The geochemistry of beryllium-7 in Chesapeake Bay

doi:10.1016/0272-7714(89)90086-3

Estuarine, Coastal and Shelf Science

Volume 28, Issue 4, April 1989, Pages 379-394

https://doi.org/10.1016/0272-7714(89)90086-3 Get rights and content

Abstract

The dissolved and particulate activities of ⁷Be in surface and deep water samples from four stations in the main stem of Chesapeake Bay were determined seven times between April, 1986 and April, 1987. Total water column concentrations ranged from < 0·05 to 3·86 dpm l⁻¹, with an average of 0·50 dpm l⁻¹. Batch adsorption experiments indicated a strong affinity of Be for Chesapeake Bay seston, but ⁷Be in the Bay was predominantly in the dissolved phase. Particulate ⁷Be activities reached 150 dpm g⁻¹, but averaged 18 dpm g⁻¹, suggesting that rapid variations in the supply of both ⁷Be and suspended particulates rarely allowed equilibrium partitioning of ⁷Be between the dissolved and particulate phases.

A ⁷Be budget for Chesapeake Bay indicated that atmospheric deposition was always the dominant source of ⁷Be in the Bay. Generally, ≥ 95% of the ⁷Be delivered to the Bay was retained by it. Most (usually > 80%) of the ⁷Be in Chesapeake Bay at any time was found in the sediments.

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Spatial and temporal patterns of sedimentation in an infilling reservoir
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Both of these radioisotopes have been used extensively in river, lake, and coastal settings (e.g. (Aalto et al., 2003; Goodbred and Kuehl, 1998; Kolker et al., 2009; Krishnaswami et al., 1980; Miller et al., 2005), but their use in reservoirs has been more limited. Sediments acquire most of their 7Be signature when they are on land (Blake et al., 2002; Dibb and Rice, 1989a; Olsen et al., 1985); thus, its presence in reservoir sediments requires delivery of watershed sediments without prolonged (>~200 days) storage in river channels and/or upstream reservoirs. 7Be has been used to investigate soil erosion delivering sediments to reservoirs (Shi et al., 2011), but its use specifically within reservoir sediments remains relatively unexplored.
River dams are ubiquitous globally, altering the connection of up- and downstream environments. In particular, they store vast amounts of sediment in their upstream reservoirs, many of which are approaching storage capacity. There is much interest in sediment management for these systems, even though quantifying sedimentation rates remains somewhat challenging. This study focuses on one such system, Conowingo Reservoir, upstream of the last and largest dam (Conowingo Dam) on the Susquehanna River before it enters Chesapeake Bay. The trapping capacity of Conowingo Reservoir has decreased since its creation in 1928, potentially increasing fluvial loading to the Bay. In addition, large flood events can scour much more than the average annual sediment load from the Reservoir bottom, delivering exceptionally large sediment loads to the Bay within days. Predicting the downstream impacts of both Reservoir infill and scour events is essential for management; however, these predictions are limited by understanding of the complex spatial and temporal variability of sedimentation. To evaluate this variability, sedimentation rates are determined throughout the Reservoir over seasonal to decadal time scales with the naturally occurring radioisotopes ⁷Be and ²¹⁰Pb (half-lives 53.3 days and 22.3 years, respectively), as well as anthropogenically produced ¹³⁷Cs (half-life 30.7 y) as an independent geochronometer. These rates are placed within the context of sediment character (mud content) and fluvial sediment supply to the Reservoir, as well as its geomorphology. In general, seasonal sedimentation rates scale with fluvial loads and reflect deposition of watershed-derived sediment. Spatial patterns of seasonal sedimentation follow those of river deltas incised by channels, while decadal-scale sedimentation rates increase downstream. Episodic event sedimentation in channels is suggested by the character of ²¹⁰Pb profiles, which indicate non-steady state sedimentation that makes application of sediment-age dating models challenging in this system. Although sediment mud content has clearly decreased as the Reservoir has filled, observed changes in sedimentation rates may reflect biases of these models rather than physical changes.
Constraining the origin of recently deposited particles using natural radionuclides 7Be and 234Th<inf>ex</inf> in deltaic sediments
2018, Continental Shelf Research
⁷Be and ²³⁴Th_ex activities were determined in sediment cores off the Rhône River mouth (Gulf of Lions), in order to trace the initial transport and deposition of riverine suspended particulate matter (SPM) and evaluate the impact of flood events through 7 cruises carried out between 2007 and 2008. Consistently high ⁷Be and ²³⁴Th_ex inventories of 2000–3000 mBq cm⁻² and 3000–5000 mBq cm⁻², respectively, were observed within a ~5 km radius off the Rhône River mouth. Their spatial distributions showed a gradual decrease with increasing distance from the Rhône River mouth, and the decrease in ⁷Be was more pronounced than that of ²³⁴Th_ex, indicating that recent riverine SPM is rapidly deposited in the area located near the river mouth. This area is also characterized by high accumulation rates determined using ¹³⁷Cs or ²¹⁰Pb_ex. Both ⁷Be and ²³⁴Th_ex inventories increased in 2008 compared to 2007, and are correlated to the cumulated SPM flux for normal and flood discharge. Moreover, the ⁷Be/²³⁴Th_ex inventory ratio appears to be a potential tracer to identify the dominant influence of recently deposited particles between terrestrial and marine waters. This ratio provides an effective tool to assess river and marine influence: Zone I at a distance inferior to 3.0 km, with ⁷Be/²³⁴Th_ex inventory ratio over 0.50 (surface area near river mouth ~ 7 km²) is dominated by riverine influence; in contrast, Zone III at a distance superior to 8.5 km, with ⁷Be/²³⁴Th_ex inventory ratio less than 0.10 (surface area off river mouth beyond 150 km²) is predominantly under a marine influence. In between, an intermediate area (Zone II at a distance between 3.0 and 8.5 km, with ⁷Be/²³⁴Th_ex inventory ratios between 0.10 and 0.50) displays a mixed influence. This zoning could help in further understanding the spreading of particle-reactive contaminants and its initial sedimentary deposition in the Gulf of Lions.
Impact of soil hydrological properties on the 7Be depth distribution and the spatial variation of 7Be inventories across a small catchment
2018, Geoderma
The natural fallout radionuclide ⁷Be is used as a tracer allowing estimates of soil redistribution on an event-based time scale. The observed ⁷Be inventory in the soil is converted to soil redistribution by comparing the ⁷Be inventory at a stable reference site to the location of interest and taking the ⁷Be depth distribution in the surface soil into account. The relaxation mass depth (h₀), describing this depth distribution, is assumed to be uniform across the study area, whereas the ⁷Be inventory at the reference site represents the balance of atmospheric ⁷Be input and radioactive decay. The ⁷Be reference inventory should, therefore, not be influenced by soil redistribution or by any variation in physico-chemical characteristics of the soil within the entire study area. Most studies to date use ⁷Be to monitor soil redistribution on the spatial scale of a hillslope. However, if the assumptions of spatially uniform fallout and rapid and irreversible sorption of ⁷Be to soil particles can be extended over a larger area, ⁷Be could be used to monitor soil redistribution at the catchment scale. In this paper, the variability in ⁷Be distribution in the soil at hillslope and catchment scales is explored and possible sources of this variability are identified. To assess the impact of variability in soil hydraulic conductivity on the depth penetration of ⁷Be in surface soil, a rainfall simulation experiment with ⁹Be spiked rainfall was performed on artificially compacted soil cores. These rainfall simulation experiments indicated a significant positive correlation between the saturated hydraulic conductivity (K_sat) and the relaxation mass depth and, thus, demonstrated that the assumption of a spatially constant relaxation mass depth is likely to be invalid. An empirical correction factor is proposed to circumvent this problem. This work demonstrates the importance of assessing variability in soil hydrological properties across a study area and is also relevant to studies concerning the vertical transport of fallout contaminants in surface soil. To assess spatial variability in fallout across a catchment and across soil types, soil trays containing different soil types were placed at three reference locations in a small catchment of 8 km² across a nine month period. The spatial variation in ⁷Be reference inventory between the sites in the catchment was not larger than the variation within one reference site (37% and 36% respectively), indicating that the uncertainty on the reference inventory will be similar over the small catchment. However, the different soil types displayed diverse ⁷Be depth profiles and total ⁷Be inventories, suggesting that a clear understanding of sorption behavior across the soil types present in a catchment is needed prior to the use of ⁷Be as a catchment-scale sediment tracer.
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Beryllium-7 (⁷Be) has been used as a sediment tracer to evaluate soil redistribution rates at hillslopes and as a tool to estimate sediment residence time in river systems. A key assumption for the use of ⁷Be as a sediment tracer is the rapid and irreversible sorption of ⁷Be upon contact with the soil particles. However, recent studies have raised questions about the validity of these assumptions. Seven soil types were selected to assess the adsorption rate of ⁷Be on the soil particles, subsequently an extraction experiment was performed to assess the rate of desorption. Next, different treatments were applied to assess the impact of soil pH, fertilizer, humic acid and organic matter on the adsorption of Be. Finally, the influence of regularly occurring cations present on the soil complex on the adsorption of Be on pure clay minerals was evaluated. The adsorption rate experiment showed a rapid and nearly complete sorption of Be for Luvisols and Cambisols under agriculture. For a temperate climate Stagnosol under forest and two highly weathered tropical Ferralsols sorption of Be was less rapid and less complete. This may result in an incomplete adsorption of ⁷Be on these three soils when runoff initiates, which could lead to an overestimation of erosion rates and sediment residence time. Additional observations were made during the extraction experiment, showing a significant loss of Be from the forest Stagnosol and a stable binding of Be to the arable soils. Of the different treatments applied, only pH showed to be of influence. Finally, Ca²⁺ and NH₄⁺ on the soil complex had only a limited effect on the adsorption of Be, while Al³⁺ in combination with a low pH inhibits the adsorption of Be on the exchange complex of the pure clay minerals. All these findings more rigorously support the use of ⁷Be as a soil redistribution tracer in arable soils in a temperate climate at a hillslope scale. The use of ⁷Be in highly weathered Ferralsols or forest rich environments should be limited to avoid overestimations of erosion rates. The spatially extended use of ⁷Be to evaluate residence times of sediments should be avoided in catchments with rapid changing environmental parameters as they might influence the sorption behavior of ⁷Be.
Assumptions and challenges in the use of fallout beryllium-7 as a soil and sediment tracer in river basins
2013, Earth-Science Reviews
Citation Excerpt :
Consequently, current knowledge with regard to the geochemical behaviour of 7Be is lacking (Mabit et al., 2008; Kaste and Baskaran, 2011). For 7Be, the tracer requirement of particle reactivity upon fallout has largely been assumed from high partition coefficients (Kd) displayed in aquatic systems, with values commonly in the range of 104 to 106 (e.g. Nyffeler et al., 1984; Hawley et al., 1986; Olsen et al., 1986; Dibb and Rice, 1989; Vogler et al., 1996; Steinmann et al., 1999; Jweda et al., 2008). Hawley et al. (1986) studied solid/liquid partitioning in freshwater lakes and demonstrated that 7Be is largely associated with the solid phase and reported Kd values of 105–106 at solid concentrations of between 0.1 and 100 mg L− 1.
This contribution reviews research surrounding the assumptions that underpin the use of beryllium-7 (⁷Be) as a soil and sediment tracer in river basins. As a cosmogenic radionuclide, the constant natural production of ⁷Be and fallout via precipitation, coupled with its ability to bind to soil and sediment particles provides the basis for its application as a conservative soil and sediment tracer. Consequently, ⁷Be has been extensively employed as a tracer across a range of spatial and temporal (event to seasonal) scales. The short half-life of ⁷Be (53.3 days) lends itself to tracing sediment dynamics over short time periods, thus, providing complementary data to medium-term estimates derived from longer lived radionuclides such as caesium-137 (¹³⁷Cs). This short half-life could provide a major advantage when considering the potential for ⁷Be to document recent effects of climate or land use change upon soil redistribution, with the latter having particular relevance for assessing the effectiveness of mitigating strategies within a catchment-wide approach to management. Although ⁷Be has been widely applied as a tracer to date, application is still in its infancy and there remains a lack of knowledge in relation to the assumptions for its use as a tracer. Specifically, our findings suggest that there are crucial information gaps with regard to ⁷Be application as an erosion tracer. Of key importance is the potential for fallout uniformity and rapid tracer sorption to be compromised under certain conditions. The assumption of irreversible sorption is likely to hold for common hillslope conditions but literature identifies the potential for tracer mobility with changing environmental parameters in the wider catchment. Further research is required to determine the likelihood of ⁷Be sorption being affected by conditions found at relevant field sites and the impact of this upon tracer applications at the catchment-scale.
Evaluating physical and biological influences on sedimentation in a tidal freshwater marsh with 7Be
2013, Estuarine, Coastal and Shelf Science
Citation Excerpt :
And it is also possible that Acatch is overestimated, since DMP is located next to an urban area that could supply large amounts of “fresh” terrigenous sediment. While there are no measurements of the 7Be activity of catchment sediments within DMP, activities of suspended particles (at depth) at the mouth of the Potomac River range from not detectable to 96.5 dpm/g (Dibb and Rice, 1989), with a median value of 15.4 dpm/g. We also calculated sedimentation rates by assuming that the value of Acatch varies spatially and temporally such that it equals the observed surficial activity at each site for each sampling period.
Key differences exist between tidal fresh- and saltwater marshes, such as the relative importance of mineral versus organic sedimentation and plant species diversity, that likely result in different drivers of sedimentation. In tidal freshwater marshes, we hypothesize that vegetation composition, along with physical marsh features (i.e., elevation and tidal channels), play a critical role in sedimentation. This hypothesis is evaluated in Dyke Marsh Preserve (Potomac River, VA, USA) by examining sediment character (grain size, organic content) and deposition rates across the marsh in spring and summer 2010 and 2011. ⁷Be is especially well suited to capture seasonal sedimentation patterns owing to its short half-life (53.3 d) and ability to assess both sediment deposition and erosion. However, its use in marshes can be challenging, especially due the presence of vegetation. In this study, ⁷Be-derived sedimentation rates are compared with sediment deposition observed on ceramic tiles to assess its utility in tidal freshwater marshes, and biophysical influences on sediment deposition are examined through statistical models. ⁷Be- and tile-derived sedimentation rates show similar spatial and temporal patterns, with highest rates occurring at sites closer to tidal channels, highlighting the importance of sediment availability. In addition, complex feedbacks between sedimentation and the plant community are discussed.

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^a: Present Address: University of New Hampshire, Institute for the Study of Earth, Oceans and Space, Science and Engineering Research Building, Durham, New Hampshire 03824, U.S.A.

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Estuarine, Coastal and Shelf Science

Abstract

References (12)

Solubility behavior of atmospheric ⁷Be in the marine environment

Marine Chemistry

Temporal and spatial distribution of beryllium-7 in the sediments of Chesapeake bay

Estuarine, Coastal and Shelf Science

The partitioning of ⁷Be in fresh water

Geochimica et Cosmochimica Acta

A kinetic approach to describe trace-element distribution between particles and solution in natural aquatic systems

Geochimica et Cosmochimica Acta

A sampler for concentrating radionuclides from natural waters

Ocean Engineering

⁷Be in Sargasso Sea and Long Island Sound waters

Journal of Geophysical Research

Cited by (42)

Spatial and temporal patterns of sedimentation in an infilling reservoir

Constraining the origin of recently deposited particles using natural radionuclides <sup>7</sup>Be and <sup>234</sup>Th<inf>ex</inf> in deltaic sediments

Impact of soil hydrological properties on the <sup>7</sup>Be depth distribution and the spatial variation of <sup>7</sup>Be inventories across a small catchment

Rapid and irreversible sorption behavior of <sup>7</sup>Be assessed to evaluate its use as a catchment sediment tracer

Assumptions and challenges in the use of fallout beryllium-7 as a soil and sediment tracer in river basins

Evaluating physical and biological influences on sedimentation in a tidal freshwater marsh with <sup>7</sup>Be

The geochemistry of beryllium-7 in Chesapeake Bay

Abstract

Marine Chemistry

Estuarine, Coastal and Shelf Science

Geochimica et Cosmochimica Acta

Geochimica et Cosmochimica Acta

Ocean Engineering

7Be in Sargasso Sea and Long Island Sound waters

Journal of Geophysical Research

⁷Be in Sargasso Sea and Long Island Sound waters