Secular Radioactive Equilibrium Research Articles

An assessment of committed effective dose and lifetime cancer risk due to the ingestion of infant milk

Infant milk is a manufactured food designed for feeding babies and infants under 12 months of age. This product is indicated when, for some reason, the baby can not be fed by breast milk. It is used in case of intolerance to milk from farmed animals and is widely used for the nutrition of newborns in artificial lactation, in place of breastfeeding. It can be used as a complete or partial substitute for human milk. In the current study, the committed effective dose and lifetime cancer risk due to ingestion of 4 K, 226Ra, and 228Ra were estimated using activity concentration obtained from high-resolution gamma spectrometry. Consumption data and dose coefficients were obtained from Brazilian Institute of Geography and Statistics and data published by ICRP 119. Infant milk and water samples were collected in the Rio de Janeiro trade, placed in a polystyrene container, and then sealed to reach the secular radioactive equilibrium condition (at least 45 days). The samples were analyzed by gamma spectrometry using a High Purity Germanium (HPGE). Detection efficiency for each sample was obtained with LabSOCS software. For infant milk samples, the highest values of activity concentrations were (242 ± 9) Bq.kg−1 for 4 K, (5 ± 1) Bq.kg−1 for 228Ra and presented values below the detection limit for 226Ra. The higher values obtained for water samples were (10 ± 1) Bq.l−1, (5 ± 1) Bq.l−1, (5.3 ± 0.1) Bq.l−1 for 4 K, 226Ra and 228Ra, respectively. These values of committed effective dose are below the values stipulated by UNSCEAR (290 μSv.y−1) and by the ICRP (1 mSv.y−1). The values obtained for lifetime cancer risk are considered negligible, once they are below the limit considered for taking action (10−4). It can be concluded that the values calculated do not present a significant risk regarding the intake of infant milk in the first two years of the child's life.

A New Methodological Approach for the Assessment of the 238U Content in Drinking Water

The radiological quality of drinking water is directly associated with the health of the population. Indeed, it is well known that the presence of radionuclides in drinking water constitutes a health risk for humans because the consumption of such water increases the likelihood of incurring cancer. For this reason, all the studies aimed at developing new methodologies for the qualitative and quantitative analysis of the radioisotopic composition of drinking water are absolutely desired by the international scientific community, as well as by the institutes that deal with the protection of public health. In this paper, a new methodological approach was developed for the evaluation of the 238U content in drinking water. A sample coming from Paola, Calabria region, southern Italy, was taken as a case study. The assessment was performed by using High Purity Germanium (HPGe) gamma-ray spectrometry, with the aim of quantifying the specific activity of the 234mPa radioisotope after a preconcentration procedure, and thus to assess the activity concentration of 238U, in the hypothesis of the secular radioactive equilibrium between it and its daughter. The obtained results were validated through the comparison with the 238U (µg/L) concentration as measured with Inductively Coupled Plasma-Mass Spectrometry (ICP-MS).

The impact of the oil and gas industry on TENORM of the desert sands in Ma’rib, central Yemen

A measurement of technologically enhanced natural occurring radioactivity materials (TENORM) in sand samples in the petroleum-producing communities of Ma'rib Province Yemen was carried out. The HPGe detector was utilized to perform the measurements. The samples were collected from 35 sites. All samples were placed in a properly cleaned container and sealed for 28 days to attain a state of secular radioactive equilibrium before analysis. The radioactivity concentrations for nuclides 40K, 232Th, 226Ra, and 238U ranged between 430.716 - 654.666, 4.347 - 43.935, 4.824 - 27.936, and 1.083 - 15.437 Bq kg-1 respectively, with averages 560.583, 20.676, 14.556 and 9.072 Bq kg-1 respectively. The results did not show any radiation risks.

Clay mineralogy insights and assessment of the natural (228Ra, 226Ra, 210Pb, 40K) and anthropogenic (137Cs) radionuclides dispersion in the estuarine and lagoon systems along the Atlantic coast of Portugal

The activity concentrations of natural (228Ra, 226Ra, 210Pb, 40K) and anthropogenic (137Cs) radionuclides were measured by gamma spectrometry on sediments collected from aquatic systems. The mineralogy of clay fractions identified by X-ray diffraction (XRD) is composed by an illitic material (detrital mica, illite/smectite, and illite/smectite/vermiculite mixed-layers) ± kaolinite and illite. The Mg2+-saturated clay fractions show three distinct illitic populations within the illitic material: detrital mica, illite/vermiculite mixed-layers, and illite/smectite. Transmission electron microscopy (TEM) images show illitic material aggregates with a flaky- and lath-shapes, pseudohexagonal platy kaolinite and lath-shape illite.The 228Ra activity concentration values range from 30.1 to 403 Bq Kg−1, the 226Ra from 24.6 to 290 Bq Kg−1 and from 44 to 368 Bq Kg−1 for 210Pb. The Ra fluxes suggest a unique source where the mixing process and radioactive decay equilibrated the losses of short- (228Ra) and long-lived (226Ra) isotopes. The 210Pb/226Ra ratio shows an average value of 2.165, indicating that the radioactive secular equilibrium (~1) was reached. A decrease of the natural radionuclide (228Ra, 226Ra, 210Pb) activity concentrations was observed in estuaries and near shore coastal sediments from north to south of Atlantic Portuguese coast. Higher 210Pb and 226Ra activity concentrations were obtained in samples composed by illitic material + kaolinite and organic matter. The illite/smectite mixed-layers and kaolinite are suitable sorbents for Ra and 210Pb isotopes. Higher 40K values (473–1001 Bq Kg−1) were measured on feldspar, mica and clay sediments provided from granite weathering (north region).The 137Cs activity values range from 1.25 to 10.1 ± 1.9 Bq Kg−1 for samples collected from the northern and central part of estuarine areas. A higher 137Cs activity (31.2 ± 3.5 Bq Kg−1) was measured in the southern part of the Atlantic coast, corresponding to a sediment with a higher amount of illite.

Radiological baseline around the Barakah Nuclear Power Plant, UAE

Destructive and non-destructive analysis techniques were used to establish a radiological baseline around the Barakah Nuclear Power Plant area in Abu Dhabi, United Arab Emirates. The natural radioactivity concentrations of 238U (226Ra), 232Th and 40K were measured for shore, soil and bottom sediment samples, using gamma spectrometry with a high-purity germanium (HPGe) detector. Alpha spectrometry was used to measure the 234U/238U ratio for some selected samples using a silicon surface-barrier detector. The measured gamma activity concentrations in shore samples are much lower compared to those in soil and bottom samples. The average activity concentrations of 238U (226Ra) are 4.43 ± 1.12, 13.54 ± 4.16 and 4.73 ± 3.01 Bq/kg in shore, soil and bottom sediment samples, respectively. The corresponding values for 232Th are 1.68 ± 0.49, 8.31 ± 3.87 and 1.83 ± 1.67 Bq/kg, and those for 40K, are 106.30 ± 50.68, 349.72 ± 107.16 and 105.23 ± 130.14 Bq/kg. The 234U/238U activity ratios span a wide range from 0.59 to 2.24, indicating a system where the daughter/parent is out of secular radioactive equilibrium. The hazard parameters, radium equivalent and absorbed dose rates, showed low levels compared to the world average level reported by the UNSCEAR in 2000. The estimated activity concentrations in this study were lower than the world average values and lower than the levels reported in nearby countries.

Radiochemical technique as a tool for determination and characterisation of El Sela ore grade uranium deposits

ABSTRACT The present study is an application of a radiochemical procedure for the determination of 238U, 235U and 234U as well as uranium isotopic ratios in El Sela ore grade samples that located at the Southeastern Desert of Egypt. This procedure is consisting of several steps include sample preparation, chemical separation and source preparation followed by counting using alpha-particle spectrometry technique. Radiochemical determination of various uranium isotopes contents indicated that El Sela uranium deposits are considered as a recent deposit where there was a large difference between radiochemically and radiometrically determined uranium. The measured 234U/238U activity ratios for most samples were approximately unity, indicating that secular radioactive equilibrium was established between 238U and 234U in the El Sela mineralised rocks. On the other hand, 238U and 234U might be equally leached due to the effect of rock-water interactions during the pluvial time by the action of surficial or underground water. The 238U/235U activity ratios of the studied mineralised granites were significantly shifted from the natural ratio 21.7, indicating that the redox conditions and alteration processes played an important role in the fractionation of 238U from 235U. The values of 234U/235U ratios in the studied granites varied between 23.08 and 34.70 in different samples, suggesting 235U migration out due to the prevailing conditions.

Monitoring the equilibration of 228Th and decay products over time after extraction from ivory

The worldwide population of elephants shows a rapid decline due to trafficking of ivory. Hence, a method is needed for the age determination to distinguish legal from illegal ivory. The analysis of 228Th and 232Th in ivory can be used for this purpose. This study provides a technique for the analysis of 228Th in ivory. The thorium was extracted from ivory and measured after various time periods applying α-spectrometry until a secular radioactive equilibrium between 228Th and its decay products was established. An equilibration period of 32 days was found. The uncertainty of the determination of the activity of 228Th could be reduced by integrating the counts of the equilibrated decay products into the determination process.

Zeolitized bottom ashes from biomass combustion as cement replacing components

In power plants biomass combustion produces large quantities of biomass bottom ash wastes. During the combustion process, the produced ashes are enriched in the radionuclides. The different enrichment of the various radionuclides within a radioactive series, such as that of 226Ra, 228Ra, 228Th, 40K, 210Pb and 137Cs results in the disturbance of radioactive secular equilibrium. The production of energy from renewable resources, such as biomass, is increasing rapidly. As the demand for bioenergy production increases, waste products from biomass combustion will increase too and will become a relevant environmental and economic problem. Therefore, environmentally friendly, and economic solutions to recycle the resulting by-products are essential. This research provides a real opportunity to save cement thus disposing the waste by using zeolitized biomass bottom ash as supplementary cementitious materials. The zeolitized products were examined by X-ray diffraction (XRD), X-ray fluorescence spectrometer (XRF) spectroscopy and high-resolution gamma spectrometry. In all investigated cases, compressive strength in hardened cement pastes and concrete samples slightly increases by replacing from 3% to 5% of Portland cement with zeolitized ashes. The results showed that natural radionuclides as 226Ra, 228Ra, 228Th, 40K are concentrated in similar way as in case of ashes obtained from combustion. However, addition of biomass leads to concentration of artificial radionuclide 137Cs that is still commonly present in biomass due to Chernobyl disaster. The observed activity concentration of 137Cs reaches the level of natural radionuclides concentration. Taking into consideration the contribution of 137Cs to gamma dose (usage of expanded activity index formula) may lead to exceeding the value of 1 in some cases. Calculated differences between activity indexes reach 13%.

Analytical evaluation of natural radionuclides and their radioactive equilibrium in raw materials and by-products

An investigation into the distribution of natural radionuclides and radioactive secular equilibrium in raw materials and by-products in a domestic distribution was conducted to deduce the optimum conditions for the analytical evaluation of natural radionuclides for 238U, 226Ra, and 232Th using a gamma-ray spectrometer and inductively coupled plasma mass spectrometer (ICP-MS). The range of the specific activities of natural radionuclides was first evaluated by analyzing 228Ac and 214Bi, which are 232Th and 226Ra indicators, respectively, in about 100 samples of raw materials and by-products through a gamma-ray spectrometer. From further experiments using several samples selected based on the results of the distribution of natural radionuclides, the validation of their analytical evaluations for the indirect measurements using a gamma-ray spectrometer and direct measurements using ICP-MS was assured by comparing their results. Chemically processed products from the raw materials, such as Zr sand and ceramic balls, were generally shown for the type of bead and particularly analyzed showing a definite disequilibrium with above a 50% difference between 238U and 226Ra in the uranium series and 232Th and 228Ra in the thorium series.

English

The metamorphosed sandstones exposures occur in two locations in Wadi Sikait. The exposed rocks in this area are ophiolitic mélange, metamorphosed sandstones, porphrytic granites invaded by post-granite dykes (lamprophyres) and quartz-fluorite veins. The uranium contents measured radiometrically range from 3 to 41.97 ppm, with an average of 12 ppm, while the chemically measured are in the range from 20 to 100 ppm and averaging 50.83 ppm. High uranium contents are mainly attributed to the presence of secondary uranium minerals (uranophane and autonite), accessory minerals (monazite, zircon, allanite and xenotime) and U-bearing minerals (muscovite, biotite, chlorite, iron oxides and clays). P- and D-factors indicate disequilibrium in U-decay due to addition of uranium in these rocks. Since radioactive secular equilibrium of the young age deposits have not yet reached, therefore, the activity ratios (AR) of 230Th/234U in the studied rocks is very small and ranges from 0.43 to 1.3. Radon exhalation rates of the studied rocks were also measured using ‘‘Sealed Can technique” and indicated the presence of subsurface and surface uranium anomaly which confirms the previous results.   Key words: Metamorphosed sandstones, uranium, minerals, Wadi Sikait.

A method for optimum PSA setting in the absence of a pure α or β emitter and its application in the determination of 237Np/233Pa

In the application of liquid scintillation counting (LSC), the α/β discrimination is carried out with the function of pulse shape analysis (PSA), which requires the setting of the optimum PSA level. The optimum PSA are usually determined by the generation of cross-over plots, whereby a pair of vials, one containing a pure α emitter and the other a pure β emitter, is counted. However, in some cases such as the determination of 237Np/233Pa, a pure α emitter or a pure β emitter is not available. Therefore, we have developed a new approach to set the optimum PSA by measuring the sample itself of mixed α/β emitters. The count rate of the sample in the α-multi-channel analyzer changes monotonically with the increase of the PSA, and there is always an inflection point which is related to the optimum PSA. By fitting the data near the inflection point with the function y=ax3+bx2+cx+d, we can obtain the optimum PSA as −b/(3a), which can be used to determine the radioactivity of 237Np/233Pa. The results obtained with this new approach were in good agreement with those obtained by HPGe γ spectrometry that was calibrated with an LSC sample of 237Np/233Pa under a radioactive secular equilibrium. The new approach is promising to be used in simultaneous determination of gross α and β emitters, especially in the absence of a pure α or β emitter.

Evaluation of Pleistocene groundwater flow through fractured tuffs using a U-series disequilibrium approach, Pahute Mesa, Nevada, USA

Groundwater flow through fractured felsic tuffs and lavas at the Nevada National Security Site represents the most likely mechanism for transport of radionuclides away from underground nuclear tests at Pahute Mesa. To help evaluate fracture flow and matrix–water exchange, we have determined U-series isotopic compositions on more than 40 drill core samples from 5 boreholes that represent discrete fracture surfaces, breccia zones, and interiors of unfractured core. The U-series approach relies on the disruption of radioactive secular equilibrium between isotopes in the uranium-series decay chain due to preferential mobilization of 234U relative to 238U, and U relative to Th. Samples from discrete fractures were obtained by milling fracture surfaces containing thin secondary mineral coatings of clays, silica, Fe–Mn oxyhydroxides, and zeolite. Intact core interiors and breccia fragments were sampled in bulk. In addition, profiles of rock matrix extending 15 to 44mm away from several fractures that show evidence of recent flow were analyzed to investigate the extent of fracture/matrix water exchange.Samples of rock matrix have 234U/238U and 230Th/238U activity ratios (AR) closest to radioactive secular equilibrium indicating only small amounts of groundwater penetrated unfractured matrix. Greater U mobility was observed in welded-tuff matrix with elevated porosity and in zeolitized bedded tuff. Samples of brecciated core were also in secular equilibrium implying a lack of long-range hydraulic connectivity in these cases. Samples of discrete fracture surfaces typically, but not always, were in radioactive disequilibrium. Many fractures had isotopic compositions plotting near the 230Th-234U 1:1 line indicating a steady-state balance between U input and removal along with radioactive decay. Numerical simulations of U-series isotope evolution indicate that 0.5 to 1 million years are required to reach steady-state compositions. Once attained, disequilibrium 234U/238U and 230Th/238U AR values can be maintained indefinitely as long as hydrological and geochemical processes remain stable. Therefore, many Pahute Mesa fractures represent stable hydrologic pathways over million-year timescales. A smaller number of samples have non-steady-state compositions indicating transient conditions in the last several hundred thousand years. In these cases, U mobility is dominated by overall gains rather than losses of U.

Radioanalytical techniques for the determination of 238U, 226Ra and 210Pb in the environment

Summary Different radiometric techniques for the determination of 238U, 226Ra and 210Pb are presented and compared in terms of detection limits with mass spectrometric techniques. It can be concluded that when samples with low activity concentrations have to be measured, the method of choice in the case of 238U should be either RNAA/INAA or alpha particle spectrometry. In the case of 226Ra and 210Pb the best performance can be expected by the alpha spectrometry, whereas drawback of waiting for establishing secular radioactive equilibrium of 210Pb with 210Po makes techniques like beta counting and LSC more attractive for the determination of 210Pb. In addition, a case study on monitoring the former uranium mine Žirovski vrh is presented along with the used methodology and the summarised measurement results.

Correcting for potential 222Rn loss in 210Pb dating of sediments from the South Adriatic Pit

Measurement of 226Ra activity concentrations is often used to estimate supported levels of 210Pb for purposes of geochronology. However, the implicit assumption that supported 210Pb and 226Ra are in secular radioactive equilibrium may not always be true because of the migration of an intermediate product, gaseous 222Rn. As a consequence, supported 210Pb activity concentration might be lower than the measured 226Ra value, which was the case in a core collected from the South Adriatic Pit. Here we present a new approach to improve the determination of supported 210Pb, which is based on correction of 226Ra activity concentrations using the average (210Pb/226Ra) activity ratio in deeper sediment layers. Using this approach, sedimentation rates for the South Adriatic Pit with a constant rate of supply model and a constant initial concentration model were 0.054 g/cm2/a and 0.070 g/cm2/a, respectively.

Radioactive secular equilibrium in 238U and 232Th series in granitoids from Greece

Granitoid rocks belonging to plutons of Greece, covering a wide range of compositions and rock-types, have been studied for their specific activity (Bq/kg) of 238U and 226Ra from 238U radioactive series and 228Ra and 228Th from 232Th radioactive series by using gamma-ray spectroscopy. Results on the radioactive secular equilibrium of both 238U and 232Th radioactive series are presented by studying the 226Ra/238U and 228Ra/228Th ratios. The majority of the samples are in radioactive secular equilibrium for 226Ra/238U±1σ. However, several samples exhibit 226Ra/238U ratios significantly different from 1. The distortion of the secular equilibrium in the 226Ra–238U isotopic system of those samples has occurred over the last 1Ma, and can be associated with post-magmatic processes, mainly rock–water interactions. All studied samples have 228Ra/228Th ratios equal to unity ±1σ. Consequently, they can be considered to be in secular equilibrium for the last 40a.

Variation in the uranium isotopic ratios234U/238U,238U/total-U and234U/total-U in Indian soil samples: Application to environmental monitoring

The uranium isotopes 238 U, 235 U and 234 U are found naturally in the environment. 238 U and 235 U are parent nuclides of two independent decay series of isotopes, while 234 U is a member of the 238 U decay chain. When decay series occur in a closed system the series tends to reach, with time, the state of secular equilibrium in which the activities of all series members are equal to the activity of its first nuclide. The activity ratio 234 U/ 238 U in natural uranium may vary as a consequence of decay chain disequilibrium due to alpha recoil and biogeochemical processes. A study based on measurement of uranium concentration and 234 U/ 238 U activity ratios in soil samples collected from Nalgonda district, Andhra Pradesh, India, a proposed mining site, was carried out to find the spatial distribution of uranium and the state of secular equilibrium of 234 U/ 238 U to examine the possibility of applying uranium concentration and uranium isotopic activity ratios to detect any hydrogeochemical changes in the environment during/post-operation. Soil samples were collected and analyzed for uranium concentration using the conventional UV fluorimetric method, showing a uranium concentration in the range of 0.7 ± 0.2 ppm to 7.9 ± 0.4 ppm with an average of 3.4 ppm, and 234 U/ 238 U activity ratios were estimated using the alpha spectrometry technique, showing an activity ratio in the range of 0.92 ± 0.11 to 1.02 ± 0.11. The 234 U/ 238 U activity ratio obtained indicated that these two uranium isotopes are in the state of secular radioactive equilibrium. The percent activity ratio of 238 U/total U and 234 U /total U is observed to vary from 47.94 ± 4.83 to 50.76 ± 4.87 and 45.80 ± 3.83 to 49.14 ± 3.99, respectively.

Quantitation of protactinium, 231Pa in abyssal carbonate

Quantitation of protactinium, 231Pa in abyssal carbonates at ∼10−12 g g−1 levels, presents certain challenges because it is the only isotope of this element that is present in the natural environment and its half-life of 32800 y is too long for useful measurement using alpha-particle or gamma-ray spectroscopy. However, because it is an isotope in the 235U decay chain, closely linked to the 238U decay chain, quantitation can be achieved relative to a carbonate standard in secular radioactive equilibrium. The 238U, 235U, 234U, 230Th and 232Th isotopes in the carbonate standard are determined with a mixed 236U–229Th spike using isotope dilution protocols and a Multi-Collector Inductively Coupled Plasma Mass Spectrometer.

Assessment of Radionuclide Concentrations and Absorbed Dose from Consumption of Community Water Supplies in Oil and Gas Producing Areas in Delta State, Nigeria

A survey of radioactivity concentration in water supplies used for domestic and industrial purposes in the oil and gas producing communities of Delta State, Nigeria was carried out using a well-calibrated High-Purity Germanium (HPGe) detector system. The study area was partitioned into ten sections and a total of two samples per partition were collected for analysis. Samples of water from a non-producing area were also collected as control. In all, a total number of forty three samples were collected and analyzed. Each sample was acidified at the rate of 10 ml of 11 M HCl per litre of water to prevent the absorption of radionuclides into the wall of the container and sealed in a properly cleaned container for at least one month so as to attain a state of secular radioactive equilibrium before analysis. The photopeaks observed with reliable regularity belong to the naturally occurring series-decay radionuclide headed by 238U and 232Th, as well as the non-series decay type 40K. The mean specific activity obtained for 40K was 49 ± 15 Bq L–1 with a range of 6 - 177 Bq L–1 while for 238U, the mean specific activity was 3 ± 1 Bq L–1 with a range of 1 - 12 Bq L–1 and the mean specific activity for 232Th was 3 ± 2 Bq L–1 with a range of 2 - 10 Bq L–1 and the total annual effective dose, which vary between 0 - 2 μSv y–1, did not show any significant health impact.

Natural radionuclides content and associated dose rates in fine-grained sediments from Patras-Rion sub-basins, Greece

The activity concentrations of the natural radionuclides (238)U, (226)Ra, (232)Th and (40)K were measured in soil samples collected from the Patras-Rion sub-basins (Southern Greece) and were found to be 28, 27, 30 and 483 Bq kg(-1), respectively. These values compare well with the average Greek and worldwide values for crustal soil and sedimentary rocks. The mean (226)Ra/(238)U activity ratio was close to 1, implying secular radioactive equilibrium in the uranium series. All soil samples have Ra(eq) values lower than the limit of 370 Bq kg(-1), indicating their safe use in brick production. The average annual terrestrial absorbed dose rate in air was 51±14 nGy h(-1), and the average annual effective dose 0.06±0.02 mSv y(-1), which is consistent with the average worldwide exposure to external terrestrial radiation outdoors (0.07 mSv y(-1)). Non-significant differences between soils with different age and depositional environments were found, which could be attributed to a common source of sediments.

Experimental evidence for 234U– 238U fractionation during granite weathering with implications for 234U/ 238U in natural waters

The daughter to parent ( 234U/ 238U) activity ratio in natural waters is often out of secular radioactive equilibrium. The major reason for this disequilibrium is related to the energetic α-decay of 238U and differential release of 234U relative to 238U. This disequilibrium originates from (1) preferential release of more loosely bound 234U from damaged mineral lattice sites or; (2) direct recoil of 234Th into surrounding media from near mineral surface boundaries, however, it is unclear which of the two mechanisms is most important in nature. To better quantify the effects of preferential release of 234U, two continuous laboratory granite leaching experiments conducted over 1100 h were performed. The leachates were characterized by declining U concentrations with time and ( 234U/ 238U) initially greater than unity (up to 1.15), which changed to below unity during leaching (∼0.95). The early elevated ( 234U/ 238U) suggests that additional 234U is released into solution by preferential release of 234U from mineral phases. However, the excess 234U constitutes a finite pool of easy leachable 234U and the ( 234U/ 238U) values become lower than unity when this pool is used up. A model based on first-order kinetics, dissolution rates and preferential release of 234U from damaged lattice sites was developed and is able to quantitatively predict the observed pattern of ( 234U/ 238U) values and U concentrations for the two granite leaching experiments. Extending the modeling to longer time scales more comparable to natural systems shows that the production of waters with high ( 234U/ 238U) ratios can be achieved in two distinct regimes (1) slow weathering where the rate of directly recoiled 234U near mineral surfaces into waters is high; (2) fast weathering where the role of incipient chemical weathering and preferential release of loosely bound 234U are important. The model is able to explain apparent opposite correlations between physical erosion rates and ( 234U/ 238U) in waters and it provides a new framework that will be useful for examining weathering regimes, their timescales and their coupling with physical erosion.