Rov- ers, 1975; Griffin et al., 1976, 1977; Cartwright et al., 1981). Today, this is a highly active field of research, and it is nearly impossible to keep up with all the scientific literature. Many 67 disposal sites have been studied that document the chemical and physical changes that can occur in the earth materials as a result of the burial of waste. Land disposal of solid wastes domestic and industrial has been practiced for many years; over the past 30 to 40 years the open burning dump has grad- ually been replaced by the sanitary landfill. Garland and Mosher (1975) estimated that in 1973 there were about 14,000 operating sanitary landfills in the United States, and Clark (1975) esti- mated that about 240 were operating in Illinois in 1973 (down from the more than 2000 known disposal sites a decade earlier). This reduction in the number of operating landfills typical throughout the United States has effectively concentrated increasing volumes of refuse at fewer sites, especially near the urban center.
68 The ever-increasing volume of waste generated by humans can be placed in the air, water, and land. The increasingly strict regulations, passed in the 1960s and 1910s and governing the discharge of pollutants to the air and surface water, placed increased emphasis on land disposal. This was only natural considering the public awareness of air and water pollution and the very clear dangers if past practices continued. Since there are interchanges between the three media, land disposal of waste ultimately discharges some of the waste products back to the water or air. It is interesting to note that the success of land disposal is judged primarily by the rate of return of the pollutants to the air or water. A thoughtful discussion of land disposal of wastes was published by the American Society for Testing and Materials (ASTM) Subcommittee D18.14 (19811. One characteristic of the sanitary landfill is the potential for production of large amounts of leachate and gas (Pohland, 1980~. The environmental consequences of the leachate and gas pro- duction have received considerable attention. This concern, in turn, has led to a variety of developments for control and treat- ment, including the concept of total isolation. The degree of control and treatment required is a function of the environ- mental sensitivity of the site and the degree of uncertainty acceptable. LEACHATE CHARACTERISTICS In the case of most municipal landfills, leachate is produced when infiltration from rainfall, surface drainage, and/or ground- TABLE 4.1 Characteristics of Leachate and Domestic Waste Waters KEROS CARTWRIGHT water inflow combine with the moisture already in the waste to exceed the liquid holding capacity (full capacity). In some cases, the compaction processes used at landfills may squeeze sufficient moisture from the refuse to exceed the liquid holding capacity and cause leachate movement prior to infiltration of moisture. There are only limited data on sanitary landfill leachate char- acteristics (Table 4.1~. There are three main variables control- ling the leachate characteristics: (1) the variability of the waste itself, (2) the climatic and hydrogeologic setting, and (3) time. The available data show the variability of leachate character- istics from site to site and from time to time. The recognition of the variability of the characteristics with time offers potential for innovative control measures and treatment practices. GAS PRODUCTION The principal gases produced by sanitary landfills (in addition to the obnoxious odor) are potentially explosive mixtures of methane (CH4) and acidification of the groundwater due to the solution of carbon dioxide (COO. Methane is considered to be the greatest problem since the acidification problem is gen- erally easily overcome by the natural buffering capacity of most geologic materials. Farquhar and Rovers (1973) studied the pattern of gas gen- eration of a "typical" sanitary landfill (Figure 4.1~. They iden- tified four phases: Phase 1, aerobic; Phase 2, anaerobic non- Constituent Rangea Ranger Ranger (mg/L) (mglL) (mg/L) 34-2,800 0.2-5,500 0. 06-1,400 0-1,000 16.5-15,600 5-4,080 2. 8-3, 770 0-7,700 0-154 0-9.9 0-5.0 1-1,826 0-1,416 Leachate Fresh Old Wastewatert 50 0.1 0.1 30 50 Ratio" Chloride (Cl) Iron (Fe) Manganese (Mn) Zinc (Zn) Magnesium (Mg) Calcium (Ca) Potassium (K) Sodium (Na) Phosphate (P) Copper (Cu) Lead (Pb) Cadmium (Cd) Sulfate (S04) Total N Conductivity (limbos) Total dissolved solids Total suspended solids pH Alkalinity as CaCO3 Hardness total Biological oxygen demand Chemical oxygen demand 100-2,400 200-1,700 1-135 100-3,800 5-130 25-500 20-500 0-42,276 0-2,685 0.7-8.5 0-20,850 0-22,800 9-54,610 0-89,520 4.0-8.5 200-5,250 100-51,000 600-800 210-325 75-125 10-30 160-250 900-1,700 295-310 450-500 742 500 49 45 277 2,136 0.5 1.6 0.4 400-650 6,000-9,000 10,000-14,000 100-700 5.2-6.4 800-4,000 3,500-5,000 7,500-10,000 16,000-22,000 197 1.5 0.16 81 254 7.35 0.5 989 9,200 12,620 327 5.2 4.96 0.1 7.51 1,400 1,144 266 7.3 14,950 22,650 81 10 40 700 200 8.0 200 see 15 5,000 490 9 43 0.7 25 13 75 45 1.6 aU. S. Environmental Protection Agency (1973). Stein et al. (1971~. CU.S. Environmental Protection Agency (1975~. Runner and Carnes (1974~.
Shallow Land Burial of Municipal Wastes LANDFILL GAS PRODUCTION PATTERN 100 0 80 an O LL =~ 60 O ~ L) O Or, 40 ~ m ,, 20 z PH A S E I, ~ , m 1 1 1 ! 1 1 l ,~ I 1 / \ C He O - ~ Hi O TIME FIGURE 4.1 Sanitary landfill gas production pattern (from Farquhar and Rovers, 1973). ~ a= methanogenic; Phase 3, anaerobic methanogenic, unsteady; and Phase 4, anaerobic methanogenic, steady. The nonme- thanogenic stage is initiated by hydrolytic processes by reduc- ing complex organic matter to soluble components by means of cellular enzymes. The microorganisms in the methanogenic stage are generally considered common bacterial inhabitants of soil and sewage. The gas generation is controlled by refuse composition, mois- ture, temperature, alkalinity, and plI (see Figure 4.2~. The rate of gas production and the length of each of the initial three phases vary considerably, depending on conditions. Most san- itary landfills will reach stable CH4 production (Phase 4) in 180 to 500 days (Farquhar and Rovers, 1973~. The initial phases may only be a few days to weeks in length. FACTOR GROUP a B TEMPERATURE r the / / ~ AERATION ~~ _ ~ / I ~ .~^tCTe 'DC ~ lentil TEATIME \ I I V me I vat" ~ // ~ CONTENT GAS _ _ pH \: ALKALINITY \ NUTRITION \- TOXIC COMPO UN DS , AIR TEMPERATURE . ATMOSPHERIC PRESSURE , ~ ~ / \~ ~ PLACEMENT AND / \\\ COVER ~ / \ \ \~ PRECIPITATION / \ TOPOG RAPH Y H Y D RO G E O LOGY RE FUSE 3~' COMPOSITION FACTORS OVER WHICH SOME CONTROL MAY BE EXERTED DURING SANITARY LANDFILL DESIGN AND OPERATION. FIGURE 4.2 Factors affecting gas production in sanitary landfills (from Farquhar and Rovers, 1973). 69 Conditions may be such as to allow gas generation, but there may be nutritional deficiencies in the refuse, which impede the microbial population and slow gas production rates. Also, it is certain that there are many materials that are toxic or inhibitory to gas-producing microorganisms, which could be disposed of into sanitary landfills. Moisture content is perhaps the most interesting controlling factor. Moisture is critical to gas generation. Gas production increases in sanitary landfills with moisture content up to sat- uration. With saturation and a rise in the water table up through the refuse, gas generation is reduced, especially the production of CH4. This in part explains the lack of gas problems at many older landfills in humid environments. HYDROGEOLOGIC CRITERIA Regulations governing the disposal of wastes including haz- ardous wastes are designed to protect human health and en- sure the quality of our water resources. It is unrealistic to assume that rigid geologic and hydrogeologic criteria can be applied over the entire United States or even to a single state. Strict application of some criteria, such as the depth from the bottom of the waste to the water table, can actually lead to the selection of unsuitable sites. Regulations governing the disposal of wastes should be applied on a site-by-site basis and should provide performance standards that the disposal site must meet. In evaluating a site, the possible effect on the environment is the most important consideration. In addition, the specific character of the wastes, the geologic materials at the proposed site, and their interaction must be carefully examined. Each regulatory agency has different rules, regulations, and guidelines that categorize wastes and landfills according to the type of wastes to be received, geologic setting, or engineering specifications. For example, landfill sites in Illinois are divided into five classes on the basis of geologic and groundwater con- ditions. Illinois regulations require disposal of hazardous wastes in class I and possibly class II sites. Class I sites require a permeability barrier at the bottom and sides of the trenches consisting of 10 ft of material with a hydraulic conductivity of 1 x 10-8 cm/see or less; class II sites require the same thick- ness of material, but the material must have a hydraulic con- ductivity of 5 x 10 - ~ cm/see or less. No standard specifications are given for making this measurement, but ASTM (1970) does have a suggested laboratory method. Laboratory measurement of such low values is difficult; the error in measurement may be quite large (see Figure 4.3>, possibly greater than the dif- ference that distinguishes the classes of sites. Field measure- ment is also difficult, time-consuming, and costly and may be no more accurate than laboratory measurement. Current Illi- nois Environmental Protection Agency (IEPA) guidelines re- quire, in addition to the specified permeability barrier at the bottom and sides of trenches, a minimum of 500 ft separating the waste from the nearest water well or body of surface water. These and other hydrogeologic requirements (such as re- quired depth from the trenches to the water table) are sub- jective; some are based on misconceptions rather than on sci- entific principles. For example, there is little point in regulating
70 FIGURE 4.3 Comparison of field and lab- oratory hydraulic conductivities at Wilson- ville, Illinois (from Johnson et al., 1983). the distance between a landfill and a water well unless other factors critical to the protection of the well (such as well con- struction, well depth, groundwater gradient, and the hydraulic conductivity of the intervening earth materials) are also con- sidered in the regulation. A performance standard should stipulate the maximum ef- fects disposal can have on surrounding land uses. For example, a standard might be written to limit the volume and concen- tration of the contaminant allowed to be discharged from the landfill and specify the water-quality criteria as related to a specific water use and/or the degree to which ambient water quality can be altered. The performance standard can be a KEROS CARTWRIGHT Idealized stratigraphic column Depth (ft) _ =_ . _ ~ IN . 7y'> At/ 7M ~ 'a\/\/ it/ ,/~\~\' '\/~\/ ,/~/ aim b/, - ,_ / '_ / /~\/\' '\/~\/ /~/, /~-/ , ~ /\' '\/~\/` /~\~' _/~_ ' ~~/ ,\/\,~ ' it\/ /~\/~1 /\/~/\/~ ,_ / '_ / At\/ 10 20 30 40 50- 60 - 70 80 - 90 ~ Average texture (%) Material sand-silt-clay Peoria Loess 5- 65- 30 Roxana Silt Sangamon Soil Vandalia Till (ablation) Hydraulic Hydraulic conductivity (cm/s) conductivity (cm/s) F ield Test1 Lab Test: 1.1 x 10-6 30-35-35 4.4x 10-7 1.6x 10-7 45-40-15 4.1 x 10-5 - 2.3 x 10-4 2.0 x 10-7 Vandalia Till (basal) fractured, few sand lenses 41-40-19 2.0x 10-4 2.0x 10-7 Vandalia Till (basal) 38-43-19 4.4x 10-7 4.1 x 10-9 few sand lenses Ban(tell)Fm 10-50-40 Pennsylvanian shale 'Water injection/recession curve tests by ISGS 2 Samples recompacted general statement specifying drinking-water standards or des- ignating specific-ion concentrations, or a combination of both. The standard should clearly specify the area at which the cri- teria for water quality are to be applied (such as at the property line or the nearest aquifer or body of surface water). If a mixing zone is acceptable (as in the case of a point-source discharge into surface water), then the performance standard should spec- ify the size of that zone. These specifications must be realistic; specifying that there must be a "zero discharge" immediately adjacent to the waste is not realistic. The performance standard presumes that it can be demon- strated in advance how fast and where the contaminants will
Shallow Land Burial of Municipal Wastes travel and that both retardation and mobilization factors are known. In fact, this is imperfectly known, and estimates are all that is available. However, predictions can be verified through the proper design ofthe monitoring system. Methods discussed in many chapters of this volume deal directly with methods to predict contaminant transport. To promulgate guidelines or regulations using design cri- teria, a regulatory agency must be sufficiently knowledgeable in its field to ensure that the particular method or design spec- ified is the only appropriate means of achieving the desired result. In waste disposal, we generally do not have sufficient knowledge to stipulate method or site characteristics that apply consistently. Thus, where design criteria are stipulated, defi- ciencies are generally present in regulations and guidelines. Each site will be different and must be considered on a site- by-site basis. Unsaturated Flow Unsaturated flow of groundwater has only recently been rec- ognized as an important factor in contaminant transport by geologists and engineers engaged in waste-disposal work. The field of soil physics has made great progress in the past 20 yr; however, little is known about this field by many practicing hydrogeologists. This was shown by the design of French drains for landfills, such as at Sheffield, Illinois (Dragonette et al., 1979), where the principles of unsaturated groundwater flow were not utilized, resulting in nonfunctioning drains. We must also consider the vapor transport of contaminants with the landfill gases. Such transport could be very rapid. Johnson and Cartwright (1978, 1980) reported on studies of sanitary landfills in the unsaturated zone. They show that the same hydrologic and geochemical processes occurred as were reported by Hughes et al. (1971) for sites below the top of the zone of saturation. Also, there are reports of rapid movement of contaminants in the unsaturated zone, which cannot be ex- plained as vapor transport. Rapid fluid movement due to cap- illary forces or gradients in the unsaturated zone must be oc- curring. Contaminant transport velocities may be many times greater (perhaps up to several orders of magnitude) than the saturated flow velocity (Cartwright, 1982~. The following may be an important factor in this regard. Consider the transport of contaminants in saturated porous media; since the effective porosity term appears in the denominator of the Darcy equa- tion, the smaller the effective porosity the faster the transport. While there is not a great deal of information about the effective porosities of fine-grained sediments, there is general consensus that it is a small percentage of the total porosity. As these sediments become unsaturated they lose only a small per- centage of their water, even at fairly high tensions. The water lost is essentially equal to the effective porosity. Thus, as the sediments lose moisture, the remaining effective porosity must become increasingly small. If an analogy to saturation can be made, the rate of contaminant transport may increase signifi- cantly, perhaps by an order of magnitude. (The whole problem is far more complex than this simple explanation; obviously much research needs to be undertaken in this area.) 71 Site Size In recent years there has been a trend from numerous, widely dispersed, small disposal sites to fewer and larger sites (Clark, 1975; Garland and Mosher, 1975~. It seems that this strategy should be used with some caution for several reasons. The attenuation capacity of any geologic materials (its capacity to remove contaminants from the water) has a limit, which if exceeded by the volume of leachate that enters the material will allow contaminants to pass through the material almost unretarded. Unfortunately, we do not know enough about the attenuation capacities of geologic materials for most leachate constituents, so we cannot clearly define this limit. Tables 4.2 and 4.3 give the mobility and relative hazard of a few common leachate constituents. Larger landfills are more likely than smaller sites to exceed this limit; therefore, smaller, more widely dis- persed sites may be less dangerous to the environment. Wastes should be segregated where possible. This may be possible to accomplish by designating sites for particular types of waste disposal. Segregation of wastes allows for better pre- diction of attenuation characteristics of the geologic material using geochemically similar waste materials in an aqueous so- lution. This is a simpler procedure than predicting attenuation characteristics for mixed wastes. Segregation also prevents in- teraction among incompatible wastes. Chemical reactions be- tween some wastes may enhance the mobility of certain toxic constituents. For example, the mobility of most heavy metals is directly related to the pH of the solution "generally, the TABLE 4.2 Ranking of Chemical Constituents in Municipal Leachate According to Their Relative Mobility through Clay Mineral Columns by Mean Attenuation Number (ATNja Mean Chemical Attenuation Constituent Number Qualitative Grouping Pb Zn Cd Hg NH4 Mg COD Na C1 R Mn Ca 99.8 97.2 97.0 96.8 58.4 54.7 38.2 37.1 29.3 21.3 15.4 10.7 -11.8 -95.4 - 656.7 High Moderate Low Negative (elusion) aThe negative ATN indicates greater concentration emerging from the columns than in the influent leachate, probably the result of ion ex- change.
72 TABLE 4.3 Increase in Salinity of an Aquifer or Small Stream from Landfill Leachatea KEROS CARTWRIGHT Hydraulic Conductivity Increase in Increase in of Liner Gradient Aquifer Salinity Stream Salinity (cm/see) (cm/cm) (ppm) (ppm) 1 x 10-8 1/1 0.8 0.002 1 x 10-6 1/10 7.7 0.02 1 x 10-4 1/100 77.0 0.2 1 x 10-2 1/1000 769.0 ~ n The salinity of the leachate, using the following assumptions, is low, typical of 20-yr-old sites, not fresh" refuse: (l) hydraulic conductivity of the aquifer is 1 x 10-0 cm/see, and its gradient is 1/1000 cm/cm; (2) there is 104 m2 of refuse; (3) aquifer is 30 m thick; (4) leachate salinity is 2000 ppm (Hughes et al., 1971); and (5) stream discharge is 1 m3/sec. more acid the solution, the more mobile the heavy metals (Griffin et al., 1976, 1977~], and organic toxicants such as PCBs, which are nearly immobile in aqueous solution, become highly mobile in organic solvents such as carbon tetrachloride (Griffin and Chou, 1981~. The present practice of segregating wastes into acids, bases, and organics is woefully inadequate. The nature and rate of degradation of wastes must also be considered; the waste may change, by some natural process, from its present form to a less complex chemical compound and less noxious form. Categorization into degradable and non- degradable wastes is desirable for all types of wastes because a time factor is added to geologic and geochemical considera- tions. The decay/decomposition process may result from ra- dioactive decay, organic decomposition, or other processes. Wastes that require a long decay/decomposition period (thou- sands of years) probably should, from a practical hydrogeologic point of view, be considered nondegradable. These wastes must be diluted to bring the levels of contaminants to acceptable levels (Table 4.4~. Water Balance The proper balance between water entering and leaving the disposal sites is critical. Wastes are buried in trenches dug in natural clay materials or in trenches having artificial or clay liners of low hydraulic conductivity that will contain the wastes and thereby protect groundwater resources. This approach can create problems in humid climates where the amount of water infiltrating naturally from the surface is greater than the amount leaving the excavation through the surrounding natural ma- terial or liner. When this excess infiltration occurs, the disposal trench fills with leachate and overflows, spilling out the sides as springs. This phenomenon is known as the bathtub effect. The bathtub eject is partly attributable to the fact that most wastes have much higher hydraulic conductivities than the natural material into which they are placed; they also have very different unsaturated soil-moisture characteristics. The hy- draulic conductivity of some wastes can be reduced by com- paction. Municipal landfill wastes are crushed by heavy equip- TABLE 4.4 Chemical Constituents in Leachate from a Landfill in Du Page County, Illinois, Ranged by Pollution Hazard Indexa - Chemical Effective Concentration Toxicity Mobility Hazard Constituent Drinking Water Standard Index Index Index NH4 862/0.5 1724.0 62.9 108,440.0 B (29.9 + 3.5)/1.0 33.4 111.8 3734.0 COD 1340/50 26.8 78.7 2109.0 Hg 0. 87/0.002 435.0 3.2 1392.0 C1 3484/250 13.9 89.3 1241.0 Ca (46.8 + 307.3)/2506 1.42 756.7 1072.0 Cd 1.95/0.01 195.0 3.0 585.0 Fe 4.2/0.3 14.0 41.6 582.0 Na 748/270 2.77 84.6 234.0 Mn (0.02 + 0.02)/0.05 0.78 195.4 153.0 K 501/250 2.00 61.8 123.0 Mg 233/250 0.93 70.7 65.7 Pb 4.46/0.05 89.2 0.2 17.8 Zn 18.8/5.0 3.76 2.8 10.5 Si 14.9/250 0.06 45.3 2.7 aSee Griffin et al. (1976) for the definition and derivation of each term Actual value not established by EPA; therefore assumed to be the same as chloride. .._ _~. . . ~~.~ ~` ~_~4 ~ - ~ 111.
Shallow Land Burial of Municipal Wastes ment or are processed and compacted with soils from the site to achieve greater and lesser hydraulic conductivity, respec- tively. If a similar procedure could be followed with toxic waste, fewer problems with the bathtub effect might occur; however, many wastes may be too dangerous to handle in this manner, and different engineering techniques may have to be used to achieve similar results. The bathtub effect occurs primarily because more moisture enters the landfill area than would infiltrate under normal, undisturbed conditions. Trench covers are generally much more permeable than the natural material or liners below the waste, and both cover and liner play an important role in controlling moisture movement through the waste. Trench Covers Trench covers, which are critical to minimizing moisture move- ment through the waste, were almost ignored until a few years ago, although careful attention has been given to landfill liners for some time. The assumption was that trench covers could be constructed to achieve a desired low hydraulic conductivity and to limit infiltration for the required period of containment or until stabilization of the wastes. However, costly, long-term programs are required to maintain most trench covers. The covers must withstand attack by vegetation, weather (freeze/ thaw, wet/dry cycles), erosion, and strain caused by consoli- dation within the trench. Wastes buried in round barrels are especially hard to deal with since it is very difficult to backfill completely between round barrels, and the voids left even- tually cause problems. Trench covers should be designed to utilize hydrogeologic concepts of saturated and unsaturated flow systems and to allow for unexpected consolidation. Re- search is now under way to design and construct a cover that will control and divert infiltration and will not lose its integrity under moderate compaction of the wastes (see Herzog et al. (1981) for a review of the problem). Properly buried and cov- ered, the wastes would be unaffected by surface effects and could meet the containment requirements with minimal mon- itoring and maintenance. Trench Liners Assuming that we can construct a trench cover to match the liner, the question arises as to which type of plastic membrane or clay should be used and, if clay is used, what type clay it should be. (Clay is the name of a large group of minerals all having, among other common characteristics, extremely small, plate-like crystals.) Clay liners can be compacted so that their hydraulic conductivities are very low; plastic membranes can have even lower hydraulic conductivities. Membranes are cur- rently in vogue and are being used in liners and covers; how- ever, little information seems to be available on the longevity of the membranes even though we are considering the iso- lation of the waste for perhaps hundreds of years. Some concern has been expressed as to whether these membranes can with- stand attack by the array of chemicals buried especially the organic solvents. Because of these concerns, clay liners have for many years 73 been considered best suited for use in burial of wastes; the expandable and chemically very active montmorillonites were usually considered the best material for liners. This type of clay was known to be subject to attack by acids, however, and recent research has shown that some organic liquids can cause cracking in the clay. Other types of clays, even though they provide less attenuation and cannot be compacted to as low a permeability as the montmorillonites, may be more stable. Also burying the wastes in a solid rather than a liquid form might solve this problem. Water leachates of the various chemicals probably would not have nearly so destructive an effect on the clay liners as would "pure" liquid chemicals. Many of these questions could be solved if expertise from the clay and chemical industry would be brought to bear on them. I still favor the use of some mixture of natural earth materials that allow very slow leakage of leachate from the site and provide attenuation of many of the leachate constituents. Many landfills especially the hazardous-waste sites now have leachate collection systems designed such that the leach- ate can be intercepted and removed if the cover leaks. This trend to engineered sites with leachate collection systems seems to be a temporary measure, particularly when used for slowly degradable or nondegradable wastes; the leachate collected will have to be disposed of at a final disposal site, perhaps at great expense. Such engineered sites may be suitable for the disposal of degradable wastes where isolation of wastes from the envi- ronment is not necessary for long periods of time. Engineered sites may be used to reduce the volume of wastes that must be transferred for final disposal. Treatment to destroy the haz- ardous leachate components may be difficult, and processing the leachate in a standard waste-treatment plant may only di- lute the hazardous substances, possibly causing the sludge from the treatment plant to become hazardous. The idea of collecting leachate from one site and redisposing of it elsewhere resem- bles "perpetual motion" and can only cause increased difficul- ties later. In addition, many of the designs that I have seen for col- lecting leachate do not correctly apply the basic principles of soil physics (the study of the movement of water in partly saturated soils and rocks). Many collection systems would allow leachate to bypass the collection system if the soil were not saturated and would only intercept leachate when a massive failure of the cover occurs. The collection system thus provides a false sense of security. MONITORING The extent of groundwater contamination can sometimes be determined through a groundwater-monitoring program. Re- liance on regional groundwater-quality-monitoring programs to identify the contamination problems will not likely be suc- cessful since the density of such monitoring would have to be so fine as to be prohibitively expensive. The occurrence of a contaminated well in a regional monitoring network indicates that either (1) the well is located at a point not representative of regional water quality (i.e., an isolated contaminated point in the aquifer, which is not representative of water quality in
74 the aquifer) or (2) water-quality deterioration in the aquifer is so extensive that it probably is too late for any reasonable action. The detection of groundwater contamination is a process by which it is determined that the groundwater quality has been altered by the activities of man. Groundwater monitoring is a process by which the extent of contamination is evaluated and the necessity of remedial action is determined. Further, mon- itoring should provide information as to what type of action, if any, is feasible. As a matter of practicality, detection and mon- itoring form a continuum and cannot be separated. Detection of groundwater contamination can be approached by system- atically searching for sources of contamination and generating new data, by reviewing all existing data and information sources, or by chance encounters. Unfortunately, the last method is the most common. The hydrogeologic monitoring conducted at waste-disposal sites should address the purpose of monitoring as well as the geology of the site to be monitored. This is seldom done, and thus much of the monitoring that is done may have little tech- nical merit. Thought must be given to the purpose for the monitoring. This purpose may be (a) to verify predictions of contaminant migration; (b) to detect contaminants in drinking- water supplies and thus protect public health; (c) to activate a contingency plan, such as a program for leachate collection; (d) to protect the operator; or (e) a cosmetic procedure to reassure the public. Each of these purposes will require a somewhat different array of monitoring points and a somewhat different sampling program. A proper monitoring system is impossible to design without a specific purpose, or purposes, in mind. The position of the monitoring points in the contaminant flow path must be determined. To do this the contaminant flow path must be clearly defined in three dimensions. Monitoring points placed in particular parts of the contaminant plume at known distances downgradient from the landfill can be used to estimate contaminant attenuation versus distance and time. This is necessary to measure the highest level of contamination, to judge the effectiveness of the site design, or to predict the future effects of contaminant migration from the site. Monitoring programs should always be associated with con- tingency plans. A program designed to detect a potential con- tamination problem must be accompanied by a program to deal with that problem if it becomes apparent. Contingency plans may, at the one extreme, consist merely of abandoning a water well and, at the other extreme, complete collection and treat- ment of all contaminants produced by the waste-disposal site. Most contingency plans should require the monitoring program to verify predictions of expected contaminant levels at selected points in the flow system at particular times and should require that critical contaminant levels be specified that require a re- sponse. A measure of our understanding of hydrogeologic and geochemical systems associated with a disposal site is the de- gree to which we can predict the response of those systems to the waste. Monitoring Methodology A monitoring methodology is an organized approach to eval- uating groundwater quality and specific groundwater contam- KEROS CARTWRIGHT ination problems, thus providing a framework for the planning and development of a technological step necessary to arrive at valid conclusions. Several methodologies exist at the site-spe- cific level, while monitoring "strategies" are being developed at the national and state levels. This discussion follows that of Naymik (1982~. A number of proposed methodologies for monitoring ground- water currently exist. No one particular methodology is the best. While some are quite general, most are tailored to a specific site, because of the differences in goals for the moni- toring programs. At the national and state levels, strategies are being developed that will be very general because of the large variation in groundwater problems. At particular sites, meth- odologies become very specific. At the national and state levels the word strategy is normally used, rather than methodology, to describe the framework for planning monitoring programs. A strategy is a general approach that is more of an administrative matter, whereas a method- ology is more specific and technological in nature. National and state strategies have not, as yet, been completed. At present, they are being developed within the guidelines of federal di- rectives. Todd et al. (1976) and Tinlin (1981) suggested methodologies consisting of a general framework for an approach to a site- specific monitoring program (Table 4.5~. They must be modi- f~ed according to the specific situation, such as contamination source, hydrogeologic situation, and political considerations. Persons involved in a monitoring program will be required to exercise professional judgment in order to apply these or any other methodology to a specific site. Verification of Contamination Any report of groundwater contamination will have to be ver- ified by sampling and analysis. Water samples have to be drawn from springs or wells. To date, there have been no sampling protocols established for springs, and none may be appropriate. Preservation of all samples will be required using the protocol established for well waters. In addition to normal careful pro- cedures for handling samples to assure correct data, legal chain- of-custody procedures are often called for in such circumstan- ces. According to Gibb et al. (1981), collecting "representative" water samples from monitoring wells is not a straightforward or easily accomplished task. Each well and surrounding envi- ronment has its own individual hydrologic and chemical char- acter. The selection of the type of sampling or pumping device; sample preparation, preservation, and storage; and sampling procedure must be tailored to the size and accessibility of the individual well, its hydrologic and chemical character, the chemical constituents of interest, the time of year, and the purpose for monitoring. The collection of representative samples was recognized early as a problem in sampling monitoring wells at waste-disposal sites, and several investigators have attempted to establish sam- pling routines. Hughes et al. (1968) noted the need for flushing monitoring wells and recommended that two well volumes be pumped. (A well volume constitutes the initial volume of water
Shallow Land Burial of Municipal Wastes TABLE 4.5 Major Sources and Causes of Groundwater Pollution and Methods of Waste Disposala 75 Category Source Common Method of Disposal Surface Seepage Dry Injec- Percolation Spreading and Pits and Stream Disposal tion Point Line Diffuse Pond Irrigation Trenches Beds Landfills Wells Wells Municipal Sewer Leakage x x NA" NA NA NA NA NA N Sewage Effluent x x x x x x x Sewage Sludge x x x x x Urban Runoff x x x x x x x Solid wastes x x x Lawn fertilizers x x Agricultural Evapotranspiration and leaching (return flow) x x Fertilizers x x Soil amendments x x Pesticides and herbicides x x Animal wastes (feedlots and dairies) x x x x x x Stockpiles x NA NA NA NA NA NA NA Industrial Cooling water x x x x Process waters x x x x Storm runoff x x x x x x Boiler blowdown x x x Stockpiles x NA NA NA NA NA NA NA Water treatment plant effluent x x x x Hydrocarbons x x x x Tanks and pipeline leaks x x NA NA NA NA NA NA NA Oilfield wastes Brines Hydrocarbons Mining Wastes x x x x x x x x x x Miscellaneous Polluted precipitation and surface water x x NA NA Septic tanks and cesspools Highway de-icing Seawater intrusion x NA x x NA NA x NA NA aFrom Todd et al. (1976). GINA, not applicable. in the well casing prior to pumping.) He also recommends, as did Mooij and Rovers (1975), that wells installed in materials of low hydraulic conductivity be pumped dry and allowed to recover before sampling. Mooij and Rovers (1975) also rec- ommended pumping five well volumes, and Fenn et al. (1977) recommended three to five well volumes before sampling. Gilkeson et al. (1977) demonstrated changes in Pb, Fe, Zn, x x x NA x NA NA X X X X X X NA NA x NA NA NA NA NA NA NA NA NA NA Cd, Cu. and Cr concentration in samples from an industrial waste site with the number of well volumes pumped. Pumping methods or mechanisms also have been shown to affect the quality of groundwater samples. Most operating water wells are sampled using the pumping system in place in the well. The bailer has been used to sample monitoring wells, and Hughes et al. (1968) and Mooij and Rovers (1975) suggested
76 that this was the preferred method of sampling. Air or gas lifts have also long been used for sampling monitoring wells (Hughes et al., 1968; Trescott and Finder, 1970; Sommerfeldt and Campbell, 1975~. However, several groups have cautioned against the exposure of groundwater to the air during sampling as it may cause chemical changes, especially in pH, alkalinity, and iron (Wood, 1970; Wallich (1977~. In recent years small-di- ameter downhole pumps have been developed that may solve many of the problems encountered with other pumping mech- anisms. CONCLUSIONS The problems discussed in this paper do not have easy answers. In most cases we must make tradeoffs between various com- peting solutions. One thing is certain: there will always be a need for land disposal of waste. Even if "complete" recycling were to occur, the sludges and residue from recycling plants and furnaces would then have to be disposed of. I believe that land burial of waste is a safe method to use with most wastes, provided that the disposal site is properly located and engi- neered, using the best technology available and a good measure of common sense. Each land-disposal site must be considered as unique and the disposal operation engineered specifically for that site. REFERENCES American Society for Testing and Materials (1970). Special procedures for testing soil and rock for engineering purposes, Am. Soc. Test. Mater., Spec. Tech. Publ. 479, pp. 141-145. American Society for Testing and Materials (1981). A hydrogeologic view of waste disposal in the shallow subsurface, Geotech. Testing J 4(2), 53-57. Brunner, D. R., and R. A. Carnes (1914). Characteristics of percolate of solid and hazardous waste deposits, presented at American Water Works Association 94th Annual Conference, Boston, Mass., 23 pp. Cartwright, K. (1982). 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