The transport and mass balance of fallout radionuclides in Brotherswater, Cumbria (UK)
This paper investigates the role of intervening transport processes on lake sediment records of the atmospherically deposited radionuclides 210Pb and 137Cs. Brotherswater is of particular interest to this issue in that its large catchment/lake area ratio and short water residence time are likely to amplify the influence of these processes, both from the catchment and through the water column. Brotherswater is also unique in being the site of two earlier multicore studies that, together with the present study, span a period of 4 decades. Measurements of fallout radionuclides were made on soil cores, suspended sediments and sediment cores, and the results combined with those from earlier studies to construct mass balances for 210Pb and 137Cs in Brotherswater. The results showed that catchment inputs accounted for 63% of 210Pb entering the lake. Further, just 47% of 210Pb entering the water column was delivered to the sediment record. For comparison, in an earlier study at nearby Blelham Tarn with a relatively smaller catchment but longer water residence time it was shown that 47% of 210Pb inputs were delivered via the catchment, 75% of which were delivered to the sediment record. Results from both sites suggest that 210Pb is predominantly transported on fine particulates with a mean particle size of 3–4 μm. Their relatively slow removal from the water column allows them to be transported relatively uniformly throughout the lake and may help account for the fact that simple 210Pb dating models are relatively reliable in spite of the complexities of the transport processes. Mass balance calculations for 137Cs are more complicated because of the variable fallout record. Measurements of 137Cs in the input stream and water column showed that catchment inputs are still significant 30 years after the last significant fallout (Chernobyl). Modelled results showed that catchment inputs delayed the date of peak inputs of weapons test fallout to the lake though by no more than 2 years. Although the results presented here are primarily concerned with fallout radionuclides and their reliability for dating, they also have implications for the use of sediment archives in reconstructing historical records of other atmospherically deposited substances such as trace metals or persistent organic pollutants.
Keywords210Pb 137Cs Mass balance Brotherswater Catchment Lake sediments Radiometric dating
In spite of their relevance to paleolimnology, there have been relatively few detailed studies of the pathways by which atmosphere contaminants deposited on the landscape are delivered to the sediment records at the bed of a lake. For the most part it is assumed that accumulating sediments provide at the very least a good qualitative record of atmospheric fallout. A more detailed understanding of transport pathways is of particular relevance to records of the fallout radionuclides 210Pb and 137Cs used to date sediments. The widely used CRS 210Pb dating model (Appleby and Oldfield 1978; Robbins 1978) assumes that a relatively constant (on yearly timescales) atmospheric flux is reflected in a constant rate of supply of 210Pb to the sediments. Chronostratigraphic dating by 137Cs matches features in the 137Cs activity versus depth record with the known historical fallout record. Both assumptions are potentially questionable at sites where a significant fraction of the supply to the sediment record consists of older catchment derived material that may have been deposited years or even decades earlier. A further uncertainty is the extent to which inputs to the water column are lost via the outflow.
A mass balance study of 210Pb in the water column of Bickford Pond (Massachusetts, USA) carried out by Benoit and Hemond (1987) based on measurements of 210Pb concentrations in inlet streams and the water column suggested that around 40% of 210Pb delivered to the water column did so indirectly from the catchment. In this particular case, since around 46% of total inputs were lost from the lake via the outflow, the amount of 210Pb delivered to the sediment record was comparable to the direct atmospheric flux. Nonetheless their results do highlight the potential influence catchment inputs may have on the timing of the supply rate and its distribution over the bed of the lake. Catchment inputs of 210Pb to Lake Geneva from the alpine Rhone watershed (Dominik et al. 1987) were found to be relatively less significant, just 17% of total inputs. A contributory factor to the lower figure here may be the smaller catchment/lake area ratio (~ 9) compared to Bickford Pond (~ 20). In a more recent study (Appleby et al. 2003) of transport pathways at Blelham Tarn (English Lake District, catchment/lake area ratio ~ 42), catchment inputs were estimated to represent around 47% of total 210Pb inputs to the lake. These inputs were however concentrated near one of the major input streams. Away from this part of the lake, the sediment record was dominated by direct atmospheric fallout, though the detailed pattern was influenced by sediment focussing.
The purpose of the present study was to investigate the transport processes controlling the fate of fallout radionuclides deposited on the catchment of Brotherswater in the English Lake District, and in particular the impact of those processes on the supply of 210Pb and 137Cs to the sediment record in the lake. Because of its large catchment/lake area ratio and small water residence time the impacts are likely to be much more visible at Brotherswater compared to other northern temperate lake systems.
Brotherswater is unique in that it has been the subject of a number of detailed studies spanning a period of 4 decades, the first of which was carried out in the late 1970s (Eakins et al. 1981, 1984). As well as investigating the potential applications of 210Pb and 137Cs for dating sediment records, this pioneering study by John Eakins and Roger Cambray (AERE Harwell) also considered the source of these radionuclides and the pathways by which they entered the sediment record. Atmospheric deposition was measured directly via the analysis of rainwater samples collected monthly from a site near the lake, and indirectly via soil cores collected from various locations in the catchment. Further analyses were carried out on lake and stream water samples collected on several occasions during the study period, and on sediment cores from different locations within the lake. Their conclusion that 210Pb enters the water column predominantly (93%) via transport from the catchment appears however to greatly overestimate the significance of this pathway, largely due to an assumption that sediments entering the lake via the main inflow, including those during flood events (90% of the total), had 210Pb concentrations similar to those on fine sediments in the water column. From this assumption they concluded that inputs of 210Pb to the lake amounted to 15 mCi year−1 (1 mCi = 3.7 × 107 Bq), of which just 1.1 mCi year−1 (7%) were attributable to direct fallout onto the lake. Further, just 14% of total inputs (2.1 mCi year−1) were incorporated in the sediment record, the remainder (12.9 mCi year−1) being lost via the outlet. Direct estimates based on their own measured concentrations in lake and stream water samples (Eakins et al. 1981, 1984) suggest a much lower figure of between 1.5 and 2.5 mCi year−1 for losses via the outflow, and in consequence a much lower figure for catchment inputs.
A second multi-core study at Brotherswater was carried out in the late 1980s and early 1990s as part of an investigation into the fate of fallout radionuclides deposited as a result of the 1986 Chernobyl accident (Bonnett et al. 1992; Hilton et al. 1992). Sediment records of the short-lived radionuclide 134Cs characteristic of Chernobyl fallout, as well as 210Pb and 137Cs, were determined in six lake sediment cores from sites similar to those sampled in the 1976/77 study. Soil cores from six different locations across the catchment were used to quantify atmospheric fallout. A further suite of six soil cores were collected in November 1991 from a flat site adjacent to the lake as part of a separate study of 210Pb and 137Cs fallout in Cumbria (Smith et al. 1997).
Brotherswater has also been the subject of a number of projects concerned with hydrological and sedimentological issues. In the 1970s Chambers (1978) carried out regular measurements of fluvial discharges into and out of the lake over a 2-year period (1975–1977), and suspended sediment inputs over a 1-year period (1976–1977). More recently (2012–2013) sediment traps were deployed to measure sediment fluxes through the water column of Brotherswater (Schillereff et al. 2016a). In the wake of Storm Desmond (December 2015), suspended sediments entering the lake have been monitored using sediment traps placed near the mouths of the main inlet stream.
In the first part of the present study, 210Pb and 137Cs records in a suite of sediment cores from Brotherswater collected during 2011–2016 were compared with those from earlier studies in order to carry out an assessment of their long-term stability and reliability as dating tools. It was evident from the results (Semertzidou et al. 2019) that fallout records in certain parts of the lake were strongly influenced by catchment inputs and that in some cases these had a significant bearing on the choice of dating model. The object of this paper is to present a more detailed account of the extent to which sediment records are impacted by catchment inputs, and also the influence on those records of losses from the lake via the outflow. Radiometric analyses carried out on soil cores, suspended sediments and bottom sediments, together with data from the earlier studies are used to establish more accurate mass balance models for both 210Pb and 137Cs in Brotherswater. This is relatively straightforward for 210Pb where in the absence of major catchment disturbances inputs and outputs on yearly timescales can reasonably be assumed to be in a steady state. The situation for 137Cs is more complicated because of its varying atmospheric flux. Global fallout from the atmospheric testing of nuclear weapons effectively began around 1953, reached a peak in 1963, and then declined rapidly following the implementation of the test ban treaty in that year. By the early 1980s it had fallen to negligible levels. Many European sites, including Brotherswater, were also subject to fallout from the Chernobyl reactor fire in 1986. Up to then, annual inputs of 137Cs to the water column will have included both contemporary fallout onto the lake, and indirect inputs via the catchment consisting of 137Cs deposited over a range of years since 1953. Inputs after 1986 consist solely of catchment derived material. Data from the different studies (1976/77, 1988/89, and 2011/16) can be used to establish mass balances for 137Cs at each of those times. From the results, simple models of transport processes from the catchment (Appleby et al. 2003) and through the water column (Appleby 1997) can be used to model the historical relationship between atmospheric fallout and inputs to the sediment record, and how this might change in the future. Catchment/lake transport parameters for 210Pb and 137Cs at Brotherswater are compared with values obtained from several other studies with a view to using these models for making estimates of catchment inputs at sites studied in less detail based on a small number of physiographic parameters such as mean annual rainfall, catchment/lake area ratio, and water residence time.
The study site
There should have been no major soil disturbance for at least 30 years,
Soils should be of a type that inhibits radionuclide migration through the soil column,
They should be on open level ground not subject to erosion or flooding by surface waters,
The soils should be relatively compact and saturated (to minimise in situ 222Rn escape),
The soil depth should be sufficient to contain the entire fallout inventory.
Transport of radionuclides from the catchment to the lake
Sediment samples collected using Time-Integrated Mass-flux Sampler (TIMS, Perks et al. 2013) sediment traps placed in both channels of the inflow stream close to its mouth were analysed for 210Pb and 137Cs. The traps were deployed and collected at 2–3 month intervals over a period of 16 months, from July 2016 through to October 2017. Samples included both fine sediments collected during periods of relatively normal flow and coarser material collected during flood events.
Radionuclide fluxes through the water column
Site A near the delta (75 m from the inflow) in ~ 12 m water depth,
Site B near the central area of the lake in ~ 17 m water depth.
Samples were collected at intervals of 1–2-months over a 17-month period, from August 2012 to December 2013 (Schillereff et al. 2016a) and analysed for 210Pb and 137Cs. Sediment masses accumulating during each collection period were used to calculate 210Pb and 137Cs fluxes through the water column.
Sediment records of fallout radionuclides
A total of 10 new sediment cores were collected from the bed of the lake during the period 2011–2016 using a short 8 cm diameter gravity corer designed to capture an intact sediment-water interface (Boyle 1995) and analysed for 210Pb and 137Cs. Figure 1 also shows their locations, together with those of cores collected during the earlier 1970s and 1980s studies. Map coordinates and water depths for the new cores are given in Semertzidou et al. (2019).
Dried sediment samples from the traps and cores were analysed for 210Pb, 226Ra, 137Cs and 241Am by direct gamma assay in the Liverpool University ERL (Environmental Radioactivity Laboratory) using Ortec HPGe GWL series well-type coaxial low background intrinsic germanium detectors (Appleby et al. 1986). Samples were placed in 5-cm-long by 1-cm-diameter nylon sample tubes sealed with flanged rubber Suba-Seals dipped in paraffin wax to prevent 222Rn escape. 210Pb was determined via its gamma emissions at 46.5 keV, and 226Ra by the 295 keV and 352 keV γ-rays emitted by its daughter isotope 214Pb following 3 weeks storage to allow 214Pb/226Ra radioactive equilibration. 137Cs and 241Am were measured by their emissions at 661.7 keV and 59.5 keV respectively. Absolute efficiencies of the detectors were determined using calibrated sources and sediment samples of known activity. Corrections were made for the effect of self-absorption of low energy γ-rays within the sample (Appleby et al. 1992), and for background radiation from the detectors themselves. Background counts of 60-h duration were carried out on each detector at regular intervals. Mean background count rates were typically around 0.5 cph (counts per hour) for the 46.5 keV peak and 1 cph for the 661.7 keV peak. An Ortec GMX series coaxial detector with similar characteristics was used for measuring larger soil core samples.
All values are relative to the date of sampling unless stated otherwise. Uncertainties have been calculated from the counting statistics.
Data analysis and modelling
Data from all three studies were assessed for their consistency and reliability and combined where appropriate. The results were used to construct mass balances for fallout radionuclides in the water column of Brotherswater detailing total inputs via direct fallout and catchment transport, and total outputs via delivery to the sediment record and losses via the outflow. Catchment/lake transport rates were used to calculate values of the 210Pb and 137Cs transport parameters for Brotherswater for the catchment/lake model developed in Appleby et al. (2003).
Mean annual 210Pb flux
Direct measurements of fallout of 210Pb on a near monthly basis were carried out at Brotherswater by Eakins et al. (1981, 1984) during a 12-month period from November 1976 to October 1977. Since the rainfall total during that period (2108 mm) was significantly below average, in estimating the mean annual 210Pb flux a correction to the amount of fallout 210Pb collected (148 Bq m−2) was made by assuming it to be proportional to rainfall. Using a revised estimate of the mean annual rainfall at Brotherswater (2609 mm year−1) based on an updated analysis of data from nearby sites (Grisedale Bridge, Brotherswater Inn, Harstop Hall, Hartsop Village), the mean atmospheric 210Pb flux at Brotherswater is calculated to be 183 Bq m−2 year−1.
In an alternative approach, measurements carried out at Esthwaite Water (10 miles to the south west) during a 16-month period from April 1997 to July 1998 showed a broadly linear relationship between monthly 210Pb deposition and monthly rainfall (Appleby et al. 2003). An analysis of the Brotherswater data shows that they follow a similar relationship with broadly similar values for the linear coefficients. Applying these relationships to the mean annual rainfall yields values for the 210Pb flux of between 177 and 188 Bq m−2 year−1.
137Cs fallout record
Results of the 137Cs measurements, summarised in ESM2, include both raw inventories (where available) and values decay corrected to 1986. The raw inventories from the 1976/77 study are essentially those published in Eakins et al. (1981, 1984) though for the two Delta sites only the mean value has been given. The decay corrected inventories include a small adjustment for the fact that weapons test fallout at the time of the 1976/77 study was incomplete. Weapons test fallout data (Cambray et al. 1989) suggest that post-1977 fallout would have contributed a further 3.25% to the total inventory.
The much higher 137Cs inventories in post-1986 soil cores (other than those from the 1989 study) are attributable to fallout from the Chernobyl accident in April/May 1986. Results from the 1989 study were clearly flawed in that they yielded 137Cs inventories significantly lower than those determined from the 1976/77 study. Results from the present study are in good agreement with those from the 1991 study. Excluding the values from cores BWSC16/1 and BWSC16/2 where the records were evidently not complete, the mean value from both these studies is 11,611 Bq m−2. Subtracting the 1986 weapons fallout inventory, fallout of 137Cs at Brotherswater due to Chernobyl is calculated to be 4001 Bq m−2. This value is consistent with the estimate made by Smith and Clarke (1989) that levels of Chernobyl 137Cs deposition in the Brotherswater catchment were between 2000 and 5000 Bq m−2.
Radionuclide concentrations on input stream sediments
210Pb and 137Cs concentrations on sediment samples from TIMS traps placed in the main Brotherswater inflow streams
Radionuclide concentrations in and fluxes through the water column
Mean 210Pb and 137Cs concentrations in the water column sediment trap samples
Sediment, 210Pb and 137Cs fluxes through the water column of Brotherswater
Water column fluxes
g cm−2 year−1
Bq m−2 year−1
Bq m−2 year−1
Fallout 210P and 137Cs transport rates to the bottom sediments
ESM4 shows 210Pb and 137Cs inventories in a total of 21 Brotherswater lake sediment cores including those from (a) the 1976/77 study, (b) the 1988/89 study and (c) during 2011/16. It also gives the mean 210Pb fluxes required to sustain the 210Pb inventories, determined by Eq. (1). Also shown are the 137Cs inventories in two cores from the early 1970s (Pennington 1981; Semertzidou et al. 2019). Results for the pre-Chernobyl study include both the raw inventories and values decay corrected to 1986. Those for the post-Chernobyl studies are all decay corrected to 1986, and also include estimates of the amounts attributable to fallout both from atmospheric nuclear weapons tests and from the 1986 Chernobyl accident. The relatively high mean value for the 1976/77 210Pb inventories is due to anomalously high inventories in the BWE and BWN cores. The 1988/9 and 2011/16 cores have very similar mean values.
Estimates of the mean weapons test 137Cs inventory (decay corrected to 1986) suggest that it has remained relatively stable, increasing slightly from just over 5000 Bq m−2 in the 1970s and 1980s to a contemporary value of 5381 Bq m−2. The Chernobyl inventory appears to have increased from 649 Bq m−2 to a contemporary value of 1084 Bq m−2. These values reflect inputs by direct fallout and catchment transport less losses via the outflow.
The fate of any chemical species entering the water column of a lake will be largely determined by its distribution between the particulate, colloidal and soluble phases. The soluble fraction and fraction attached to colloidal size particle (< 0.4 μm) are essentially transported with the aqueous phase. The fate of the particulate fraction, and in particular the extent to which it is deposited on the bed of the lake or lost via the outflow, is largely determined by factors controlling the transport of suspended sediments through the lake. These include the particle-size distribution, water depth, and water residence time. Since the main unknowns concerning the source of radionuclides to the sediment record are catchment inputs and outflow losses we begin with a brief discussion of water balance and sediment transport in Brotherswater.
Chambers (1978) carried out weekly measurements of the discharge through streams flowing into and out of Brotherswater over a 2-year period from 25th September 1975 to 25th September 1977. Estimates of the mean annual discharge rates based on a simple numerical procedure for integrating the individual measurements yielded values of 24.75 × 106 m3 year−1 and 28.35 × 106 m3 year−1 for the mean inflow rate and outflow rates respectively. The difference between the two can be attributed to additional inputs via direct rainfall onto the lake (0.47 × 106 m3 year−1) and groundwater flow (3.13 × 106 m3 year−1). The calculations necessarily underestimated the impact of flood events that took place between flow measurements. Although it was estimated that these might have accounted for an additional inflow of 19.05 × 106 m3 year−1, the procedure was acknowledged by Chambers to be highly uncertain and give no better than an order of magnitude of the true figure. Daily precipitation data from the nearby Grisedale Bridge monitoring station (Met Office 2006) indicate that 36% of rainfall during the period monitored by Chambers fell on days with 25 mm of rain or more.
Water balance estimates for Brotherswater
106 m3 year−1
Direct rainfall onto the lake
The input of suspended sediments, the main vector for transporting fallout 210Pb through the water column of a lake, were also monitored weekly by Chambers over a period of about 1 year, from 11 October 1976 to 25 November 1977. The calculated load based on these measurements, 81 × 103 kg year−1, was however thought to greatly underestimate the total load. Particle concentrations were found to be up to three orders of magnitude higher during flood events, most of which were not picked up by the weekly monitoring.
The extent to which sediments entering the water column of Brotherswater accumulate in the sediment record can be determined using 210Pb records from the suite of cores collected during this and earlier studies, shown in Fig. 1. Sedimentation rates determined from the 210Pb records were consistent with chronostratigraphic dates determined from the 137Cs records (Semertzidou et al. 2019). A good estimate of the mean long-term accumulation rate at any individual site is given by the 90% 210Pb equilibrium depth, corresponding to 75 years accumulation. Mean sedimentation rates determined in this way ranged from 0.173 g cm−2 year−1 at BW11/2 (relatively near the mouth of the main inlet stream) to 0.026 g cm−2 year−1 at BW16/SE2 (towards the south-eastern corner of the lake) and are consistent with the sediment trap data which suggest sediment fluxes through the water column of 0.093 g cm−2 year−1 at Site A near the inflow stream and 0.056 g cm−2 year−1 at Site B near the centre of the lake. Integrating over all the data, the mean accumulation rate was calculated to be 0.059 g cm−2 year−1 giving a mean whole basin accumulation rate of 106 × 103 kg year−1.
Because of the buffering effect of the lake, particle concentrations at the outflow will be much more uniform than at the inflow. Suspended sediment concentrations in samples collected from the outflow during weekly monitoring (Eakins et al. 1981, 1984) had a mean value of 1.1 mg L−1. Assuming this to be typical of the total discharge through the outflow (40.9 × 106 m3 year−1), the total sediment loss via the outflow is calculated to be 46 × 103 kg year−1. The combined total (152 × 103 kg year−1) of losses via sedimentation and outflow is nearly double Chambers measured value of inputs during normal flow conditions (81 × 103 kg year−1) and probably a good measure of the total input of relatively fine sediments from the catchment. The higher figure (≈ 1300 × 103 kg year−1) suggested by Chambers when flood events are included, if correct would almost certainly be mainly due to inputs of relatively coarse material (sand and gravel) mobilised during flood events and deposited in a delta relatively close to the point of entry to the lake. That such material is unlikely to contribute significantly to catchment inputs of 210Pb is supported by results from a recent (2017) core collected from the delta. The mean 210Pb supply rate to this core (251 Bq m−2 year−1) was just 6% higher than the whole lake mean.
210Pb mass balance
From the estimated values of the atmospheric flux (183 Bq m−2 year−1) and mean 210Pb supply rate to the sediment record (236 Bq m−2 year−1) the total amounts of 210Pb entering the water column of Brotherswater via direct fallout onto its surface and exiting via deposition over the bed of the lake are calculated to be 3.30 × 107 Bq year−1 and 4.26 × 107 Bq year−1 respectively. The difference between these figures, 0.96 × 107 Bq year−1 is a measure of the extent to which inputs from the catchment exceed losses via the outflow. In the study carried out by Eakins et al. it was assumed that 210Pb concentrations on all sediments entering the lake (including those during storm events) were similar to those on suspended sediments in the water column, estimated to be 10.3 pCi g−1 (381 Bq kg−1). Measurements carried out during the present study suggest a much lower figure, particularly during flood events. Unsupported 210Pb concentrations (Table 1) varied widely from as little as 13 Bq kg−1 to more than 200 Bq kg−1. Sediments transported during flood events include large quantities of relatively coarse material containing relatively little 210Pb, exemplified by the October 2016b and October 2017b samples. Excluding these, the mean unsupported 210Pb concentration on the remaining mainly fine-grained material was around 109 Bq kg−1. Assuming this to be typical of the estimated 152 × 103 kg year−1 of fine sediments entering the lake via the main inflow, and again supposing that 25% of 210Pb was on colloidal material and effectively in solution, the contribution of this material to the amount of catchment derived 210Pb entering the lake is calculated to be 2.21 × 107 Bq y−1. Assuming further that the coarse sediments transported during flood events (up to 1200 × 103 kg year−1 according to Chambers’ estimates) had a minimal unsupported 210Pb concentration of 15 Bq kg−1 (the mean value of the October b samples), total inputs of catchment derived 210Pb are calculated to be 4.70 × 107 Bq year−1. An alternative estimate can be made using the measurements carried out by Eakins et al. (1981, 1984) on water samples from the Brotherswater inlet stream. Concentrations on samples collected on five occasions during July 1976 to November 1977 under both low flow conditions and following heavy rain ranged from 0.5 to 2.6 Bq kL−1 (14–70 fCi L−1) with a mean value of 1.5 Bq kL−1 (41 fCi L−1). Multiplying by the estimated discharge rate through the inflow of 37.3 × 106 m3 year−1, this calculation suggests an annual rate of supply of 210Pb from the catchment of 5.69 × 107 Bq year−1. The average of these two estimates (5.19 × 107 Bq year−1) puts the value for total mean annual inputs of 210Pb to the water column at 8.49 × 107 Bq year−1.
Eakins et al. (1981, 1984) carried out measurements of 210Pb in water samples collected from the Brotherswater outflow stream on five occasions during July 1976 to November 1977 under both low flow conditions and following heavy rain. Concentrations ranged from 0.9 to 3.2 Bq kL−1 (23–87 fCi L−1) with a mean value of 1.8 Bq kL−1 (49 fCi L−1). Multiplying by the estimated mean outflow rate (40.9 × 106 m3 year−1), one estimate of the annual rate of loss of 210Pb from the lake via the outflow is calculated to be 7.36 × 107 Bq year−1. Because of the relative sparsity of the measurements and their wide range of values this figure does however have a large uncertainty. An alternative estimate can be made using measured values of the 210Pb concentrations on suspended particulates in the water column. Unsupported 210Pb concentrations on samples from sediment Trap B near the centre of the lake were significantly higher than those in samples from Trap A, closer to the inflow, presumably due to the progressive settling out of coarser particles with lower concentrations. Mean concentrations in Trap B samples (Table 2) ranged from 423 Bq kg−1 in the base-level collector to 566 Bq kg−1 in the top-level collector. Assuming the latter value to be more typical of suspended sediments reaching the outflow, multiplying by the estimated sediment load (46 × 103 kg year−1), and also allowing for the estimated 25% of 210Pb on fine material that is essentially transported with the aqueous phase (Eakins et al. 1981, 1984), this method gives a lower value of 3.44 × 107 Bq year−1 for the 210Pb outflow rate. From the mean value (5.40 × 107 Bq year−1) total mean annual losses of 210Pb from the water column are put at 9.66 × 107 Bq year−1. The discrepancy between this figure and the above estimate for total inputs to the water column (an imbalance of 1.17 × 107 Bq year−1) is mainly due to uncertainties in the values of the catchment inputs and outflow losses. Making proportionate corrections to these terms gives a corrected figure of 9.06 × 107 Bq y−1 for total inputs to and losses from the water column. It follows from this that catchment inputs must be around 5.76 × 107 Bq year−1 and outflow losses 4.80 × 107 Bq year−1. The mass balance requirements coupled with relatively low uncertainties in the atmospheric flux (~ 3%) and supply rate to the sediments (~ 6%) place a relatively strong constraint on the apparently large uncertainties in these values. Assuming them to be comparable, since their difference has an uncertainty of around 0.25 × 107 Bq year−1, they are each likely to have an uncertainty of around 8%.
Mean particle size for sediments transporting 210Pb
Figure 6 also plots corresponding values of the sediment transfer fraction for Blelham Tarn determined from the relevant lake data for that site. The empirically determined value of 0.75 suggests that there too 210Pb is mainly transported on particulates with a mean size of around 3–4 μm.
Catchment/lake transport parameters
137Cs mass balance and transport
Inputs of 137Cs to Brotherswater
137Cs concentrations on sediment samples from traps placed in the main Brotherswater inflow stream during the period from July 2016 through to October 2017 (Table 1) varied from 12 Bq kg−1 to 42 Bq kg−1. Excluding the relatively coarse material transported during flood events (October 2016b and October 2017b) the mean 137Cs concentration on the remaining mainly fine-grained material was around 30 Bq kg−1. Assuming this to be typical of the estimated 152 × 103 kg year−1 of fine sediments entering the lake via the main inflow, and that concentrations on the October 2016b and October 2017b samples (12 Bq kg−1) are typical of coarse sediments, inputs of 137Cs on suspended sediments entering the lake during this period are estimated to be 1.94 × 107 Bq year−1. Total inputs will however also include a substantial fraction present in the essentially aqueous (< 0.45 μm) phase. Measurements of 137Cs concentrations in lake waters (Liverpool University ERRC data base) yielded values of the particulate fraction fD ranging from as little as 11% to as much as 40%, with a mean value of around 25%. Results from river waters suggest significantly higher values. Measurements carried out at five sites in the Dnieper river system between 1987 and 1993 (Sansone et al. 1996) yielded mean values of fD ranging from 26 to 34% with a mean value of 30%. During that period values of the particulate fraction remained relatively constant in spite of an 80% decline in concentrations. Results from a number of rivers in Belarus, Austria, Lithunia (Monte et al. 2002), although more varied had a similar mean value (34%). Since the main driver for higher 137Cs concentrations during flood events will be higher concentrations of suspended sediments remobilised from the bed of the stream or eroded from catchment soils adjacent to the stream, concentrations in the essentially aqueous phase are likely to be comparable to those during normal flow. Assuming a value of 30% for the particulate fraction fD during normal flow, inputs of 137Cs aqueous inputs by 2017 are estimated to be 2.32 × 107 Bq year−1, and total inputs 4.26 × 107 Bq year−1. Matching this empirical value to the modelled value for the beginning of 2017 gives a slightly lower value for k of 0.0039 year−0.5. Using the mean value 0.0043 year−0.5 as a best value for k, Fig. 8 also shows modelled values of catchment inputs of 137Cs to Brotherswater from 1954 through to 2050 and compares them with the empirical values for 1976/77 and 2016/17. The results suggest that inputs of weapons fallout from the catchment peaked slightly later than direct inputs, and at their peak were comparable to direct inputs. From the late 1960s onwards catchment inputs were the dominant source of 137Cs entering Brotherswater, apart from 1986 when there was a brief but intense episode of fallout from the Chernobyl accident. The impact of catchment inputs when added to direct inputs was to significantly broaden the early 1960s peak delivery of 137Cs to Brotherswater (Fig. 8).
Transport of 137Cs through the water column
Cumulative inputs of 137Cs to the water column of Brotherswater, and to the sediment record
In spite of many large uncertainties, mainly associated with the difficulty in determining the precise impact of large flood events, the results presented here confirm earlier results that in lakes with large catchments, a significant proportion of fallout 210Pb and 137Cs (and presumably other atmospheric contaminants) entering the water column consists of older material that may have been deposited some years earlier on the catchment. Catchment transport is estimated to account for 67% of 210Pb inputs to Brotherswater, a little higher than the figure for Blelham Tarn (47%), perhaps reflecting the relatively larger catchment at Brotherswater. Although substantial it is significantly lower than the 93% suggested in the earlier study by Eakins et al. (1981, 1984). It does however amount to no more than 2.4% of annual fallout onto the catchment. According to the model used here this fraction is determined by the catchment/lake transport parameter k (Eq. 9). Its calculated value at Brotherswater (0.0024 year−0.5) is similar to the value for Blelham Tarn (0.0022 year−0.5) in spite of the relatively larger catchment.
The fraction of 210Pb entering the water column transferred to the sediment record is largely controlled by the water residence time, lake depth and suspended sediment particle size. Because of the short water residence time a relatively large fraction (53%) of the 210Pb inputs at Brotherswater is lost via the outflow, with just 47% entering the sediment record. The significantly higher fraction (75%) entering the sediment record at Blelham Tarn can be attributed to its longer residence time in the water column. At both sites, a comparison between the modelled and empirical values suggests that 210Pb is mainly transported on relatively fine particulates with a mean particle size of around 3–4 μm. Their relatively slow removal from the water column does however allow them to be transported relatively uniformly throughout the lake.
The sediment trap results highlight the extent to which significant quantities of 137Cs deposited on the catchment of Brotherswater continue to be transported to the lake 30 years after the last fallout. The catchment/lake transport parameter for 137Cs was estimated to be 0.0043 year−0.5, significantly higher than the value for 210Pb but lower than the value for 137Cs at Blelham Tarn (0.0060 year−0.5). Because of the higher proportion of 137Cs in the essentially aqueous phase (passing a 0.45 μm filter) a much greater fraction of 137Cs entering the water column was lost via the outflow, with only about a sixth being retained in the sediment record. Although catchment inputs soon exceeded direct inputs from the atmosphere, their impact was to delay the date of peak inputs to the lake by no more than one or 2 years. The results do however highlight the potential importance of the catchment pathway in interpreting sediment records of atmospheric pollution, particularly in lakes with large catchments.
The complexity of the relationship between atmospheric fallout and sediment records is of particular relevance to 210Pb in view of its widespread use for dating these natural archives. This is highlighted in the results presented in Semertzidou et al. (2019). These showed that in parts of the lake heavily impacted by catchment inputs the simple 210Pb dating models are not necessarily applicable and may need to be applied in a piecewise way using chronostratigraphic dates as reference points. In areas of the lake not so affected the simple models gave good results except where the record was disturbed e.g.by local slump events.
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