M. Winters Shaw Environmental & Infrastructure 100 West Hunter Avenue, Maywood, NJ 07607

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1 USACE FUSRAP Mywood Tem Develops Mechnism to Evlute Residul Rdon Exposure Potentil t Vicinity Properties Where Remedition of Accessible Contmintion hs been Completed M. Winters Shw Environmentl & Infrstructure 100 West Hunter Avenue, Mywood, NJ S. Wlnicki Sfety & Ecology, Inc. 100 West Hunter Avenue, Mywood, NJ D. Hys Unites Sttes Army Corps of Engineers 100 West Hunter Avenue, Mywood, NJ ABSTRACT The Mywood FUSRAP Tem is obligted, under its pproved remedy selection decision document, to demonstrte substntive complince with New Jersey Administrtive Code 7:28-12()2, estblishing n indoor limit of three PicoCuries per liter bove bckground for rdon-222 (Rn-222). The Mywood Tem explores vrious venues for deling with the rdon issue nd provides n lterntive for demonstrting substntive complince with the rdon remedition stndrd by nswering the question: In certin conservtive situtions, cn complince with the rdon stndrd be demonstrted without performing monitoring? While monitoring my be the most definitive method for demonstrting complince, logicl rgument cn be mde tht when rdiologicl remedition removes the potentil source for Rn-222 bove bckground, monitoring is unnecessry. This position is defended through the use of historicl physicl rdon mesurements which illustrte tht indoor rdon ws not pre-remedition problem, nd postremedition soil smpling dt which demonstrte tht the source of the potentilly elevted Rn- 222 levels hve been successfully mitigted. Monitoring recommendtions re mde for situtions where insufficient dt exists to mke definitive determintions or when unremedited sources ffecting hbitble structures remin on given property. Additionl informtion regrding recommended techniques nd references for effective monitoring of indoor rdon re included in this pper. This pper my benefit tems tht hve similr regultory commitments nd/or hve need to mke ssessments of rdon exposure potentil bsed upon historicl monitoring dt nd vilble soils concentrtion dt. 1

2 INTRODUCTION The United Sttes Army Corps of Engineers (USACE) hs contrcted Shw Environmentl, Inc. (Shw) to remedite Phse II of the Formerly Utilized Sites Remedil Action Progrm (FUSRAP) Mywood Superfund Site (FMSS). Rre erth nd thorium processing opertions t the site during the first hlf of the lst century creted wstes contining thorium, rdium, nd urnium which contminted the environment primrily s buril pits nd secondrily through trnsport vi surfce/storm wter nd use of the mteril s fill. Phse II includes remedition of the remining FMSS commercil nd governmentl properties The gseous decy products of the primordil rdionuclides present in Mywood FUSRAP soils re routinely evluted to ssess outdoor environmentl levels. They hve lso been evluted in the pst to ssess indoor exposure levels to members of the public who work/visit commercil buildings tht in some cses were constructed bove or in the immedite vicinity of FUSRAP rdioctive wste. The Mywood Tem is currently evluting residul rdon risk t properties where, to the extent prcticble, contminted soil removl ctions hve been completed. This pper discusses the elements of the evlution process nd the decision mking tools used by the Tem to mke protective determintions for properties in the current remedil phse of the project (Phse II-Commercil & Governmentl Properties). The FMSS Phse II remedil ctions re being executed in ccordnce with the Record of Decision for Soils nd Buildings t the FUSRAP Mywood Superfund Site (ROD) [1], which hs identified the New Jersey Rule N.J.A.C 7:28-12 Soil Remedition Stndrds for Rdioctive Mterils [2]s n pplicble or relevnt nd pproprite requirement (ARAR). The ROD sttes the following regrding Rn-222: Indoor rdon ir concentrtions will meet the 3 PicoCuries per liter (pci/l) rdon- 222 (Rn-222) limit specified in the NJAC 7: ()2 t ll properties ddressed in this ROD. [1] New Jersey Administrtive Code (NJAC) sttes the following: 7: Rdition dose stndrds pplicble to remedition of rdioctive contmintion of ll rel property () Sites shll be remedited so tht the incrementl rdition dose to ny person from ny residul rdioctive contmintion t the site bove tht due to nturl bckground rdionuclide concentrtion, under either n unrestricted use remedil ction, limited restricted use remedil ction, or restricted use remedil ction, shll be s specified below: 1. For the sum of nnul externl gmm rdition dose (in effective dose equivlent) nd intke dose (in committed effective dose equivlent)*, including the groundwter pthwy*: 15 millirem (0.15 millisievert) totl nnul effective dose equivlent (15 mrem/yr TEDE). 2

3 2. For rdon*-222*: three picocuries per liter (pci/l) of rdon gs (111 Bq/m3). [2] In order to logiclly ssess the rel potentil for properties with/without residul rdioctive soils contmintion to exceed the ROD criteri for Rn-222 it is necessry to nswer severl bsic questions: Is there historicl evidence tht Rn-222 levels in hbitble structures, prior to remedition, exceeded ROD criteri? Is there evidence of residul rdioctive contmintion under or immeditely djcent to hbitble structure? If so, how will the residul risk be evluted? Using set of conservtive ssumptions, cn post-remedition finl sttus survey (FSS) soil smpling dt for rdium-226 (R-226) be used to definitively ssert tht indoor Rn- 222 concentrtions would be below ROD criteri? Wht situtions would be pproprite / inpproprite for ppliction of this methodology? Once determintion is mde regrding the necessity of physicl monitoring to demonstrte complince with ccepted stndrds, remedition tem cn move forwrd. The finl section of the pper introduces reders to selection of the fundmentl guidnce/requirements for n effective indoor rdon monitoring regime. DISCUSSION Evluting Historicl Rdon Monitoring Dt to Assess Residul Rdon Exposure Potentil Is there historicl evidence tht Rn-222 levels in the occupied fcilities, prior to remedition, exceeded ROD criteri? The simple nswer is no. The most comprehensive evlution to dte of indoor rdon levels t Mywood site properties ws conducted by the U.S. Deprtment of Energy (DOE) in The report, titled Results of Rdon nd Gmm Rdition Mesurements t 19 Commercil nd Governmentl Properties of the Mywood Site [3], is included s prt of the Mywood Soils Fesibility Study [4]. The report presented the results of ctivted chrcol cnister smpling conducted in commercil/government vicinity property buildings t the FUSRAP Mywood Site. The study ws thorough nd performed with n cceptble level of dt qulity. The only notble limittion is tht the durtion of smpling (i.e., seven dys) would fil to cpture sesonl vribility nd its effect on building rdon levels. Tble I presents the results for rdon/thoron monitoring from the 1994 DOE Study [3]. 3

4 Tble I. Historicl Indoor Rdon Monitoring Results from FMSS Phse II (Commercil & Governmentl) Properties Property Phse II Property ID# Gross Rn-222 Results (pci/l) Gross Rn-220 Results (pci/l) 160/174 Essex Street 4A 0.2 to Essex Street 5B 0.2 to 0.6 < N. NJ Stte Route 17 6A 0.2 to N. NJ Stte Route 17 6A 0.2 to N. NJ Stte Route 17 6A 0.2 to Stte Rt. 17 (office) 6D 1.3 < Stte Rt. 17 (storge) 6D W. Hunter Ave. (Stepn) 10A 0.2 to 3.4 < Stte Rt. 17 6C 0.2 to <0.3 < Stte Rt. 17 6B 0.2 to Industril Rod 2C 0.3 to 0.5 < Gregg Street 3A 0.2 to Hncock Street 2B 0.2 to 0.4 < Hncock Street 2A 0.4 to 0.5 < West Howcroft Rod 8A 0.2 to 0.4 < Sidney/88 Money Street 1A 0.3 to Stte Rt. 17 5C 0.3 to 0.4 < Mywood Avenue 9A 0.4 or less < Mywood Avenue 11A 0.4 to 1.7 < Mill Street, Lodi NJ 2D 0.3 to 0.6 Thoron nlysis of chrcol cnisters not conducted for this building The results of the 1994 rdon study identified no buildings tht exceeded n ction level. The DOE used the ction level of three pci/l bove bckground for both Rn-222 nd rdon-220 (Rn- 220) (evluted seprtely). The mximum result, uncorrected for bckground, ws identified in Building 3 on the Stepn Compny Property (3.4 pci/l for Rn-222, <2 pci/l for Rn-220). Stepn Building 3 cn be resonbly considered worst-cse scenrio from potentil source stndpoint since it ws the only hbitble structure built directly bove one of three licensed rdioctive wste buril pits on the site. Other structures on the FMSS tht re known or suspected to hve residul rdioctivity directly below them were constructed bove lluvil deposition res or bove res bckfilled with fill contining wste mterils blended with regulr soils. Furthermore, the lens thickness of contminted soils under the Building 3 foundtion is believed to be greter thn under ny other structure where contmintion is known or suspected to be present (i.e., 12 or more feet in thickness). It should be noted tht Building 3 ws demolished to the foundtion level in 2007 by the Stepn Compny in dvnce of upcoming soils remedition by the Mywood Tem nd is no longer considered hbitble structure. 4

5 Evluting Indoor Rn-222 Exposure Potentil Bsed on Residul Outdoor Rdium-226 Concentrtions in Soil In order to ssess rdon potentil from residul soils fter remedition, there must be resonble mechnism for ccomplishing the following: Implementing resonble process by which residul R-226 soil concentrtions in pci/g cn be relted to expected Rn-222 concentrtions in indoor ir in order to ssess n ction level corresponding to the ccepted limit of three pci/l bove bckground for Rn-222; nd Quntifying residul soil concentrtion verges nd confidence intervls of R-226 (the prent rdionuclide to Rn-222 nd Mywood Rdionuclide of Concern) on the property. Severl studies hve suggested correltion between rdium in soil nd indoor rdon concentrtions [5, 6, 7]; while there is no bsolute correltion due to the myrid of fctors which contribute to indoor rdon concentrtions, the conclusion cn be mde bsed on these studies tht indoor rdon potentil cn be resonbly estimted bsed upon the rdium in soil content. In the Development of Generic Stndrds for Remedition of Rdioctively Contminted Soils [4], New Jersey suggests conservtive rtio of indoor rdon to rdium in soil of 1.5 pci/l rdon per pci/g rdium. This rtio ws developed by tking the geometric men for indoor rdon in New Jersey homes nd compring it to the sttewide verge rdium in soil content. The New Jersey vlue, with which the Mywood Tem concurs, is supported by n Environmentl Protection Agency (EPA) study [6] which found tht the rtio of Rn-222 (pci/l) to R-226 (pci/g) vried bsed on soil types between 0.2 nd 2.4 pci/l per pci/g, with n verge rtio of 1.1 pci/l per pci/g. The New Jersey rtio is more conservtive then the verge rtio from the EPA study nd is therefore used for the purposes of this pper. Regionl Bckground Prior to ny discussion regrding complince with remedition stndrds, it is impertive tht bckground levels of rdionuclides re estblished, for both soil (R-226 & Th-232) nd ir (Rn- 220 & Rn-222). This is necessry becuse ll remedition stndrds re presented s llowble increments bove bckground. Prior to the ROD-driven remedition of FMSS properties, Bckground Investigtion Report [8] ws prepred for the USACE by Shw with the intent of estblishing regionl bckground vlues for rdionuclide(s) of concern. Results from this regionl study estblished the verge bckground concentrtion of rdium in soil s / pci/g (1σ). The verge bckground concentrtion of Th-232 ws estblished s / pci/g (1σ). These vlues were derived specificlly for the FMSS in ccordnce with the Multi-Agency Rdition Survey nd Site Investigtion Mnul (MARSSIM) [9] guidnce for determining regionl bckground, nd s such will be used s pproprite bckground vlues for the purposes of this pper. Estblishing Conservtive Soils R-226 Action Level 5

6 The Technicl Bsis Document for N.J.A.C. 7:28-12 [4] provides technicl justifiction nd ssumptions to estblish n indoor rdon to soil rdium rtio (RRR) of 1.5 pci/l. This is the most conservtive rtio considered in the document [4] nd gives no considertion to either the verticl extent of the R-226 contmintion, or to ny clen lyer which might exist between the slb on grde/bsement nd the R-226 lyer. Using this rtio provides n llowble concentrtion of R-226 in soil in order to control the indoor Rn-222 concentrtion to less thn 3.0 pci/l. Since the Rn-222 clenup criteri presented in the ROD is 3 pci/l bove bckground, n pproprite vlue for regionl Rn-222 bckground is dded. Regionl bckground concentrtion for indoor rdon in New Jersey homes is 1.35 pci/l. Appliction of the RRR to the rdon remedition stndrd of 3.0 pci/l bove bckground yields the llowble R-226 concentrtion in soil: R-226 (llowble [soil]) = (Rn-222 (llowble) + Rn-222 (bckground) ) / (RRR) R-226 (llowble [soil]) = (3.0 pci/l pci/l) / (1.5 pci/l per pci/g) R-226 (llowble [soil]) = (4.35 pci/l) / (1.5 pci/l per pci/g) R-226 (llowble [soil]) = 2.9 pci/g Bsed on the forementioned correltion rtio between rdium in soil nd rdon, s well s the cceptble bckground nd llowble increment of rdon in ir, the ction level for R-226 in soil is estblished s 2.9 pci/g. In terms of rdon, it is evident from the bove discussion tht the rdon remedil ction objective is 4.35 pci/l. Quntifying Residul Soil Concentrtion Averges nd Confidence Intervls for R-226 The Mywood Tem hs selected post-remedition FSS systemtic smpling dt (ll results, regrdless of depth) for use in determining residul R-226 concentrtions in Phse II Property soils. Systemtic smple results re collected in tringulr grid pttern with rndom strting point, on survey unit by survey unit bsis, s recommended by the MARSSIM. Although FSS soil smples re collected to determine complince with ROD soils clenup criteri, their unbised nture nd resonble frequency of collection re suitble for use in this ppliction. A descriptive sttisticl dt evlution of the dt collected is mde up of the following elements: Tble II presents property-specific dt nd summry sttistics for R-226 from ll systemtic smple loction results (excluding qulity control results) collected from MARSSIM Clss I/II FSS survey units successfully remedited to the ROD Unrestricted Use Soils Clenup Criteri (i.e., 5 pci/g bove bckground for the sum of R-226 nd Th-232 nd 100 pci/g Totl Urnium). The men, medin, mximum, stndrd devition, nd upper 95% confidence intervl (UCI95) of the men is determined for ech Phse II property nd for the overll dtset. For this ppliction the UCI95 for the 6

7 dtsets re clculted by dded the 2σ dtset uncertinty to the higher of the men or medin vlues. Tble II. Summry Sttistics for FSS Systemtic Smpling Results from Unrestricted Use Survey Units within the FMSS (Phse II) Property ID Totl # of Systemtic FSS Smples Medin R-226 (pci/g) Men R-226 (pci/g) 1σ Uncertinty (pci/g) Mximum R-226 (pci/g) UCI 95% R-226 (pci/g) 1A A B C D A A B C B C A B C D A Totl Figure 1 presents grphicl plot of the dt presented in Tble II in reltion to both the regionl bckground rnge (UCI95) for the FMSS [7] nd the derived ction level of 2.9 pci/g. 7

8 4.000 FSS Systemtic Smple Dt Rdium (pci/g Individul Smple Results Smple results Avg. Bkgd Bkgd. UCI95% Derived Action Level Fig. 1. Comprison of Rdium in Soil Results to Bckground nd Rdon Potentil Action Level Evluting Descriptive Soil Dt Sttistics nd Associted Indoor Rdon Exposure Potentil Considering tht the UCI95 of ech property s R-226 dtset flls below the trigger limit of 2.9 pci/g, high sttisticl confidence exists tht indoor rdon levels in ssocited hbitble structures would be below the 4.35 pci/l bckground djusted limit for Rn-222. In ddition, of the 1,874 FSS systemtic smples presented in this study, greter thn 99% of the results fell within the upper 95% confidence intervl of regionl R-226 bckground (1.8 pci/g) nd only single result exceeded the 2.9 pci/g R-226 trigger limit. Bsed on the definitive evidence of limited residul R-226 potentil in survey units 1 remedited to the unrestricted use soils clenup criteri, the Mywood Tem sserts tht no supplementl indoor complince monitoring for Rn-222 is necessry in survey units successfully remedited to the ROD Unrestricted Use Soils Clenup Criteri. (i.e., clen ). This stted position pplies to ny hbitble structure tht currently exists within clen survey units or one tht is constructed 1 Survey Units refer to the bsic division used for determining regultory relese sttus vi Finl Sttus Survey. 8

9 in the future bove clen survey unit, nd is in effect regrdless of the overll property sttus (i.e., unrestricted/restricted use). Evluting Residul Inccessible Contmintion nd other Restricted Use Scenrios to Assess Residul Rn-222 Exposure Potentil nd Estblish n Approprite Monitoring Regime Considering tht the primry indoor rdon potentil comes the soils directly under or djcent to structure, indoor rdon monitoring should not be required in hbitble building surrounded by clen survey units nd built upon non-rdiologiclly impcted soils, just becuse n re of Restricted Use (i.e., clenup to Restricted Use limits or residul inccessible res of contmintion) exists somewhere on the property. An exmple would be n inccessible utility corridor tht psses through property vi n esement nd is contminted in the subsurfce. If hbitble structures re not constructed or present over the contminted utility corridor AND survey units ffilited with structure meet the unrestricted use criteri, indoor rdon monitoring is not required. The Mywood Tem mkes no ssertion tht survey units remedited to the ROD Restricted Use Clenup Criteri exhibit residul Rn-222 hzrd potentil. It is merely the position of the Mywood Tem tht insufficient dt exists to definitively support the position tht survey units remedited to restricted use levels retin no residul rdon exposure potentil. The Restricted Use Clenup Criteri differs from the Unrestricted Use Clenup Criteri only in tht residul rdioctivity below depth of 15 centimeters my be s high s 15 pci/g bove bckground for the sum of R-226 nd Th-232, provided minimum of one foot of clen fill cp is plced over the residul rdioctivity. However, it is ssumed tht higher percentge of systemtic smple results exceeding 2.9 pci/g would be present in survey units remedited to the Restricted Use Criteri. Becuse of the lck of definitive dt evlution for ll Restricted Use survey units, unit-specific RRR evlution which considers the chrcteristics of the residul rdioctive lens nd the clen lyer thickness for estblishing correction fctors is needed to more firly ssess the indoor rdon potentil. The correction fctors re pplied to the 1.5 pci/l per pci/g rtio nd essentilly lower this rtio bsed upon the thickness of the clen lyer of bckfill s well s the verticl extent of residul rdioctive contmintion. Applying this property-specific rtio to the residul R-226 concentrtion yields n pproprite vlue for indoor rdon potentil in pci/l. Comprison of this vlue to the remedil ction objective of 4.35 pci/l for Rn-222 would be used to determine whether rdon monitoring is wrrnted for ffected structures. A survey unitspecific nlysis could be supported by one or more rounds of physicl rdon monitoring of existing structures, if necessry. Subsequent evlution of the rdon exposure potentil from survey units remedited to the ROD Restricted Use Soils Clenup Criteri is likely to be performed by the Mywood Tem in the future, s dditionl FSS dt becomes vilble. The ROD t Section M.2 includes the following requirement under the Description of the Selected Remedy : Periodic Rn-222 Monitoring of structures over inccessible soils to ensure tht the structure continues to provide dequte protection from these soils; mitigtion of Rn-222 (e.g., seling foundtion crcks, supplementing existing ventiltion systems, etc.) would be performed if indoor ir levels exceed 3 pci/l bove bckground). [1] 9

10 The Mywood Tem mplifies the ROD text with the understnding tht if residul contmintion bove the property-specific soils clen-up criteri remins under the foundtion of hbitble structure or in direct contct with n exterior wll, periodic monitoring should be conducted to ssess on-going rdon risk potentil. A round of monitoring is currently being plnned for those properties where remedition of ccessible soils hs been completed nd inccessible contmintion remins s previously described. Given the historicl dt collected nd the subsequent removl of significnt source term in nerby ccessible soils, it is not expected tht monitoring results will exceed the limits estblished in the ROD; however, pproprite mitigting ctions will be undertken in the event tht results exceed criteri. Any likely chnge to indoor rdon levels would most likely be ttributble to some chnge or degrdtion of the foundtion or exterior sub-grde wll llowing for esier penetrtion of Rn- 222 into the structure. SUMMARY The evlution of historicl Rn-222 monitoring dt nd systemtic FSS smple results, coupled with resonble ssessment of rdon exposure potentil from residul soil R-226 concentrtions leds to the following conclusions: Regrdless of the overll property sttus, hbitble structures (current nd future) within survey units remedited to Unrestricted Use Criteri retin insignificnt residul indoor Rn-222 exposure risk nd require no further monitoring. Hbitble structures (current nd future) within survey units remedited to Restricted Use Criteri my retin residul indoor Rn-222 exposure risk. Monitoring, or more detiled nlysis which considers ppliction of correction fctors bsed on the chrcteristics of the residul rdioctivity in soils, is necessry to effectively mesure exposure risk potentil. Hbitble structures (current nd future) bove known res of inccessible residul contmintion in excess of the property-specific ROD soils clenup criteri should be monitored for Rn-222 to confirm historicl dt nd ssess potentil impct cused by structurl chnges nd time degrdtion of the fcility. Additionl dt collection efforts re plnned for hbitble structures of the FMSS s remedition efforts re completed. The demonstrtion of complince with rdon remedition stndrds cn be ccomplished through n nlysis of residul R-226 concentrtions in soil provided tht enough soil dt exists, such s the cse with FUSRAP nd other Superfund clenup sites using MARSSIM-bsed Finl Sttus Survey. The use of soil nlysis my result in cost svings, time svings, nd is preferentil in the event site personnel re unble to gin property ccess. In some cses rdon mesurements or monitoring my be recommended or required. Considering the reltively smll cost of indoor rdon monitoring, potentilly hbitble structures should be monitored where 10

11 there is greter uncertinty regrding residul rdon. The frequency of subsequent monitoring evolutions (e.g. one-time, nnully, etc.) is left to gencies responsible for the monitoring nd ssocited stkeholders (i.e., regultors nd property owners). DISCLAIMERS Beyond the post-remedition round of rdon monitoring, decisions relted to subsequent monitoring frequencies re left to the gency responsible for long-term stewrdship of the Mywood site nd re not ddressed by this pper. The mission of the Mywood Tem is to meet the remedil ction objectives estblished in the Record of Decision for Soils nd Buildings t the FUSRAP Mywood Superfund Site (ROD) [1]. The complex scientific studies nd evlutions necessry to fully understnd the mechnics of gseous decy product trnsport in the regionl environment re beyond the scope of this mission nd s such, this pper. The gols of this pper re to communicte the Mywood Tems understnding of the rdionuclides present on site nd how tht informtion is used to mke logicl ssessments regrding residul rdon exposure potentil t properties where, to the extent prcticble, the remedil ction objectives hve been chieved. There is no pplicble remedition stndrd under the FUSRAP Mywood Site Soils ROD for Rn-220 (i.e., thoron). Discussions nd dt relted to Th-232 nd its gseous decy progeny, Rn-220, re included to provide dditionl informtion to the reder. REFERENCES [1] United Sttes Army Corps of Engineers, Record of Decision for Soils nd Buildings t the FUSRAP Mywood Superfund Site. Prepred with technicl ssistnce from Stone & Webster, Inc. August [2] New Jersey Deprtment of Environmentl Protection, Soil Remedition Stndrds for Rdioctive Mterils, Adopted New Rule: N.J.A.C. 7: August [3] United Sttes Deprtment of Energy, Results of Rdon nd Gmm Rdition Mesurements t 19 Commercil nd Governmentl Properties of the Mywood Site. August (USACE Administrtive Record Document Number MISS-108). [4] United Sttes Army Corps of Engineers, Fesibility Study for Soils nd Buildings t the FUSRAP Mywood Superfund Site. Prepred with technicl ssistnce from Stone & Webster, Inc. August [5] Cecili Edsfeldt, Royl Institute of Technology, The Rdium Distribution in some Swedish Soils nd its Effect on Rdon Emntion. August

12 [6] Kirk K. Nielson, Vern C. Rogers, nd Rodger B. Holt, United Sttes Environmentl Protection Agency, Site-Specific Protocol for Mesuring Soil Rdon Potentils for Florid Houses. EPA/600/SR-96/045. August [7] New Jersey Deprtment of Environmentl Protection, Development of Generic Stndrds for Remedition of Rdioctively Contminted Soils in New Jerseys A Pthwys Anlysis Approch. Technicl Bsis Document for N.J.A.C. 7: July [8] United Sttes Army Corps of Engineers, Bckground Study Investigtion Report, Revision 1, Prepred for the USACE by Shw Environmentl, Inc. My [9] United Sttes Nucler Regultory Commission. Multi-Agency Rdition Survey nd Site Investigtion Mnul (MARSSIM). NUREG-1575, Revision 1. August