Environ. Sci. Technol. 2001, 35, 4470-4476
129
I and 127I Transport in the Mississippi River
S . D . O K T A Y , †,‡ P . H . S A N T S C H I , * ,† J. E. MORAN,§ AND PANKAJ SHARMA# Department of Oceanography, Texas A&M University, 5007 Ave U, Galveston, Texas 77551, Lawrence Livermore National Laboratory, 231, L-P.O. Box 808, Livermore, California 94550, and PRIME Lab, Purdue University, 1396 Physics Building, West Lafayette, Indiana 47907
The watershed processes which control 129I/127I ratios, 129I and 127I concentrations, and speciation of iodine isotopes were studied through an investigation into the variability of these parameters in the Mississippi River near New Orleans, undertaken in 1996-1998. Analyses of suspended particulate matter (SPM) revealed a greater percent association of 127I than of 129I, resulting in lower 129I/127I ratios in SPM than in surrounding water. Furthermore, crossflow ultrafiltration showed that organo-iodine was the dominant form for both isotopes, with 70-85% of these isotopes found in the 0.45 µm filter-passing fraction associated with colloidal macromolecular organic matter. 129I showed a weak correlation, 127I no correlation, and 129I/127I ratios a strong inverse correlation with river flow rate. Inverse correlations between 129I/127I ratios and river flow rates can be best explained by rainwater and evapotranspiration dominated ratios at base flow and a lowering of the isotope ratios during higher flow due to extra inputs of 127I from soil weathering. We postulate that different equilibration times for 127I and 129I as well as for bombproduced 129I and reprocessing-produced 129I are responsible for these fractionation effects and the differential mobilities of these isotopes in the Mississippi River watershed.
Introduction Iodine is a biophilic element, occurring in the environment primarily as 127I (natural abundance nearly 100%) with several radioisotopes of which the only long-lived isotope is 129I (halflife ) 15.6 × 106 years). 129I is produced by cosmic ray-induced spallation of Xe in the atmosphere, by spontaneous fission of 238U in the earth’s crust, and by human activities, such as nuclear bomb testing and nuclear fuel reprocessing (1). The amount of natural 129I in the surface environment is approximately 100 kg (2-4), while atmospheric bomb testing has contributed an additional 150 kg, equivalent to about 27.3 Ci (5), and the Chernobyl reactor accident 1.3 kg (6). The dominant sources of 129I in recent years are fuel reprocessing plants located at Cap de La Hague, France, and Sellafield (formerly Windscale), England. These plants have contributed about 2360 kg or 420 Ci of 129I between 1966 and * Corresponding author phone: (409)740-4476; fax: (409)740-4786; e-mail:
[email protected]. † Texas A&M University. ‡ Present address: University of Massachusetts-Boston, Environmental, Coastal and Ocean Sciences Department, 100 Morrissey Blvd., Boston, MA 02125. § Lawrence Livermore National Laboratory. # Purdue University. 4470
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1997 (4). Their atmospheric releases have increased in the past decade, providing the largest point source for 129I in the surface environment. These releases have completely overwhelmed the natural background ratios of 129I/127I. Wagner et al. (7) documented these combined releases of 129I since the 1950s in an alpine ice core, which shows increasing 129I concentrations from the 1970s to the present, in contrast to most other bomb fallout nuclides, which show a peak in 1963. Even relatively small amounts of atmospherically delivered 129 I transported from the Sellafield/La Hague region can drastically change 129I/127I ratios in surface waters, soils, and biota in the U.S. (8, 9). Elevated 129I/127I ratios in surface environments of the U.S. have recently been reported (814). Moran et al. (8, 9) found higher 129I/127I ratios in meteoric water and epiphytes in the continental U.S. as compared to coastal seawater, indicating that atmospheric transport is an important distributor for surface 129I. At present, there are very little 129I data from freshwater and coastal environments. Many measurements that do exist come from areas around known anthropogenic sources (e.g., refs 14 and 15). In these cases, the study goals were to determine the dose of 129I to humans after accidental releases such as Chernobyl or to investigate the transport of 129I after releases from fuel reprocessing plants. Most of the research conducted by Moran et al. (8, 9, 12), and the results presented here, focus on determining 129I levels in large-scale reservoirs away from point sources to evaluate global or regional transport. Previously reported 129I concentrations in Mississippi River water were 4 × 107 atoms/L (10, 11). However, it was not known whether this value is representative. Furthermore, this value was deemed relatively high for a river without a reprocessing plant in its drainage basin and was also higher than the 129I concentration in Gulf of Mexico surface waters of 1.8 × 107 atoms/L. Iodine in rivers can come from two primary sources: oceanic iodine delivered atmospherically to river basins and watersheds and iodine weathered from soils and rocks. The Mississippi River integrates atmospheric fallout from 40% of the surface area of the contiguous U.S.A. and delivers approximately 50% of the total freshwater runoff from the continental U.S. to the Gulf of Mexico (16). Therefore, the Mississippi River should contain iodine from both sources. As part of detailed survey of the major rivers of the U.S.A. (17), we present here a 2-year survey of 129I and 127I in the Mississippi River undertaken to determine the seasonal processes that control the observed variations in 129I and 127I concentrations, such as evapotranspiration and soil weathering rates in drainage basins, and iodine speciation. The biophilic nature of iodine makes it likely that some fraction exists in a form bound to organic matter. Benner et al. (18) isolated colloidal material in the Mississippi River and nearby open Gulf waters and documented the organic nature of colloids in Mississippi River water. Benner et al. (18) further documented that the average molecular size of riverine dissolved organic matter (DOM) recovered by tangentialflow ultrafiltration is considerably larger than for marine DOM. Current knowledge of DOM cycling in the Mississippi River estuary and Gulf of Mexico is summarized in Guo et al. (19). Warwick et al. (20) suggested that strong, covalent carboniodine bonds produced by electrophilic substitutions into phenolic moieties efficiently bind iodine onto humic material. River humic substances are categorized as having mostly aromatic carbon compounds (21) with a carbon concentration of 40-50% (up to 65%) and generally lower concentra10.1021/es0109444 CCC: $20.00
2001 American Chemical Society Published on Web 10/16/2001
FIGURE 1. Sampling site on Mississippi River (Audubon Park, New Orleans). Discharge data provided by the US Army Corps of Engineers in New Orleans (USACE - NO) from Tarbert Landing, MS; 460 km upstream of the river mouth, but below the point of any significant additional input. tions of hydrogen and nitrogen. The composition of DOC in a typical river with a DOC concentration of 5 mg/L is 40% fulvic, 30% hydrophilic, and 10% humic acids (21). To determine the importance of organically associated iodine, the fractions of 129I and 127I associated with colloids were determined. Colloids are microparticles and macromolecules in the size range of 1 nm to 1 µm and are composed mostly of organic components in surface environments (19, 22-28). Colloids contain both phytoplankton-derived and soil-derived biomarkers (29-31). Any iodine found associated with colloids is thus likely associated with colloidal macromolecular organic matter.
Methodology Water Samples. Samples were collected and analyzed for 129I and 127I at approximately bimonthly intervals in the Mississippi River at Audubon Park in New Orleans (Figure 1). Discharge information was provided by the Army Corps of Engineers (USACE)-New Orleans district gauged at Tarbert Landing, MI (USGS site location 07295100) approximately 460 km upstream of the river mouth but below the point of any significant inputs (USACE, John Miller, personal communication). The sum of the discharge at Tarbert Landing and the discharge diverted to the Atchafalaya River represents the total input of the Mississippi River to the Gulf of Mexico. Daily discharge rates were available for the entire 19941998 period studied. Both filtered and unfiltered water samples were collected and analyzed to estimate the amount of iodine associated with riverine particulate matter. A 0.45 µm pore size Nuclepore filter cartridge was used for filtration. A 20 L, 18.0 Mohm ultrapure water sample was also filtered through the cartridge and processed to establish “field” sampling blanks. Approximately 20 L of unfiltered and filtered water were collected each time, except for the August 1997 sampling trip when only unfiltered samples were collected. These samples were split into two 8-10 L samples. Aliquots of the unfiltered water were obtained for suspended matter determination, while aliquots of both filtered and unfiltered water were collected for 127I measurement by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Iodine extraction and measurement of river water samples were as follows. Before the rotary distillation preconcentration step, 10 mL of a 10 mM NaHSO3 solution was added for each liter processed to reduce any IO3- present to I-. Samples
were then concentrated to a volume of about 500 mL via rotary vacuum distillation. ICP-MS measurement of 127I on aliquots of concentrated samples showed that losses during rotary evaporation were 5-20%. Low 129I (129I/127I ) 80 × 10-15) carrier iodine (2-4 mg) was then added to the samples, which were subsequently acidified to a pH of 1.5-2.0 with concentrated HNO3, and 5-15 mL of CCl4 was introduced as the extraction medium. The I- was oxidized with approximately 5 mL of H2O2 to I2, which is soluble in the CCl4. H2O2 also was used to help oxidize some of the dissolved organic matter. After multiple CCl4 extractions, 20-30 mL of a 1 M NH4OH-HCl solution was added to reduce any IO3- present; this portion was also extracted into CCl4. The CCl4 extracts were combined, and the iodine was back-extracted into the aqueous phase using a 1 M NaHSO3/H2SO4 acid solution. Two milligrams of Cl(as aqueous NaCl) as well as 2 mg of Ag was added, and AgI and AgCl were coprecipitated using AgNO3. The AgCl was dissolved using NH4OH, and the remaining AgI was centrifuged, rinsed with deionized water, and dried. The finished target sample was typically a AgI pellet weighing 2-5 mg which was sent to the accelerator mass spectrometer (AMS) facility at the PRIME Lab of Purdue University for 129I/127I determination (32). Reagent blanks for 129I/127I were established by CCl4 extraction of the carrier material alone. Blanks for 129I/127I from rotary vacuum distillation, centrifugation, transport, and storage were also measured and found to be within acceptable ranges. 129I/127I in blanks averaged about 0.1-0.3 × 10-12 and were subtracted when appropriate. The detection limit for AMS is approximately 5 × 10-15 for 129I/127I (32). All of the raw 129I/127I ratios determined for river and colloidal samples were >500 × 10-15, significantly above the detection limit; therefore, errors due to counting statistics for the AMS detector are relatively small, with one sigma errors less than 10%. Ultrafiltration. Mississippi River water collected at the Audubon Park, New Orleans site in June of 1997 was processed using cross-flow ultrafiltration (33, 34) to determine 129I and 127I concentrations in five operationally defined categories: (1) unfiltered water, (2) 0.45 µm filter-passing fraction, (3) particulate (>0.45 µm), (4) retentate or colloidal (1 nm-0.45 µm), and (5) permeate (e 1 nm). A 1 nm pore size membrane rejects about 50% of molecules with 1 kDa molecular weight and, thus, has a 1 kDa molecular weight cutoff; such a membrane retains the largest fraction of colloids, including the lowest size range of colloidal macromolecular organic matter (34). Using a Masterflex peristaltic pump, 60 L of river water was pumped through an acid-cleaned 0.45 µm Nuclepore filter cartridge and subsequently collected in acid-cleaned and sample-rinsed polycarbonate 20 L carboys at the Audubon Park site (Figure 1) on June 23, 1997. Fifty-seven of the 60 L of unfiltered water that was brought back to Galveston was used for 0.45 µm filtration and cross-flow ultrafiltration using an Amicon DC10 fitted with two 1 kDa cutoff cartridges (Amicon, S10N1). Ultrafiltration cartridges were thoroughly sequentially cleaned prior to use with laboratory detergent (Micro), NaOH, HCl, and other cleaning solutions as recommended by the manufacturer. In addition, large volumes of high purity (18 Mohm) deionized water were used to flush the system, and then a small amount of prefiltered river water was pumped through the system and discarded to condition the cartridges. A more complete description of the cross-flow ultrafiltration procedure is given in refs 24, 33, 34. The concentration factor, i.e., the ratio of the initial volume to the retentate (containing the colloids) volume, was equal to 19.3 in this experiment. This concentration factor does not include the rotary evaporation concentration factor. VOL. 35, NO. 22, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Iodine and Suspended Particulate Matter (SPM) Data from the New Orleans Sampling Site and Water Discharge (Flow Rate) Data for the Mississippi River at Tarbert Landing, MSa date and sample IDb
Julian date
monthly flow rate (m3/s)
daily discharge (m3/s)
May/4/96 Jun/15/96F1 Jun/15/96F2 Jun/15/96U Nov/20/96U Jan/16/97F Jan/16/97U Feb/9/97U Mar/20/97F Mar/20/97U Apr/29/97U1 Apr/29/97F Jun/23/97U1c Jun/23/97U2c Jun/23/97F1c Jun/23/97F2c Au/11/97U#1 Au/11/97U#1 Sep/16/97F#2 Sep/16/97F#1 Sep/16/97U Apr/10/98F#1 Apr/10/98F#2
125 167 167 167 325 016 016 040 079 079 119 119 174 174 174 174 223 223 259 259 259 100 100
24300 27800 27800 27800 14600 17700 17700 19800 35300 35300 32700 32700 21300 21300 21300 21300 8260 8260 6540 6540 6540 26200 26200
20900 28200 28200 28200 17700 17500 17500 22600 40000 40000 26500 26500 23200 23200 23200 23200 7920 7920 5660 5660 5660 27300 27300
a
SD ) standard deviation.
TABLE 2. 129I/127I Ratios and River Water Samplesa
b
SPM, (mg/L)
1 SDa
115.5 115.5 115.5 112.7
4.5 4.5 4.5 3.2
125.7 185.2
5 29.8
70.4 70.4 76.2 76.2 28.8
0.85 0.85 30 30 5.35
127I (µg/L)
1 SDa
129I/127I
(10-9)
1 SDa
129I (107 atoms/L)
1 SDa
5.6 5.8 5.8 5.9 16.9 3.6 3.8 11.0 12.4 12.2 4.2 3.7 2.5 2.5 2.2 2.2 5.7 5.7 6.8 6.8 6.8 4.27 4.27
0.35 0.30 0.30 0.5 0.75 0.07 0.05 0.83 0.70 0.74 0.3 0.07 0.3 0.3 0.02 0.02 0.12 0.12 0.07 0.07 0.04 0.49 0.49
2.2 1.71 1.58 1.44 4.22 5.12 3.40 1.78 1.13 1.18 1.48 2.01 3.98 4.82 4.78 4.86 3.17 3.44 4.36 5.19 4.18 1.88 1.83
0.34 0.12 0.11 0.11 0.27 0.24 0.13 0.17 0.09 0.10 0.11 0.07 0.49 0.61 0.13 0.11 0.13 0.13 0.13 0.17 0.13 0.22 0.22
5.8 4.7 4.4 4.0 33.8 8.7 6.1 9.3 6.7 6.9 3.0 3.5 4.7 5.7 5.0 5.1 8.6 9.3 14.1 16.7 13.5 3.8 3.7
0.9 0.32 0.30 0.32 2.2 0.2 0.23 0.91 0.52 0.59 0.22 0.12 0.58 0.72 0.13 0.12 0.36 0.35 0.41 0.54 0.41 0.41 0.44
U ) unfiltered water sample; F ) 0.45 µm filter-passing fraction. c Ultrafiltration samples.
127I
and
129I
Concentrations in Replicated 0.45 µm Filtered Water Samples from the Mississippi
sample I.D.
category
127I (µg/L)
1 SDc (µg/L)
129I/127I (10-9)
1 SDc (10-9)
129I (107 atoms/L)
1 SDc (107 atoms/L)
0.45 µm filtered #1 0.45 µm filtered #2 unfiltered #1 unfiltered #2 permeate ) 1 kDa retentate ) 1 kDab particulate ) 0.45 µm
2 2 1 1 5 4 3
2.2 2.2 2.5 2.5 0.5 1.6 0.53
0.3 0.3 0.4 0.4 0.1 0.3 0.05
4.8 4.9 4.0 4.8 3.2 2.0 1.8
0.2 0.7 0.5 0.6 0.15 0.1 0.30
5.0 5.0 5.1 6.2 0.75 1.5 0.44
0.25 0.2 0.7 0.8 0.16 0.1 0.07
a One of the samples (#) was further cross-flow ultrafiltered through a membrane with a 1 kDa nominal molecular weight cutoff. Particles were recovered and analyzed separately. All samples were collected on June 23, 1997 at Audubon Park, New Orleans. b Sample was probably affected by lowered recovery of 129I. Therefore, 129I in that sample was calculated by difference. This would increase the 129I concentration in that sample to 4.25 × 107 atoms/L and the 129I/127I ratio to (5.6 ( 0.6) × 10-9. c SD ) standard deviation.
Typically, concentration factors above 10 are adequate for representative and thorough preconcentration of colloids (33, 34). A 5 mL subsample was taken from the retentate for ICP-MS analysis after the ultrafiltration process was complete. The well-mixed retentate and permeate were further concentrated to 350 and 325 mL, respectively, using rotary evaporation distillation, resulting in overall concentration factors of about 165 for both fractions. This high concentration factor resulted in a substantial concentration of natural organic matter in the retentate, which could have had an adverse effect on extraction efficiencies due to the formation of a waxy layer on the interface of the aqueous/organic phases. It could have prevented the extraction and/or back extraction of some of the iodine species. However, for “normal”, ambient samples, such as river, rain, and sediments, the organic matter concentration was low enough to minimize this extraction problem. All fractions were then processed for 129I in the same manner mentioned above. Samples of the back extract were also taken for 127I measurement on the ICP-MS to establish extraction efficiency of the carrier and sample. As will be detailed below, extraction efficiencies for organic-rich colloidal material were significantly lower than for ambient river water, rainwater, and sediment samples. For most of the samples analyzed by refs 8-13 and 35, the amount of 127I remaining after back 4472
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extraction was less than 5%, with typical residual 127I concentrations close to ICP-MS detection limits of 0.2-0.5 µg/L.
Results and Discussion Association of 127I and 129I with Colloidal Macromolecular Organic Matter and Particles. Table 1 shows the results of iodine and 129I determinations as well as ancillary data from the 2-year seasonal study of the Mississippi River. Table 2 gives the results of the 127I and 129I determinations of size fractionated water samples, including particles and colloids, sampled on June 23, 1997. Mass balance calculations for 127I listed in Table 2 indicate a 93 ( 8% recovery, with only 7% lost during ultrafiltration and distillation of the samples. Seventy-two percent of the total 127I was found in the retentate or colloidal fraction, while 21% was in the permeate fraction. The 2σ standard deviation of stable iodine measurements was typically 10%, so this apparent loss is within the error of the measurements. This remarkably high concentration of colloidal iodine associated with macromolecular organic matter represents an unprecedented enrichment of iodine in natural organic matter. If we assume that colloidal organic carbon (COC) concentration is about 300 µM OC/L (18, 19), equivalent to
about 3.6 mg DOC/L, and 72% of DOC is made of COC, with colloidal organic matter (COM) to COC ratio of 2.5 (e.g., refs 19 and 22), this results in about 6.5 mg of COM/L. In this water sample, there was 1.6 µg/L of colloidal 127I (Table 2) and 0.5 µg/L of 127I in the permeate, which was likely composed of primarily inorganic iodine. The 127I concentration in COM was therefore about 246 mg/kg OM, and the enrichment factor, EF, of 127I in COM was 5 × 105 L/kg {EF ) (1.6 µg/L)/((0.5 µg/L)(6.5 × 10-6 kg OM/L)). This enrichment is surprisingly high, i.e., 1-3 orders of magnitude higher than for marine organic matter (36). 129 I enrichment is equally high, but the calculation is more tenuous. The combined 129I concentrations (2.3 × 107 atoms/ L) in the permeate and the retentate (Table 2) equal only 46% of the average 129I concentration (5 × 107 atoms/L) in the 1 µm filtered and 0.45 µm filtered river water. Therefore, only one-half of the expected 129I concentration could be accounted for. This apparent 129I loss is likely due to difficulties of 129I extraction and back-extraction of concentrated organic matter rich phases, as mentioned before. Due to these 129I losses, 129I associated with macromolecular organic matter was estimated from the permeate and the 0.45 µm filter-passing fraction (an approach used when the colloidal fraction is compromised; e.g., refs 37 and 38), as follows: The 129I in the retentate fraction (Rc) was estimated to be equal to the 129I concentration in the 0.45 µm filterpassing fraction minus the 129I concentration in the permeate. Thus, Rc ) (5 ( 0.5)-(0.75 ( 0.16) × 107 atoms 129I/L) (4.25 ( 0.24) × 107 atoms 129I/L. The 129I in the colloidal (1 kDa0.45 µm) fraction would then be equal to 85% of the 0.45 µm filter-passing fraction. Thus, 129I was also strongly associated with colloidal macromolecular organic matter, with an EF value similar to that of 127I (i.e., 8.7 × 105 for 129I vs 5 × 105 L/kg for 127I). This suggests that the fate and transport of 127I and 129I in the riverine environment is closely linked to that of organic carbon (in all likelihood through associations with humic substances). 129I/127I ratios and stable iodine concentrations in filtered versus unfiltered river water differed by only 10%. Concentrations of 129I in filter-passing fractions were usually within 10% of the unfiltered sample values (Table 2). To get a more precise estimate of the particulate 129I concentration, suspended matter was also filtered through a 0.45 µm Nuclepore filter and recovered by back flushing (reverse-flow) the cartridge with deionized 18.0 Mohm water. When this particulate fraction was analyzed for stable iodine (Table 2), the 127I concentration was approximately 20% of the unfiltered water value, i.e., 0.53 ( 0.05 µg/L for particulate iodine versus 2.5 ( 0.4 µg/L 127I for the unfiltered water. The 129I/127I ratio for the backflushed particles was significantly lower than for 0.45 µm filter-passing fraction, but when the 129I concentrations are compared (Table 2), the backflushed particles comprise only 9% of the total 129I concentration. The proportion of 129I in the isolated particles (9%) is thus within the difference between unfiltered and filtered river water (10%). Iodine and 129I concentrations in suspended particulate matter (SPM) can be calculated from the data given in Tables 1 and 2 and using a SPM concentration of 70 mg/L to equal 7.3 µg/g of 127I and 6.3 × 107 at/g of 129I, respectively. These concentrations are significantly different from those measured in surface sediments of the Mississippi River delta (35), which were 11.5 µg/g and 8.5 × 106 at/g, respectively. Obviously, concentrations in fine suspended particulate matter are not a good predictor for larger particles, which more easily settle out in the river delta region. The filtration and ultrafiltration methods used here should be applied in other areas in order to investigate the behavior of particulate versus colloidal and dissolved forms of the iodine isotopes in different aquatic environments. However,
FIGURE 2. 129I in the Mississippi River at Audubon Park, New Orleans vs flow rate, i.e., vs daily discharge of Mississippi River measured at Tarbert Landing, MS, with comparison to average rainwater values from ref 8. the lower 129I/127I ratio observed in riverine suspended particulate matter versus the ratio found in filter-passing water suggests a depletion of 129I in riverine particles and sediments compared to the “dissolved” ()0.45 µm) phase. This observation agrees well with the lower 129I/127I ratios in recently deposited surface sediments from the Mississippi River delta region compared to the ratios measured in the river water from which they originated (35). River Water. Table 1 displays the 127I and 129I data, along with suspended particulate matter (SPM) concentrations and Mississippi River discharge rates for each of the sampling dates. Multiple listings per date originate from the inclusion of duplicate samples and the collection of unfiltered samples and 0.45 µm filter-passing samples. Figure 2 shows that the concentration of 129I is weakly and inversely correlated to river flow (p ) 0.02). We do not have a good explanation for the Nov. 1996 value, which shows anomalously high 129I concentrations at average flow rate, and which is included in the correlation, except that it occurred in the fall, at maximum evapotranspiration rates (see below). However, stable 127I concentrations and river flow rates are not significantly correlated (p ) 0.50, not shown), suggesting that sources and erosional behavior of 127I are probably different from those of 129I. While 129I concentration values were consistently high in the fall when the river is at base flow, values were considerably lower in the spring when flow rates were high (Figure 2). The same is not true for 127I, as 127I values varied erratically with season (Table 2). While 127I can also be enriched in the river water through weathering reactions and, thus, has a source other than meteoric water, the same is not true for 129I. It has only one major source, atmospheric precipitation through wet deposition. Dry deposition of iodine would add at most 20% to the total flux (39) during dew and fog conditions. In Mediterranean climates, dry deposition of 129I was 2 orders of magnitude lower than wet deposition (40). Soil leachates of 129I from bomb fallout are likely lower in concentration. This is deduced from the concentrations of bomb-produced 129I in Mississippi River delta sediments (35), which are rather low, as compared to particulate 129I concentrations in the river water. In addition, the inverse correlation of 129I with flow rate that we observe would argue against such soil leaching effects to be important. Such 129I leaching would be expected to be highest at high runoff, which is opposite from what is observed. 129I should be equivalent to 129I concentrations in rainwater, in the absence of any soil sorption, release, and/or evaporation effects. Sorption during interflow, if present, would tend to decrease the concentration of iodine as well as 129I. 129I concentrations in all Mississippi River water samples were either equal to or higher than those in rainwater. This requires either a concentration effect, or additional VOL. 35, NO. 22, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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sources, but does not permit significant removal of 129I due to soil uptake during interflow and bank storage of nearsurface groundwater. Additional anthropogenic sources of 129I can be excluded in the Mississippi River drainage basin. Low soil uptake of 129I is not unreasonable for samples from near the river mouth in a large river drainage basin such as that of the Mississippi River. The Mississippi drains more than 40% of the continental U.S.A., thus integrating different climatic regimes. Therefore, there will frequently be rainfall events somewhere, even during base flow, leading to the dominance of surface runoff over groundwater exfiltration. Groundwater making up a major fraction of streamflow (at base flow) is documented only for smaller watersheds (e.g., refs 41-43). In the absence of significant sorption, evapotranspiration (ET), which is highest during the summer and fall months, concentrates rainwater constituents in soils. Relatively large amounts of surface water are used for irrigation in agriculture in the Mississippi basin, and ET is increased when river water is used for irrigation purposes. Since 129I concentrations are all equal or higher than in rainwater, ET can thus account for the higher 129I concentrations during times of average and low flow rates (Figure 2). The same concentration factor caused by ET also has to apply to 127I, and any excess over that expected from ET can be attributed to soil leaching. This should thus allow estimation of the contribution of 127I from soil weathering. The database for 129I in rainwater for the continental U.S.A. is quite limited, consisting of 14 values measured in three locations (8). Furthermore, the large sample size requirement (approximately 4 L) may result in bias toward large rain events and relatively low 129I concentrations. A more thorough survey of 129I in precipitation in locations distant from nuclear fuel reprocessing facilities is needed in order to definitively assess the processes that are at work in the complex soil-river system. Ancillary data, such as chloride or stable isotopes of the water molecule, would help corroborate interpretations based on 129I. The following analysis, however, is based on the rainwater data at hand and suggests a plausible mechanism and way of quantifying 129I concentration factors in Mississippi River water. Measured rainwater concentrations of 129I and 127I over the continental U.S.A. (8) are in fact quite homogeneous and vary only over 1 order of magnitude, and 12 out of 14 values vary by just a factor of 5, with no obvious seasonal trend. Spatial variations in the atmospheric flux of 129I and 127I are integrated over the entire watershed, and temporal variations in rainwater concentrations of 129I, caused by reprocessing releases from La Hague, France over the past decade, are thus relatively small (44, 45). This is in contrast to rainwater concentrations of 129I in Europe (46, 47), which are orders of magnitude higher, and can vary over 2-3 orders of magnitude, depending on air circulation patterns and trajectories from the nuclear reprocessing sources. Furthermore, 129I in rainwater from the Mississippi River drainage basin (8) were close to the average values over the U.S.A. Assuming average rainwater concentrations over the continental U.S.A. for 127I of 2.5 ( 0.6 µg/L, and for 129I of 2.7 ( 0.4 × 107 atoms/L (8), and similar evapotranspirative enrichment for 129I and 127I in river water, the original (evaporation corrected) 127I concentration can be expressed as
ET ) 129I(river)/129I(rain)
(1a)
[127I(weathering)] ) [127I(river)]/ET - [127I(rain)] (1b) [127I(weathering)] ) [127I(river)] - [127I(rain)] × ET (1c) The difference between eq 1b and 1c is the time sequence of concentration by ET and removal or addition of 127I in 4474
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FIGURE 3. Relationship between the evaporation corrected, weathering-related 127I and the river flow rate. Negative values means removal to soils.
FIGURE 4. Inverse relationship between discharge.
129 127
I/ I ratio and river
soils. If it is assumed that the rainwater composition is first altered by sources and sinks and then concentrated by ET, eq 1b results. In eq 1c, on the other hand, it is assumed that rainwater is first evaporated, and then weathering products are added to or subtracted from the water. Consequently, [127I(weathering)] is calculated in rainwater equivalents from eq 1b and in river water equivalents from eq 1c. ET, calculated from 129I river and average rainwater data (as 129I(river)/129I(rain)), varies by factors of 2-12. The average ET value of 3 ( 0.5 is close to the value of 3.8, expected from mass balance considerations (the ratio of average rainfall in the drainage basin (0.75 m/yr × 3 × 1012 m2) to average river discharge rate (6 × 1011 m3/yr)). Equation 1a predicts that because river water concentrations of 129I are inversely related to river flow rate, so are ET factors. Flow rate here is a proxy for mean temperature and seasonality in the drainage basin, which are inversely related to each other. Using the above relationships (eqs 1b and 1c), the weathering-related, evaporation-corrected 127I concentration has a negative sign (soil removal) at base flow and a positive sign at higher flow rates (soil leaching) and exhibits a significant correlation with flow rate (Figure 3). When comparing this 127I concentration to the average rainwater concentration of 2.5 ( 0.6 µg/L from Moran et al. (8), it appears that for flow rates higher than base flow, significant amounts of soil erosion-derived 127I (i.e., 127I(weathering), eqs 1b and 1c) are added to the river water, in proportion to the flow rate. Of course, removal of iodine and 129I in soils makes our estimate of ET a minimum estimate. Most importantly, eqs 1b and 1c not only determine and constrain the magnitude of the ET factor but also the magnitude of the sum of additional source and sink terms. Interestingly, 129I/127I ratios are significantly (p < 0.001) and inversely correlated to river flow (Figure 4), suggesting that materials containing different 129I/127I ratios are present in river water at different flow rates. The significant correlation
between river flow rates and the 129I/127I ratios may be caused by the following two main processes: During base flow, 129I and 127I are being similarly concentrated by ET, with only small amounts of 127I removed to soils, as 129I/127I ratios are close to the average ratio in rainwater (3.8 ( 1 × 10-9 calculated from individual ratios, 2.4 ( 0.65 × 10-9 from individual average concentrations of 129I and 127I (8)). During times of higher flow rates (i.e., spring time), extra inputs of 127 I from soil weathering reactions and sediment resuspension cause a decrease in the 129I/127I ratio. As would be expected from a rainwater source with a relatively narrow concentration range, daily 129I fluxes are quite constant (19 of the 23 values are within a factor of 2), leading to an inverse correlation between 129I concentrations and flow rate. The relatively constant 129I flux is consistent with the narrow range of 129I concentrations observed in rainwater, its main source. In addition, it is consistent with the fact that the atmospheric reprocessing emissions of 129I in Europe have been relatively constant over the sampling period (44, 45). Using the inverse correlation of 129I concentrations and the daily flow rates for 1997, an annual flux (F) of 129I can be estimated from the daily flow rates (Qi), as
F)
∑Fi, with Fi ) (a + bQi)Qi, i ) 1 to 365, and a,b )
intercept and slope (2)
A flux of 15 g per year is obtained for this major river without any nuclear reprocessing plants in its drainage basin. This flux of 15 g/yr can be compared with an estimated flux from a river that contains reprocessing effluents, i.e., the Ob river (Russia), for which a 129I flux of 235 g/yr was previously reported (48, 49). Atmospherically delivered iodine is the major terrestrial environmental source for both stable 127I (from sea spray) and 129I (from reprocessing plant emissions). Other sources of 127I include iodine weathered from soils, iodine volatilized from plants, and the degradation of organic matter. As anthropogenic (bomb and reprocessing produced) 129I spreads throughout the surface environment, organic matter acquires a 129I/127I ratio, which, over long periods of time, approaches that of the main source materials. Terrestrial organic matter exhibits a higher ratio of 129I/127I than marine organic matter with the same atmospheric input due to shallow penetration depths (i.e., ∼8 cm (14); and 10 cm (49); vs 500 m (11)) and lower 127I concentrations. The transport and reactions of these iodine isotopes thus not only depend on the chemical forms of iodine and nature of the carbon with which it is associated, i.e., whether the carbon is refractory or labile, but also on the time for equilibration. If the organic matter is easily degraded or released during soil erosion and re-suspension, the associated iodine will also be released. On the other hand, iodine associated with refractory organic carbon will travel with refractory particles through the watershed (50) and will be deposited in river delta sediments (35). As 129I from bomb fallout and more recent reprocessing sources had different lengths of time to equilibrate with soil organic matter pools, such as humic material (20, 51, 52), the two sources must have different mobilities in riverine watersheds. Because of the longer equilibration times for 129I from bomb fallout, only this source has so far appeared in delta sediments of the Mississippi River (35). Thus, the source functions of 129I are likely imprinted in the different organic matter fractions, particulate, macromolecular (colloidal), and dissolved. Wershofen and Aumann (53) and UNSCEAR (54) observed that most 129I emitted by nuclear reprocessing plants is in the relatively inert methyl iodide form. In contrast, bomb produced 129I had to be, at least initially, in an inorganic form due to the absence of any organic carbon source to react with. In the atmosphere, both sources could have been
further converted into methyl iodide or other forms. Methyl iodide is less reactive with soil organic and humic matter (51), while inorganic forms of I (I-, IO3-, I2) tend to react more strongly with soil organic matter (20, 51, 52). Schmitz and Aumann (55) observed that only a small fraction of natural iodine 127I (2-4%), but a large fraction of the anthropogenic 129I (38-49%), is water-soluble. If this is true for the Mississippi River watershed, it would lead to a disproportionately higher runoff of 129I compared to 127I, leading to lower 129I/127I ratios in eroded soil particles, some of which eventually get deposited in river and delta sediments. In addition, one would expect chemical and physical fractionation to occur not only between 129I and 127I but also between organic and inorganic forms of I and its isotopes in soils. Thus, any further progress in our understanding of 129I in the environment rests on a better characterization of the major chemical and physical forms of 129I and 127I in the different environmental compartments. The results of this first systematic study of 129I and 127I in a major world river, the Mississippi River, indicate that 129I concentrations are either close to or higher than in rainwater, thus requiring either a concentration effect or additional sources, but do not permit significant removal of 129I due to soil uptake. Additional sources of 129I can be excluded in the Mississippi River drainage basin. Furthermore, the 129I data do not support significant removal of 129I during interflow and bank storage of near-surface groundwater. The situation here is different from that in wetter climates and smaller watersheds, such as those in Europe (46, 47), where riverine 129I concentrations are always lower than those in rainwater. The use of 129I in the Mississippi River reveals concentration effects due to ET, which facilitates the distinction between atmospherically/marine-derived 127I and soil-weatheringderived 127I. The dose estimate of 129I to humans depends on a detailed knowledge of the relative contribution of these two sources for I (56). However, further progress will require more research into the major chemical forms of iodine and its isotopes, especially its organic forms, as well as reactions leading to release of iodine from soils during high river flow. Only when iodine speciation for the two isotopes is fully understood, can 129I/127I ratios be used as a tracer for specific organic matter fractions, which could greatly improve our understanding of terrestrial organic matter inputs into marine environments.
Acknowledgments We would like to acknowledge the operations crew at PRIME Lab, for producing high quality isotope ratio measurements on samples with highly variable ratios. We also wish to thank student assistants Trish Sullivan, Henry Coward, and Anthony Villarino who helped process these samples. This work was supported, in part, by the Coordinating Board of Texas (Advanced Research Project) and the Texas Institute of Oceanography.
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Received for review May 4, 2001. Revised manuscript received September 4, 2001. Accepted September 5, 2001. ES0109444
