Acid Mine Drainage

Mod­i­fied and updated by Chris­ti­an Wolk­ersdor­fer based on a text pub­lished by the Office of Sur­face Min­ing Reclam­a­tion and Enforce­ment (OSMRE).

Acid mine drain­age (AMD) has been a det­ri­ment­al by-product of coal min­ing for many years. Cur­rently, acid mine drain­age con­tin­ues to pose a poten­tial prob­lem in some areas, des­pite improved pre­dic­tion and pre­ven­tion tech­niques.

Acid Mine Drainage Research

Acid mine drain­age (AMD) has been the sub­ject of intens­ive research since the 1960s. Research efforts have pro­ceeded recently through the Mine Drain­age Tech­no­logy Ini­ti­at­ive (MDTI), the Inter­na­tion­al Net­work of Acid Pre­ven­tion (INAP), the Mine Envir­on­ment Neut­ral Drain­age (MEND) pro­gram and the Inter­na­tion­al Mine Water Asso­ci­ation (IMWA). The major areas of research on AMD are Pre­dic­tion of AMD and Prevention/Mitigation.

The fol­low­ing is an over­view of inform­a­tion on some major acid mine drain­age top­ics.

Factors controlling the formation of AMD

The form­a­tion of acid drain­age is a com­plex geo­chem­ic­al and micro­bi­ally medi­ated pro­cess. The acid load ulti­mately gen­er­ated from a mine site is primar­ily a func­tion of the fol­low­ing factors:

Pyrite Weathering

Chemistry of Pyrite Weathering

A com­plex series of chem­ic­al weath­er­ing reac­tions are spon­tan­eously ini­ti­ated when sur­face min­ing activ­it­ies expose spoil mater­i­als to an oxid­iz­ing envir­on­ment (Deutsch, 1997). The min­er­al assemblages con­tained in the spoil are not in equi­lib­ri­um with the oxid­iz­ing envir­on­ment and almost imme­di­ately begin weath­er­ing and min­er­al trans­form­a­tions. The reac­tions are ana­log­ous to “geo­lo­gic weath­er­ing” which takes place over exten­ded peri­ods of time (i.e., hun­dreds to thou­sands of years) but the rates of reac­tion are orders of mag­nitude great­er than in “nat­ur­al” weath­er­ing sys­tems. The accel­er­ated reac­tion rates can release dam­aging quant­it­ies of acid­ity, metals, and oth­er sol­uble com­pon­ents into the envir­on­ment. The pyr­ite oxid­a­tion pro­cess has been extens­ively stud­ied and has been reviewed by Nord­strom (1979). For pur­poses of this descrip­tion, the term “pyr­ite” is used to col­lect­ively refer to all iron disulf­ide min­er­als.‌‌‌‌‌‌‌

The fol­low­ing equa­tions show the gen­er­ally accep­ted sequence of pyr­ite reac­tions:

2 FeS₂ + 7 O₂ + 2 H₂O → 2 Fe²⁺ + 4 SO₄^2- + 4 H⁺

4 Fe²⁺ + O₂ + 4 H⁺ → 4 Fe³⁺ + 2 H₂O

4 Fe³⁺ + 12 H₂O → 4 Fe(OH)₃ + 12 H⁺

FeS₂ + 14 Fe³⁺ + 8 H₂O → 15 Fe²⁺ +2 SO₄^2- + 16 H⁺

In the ini­tial step, pyr­ite reacts with oxy­gen and water to pro­duce fer­rous iron, sulfate and acid­ity. The second step involves the con­ver­sion of fer­rous iron to fer­ric iron. This second reac­tion has been termed the “rate-determ­in­ing” step for the over­all sequence.

The third step involves the hydro­lys­is of fer­ric iron with water to form the sol­id fer­ric hydrox­ide (fer­ri­hydrite) and the release of addi­tion­al acid­ity. This third reac­tion is pH-depend­ent. Under very acid con­di­tions of less than about pH 3.5, the sol­id min­er­al does not form and fer­ric iron remains in solu­tion. At high­er pH val­ues, a pre­cip­it­ate forms, com­monly referred to as “yel­low boy.”

The fourth step involves the oxid­a­tion of addi­tion­al pyr­ite by fer­ric iron. The fer­ric iron is gen­er­ated by the ini­tial oxid­a­tion reac­tions in steps one and two. This cyc­lic propaga­tion of acid gen­er­a­tion by iron takes place very rap­idly and con­tin­ues until the sup­ply of fer­ric iron or pyr­ite is exhausted. Oxy­gen is not required for the fourth reac­tion to occur.

The over­all pyr­ite reac­tion series is among the most acid-pro­du­cing of all weath­er­ing pro­cesses in nature.

Microbiological Controls

The pyr­ite weath­er­ing pro­cess is a series of chem­ic­al reac­tions but also has an import­ant micro­bi­o­lo­gic­al com­pon­ent. The con­ver­sion of fer­rous to fer­ric iron in the over­all pyr­ite reac­tion sequence has been described as the “rate-determ­in­ing step” (Sing­er and Stumm, 1970). This con­ver­sion can be greatly accel­er­ated by a spe­cies of bac­teria, Acidi­th­ioba­cil­lus ferrox­id­ans. This bac­teria and sev­er­al oth­er spe­cies thought to be involved in pyr­ite weath­er­ing are wide­spread in the envir­on­ment. A. ferrox­id­ans has been shown to increase the iron con­ver­sion reac­tion rate by a factor of hun­dreds to as much as one mil­lion times (Sing­er and Stumm, 1970; Nord­strom, 1979).

The activ­ity of these bac­teria is pH-depend­ent on optim­al con­di­tions in the range of pH 2 to 3. Thus, once pyr­ite oxid­a­tion and acid pro­duc­tion have begun, con­di­tions are favour­able for bac­teria to fur­ther accel­er­ate the reac­tion rate. At pH val­ues of about 6 and above, bac­teri­al activ­ity is thought to be not rel­ev­ant or com­par­able to abi­ot­ic reac­tion rates. The cata­lyz­ing effect of the bac­teria effect­ively removes con­straints on pyr­ite weath­er­ing and allows the reac­tions to pro­ceed rap­idly. The role of microbes in pyr­ite oxid­a­tion is described in more detail by Klein­mann et al. (1981) and Nord­strom (1979).

Depositional Environment

Paleoen­vir­on­ments under which coal-bear­ing rocks formed can be char­ac­ter­ized into three gen­er­al cat­egor­ies: mar­ine; fresh­wa­ter; and brack­ish. Stud­ies of Pennsylvani­an-age coal-bear­ing rocks have shown that the paleoen­vir­on­ment can be used to broadly define acid drain­age poten­tial (Skousen et al., 1998, Brady et al. 1988; Horn­ber­ger et al. 1981). Rocks formed in brack­ish water con­di­tions are gen­er­ally most prone to acid pro­duc­tion; fresh­wa­ter sys­tems usu­ally pro­duce non-acid water, and mar­ine sys­tems pro­duce vari­able drain­age qual­ity. In some coal meas­ures, the paleoen­vir­on­ment var­ies lat­er­ally and ver­tic­ally with­in a single mine site and is a con­trolling factor in the inher­ent dis­tri­bu­tion of pyr­ite and car­bon­ates.

Acid/Base Balance and Reaction Rates

Drain­age and spoil qual­ity are a product of two com­pet­ing pro­cesses: acid form­a­tion from pyr­ite oxid­a­tion, and gen­er­a­tion of alka­lin­ity from the dis­sol­u­tion of car­bon­ates and oth­er basic min­er­als.

The acid gen­er­a­tion pro­cess con­sists of three phases: ini­ti­ation; propaga­tion; and ter­min­a­tion. The ini­ti­ation phase can begin as soon as pyr­it­ic mater­i­als are exposed to an oxid­iz­ing envir­on­ment, how­ever, the acid load gen­er­ated is rel­at­ively small. In the propaga­tion phase, acid pro­duc­tion increases rap­idly. In the ter­min­a­tion phase, acid pro­duc­tion gradu­ally declines. The actu­al times asso­ci­ated with these phases are, at present, ill-defined, but appear to be on the order of years to dec­ades. Mod­el­ling pre­dic­tions and com­par­is­on to a lim­ited num­ber of field sites indic­ate the peak acid load occurs 5 to 10 years after min­ing, fol­lowed by a gradu­al decline over 20 to 40 years (Ziemkiewicz et al., 1991, Hart et al., 1991). The same stud­ies pro­ject very long decay curves for coal refuse (bey­ond 50 years) before acid leachate is depleted. Reli­able acid generation/depletion pre­dic­tions for under­ground mine dis­charges are not avail­able.

The over­all acid-pro­du­cing pro­cess can pro­ceed very rap­idly with few chem­ic­al con­straints. In con­trast, dis­sol­u­tion or reac­tion rates of many com­mon min­er­als are gen­er­ally slow due to sol­u­bil­ity lim­it­a­tions. Pro­duc­tion of alka­lin­ity tends to attain a con­stant value or level off with time so that the rate of acid pro­duc­tion com­monly may exceed the pro­duc­tion of alka­lin­ity.

The trends in reac­tion rates can be off­set or enhanced by the mass bal­ance between acid and alkaline-pro­du­cing min­er­als. A gen­er­al rela­tion between acid and basic min­er­als and res­ult­ant drain­age qual­ity is described as fol­lows:

• Low pyr­ite, low base con­tent – Drain­age may con­tain low levels of acid­ity, or maybe non-acid. Low con­cen­tra­tions of dis­solved metals.
• Low pyr­ite, high base con­tent – Drain­age is alkaline with low con­cen­tra­tions of dis­solved metals.
• High pyr­ite, low base con­tent – Drain­age is acid with high con­cen­tra­tions of dis­solved metals.
• High pyr­ite, high base con­tent – Drain­age is usu­ally alkaline, occa­sion­ally acid, with high con­cen­tra­tions of dis­solved metals.

The con­di­tions most con­du­cive to acid form­a­tion are high pyr­ite con­tents with little base mater­i­al present. Con­versely, an excess of base rel­at­ive to pyr­ite is most likely to pre­clude acid form­a­tion. Sites con­tain­ing low quant­it­ies of pyr­ite and bases pro­duce the most vari­able drain­age qual­ity and are the most dif­fi­cult to assess with pre-min­ing pre­dict­ive tech­niques.

Lithologic Controls

Litho­logy or rock type also influ­ences spoil and drain­age qual­ity. Phys­ic­al char­ac­ter­ist­ics of the rock, such as poros­ity, and access­ory min­er­als can exert vari­ous con­straints or enhance­ments to the over­all chem­ic­al weath­er­ing pro­cess. For example, pyr­it­ic sand­stones tend to release their acid load rap­idly (Ziemkiewicz, 1991). Argil­la­ceous rocks tend to release their acid load over a longer peri­od of time. Access­ory min­er­als (clays and oth­er silic­ates) may dis­solve, form new min­er­als, or atten­u­ate the acid and alkaline weath­er­ing products.

Mineralogical Controls

The min­er­al pyr­ite occurs in sev­er­al dif­fer­ent mor­pho­lo­gic­al forms and a range of grain sizes. The “framboid­al” form is con­sidered highly react­ive and is char­ac­ter­ized by a small grain size and large sur­face area (Caruc­cio et al., 1977). Pyr­ite can occur in grain sizes ran­ging from invis­ible to the eye up to sev­er­al inches. Framboids and oth­er fine-grained pyr­ites with a large sur­face area are much more chem­ic­ally react­ive than the coars­er forms (Evan­gelou, 1995). The react­iv­ity of fine-grained pyr­ites reflects the fact that acid-gen­er­at­ing reac­tions occur at the min­er­al sur­face.

Mine site Hydrologic Conditions

Mine site hydro­logy plays a crit­ic­al role in determ­in­ing drain­age qual­ity, yet the flow mech­an­ics of ground­wa­ter in spoils are among the least under­stood aspects of AMD. The products of pyr­ite oxid­a­tion are free acid and sol­uble acid salts. If no per­col­at­ing water is present, the acid salts gen­er­ated from the lim­ited avail­able mois­ture simply reside with­in the spoil. When excess mois­ture is present, the acid weath­er­ing products are dis­solved and trans­por­ted with the water mov­ing through the mater­i­al.

The chem­istry of ground-water dis­charges can vary depend­ing on the degree of flush­ing (Snyder and Caruc­cio, 1988) and the time since the last pre­cip­it­a­tion event. Ground-water dis­charge can be “flashy” or rap­id shal­low inter­flow asso­ci­ated with high-intens­ity short-dur­a­tion pre­cip­it­a­tion events or base flow. Under­ground mine dis­charges that drain from large-volume pool stor­age typ­ic­ally exhib­it a muted or sea­son­al response to pre­cip­it­a­tion pat­terns.

The pos­i­tion of a water table with­in the spoil also influ­ences drain­age qual­ity. Water table elev­a­tions in spoils fluc­tu­ate in response to sea­son­al con­di­tions form­ing a zone of cyc­lic wet­ting and dry­ing. This provides optim­al con­di­tions for the oxid­a­tion and sub­sequent leach­ing of pyr­ite and asso­ci­ated weath­er­ing products. Ground-water flow paths and the loc­a­tion and elev­a­tion of sat­ur­ated zones are often dif­fi­cult to pre­dict in mine spoils.

Numer­ous chem­ic­al, phys­ic­al and bio­lo­gic­al factors inter­act to con­trol the qual­ity of mine drain­age. Although the basic pro­cesses of acid mine drain­age form­a­tion are uni­ver­sal, the import­ance of any single con­trolling factor is fre­quently spe­cif­ic to mine site con­di­tions.‌‌

Prediction of AMD‌

Pre­dic­tion of acid gen­er­a­tion based on geo­chem­ic­al ana­lys­is has been prac­tised for about 25 years. The most widely used meth­od, Acid/Base Account­ing (Sobek et al., 1978), quant­it­at­ively bal­ances pyr­ite against car­bon­ates and oth­er alkaline mater­i­als. Its ori­gin­al use was to identi­fy top­soil sub­sti­tutes and root zone media, not a quant­it­at­ive pre­dict­or of drain­age qual­ity. As a water qual­ity pre­dict­or, it has been accur­ate in some instances and mis­lead­ing in oth­ers (Erick­son and Heiden, 1988). Research has, there­fore, con­tin­ued on improv­ing pre­dict­ive meth­ods.

A vari­ety of sim­u­lated weath­er­ing tests have been developed and stud­ied as drain­age qual­ity pre­dict­ors (Caruc­cio, 1967; Sturey et al., 1982; Renton et al., 1988) Test details dif­fer, but all meth­ods attempt to mim­ic cyc­lic wetting/drying and flush­ing of spoil piles. Cur­rently, there is no con­sensus on which meth­od most accur­ately reflects field con­di­tions. Ques­tions have also aris­en regard­ing the length of labor­at­ory test time and extra­pol­a­tion to field weath­er­ing time.

It has been observed that pyr­ites of dif­fer­ent ori­gins can exhib­it vary­ing levels of react­iv­ity. Labor­at­ory stud­ies have been con­duc­ted (Ham­mack et al., 1988) to determ­ine why cer­tain pyr­ites are more chem­ic­ally react­ive. “Evolved gas ana­lys­is,” which involves the thermal decom­pos­i­tion of sul­fur com­pounds under con­trolled con­di­tions, has been used to char­ac­ter­ize pyr­ite react­iv­ity. The basic premise is that lower tem­per­at­ure decom­pos­i­tion reflects unstable and more react­ive pyr­ite. Cur­rent research efforts (Sheetz, 1990) are focused on X‑ray dif­frac­tion stud­ies of subtle dif­fer­ences in crys­tal struc­ture and pos­sible trace inclu­sions in the crys­tal lat­tice, in com­bin­a­tion with evolved gas ana­lys­is. The goal of these research efforts is to identi­fy the con­trolling factors and devel­op a repro­du­cible test that dis­crim­in­ates react­ive and non­re­act­ive pyr­ites.

Com­puter mod­els are anoth­er approach to the pre­dic­tion of acid gen­er­a­tion. Most of these mod­els incor­por­ate a num­ber of chem­ic­al and phys­ic­al para­met­ers to describe the chem­ic­al reac­tions of acid gen­er­a­tion, micro­bi­al cata­lys­is and leach­ing (trans­port) of the weath­er­ing products (Jaynes, 1991; Scharer et al., 1991). Many of these para­met­ers are dif­fi­cult to meas­ure or must be estim­ated and veri­fic­a­tion is gen­er­ally lack­ing. One mod­el uses a “lumped vari­able” approach, rather than a large num­ber of indi­vidu­al para­met­ers (Rymer et al., 1990; Hart et al., 1991). One com­bined vari­able estim­ates acid gen­er­a­tion, and a second vari­able accounts for the leach­ing of weath­er­ing products. This mod­el is still under­go­ing test­ing and veri­fic­a­tion.

Prevention/Mitigation of AMD

Research on acid pre­ven­tion and mit­ig­a­tion has focused on three main areas: chem­ic­al inhib­i­tion of the acid-gen­er­at­ing reac­tions; inhib­i­tion of the microbes respons­ible for cata­lyz­ing the acid-gen­er­at­ing reac­tions; and phys­ic­al or geo­tech­nic­al treat­ments to min­im­ize water con­tact and leach­ing.

Chemical Methods

Alkaline Addition

Alkaline place­ment strategies involve either mix­ing dir­ectly with pyr­it­ic mater­i­al or con­cen­trated place­ment to cre­ate a highly alkaline envir­on­ment. Dir­ect mix­ing places alkaline mater­i­als in intim­ate con­tact with pyr­it­ic spoil to inhib­it acid form­a­tion and neut­ral­ize any gen­er­ated acid­ity in situ. Alkaline addi­tion case stud­ies have been repor­ted by Brady et al., 1990. “Alkaline recharge” employs trenches loaded with alkaline mater­i­al, usu­ally a com­bin­a­tion of sol­uble sodi­um car­bon­ate and crushed lime­stone.

The strategy is to charge infilt­rat­ing waters with high doses of alka­lin­ity suf­fi­cient to over­whelm any acid pro­duced with­in the back­fill. This approach is highly depend­ent on the place­ment of the alkaline trenches to provide max­im­um inflow to the acid-pro­du­cing zones. An alkaline recharge case study has been repor­ted by Caruc­cio and Geidel (1989). A third vari­ant of the alkaline place­ment tech­nique is encap­su­la­tion with alkaline mater­i­al above and below the acid-pro­du­cing zone.‌

Alkaline Agents‌

The bene­fits of adding lime (cal­ci­um car­bon­ate) and oth­er alkaline agents have long been recog­nized in mit­ig­at­ing acid drain­age. How­ever, the com­plex chem­istry of spoil mater­i­als has giv­en vary­ing levels of effect­ive­ness in alkaline addi­tion stud­ies.

Dir­ect mix­ing and con­tact with pyr­it­ic mater­i­als appear most effect­ive but an optim­um lime-to-pyr­ite ratio remains unknown.

Indir­ect treat­ments such as alkaline recharge (Caruc­cio and Geidel, 1989) and bore­hole injec­tion (Aljoe and Hawkins, 1991; Lad­wig et al., 1985) have also yiel­ded mixed res­ults. Field stud­ies of alkaline addi­tion (Brady et al., 1990) have been con­duc­ted but it has been dif­fi­cult to identi­fy defin­it­ive cause-and-effect rela­tion­ships. Fur­ther research is con­tinu­ing in this area.

Phosphate

The applic­a­tion of rock phos­phate is anoth­er tech­nique under study as a pyr­ite oxid­a­tion inhib­it­or (Renton et al., 1988; Evan­gelou et al., 1991). Pyr­ite weath­er­ing ulti­mately pro­duces free fer­ric iron which acts to oxid­ize addi­tion­al pyr­ite, thus estab­lish­ing a cyc­lic and self-propagat­ing series of reac­tions. Dis­sol­u­tion of rock phos­phate in acid media releases highly react­ive phos­phate ions, which will com­bine with iron to form insol­uble iron phos­phate com­pounds. The form­a­tion of insol­uble iron phos­phates would halt or inhib­it the cyc­lic reac­tion of iron and pyr­ite. Phos­phate treat­ment has effect­ively reduced acid gen­er­a­tion in labor­at­ory stud­ies; one field study showed a reduc­tion of about sev­enty per cent in acid load com­pared to a con­trol (Meek, 1991).

For reas­ons not yet com­pletely under­stood, an applic­a­tion rate of about two to three per cent rock phos­phate provides the most effect­ive con­trol. Thor­ough mix­ing of phos­phate and pyr­it­ic mater­i­al also appears neces­sary for effect­ive treat­ment. Fur­ther research is con­tinu­ing in this area.

Coatings and Sealants

Oth­er ongo­ing research activ­it­ies focus on the sur­face chem­istry of pyr­ite and the devel­op­ment of vari­ous types of seal­ers, coat­ings and inhib­it­ors to halt acid pro­duc­tion.

Biological Agents/Bactericides

The cata­lyt­ic role of bac­teria in pyr­ite oxid­a­tion has been well doc­u­mented (Klein­mann et al., 1981). Many com­pounds have been screened as select­ive bac­ter­icides and the anion­ic sur­fact­ants sodi­um lauryl sulfate and alkyl ben­zene sulf­on­ate are con­sidered to be the most reli­able inhib­it­ors. The applic­a­tion of bac­ter­icides has reduced acid load­ing in field exper­i­ments. Bac­ter­icides are gen­er­ally water sol­uble and will leach from the spoil. Cur­rently, the time required for the leach­ing of bac­ter­icides is uncer­tain. It is also unclear wheth­er the sul­fur and iron oxid­iz­ing bac­teria will repop­u­late the spoil and cata­lyze the acid-pro­du­cing reac­tions when the bac­ter­icide is depleted.

Physical or Geochemical Treatments

Controlled Placement‌

Con­trolled place­ment (spe­cial hand­ling) is a pre­vent­at­ive meas­ure involving the place­ment of pyr­it­ic or alkaline mater­i­al dur­ing min­ing to min­im­ize or neut­ral­ize the form­a­tion of AMD. Accord­ing to the gen­er­ally accep­ted chem­ic­al equa­tions for pyr­ite oxid­a­tion, oxy­gen and water are neces­sary to ini­ti­ate acid form­a­tion. Exclu­sion of either react­ant should pre­clude or inhib­it acid pro­duc­tion. Place­ment of pyr­it­ic mater­i­al encom­passes either an attempt to exclude oxy­gen, usu­ally by com­plete sub­mer­gence below the water table; or an attempt to isol­ate the mater­i­al from water con­tact to avoid leach­ing of acid salts. Place­ment of alkaline mater­i­als has a two­fold role:

1. inhib­i­tion of the acid-form­ing reac­tions by main­tain­ing neut­ral to alkaline pH; and
2. neut­ral­iz­a­tion of any acid formed.

Submergence

Sub­mer­gence relies on sev­er­al physico-chem­ic­al phe­nom­ena for suc­cess. Oxy­gen dif­fuses very slowly and has lim­ited sol­u­bil­ity in water. For this approach to suc­ceed, a stag­nant or no flow con­di­tion and a rel­at­ively thick sat­ur­ated zone appear crit­ic­al. Stag­nant flow con­di­tions lead­ing to the devel­op­ment of anox­ic (oxy­gen-free) con­di­tions and a sat­ur­ated thick­ness on the order of sev­er­al tens of feet appear to effect­ively cur­tail oxy­gen dif­fu­sion. This approach is most suc­cess­ful in large mines in flat ter­rain where ground-water gradi­ents are low, the sat­ur­ated zone is thick, and aquifers are of large areal extent. Ham­mack and Watzlaf (1990) con­cluded that a water cov­er to main­tain oxy­gen below a par­tial pres­sure of one per cent is neces­sary to inhib­it pyr­ite oxid­a­tion.

Sub­mer­gence is gen­er­ally not used in the hilly ter­rain of Appalachia, where gradi­ents and flow velo­cit­ies are too great to achieve stag­nant, anox­ic con­di­tions. In these situ­ations, sub­mer­gence may be coun­ter­pro­duct­ive and actu­ally enhance the pro­duc­tion and leach­ing of acid products. Sub­mer­gence or flood­ing is also applied to pre­vent AMD from under­ground mines. Key con­sid­er­a­tions include:

• Wheth­er the mine is loc­ated above or below drain­age.
• The abil­ity of mine seals and out­crop bar­ri­ers to pre­vent seep­age.
• Poten­tial for mine seals and out­crop bar­ri­ers to fail under hydraul­ic pres­sure.

In gen­er­al, flood­ing to pre­vent AMD is believed to be more suc­cess­ful in below-drain­age mines. It is assumed that com­plete flood­ing elim­in­ates oxy­gen and halts or severely cur­tails acid gen­er­a­tion, the mine pool is stable and little or no dis­charge occurs (Kim et al., 1982).

Flood­ing of above drain­age mines is also prac­tised typ­ic­ally through the use of “wet” seals, which allow water to drain but exclude air entry. Kim et al., 1982 con­cluded that seal­ing and flood­ing above drain­age mines does reduce acid load­ing but is tech­nic­ally more dif­fi­cult and less effect­ive than oth­er meth­ods in AMD pre­ven­tion. Mon­it­or­ing stud­ies of sealed mines indic­ate a gen­er­al decrease in pol­lut­ant load­ing 10 to 25 years after min­ing (Borek et al., 1991), but it is unclear if the decreases were due to mine seal­ing or “nat­ur­al phe­nom­ena”. While pol­lut­ant load­ing decreased, and water qual­ity remained well out­side accep­ted water qual­ity stand­ards for mine drain­age.

OSM’s rules do not spe­cific­ally address cri­ter­ia for out­crop bar­ri­er thick­ness for flood­ing under­ground works. A con­sensus “stand­ard” engin­eer­ing design approach to out­crop bar­ri­ers and seals is also lack­ing. A con­tract report to the U.S. Bur­eau of Mines (Dames and Moore, 1981) dis­cusses the factors affect­ing the sta­bil­ity of out­crop bar­ri­ers. Out­crop bar­ri­ers should be wide enough to pre­vent seep­age and have suf­fi­cient over­bur­den to pre­vent fail­ure (blo­wout). Cur­tain grout­ing, relief wells and com­part­ment­al­ized bar­ri­ers are sev­er­al of the tech­niques sug­ges­ted for con­trolling AMD dis­charges.

Isolation Above the Water Table‌

Place­ment of pyr­it­ic mater­i­al above a water table is an attempt to isol­ate the mater­i­al from con­tact with water and pre­clude the leach­ing of acid-weath­er­ing products. Com­pac­tion and cap­ping with clay or oth­er mater­i­als may also be employed to reduce per­meab­il­ity. In prac­tice, it has proven very dif­fi­cult to com­pletely isol­ate spoil mater­i­als from water con­tact. Clay caps and oth­er flow bar­ri­ers are prone to leak­age, and the sporad­ic infilt­ra­tion of rain or snow­melt may peri­od­ic­ally leach the spoil. The cap­ping approach can be exten­ded to com­plete encap­su­la­tion on the top, bot­tom and sides as a fur­ther effort to isol­ate the mater­i­als from water con­tact. Skousen et al., 1987 give a gen­er­al review of isol­a­tion and cap­ping and oth­er pre­vent­ive tech­niques for hand­ling pyr­it­ic spoil.

Encapsulation/Physical Barriers

Tech­niques to isol­ate or encap­su­late pyr­it­ic mater­i­al include the use of fly ash, cement, benton­ite, and oth­er clays; these are a few of the mater­i­als stud­ied as seal­ants and flow bar­ri­ers by Skousen et al. (1987) and Bowders and Chi­ado (1990). Suc­cess­ful applic­a­tion of these meth­ods in the field is heav­ily depend­ent on good engin­eer­ing and con­struc­tion prac­tices and site con­di­tions. Oth­er invest­ig­a­tions have attemp­ted bore­hole injec­tion to isol­ate bur­ied pyr­it­ic mater­i­al. Research is ongo­ing in this area and may escal­ate as sol­id waste dis­pos­al rules become more strin­gent.

Water Management

Water man­age­ment strategies both dur­ing and after min­ing are anoth­er option for redu­cing acid gen­er­a­tion. Water man­age­ment can include the fol­low­ing:

• Act­ive min­ing oper­a­tions can incor­por­ate diver­sions to route sur­face drain­age away from pyr­it­ic mater­i­al or through alkaline mater­i­al.
• Spoil mater­i­al can be placed and rough-graded to pre­vent pond­ing and sub­sequent infilt­ra­tion.
• Prompt remov­al of pit water can lessen the amount and sever­ity of acid gen­er­ated.
• Pol­luted pit water can be isol­ated from non-con­tam­in­ated sources (no com­ming­ling) to reduce the quant­ity of water requir­ing treat­ment.
• Con­struc­ted under­drain sys­tems can be used to route water away from con­tact with acid-form­ing mater­i­al.

Spe­cial hand­ling (con­trolled place­ment), alkaline place­ment and water man­age­ment strategies alone or in com­bin­a­tion can sub­stan­tially reduce or mit­ig­ate the gen­er­a­tion of acid drain­age. Optim­al strategies are site-spe­cif­ic and a func­tion of geo­logy, topo­graphy, hydro­logy, min­ing meth­od and cost-effect­ive­ness.

Mine-spoil Hydrology‌

Although some­times not con­sidered an AMD research top­ic, mine spoil hydro­logy plays a cru­cial role in determ­in­ing drain­age qual­ity. Rel­at­ively few stud­ies of hydro­geo­lo­gic pro­cesses have been con­duc­ted in the con­text of con­trolling mine drain­age qual­ity, and it is a sub­ject in need of fur­ther invest­ig­a­tion. Much use­ful research has been con­duc­ted in pre­dict­ive and pre­vent­at­ive acid drain­age tech­niques. No uni­ver­sally effect­ive tech­no­lo­gies have yet been developed, how­ever.

Treatment Methods and Costs

The least costly and most effect­ive meth­od of con­trolling Acid Mine Drain­age is to pre­vent its ini­tial form­a­tion. This usu­ally can be accom­plished by applic­a­tion of the prin­ciples and pro­ced­ures described in the pre­vi­ous sec­tion and care­ful mine plan­ning.

On occa­sion, des­pite the applic­a­tion of sound min­ing and reclam­a­tion prin­ciples, Acid Mine Drain­age will be formed and must be treated to meet exist­ing Fed­er­al and State stand­ards before it is released from the mine site. Pri­or to open­ing the mine, the oper­at­or should eval­u­ate the poten­tial for cre­at­ing Acid Mine Drain­age that would require treat­ment and should become famil­i­ar with the extent of the costs that such treat­ment might impose. Con­sid­er­a­tion should also be giv­en to the pos­sib­il­ity that treat­ment might have to be con­tin­ued well into the future, spe­cific­ally until such time that the dis­charge meets efflu­ent stand­ards without treat­ment.

Treat­ment, as nor­mally applied to Acid Mine Drain­age, involves chem­ic­al neut­ral­iz­a­tion of the acid­ity fol­lowed by pre­cip­it­a­tion of iron and oth­er sus­pen­ded solids. Treat­ment sys­tems include:

1. equip­ment for feed­ing the neut­ral­iz­ing agent to the Acid Mine Drain­age
2. means for mix­ing the two streams (Acid Mine Drain­age and neut­ral­iz­ing agent)
3. pro­ced­ures for ensur­ing iron oxid­a­tion
4. set­tling ponds for remov­ing iron, man­ganese, and oth­er co-pre­cip­it­ates

A num­ber of factors dic­tate the level of soph­ist­ic­a­tion of the treat­ment sys­tem that is neces­sary to ensure that efflu­ent stand­ards will be met. These factors include the chem­ic­al char­ac­ter­ist­ics of the Acid Mine Drain­age, the quant­ity to be treated, cli­mate, ter­rain, sludge char­ac­ter­ist­ics, and the pro­jec­ted life of the plant. The chem­ic­als usu­ally used for Acid Mine Drain­age treat­ment include lime­stone, hydrated lime, soda ash, caustic soda, and ammo­nia. The fol­low­ing dis­cus­sion high­lights some of the char­ac­ter­ist­ics of each of these neut­ral­iz­ing agents.

Limestone (calcium carbonate)

The cal­ci­um con­tent of lime­stone should be as high as pos­sible (Dolo­mit­ic lime­stones are less react­ive and gen­er­ally inef­fect­ive in treat­ing Acid Mine Drain­age.) Advant­ages of using lime­stone include low cost, ease of use, and form­a­tion of a dense, eas­ily handled, sludge. Dis­ad­vant­ages include slow reac­tion time, loss in effi­ciency of the sys­tem because of coat­ing of the lime­stone particles with iron pre­cip­it­ates, dif­fi­culty in treat­ing Acid Mine Drain­age with a high fer­rous-fer­ric ratio, and inef­fect­ive­ness in remov­ing man­ganese. Lime­stone treat­ment is gen­er­ally not effect­ive for acid­it­ies exceed­ing 50 mg/L.

Hydrated Lime (calcium hydroxide)

Hydrated lime is nor­mally the neut­ral­iz­ing agent of choice by the coal min­ing industry because it is easy and safe to use, effect­ive, and rel­at­ively inex­pens­ive. The major dis­ad­vant­ages are the volu­min­ous sludge that is pro­duced (when com­pared to lime­stone) and the high ini­tial costs that are incurred because of the size of the treat­ment plant.

Soda Ash (sodium carbonate)

Soda ash bri­quettes are espe­cially effect­ive for treat­ing small Acid Mine Drain­age flows in remote areas. Major dis­ad­vant­ages are high­er reagent cost (rel­at­ive to lime­stone) and poor set­tling prop­er­ties of the sludge.

Caustic Soda (sodium hydroxide)‌

Caustic soda is espe­cially effect­ive for treat­ing low flows in remote loc­a­tions and for treat­ing Acid Mine Drain­age hav­ing a high man­ganese con­tent. Major dis­ad­vant­ages are its high cost, the dangers involved with hand­ling the chem­ic­al, poor sludge prop­er­ties, and freez­ing prob­lems in cold weath­er.

Ammonia

Anhyd­rous ammo­nia is effect­ive in treat­ing Acid Mine Drain­age hav­ing a high fer­rous iron and/or man­ganese con­tent. Ammo­nia costs less than caustic soda and has many of the same advant­ages. How­ever, ammo­nia is dif­fi­cult and dan­ger­ous to use and can affect bio­lo­gic­al con­di­tions down­stream from the min­ing oper­a­tion. The pos­sible off-site effects are tox­icity to fish and oth­er aquat­ic life forms, eutroph­ic­a­tion and nitri­fic­a­tion. Fish spe­cies gen­er­ally exhib­it low tol­er­ance to uni­on­ized ammo­nia and tox­icity levels can be affected by pH, tem­per­at­ure, dis­solved oxy­gen and oth­er factors. A more com­plete review of ammo­nia treat­ment of mine drain­age is giv­en by Faulkner (1991). Ammo­nia use is not allowed in all States and, where per­mit­ted, addi­tion­al mon­it­or­ing is required.

Constructed Wetlands

Con­struc­ted wet­lands util­ize soil- and water-borne microbes asso­ci­ated with wet­land plants to remove dis­solved metals from mine drain­age. Ini­tial design and con­struc­tion costs may be sub­stan­tial, ran­ging into tens of thou­sands of dol­lars. Unlike chem­ic­al treat­ment, how­ever, wet­lands are pass­ive sys­tems requir­ing little or no con­tinu­ing main­ten­ance. This is a rel­at­ively new treat­ment tech­no­logy with many spe­cif­ic mech­an­isms and main­ten­ance require­ments not yet fully under­stood. Optim­um siz­ing and con­fig­ur­a­tion cri­ter­ia are still under study. Sea­son­al vari­ations in metals remov­al effi­ciency have been noted with less­er amounts removed in cold weath­er. Wet­lands are gen­er­ally more effect­ive in remov­ing iron than man­ganese. The greatest util­ity of wet­lands appears to be in the treat­ment of small flows of a few gal­lons per minute.

Treatment Costs

For Acid Mine Drain­age treat­ment costs, OSM has developed AMDTreat, a com­puter pro­gram to cal­cu­late long-term costs for treat­ing mine drain­age, avail­able for down­load at: www.osmre.gov

Technical references on Acid Mine Drainage‌

Aljoe W.W. and J.W. Hawkins, 1991, “Hydro­lo­gic Char­ac­ter­iz­a­tion and In-Situ Neut­ral­iz­a­tion of Acid­ic Mine Pools in Aban­doned Under­ground Coal Mines,” in Pro­ceed­ings Second Inter­na­tion­al Con­fer­ence on the Abate­ment of Acid­ic Drain­age, Septem­ber 16–18, 1991, Montreal, Canada, Volume 1, p 69–90.

Borek S. L., T. E. Ack­man, G. P. Watzlaf, R. W. Ham­mack, J. P. Lipscomb, 1991, “The Long-Term Eval­u­ation of Mine Seals Con­struc­ted in Ran­dolph County, W.V. in 1967,” in Pro­ceed­ings Twelfth Annu­al West Vir­gin­ia Sur­face Mine Drain­age Task Force Sym­posi­um, April 3–4, 1991, Mor­gan­town, West Vir­gin­ia.

Bowders, J. and E. Chi­ado, 1990, ” Engin­eer­ing Eval­u­ation of Waste Phos­phat­ic Clay for Pro­du­cing Low Per­meab­il­ity Bar­ri­ers,” in Pro­ceed­ings 1990 Min­ing and Reclam­a­tion Con­fer­ence and Exhib­i­tion, Volume 1, p 11–18, West Vir­gin­ia Uni­ver­sity.

Brady, K. B., M. Smith, R. Beam and C. Cra­votta III, 1990, “Effect­ive­ness of Addi­tion of Alkaline Mater­i­als at Sur­face Coal Mines in Pre­vent­ing and Abat­ing Acid Mine Drain­age: Part 2 Mine Site Case Stud­ies,” in Pro­ceed­ings of the 1990 Min­ing and Reclam­a­tion Con­fer­ence and Exhib­i­tion, Volume 1, p 227–242, West Vir­gin­ia Uni­ver­sity.

Brady K.B., J.R. Shaul­is and V.W. Sekma, 1988, “A Study of Mine Drain­age Qual­ity and Pre­dic­tion Using Over­bur­den Ana­lys­is and Paleoen­vir­on­ment­al Recon­struc­tions, Fay­ette County, Pennsylvania,” in Con­fer­ence Pro­ceed­ings, Mine Drain­age and Sur­face Mine Reclam­a­tion, U.S. Bur­eau of Mines Inform­a­tion Cir­cu­lar 9183, p 33–44.

Caruc­cio F. T. and G. Gediel, 1989, “Water Man­age­ment Strategies in Abat­ing Acid Mine Drain­age – Is Water Diver­sion Really Bene­fi­cial?,” in Pro­ceed­ings 1989 Mul­tina­tion­al Con­fer­ence on Mine Plan­ning and Design, Uni­ver­sity of Ken­tucky, Lex­ing­ton, Ken­tucky.

Caruc­cio, F.T., J.C. Ferm, J. Horne, G. Geidel, B. Bagenz, 1977, “Paleoen­vir­on­ment of Coal and Its Rela­tion to Drain­age Qual­ity,” pre­pared for U.S. Envir­on­ment­al Pro­tec­tion Agency, EPA-600/7–77-067, Cin­cin­nati, Ohio.

Caruc­cio F. T., 1967, “An Eval­u­ation of Factors Influ­en­cing Acid Mine Drain­age Pro­duc­tion from Vari­ous Strata of the Allegheny Group and the Ground Water Inter­ac­tions in Selec­ted Areas of West­ern Pennsylvania,” Ph.D. Dis­ser­ta­tion, The Pennsylvania State Uni­ver­sity, State Col­lege, Pennsylvania.

Dames and Moore, 1981, “Out­crop Bar­ri­er Design Guidelines For Appalachi­an Coal Mines,” pre­pared for the U.S. Bur­eau of Mines, Con­tract J0395069, Bur­eau of Mines Open File Report 134–81.

Deutsch, W. J., Ground­wa­ter Geo­chem­istry: Fun­da­ment­als and Applic­a­tions to Con­tam­in­a­tion, 1997, CRC Press, N.Y., 221 p.

Erick­son P. and R. Heiden, 1988, “Eval­u­ation of Over­bur­den Ana­lyt­ic­al Meth­ods as a Means to Pre­dict Post-Min­ing Coal Mine Drain­age Qual­ity,” in Con­fer­ence Pro­ceed­ings Mine Drain­age and Sur­face Mine Reclam­a­tion, U.S. Bur­eau of Mines Inform­a­tion Cir­cu­lar 9183, Volume 1, p 11–20.

Evan­gelou, V., U. Sain­ju and E. Portig, 1991, “Some Con­sid­er­a­tions When Apply­ing Limestone/Rock Phos­phate Mater­i­als on to Acid Pyr­it­ic Spoils,” in Pro­ceed­ings Twelfth Annu­al West Vir­gin­ia Sur­face Mine Drain­age Task Force Sym­posi­um, April 3–4, 1991, Mor­gan­town, West Vir­gin­ia.

Evan­gelou, V.P., 1995. Pyr­ite Oxid­a­tion and its Con­trol. CRC Press, N.Y., 293 p.

Faulkner, B. (ed.), 1991, “Hand­book for Use of Ammo­nia in Treat­ing Mine Waters,” West Vir­gin­ia Min­ing and Reclam­a­tion Asso­ci­ation, Char­le­ston, West Vir­gin­ia.

Ham­mack R. W. and G. R. Watzlaf, 1990, “The Effect of Oxy­gen on Pyr­ite Oxid­a­tion,” in Pro­ceed­ings of the 1990 Min­ing and Reclam­a­tion Con­fer­ence and Exhib­i­tion, Volume 1, p 257–264, West Vir­gin­ia Uni­ver­sity.

Ham­mack, R. W., R. Lai and J. R. Diehl, 1988, “Meth­ods for Determ­in­ing Fun­da­ment­al Chem­ic­al Dif­fer­ences Between Iron Disulf­ides from Dif­fer­ent Geo­lo­gic Proven­ances,” in Con­fer­ence Pro­ceed­ings Mine Drain­age and Sur­face Mine Reclam­a­tion, U.S. Bur­eau of Mines Inform­a­tion Cir­cu­lar 9183, p 136–146.

Hart, W. M., K. Batarseh, G. P. Swaney and A. H. Stil­ler, 1991, “A Rig­or­ous Mod­el to Pre­dict the AMD Pro­duc­tion Rate of Mine Waste Rock,” in Pro­ceed­ings Second Inter­na­tion­al Con­fer­ence On The Abate­ment Of Acid­ic Drain­age, Septem­ber 16–18, 1991, Montreal, Canada, Volume 2, p 257–270.

Hart W.H., K.I. Batarseh, G.P. Swaney and A.H. Stil­ler, “A Rig­or­ous Mod­el to Pre­dict the AMD Pro­duc­tion of Mine Waste Rock,” in Pro­ceed­ings Second Inter­na­tion­al Con­fer­ence on the Abate­ment of Acid­ic Drain­age, Septem­ber 16–18, 1991, Montreal Canada, Volume 2, p 257–270.

Horn­ber­ger, R.J., R.R. Par­izek and E.G. Wil­li­ams, 1981, “Delin­eation of Acid Mine Drain­age Poten­tial of Coal Bear­ing Strata of the Pott­s­ville and Allegheny Groups in West­ern Pennsylvania,” Research Report OWRT Pro­ject B‑097-PA, the Pennsylvania State Uni­ver­sity, Uni­ver­sity Park Pennsylvania.

Jaynes, D. B., 1991, “Mod­el­ing Acid Mine Drain­age from Reclaimed Coal Strip Mines,” in Pro­ceed­ings Second Inter­na­tion­al Con­fer­ence on the Abate­ment of Acid­ic Drain­age, Septem­ber 16–18, 1991, Montreal, Canada, Volume 2, p 191–210.

Kim, A., B. Hei­sey, R. L. P. Klein­mann and M. Duel, 1982, “Acid Mine Drain­age: Con­trol and Abate­ment Research,” U.S. Bur­eau of Mines Inform­a­tion Cir­cu­lar 8905.

Klein­mann R.L.P., D.A. Crerar and R.R. Pacelli, 1981, “Biogeo­chem­istry of Acid Mine Drain­age and a Meth­od to Con­trol Acid Form­a­tion,” Min­ing Engin­eer­ing, March 1981.

Lad­wig, K., P. Erick­son and R. Klein­mann, 1985, Alkaline Injec­tion: An Over­view of Recent Work,” in Con­trol of Acid Mine Drain­age, Pro­ceed­ings of a Tech­no­logy Trans­fer Sem­in­ar, U.S. Bur­eau of Mines Inform­a­tion Cir­cu­lar 9027.

Meek A., 1991, “Assess­ment of Acid Pre­vent­at­ive Tech­niques at the Island Creek Min­ing Co. Ten­mile Site,” in Pro­ceed­ings Twelfth Annu­al West Vir­gin­ia Sur­face Mine Drain­age Task Force Sym­posi­um, April 3–4, 1991, Mor­gan­town, West Vir­gin­ia.

Nord­strom, D. K., 1979, “Aqueous Pyr­ite Oxid­a­tion and the Con­sequent Form­a­tion of Sec­ond­ary Iron Min­er­als,” in “Acid Sulfate Weath­er­ing,” Soil Sci­ence Soci­ety of Amer­ica Spe­cial Pub­lic­a­tion No. 10, p 37–56.

Renton, J., A. H. Stil­ler and T. E. Rymer, 1988, “The Use of Phos­phate Mater­i­als as Ameli­or­ants for Acid Mine Drain­age,” in Con­fer­ence Pro­ceed­ings Mine Drain­age and Sur­face Mine Reclam­a­tion, U.S. Bur­eau of Mines Inform­a­tion Cir­cu­lar 9183, p 67–75.

Renton, J. J., T. E. Rymer and A. H. Stil­ler, 1988, “A Labor­at­ory Pro­ced­ure to Eval­u­ate the Acid Pro­du­cing Poten­tial of Coal Asso­ci­ated Rocks,” Min­ing Sci­ence and Tech­no­logy, Vol 7, p 227–235, Elsevi­er, Ams­ter­dam.

Rymer, T., A. Stil­ler, W. Hart and J. Renton, 1990, “Some Aspects of SSPE/PSM Mod­el­ing for Quant­it­at­ive Assess­ment of Dis­turbed Hydro­lo­gic Sys­tems,” in Pro­ceed­ings 1990 Min­ing and Reclam­a­tion Con­fer­ence and Exhib­i­tion, Volume 1, p 61–68, West Vir­gin­ia Uni­ver­sity.

Scharer, J. M., V. Garga, R. Smith and B. E. Hal­bert, 1991, “Use of Steady State Mod­els for Assess­ing Acid Gen­er­a­tion in Pyr­it­ic Mine Tail­ings,” in Pro­ceed­ings Second Inter­na­tion­al Con­fer­ence on the Abate­ment of Acid­ic Drain­age, Septem­ber 16–18, 1991, Montreal, Canada, Volume 2, p 211–230.

Sheetz, B.E., 1990, “Improv­ing The Char­ac­ter­iz­a­tion Of Sul­fur In Over­bur­dens and Coals Using State-of-the-Art Tech­no­logy,” First Annu­al Pro­gress Report to the Office of Sur­face Min­ing, Grant 996421, Mater­i­als Research Labor­at­ory, the Pennsylvania State Uni­ver­sity, Uni­ver­sity Park, Pennsylvania.

Sing­er, P.C. and W. Stumm, 1970, “Acid Mine Drain­age: The Rate Determ­in­ing Step,” Sci­ence 167; p 1121–1123.

Skousen, J. G., K. Pol­it­an, T. Hilton and A. Meek, 1990, “Acid Mine Drain­age Treat­ment Sys­tems: Chem­ic­als and Costs”, in Green Lands, Vol 20, No.4, p 31–37, Fall 1990, West Vir­gin­ia Min­ing and Reclam­a­tion Asso­ci­ation, Char­le­ston West Vir­gin­ia.

Skousen, J. G., J. C. Sencin­diver and R. M. Smith, 1987, “A Review of pro­ced­ures For Sur­face Min­ing and Reclam­a­tion in Areas with Acid-pro­du­cing Mater­i­als,” in cooper­a­tion with The West Vir­gin­ia Sur­face Mine drain­age Task Force, the West Vir­gin­ia Uni­ver­sity Energy and Water Research Cen­ter and the West Vir­gin­ia Min­ing and Reclam­a­tion Asso­ci­ation, 39 p, West Vir­gin­ia Uni­ver­sity Energy and Water Research Cen­ter.

Snyder, D. T., and F. T. Caruc­cio, 1988, ” The Par­ti­tion­ing of Flow Com­pon­ents of Acid­ic Seeps from Sur­face Coal Mines and the Iden­ti­fic­a­tion of Acid Pro­du­cing Hori­zons with­in the Back­fill,” in Con­fer­ence Pro­ceed­ings Mine Drain­age and Sur­face Mine Reclam­a­tion, U.S. Bur­eau of Mines Inform­a­tion Cir­cu­lar 9183, p 59–66.

Sobek A. A., W. A. Schuller, J. R. Free­man and R. M. Smith, 1978, “Field and Labor­at­ory Meth­ods Applic­able to Over­bur­dens and Minesoils,” pre­pared for U.S. Envir­on­ment­al Pro­tec­tion Agency, EPA- 600/2–78-054, Cin­cin­nati, Ohio.

Sturey C. S., J. R. Free­man, T. A. Keeney, and J. Sturm, 1982, “Over­bur­den Ana­lys­is by Acid-Base Account­ing and Sim­u­lated Weath­er­ing Stud­ies as a Means of Determ­in­ing the Prob­able Hydro­lo­gic Con­sequences of Min­ing and Reclam­a­tion,” in Pro­ceed­ings Sym­posi­um on Sur­face Min­ing, Hydro­logy, Sed­i­mento­logy, and Reclam­a­tion, Uni­ver­sity of Ken­tucky, Lex­ing­ton, Ken­tucky.

Tetch­er J. J., T. T. Phipps and J. G. Skousen, “Cost Ana­lys­is For Treat­ing Acid Mine Drain­age from Coal Mines in the U.S.,” in Pro­ceed­ings Second Inter­na­tion­al Con­fer­ence on the Abate­ment of Acid­ic Drain­age, Septem­ber 16–18, 1991, Montreal, Canada, Volume 1, p 561–574.

Ziemkiewicz, P., J. Renton and T. Rymer, 1991, “Pre­dic­tion and Con­trol of Acid Mine Drain­age: Effect of Rock Type and Amend­ment,” in Pro­ceed­ings Twelfth Annu­al West Vir­gin­ia Sur­face Mine Drain­age Task Force Sym­posi­um, April 3–4, 1991, Mor­gan­town, West Vir­gin­ia.