Fallout radionuclides to investigate soil erosion:An overview

Lionel Mabit Institute of Environmental Geosciences, University of Basel

Soil Degradation

80% of degraded land is located in developing countries !!!

From: Oldeman et al. (1990)

Methods for erosion measurement

Modeling (USLE, Erosion 3D, Eurosem, AGNPS..)

Erosion pins Erosion plots

Indirect methods (sediment deposits)

Nuclear techniques(radioisotopes)

Reasons for using radioisotopes

The fallout is universal

Strongly fixed to soil particles

Integration of climatic variability

Only one sampling required to estimate erosion processes

Sediment budgets can be calculated at different scales

Radioisotopes for erosion studies

From: Zupanc and Mabit (2010)

Radioisotopes for erosion studies

Nuclide Origin Half-life γ-Energy

137Cs Artificial 30 y 661 keV

7Be Natural 53 d 477 keV

210Pb Natural 22 y 46 keV

Guiding principle of 137Cs method

Initial fallout of 137Cs

Reference site (RS) e.g: 2000 Bq/m²

Erosion

Stable

Deposition

♦ Samples

137Cs level higher than RS e.g: 3000 Bq/m²

137Cs level lower than RS e.g: 1600 Bq/m²

137Cs level around RS e.g: 2100 Bq/m²

In the field : Sampling with soil corer or cylinder

Establish a sampling strategy1 - Vertical and horizontal sampling (transect..)

Pennock and Appleby, 2002

Grid Sampling

Pennock and Appleby, 2002

Transect Sampling

Lab 1 (pre-preparation) : Drying and sieving

Lab 2 (measurement) : Gamma spectroscopy

Gamma detector

Gamma spectroscopy analysis

Conversion Bq/m² ± t/ha/yr (Models)

Conversion Bq/kg Bq/m²

Case study on the application of FRN (137Cs)

180ha

From: Bernard et al. (1998); Mabit et al. (1998 and 1999)

Study of the Vierzy watershed (France)

180 ha

Temperate Climate

~ 700 mm/yr

~ 140-150 m

Slopes ~ 2 %

Silty loam

Land use

Openfield

Dry thalweg

Vierzy watershed land use

Potato 10 %

Other 15 % Winter wheat 45 %

Sugar beet 30 %

AugustJune

December–May(Bare Soil)

October / November

(Harvest)

March

September-February

(Bare Soil)

June

November-May(Bare Soil)

August

Map of the soil movement in

Vierzy

Model

Sampling and geostatistical

analysis

-18 -15 -12 -9 -6 -3 0 3 6 9 12 15 18 20 2

154 m

146 m

154 m

Mouvement des sols en tonnes/hectare/an

500 m

N

EROSION DEPOSITION

Soil movement in t/ha/yr

-18 -15 -12 -9 -6 -3 0 3 6 9 12 15 18 20 21

154 m

146 m

154 m

Mouvements des sols en tonnes/hectare/an

500 m

N

EROSION DÉPOSITION

Soil movement in t/ha/yr

Eroding area% total area 45Mean erosion (t/ha /yr) 6

Sedimentation area% total area 15Mean deposition (t/ha /yr) 7

Total areaNet output (t/ha/yr) 2SDR (%) 60

Stable area% total area 40

2 t/ha/yr

Sediment budget of the watershed

OM (%) = 0,0002 137Cs (Bq m-2) + 0.96 (n = 30 ; r = 0,79 ; p < 0,001)

1.01.11.21.31.41.51.61.71.81.92.0

1000 1500 2000 2500 3000 3500 4000 4500

137Cs (Bq/m2)

OM

(%)

ErosionStableDeposition

Erosion vs soil quality

From: Mabit and Bernard (1998)

Case study on the application of FRN (137Cs) +GIS

~ 200km2

From: Mabit et al. (2007 and 2011)

Boyer River watershed ~ 200 km2

60% of the basin area is cultivated

Affected by a degradation of the soil and water resources

Boyer River watershed (Canada)

0102030405060708090

61 63 65 67 69 71 73 75 77 79 81 83 85 87Year

Ton

s

Use of GIS and 137Cs

5 Soil–slope combinations

GIS: identify sectors with similar agri-environmental conditions (soil, slope, land use)

MBM + IDW2

Land use: forest and agricultural area

14 reference areas (n=42)

2860 Bq m-2 ; CV of 21% (n = 42)

Min 3 representatives fields / isosector

3 transects ~ 10 samples/transect

Summary of results

GIS/137Cs

Overall sediment production = 2.8 t ha-1 yr-1

28% of the arable lands of the watershed present erosion rateshigher than 6 t ha-1 yr-1

Sediment production per isosectors

Conclusion and perspectivesFRNs are very useful tools for improving knowledge about erosion processes;

The use of 137Cs as soil tracer in Alpine areas : current status

(ii) Test 239+240Pu more homogenous distribution - Nuclear bomb tests origins

(i) Re-sampling method for 137Cs based on the samples collected in 2007

(i) to localize degraded area and (ii) to quantify the magnitude of soil erosion

Heterogeneity of the initial fallout originating from Chernobyl

Successfully applied in 2010 and 2012 (e.g. Schaub et al., 2010)

Meusburger et al., 2010

i. Bernard, C., Mabit L., Wicherek, S., Laverdière, M.R. (1998). Long-term soil redistribution in a small French watershed as estimated from 137Cs data. Journal of Environmental Quality, 27(5), 1178-1183.

ii. Mabit, L. (2011). Erosion/deposition data derived from fallout radionuclides (FRNs) using geostatistics. In: Impact of soil conservation measures on erosion control and soil quality. IAEA-TECDOC-1665. pp. 185-194.

iii. Mabit, L., Benmansour, M., Walling D.E. (2008). Comparative advantages and limitations of Fallout radionuclides (137Cs, 210Pb and 7Be) to assess soil erosion and sedimentation. Journal of Environmental Radioactivity, 99(12), 1799-1807.

iv. Mabit, L., Bernard, C. (1998). Relationship between soil inventories and chemical properties in a small intensively cropped watershed. Comptes Rendus de l’Académie des Sciences, Série IIa, Earth and Planetary Sciences, 327 (8), 527-532.

v. Mabit, L., Bernard, C., Laverdière, M.R. (2007). Assessment of erosion in the Boyer River watershed (Canada) using a GIS oriented sampling strategy and 137Cs measurements. Catena, 71(2), 242-249.

vi. Mabit, L., Bernard, C., Laverdière, M.R., Wicherek, S. (1999). Assessment of water erosion in a small agricultural basin of the St.Lawrence river watershed. Hydrobiologia, 410, 263-268.

vii. Mabit, L., Bernard, C., Laverdière, M.R., Wicherek, S. (1998). Spatialisation et cartographie des risques érosifs à l’échelled’un bassin versant agricole par un radio-isotope (137Cs). Étude et Gestion des sols, 5 (3), 171-180.

viii. Zupanc, V., Mabit, L. (2010). Nuclear techniques support to assess erosion and sedimentation processes: preliminary results of the use of 137Cs as soil tracer in Slovenia. Dela, 33, 21-36.