Legacy of anthropogenic lead in urban soils: Co-occurrence with metal(loids) and fallout radionuclides, isotopic fingerprinting, and in vitro bioaccessibility.

Anthropogenic lead (Pb) in soils poses risks to human health, particularly to the neuropsychological development of exposed children. Delineating the sources and potential bioavailability of soil Pb, as well as its relationship with other contaminants is critical in mitigating potential human exposure. Here, we present an integrative geochemical analysis of total elemental concentrations, radionuclides of 137Cs and 210Pb, Pb isotopic compositions, and in vitro bioaccessibility of Pb in surface soils sampled from different locations near Durham, North Carolina. Elevated Pb (>400 mg/kg) was commonly observed in soils from urban areas (i.e., near residential house foundation and along urban streets), which co-occurred with other potentially toxic metal(loids) such as Zn, Cd, and Sb. In contrast, soils from city parks and suburban areas had systematically lower concentrations of metal(loids) that were comparable to geological background. The activities of 137Cs and excess 210Pb, coupled with their correlations with Pb and co-occurring metal(loids) were used to indicate the persistence and remobilization of historical atmospherically deposited contaminants. Coupled with total Pb concentrations, the soil Pb isotopic compositions further indicated that house foundation soils had significant input of legacy lead-based paint (mean = 1.1895 and 2.0618 for 206Pb/207Pb and 208Pb/206Pb, respectively), whereas urban streetside soils exhibited a clear mixed origin, dominantly of legacy leaded gasoline (1.2034 and 2.0416) and atmospheric deposition (1.2004-1.2055 and 2.0484-2.0525). The in vitro bioaccessibility of Pb in contaminated urban soils furthermore revealed that more than half of Pb in the contaminated soils was potentially bioavailable, whose Pb isotope ratios were identical to that of bulk soils, demonstrating the utility of using Pb isotopes for tracking human exposure to anthropogenic Pb in soils and house dust. Overall, this study demonstrated a holistic assessment for comprehensively understanding anthropogenic Pb in urban soils, including its co-occurrence with other toxic contaminants, dominant sources, and potential bioavailability upon human exposure.

Sorption Behavior and Aerosol-Particulate Transitions of 7Be, 10Be, and 210Pb: A Basis for Fallout Radionuclide Chronometry.

We investigated the partitioning of 7Be, 10Be, and 210Pb aerosols between operationally dissolved and >0.5 µm particulate fractions in wet and dry atmospheric deposition. Bulk deposition in situ-log(KD) averaged 4.27 ± 0.46 for 7Be and 4.79 ± 0.59 for 210Pb (±SD, n = 163), with corresponding activity-fractions particulate (fP) = 24 and 48%. KD was inversely correlated with particulate mass concentration (pC), a particle concentration effect (p.c.e.) that indicates that dissolved 7Be and 210Pb are bound to submicron colloids. Experimental desorption-KD was higher than in situ by a factor of 20 for 7Be and 4 for 210Pb (n = 27), indicating that FRN sorption to particulates was irreversible. 7Be:10Be ratios confirmed that colloidal and particulate fractions were geochemically distinct, with corresponding ages of 120 ± 30 and 260 ± 45 days, respectively [mean ± SE, n = 9, p = 0.011]. Fractions particulate fBe7, fBe10, and fPb210 each increased with 7Be:10Be bulk age, a particle-age effect (p.a.e). In multiple regression, fBe7 was best predicted by N, Mn, Al, and Ni [R2 = 0.75, p < 0.0001], whereas fPb relied on N, S, Fe, and Mn [R2 = 0.69, p < 0.0001]. Despite differences in magnitude and controls on partitioning, the ratio fBe:fPb converged to 1 with pC in the range of 10-100 mg L-1. Given sufficient solid surfaces, irreversible sorption and p.a.e. form a basis for 7Be:210Pb chronometry of aerosol biogeochemical cycling.

Determination of sedimentation, diffusion, and mixing rates in coastal sediments of the eastern Red Sea via natural and anthropogenic fallout radionuclides.

The Red Sea is a unique ecosystem with high biodiversity in one of the warmest regions of the world. In the last five decades, Red Sea coastal development has rapidly increased. Sediments from continental margins are delivered to depths by advection and diffusion-like processes which are difficult to quantify yet provide invaluable data to researchers. Beryllium-7, lead-210 and ceseium-137 were analyzed from sediment cores from the near-coast Red Sea near Jeddah, Saudi Arabia. The results of this work are the first estimates of diffusion, mixing, and sedimentation rates of the Red Sea coastal sediments. Maximum chemical diffusion and particle mixing rates range from 69.1 to 380cm-2y-1 and 2.54 to 6.80cm-2y-1, respectively. Sedimentation rate is constrained to approximately 0.6cm/yr via multiple methods. These data provide baselines for tracking changes in various environmental problems including erosion, marine benthic ecosystem silting, and particle-bound contaminant delivery to the seafloor.

Reconstructing the history of mining and remediation in the Coeur d'Alene, Idaho Mining District using lake sediments.

Mining that began in the late 1800s intensified during World War II contaminating Lake Coeur d'Alene sediments with potentially toxic elements. We used 80y of the sediment record to reconstruct metal(loid) loadings to the lake and quantitatively evaluate the effectiveness of tailings management. Sediment core analysis for pollen, chronological markers, and metal(loid)s permitted stratigraphic reconstruction showing that contaminant loading decreased after tailings pond construction, but that most metal(loid) concentrations exceed recommended limits. Arsenic concentrations (250-450 mg kg(-)(1)) at the sediment-water interface are potentially toxic; however, low P concentrations in recent sediments (1.0-1.4 mg kg(-)(1)) inhibit eutrophication and the concomitant release of soluble As. Zinc (3 g kg(-)(1)), Cd (10 mg kg(-)(1)), Ag (10 mg kg(-)(1)), and Cu (90 mg kg(-)(1)) concentrations are now lower than in sediments deposited during active mining, but remain an environmental concern. Sedimentary Cr and Pb concentrations have not changed in the last 50y, because tailings continue to enter the lake. Although modern Cr concentrations (40 mg kg(-)(1)) are unlikely to cause toxicity, current Pb concentrations (4 g kg(-)(1)) exceed acceptable limits, creating challenges for remediation. Strategies to manage other mining-contaminated watersheds should include consideration of elemental differences when evaluating remediation effectiveness.

Quantitative retention of atmospherically deposited elements by native vegetation is traced by the fallout radionuclides 7Be and 210Pb.

Atmospheric deposition is the primary mechanism by which remote environments are impacted by anthropogenic contaminants. Vegetation plays a critical role in intercepting atmospheric aerosols, thereby regulating the timing and magnitude of both contaminant and nutrient delivery to underlying soils. However, quantitative models describing the fate of atmospherically derived elements on vegetation are limited by a lack of long-term measurements of both atmospheric flux and foliar concentrations. We addressed this gap in understanding by quantifying weekly atmospheric deposition of the naturally occurring radionuclide tracers (7)Be and (210)Pb, as well as their activities in leaves of colocated trees, for three years in New Hampshire, U.S. The accumulation of both (7)Be and (210)Pb in deciduous and coniferous vegetation is predicted by a model that is based solely on measured atmospheric fluxes, duration of leaf exposure, and radioactive decay. Any "wash off" processes that remove (7)Be and (210)Pb from foliage operate with a maximum half-time of greater than 370 days (P > 99%), which is an order of magnitude longer than previously assumed. The retention of both (7)Be and (210)Pb on leaves is thus quantitative and permanent, coupling the fate of (7)Be, (210)Pb and similar atmospheric species to that of the leaf matter itself. These findings demonstrate that the long-standing paradigm of a short "environmental half-life" for atmospheric contaminants deposited on natural surfaces must be re-evaluated.

Incorporation of radiometric tracers in peat and implications for estimating accumulation rates.

Accurate dating of peat accumulation is essential for quantitatively reconstructing past changes in atmospheric metal deposition and carbon burial. By analyzing fallout radionuclides (210)Pb, (137)Cs, (241)Am, and (7)Be, and total Pb and Hg in 5 cores from two Swedish peatlands we addressed the consequence of estimating accumulation rates due to downwashing of atmospherically supplied elements within peat. The detection of (7)Be down to 18-20 cm for some cores, and the broad vertical distribution of (241)Am without a well-defined peak, suggest some downward transport by percolating rainwater and smearing of atmospherically deposited elements in the uppermost peat layers. Application of the CRS age-depth model leads to unrealistic peat mass accumulation rates (400-600 g m(-2) yr(-1)), and inaccurate estimates of past Pb and Hg deposition rates and trends, based on comparisons to deposition monitoring data (forest moss biomonitoring and wet deposition). After applying a newly proposed IP-CRS model that assumes a potential downward transport of (210)Pb through the uppermost peat layers, recent peat accumulation rates (200-300 g m(-2) yr(-1)) comparable to published values were obtained. Furthermore, the rates and temporal trends in Pb and Hg accumulation correspond more closely to monitoring data, although some off-set is still evident. We suggest that downwashing can be successfully traced using (7)Be, and if this information is incorporated into age-depth models, better calibration of peat records with monitoring data and better quantitative estimates of peat accumulation and past deposition are possible, although more work is needed to characterize how downwashing may vary between seasons or years.

Spatial and vertical distribution of mercury in upland forest soils across the northeastern United States.

Assessing current Hg pools in forest soils of the northeastern U.S. is important for monitoring changes in Hg cycling. The forest floor, upper and lower mineral horizons were sampled at 17 long-term upland forest sites across the northeastern U.S. in 2011. Forest floor Hg concentration was similar across the study region (274 ± 13 µg kg(-1)) while Hg amount at northern sites (39 ± 6 g ha(-1)) was significantly greater than at western sites (11 ± 4 g ha(-1)). Forest floor Hg was correlated with soil organic matter, soil pH, latitude and mean annual precipitation and these variables explained approximately 70% of the variability when multiple regressed. Mercury concentration and amount in the lower mineral soil was correlated with Fe, soil organic matter and latitude, corresponding with Bs horizons of Spodosols (Podzols). Our analysis shows the importance of regional and soil properties on Hg accumulation in forest soils.

Measuring environmental change in forest ecosystems by repeated soil sampling: a north american perspective.

Environmental change is monitored in North America through repeated measurements of weather, stream and river flow, air and water quality, and most recently, soil properties. Some skepticism remains, however, about whether repeated soil sampling can effectively distinguish between temporal and spatial variability, and efforts to document soil change in forest ecosystems through repeated measurements are largely nascent and uncoordinated. In eastern North America, repeated soil sampling has begun to provide valuable information on environmental problems such as air pollution. This review synthesizes the current state of the science to further the development and use of soil resampling as an integral method for recording and understanding environmental change in forested settings. The origins of soil resampling reach back to the 19th century in England and Russia. The concepts and methodologies involved in forest soil resampling are reviewed and evaluated through a discussion of how temporal and spatial variability can be addressed with a variety of sampling approaches. Key resampling studies demonstrate the type of results that can be obtained through differing approaches. Ongoing, large-scale issues such as recovery from acidification, long-term N deposition, C sequestration, effects of climate change, impacts from invasive species, and the increasing intensification of soil management all warrant the use of soil resampling as an essential tool for environmental monitoring and assessment. Furthermore, with better awareness of the value of soil resampling, studies can be designed with a long-term perspective so that information can be efficiently obtained well into the future to address problems that have not yet surfaced.

Threshold increases in soil lead and mercury from tropospheric deposition across an elevational gradient.

Atmospheric deposition is the primary mechanism by which remote ecosystems are contaminated, but few data sets show how fluxes change and control soil metal burdens at the landform scale. We present mercury (Hg), lead ((210)Pb and total Pb), and cosmogenic beryllium-7 ((7)Be) measurements in organic (O) soil horizons at high-resolution elevation intervals of ∼60 m from 540 to 1160 m on Camels Hump in northern Vermont, USA. Across this gradient, average O horizon Hg ranges from 0.99 mg m(-2) in the low elevation deciduous forest zone to 7.6 mg m(-2) in the higher elevation coniferous forest at 1030 m. We measure two pronounced threshold increases in soil metal burdens above 801 and 934 m, corresponding to the two most common altitudes of cloud base, which coincide with changes in vegetation species. Lead-210, a unique tracer of tropospheric deposition, also increased from 3200 Bq m(-2) to 11500 Bq m(-2) in O horizons, exhibiting threshold responses at the same elevations as Hg and total Pb. Concentrations of (210)Pb and Hg in foliage double from 760 to 900 m elevation, indicating enhanced deposition across the transition from deciduous to coniferous forest. In contrast, (7)Be is constant across the entire elevational gradient because of its upper atmospheric source. This indicates that the effects of orographic precipitation have a smaller control on soil contaminant burdens than the coupled cloudwater deposition-vegetation scavenging effect in the presence of upwind sources. By measuring soil contaminants and unique tracers of atmospheric deposition, we show that tropospheric fluxes of Hg and Pb are higher by a factor of 2 in high-elevation coniferous forests than in adjacent lowlands. Total O horizon Hg and Pb burdens increase by over 4-fold with elevation because of the compounding effects of enhanced deposition and longer metal residence times at higher elevations (>50 years).

Lead sequestration and species redistribution during soil organic matter decomposition.

The turnover of soil organic matter (SOM) maintains a dynamic chemical environment in the forest floor that can impact metal speciation on relatively short timescales. Here we measure the speciation of Pb in controlled and natural organic (O) soil horizons to quantify changes in metal partitioning during SOM decomposition in different forest litters. We provide a link between the sequestration of pollutant Pb in O-horizons, estimated by forest floor Pb inventories, and speciation using synchrotron-based X-rayfluorescence and X-ray absorption spectroscopy. When Pb was introduced to fresh forest O(i) samples, it adsorbed primarily to SOM surfaces, but as decomposition progressed over two years in controlled experiments, up to 60% of the Pb was redistributed to pedogenic birnessite and ferrihydrite surfaces. In addition, a significant fraction of pollutant Pb in natural soil profiles was associated with similar mineral phases (approximately 20-35%) and SOM (-65-80%). Conifer forests have at least 2-fold higher Pb burdens in the forest floor relative to deciduous forests due to more efficient atmospheric scavenging and slower organic matter turnover. We demonstrate that pedogenic minerals play an important role in surface soil Pb sequestration, particularly in deciduous forests, and should be considered in any assessment of pollutant Pb mobility.

Determining 234Th and 238U in rocks, soils, and sediments via the doublet gamma at 92.5 keV.

Efficient and accurate measurements of uranium (U) and U-series radionuclides in earth's materials are needed to assess its environmental impact, reconstruct geochemical histories, and quantify heat production in the crust. To date, measurements of 234Th and 238U by gamma spectrometry have relied on the quantification of 234Th gamma emissions at approximately 63 keV (absolute intensity = 3.7%) and the (234m)Pa gamma at 1001 keV (absolute intensity = 0.84%). However, use of the 63 keV emissions can be hampered by 232Th interferences and self absorption, and the 1001 keV photon has a very low yield. Here we describe the effective use of the 234Th doublet gamma emission at approximately 92.5 keV (total absolute intensity = 4.8%) for 234Th and 238U measurements. This doublet has been largely ignored because of its proximity to the Th K(alpha1) (93.3 keV) and thus its vulnerability to a Th self-fluorescence interference. We demonstrate that additions of U and 40K to a Th ore sample do not increase the count rate at 92-93 keV above that which would be expected from the associated additions of U and 234Th. We also show that the Th self-fluorescence interference appears to be an anomaly associated only with the analysis of relatively thick (>1 mm) Th minerals, and suggest that fluorescence will not complicate the 92-93 keV region in most environmental samples. A review of decay data reveals that Th K(alpha1) X-rays associated with the decay of 235U and 228Ac can significantly interfere with quantification of the 92.5 keV 234Th doublet. We show that simple experimentally-derived correction factors for these X-rays can be used to accurately determine 234Th using its strongest gammas, resulting in higher count rates and smaller self-absorption corrections relative to the traditional analytical lines. Total 1 sigma analytical error associated with U measurements at 92.5 keV ranged from 1 to 9% and is dominated by the relative size of the 228Ac interference. Detection limits for U in environmental samples using this technique are on the order of 50 ppb.

Using stable and radioactive isotopes to trace atmospherically deposited Pb in montane forest soils.

Atmospheric deposition of lead (Pb) throughout the 1900s resulted in elevated amounts of this toxic metal even in remote forest soils of the northeastern United States. Soils can act as a net sink for metals and thus minimize groundwater and surface water contamination. Recent studies utilizing forest floor temporal data and models of total Pb in precipitation, surface soils, and streams have estimated the time scale of Pb release from soils. However, due to the limited availability and spatial variability of forest floor survey data, other methods for quantifying anthropogenic Pb movement are needed. This study uses the isotopic composition (206Pb/207Pb) of soil Pb and measurements of 210Pb and 226Ra to directly trace the transit of atmospherically deposited Pb in the soil profile. We also report on the recovery of an enriched 207Pb dose applied in 1984 to the surface of a soil plot in the coniferous forest at Camels Hump in Vermont. The isotopic composition of soil Pb in low elevation deciduous forests suggests that approximately 65% of the original atmospheric Pb load has migrated from the forest floor to the upper 10 cm of the mineral soil. Higher elevation sites with coniferous vegetation have thicker forest floors, which have prevented significant amounts of Pb from entering the mineral soil. After 17 years, the soil organic horizon in the coniferous zone prevented any penetration of the applied Pb into the mineral soil. Using 210Pb budgets in different soil compartments, we determine forest floor response times for atmospherically delivered Pb to be approximately 60 years in the low elevation deciduous forest zone and 150 years for the high elevation spruce-fir forest zone at Camels Hump. According to its distribution in the soil profile, we conclude that a dispersed release of anthropogenic Pb to groundwater and surface water is possible this century. Our results also offer independent confirmation of Pb deposition models previously generated for the region.