Author Institution: Department of Agronomy, Ohio Agricultural Experiment Station, Wooster, Ohi

Struthers, Paul H.

The Ohio Journal of Science

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CHEMICAL WEATHERING OF STRIP-MINE SPOILSP. H. STRUTHERSDepartment of Agronomy, Ohio Agricultural Experiment Station, Wooster, OhioThe aim of strip-mine reclamation has been defined as returning the disturbedland to a useful and productive condition as soon as possible. Fundamental tothis is the natural process of soil formation. Sometimes we tend to forget thatsoil formation is natural and would proceed even if man suddenly ceased to exist.On strip-mine spoils, however, without our help new spoil would not necessarily beformed in the location or with the speed we desire. Nor would the new topographyalways be what we want. It is in these areas that man can work with nature,by grading and revegetating strip-mine spoils, to stimulate and direct the physical,chemical, and biological forces of soil formation.We have been unaware, also, of the intensity with which these forces act.When we hear that it takes a thousand years to form an inch of topsoil, we arelulled into thinking of very gradual changes. Actually, the changes are slow onlyas the maturity of a soil is approached. In spoil banks, the quick developmentof acidity illustrates that very rapid chemical reactions occur as the exposed rockbegins to come into equilibrium with its new environment.In order to obtain quantitative information about the changes which takeplace during the weathering of fresh strip-mine spoil materials, an experiment wasinitiated on January 1, 1959, at the Ohio Agricultural Experiment Station. Oneof the objectives was to investigate the chemical changes in spoil samples sub-jected to natural weathering processes. These changes were assessed by periodic-ally analyzing the drainage waters for the resulting soluble salts.At the outset, it may be well to review briefly some of the concepts involved.Acidity in strip-mine spoils originates from iron disulfi.de particles in the rock,which oxidize when exposed to air and moisture, forming acid iron sulfate andsulfuric acid. The acids in turn attack other rock fragments, forming solublesalts. If enough limestone and other basic materials are present in the rock, theacid is neutralized, and mostly neutral salts are formed. Without sufficient lime,the acid sulfate salts of iron and aluminum are produced, causing the acidity oftenencountered in spoil banks. (This is quite different from soil acidity caused byexchangeable or adsorbed ions, in which salts are not involved.) Rain water thatsoaks into spoil banks hastens spoil improvement by leaching the salts from thesurface layers. Flowing surface water, on the other hand, actually retards soilformation by eroding away the already leached and weathered spoil material andcontinually exposing new raw rock to be oxidized.THE EXPERIMENTAL ARRANGEMENTNineteen spoil samples were placed in the ground in plastic lysimeters one footin diameter and four feet deep (fig. 1). One soil sample (Wooster silt loam) wasalso included for comparative purposes. The spoil samples were obtained duringactive strip-mining operations and represent a mixture of the rock strata beingremoved at the time of sampling. A diversity of spoil types was obtained, in-cluding ten calcareous and nine acidic samples with a wide range of physicalcharacteristics. Of the acid samples, five proved to be toxic and one marginalwith respect to plant survival. Table 1 shows the source and description of eachsample.The spoil samples were exposed to natural weather conditions continually fromthe beginning of the experiment. Natural precipitation furnished the moistureTHE OHIO JOURNAL OF SCIENCE 64(2): 125, March, 1964.126 P. H. STRUTHERS Vol. 64TABLE ISample description and sourceDescription Coal seam strip mined CountyCalcareousCalcareous clay shaleToxic shale, fine sandstoneToxic silty clay shaleCalcareous clay shaleCalcareous shale, coarse sandstoneCalcareous clay shaleCalcareous hard shale, limestoneAcid silty clay shaleAcid shale, coarse sandstoneAcid silty clay shaleMarginal (very acid) shale, sandstoneToxic silty clay shaleCalcareous silty clay shaleCalcareous silty clay shaleToxic shale, sandstoneToxic soft clay shaleCalcareous soft clay shaleCalcareous shale, limestoneAcid soil, Wooster silt loamMeigs Creek No. 9Meigs Creek No. 9Meigs Creek No. 9Meigs Creek No. 9Pittsburgh No. 8Pittsburgh No. 8Pittsburgh No. 8Upper Freeport No. 7Middle Kittanning No. 6Middle Kittanning No. 6Middle Kittanning No. 6Middle Kittanning No. 6Middle Kittanning No. 6Middle Kittanning No. 6Middle Kittanning No. 6Lower Kittanning No. 5Lower Kittanning No. 5Clarion No. 4aBrookville No. 4BelmontMorganNobleGuernseyHarrisonHarrisonJeffersonColumbianaCoshoctonCoshoctonPerryPerryPerryVintonVintonMahoningStarkVintonStarkWaynewhich percolated through the samples, the leachate draining freely into collectionflasks at the bottom. The liquid leachates, some colored brown with iron salts,were collected at about monthly intervals during the first year, and less oftensubsequently. The immediate objective was to measure the amounts of saltsproduced, and to note changes as time passed. Since most sulfate salts formcrystals containing water, all were dried at 610 F before weighing.OBSERVATIONS AND RESULTSAs the study progressed, wide variation in rainfall caused fluctuations in theeffluent leachate and salt yields. Figure 2 shows the pattern of rain distributionduring different seasons and years, together with leachate and salt productionfrom the marginal spoil sample. The first year was wetter than normal, withenough rain to leach out the salts as they formed. The salt yields from the sam-ples were much greater than expected. In the first 11.5 months the five toxicsamples yielded an average of 60 tons of salts per acre, one producing more than130 tons. The marginal spoil yielded 10.3 tons per acre in the same period, theother spoil samples yielding lesser amounts down to 0.45 tons. By comparisononly 320 pounds of salts per acre were leached from the soil sample.In contrast with the first year, the second was very dry; rainfall was 27 percent below the annual average of 38 inches. Late in the year, for periods lastingup to six months, some samples yielded no leachate at all until flushed by rainsduring the following spring. Consequently, the salt yields for the second calendaryear were inordinately low, and could not properly be compared with first-yearyields. When a similar pattern was noted the third year, however, a basis forvalid comparisons became apparent: annual increments of salt yield could bestbe measured after spring rains from one year to the next, rather than on a calendar-year basis. Fortunately for the experiment, the first year with its unusually highrainfall proved to be exceptional and could be included with no loss of initialdata for the first spring.No. 2 CHEMICAL WEATHERING OF STRIP-MINE SPOILS800_127FIGURE 1. Lower part of plastic lysimetercontaining marginal No. 6 spoil. Roots areof alder (Alnus glutinosa) nearing 3-ft depthby midsummer.FIGURE 3. Annual salt yields from four ftof spoil materials, showing changes withduration of weathering. Points marked Xidentify the marginal sample.19620.05Four Foot Depth of Spoil. No-6 Coal Seam, Perry Co., OhioFIGURE 2. Daily rainfall rates and corresponding drainage and salt-yield rates for marginalNo. 6 spoil.128 P. H. STRUTHERS Vol. 64TRENDS IN SALT YIELDHaving extablished the basis for observing changes with time, definite trendscould be observed in salt yields from the different spoil samples. Figure 3 showsthe results plotted for several samples which were selected to illustrate the widerange in yields observed, and the different trends noted. Data for three toxicspoils are shown at the top of the figure. Curves for the two No. 6 coal spoilsfrom different sites in Perry County exemplify the variability which may occur inoverburden from the same coal seam. Data for the marginal spoil sample aredesignated with points marked X; these points represent decreasing yields of 10.3,9.1, 8.3 and 8.17 tons of salt per acre per year. Only the last three quantities areadditive, however, since the first two represent overlapping increments. In eachcurve the first point represents salt yield for the initial 11.5 months, and subsequentpoints are for annual increments ending about March, after the spring flush ofdrainage. In the foregoing case, the cumulative salt yield amounted to 28.3 tonsper acre in three and one-quarter years.Variations in slope of the long lines in the graphs indicate different rates ofsalt production with the passage of time. Most spoil samples showed progressivelydecreasing salt yields. The dip in the curve for the No. 8 coal spoil is probably anexaggerated effect of the low rainfall during the second year, rather than an indi-cation of a reversal in the rate of salt production. The real exception to the gen-eral trend is the increasing salt yields from the toxic No. 9 coal spoil, shown nearthe top of the graph. In three years, the annual increment per acre rose from38 to 105 tons. However the rate of salt yield is gradually decreasing, and adecrease in the quantity of salt will follow after a few more years.These results corroborate field observations that most spoil banks improvewith time, but some appear to become more and more toxic the first few yearsafter being formed. An increase in toxicity develops when there is considerableexcess potential acid relative to the basic minerals in overburden material, withthe result that the rock minerals—including more polysulfides—are attacked byincreasingly stronger acid products.The toxic samples all had higher salt yields and lower pH levels (below 3.0)than the nontoxic spoils. However, no relationship was found within either groupbetween amount of salts produced and the spoil reaction or pH. This was trueeven when comparing alkaline and acid samples. The highest and lowest yieldsamong the nontoxic samples were both from acid samples, shown by curves forthe two No. 6 spoils. Respectively adjacent curves for the calcareous No. 8and No. 4 spoils indicate almost equally high and low salt production from sampleshaving alkaline reactions.COMPOSITION OF SALTSIn addition to a measure of the total salts leached from spoils, the compositionof the salts was also determined. Chemical analyses showed considerable varia-tion in the mixture of salts from the different samples. Summarized results offirst-year analyses of four samples are shown in figure 4, the relative weights beingindicated cubically. A similarity in both array and relative amounts of the metalions can be noted between the soil sample at the top and the next two spoil samples.In all of these, calcium was the major metal component of the salt, exceedingmagnesium and other cations, while aluminum, manganese, and iron were rela-tively minor constituents. Plants were found to grow successfully in all three ofthese materials.The fourth illustration in the figure, however, showing results for a toxicsample, presents an entirely different picture. The total salt was much greater;among the cations, calcium was exceeded by magnesium, and potassium wasrelatively low. Iron, aluminum, and manganese formed a considerable portionNo. 2 CHEMICAL WEATHERING OF STRIP-MINE SPOILS 129of the metal ions. These findings appear to be generally characteristic of toxicspoils.A comparison of first- and second-year results are shown by linear bar graphin figure 5. Only cations contributing 1 per cent or more to the salts are in-cluded. Considerable changes took place in salt composition as the samples con-tinued to weather. Calcium, being the most active cation, showed evidence ofsteady progress toward depletion under acid conditions. Less was leached thesecond year than the first, the greatest reduction occurring in the toxic No. 9spoil. Magnesium yield also decreased the second year in the toxic sample, butincreased in the marginal No. 6 spoil. A marked increase in the production of300T o t a l S a l t s C a l c i u m M a g n e s i u m S o d i u m P o t a s s i u m A l u m i n u m M a n g a n e s e I r o nFIGURE 4. First-year yields of leached salts and metal ion components, in pounds, from fouracre-ft of soil and spoil materials.iron and aluminum occurred in toxic spoils as the basic minerals neared deple-tion. This constituted a characteristic difference between toxic and nontoxicspoils.The different percentage ranges, shown by the lengths of the bars in the graph,are probably indicative of somewhat different anion composition in the salts.Although most strip-mine salts are sulfates, nontoxic spoils no doubt also contributeappreciable amounts of carbonates and bicarbonates to their leachate solutions,130 P. H. STRUTHERS Vol. 64thus tending to increase the basic cation percentage. On the other hand, toxicspoils may yield some free sulfuric acid along with the salts, thus decreasing thecation concentration. These anionic influences varied with changes in cationcomposition upon continued exposure to weathering.10Percent Metal Ions in20Leached SaltsFIGURE 5. Changes in major components (over 1 per cent) of salts leached during consecutiveyears from toxic and marginal spoil samples.SUMMARYQuantitative analytical information has become available for the first timeregarding the chemical weathering of coal strip-mine spoil materials, which canbe summarized in the following observations:1. Intensive chemical weathering of spoils began immediately upon exposureto air and moisture.2. Large quantities of sulfate salts were produced in the top four feet of spoilmaterials the first year. Quantities measured varied from 900 pounds tomore than 130 tons per acre.3. Highest salt concentrations in spoils occurred in late summer and fall,lowest in early spring.4. Soluble mineral plant nutrients were generally more abundant in spoilsthan in Ohio soils.5. Salt production decreased in most spoils after the first year. The saltyield increased over a period of three years in one sample toxic to plants.6. Salt production was not related to spoil reaction (pH) among either toxicor nontoxic samples.7. Toxic spoils were characterized chemically as follows:a. Very high acidity with pH levels below 3.0.b. Very high salt content.c. Limited and quickly depleted amounts of calcium and magnesium, withlow Ca/Mg ratios.d. High levels of soluble aluminum, iron and manganese salts.The findings have practical significance in the reclamation of strip-mined land.No. 2 CHEMICAL WEATHERING OF STRIP-MINE SPOILS 131Plant establishment has been observed to be more successful in the spring than atother seasons; it is now evident that one of the reasons is the lower salt content ofspoils after leaching by winter and early spring rains. Grading in a manner toencourage maximum rain infiltration and leaching throughout the year also assumesgreater significance as a means to promote spoil amelioration and plant survival.After salt concentrations in toxic spoils have been reduced by chemical weatheringand leaching, it appears probable that moderate applications of lime can restorethe mineral balance and decrease the acidity to permit the establishment of plantcover.Considering the great chemical reactivity in spoil banks, pride may well betaken in the progress and achievements which have been made in strip-minereclamation.QUESTIONS DIRECTED TO DR. STRUTHERSI1. Why do you say grading is necessary when test plots indicate tree growth isbetter on ungraded areas?Answer:There are a number of considerations involved. Perhaps if growth were theonly objective, grading might not be necessary. However, if the trees are to beharvested, or any other use made of the forested land, grading would certainlyfacilitate access to such areas. Also, other uses are sometimes made of strip-mined land, as for pasture and forage crops, in which case grading would cer-tainly be an advantage. My main thought, however, was that proper gradingcan prevent excessive erosion, decrease water runoff, and increase leaching, thusleaving the surface in a condition for more rapid soil formation, and leading toeven greater productivity.2. In Pennsylvania a comparable pH reading (3.5 to 4.5) can be obtained fromthe same plots after ten years. Five of nine plots increased in acidity slightlyand four decreased slightly. Do you have an explanation as to why thesesoils apparently stay the same?Answer:This situation is probably a common occurrence, and involves an importantconcept. The soil has been left acid because most of the basic minerals havebeen leached out; it will remain acid and unproductive until they are replaced,even after the formation of acid sulfates has virtually ceased. In this case, pH isnot a good indicator of change.One of the objectives of my study is to determine how long it takes to reachthis nearly static situation in spoils. Once it has been reached or approached,the soil acidity can be quickly corrected by the addition of lime, and the produc-tivity restored with fertilizers. If man does not do this, the same additions willoccur naturally over a long period of time, with the gradual accumulation and de-cay of dead plant material such as leaves. Surface erosion, of course, would delaythis slow process even further, which emphasizes the importance of properlygrading our spoil banks.

Chemical Weathering of Strip-Mine Spoils

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Chemical Weathering of Strip-Mine Spoils

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