B. Radium Isotope Mass Balance Model Used to Derive Submarine Groundwater Discharge

Transcription

1 Supplementary Materials Lingering radioactivity at the Bikini and Enewetak Atolls Ken Buesseler a *, Matthew A. Charette a, Steven Pike a, Paul Henderson a, Lauren Kipp a a Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole MA USA * corresponding author. kbuesseler@whoi.edu A. Supplemental Text: Water Mass Age Model Derived from Radium Isotopes We calculated water mass ages, which we assume to be equivalent to the replacement time (T w ) of water in the lagoons using the model developed by Moore (2000) and modified by Charette et al. (2001): 223 Ra ex 226 Ra ex obs = 223 Ra ex 226 Ra ex i e λ 223T w (1) where the Ra isotope excess ex values are the measured values corrected for their offshore (North Pacific Ocean) endmember and 223 is the decay constant for 223 Ra. The ratio on the left hand side of the equation ( obs ) is the observed or measured value within the lagoon surface waters while the Ra isotope ratio on the right hand side is the initial ( i ) value, which in this case is what we measured in average groundwater from the lagoon islands. This method takes advantage of the relative decay rates of the two isotopes as they mix away from the source (SGD in this case). It also assumes that the activity ratio in SGD is constant over time (steady state). The process in equation 1 of normalizing the shortlived isotope (here 223 Ra) to the long lived isotope (here 226 Ra) eliminates mixing affects such that decreases in the observed ratio are due to decay alone. The derivation of equation 1 requires that both isotopes decrease to zero as time approaches infinity. Therefore, in the case of 223 Ra, we use 223 Ra ex, which is 223 Ra not supported by decay of 227 Ac in the water column and is typically well below detection in the open surface ocean. For 226 Ra, minimum North Pacific Ocean surface values are no less than 0.83 Bq m 3 (Huh and Ku, 1998) such that we define 226 Ra ex as 226 Ra values corrected for this open ocean background activity. For the initial 223 Ra/ 226 Ra activity ratio, we used the average of groundwater samples from islands in both lagoons (0.19; n=16). Surface water 223 Ra/ 226 Ra ratios were all lower, reflecting decay of 223 Ra relative to 226 Ra due to aging within the lagoons. The 11.4 day half life of 223 Ra dictates that this method is useful for water mass ages below ~2 months. All surface water stations in Enewetak and Bikini were found to be below this threshold except for one, which was not included in the T w average. B. Radium Isotope Mass Balance Model Used to Derive Submarine Groundwater Discharge Submarine groundwater discharge to the Enewetak and Bikini lagoons was estimated using a steadystate mass balance model as described in Charette et al. (2008): F Ra =I Ra λ+ 1 T w (2)

2 where F Ra is the flux of a radium isotope to the lagoon water column from SGD, I Ra is the radium isotope inventory in the water column, and is the decay constant for a particular radium isotope. The data used in the model were collected in the shallow surf zone off of Bikini and Runit Islands; three Ra isotopes were easily detected in the water column there such that we are able to estimate SGD for each isotope. SGD fluxes in the main text are the average of the three Ra isotope estimates. The model assumes that the main source of Ra to the surf zone is SGD, and that the only Ra loss terms are mixing and decay (Colbert and Hammond, 2008). To obtain the Ra isotope inventory (I Ra ; Bq m 2 ), we corrected the average of the surf zone activities (Bq m 3 ) from Runit and Bikini for the background Pacific Ocean activity (stations measured outside each lagoon) and multiplied these values by the surf zone water depth (2 m). The T w values used in the model are those calculated for the surf zone samples only (not the entire lagoon). To convert the SGD derived Ra flux (F Ra ; Bq m 2 d 1 ) to a water flux, we divide by the average Ra isotope activity (Bq m 3 ) in island groundwater. The SGD rates are thus in units of m 3 m 2 d 1 or cm d 1 as reported in the text. B. References for supplemental document Charette, M. A.; Moore, W. S.; Burnett, W. C. Chapter 5 Uranium and Thorium Series Nuclides as Tracers of Submarine Groundwater Discharge. Radioactivity in the Environment. 2008, pp Charette, M. A.; Buesseler, K. O.; Andrews, J. E. Utility of radium isotopes for evaluating the input and transport of groundwater derived nitrogen to a Cape Cod estuary. Limnol. Oceanogr. 2001, 46 (2), Colbert, S. L.; Hammond, D. E. Shoreline and seafloor fluxes of water and short lived Ra isotopes to surface water of San Pedro Bay, CA. Mar. Chem. 2008, 108 (1 2), Huh, C. A.; Ku, T. L. A 2 D section of 228Ra and 226Ra in the Northeast Pacific. Oceanol. Acta 1998, 21 (4), Moore, W. S. Ages of continental shelf waters determined from 223Ra and 224Ra. J. Geophys. Res. 2000, 105,

3 B. Supplemental Figures Figure S Cs vs salinity in both groundwater and at Bikini (blue circles) and Enewetak (open triangles). Errors on 137 Cs are smaller than the symbol size.

4 Figure S2. Time series sampling off Bikini Island of 137 Cs and 223 Ra and corresponding tidal height

5 Figure S3. Distribution maps of radium derived ages (days) for Bikini and Enewetak lagoon waters

6 Table S1. Complete water data PART 1 (Bq/m 3 ) (Bq/m 3 ) (Bq/m 3 ) (Bq/m 3 ) updated July 13, 2017 coll date coll time decimal degrees Diss. O2 137 Cs Tritium* SigTritium 224 Raxs error 223 Raxs error 226 Ra error 228 Ra error B# collection type (local) (local) depth (m) Lat N Lon E Temp Salinity (% sat) Bq/m 3 +/ T.U. Bq/m 3 +/ Bq/m 3 +/ Bq/m 3 +/ Bq/m 3 +/ Bikini B200 B barrel 18 Jan 15 10: B204 A barrel 18 Jan 15 16: B210 barrel time series 19 Jan 15 1: B211 barrel time series 19 Jan 15 3: B208 barrel time series 19 Jan 15 10: B209 barrel time series 19 Jan 15 11: B213 barrel 20 Jan 15 8: B214 barrel 20 Jan 15 9: B215 barrel 20 Jan 15 10: B222 barrel 20 Jan 15 17: B221 CTD 20 Jan 15 17: B220 CTD 20 Jan 15 17: B219 CTD 20 Jan 15 17: B218 CTD 20 Jan 15 17: B510 surf zone 19 Jan 15 18: groundwater samples B501 well 17 Jan 15 18: bd B502 well 17 Jan 15 19: B503 well 17 Jan 15 20: B504 well 17 Jan 15 21: bd B505 cistern 17 Jan 15 23: B507 well 18 Jan 15 21: bd B508 well 18 Jan 15 22: bd B506 beach push point 18 Jan 15 18: B511 beach push point 19 Jan 15 20: B512 beach push point 19 Jan 15 21: bd B513 beach push point 19 Jan 15 22: B514 beach push point 19 Jan 15 23: bd Bikini to Enewetak B223 barrel 21 Jan 15 9: bd B224 barrel 21 Jan 15 17: bd B225 barrel 22 Jan 15 6: Enewetak B228 barrel time series 23 Jan 15 10: B229 barrel time series 23 Jan 15 12: bd B230 barrel time series 23 Jan 15 14: B231 barrel time series 23 Jan 15 16: B205 cubitainer 22 Jan 15 10: B206 cubitainer 22 Jan 15 11: B207 cubitainer 22 Jan 15 12: B226 barrel 22 Jan 15 15: B227 barrel 22 Jan 15 18: B232 cubitainer 23 Jan 15 12: B233 cubitainer 23 Jan 15 12: B236 barrel 24 Jan 15 8: B237 barrel 24 Jan 15 9: B244 barrel 24 Jan 15 13: B245 CTD 24 Jan 15 13: B246 CTD 24 Jan 15 13: B247 CTD 24 Jan 15 13: B527 surf zone 23 Jan 15 19: bd groundwater samples B515 cistern 21 Jan 15 16: B516 cistern 21 Jan 15 17: bd bd bd B517 cistern 21 Jan 15 18: bd B518 cistern 21 Jan 15 19: bd B523 well 22 Jan 15 18: bd bd B525 well 22 Jan 15 21: bd B522 beach push point 22 Jan 15 17: bd B524 beach push point 22 Jan 15 20: bd B528 well 23 Jan 15 21: bd B526 beach push point 23 Jan 15 18: bd Outside Marshalls Guam to Majuro B139 CTD 1 Jan 15 15: B140 CTD 1 Jan 15 15: B141 CTD 1 Jan 15 15: B142 CTD 1 Jan 15 15: B143 CTD 1 Jan 15 15: B144 CTD 1 Jan 15 15: B109 CTD 1 Jan 15 13: bd B110 CTD 1 Jan 15 13: bd B111 CTD 1 Jan 15 13: B112 CTD 1 Jan 15 13: bd B113 CTD 1 Jan 15 13: bd B114 CTD 1 Jan 15 13: bd CTD samples use T & S from sensors Barrels and GW use salinity from Salt bottles or YSI sensors, and YSI sensor if temp noted All 241 Pu decay corrected to sampling date All Pu data from TIMS

7 Table S1. Complete w PART 2 updated July 13, Pu 240 Pu 239,240 Pu 241 Pu 240 Pu/ 239 Pu 241 Pu/ 239 Pu 241 Pu/ 239,240 Pu 239,240 Pu/ 137 Cs B# mbq/m 3 +/ mbq/m 3 +/ mbq/m 3 +/ mbq/m 3 +/ atom +/ atom +/ Bq/Bq +/ Bq/Bq +/ Bikini B200 B B204 A B210 B211 B B B B214 B B B221 B B219 B218 B groundwater samples B501 B502 B503 a B504 b B505 c B507 B508 B B511 B512 B B514 Bikini to Enewetak B223 B bd B225 d Enewetak B B229 B B B205 B206 B207 B226 B227 e B232 B233 B236 f B237 B B bd B246 B B groundwater samples B515 B516 g B bd B518 B523 B B B bd B528 h B526 Outside Marshalls Guam to Majuro B139 B bd B bd B bd B B144 B109 B110 i bd B111 j bd B112 B113 B114 CTD samples use T & S from sensors Barrels and GW use salinity from Salt bottles or YSI sensors, and YSI sensor if temp noted All 241 Pu decay corrected to sampling date All Pu data from TIMS a No Cs collected b No Cs collected c No Pu activity data d 239 Pu error too large for 241/239 ratio e No Pu activity data f No Pu activity data g No Cs collected h No Pu activity data i Cs = 0 agrees with tritium j Cs value is suspect, closer to zero

8 Table S2. Complete sediment core data Core Avg Depth 137 Cs 137 Cs 241 Am 241 Am 207 Bi avg 239 Pu 240 Pu 239 Pu+ 240 Pu 240/ Pu 238/239,240 Pu/Cs area cm Bq/kg +/ Bq/kg +/ Bq/kg +/ Bq/kg +/ Bq/kg +/ Bq/kg +/ atom +/ Bq/kg +/ activity activity Bikini shore BD BD BD 20.5 BD 71 5 BD Bravo crater BD BD Runit Shore BD BD 4.5 BD BD BD BD BD BD Runit Shore Mike crater BD BD BD BD BD BD Pu, 240 Pu, 241 Pu by ICPMS 238 Pu by alpha 241 Pu decay corrected to June 2015 sampling 137 Cs from 661 kev; 241 Am from 59.6 kev and 207 Bi is avg. from 569 & 1063 kev

9 Table S3. Summary of Pu and Cs fluxes into and out of the lagoon Bikini Enewetak Pu Cs Pu Cs method Bq/d Bq/d Bq/d Bq/d lagoon residence times a 6.6E E E E+09 SGD b 1.6E E E E+04 a from Ra residence times lagoon b From Ra in SGD calc