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

Nick Ryken (UGent) , Bashar Al-Barri (UGent) , W Blake, A Taylor, Filip Tack (UGent) , Samuel Bodé (UGent) , Pascal Boeckx (UGent) and Ann Verdoodt (UGent)
(2018) GEODERMA. 318. p.88-98
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Abstract
The natural fallout radionuclide Be-7 is used as a tracer allowing estimates of soil redistribution on an event-based time scale. The observed Be-7 inventory in the soil is converted to soil redistribution by comparing the Be-7 inventory at a stable reference site to the location of interest and taking the Be-7 depth distribution in the surface soil into account. The relaxation mass depth (h(0)), describing this depth distribution, is assumed to be uniform across the study area, whereas the Be-7 inventory at the reference site represents the balance of atmospheric Be-7 input and radioactive decay. The Be-7 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 Be-7 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 Be-7 to soil particles can be extended over a larger area, Be-7 could be used to monitor soil redistribution at the catchment scale. In this paper, the variability in Be-7 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 Be-7 in surface soil, a rainfall simulation experiment with Be-9 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 km(2) across a nine month period. The spatial variation in Be-7 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 Be-7 depth profiles and total Be-7 inventories, suggesting that a clear understanding of sorption behavior across the soil types present in a catchment is needed prior to the use of Be-7 as a catchment scale sediment tracer.
Keywords
Beryllium-7, Erosion tracing, Hydraulic conductivity, Be-7 depth distributions, FALLOUT RADIONUCLIDES CS-137, EROSION RATES, HYDRAULIC-PROPERTIES, SEDIMENT TRACER, EXTENDED PERIOD, BERYLLIUM-7, VARIABILITY, RAINFALL, PB-210(EX), LAND

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Citation

Please use this url to cite or link to this publication:

Chicago
Ryken, Nick, Bashar Al-Barri, W Blake, A Taylor, Filip Tack, Samuel Bodé, Pascal Boeckx, and Ann Verdoodt. 2018. “Impact of Soil Hydrological Properties on the 7Be Depth Distribution and the Spatial Variation of 7Be Inventories Across a Small Catchment.” Geoderma 318: 88–98.
APA
Ryken, Nick, Al-Barri, B., Blake, W., Taylor, A., Tack, F., Bodé, S., Boeckx, P., et al. (2018). Impact of soil hydrological properties on the 7Be depth distribution and the spatial variation of 7Be inventories across a small catchment. GEODERMA, 318, 88–98.
Vancouver
1.
Ryken N, Al-Barri B, Blake W, Taylor A, Tack F, Bodé S, et al. Impact of soil hydrological properties on the 7Be depth distribution and the spatial variation of 7Be inventories across a small catchment. GEODERMA. 2018;318:88–98.
MLA
Ryken, Nick, Bashar Al-Barri, W Blake, et al. “Impact of Soil Hydrological Properties on the 7Be Depth Distribution and the Spatial Variation of 7Be Inventories Across a Small Catchment.” GEODERMA 318 (2018): 88–98. Print.
@article{8547020,
  abstract     = {The natural fallout radionuclide Be-7 is used as a tracer allowing estimates of soil redistribution on an event-based time scale. The observed Be-7 inventory in the soil is converted to soil redistribution by comparing the Be-7 inventory at a stable reference site to the location of interest and taking the Be-7 depth distribution in the surface soil into account. The relaxation mass depth (h(0)), describing this depth distribution, is assumed to be uniform across the study area, whereas the Be-7 inventory at the reference site represents the balance of atmospheric Be-7 input and radioactive decay. The Be-7 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 Be-7 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 Be-7 to soil particles can be extended over a larger area, Be-7 could be used to monitor soil redistribution at the catchment scale. In this paper, the variability in Be-7 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 Be-7 in surface soil, a rainfall simulation experiment with Be-9 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 km(2) across a nine month period. The spatial variation in Be-7 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 Be-7 depth profiles and total Be-7 inventories, suggesting that a clear understanding of sorption behavior across the soil types present in a catchment is needed prior to the use of Be-7 as a catchment scale sediment tracer.},
  author       = {Ryken, Nick and Al-Barri, Bashar and Blake, W and Taylor, A and Tack, Filip and Bod{\'e}, Samuel and Boeckx, Pascal and Verdoodt, Ann},
  issn         = {0016-7061},
  journal      = {GEODERMA},
  keyword      = {Beryllium-7,Erosion tracing,Hydraulic conductivity,Be-7 depth distributions,FALLOUT RADIONUCLIDES CS-137,EROSION RATES,HYDRAULIC-PROPERTIES,SEDIMENT TRACER,EXTENDED PERIOD,BERYLLIUM-7,VARIABILITY,RAINFALL,PB-210(EX),LAND},
  language     = {eng},
  pages        = {88--98},
  title        = {Impact of soil hydrological properties on the 7Be depth distribution and the spatial variation of 7Be inventories across a small catchment},
  url          = {http://dx.doi.org/10.1016/j.geoderma.2017.12.036},
  volume       = {318},
  year         = {2018},
}

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