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EXPLANATION OF SIGNIFICANT DIFFERENCES RECORD OF DECISION (ROD)-- WHITEV'^OOD CREEK 0 m^

DECLARATIONS

Considering the new information that has been developed and the changes that hsve been made to the selected remedy chosen in ths Harch 30, 1990 ROD, EPA has determined that the remedy remains protective of human health and the environrcentj complies with Federal and State requirements that are applicable or relevant and appropriate to this remedial action, except those for which a waiver is invoked, and is ccst-effective. In addition, the revised remedy utilizes permanent solutions and alternative treatment technolo-jies to the maximum extent practicable for this Site,

C -fc Jaine's j / . " Schemer Re4ij>nal Adminis t ra tor

Date

EXPLANATION OF SIGNIFICANT

Whitev/ood Creek Super fund Site

Lawrence^ Meade, and Butte Count ies, South Dakota

United States EnvironrTiental Pfotsction Agency, Region V̂ June IDQ-i

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The purpose of ihis documen! is lo explain the significant ciiflercnccs bciwcen the remedy chosen in tlie Record of Decision (ROD)-.Hipicii by lhc U.S. Environmcnia! ProTccEion Agenc)' (EPA) on March 30,1990, and l,hc rcmcdywhich wiJi he implcmCDtcti at ihe Whil€wood Creek Syperfursd Siie (SUc'; iocaicii in west ccnirai Soulh Dakoia. ^Te^m^ appearing in italics are dciined ir. ihc glossary-) EPA is the iead agencv' at ihc Site wjih assistance from lhe Stale of South Dakoia's Dcpanmen! oI~ Er.vironrnenl and Nsiural Resources which is the support agency a! ihe Sue.

The Site is currently in the remedial design phase of ihe SuperTund cleanup process. Since design aciiviiics began, subsequent to Uie signing of the ROD. new informalion has been Gbiained \vhich has resulted in the need for tht.s ExpUmaiion ofSignificani Differences (ESD).

This ESD provides a brief b3ci;ground on lhc Siie. describes lhc original remedy seiecte-d in Ihe ROD. and expbins Ihc ways in which the modiiied remedy differs from ihcorigina!. U also providcsa summary of ihcsuppors agcncN-'s commeriis on the changes lo the remedy, discusses the modified remedy'.^ compliance with nil legal reqiiiremcnss, and provides deiail.s on howyou can obtain more information orsubmit aimmcnis on lhc modified remedy.

n i i s document presents on!y a summary nl'ihe changes u> Ihe remedy and a synopsis of informafion on the Site. The adminisiraih'e record, which conlains this ESD and the complete dccumcniation, is avaiiahic for public review at the iocaiioti-s indicated bciow.

DIFFERENCES, This ESO dsscf toes two changssf o the remedy • • • j thai will be impiemsnted at the Whttewcod

1. /i/se?i/c-contsininated msterials removed from fesidsntlaf areas wii! bs disposed in an on-site faciijt'/ instead of an off-sHeJaciiity,

2. The term "existing resk used in \h&- ROD to describe those areas of the Site which wHt undergo soils .ciesnup during remedsa!action,-is to refertoareas in which residsn'iat land use is occurring at the effective date of county isn6 use: ordinances required i

lliis ESD is prepared in mifiUmeni oJ" EPA's public pLitncipaiion rcsponbihiliiies iindcr Seciinn H7(c) of the Cumprehensive Environmental Re.-;ponse, Compematjon, and Lmbiiky Acl of 19S0, 42 USC % 0 ! ci seq. tCERCLA, more commonly referred in as Superfund), as amended by the Superfurui Amendmenis and Reauihorization Acl of 1986 and Section 3O0.435(c)(2}(i) of the National Contingency Plan, 40 CFR Pan 500.

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ADMlMiSlRATiVE RECORD LOCATiONS

Lawrence Coutijy Registry of Dccd^ 6 Carney Sirect Deadwood, South Dakota 57732 Hours: M - F S : 0 O A M - 5 ; C " X J P M Phone: (605) 578-3930

EPA Superfiiiid Records Censer 999 !8!h Street. Suite 5(X) Denver, CO S0202 f Jours; M-F S:00 / \M - 4:.'̂ n PM Phone: (30.^) 293-1^07 TolS-FrccN'o: !-SiX)-75fJ-4372, est. !fi07

COMMEMTS OH INQUIRIES

EPA encourages the pubiic to .^uhmii their ajmmeni.s or questions about the modi/led remedy. Pieasu subi;,.; c\)mmcnis by July 30, !99I fo:

Michael H. McCency Rgmcdsal Projcci Manager U.S- EnviroBmenial Prtncellon Agencv Mali Code: SHWM-SR 9̂ jy ISEbSiEcc!, StiileSfK! DenvxT, Coiorado 80202 Phn!!c:{303)29-i-7!69 Toll-Free No: 1 8ili>-75y-43~2. est. 7169

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The Whiiewood Creek Superfund Silt; is kscalcd in west central South Dakota (Figssre 1). The Site encompasses approximately 2.f)(X) acres aioag IS miles of the Whitcwood Creek floodpSain in Lawrence, Meade, and Bune Coursiics from the Crook City Bridge io the coRnuence of Whitcwood Creek and the Be]icFourche River. Disposaiofmincifii/yig.'! cont3ining3rsen[can(lothennctaiSFesiihcdina)ntamEr!atior. ol"soiis,.ra7^c£ waier, and ff-ound kvarsr ihroughoui portions of thcSite, TheSitcwas placed on EPA'.s sSmionaiFrionziesLisi fiVPl,) of hazardous waste sites in September 1983 making 15 eligible for cleanup under CERCLA, oi Stiperfund, as amended.

In a thrce-parxy agreement, the Hoirsestakc Mining Company (Hotncstake) , a potentially responsible party for containirtasionaftheSiie, EPA, and iheStaieof South Dakosa (the State) investigated the Site io dc:ermine the extent and nature of the corst ami nation. This work, along with other related studies conducted at the Sife, is ducumcrded in the remedial Lnvesngation report which is availabje at the informalion centers in Deadwood and Denver (issled ott page 1). Results of the^^c studies indicate that unacceptabJe levels of arsenic contaniinaucn exist in aliuvia! ground water, taiUngs deposits and residential soils located wi;hjn lite Siic. ant! the surface water of Whitewood Creek.

Beginningini9SS,Homestake evaluated cleanup alternatives under the oversight of EPA and the State. Tliis work was completed in December of 19S9 and is documented iss the fensibiliiy smdy rejjpn which may siso be found in the information cenicrs. Following ihe studies and pubiic comrnent on the proposed remedial aiicrnatsves, EPA, in accordance wiih Superfund reguiaiions, selected a remedial action to be implemented. The selected remedy is set forth in the ROD.

After signing the ROD, negotiations began bclwccn EPAand Homestake for cleantip of ihe Site. In Asigust of 1990. Homestake signed an agreement with EPA in whicii it agreed to:

1. payS375,{XK)in past costsincurred by EPAat the Site;

2. ufider EPA and Siate oversight, conduct remedial design and remedial action at the Site in accordance wiEh'thcROD;and

3. pay sii future costs incarred by EP.A at the Stie.

This agreement, in the iorm of 'd consent decree. VJ3% formaSly entered by the U.S. District Court for South Dakota, Western Division, on April 4, 1991.

Remedial design activities at the Site began in Scptenibcr 1950. in the course of condiscting these activijic;, EPA has obtained new infurnsatfon which has resuHod in the need for this ESD.

r/OF THE I OF OEGISI

The objective ofthc remedy sclccicd iainc ROD is lo reduce huma!i exposure to arscnic-coniamsn;ned tailings, soils, and grouad water at ihc Whittwooti Creek Superftind Site. This remedy consists of the covering and/or removal of aiiuaminaied soiEs at existing resideniial areas and establishntent of insViiutional controls to restrict access to tailings deposits and ground water. ImplcnseniatJon of these measure!; vdU reduce the risk to public health presented by residential soiis, laiJings depc^its, and aliuvial groutid water coEitamifiated with arsenic.

The sTjaJor components of the selected remedy ijiciude;

© Cover and/or remove soils in the existing residentia! areas containing arsenic levels of 100 mUliff-ams per kiio^an-i (mg/kg) or greater; contaminated materials removed during this activity wo iiSd bcdis posed in an off-sile disposal facility approved by EPA and the Siate;

a Restrict future devciopmeni in the tOO-yenrflifCidptain, the tailings deposits, and areas coniaining tailings-impaaed soils through ojiinty ordinances regulating land use;

e Prohibit excavation of tailings deposits for other uses and prohibit excavation of remediated areas through coiinty ordinance, aUhough mining would be allowed Subject to the reguiaiions of the State of South Dakota;

# Refine knowledge of the extent of wntaminatiosi and delineate the 100-year fioodplain. Provide detailed majis to define Site boundaries and specify scitvities lo support iniplementation of county ordinances;

9 Set up an educational program to inform people abou! hazards presenicd at the Site tiod ways io decrease their personal exposure;

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DESCRlFriGN OF SfSNIFfCANT

The significant differences beiv.'ecn the rcntcdy described in lhe ROD and in this ESD arc:

1. Contaminated maicrials removed from the residcna\^ during remediation will be disposed of in an on-si;e facility instead of an off-site faciHiy.

2. The term "existing residential areas" is to refer lo areas within the Site where residential land use is occurring a> of the effective date of county land u.se ordinances. 'Phis tctm was not explicitiy defined in the ROD.

All other aspects of the 199Q selected remedy, as described above, remain the same. A more detailed description of the revised componcnis to !he remedy follows.

The ROD specified thai arsefiic-contammaled msicnals removed from the residences during remediation would be disposed in an off-site, permitted storage faciiiiy, such as the Grizzly Gulch Tailings impoundtnent located near Lead. South Dakota. A specific faciitty was to be chosen during remedial design.

New Information S i n c e t h e ROD

Since the issi^ance of the ROD, preUminar\- remedial design work has been condtsctcd for the residential clearjup task. Homcstake's remedial design engineer has esiimaied thst ihc quantity of materials lo be disposed w1i! be ^css ihan lOMiQ cubic yards. This estimate is less than one-third (he amount f/.0,0-00 cubic yards) estimated during development of the ROD. The lesser amount of disposal materials is primarily due to lhe fact thsL based on discussions wjih residents regarding their land use habii.'i. a smailcr area araunJ each of the homes is to be remediated than was estimated ii5 ihc feasibility study.

TTie M o d m s d Disposal Plan

EPA's modiilcd disposal plan involves constmciion of an OTS-site disposal area on properly owned and uontroiled by Komestake situated at the northern end off he Supertur.-i Site near the confluence of WhiEcwnod Creek anJ the BcHc Fourehe River (Figure 1).

Tfiis disposal area wiii be designed andoperafcd in sccordance with al! federal and %VAhzappiicahk or r&ievaru ami appropriate requirertients (ARARs). except for those in which a waiver is invoked. The disposal area, which will be approxiitsalely 7.5 acres in size, ha.s been designed to be situated on an ovcrbasik deposit of mine tailings. These mine tailings have been shown

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BELLE FOURCHc RIVER CK. -ON - SITE ^ . DISPOSAL ^ W FACSi-ITV

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lo contain LO!uvn!raHi>ns iifurMinit; ran^'ing trisnj Hy-V m^ku tolO.tHKimg.-'ki^icons.smtni'icdmaierialsEobcUisposcdat ihc Si?e arc csEimati;d!rsa>!ii:.iuKina\cr:!i;c arsenic coTiccnuaimn of^Om^lcg)-

ConlJininaied soii.s and gravel rcmovt-d from resideniial properties will be placed on the disposal ar^a and rcve^ciateU with native grasses. Dcplhsofihisfiii material wili range from six inches to four feci, Other consuuciion debris, stich as removed fences and trees, will he con.soiid;ttcd adiaccni Jc; the fill area.

Dispo.sai maicriais v.-il) be iransported to the lacility in accordance with all federal and state AH..'\Rs. Truck loads wiii be covered to prevent windblown dust. When disposal has been eompleied, a security fence will be CiJnsimcied around the facility in order to restrict site access. Under the modified disposal plan, the on-site dispo.'^a! facility will be available to homeowners and devftopers for dispossi of arsenic-contaminated materiai removed from Site propcriics during future construction activities.

Additional details about the modified di.'iposal facility can be found in the Disposal Pian design dac-amcnis ctintained io the administrative record for the Site.

JustJficafen for the Change

EPA modified the original disposal pian for the foiiowing reasons:

1. 1~hc reduced distance for the dfspo.-.al haul roole will expedite the cleanup schedule by shortening the turn-around time for disposial acii\ilies. This wiH not only facJHsate the process of achieving the cScaiiupobi.xiives at the Site but will also reduce the overall cost of the remedy.

2. The njodifieddisposalremcdywill help reduce poteniial short-term risScs associated with iransponatson of coniaminaEed materials since the majcrsals wiii be transported shorter distances, on rural county roads with significantly lower traffic volumes, and through areas more sparsely populated than the routes reqEiircd for off-siic di.'^posal.

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3. The reiauvely small area needed to pbcc tli materials (7.5 acres) can be easily designed and constructed on-site in accordance wiih state and federal landfill requirements arid alt other ARARs.

Additional details rcgardiiigtheju.siificaiion for this modified disposal plastand a dcicrminatlon of the A R . ' ^ R requirements associated with ihcdisposa! plan can he found in Homcsiiikc's petition for ihc change ssibmiiied to EPA in a documeni entitled On-Site J3!spos.-i! Plan for Conntminaicd MajcrialSj VVhiteucH'd Creek Siiperftrnd Site. April 11. 1991. located in the acminisiraiivir rca>rd for ihc Site.

ITiis ESD IS ;iUo being used lo clarify and define tht term "existing rcskieniiai areas" â used in the ROD, The remedy chosen in ihe ROD specifics that soi! ir. existing resident';)! areas containing arsenic cotseentraiions of KX) jng-'l̂ g cr grcascrshail be cohered or e.xeavalcd. Though iioi defined in ihcROD the term "'existing resideniial areas" [."npUciity refers to areas where residential land use wiis occurring at the lime ofsigningofihcROD. However, two problems exis! with this inierpresaiioo;

1. At ihc lime the ROD was signed, it was not compietciy kno>ATi which residential areas coniained arsenic-eontamiftated soils. The extent of arsenic coniamjnaiioit was not fully known and ihercfore further site characierization activities were specified to take place during design Oi the remedv (after the signing of Ihc ROD).

2. Residential conslrucuon occurring in the Superfund Sitcaftcnhesigoingofihe ROD but before the effective date of county land use ordinances cnui.1 lake place without conducting soils cleanup activities. Therefore, a residence could be developed and continsje lo exist on an un remediated, arsenic-contaminated area (After county land use nfdtnariccs arc in place, as required by the ROD.il will beadevclopcr'sresponsibsiiiyioensurc that new residential areas do not contain surface .soits contaminated with arsenic above the 100 mg/kg action levei)

!n order lo correct these problems and to effectively achieve tbe remedial objectives set ous in the ROD, EPA is defining "existing residential areas" lo be those areas in which fcsidentiaHaiid use jsoccurring upon thccffeciivc date of the couniy land use otdinanccs requiECd by the ROD,

Since the signing of the ROD, remedial design site characteri/ation activities have occurred including soi! sampHng in known residential areas. To date, twenty-three wideW scattered residentia! areas have been identified as containjngarsenic-contaminatedsonsat levels above the 100 mg/T(g aclion level If, at thceffectivcdatcof county land use ordinances, land tjse in any of these areas has changed from residential to rton-residcntiai, such areas will not be required to undergo soils cleanyp during remedial action. Human exposiire to arscttic coiitam.ination in such areas y-AW be addressed through county Sand use ordinances.

Conversely, if any additional residential land development (since the signing of the ROD) occurs within she Superfund Siic before the effective date ofcounty land use ordinances, the property would be subject to soils cleanup acli^tics in accordance with ihc ROD and this r^SD.

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in order to expedite tht; cleanup activities at the Sue, some of lhe tweniy-lhrcercsidejniahiicasakeadycharaclcrixcditt the remediai dcsica process are scheduled to be remediated prior to cnactmeni of couniy land tise ordinances. This soils cleanup is .scheduled U) begin in the summer of ]*^1 iu resideniial areas where EPA can reasonably determine, through coordination wiih the rcsidcnis, ihat residentia! u.se of the area will not change.

rUMMARY OF SiGMir ICAf^T

The major differences between the original ROD Temedy-^and'the modified remedy in this ESD are summarized as toiiows:

Originaî Remad}̂

s off-sita cjlsposa! o-f arsenic-contaminaisci materials removed front residences

# "existing resitJenti^ areas" not expiieitly ĉ efsnsd

Modified Remedy

on-site •.materisis removed from residences

"existing restctential areas" is to refer to areas within the Site where residential isnd use is occurring ss of the eff ectî /e date ct county land use

'Hie South Dakota Department of Environment and Natural Resources has reviewed this ESD and supports implementation of the m(5diflcd remedy as set forth herein.

STATUTORY DETERMINATIONS

Considering thenew information thai has been developed and the changes that have been made to the sciecied remedy, EPA and the State believe ihal the remedy remains protective of human health and the environment.complies With federal and Siaie rcquirenicnts that are applicable or relevant and appropriate Io this remedial a-:;[ion. except those for which a waiver is invoked, and is cost-cffcenvc. In addition, the revised remedv uliiizcs permanent solt^iions and alternative trcatmcnnechnologic^io the maximtsm extent practicable for this Site.

I&J-Year Fkxsdplnm: AD area that would be covered by wstcr dtifing a OIKKS evens estimated to occur orice every 100 vears.

h An amount of a ainlaminani in sojj, air. or water at v-lijch EPA believes a response is necessary. .Action levels V8r>' from site lo site and even within siies. based on poteniial cxpc'^ures.

AdmhusiraiJve Record: The body of docuirienis lipon which EPA bases a cleanup decision about a Superfund site. By law, the administrai ive record file, which is the file ainiaining the documctt's used in .selecting the remedy for a site, must be made avaiiable to the ptsblic ;u a reposiiory located near the Superfund site.

}: Refers to the federal and stale requirements that a selected remedy is required to attain. M. includes reqiiiremcsis such 3,s allowable air emis",ions limits and allowable levels of contaminants in site mcaia {.';uch as soils and waler).

Aquifer A layer of rock or soil below the ground surface that can supply usable quantities of water to wells and springs. Aquifers can be a source of water for drinking and other uses.

Arsenic: Thecontaminani of most significant environmental concern at ihe Whitcwood Creek Superfund Site, arsenic occurs in many forms. At the Whitcwood Creek Site,ii occurs prsncipaiiy in the form of arsenopyTitc (a naturally occurring anicnic-sulfidemineraijwhich is present in theorc body where 3old veins are fnond. Tlte tailings deposits at the Site contain :onccntrations of arsenic significantly above levels in 1!neonlamistated alluvia! soils. Dissolved arsenic is rapidly absorbed into the body following ingestion or inhalation and can affect the cardiova.'icular .siystcm, skin, or lungs. More detailed informalion regarding the health effects of arsenic may be found in the U.S. EPA Report No. S25/.V87/013. November i9S".

CoTT.pensatkiti, a n d Liabuky Acl of 1989 (CERCLA or Superfimd): A law passed in 1980 that establishes a program to identify abandoned hazardous waste sites, ensure that they arc cleaned up, evaluate damages to natural resources, and create clainis procedures for parties who cleaned up the sites.

CtmsetU X>ecrse: A legal and enforceable agreement signed by the Uaiied States and the potesitially responsible parties and entered as a court order by a judge. Tne decree at Ihe Vr'hitewood Creek Superfund Site describes activities to be conducted during the run^cdia! design and remedial action phases of site work.

Expianul.ion. afSignificarU Differences (ESD): Refers to a requfrerr.en! of Section U7(c)ofCHRCLA. as amended, and the NCP, Section 3(Xa4.i5(c){2){i), chat requires the lead

agency, foUowittg adoption of she ROD, to document and explain any signtficanl changes to the ROD. The ESD and supporting infortnatiojt must be made available to the public in the administrative record and information repository for the site. In addition, a public nosice summarizing the ESD must be published in a major local newspaper of genera! circulation.

FeasibsMty Study (FS): Astuoy required under Superfund In which aitematives for cleaniug up site contamination are idealified, screened, and rampared.

Ground Water: Underground water that fills pores m .soils or openings in rock to the point of saturation.

Insti tutions! Conso l s : At the Whitcwood Creek Superftind Stic, ihisicrni refers to legal, non-engineering methods used to prevent or restrict u.scof. or access to, contaminated soils ana grcjuitd water. In general, institutional controls may lake the form of rules, reguiatioits, laws, or covenants such as county or city ordinances, building permits, or other appropriate measures, as ncccssary.

MUligramsper ^ j l o ^ a m (mglkg): A u n i t o f measyremeni contmonly used to express low concentrations of contaminants. Thi&measuremcntisihceauivaientofonepart per million (ppm).

Nat iona l Conimgerury P lan : A body of federal regulations governing Ihe inipiemcntation of CERCLA-

MaiionalPnoridesList : EPA~s list of lop-prioriiy ha?.ardous waste sites that are eligible for investigation and cleanup under the federal Superfund program.

Polemmliy .Respons-ible Party: An individual, company, or governmentbodyider.linedas potentially liable for cleanup of

hazardoiissubstaneesatasiic. Undi.:rthe Superfund progslntl.* EPA may hold liable any pany that hiss generated or irastsporicd hawrdoussubsiances, as well as t ho.se who ow-aed or operated a dL':posal faciliiy, or ihme who cujrcfitly own such facilities.

' Ol usc i smn \ R O D ) : A public document that sets forth and explains the cleanup aUernative(s) to be used at a Supsrfsind site. The ROD ES generally ^ased on information from the remedial investigation and fe^ibiliQ'studies, public commenis, and community concerns.

The actual construction or implcmcntatiot» phase of Superfund work (iL=rmg which the seiccied remedy is put ittto place.

Remedml D s s i ^ - The engineeringphase of Superfund work following the Record of Decision that inclxid^ technical analysis and proccdtires which result in a aetsUed set of platis, technical drawings, and specifications for implementing the

' remedy durina the remedial action Dhase of work.

tion (RJ): A stody required uj^der Superfund thai is conducted in order to identify the types, smounis, and locations of contaminaiioF5 a! a site.

Superfimd Amendmenl-i arsd Remuhorizaiion Act of 1986: .^ isw passed in 1986 thai reauthorizes the Superfund law.

Surface Waier: Bouic>: of wa^er thai are above ground, such as rivers.streamsandlakes.asweli as precipitation (rainwater or snow melt) flowing on the ground.

Tail ing- Tne portion of mineral ores thai is separated out durang the miUJngof ore and disposed.

Michael H, McCcney Remedial Proiect Manager U.S. Environmental Protection Agency Mai! Code: 8HWM-5R 999 18th Street, Suite 500 Denver, Colorado S0202

Whitev '̂Gcd Creek Superfund Site, South Dakota

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