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WATER RESOURCES RESEARCH GRANT PROPOSAL

Title: Test of a potential method to date recharge and surface waters / ground water interactions using anthropogenic 129Iodine and 127Iodine species, some of them chemical analogs for nitrate

Focus categories:

Keywords: 129Iodine, recharge, infiltration, exfiltration, nitrate, iodate, iodide, radioisotope and isotope tracers, mass balance, chloride, 222Radon, 210Polonium

Duration: 3/1/00 to 2/28/01

Federal funds requested: $25,000

Non-federal funds pledged: $50,042 (1. Coordinating Board, Texas Advanced Research Program - Reconstruction of Terrestrial 129I Inputs into Marine Environments, 1998-2000; $96,358 total, $20,000 remaining funds in Acct. 161971. 2. Interdisciplinary account 02-134311).

Principal investigators:

Texas A&M University

Peter Santschi, Professor, Department of Oceanography, TAMUG
Environmental isotopes, http://loer.tamug.tamu.edu

Bruce Herbert, Associate Professor, Department of Geology & Geophysics , TAMU Soils and aqueous geochemistry,
http://geoweb.tamu.edu/faculty/herbert/herbert.html

Kathy Schwehr, Graduate Student, Department of Oceanography, TAMUG

Congressional District: Field work: District 11; University: Districts 8 and 9

 

Statement of critical Texas water problems

Need: To evaluate surface water/ground water interactions, exfiltration, infiltration, and recharge rates, to trace and date nitrate cycling

Who: Texas Water Resources Institute and Texas Water Development Board

Why: To provide input for Ground Water Availability Modeling and water resource management and to evaluate cycling of nitrate pollutants.

Results, benefits: The 129Iodate species may provide an analog for studying the cycling of nitrate, a contaminant with serious side effects, at a study site in Gorman Cave, Colorado Bend State Park. Furthermore, 129I/127I may provide aquifer water residence times. This application will have no adverse environmental or ecological impacts. No injections or artificial tracers are needed to use anthropogenic 129Iodine and its species (iodate, iodine, and organo-iodine). Other studies using the variations in the isotopic composition of nitrogen have had unexpected results that are difficult to interpret (Bohlke et al., 1992) or have methodologies that are difficult to use (Kreitler, 1975).

Application of the 129Iodine method may provide a new dating tool for recent ground waters (Santschi et al., 1999). There is a high potential that needed information can be provided that no other method can. For instance, preliminary data gathered from a well characterized aquifer in Glattfelden, Switzerland, indicate a close correlation between 129I/127I and water residence time (Santschi et al., 1999). The use of 226Radium and its daughters, 222Radon (half-life 3.82 days), and 210Polonium (half-life 138 days), is limited to specific areas where concentrations are high enough to measure, like those proposed here. Other tracers, such as 3H, 14C, and CFCs, only have limited utility in groundwater hydrology. For example, tritium (3H, half-life 12.5 years) concentrations that were measurable after nuclear bomb tests are now scarcely discernable from background levels. The long half-life of radiocarbon, 14C, (half-life 5730 years) precludes its use for dating younger waters. The application of the radiocarbon method also requires a correct evaluation of the initial 14C activity of the total dissolved inorganic carbon and a multitude of processes that may produce a dilution or enrichment of the 14C concentration resulting in an apparent, unrealistic age (Tenu, 1987; Vogel and Ehhalt, 1963). Corrections for dilution to 14C concentration in shallow Texas aquifers pose an especial challenge due to the presence of methane (Grossman et al., 1989; Geyh and Kunzl, 1981). Chlorofluorocarbons (CFCs) are not chemically analogous to nitrates and are retained in organic fractions, so are not suitable for studying nitrates as they could be associated with organic matter. Also, CFCs may show variability due to local input and due to solubility changes that are very sensitive to aquifer temperature (Clark and Fritz, 1997).

Information to be gained: 129Iodine, 222Radon and 210Polonium will be evaluated for their suitability to date recently infiltrating or exfiltrating waters in cave pools and springs at Gorman Cave, Colorado Bend State Park. The 129Iodate will be tested for its analogy to nitrate for tracing interactions between surface and ground waters. Geochemical processes in these waters will be evaluated to verify mass balance, mixing, transport, and adsorption models. Data to be assessed will include concentrations of 129I/127I ratios for total iodine, iodate, iodide, and organic iodine, dissolved organic carbon, dissolved inorganic carbon, alkalinity, pH, sulphate and sulfide, nitrate, nitrite, ammonium, urea, chloride, phosphate, silicate, iron, manganese, 222Rn, and 210Po, the oxygen and deuterium isotopic composition for these waters, temperature, dissolved oxygen, and total dissolved solids.

How it will be used: This study provides a systematic pilot test for a promising new dating tool for recent waters (1 to 50 years old), delineating interactions between surface and ground waters, and assessing how far iodate can be used as an analog for the fate of nitrate. Information will be appraised by comparing of a suite of aqueous geochemical and isotopic indicators in the Gorman Cave aquifer. Evaluation of the 129I method in an area where alternative radioisotope dating is available (226Ra) will show its utility throughout other areas. Determination of the interactions and rates for infiltration of meteoric and surface waters to ground waters and exfiltration of ground waters to surface waters will be an important element for the Groundwater Availability Modeling effort. Success of this pilot project will help to extend its applicability more statewide.

Nature: The radioisotope 129Iodine (half-life 15.7 Ma) is naturally produced at about the same rate by cosmogenic spallation Xenon in the atmosphere as it is produced by subsurface fissionogenic decay from 238Uranium. These sources have contributed approximately 80 kg or 16 Ci to the earth's inventory of 129I. The long half-life of 129I and the dominance of sea-spray iodine have produced a well-mixed isotopic ratio for 129Iodine to stable 127Iodine (129I/127I) of an average 1500 * 10-15 in all global reservoirs (Moran et al., 1998; Fabryka-Martin et al., 1985). Anthropogenic inputs resulting from nuclear weapons testing and continued release from nuclear reprocessing facilities have raised the isotopic ratio by several orders of magnitude (Oktay et al., 1999; Moran et al., 1998; Smith et al., 1998; Kilius et al., 1994; Rao et al., 1994; and Yiou et al., 1994). Nuclear weapons tests added about 50 kg or 10 Ci to the background level Nuclear reprocessing from Sellafield and La Hague, the major input sources, have cumulatively released approximately 1440 kg or about 250 Ci from 1966 to 1994. Since 1994, releases from the facilities have been about 200 kg or 40 Ci per year. Some sources indicate that La Hague facility has steeply increased production (Moran et al, 1999a). Regional measurements in Texas and the United States show that concentrations of 129Iodine in rains, air-plants, surface soils, and rivers are not environmentally threatening, but may provide a tool to measure an increase in concentration with time. The changes in concentration of 129Iodine are expected to date water from the present to 50 years old +10%. Variations in concentrations due to interactions between ground and surface waters and mixing will be resolved through coincident measurement of the oxygen and hydrogen isotopic composition of water, d18O and dD, ionic ratio comparisons, i.e., NO3- /Cl-, and ages of waters. The proposed pilot project area at Gorman Cave, Colorado Bend State Park, is enriched in 222Radon, which has a half-life of 3.82 days, (Baker, 1999; Hoehn and Gunten 1989; Hoehn and Santschi, 1987; Cech et al., 1987) and 210Polonium, half-life 138 days, (Burnett et al., 1997; Burnett et al., 1988), which will be use to verify the resultant 129Iodine date.

The chemistry of meteoric precipitation and surface waters from rivers and springs at Gorman Cave, Colorado Bend State Park will be compared to infiltrated waters from dripping speleothems, cave pools, and springs exfiltrating from the cave into the Colorado River to test the proposed method. The study site, Figure 1, Gorman Cave, Colorado Bend State Park, was selected for its access, the potential for a secondary method of dating waters, and because it is a source of nitrates after bat roosting season.

The elevated natural radioactivity in the proposed study site is due to the geologic history of the area, Figure 1. Radioactive minerals in the uranium series are associated with the Precambrian igneous and metamorphic rocks of the Llano Uplift, central Texas. These rock units have been uplifted, exposed, and eroded, forming parent materials for the overlying Cambrian Hickory Sandstone aquifer. Some parent material may also be present in the Cambrian and Ordovician Ellenburger-San Saba aquifer, but more influential is the proximity of radionuclides in the uplifted rock and the migration of daughter products (Baker, 1999; Cech et al., 1988).

Iodate, a chemical analog to nitrate (Wong, 1991), and organic forms of iodine and nitrogen exist in aerobic (oxygen-rich), unconfined waters (Langmuir,1997). Plants fix nitrogen from the atmosphere and ammonium from the soils into organic biomass. As the biomass decays, ammonium is released, some as volatile gases while the balance may be oxidized by nitrification to nitrate. Commercial urea fertilizers also follow a similar nitrification process.

 

Figure 1. Map of Gorman Cave (Colorado Bend State Park) study site.

Figure 1. Map of Gorman Cave (Colorado Bend State Park) study site. In the upper view, the red box is the location of the proposed study site. Colorado Bend State Park is at the junction of San Saba, Lampasas, and Burnet Counties. Austin is shown as a green dot. The yellow area shows where there is elevated natural Radium concentration in the ground water, after Cech et al., 1988. The lower left map shows the Llano Uplift region of Precambian igneous and metamorphic rock associated with Uranium series minerals and daughter radionuclides. Younger Paleozoic units and their eroded material overlie the igneous and metamorphic rock in San Saba and Lampasas Counties. The lower right map shows the relative positions of the Hickory Sands, Ellenburger-SanSaba, and the Marble Falls Aquifers (Baker, 1999). Daughter decay products migrate into these aquifers. The cave system is in the Ellenburger-San Saba Formations.

Nitrate and iodate reduction or uptake by organisms may occur simultaneously (Wong, 1991). In anaerobic (oxygen-depleted) or confined water systems, the reduced species are iodide, nitrite, nitrous oxide (minor concentration) and nitrogen gas, and ammonia. The reduction process, denitrification, requires an organic substrate (dissolved organic carbon) or may use electron sources such as manganese, iron, sulphide, and methane (Clark and Fritz, 1997).

Since the nature of the iodine-nitrogen chemistry is heavily dependent on reduction and oxidation, the redox chemistry of the waters and important mineral species will be analyzed. The ratio of the radioisotope, 129I, will be compared to the stable 127I, and for the iodine species (iodate, iodide, and organic iodine). Nitrate, ammonium, nitrite, urea, sulphate, sulfide, phosphate, and chloride concentrations as well as dissolved organic carbon concentrations will portray the redox conditions essential to the nitrate cycling processes. These ions may be normalized to the inert chloride ion to show mixing, mass balance, and transport (Stigter et al., 1998). Iron, manganese, and silicate concentrations add information regarding adsorption. Dissolved oxygen, alkalinity, pH, temperature as measured in the field will also help define distributions of reduction and oxidation zones (Szabo and Zapecza, 1988). Total dissolved solids are an additional indicator of transport distance with higher concentrations signifying increased transport distance.

Scope: This study is designed to be a pilot project using a controlled area to assess the utility of conceptual models for delineating the interaction between surface and ground waters through the use of 129Iodine and its speciation relationships. Also, the potential for use of iodate as a nitrate analog will be assessed. 222Radon and 210Polonium will be used to confirm water ages obtained using the 129Iodine, 129Iodide, and 129Iodate system.

Objectives: The major objectives are to evaluate the use of the 129Iodine system for evaluating surface and ground water interactions, assessing the use of iodate as an analog for nitrate cycling, and for dating the waters.

Methods

Sampling: Waters in the Colorado Bend State Park study site area will be collected from rains, springs above the cave system, dripping speleothems and cave pools, springs exfiltrating from the cave into the Colorado River, and from the Colorado River. Collection time intervals will be adequate to sample a variety of changes in rainfall, temperature, and bat guano concentration. Samples will be filtered and analysis will be carried out on both the filtrate (particulate fraction) and permeate (dissolved fraction) where possible to collect enough sample.  Chemical species will be preserved following protocol for each type of sample via filtering (removes most bacteria, suspended clays and most iron and manganese oxy-hydroxides), keeping away from light, transporting with dry ice/ ice or freezing where appropriate, and analysis will be conducted as soon as possible.

Analytical techniques: Aqueous iodine species will be separated using anion exchange liquid chromatography and elution with sodium nitrate (Reifenhauser and Heumann, 1990; Wong and Brewer, 1976). Organic fractions will be denatured using hydrogen peroxide and ultraviolet light irradiation to separate out the iodine species. In this fashion, the concentration of iodine can be determined for total iodine, iodate, iodide, and organic iodine. 129I will also be measured in the same species using novel techniques worked out in our laboratory.

Stable 127I will be analyzed in a small aliquot taken from the species-separated sample volume. The remaining volume will be prepared as a AgI pellet of target material for Accelerator Mass Spectrometry (AMS) analysis of 129I (Sharma et al., 1997) at Purdue University. The method for precipitating the AgI pellet is modified from Fehn et al (1992) wherein a known amount of “carrier” iodine that has been established as having a low 129I ratio ( 129I/127I ratio of 80x10-15) is added to each sample. The detection limit for AMS is approximately 1 x 10-14 for 129I/127I (Sharma et al., 1997). One sigma counting errors are generally less than 10%. Stable 127I concentration measurements will be done by ICP-MS (Oktay, 1999). Reagent blanks will be established by carrying out the chromatographic separation process and AgI pellet precipitation on the carrier material alone. Dissolved organic carbon (DOC) will be analyzed using the high temperature combustion oxidation method using a MQTOC Analyzer 1001 or Shimadzu-5000 Instrument (Guo et al., 1995). Nutrient ion concentrations for nitrate, urea, ammonium, nitrite, phosphate, and silicate will be determined photometrically using autoanalyzer techniques similar to those of Grasshofft et al., 1993. Sulphate, sulfide, and chloride concentrations will be performed on Dionex BIOLC Ion Chromatograph equipped with conductivity detector. Iron and manganese concentrations will be established through analysis on a Graphite Furnace Atomic Absorption Spectrophotometric Instrument (GF-AAS). The oxygen and deuterium isotopic composition of waters will be analyzed on a Finnegan Mat Delta-E after extraction in the University of Houston Stable Isotopic Laboratory.

Water dating using 222Rn will be done using the column extraction and liquid scintillation counting procedure described by Hoehn and Gunten, 1989. The method of Burnett et al., 1997, and Burnett et al., 1988, will be used to extract and measure the activity of 210Po.

Analytical techniques will then be employed to interpret the data and develop models for surface/ground water interactions, transport, adsorption, and nitrate cycling (Corbett et al., 1997; Batson et al, 1996; Hoehn and Santschi, 1987).

All methods and instruments mentioned in this proposal, except AMS and O/H isotopic analysis, are in use in our lab. AMS analysis will be carried out by P. Sharma at Purdue University, with whom we have collaborated for more than 5 years, and Jim Lawrence at University of Houston, the former masters thesis advisor of graduate student Kathy Schwehr.

The research facilities available at the Laboratory of Oceanographic and Environmental Research for this project can be viewed at http://loer.tamug.tamu.edu/facilities.


Related research: The proposed research will take advantage of the insights from recent research which elucidated the geochemical processes which control 129I and 127I concentrations in meteoric water, soils, river and ground water and river delta sediments. The next step in 129I research requires a comparison of 129I speciation to that of 127I in different environmental reservoirs. The following are highlights of past and current research carried out by our group which is related to the proposed research:

1) Anthropogenic 129I can be used to elucidate currents and vertical mixing rates in the Gulf of Mexico (Schink et al., 1995b), and in the Middle Atlantic Bight (Santschi et al., 1996).

2) 129I/127I ratios in meteoric and river water from the USA, away from known sources of 129I, are high, strongly suggesting that we currently experience the fallout from European (and Russian nuclear) reprocessing releases (Moran et al., 1999a,b).

3) Groundwater ages derived from 129I/127I ratios in near-surface groundwater in a river-fed aquifer in Europe agreed with residence times derived from tritium and radon (Santschi et al., 1999, Figure 2), suggesting that 129I has the potential to be used for dating near-surface groundwater.

 

Figure 2. Results from preliminary results of 129I and 127I determinations in a Swiss aquifer, demonstrating a close relationship between Iodine concentrations and isotope ratios with water residence time (Santschi et al., 1999).

Figure 2. Results from preliminary results of 129I and 127I determinations in a Swiss aquifer, demonstrating a close relationship between Iodine concentrations and isotope ratios with water residence time (Santschi et al., 1999).

4) 129I/127I ratios in whole Mississippi River water were consistently high, correlated inversely with flow rate, and most radioactive and stable iodine in the water was present in a dissolved but high molecular weight organic phase that could be separated by cross-flow ultrafiltration (Oktay, 1999). The 129I/127I ratios in the river water appear to be regulated by erosional and washout processes in drainage basin soils (Oktay, 1999).

5) 129I/127I ratios in Mississippi River surface sediment were about an order of magnitude lower than in the river, and were very closely tracking the profiles of the bomb fallout nuclides (Oktay et al., 1999).

The proposed research will make use of the recent insights into the geochemical cycling of 129I and 127I in terrestrial and marine environments which Santschi’s group gained over the past several years. Herbert’s group’s experience in soil and aquifer processes will be critical to establish iodine isotopes as geochemical and hydrological tracers.

Information transfer: The results will be presented at a national meeting and used to prepare a future manuscript and a more comprehensive proposal to a federal agency (e.g., NSF-Hydrology).

Training potential: This study project will be part of the Ph.D. dissertation of Kathy Schwehr, majoring in the field of chemical oceanography. She holds a masters degree in Geology, and has extensive experience in hydrogeology. In addition, undergraduate students will participate in some aspects of the proposed research, as independent 485 projects.

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