Relation between "terra rossa" from the Apulia aquifer of Italy and the radon content of groundwater: Experimental results and their applicability to radon occurrence in the aquifer

 The radon-222 (²²²Rn) activity in groundwater of the Apulian karstic aquifer in southern Italy is as great as 500 Becquerel per liter (Bq/L) locally. Normal radium-226 (²²⁶Ra) activity in the limestone and calcareous dolomites of the aquifer is not enough to explain such a high level. Laboratory investigations identified high ²²⁶Ra activity in the "terra rossa," the residuum occupying fissures and cavities in the bedrock, and also the relation between (1) ²²⁶Ra-bearing bedrock and "terra rossa" and (2) ²²²Rn in water. The "terra rossa" is the primary source of the radon in the groundwater. The experimental results show the need to characterize the "terra rossa" of Apulia on the basis of ²²⁶Ra activity and also to study the distribution and variations in ²²²Rn activity over time in the aquifer. Received, January 1996 · Revised, March 1997, September 1997, June 1998 · Accepted, July 1998     

On 226Ra and 222Rn concentrations in the brackish waters of coastal aquifers: lab-investigations and confirmation in the carbonate aquifer of Brindisi (Italy)

The ground waters circulating in the Apulian mesozoic carbonate aquifer, of coastal type, show high concentrations of ²²²Rn everywhere. Considering their variation during the different phases of a hydrological year, such high concentration values can reach activity of 20 Bq/L, in the more internal zones of the aquifer. Moreover, it is often observed that, in correspondence of wells and springs nearest the coast, the concentrations of radioactive gas reach values greater than 400 Bq/L and vary considerably during the course of a day and with withdrawals. The research carried out over the last few years, has confirmed that ²²⁶Ra and ²²²Rn concentrations in the karst groundwater of Apulia, are mainly related to the occurrence of Terra Rossa inside the aquifer and the capacity of these paleosols to fix the salts of ²²⁶Ra coming from the dissolution of the calcareous and calcareous-dolomitic rocks. This paper shows the results of the analysis performed to define ²²²Rn increase in the brackish waters that come in contact with carbonate rocks and terra rossa. It also indicates the results of surveys performed in a coastal zone with well-known hydrogeological features. The controls performed during one hydrological year, have confirmed the relationships between the salt content of the ground waters and the enrichment of ²²²Rn and have highlighted that the manner of increase of this radioisotope is related to cases of ionic exchange and adsorption regulated by the dynamics of marine intrusion.     

Ra-226 and Rn-222 in saline water compartments of the Aral Sea region

The Aral Sea has been shrinking since 1963 due to extensive irrigation and the corresponding decline in the river water inflow. Understanding of the current hydrological situation demands an improved understanding of the surface water/groundwater dynamics in the region. ²²²Rn and ²²⁶Ra measurements can be used to trace groundwater discharge into surface waters. Data of these radiometric parameters were not previously available for the study region. We determined ²²²Rn activities after liquid phase extraction using Liquid Scintillation Counting (LSC) with peak-length discrimination and analyzed ²²⁶Ra concentrations in different water compartments of the Amu Darya Delta (surface waters, unconfined groundwater, artesian water, and water profiles from the closed Large Aral Sea (western basin).The water samples comprise a salinity range between 1 and 263 g/l. The seasonal dynamics of solid/water interaction under an arid climate regime force the hydrochemical evolution of the unconfined groundwater in the Amu Darya Delta to high-salinity Na(Mg)Cl(SO₄) water types. The dissolved radium concentrations in the waters were mostly very low due to mineral over-saturation, extensive co-precipitation of radium and adsorption of radium on coexisting solid substrates.The analysis of very low ²²⁶Ra concentrations (<10 ppq) at remote study sites is a challenge. We used the water samples to test and improve different analytical methods. In particular, we modified a procedure developed for the α-spectrometric determination of ²²⁶Ra after solid phase extraction of radium using 3M Empore™ High Performance Extraction Disks (Purkl, 2002) for the analysis of the radionuclide using an ICP sector field mass spectrometer. The ²²⁶Ra concentration of 17 unconfined groundwater samples ranged between 0.2 and 5 ppq, and that of 28 artesian waters between <0.2 and 13 ppq. The ICP-MS results conformed satisfactorily to analytical results based on γ-measurements of the ²²²Rn ingrowth after purging and trapping on super-cooled charcoal. The ²²⁶Ra concentrations were positively correlated with the salinity and the dissolved NaCl concentrations. The occurrence of unusually high ²²⁶Ra activities is explained by radium release from adsorption sites with increasing salinity. The inferred spatial variability of ²²²Rn in the Aral Sea and of ²²²Rn and ²²⁶Ra in the groundwater of the Amu Darya Delta is discussed in the context of our own previous hydrochemical studies in the study sites. Relatively low ²²²Rn activities in the unconfined GW (1–9.5 Bq/l) indicate the alluvial sediments hosting the GW to be a low-²³⁸U(²²⁶Ra) substrate. Positive correlations between U and ²²⁶Ra, and U and ²²²Rn are likely related to locally deposited Fe(Mn)OOH precipitates. The ²²²Rn activity of the GW, however, distinctly exceeds the ²²²Rn concentration in the Aral Sea (10 mBq/l), in principle, making ²²²Rn a sensitive tracer for the inflow of GW. The high water volume of the Large Aral Sea and wind induced mixing of its water body, however, hamper the detection of local groundwater inflow.     

Ra and Rn isotopes as natural tracers of submarine groundwater discharge in the patagonian coastal zone (Argentina): an initial assessment

Submarine groundwater discharge (SGD) is herein recognized as a significant pathway of material transport from land to the coastal SW Atlantic Ocean and thus, it can be a relevant factor affecting the marine biogeochemical cycles in the region. This paper focuses on the initial measurements of ²²⁶Ra, ²²⁸Ra and ²²²Rn made in Patagonia’s coastal zone of Chubut and Santa Cruz provinces (42°S–48°S, Argentina). ²²⁶Ra activity ranged from 2.9 to 73.5 dpm 100 L^?1, and ²²⁸Ra activity ranged from 11.9 to 311.0 dpm 100 L^?1 in groundwater wells. The radium activities found in Patagonia’s marine coastal regions and adjacent shelf indicate significant enrichment throughout the coastal waters. Groundwater samples presented the largest ²²²Rn activity and ranged from 2.66 to 1083 dpm L^?1. Conversely, in the coastal marine environment, the ²²²Rn activity ranged from 1.03 to 6.23 dpm L^?1. The Patagonian coastal aquifer showed a larger enrichment in ²²⁸Ra than in ²²⁶Ra, which is a typical feature for sites where SGD is dominant, probably playing a significant role in the biogeochemistry of these coastal waters.     

The transport of U- and Th-series nuclides in sandy confined aquifers

Abundances of ²³⁸U, ²³⁴U, ²³²Th, ²²⁶Ra, ²²⁸Ra, ²²⁴Ra, and ²²²Rn were measured in groundwaters of the Ojo Alamo aquifer in northwest New Mexico. This is an arid area with annual precipitation of ∼22 cm. The purpose was to investigate the transport of U-Th series nuclides and their daughter products in an old, slow-moving groundwater mass as a means of understanding water-rock interactions and to compare the results with a temperate zone aquifer. It was found that ²³²Th is approximately at saturation and supports the view of Tricca et al. (2001) that Th is precipitated irreversibly upon weathering, leaving surface coatings of ²³²Th and ²³⁰Th on aquifer grains. Uranium in the aquifer waters has very high [²³⁴U/²³⁸U] ∼ 9 and low ²³⁸U concentrations. These levels can be explained by low weathering rates in the aquifer (w_238U ∼ 2 × 10⁻¹⁸ to 2 × 10⁻¹⁷s⁻¹) using a continuous flow, water-rock interaction model. The Ra isotopes are roughly in secular equilibrium despite their very different mean lifetimes. The ²²²Rn and ²²⁸Ra isotopes in the aquifer correspond to ∼10% of the net production rate of the bulk rock. This is interpreted to reflect an earlier formed irreversible surface coating of Th that provides Ra and Rn to the aquifer waters. The surface waters that appear to be feeding the aquifer have low [²³⁴U/²³⁸U] and high ²³⁸U concentrations. The flow model shows that it is not possible to obtain the high [²³⁴U/²³⁸U] and low [²³⁸U] values in the aquifer from a source like the present vadose zone input. It follows that the old aquifer waters studied cannot be fed by the present vadose zone input unless they are greatly diluted with waters with very low U concentrations. If the present sampling of vadose zone sources is representative of the present input, then this requires that there was a major change in water input with much larger rainfall some several thousand years ago. This may represent a climatic change in the Southwest.     

Factors controlling the groundwater transport of U, Th, Ra, and Rn

A model for the groundwater transport of naturally occurring U, Th, Ra, and Rn nuclides in the²³⁸U and²³²Th decay series is discussed. The model developed here takes into account transport by advection and the physico-chemical processes of weathering, decay, α-recoil, and sorption at the water-rock interface. It describes the evolution along a flowline of the activities of the²³⁸U and²³²Th decay series nuclides in groundwater. Simple sets of relationships governing the activities of the various species in solution are derived, and these can be used both to calculate effective retardation factors and to interpret groundwater data. For the activities of each nuclide, a general solution to the transport equation has been obtained, which shows that the activities reach a constant value after a distance ϰ_i, characteristic of each nuclide. Where ϰ_i is much longer than the aquifer length, (for²³⁸U,²³⁴U, and²³²Th), the activities grow linearly with distance. Where gKⁱ is short compared to the aquifer length, (for²³⁴Th,²³⁰Th,²²⁸Th,²²⁸Ra, and²²⁴Ra), the activities rapidly reach a constant or quasi-constant activity value. For²²⁶Ra and²²²Rn, the limiting activity is reached after 1 km. High δ²³⁴U values (proportional to the ratio^ɛ234Th/^W238U) can be obtained through high recoil fraction and/or low weathering rates. The activity ratios²³⁰Th/²³²Th,²²⁸Ra/²²⁶Ra and²²⁴Ra/²²⁶Ra have been considered in the cases where either weathering or recoil is the predominant process of input from the mineral grain. Typical values for weathering rates and recoil fractions for a sandy aquifer indicate that recoil is the dominant process for Th isotopic ratios in the water. Measured data for Ra isotope activity ratios indicate that recoil is the process generally controlling the Ra isotopic composition in water. Higher isotopic ratios can be explained by different desorption kinetics of Ra. However, the model does not provide an explanation for²²⁸Ra/²²⁶Ra and²²⁴Ra/²²⁶Ra activity ratios less than unity. From the model, the highest²²²Rn emanation equals 2ɛ. This is in agreement with the hypothesis that²²²Rn activity can be used as a first approximation for input by recoil (Krishnaswamiet al 1982). However, high²²²Rn emanation cannot be explained by production from the surface layer as formulated in the model. Other possibilities involve models including surface precipitation, where the surface layer is not in steady-state.     

Relationships between radium and radon occurrence and hydrochemistry in fresh groundwater from fractured crystalline rocks, North Carolina (USA)

Naturally-occurring radionuclides (uranium, radium, and radon), major dissolved constituents, and trace elements were investigated in fresh groundwater in 117 wells in fractured crystalline rocks from the Piedmont region (North Carolina, USA). Chemical variations show a general transition between two water types: (1) slightly acidic (pH 5.0–6.0), oxic, low-total dissolved solids (TDS) waters, and (2) near neutral, oxic to anoxic, higher-TDS waters. The uranium, radium, and radon levels in groundwater associated with granite (Rolesville Granite) are systematically higher than other rock types (gneiss, metasedimentary, and metavolcanic rocks). Water chemistry plays a secondary role on radium and radon distributions as the ²²²Rn/²²⁶Ra activity ratio is correlated with redox-sensitive solutes such as dissolved oxygen and Mn concentrations, as well as overall dissolved solids content including major divalent cations and Ba. Since ²²⁴Ra/²²⁸Ra activity ratios in groundwater are close to 1, we suggest that mobilization of Ra and Rn is controlled by alpha recoil processes from parent nuclides on fracture surfaces, ruling out Ra sources from mineral dissolution or significant long-distance Ra transport. Alpha recoil is balanced by Ra adsorption that is influenced by redox conditions and/or ion concentrations, resulting in an approximately one order of magnitude decrease (~ 20,000 to ~ 2000) in the apparent Ra distribution coefficient between oxygen-saturated and anoxic conditions and also across the range of dissolved ion concentrations (up to ~ 7 mM). Thus, the U and Th content of rocks is the primary control on observed Ra and Rn activities in groundwater in fractured crystalline rocks, and in addition, linked dissolved solids concentrations and redox conditions impart a secondary control.     

Ground water geochemistry of 228Ra, 226Ra and 222Rn

²²⁸Ra, ²²⁶Ra, and ²²²Rn activities were determined on over 150 ground water samples collected from drilled, public water supply wells throughout South Carolina. A wide range of aquifer lithologies were sampled including the crystalline rocks of the Piedmont and sedimentary deposits of the Coastal Plain. A significant linear relationship between log ²²⁸Ra and log ²²⁶Ra (n = 182, r = 0.83) was indistinguishable between Piedmont and Coastal Plain ground water. Median ${}^{228}\text{Ra}{}^{226}\text{Ra}$ activity ratios for the Piedmont, 1.2, and Coastal Plain, 1.3, ground water are close to estimated average crustal ${}^{232}\text{Th}{}^{238}\text{U}$ activity ratios of 1.2 to 1.5 corresponding to Th/U weight ratios of 3.5 to 4.5. A linear correlation was also found between log ²²²Rn and log ²²⁶Ra for Piedmont (n = 68, r = 0.62) and Coastal Plain (n = 89, r = 0.64) ground water. However, the median ${}^{222}\text{Rn}{}^{226}\text{Ra}$ activity ratio for Piedmont ground water, 6100, was much higher than for Coastal Plain ground water, 230. Higher excess ²²²Rn activities may be due to greater retention of ²²⁶Ra by the chemically active Piedmont aquifers compared to the more inert sand aquifers sampled in the Coastal Plain. The relationship between log ²²⁸Ra and log ²²⁶Ra was used to predict total Ra (${}^{228}\text{Ra +}{}^{226}\text{Ra}$) distributions in Appalachian and Atlantic and Gulf Coastal Plain ground water. Predictions estimate that 2.4% of Appalachian and 5.3% of Atlantic and Gulf Coastal Plain ground water supplies contain total Ra activities in excess of the 5 pCi/l limit established by the U.S. Environmental Protection Agency. These predictions also indicate that 40–50% of these ground water wells may be overlooked using the presently suggested screening activity of 3.0 pCi/l of ²²⁶Ra for ²²⁸Ra analysis.     

Radon-222 in medicinal groundwaters of Szczawno Zdrój (Sudety Mountains, SW Poland)

 Radon is a significant component of the groundwaters that discharge in the springs of Szczawno Zdrój and are recognized as medicinal. However, among the five exploited springs adjoining each other, it is only in Marta Spring that radon occurs in large concentrations (up to 325.6 Bq/dm³). Therefore, the authors have made an attempt to describe and clarify this fact. They found out from their own research and archival data that ²²²Rn dissolves in the waters of Marta Spring after acidulous waters of deep circulation have mixed with poorly mineralized shallow waters in their outflow zone. The genesis of the gas is determined by the content of its parent nuclide, ²²⁶Ra, in the sandstones in the vicinity of the intake. The volume of the rocks providing radon to the waters of this intake has been estimated at several hundred cubic metres. No seasonal fluctuations in radon concentration have been observed and ²²²Rn concentration changes do not seem to be influenced by changes in the concentration of other chemical components of the waters or by the discharge of the intake. The process of dissolving ²²²Rn in the medicinal groundwaters of Marta Spring is the last, the shortest, and the most local of the processes that form the chemical composition and the physical properties of these waters. Received: 7 January 2000 · Accepted: 12 August 2000     

The relationship of soil 210Po and 210Pb geochemical dispersion patterns to uranium mineralization

²¹⁰Po and ²¹⁰Pb measurements of soils delineated uranium anomalies at three out of four test sites in Ontario, Canada. Measurements were made of ²¹⁰Po in solutions produced by both complete digestion and partial leaching of soil samples. Direct plating of ²¹⁰Po onto metal plates was followed by measuring the alpha activity. Subsequent plating of ²¹⁰Po in-grown from ²¹⁰Pb in solution several months later confirmed the anomalies.The ²¹⁰Po and ²¹⁰Pb anomalies at three of the test sites coincided with ²²⁶Ra and ²²²Rn anomalies. Samples from the fourth uranium occurrence associated with a known ²²²Rn anomaly failed to show either a ²²⁶Ra anomaly on the one hand, or ²¹⁰Po or ²¹⁰Pb anomalies on the other. This suggests that the ²¹⁰Po and ²¹⁰Pb anomalies were probably produced by the decay of ²²⁶Ra contained within secondary dispersion haloes.Although anomalies due to the ²¹⁰Po and ²¹⁰Pb products of ²²²Rn have now been documented, prospecting methods based on their use as direct tracers of the migration paths of ²²²Rn require much further development.     

The relationship of soil Po and Pb geochemical dispersion patterns to uranium mineralization

²¹⁰Po and ²¹⁰Pb measurements of soils delineated uranium anomalies at three out of four test sites in Ontario, Canada. Measurements were made of ²¹⁰Po in solutions produced by both complete digestion and partial leaching of soil samples. Direct plating of ²¹⁰Po onto metal plates was followed by measuring the alpha activity. Subsequent plating of ²¹⁰Po in-grown from ²¹⁰Pb in solution several months later confirmed the anomalies.The ²¹⁰Po and ²¹⁰Pb anomalies at three of the test sites coincided with ²²⁶Ra and ²²²Rn anomalies. Samples from the fourth uranium occurrence associated with a known ²²²Rn anomaly failed to show either a ²²⁶Ra anomaly on the one hand, or ²¹⁰Po or ²¹⁰Pb anomalies on the other. This suggests that the ²¹⁰Po and ²¹⁰Pb anomalies were probably produced by the decay of ²²⁶Ra contained within secondary dispersion haloes.Although anomalies due to the ²¹⁰Po and ²¹⁰Pb products of ²²²Rn have now been documented, prospecting methods based on their use as direct tracers of the migration paths of ²²²Rn require much further development.     

Lead isotopic compositions in fault gouges and their parent rocks: implications for ancient fault activities

This is the first study on Pb stable isotopes in fault gouges and their parent rocks. We analyzed the composition of Pb isotopes and contents of U and Pb in 10 pairs of fault gouges and their parent rocks collected along several active faults in central Japan. Thorium-232-²⁰⁸Pb ages of two fault systems were determined as pre-Tertiary, which are consistent with the data from KAr ages and geological considerations.Naturally, the²³⁵U²⁰⁷Pb system is of little use for dating because the magnitude of difference in²⁰⁷Pb/²⁰⁴Pb between gouges and parent rocks is too small. It is found that the²⁰⁶Pb/²⁰⁴Pb can indicate the contribution of²⁰⁶Pb resulting from excess supplies of²²⁶Ra and²²²Rn along the fault. The excess²⁰⁶Pb accumulation rate corresponds to the average²²²Rn concentration in soil gas or groundwater through geological time since the gouge formation. A comparison of Quaternary fault activity and estimated Tertiary activity reveals the characteristics of each fault system.     

Radionuclides in ground water of the Carson River Basin,western Nevada and eastern California,U.S.A.

Ground water is the main source of domestic and public supply in the Carson River Basin. Ground water originates as precipitation primarily in the Sierra Nevada in the western part of Carson and Eagle Valleys, and flows down gradient in the direction of the Carson River through Dayton and Churchill Valleys to a terminal sink in the Carson Desert. Because radionuclides dissolved in ground water can pose a threat to human health, the distribution and sources of several naturally occurring radionuclides that contribute to gross-alpha and gross-beta activities in the study area were investigated. Generally, alpha and beta activities and U concentration increase from the up-gradient to down-gradient hydrographic areas of the Carson River Basin, whereas²²²Rn concentration decreases. Both²²⁶Ra and²²⁸Ra concentrations are similar throughout the study area. Alpha and beta activities and U concentration commonly exceed 100 pCi/l in the Carson Desert at the distal end of the flow system. Radon-222 commonly exceeds 2,000 pCi/l in the western part of Carson and Eagle Valleys adjacent to the Sierra Nevada. Radium-226 and²²⁸Ra concentrations are <5pCi/l. Four ground water samples were analyzed for²¹⁰Po and one sample contained a high concentration of 21 pCi/l. Seven samples were analyzed for²¹⁰Pb; six contained <3pCi/l and one contained 12 pCi/l. Thorium-230 was detected at concentrations of 0.15 and 0.20 pCi/l in two of four samples.Alpha-emitting radionuclides in the ground water originated from the dissolution of U-rich granitic rocks in the Sierra Nevada by CO₂, oxygenated water. Dissolution of primary minerals, mainly titanite (sphene) in the granitic rocks, releases U to the water. Dissolved U is probably removed from the water by adsorption on Fe- and Mn-oxide coatings on fracture surfaces and fine-grained sediment, by adsorption on organic matter, and by coprecipitation with Fe and Mn oxides. These coated sediments are transported throughout the basin by fluvial processes. Thus, U is transported as dissolved and adsorbed species. A rise in the water table in the Carson Desert because of irrigation has resulted in the oxidation of U-rich organic matter and dissolution of U-bearing coatings on sediments, producing unusually high U concentration in the ground water.Alpha activity in the ground water is almost entirely from the decay of U dissolved in the water. Beta activity in ground water samples is primarily from the decay of⁴⁰K dissolved in the water and ingrowth of²³⁸U progeny in the sample before analysis. Approximately one-half of the measured beta activity may not be present in ground water in the aquifer, but instead is produced in the sample after collection and before analysis. Potassium-40 is primarily from the dissolution of K-containing minerals, probably K-feldspar and biotite. Radon-222 is primarily from the decay of²²⁶Ra in the aquifer materials. Radium in the ground water is thought to be mainly from alpha recoil associated with the decay of Th in the aquifer material. Some Ra may be from dissolution (or desorption) or Ra-rich coatings on sediments.     

Geostatistical techniques application to dissolved radon hazard mapping: An example from the western sector of the Sabatini Volcanic District and the Tolfa Mountains (central Italy)

Dissolved Rn was determined in 192 samples collected from cold shallow volcanic and sedimentary aquifers, deep thermal aquifers and from waters associated with bubbling gases in the western sector of the Sabatini Volcanic District and the Tolfa Mountains (central Italy). Shallow aquifers hosted in the Quaternary volcanic complexes show values ranging from 1.0 to 352 Bq/L (median value 55 and inter-quartile distance 62 Bq/L), while waters circulating within the permeable horizons of the sandy-to-clayey sediments of the Tolfa flysch have values from 1.0 to 44 Bq/L (median value 6.9 and inter-quartile distance 8.1 Bq/L). Thermal waters are hosted in the Mesozoic carbonate formations and move towards the surface along faults. Here, dissolved Rn values range from 0 to 37 Bq/L (median value 3.0 and inter-quartile distance 9.5 Bq/L). Waters associated with bubbling gases show dissolved Rn contents ranging from 2.0 to 48 Bq/L (median value 6.2 and inter-quartile distance 23 Bq/L). Those results suggest that lithology is the main factor affecting the Rn contents in shallow aquifers, due to the high levels of Rn progenitors U and Ra in the volcanic rocks relative to sedimentary units. The influence of other factors such as the presence of a fracture network, seasonal flow variations, type of discharge (from well or spring) was also investigated. Radon contents of thermal waters result from mixing with shallow waters (from both volcanic and sedimentary rock aquifers) and decrease of Rn solubility with temperature, while for bubbling pools the effects of strong degassing were also considered.In terms of health hazard from direct ingestion of Rn-rich waters, 20.8% of those circulating within the volcanic aquifer show values higher than the recommended value of 100 Bq/L, while none of those circulating within the sedimentary aquifers exceed the threshold value. Geostatistical techniques were used for the elaboration of contour maps by using variogram models and kriging estimation aimed at defining the areas where a potential health hazard due to the direct ingestion of Rn-rich waters and to inhalation of air following degassing of Rn from waters may be expected.     

Radium geochemistry of ground waters in Paleozoic carbonate aquifers,midcontinent, USA

The purpose of this study was to elucidate the processes controlling the distribution and behavior of the longer-lived Ra isotopes in continuous Paleozoic carbonate aquifers of parts of Missouri, Kansas, and Oklahoma. Activities of (²²⁸Ra) and (²²⁶Ra) were analyzed in fresh and saline ground waters, brines, and rocks. The fluids have a wide salinity range (200–250,000 mg l⁻¹ total dissolved solids). The (²²⁶Ra) activity ranges from 0.66–7660 dpm kg⁻¹ and correlates with salinity and other alkaline earth element (Ca, Sr, and Ba) concentrations. The range of (²²⁸Ra:²²⁶Ra) ratios in the fluids (0.06–1.48) is similar to that in the aquifer rocks (0.21–1.53). The relatively low mean fluid (²²⁸Ra:²²⁶Ra) ratio (0.30) reflects the low Th:U ratio of the predominant carbonate aquifer rock. Radium occurs mostly (≥77%) as Ra²⁺ species in the fluids. Salinity-dependent sorption–desorption processes (with log K values from 10⁰–10⁴ and negatively correlated with salinity), involving Th-enriched surface coatings on aquifer flow channels, can explain the rapid solid–fluid transfer of Ra isotopes in the system and the correlation of Ra with salinity.     

Characterizing vertical contaminant distribution in a thick unconfined aquifer,Hanford site,Washington, USA

Drilling-intensive aquifer characterization techniques have been used to obtain depth-discrete water samples from a thick, hydrogeologically continuous unconfined aquifer system; groundwater results indicate that carbon tetrachloride contamination is widespread and extends deeper and at concentrations much higher than detected in monitoring networks at the water table. Carbon tetrachloride, a legacy waste, was used in the plutonium extraction process at the Hanford site in south-central Washington State. Vertical, depth-discrete groundwater samples were collected during well drilling throughout a 28-km² region to determine the concentration of carbon tetrachloride present as a dissolved phase in the aquifer. Results indicate that high concentrations of carbon tetrachloride, three orders of magnitude above the allowable regulatory limit, are present at depths greater than 25 m beneath the water table. In support of future efforts to remediate the carbon tetrachloride contamination, it is imperative to locate the remaining chemical inventory, determine the vertical as well as the lateral distribution of this contaminant and its physical form. Depth-discrete aquifer characterization throughout the uppermost-unconfined aquifer system is providing this information and improving the understanding of the contaminant distribution and the hydrogeologic framework through which it moves.

Mapping of groundwater quality in the Turonian aquifer of Oum Er-Rabia Basin, Morocco: a case study

This study takes the groundwater of the Moroccan limestone aquifer of Oum Er-Rabia as an example of statistical and cartographical approaches in water resources management. Statistical analyses based on frequency distribution and PCA methods revealed the homogeneity of waters with the existence of abnormal points and have helped to assess correlations between the studied variables. The mapping approach illustrated that waters are influenced by the lithology of the surrounding rocks and are of Ca–Mg–HCO₃, Ca–Mg–Cl–SO₄, and mixed types according to the Piper classification. The quality of water is of high to medium, north of the basin, but it is of medium to bad, NE and south, due to excessive contents of chloride, sulfate and nitrate. According to the US Salinity Laboratory classification, water used for irrigation in the eastern and the southern parts of the basin should take into consideration the drainage conditions, the nature of plants and the addition of gypsum doses.

Is 222Rn a suitable tracer of stream–groundwater interactions? A case study in central Italy

River water infiltration into an unconfined porous aquifer (∼73% gravels, ∼12% sands, ∼15% silts and clays) in the Petrignano d’Assisi plain, central Italy, was traced combining isotopic techniques (²²²Rn) with hydrochemical and hydrogeologic techniques in order to characterize the system under study. The ²²²Rn gave information about the river water residence times within the aquifer and hydrochemical data, in a two-component mixing model, which allowed estimating the extent of mixing between surface waters and groundwater in wells at increasing distances from the river. The mixing measured in the well closer to the riverbank indicated a higher contribution of river water (up to 99%) during the groundwater recession phase and a moderate contribution (up to 64%) during the recharge phase. A model describing ²²²Rn concentrations in groundwater as the result of both parent/daughter nuclide equilibrium and mixing process (²²²Rn mixing/saturation model) was used to describe observed Rn concentrations and mixing index trends with the aim of evaluating water mean infiltration velocities along the transect. The stream bank infiltration velocities obtained by the model ranged from 1 m day⁻¹ during groundwater recharge periods, when river water infiltration is lower, to 39 m day⁻¹ during recession phases, when river water infiltration is larger.     

Ra/Ra and Ra/Ba ratios in the Western Mediterranean Sea: Barite formation and transport in the water column

²²⁶Ra, ²²⁸Ra and Ba distributions as well as ²²⁸Ra/²²⁶Ra and ²²⁶Ra/Ba ratios were measured in seawater, suspended and sinking particles at the DYFAMED station in the Western Mediterranean Sea at different seasons of year 2003 in order to track the build-up and fate of barite through time. The study of the ²²⁸Ra_ex/²²⁶Ra_ex ratios (Ra_ex = Ra activities corrected for the lithogenic Ra) of suspended particles suggests that Ba_ex (Ba_ex = Ba concentrations corrected for the lithogenic Ba, mostly barite) formation takes place not only in the upper 500 m of the water column but also deeper (i.e. throughout the mesopelagic layer). Temporal changes in the ²²⁸Ra_ex/²²⁶Ra_ex ratios of sinking particles collected at 1000 m depth likely reflect changes in the relative proportion of barite originating from the upper water column (with a high ²²⁸Ra/²²⁶Ra ratio) and formed in the mesopelagic layer (with a low ²²⁸Ra/²²⁶Ra ratio). ²²⁸Ra_ex/²²⁶Ra_ex ratios measured in sinking particles collected in the 1000 m-trap in April and May suggest that barite predominantly formed in the upper water column during that period, while barite found outside the phytoplankton bloom period (February and June) appears to form deeper in the water column. Combining ratios of both the suspended and sinking particles provides information on aggregation/disaggregation processes. High ²²⁶Ra_ex/Ba_ex ratios were also found in suspended particles collected in the upper 500 m of the water column. Because celestite is expected to be enriched in Ra [Bernstein R. E., Byrne R. H. and Schijf J. (1998) Acantharians: a missing link in the oceanic biogeochemistry of barium. Deep-Sea Res. II45, 491-505], acantharian skeletons may contribute to these high ratios in shallow waters. The formation of both acantharian skeletons and barite enriched in ²²⁶Ra may thus contribute to the decrease in the dissolved ²²⁶Ra activity and ²²⁶Ra/Ba ratios of surface waters observed between February and June 2003 at the DYFAMED station.