SPATIAL VARIABILITY OF CS-137 AND THE INFLUENCE OF SAMPLING ON ESTIMATES OF SEDIMENT REDISTRIBUTION

The principal objectives of this study were to assess the spatial variability of Cs-137 within a cultivated field, and to examine the influence of grid size and positioning on estimates of sediment redistribution. A 11 x 11 grid (6.25 ha) was sampled in a gently sloping cultivated field in central Saskatchewan, Canada. The mean Cs-137 areal activity in the cultivated field was 1910 Bq m-2, with a coefficient of variation (CV) of 28%. Spatial variability within the cultivated field was 55% greater than that in a nearby undisturbed (native) field, and this was assumed to result from the combination of wind erosion, localized water erosion, and possibly tillage redistribution. Lag one serial correlation was noted for two cultivated transects, and one downslope transect showed a positive linear relationship between Cs-137 activity and distance. Therefore, estimation of an average Cs-137 areal activity based on only one 11-point transect in the cultivated field was inappropriate, and a sample size of 32 would be required to estimate the population mean with 95% confidence and an allowable error of +/-10%. In the cultivated field redistribution of Cs-137 was used to assess sediment redistribution over the past 35 years. Sample locations in the cultivated field with Cs-137 areal activities less than the lower 95% confidence limit of the mean for the undisturbed field were considered to be eroded, while locations with activities greater than the upper 95% confidence limit were considered to be depositional. The cultivated grid was subdivided into four 6 x 6 grids to determine the influence of grid size and positioning on sediment redistribution. Statistical tests indicated that there were no significant differences between grids, and it was concluded that a 6 x 6 grid provided an optimal sample size, regardless of position, to assess the relative degradation of the cultivated field.