### UNITE D STATE S ENVIRONMENTA L PROTECTIO N AGENC Y REGIO N VII I 99 9 18t h STREE T- SUIT E 50 0 ^ D OCUlTien t I D DENVER , COLORAD O 80202-246 6 &K':!?.S.,,imiiiiiiiiiiilll\lllllllllllll l 

### 203809 2

FINA L POLLUTIO N

REPOR T Midval e Sla g OU 2 Abandone d Well s



### I . HEADIN

G Date : Subject : From : 

### To : POLRE P No , 2/3/9 7

Midval e Sla g NP L Sit e Mik e Zimmerman , OS C _ Ann e Spencer , EPAHeadqu'art'er s Emergenc yRespons e FINA L POLRE P - # 2 (OU2 ) Abandone d Well s

II . BACKGROUND Sit e No. : CERCLI S No : Respons e Authority : Stat e Notification : Actio n Memorandum : Star t Date : Completio n Date : 

### 7 1 UTD08183427 7 CERCLA UDE Q Notifie d 07/26/9 6 10/3/9 6 11/8/9 6
III . SIT E INFORMATIO N A . Inciden t Categor y Time-Critical , NP L B . Sit e Descriptio n 1 . Sit e Descriptio n Th e entir e Midval e Sla g sit e consist s o f abou t 53 0 acres ; OU 2 cover s abou t 20 0 acre s o f th e Site . Asmelte r operate d o n OU 2 from189 7 unti l 1958 . Ove r th e years, , i t change d froma coppe r t o a lea d smeltin g facility , producin g gold/lead/silve r bullio n fromsulfid e ores . Fiv e well s wer e installe d o n OU 2 i n 193 6 i n th e Dee p Principa l Aquife r fo r th e purpos e o f supplyin g wate r t o th e smelter . Durin g th e demolitio n o f th e smelte r i n 1960 , th e surfac e casing s o f

Primed on Recycled

Paperth e well s wer e damage d an d severa l o f th e wel l head s wer e covere d wit h demolitio n materials . I t appear s cha t th e well s remaine d i n a n unprotecte d conditio n fo r manyyears , makin g i t likel ytha t surfac e wate r an d windblow n smelte r wast e material s entere d th e casings . 2 . Descriptio n o f Threa t A s a resul t o f damaged/deteriorate d surfac e casin g o f 5 well s o n th e OU 2 site , surfac e wate r and windblown smelte r wast e material s hav e entere d th e wells , and the yno wsho w elevate d concentration s o f severa l metals , especiall ybariu m and cadmium . Sinc e ther e ar e abundan t concentration s o f lead , arsenic , copper , silver , chromium , and zin c i n th e slag , dross , and baghous e dus t foun d o n th e Sit e fro m previou s smelte r operations , i t i s possibl e tha t additio n metal s coul d migrat e int o th e well s i n th e future . Becaus e thes e well s ma yb e actin g a s conduit s fo r contaminatio n t o th e underlyin g aquifer , the ywil l b e abandone d an d plugged . C . Sit e Evaluatio n Result s Durin g th e EE/CAfiel d investigatio n i n 1993 , th e surfac e casing s fo r thes e 5 forme r water-suppl ywell s wer e located . Tw o o f th e well s wer e sample d an d showe d elevate d concentration s greate r tha n th e Maximu mContaminan t Leve l (MCL ) fo r tota l and dissolve d bariuman d cadmium . Thes e elevate d concentration s migh t b e accounte d fo r b ysurfac e wate r infiltratio n du e t o severel ydeteriorate d wel l casing s a t th e surface . IV . RESPONS E INFORMATION A . Situatio n 1 . Curren t Situatio n O n Octobe r 3 , 1996 , th e EP AERC S Contracto r mobilize d t o th e Sit e t o begi n th e pluggin g an d abandonmen t o f th e fiv e (5 ) wells . Th e wel l stem s o n th e nin e (9 ) monitorin g well s wil l b e remove d an d th e well s wil l b e plugge d an d abandoned . Whe n thi s i s complete d i t wil l remov e a condui t fo r contaminatio n and a n obstacl e t o futur e remediation . 2 . Fina l Dispositio n Th e plasti c casin g from th e nin e (9 ) monitorin g well s ha s bee nremove do r "drille dup " durin g reentr y drilling , an dth e hole s hav e bee ncompletel y fille dwit ha cement bentonit e mixture . Th e remove dplasti c pil e an dcemen t pads-shor t casing s use dfo r surfac e protectio nfrom th e monitorin g well s ar e stage don-site .

### Fou r o f th e fiv e deepe r (±350' ) wate r suppl ywell s wer e over-drille d wit h a n oversho t - i.e. , a large r diamete r dril l strin g wit h cutting teet h welde d o n th e botto mjoint . Th e fift h well , whic h i s locate d onl ya fe wfee t fro mth e fourth , ha d bee n previousl yplugge d and abandone d - th e to p o f th e cemen t plu g wa s encountere d a t 128 ' . Onl y120 ' o f origina l casing wa s recovere d fromal l four well s and i t wa s foun d t o b e badl ycorroded . Th e recovere d pip e i s als o stage d on-sit e nea r th e monitoring well' s debris . Th e fou r hole s wer e completel yfille d wit h approximatel y10 0 sack s o f th e cement-bentonit e mixtur e pe r well . V . Cos t Informatio n Th e cost s t o dat e ar e $113,260.49 . Th e cos t mayincreas e slightl yafte r a surveyverificatio n o f al l wel l locations .

## REGIO NV mSTAT EAGENC Y RRT CONTACT S POLREPDISTRIBUTIO N FA X SHEE T

TO : SE ELIS T BELOW
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. Date : Comment s L. *. oL o n



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