| 0.0.1 1. Introduction to soil erosion measurement by radionuclides
Soil erosion by water, wind and tillage affects both agriculture and the natural environment.
Studying this phenomenon would be one of the advancements in science. Soil erosion occurs
worldwide and since the last two decades it has been a main topic of discussion all over the
world. The use of environmental radionuclides such as 90Sr, 137Cs to study medium term soil
erosion (40 yrs) started in the early 1990’s. Using these new techniques better knowledge
about erosion can be gained and this knowledge can be implemented for erosion risk
management.
The erosion and sedimentation study by using man-made and natural radioisotopes is
a key technique, which has developed over the past 30 years. Fallout 137Cs and
Cosmogenic 7Be are radionuclides that have been used to provide independent
measurements of soil-erosion and sediment-deposition rates and patterns [1] [2] [3]
[4].
Erosion measurements using radionuclides 137Cs, 7Be
Caesium-137 from atmospheric nuclear-weapons tests in the 1950s and 1960s (Fig.1) is a
unique tracer of erosion and sedimentation, since there are no natural sources of 137Cs.
Unique events such as the Chernobyl accident in April 1986 caused regional dispersal of
137Cs that affects the total global deposition budget. This yearly pattern of fallout can be used
to develop a chronology of deposition horizons in lakes, reservoirs, and floodplains. 137Cs
can be easily measured by gamma spectroscopy. Using 137Cs is a fast and cheap method to
study erosion-deposition processes compared to the traditional methods like silt
bags.
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Figure 1: Global 137Cs fallout (Modified from SAAS Bulletin 353, Part E, DDR,
1986)
When 137Cs, 7Be reach the soil surface by wet and dry deposition, they are quickly and
strongly adsorbed by ion exchange and are essentially non exchangeable in most
environments. Each radionuclide is distributed differently in the soil because of
differences in half-lives (30 yrs for 137Cs and 53 days for 7Be), delivery rates, delivery
histories, and land use (Fig. 2). An Physical processes, such as water and wind, are the
dominant factors moving 137Cs, 7Be tagged soil particles within and between landscape
compartments.
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Figure 2: Generalized sketch illustrating the distributions of 137Cs and 7Be in tilled
and undisturbed soils
2 Erosion study at Young Moraine regions of Germany
Recently, a study had been performed to evaluate erosion rates in a typical pattern of
landscapes in the Young Moraine regions of North-East Germany [5]. The 137Cs
concentrations were measured at selected sampling points of various study sites.
Among the areas selected for sampling was Basedow, which is a cultivated area,
situated north of Berlin. During a master thesis study at university of Bremen in the
academic year 2008-2009 [6] a second sampling campaign was performed at the same
study site and 137Cs and 7Be concentrations were measured. Two mathematical
models (a proportional model and a mass balance model) were applied to estimate
erosion or deposition rates giving a distinction between uncultivated or essentially
undisturbed soils and cultivated or soils under permanent pasture (Fig.3A). An
improved depositional model was developed during this study. The simulation results
from this model are presented in Fig.4. Due to the half-life (53.2 days) of 7Be, a
mass balance model was developed and used to calculate erosion rates from 7Be
(Fig.3B).
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Figure 3: A: Erosion rates for 137Cs calculated by mass balance model.
B: Erosion rates calculated with mass balance model using the 7Be data at Basedow
(2008).
The results verify that there is long term erosion as a result of wind, water and agricultural
practices. The annual erosion rates at Basedow calculated using a mass balance and a
proportional model were in the range between 30-50 t ha-1yr-1. These values were
comparable to the erosion rates calculated in the previous study [5] by the models mentioned
above.
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Figure 4: Profiles of sediment calculated for different erosion rates by Cs-137 within
the ploughed soil
3 Conclusions and outlook
Erosion rates for agricultural soils at Young Moraine regions of North-East Germany
were determined by using two radionuclides, 137Cs and 7Be. In combination, the two
radionuclides provide a valuable means of investigating soil erosion and assessing erosion
risk in the study area. Potentials and limitations of the erosion measurement techniques using
radiotracers are discussed in this study. The models used to quantify erosion rates using 137Cs
and 7Be were studied. Erosion rates calculated by theses models are difficult to measure over
a period of 50 years. A validation of these erosion rates for the time period (50 years) used
in the 137Cs-based models will give a new perspective to the use of soil erosion
modeling.
Most of the regions in India are suffering from high erosion rates [7]. By using the new
techniques in erosion quantification the land management practices can be improved and the
erosion risk can be reduced in India. |