Transportation Research Recordl 892 25 Its ReLation to Drainage Quality. U.S. Envi- wastes. Office of Technology Transfer, U.S. ronrnental Protection Agency, Cíncinnatir Rept. Environmental Protection Àgency' RePt. EPA-600/ EPA-6OO /7 -77 -067, 1977 . 4-79-020, L979. 16. ASTî4 Method for Colu¡nn Leaching of lfaste Mate- ríats. committee D-34.02 on tfaste Dísposal, AST!¿1, Philadelphia, Draft Rept. 198I. Publication of this paper sponsored by ûmmittee on Physícochemical Phe- L"t, Methods for Chernical Analysis' of Water and nomenø in Soils. Identifrcation of Source Materials for Acid Leachates in Maryland Coastal Plain D.P. WAGNER, D.S. FANNING, AND J.E. FOSS Acid leachates are produced ¡n the ox¡dation of naturally ocanrring, sulfide- course of construction activíties to avoid or con- bear¡ng sed¡ments d¡str¡buted throughout much of the Maryland Coastal trot åcid sulfatê Problerns. Plain. Geologic ages for the sed¡ments span from Lower Cretaceous through Tertiary. When these s€d¡ments are exposed to the atmosphere, sulfuric acid MATERIALS AND METHODS is produced in quant¡t¡es suffic¡ent to prohibit plant growth, dissolve con' c{ete, and @rrode metal. lnitial pH values of near neutral or above may drop Study sites were selected on the basis of norpholog- to as low as 2 after the sulfidic sediments undergo oxidat¡on. ln add¡tion to ical properties observed ín the fieId. Sites were pH, characteristics useful ¡n ¡dentifying sulfide-bearing Coastal Pla¡n sediments located by reconnaissance of areas where outcropping include sulfur content, sediment morphology, presence of sulfide or sulfate geologic for¡nations r,tere suspected of containing minerals, and morphology of surface soils formed from the sed¡ments. Un- and secliment samples were retrieved oxidized sulfidic sed¡ments are mostly dark colored, Typical colors include sulfides. soil black (5Y 2.5111, yay (l0YR 5/11, or dark gray 6Y 4/1). Pyrite has been from road cuts, hancl-dug pits, and hand borings. pâssed 10- ident¡fied as the pr¡nc¡pal sulfide mineral present ¡n the sediments. Pyrite samples were air-dried and through a a morphology ranges from large megascopic crystals assoc¡ated with Lower mesh (2-nm) sieve. Soil pH was measured by using Cretaceous l¡gn¡t¡c deposits to mictoscopic framboids common in Upper l:1 ratio of soil to wåter. Identification of sul- CÌetaceous and Tert¡ary format¡ons. Sulfate minerals formed from pyrite fur minerals was accomplished by either scanning oxidat¡on are useful field indicators of ac¡d{enerat¡ng sed¡ments. Sulfate electron microscopy with energy-dispersive x-ray m¡nerals that have been identified in acidic sediments include rozenite, nicroanalysis or X-ray diffraction. X-ray diffrac- szomolnokite, ferrohexahydrite, copiapite, gypsum, and ¡arosite. Jarosite is tion analyses were performed by using a Phillips a highly persistent m¡neral and has often been observed in naturally weathered compensating slit and of diffractometer with a 2-theta soil profiles formed from sulfide-bearing sediments. The identification graphite crystal monochroneter. concentrâtions of ¡arosite in near-surface soil hor¡zons thus may serve as an indicatíon of under- x-ray lying sediments with ac¡dSenerating potential. sulfur and free iron were determinecl by the spectroscopic procedures of Snow l!Z) and Fanning and others (13), respèctivelY. The generation of excessive amounts of sulfuric acid often becomes a severe problen when excavation R¡SULTS AND DISCUSSION activities cause the exposure of suJ-fide-bearing rocks and sediments to the oxidizing environment of Properties of Sulfidic Strata the earthrs surface. One of the most conmon ex- amples of this phenomenon is thê well-known problen At least seven geologic formations in the Maryland of acid mine clrainage associated with coal minÍng coastal Plain vrere found to contain subsurface sul- excavations. Interception of sulfi<ie-bearing strata fide-bearing strata bhat, when exposeal to the at¡no- by earth-moving operations is, however' not a hazard sphere, were capable of proclucinq high amounts of unique only to coal or other mining activities. sulfuric acid. These sediments vrere found through- Nunerous reports (1-g) have described thè occur- out much of the western and central portions of the rences of sulfidic strata across a wide spectrum of Maryland coastal Plain. The general properties of geologic seetings. the sulfidic strata are given in Table l. Soil materiats that have undergone sulfide oxida- As is apparent from Table I' a connon property tiÕn and have excessivety low pH values are comnonly shared by each of the sedirnent types was dark color- referred to as acid sulfate soils or cat clays. rn ation. Dark colors for these materials probably the past' these terns have been used principally for result from the presence of organic conpoun<is asso- identifying acid-generating soils in tidal areas of ciatecl with reduced sulfidic strata as wefÌ as dark- the wortd. RecentÌy' investigâtors have also found ness of metallic sulfides (mostly pyríte) thern- it appropriate Èo appl-y these terms to upland Coast- selves. In applying Munsell soil color notation for aI PIain soils that disptay features derived from describing chroma and va1ue, sulfide-rich materials sulfide oxidation processes. With studies of acid generally have chro¡nas of I or less and values of 4 sulfate features in upland Coastal Plain soils (9- or less, 1I) has cone the recognition of the widespread na- Beyond color, hosrever, few other similarities ture of sulfides in many subsurface Coastal PIain existed for Èhe sulfide-rich strata. Textures strata. Because of the hazards these sedirnents pose ranged from loany sand to clay' and geologic ages to building materials and ecosystems v¡hen exposed to for the materials span frorn Lower Cretaceous through the atmosphere by excavation, identifícation of sul- Miocêne. In addition, it nust be emphasized that fidic stratâ is an irnportant first step in the the fornations listed in TabIe I are generally not 26 Transportation Research Record 892 Table 1, General character¡stiæ of Typical (moist) sulfidic strata in Maryland Coastal Color Plain formations. Common Formation Geologic Age Munsell Name Textural Classa Other Feature Potomac Group l-ower Cretaceous rOYR 2.s/l Black Silty clay ioam to clay Lienitic lOYR s/l Gray Magothy Upper Cretaceous 5Y 2.s11 Black Sandy loam Glauconitic Mat¿wan Upper Cretaceous 5Y 2.s 11 Black Sandy loam to loam Glauconitic Monmouth Upper Cretaceous sY 2.511 Black Fine sandy loam to sandy Glq"qq.{t. clãy-o'm Aquia Paleocene sY 4ll Dark gray Fine sandy loam to loamy Glauconitic sand Nanjemoy Eocene sY 411 Dtrk gray Fine sandy loam Glauconitic sY 3/1 Very dark $ay Calvert Miocene sGY 411 Dækgreen- Silt loam ish gray aU.S, Department of Agriculture classifícation. Table 2. Chemical propeñies and color w¡th depth in an oxidizing sulfidic ters of pyrite crystals known as framboiils. Pyrite sediment. franboids were iilentified by scanning electron microscopy in a sample of the Monmouth Fornation of Percentage of Upper Cretaceous age. Fra¡nboidal pyrite, perhaps Depth because of its smaller size ancl greater surface (cm) pH S FeDa Munsell Common Name area, is considered to be ¡nore reactive than rnega- scopic pyrite (14). Oxidized 0-6 2.7 o.22 1.72 lOYR 4/4 Dark yellowish ties of oxidized Strata brown 6-20 2.7 0.58 1.11 sY 312 Dark olive gray Properties so far discussed have been for unoxiilized 20-30 -t-/, 0.60 0.92 sY 312 Dark olive gray strata only, and sulfidic strata exposed to oxidiz- Mostly Unoxidized ing conditions undergo considerable morphological ancl chemical alterations. This is true for both 30-35 4,1 1.60 0.91 sY 2.s 11 Black naturally weathered sediments as vre1l as those that 35-46 7.7 1.59 0.87 sY 2.5ll Black have undergone artificially inducetl oxidation due to 46-56 8.1 t.47 0.ss sY 2.sll Black excåvâtion or drainage. s6-66 8.3 |.54 0.s8 5Y 2.s ll Black In terms of general appearance, oxidized sedí- 66-82 8.3 r.5 8 o.4s 5Y 2.s 11 Black 82-97 8.4 t.63 0.27 sY 2.511 Black nents are distinguished from originally sulfidic aDithionite materials by usually more re¿ldish hues, lighter extractable Fe, color values, higher chromas, and mottling or stain- ing by iron oxides and sulfates. Chemically, dif- ferences âre dependent on the original chemical com- characterized by high concentrations of sulfides position of the sulfide-rich sedíment, especially throughout. In nany instances, sulfide-bearing sulfur content, and on the degree to which oxidation strata nay be thin and lateralLy discontinuous. Such has progressed. Although other factors such as sul- characteristics are particularly t'rue for sulfide- fide for¡n or natural acid neutralizing capacity of rich beds within the Potomac croup of sediments. the sedÍmênt can be important., it is generally Èrue Glauconitic sedínents were found to be the nost uni- that the higher the original concentration of sul- fornly sulfidic in unoxidized zones. fides in the sêdiment, the greaÈer hrill be the Measurement of pH of fresh sulfidic sarnples is of amount of free acial generated. As long as con¿li- little use in ídentifying materials with harmful tions are sufficiently aerobic, sulfuric acid gen- acid-generating potentials. Freshly obtained, un- eration will continue until the supply of oxidizable oxi¿lized samples of sulfidic sedinents vrere found to sulfides is exhaustetl. have pH values thaÈ ranged frôn 5 to more than 8. Table 2 gives the characteristics of a soil pro- I,fhen oxiilized, the same sediments had pH valuês of file in rrhich both oxidized and unoxídized materíals Iess than 3.