new concepts in soil survey interpretations for on-site disposal of septic tank effluent1

BOUMA: SOIL SURVEY INTERPRETATIONS FOR ON-SITE DISPOSAL OF SEPTIC TANK EFFLUENTS 941

8. Jackson, M. L. 1956. Soil chemical analysis; advancedcourse, p. 31-168. Dep. of Soils, University of Wisconsin,Madison.

9. Jackson, M. L. 1960. Soil chemical analysis. Prentice Hall.Inc., Englewood Cliffs, N.J. 498 p.

10. Joffe, J. S., and A. J. Pugh. 1934. Soil profile studies: VI.Distribution of titanium in soils with special reference topodzols. Soil Sci. 38:245-257.

11. Joffe, J. S. 1949. Pedology. Pedology Publications, NewBrunswick, N.J. 662 p.

12. Kanehiro, Y., and A. T. Chang. 1956. Cation exchangeproperties of the Hawaiian great soil groups. Hawaii Agr.Exp. Sta. Tech. Bull. 31, p. 27.

13. Karim, A. 1953. The pedological significance of titanium.J. Soil Sci. 4:56-58.

14. Kilmer, V. J. 1960. Estimation of free iron oxide in soils.Soil Sci. Soc. Amer. Proc. 24:420-421.

15. Piper, C. S. 1950. Soil and plant analysis. IntersciencePublications, Inc., New York, N.Y. 368 p.

16. Ruhe, R. V., et al. 1965. Nature of soil parent materialsin Ewa, Waipahu area, Oahu, Hawaii. Soil Sci. Soc. Amer.Proc. 29:282-287.

17. Saunders, W. M. H. 1959. On gleying. N. Z. Soil News 2:58-60.

18. Shapiro, L., and W. M. Brannock. 1956. Rapid analysis ofsilicate rocks. U.S. Geol. Surv. Bull. 1036-C. 56 p.

19. Shapiro, L., and W. M. Brannock. 1962. Rapid analysis ofsilicate, carbonate and phosphate rocks. U.S. Geol. Surv.Bull. 1144-A. 56 p.

20. Soil Survey Staff. 1973. Soil taxonomy. Preliminaryabridged text. USDA Soil Conservation Service, Washing-ton, D.C. 330 p.

21. Worrall, W. E., and A. E. Cooper. 1966. The ionic com-position of disordered kaolinite. Min. Soc. Cir. 80, p. 1.

New Concepts in Soil Survey Interpretations for On-Site Disposal of Septic Tank Effluent1

J. BOUMA2

ABSTRACT

Soil survey interpretations for on-site disposal of septic tankeffluent are made in terms of soil limitations using existing tech-nology. New technology, based on a detailed analysis of liquidmovement and associated purification, can be used to overcomesevere and very severe limitations and to reduce slight andmoderate limitations. Experimental data obtained and tech-nology derived from single experimental innovative disposalsystems are relevant only if extrapolations can be made to othersoils shown to be identical. Three procedures of extrapolationare discussed: (i) detailed on-site spot checks of key properties;( i i ) on-site taxonomic soil classifications and (i i i) taxonomicsoil classifications at the experimental site followed by extra-polation to mapping units named after the same soil series.Largely unknown variability of key properties for liquid wastedisposal in soil series or in mapping units may reduce the prac-tical value of the latter two procedures but potential advan-tages are (i) reductions of expensive on-site inspections and(i i ) use of soil maps for showing potential changes in land-usepatterns following introduction of innovative technology. Thisapproach emphasizes soil potential rather than soil limitations.

Additional Index Words: soil variability, waste-purification,land-use planning.

SOIL SURVEY INTERPRETATIONS for on-site disposal ofseptic tank effluent follow a general procedure of

defining different degrees of limitations for soils relating toabsorption and purification of effluent (16). "Slight," "mod-erate," "severe," and "very severe" limitations describe asequence of increasing difficulties to be encountered when

1 Contribution from the Wis. Geol. Nat. Hist. Surv., Univ.Ext., Madison, Wis. and the Soil Sci. Dept., Univ. of Wis.,Madison, 53706. This research is part of the Small Scale WasteManagement Project, Univ. of Wis., Madison, funded by theState of Wisconsin and the Upper Great Lakes Regional Comm.and the EPA (grant 802874-01-0). Received 8 May 1974. Ap-proved 12 Aug. 1974.

2 Associate Professor of Soil Science.

existing septic tank technology is applied to construct on-site disposal systems. Construction in soils having slight ormoderate limitations is considered feasible, but soils withsevere or very severe limitations generally cannot be used.More specifically, the "severe" limitation implies that majorsoil reclamation, special design or intensive maintenancewould be required to construct a satisfactory system (16).The soil survey approach follows some attractive, realisticassumptions: (i) satisfactory on-site disposal and treatmentof septic tank effluent is possible by means of soil absorp-tion in soils with slight limitations. However, problems mayoccur even in these soils due to construction practices orother causes not directly related to the soil. Limitations aretherefore always present although they generally can beovercome with present technology; (ii) satisfactory on-sitedisposal is not necessarily impossible on soils with severeor very severe limitations, but even a professional applica-tion of available technology will generally not result in a sat-isfactory system. The absolute impossibility of on-site dis-posal is thus never implied, but a practical limit is suggestedat least for the present time.

The assumption of satisfactory disposal, which requiresthat raw effluent never surfaces but always disappears in thesoil, is much easier to visualize than the assumption of satis-factory treatment, unless, of course, severe pollution ofgroundwater wells occurs with visible results. Treatmentoccurs unseen underground, and results from different com-plex interrelated processes of filtration, absorption, and oxi-dation that reduce the content of contaminants in the waste.The lower boundary of the treatment system in the soil isgenerally not defined nor known. Does it, for example, endat the groundwater level, if present, or is the ground wateritself, by its diluting action, part of the treatment system?

Conclusions as to treatment of effluent during and aftersoil percolation have therefore often been based on column-study data, which may have been generated under unrepre-sentative conditions (10) or on data derived from somewell points placed near disposal systems in ground water.

942 SOIL SCI. SOC. AMER. PROC., VOL. 38, 1974

This may be unsatisfactory because natural flow patternscan be quite complex (20). Despite these difficulties, ratherbroad minimum criteria have been established for soils andthese are being used as follows to estimate soil suitabilityfor on-site septic tank disposal (16, 17, 22): (i) the hydrau-lic capacity of soil to transmit liquid is generally expressedby the percolation rate which should be faster than 60 min/inch; (ii) at least 90 cm of unsaturated soil should be pres-ent between the bottom of the seepage system and a highgroundwater table or bedrock. Bedrock can be observed atall times, but the groundwater level may only be high in wetperiods of the year. A seepage system has to function at alltimes and the highest groundwater level is therefore mostrelevant. Many health codes allow observation of the high-est level of soil mottling to be used as an indicator for theseasonally highest water level (22); (iii) limiting site char-acteristics such as excessive slopes and location in flood-plains are considered also (22). Following these criteria,unsuitable soils can be defined as having severe or very se-vere limitations in soil survey interpretations. Interpretivemaps can be prepared based on the soil maps, showingareas occupied by these soils and maps of this type can be

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Fig. 1—Hydraulic conductivity curves, measured in situ withthe crust test procedure for some major soil horizons (Curve1 = C horizon, Plainfield loamy sand, Curve 2 = IIC hori-zon and Curve 3 = B2 horizon of Batavia silt loam, Curve4 = B2 of Hibbing loam). Asterisks indicate measured ten-sions below ponded seepage beds or soil cores. Data for thesilt loam are not available yet.

very useful for land-use planning purposes, based on cur-rent health regulations (1, 2, 21). However, many practicalquestions are currently being raised regarding the availabil-ity of alternative waste disposal procedures for soils withsevere or very severe limitations. ELecent research, based oninnovative test procedures and a more strict analysis ofliquid-waste disposal problems, has indicated that such al-ternatives may be available and a general discussion of thesewill be presented in the next section, This development maypresent the opportunity to expand the current interpretationconcept by not only describing soil limitations for currenttechnology for certain soil types but by also defining specificalternative "construction and management packages" that,when applied, would overcome these limitations, so as tomake satisfactory on-site disposal a practical reality in spe-cific areas that can be delineated on soil maps. This ap-proach is not necessarily limited to soils that are now con-sidered to have severe or very severe limitations. The verydefinition of "slight" limitations implies that problems canarise if mistakes are made in designing, constructing orloading a system in a soil with a "slight" limitation. A(small) "package" can be defined for these soils to assureconstruction of a good system. Generally, soils with moder-ate limitations could be expected to need a more elaborate"package." Basically, however, modifications or refine-ments of current practices should be adequate for soils withslight or moderate, limitations, whereas drastically new ap-proaches would be needed for soils now classified as havingsevere or very severe limitations. This approach to interpre-tation offers a major advantage in that derived maps couldbe more useful by not only showing limitations for use ofcurrent technology but, more importantly, by also showingpotential implications of applying innovative technology.Rather than describing limitations only, emphasis would beshifted to defining soil potential and to defining means forrealizing it. The purpose of this paper is to discuss possibleimplications of these concepts based on experience beingobtained in the Small Scale Waste Management Project inWisconsin. Technical aspects of innovative on-site disposaland treatment of septic tank effluent will be briefly reviewedand implications for soil suitability classifications and futurerequirements for soil survey research will be discussed.

CRITERIA AND METHODS

Use of Soil for Disposal and Treatment of Liquid WasteThe capacity of a subsurface seepage bed to accept liquid

waste cannot be adequately expressed by the percolation test(5, 6). But modern physical tests can be used to predict infil-tration into freshly exposed or clogged soil when flow occurs insaturated or unsaturated soil (6). The availability of a hydraulicconductivity (K) curve, which expresses an infinite number ofcharacteristic permeabilities corresponding with different mois-ture contents, allows application of the engineering concept touse of the soil: soil behavior is determined by, and can there-fore be manipulated by, construction and management tech-niques which for liquid waste disposal refer mainly to frequencyand methods of effluent application and loading rates. Flowrates in the soil are related to the degree of purification whichresults from processes of filtration, absorption, and oxidation(6, 14). The final desirable degree of purification has neverbeen specifically defined. Generally, fecal indicators should beremoved and concentrations of suspended solids, BOD, nitrogen


and phosphorous should be adequately reduced. The levels towhich reductions should occur in a limited volume of soil aresubject to continuous debate, but the trend is towards definitionof ever lower values. Test procedures may be debated also. Forexample, use of fecal indicators to detect fecal, bacterial con-tamination sometimes offers problems because some indicatorspecies may occur naturally in the soil. Ratios of organismsrather than absolute numbers may be most meaningful (11).These examples illustrate that: (i) standards for effluent purifi-cation and methods to determine them are not rigidly estab-lished and (ii) standards are quite diverse, as they relate to dif-ferent compounds in the waste. Column studies, using large un-disturbed cores, and field monitoring have shown that soil orsoil materials can be very effective as a purifier of liquid waste(6, 10, 12, 13, 14). Exploring the potential of soil for use as amedium for disposal and treatment of liquid waste involves:(i) a quantitative analysis of the range of natural and potentialhydraulic conditions in a soil or in soil material; (ii) evaluationof the associated purifying potential, followed by (iii) selectionof optimum hydraulic conditions in soil for disposal and treat-ment, and determination of construction and management tech-niques for achieving these optimum conditions, which are boundto be different for different soils. This three-point analysis willbe made in the following sections for a wide range of differentsoils, currently classified as having slight to severe limitationsfor on-site liquid waste disposal.

Present technology

HYDRAULIC CONDITIONS

Hydraulic conditions in soil during waste disposal can be pre-dicted in quantitative terms by using hydraulic conductivity(K) and moisture retention data. Representative curves are pre-sented in Fig. 1. The maximum steady infiltration rate is equalto K at saturation in a homogeneous, vertical one-dimensionalflow system. Infiltration rates decrease upon desaturation of thesoil. Desaturation may be caused by formation of barriers toflow at the infiltrative surface. Barriers may be formed by eithercompaction or puddling during construction under wet condi-tions or by accumulation of suspended solids and associatedbacterial action during use of the seepage bed (6, 14). "Effec-tive" infiltration rates have to be known so as to enable calcula-tion of minimum required seepage areas. Clogging in sands re-sulted in equilibrium flow rates of approximately 5 cm/day (6,19). Clogging in sandy loams resulted in lower flow rates of5 mm/day (6), whereas flow rates in silty clay loams, derivedfrom column studies, averaged around 6 mm/day (10). Theserates, shown as asterisks in Fig. 1, indicate that flow throughclogged layers or crusts never stops completely. Satisfactory ab-sorption and lack of undesirable surface seepage can beachieved if seepage areas are made sufficiently large. However,economical and technical considerations make very large sizes,exceeding approximately 108 m2 (1,200 square feet), unattrac-tive. This figure translates into a critical "effective" infiltrationrate of approximately 1 cm/day at a total loading of 1000liters/day for an "average" family. All soils tested in situ inWisconsin so far had .K-sat values exceeding 1 cm/day, butslight crusting or clogging would reduce infiltration rates below

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Fig- 2—Schematic diagram of on-site disposal systems of septic tank effluent, comparing present and new technology (see text).Percentages indicate approximate areas of Wisconsin occupied by the described soil conditions.


this level in several clayey soils (Fig. 1). The testing programis being continued and soils may yet be found with K-sat valuesbelow 1 cm/day.

DEGREE OF PURIFICATION

The degree of purification is related to the flow regime. Oc-currence of unsaturated flow is favorable for achieving purifica-tion because liquid moves relatively slowly through the finerpores enhancing filtration and absorption whereas larger poresare filled with air, enhancing oxidation. For example, fecal indi-cators have been shown to move freely for relatively large dis-tances in ground water in sands if seepage beds were submergedin the ground water (6, 14), whereas removal occurred afterpercolation through only 10 cm during unsaturated flow (6).Criteria for purification are diverse and sometimes soils canonly achieve limited purification. For example, systems in sandcan remove fecal indicators and BOD effectively but nitrogen,oxidized as nitrates, may move freely into the ground waterwhereas the absorption capacity for phosphorous may be lim-ited (6, 13). Satisfactory purification can be achieved by perco-lation through slowly permeable clayey soils at moisture con-tents only slightly below saturation due to the relatively slowmovement and the high absorptive capacity of the soil (10).However, the unsaturated zones with low tension below theseepage bed should have a minimum depth of 60 cm to ensureadequate downward movement and filtration. Ground water,therefore, should preferably not be higher than that level.

DESIGNS FOR INNOVATIVE SYSTEMS

Designs for innovative systems, to be presented as "construc-tion and management packages," are schematically presentedin Fig. 2 for major soil groups in Wisconsin. The diagram illus-trates a concept and does not at this time reflect proposed prac-tical changes in the health code allowing on-site disposal invirtually all soils. Detailed discussions of these systems havebeen and will be reported elsewhere and only a review will bepresented here. Newly designed effluent distribution systems areused to avoid uneven loading of the seepage bed during applica-tion of liquid (9). Thin permeable soils over creviced bedrock(Fig. 2, no. 1) offer inadequate purification resulting in patho-genic well pollution. Mound systems, in which effluent ispumped into a covered seepage bed on top of 60 cm of sandyfill placed on the original soil surface, have been developed asan alternative (7). Seepage beds in sands (Curve 1 in Fig. 1)with the highest water table at least 1.80 m below the soil sur-face should be sized for an infiltration rate of a maximum 5cm/day. Conventional systems in sands work satisfactorily ex-cept for lack of nitrogen removal and housing densities and wellconstruction practices must be controlled to avoid health oreutrophication problems (19, 20). Research is in progress todevelop innovative systems incorporating denitrification andalso improved P removal (Fig. 2, no. 2). Formation of barriersto flow in seepage beds caused by either puddling during con-struction or by biological clogging is a serious problem in loamyand clayey soils (Curves 2, 3, 4; Fig. 1). Excavation of seepagebeds in clayey soils often results in compaction of future infiltra-tive surfaces due to driving over the bottom of the bed. Thisproblem can be reduced by excavating a series of smallertrenches and this procedure is therefore recommended. Biologi-cal clogging can be reduced by introducing periods of aeration,when clogging components break down, between intermittentapplications ("dosing") of effluent. This can be achieved bydosing in one single trench-system (Fig. 2, no. 3.1) or by usingtwo systems intermittently (Fig. 2, no. 3.2) (8). Slowly perme-able soils (Curve 4, Fig. 1) with the highest ground water atleast 1.50 m below the soil surface can use large subsurfacetrench systems (Fig. 2, no. 4.1) when .K-sat values exceed 1cm/day in all layers below the bottom of the trench. Highergroundwater levels in these and in more permeable soils cansometimes be lowered by using drainage systems. A mound sys-tem (Fig. 2, no. 4.2: slowly permeable soil and 5b: permeable

soil) in which effluent is pumped intermittently into a seepagebed on top of 60 cm of fill which covers the plowed surface ofthe original soil can be built as an alternative. The mound sys-tem avoids problems associated with soil excavation, providestreatment of effluent during percolation through the sandy fillwhich acts as a sandfilter and allows this liquid to enter the un-derlying soil in surface horizons which are usually most perme-able (6). Mound systems are not recommended where the high-est watertable level is closer than :'>0 cm to the soil surface.Generally, seasonally perched water tables are considered to beequivalent to real water tables. However, removing a perchedwater table may be easier than lowering a real water table, cer-tainly when the former occurs on top of slowly permeable hori-zons over unsaturated, permeable sediments. Occurrence oflow-chroma mottling often indicates soil saturation and presenceof ground water (15). However, soils were described in whichlow-chroma mottling in the lower part of a slowly permeable siltcap overlying outwash sand did not indicate saturated condi-tions. Excavation of the silt cap and construction in the sandwas recommended (Fig. 2, 5a) (18).

Field monitoring data is still being collected for these systemswhich are still considered experimental, but results are encour-aging and a discussion of possible implications for soil surveyinterpretations and soil research is therefore considered appro-priate at this time.

Implications for Soil Survey Interpretations

Different innovative on-site disposal and treatment systemshave been described that can be used to overcome limitationsof different soils for the conventional system consisting of septictank and subsurface seepage bed. These innovative systems arecurrently tested under field conditions and data obtained willonly be relevant if extrapolations can be made to other soils thatare somehow characterized as being identical. Three procedurescan be followed here: (i) the soil at experimental site A is char-acterized in the field at some observation points by emphasizingkey properties that have been defined as critical for achievingsatisfactory movement and purification of liquid waste, as dis-cussed. The same characterization will have to be made at anyfuture location (B) where a new system is to be built to deter-mine whether the soils are identical. If so, within limits, tech-nology used at Site A is extrapolated to Site B; (ii) the soil atexperimental Site A is observed and classified in the field atsome observation points following taxonomic schemes of theCooperative Soil Survey. If the taxonomic classification of a soilat any new location B is identical to the one at Site A, technol-ogy is extrapolated from A to B; (iii) the soil at experimentalSite A is observed and classified in the field as in (ii), and it isassumed that sites occurring elsewhere in mapping units namedfor the same soil series have identical key properties. Technol-ogy is extrapolated if Site B occurs in a mapping unit named forthe soil series observed at Site A.

Key properties, discussed in the previous sections, were: (i)hydraulic conductivity characteristics, which are a function ofsoil texture and structure; (ii) occurrence of ground water; (iii)occurrence of bedrock; and (iv) site characteristics, such asslope and landscape location. These data are available for allour experimental sites. Application of the first extrapolationprocedure would require measurement of K curves at any newsite and several soil borings to study key soil properties andvariability in an area of perhaps 90 m2 in which a seepage sys-tem is to be built. These auger holes would serve for observationof groundwater or bedrock levels, if present, or of indicative soilmorphological features such as mottling. This procedure is cur-rently required in Wisconsin by state law (22) except that per-colation rates are measured rather than K curves. The proce-dure is effective but time-consuming, and extrapolations areconfined to isolated, single sites. Application of the second ex-trapolation procedure assumes that taxonomic classifications,based on pedological criteria, adequately reflect key soil proper-ties for liquid waste disposal. This is realistic, within limits, forgroundwater or bedrock levels, since soil series descriptions list


a range of observed diagnostic features. However, hydraulicconductivities of pedons within a series, or phases thereof, mayvary (3, 4). Moreover, textural differences may occur in thesame soil series at depths exceeding 1.50 m which may not bereflected in separate phases, and this may strongly affect liquidmovement from a subsurface seepage trench. For example, soilsin glaciated areas may have formed in a relatively homogeneousloess cover of 1.50 m over heterogeneous sandy and clayey gla-cial deposits. Diagnostic surface and subsurface horizons maythen be identical in identical taxonomic units at different loca-tions, even though rates of movement through the subsoil maybe quite different, so much so that several experimental designsfor seepage systems may be required. However, a routine appli-cation of the second extrapolation procedure would make avail-able increasingly detailed data on magnitude and variability ofkey properties in soil series, or phases thereof, assuming that on-site measurements needed for extrapolation procedure 1 aremade as well. This could mean that on-site measurements wouldnot be needed anymore for well-characterized taxonomic soilseries with proven low variability at some future time. The re-quired procedure would then be to: (i) classify a soil at anyprospective new site following taxonomic schemes of the Coop-erative Soil Survey; (ii) check the variability of key propertiesmeasured elsewhere in this series, and (iii) determine whetherthis variability is sufficiently low to allow direct interpretationof site suitability or whether variability has been so high thaton-site determination of key properties is still necessary. Soilseries with proven low variability of key properties could bedirectly associated with certain types of innovative systemsshown in Fig. 2.

The third extrapolation procedure is more difficult to acceptfrom a conceptual point of view. Soil mapping units are carto-graphic units which delineate areas within landscapes and whichare normally named for the dominant series within the mappingunit. This unit would contain the named series, other series thatdo not significantly differ from the named series with respectto "use and management" and some percentage of contrastinginclusions (15-20% often cited). Use of this procedure canonly be satisfactory if mapping units named for a soil series havefew inclusions and if the soil series itself has a low variability ofkey properties, as discussed in the context of the second ex-trapolation procedure. Another problem is one of scale posedby use of detailed soil maps which do not allow showing sepa-rate areas if smaller than approximately 1 ha (2 acres). How-ever, a seepage area of 90 m2 (appr. 0.01 ha) is consideredlarge and this difference shows that this third procedure of ex-trapolation is not acceptable except when applied to very homo-geneous soil mapping units. However, few data are available onthe variability of soil mapping units and the third extrapolationprocedure can only become viable if more data of this type aregenerated. The discussion of the third extrapolation procedureis included here because it is being applied already to test sitesuitability for conventional systems (22) and to show the effectof introducing innovative technology on land-use patterns (1,2, 21). For example, the current Wisconsin administrative codeallows denial of a permit for a conventional septic tank on thebasis of a "severe" or "very severe" interpretive rating of a soilshown to occur on a specific site on a soil map. However, theproperty owner is permitted to present evidence to overrule anydenial made on the basis of the soil map interpretation (22,p. 274t).

Limitations of extrapolation procedures 2 and 3 should notlead to the conclusion that the first procedure of extrapolationis the only viable one because the other procedures have somespecific and attractive advantages: (i) on-site K measurementsand observations, needed for extrapolation procedure 1, arerelatively costly and time consuming, whereas using limited on-site observations needed for soil taxonomy (extrapolation pro-cedure 2) would be much more economical; (ii) soil maps canbe used, assuming that soil mapping units are reasonably homo-geneous, to show the potential future impact of innovative tech-nology on land use patterns before any development has takenplace (1, 2, 21). Many soils not suitable for on-site disposal

now may be used in the future and this has implications whichdepend on how large an area will potentially be affected andwhere such areas occur. If this analysis indicates the potentialfor major change in large areas, attempts may be initiated earlyto create new zoning laws if such developments are consideredundesirable. If, on the other hand, this analysis indicates thatfuture effects will be minimal, concern can be channeled in timeto more worthy causes.

Variability of key properties in taxonomic soil series or soilmapping units is a critical factor because advantages of their usediminish strongly as they become more heterogeneous. Obtain-ing key soil properties, as defined, by on-site inspections and byapplying K measurements in situ (6) and measuring ground-water (or perched water) levels during the year will take timeand current soil characterization procedures will have to be usedas long as more detailed data are not available. For example,a percolation rate of 120 min/inch is temporarily assumed tobe equivalent to a K value of 1 cm/day and soil mottling char-acteristics are used to estimate groundwater fluctuations, eventhough limitations of these procedures are recognized (18).

CONCLUSIONS

A future soil characterization program for on-site liquidwaste disposal should allow the definition of certain soilseries that are relatively homogeneous in terms of their keyproperties. This requires a routine application of the taxo-nomic soil classification system at any site where a disposalsystem is to be built and where detailed tests are made. Thepurpose is to allow extrapolation procedure 2, as discussed,for relatively homogeneous soil series. In addition, moredata should be generated on the variability of soil mappingunits named for specific series because use of soil maps forregulatory purposes and for purposes of demonstrating theimpact of certain new technologies on land-use patterns, isbased on the inadequately tested assumption that mappingunits are reasonably homogeneous. Definition of certain"homogeneous" mapping units could allow extrapolationprocedure 3, as discussed, but this largely untested proce-dure is as yet not very attractive.

The feasibility of using soil survey interpretations basedon "potential" rather than "limitations" will depend on thedegree of variability of key soil properties in specific taxo-nomic soil series or in mapping units. Too much variabilitywould not allow general use of either soil taxonomy or soilmaps for extrapolations of rather elaborate innovative "con-struction and management packages" (Fig. 2) developed atspecific sites to overcome limitations encountered whenusing present technology.

ACKNOWLEDGMENT

The author acknowledges helpful discussions with M. T.Beatty (Univ. Ext., Madison) and A. J. Klingelhoets (Soil Cons.Service, Madison).

new concepts in soil survey interpretations for on-site disposal of septic tank effluent1

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