USING FALLOUT RADIONUCLIDES
3. RESULTS AND DISCUSSION 1. Reference Site
3.2. Slope Transects
3.2.1. NARC managed sub-catchment
Data on soil cores collected from the different slope transects and valley floor are given in Tables 1-3.
The values of 137Cs inventory obtained for 16 bulk cores ranged from 232 to 11391 Bq m-2. Except for three locations (almost flat areas and thick bushes in between the slope), all the values were lower than the reference value of 3280 Bq m-2. The significant reduction in inventory evident for most cores indicates that most of the sampling points have experienced appreciable net erosion over the period since the commencement of 137Cs fallout in the mid 1950s.
Of the 24 bulk cores collected along the transect on hill 2, only 3 had 137Cs inventories higher than the reference inventory indicating net soil accumulation, while 21 had lower 137Cs inventories, indicating net soil loss. These three locations with deposition were on a reasonably flat area along the transect. The rest of the points indicated the presence of erosion.
Very high erosion rates were observed at the lower portion of the transect because of low vegetation cover, steep slope gradient and accelerated run-off.
The results obtained at hill 3 indicated less erosion at the upper slope position because of good plant cover, the absence of walking tracks and a relatively lower slope gradient. The
137Cs inventory obtained for 12 bulk cores ranged from 74 to 8691 Bq m-2. However, the erosion accelerated downwards because of steeper slope gradients, accelerated run-off and less vegetation, especially at the lower part of the hill.
TABLE 1. CAESIUM-137 INVENTORIES AND SOIL REDISTRIBUTION AT THE TRANSECT ALONG HILL 1
Sample No Inventory
(Bq m-2)
Soil redistribution (t ha-1 a-1) (Using Profile Distribution Model)
1 1652 -11.3
2 1439 -13.5
3 2728 -3.0
4 5303 10.2
5 3258 -0.1
6 515 -30.4
7 1958 -8.5
8 9433 31.3
9 11391 41.2
10 232 -43.5
11 1394 -14.0
12 637 -26.9
13 1122 -17.6
14 1760 -10.2
15 565 -28.9
16 1363 -14.4
TABLE 2. CAESIUM-137 INVENTORIES AND SOIL REDISTRIBUTION AT THE TRANSECT ALONG HILL 2
Sample N° Inventory
(Bq m-2)
Soil redistribution (t ha-1 a-1) (Using Profile Distribution Model)
1 3910 3.2
2 376 -35.6
3 809 -23.0
4 2531 -4.3
5 7788 23.0
6 1617 -11.6
7 9405 31.3
8 1889 -9.1
9 9143 29.9
10 3121 -0.8
11 2898 -2.0
12 1203 -16.5
13 322 -38.1
14 1525 -12.6
15 1125 -17.6
16 1861 -9.3
17 1247 -15.9
18 2813 -2.5
19 317 -38.4
20 417 -33.9
21 218 -44.5
22 622 -27.3
23 711 -25.1
24 612 -27.6
TABLE 3. CAESIUM-137 INVENTORIES AND SOIL REDISTRIBUTION AT THE TRANSECT ALONG HILL 3
Sample N° Inventory
(Bq m-2)
Soil redistribution (t ha-1 a-1) (Using Profile Distribution Model)
1 1673 -11.1
2 4126 4.3
3 8691 27.7
4 2658 -3.5
5 2828 -2.4
6 7962 23.9
7 3882 3.1
8 3555 1.4
9 472 -31.8
10 2377 -5.3
11 635 -27.0
12 74 -62.2
Hill 1 Transect
FIG. 5. Soil redistribution along the Hill 1 transect.
Hill 2 Transect
FIG. 6. Redistribution along the Hill 2 transect.
Hill 3 Transect
FIG. 7. Redistribution along the Hill 3 transect.
The average rate of soil loss from the NARC managed subcatchment 2 was -10.2 t ha-1 a-1. However, the soil redeposition within the catchment is ~ 30% whereas the rest of the eroded soil is delivered to the channel.
3.2.2. Unmanaged area outside NARC sub-catchment
On the other hand, outside the NARC managed site different land use practices had different impacts on the redistribution of soil. For example, the cores taken from the top of the deforested hill down to the river Kurang bank had very low 137Cs inventories showing very high erosion rates. The average redistribution rate along the transect using profile distribution model was -31.9 t ha-1 a-1. The results are shown below in Table 4. Another profile taken from the adjacent deforested hill with relatively small slope gradient gave an average redistribution value of 21.4 t ha-1 a-1 (Table 5).
In a similar way the low 137Cs inventories at the bottom of hill slope adjacent to a road under construction (where the natural land structure has been changed due to bulldozing, tree cutting etc.) show severe erosion and the results are shown in Table 6.
TABLE 4. CAESIUM-137 INVENTORIES AND SOIL REDISTRIBUTION AT THE
TABLE 6: CAESIUM-137 INVENTORIES AND SOIL REDISTRIBUTION AT TRANSECT AT THE LOWER SLOPE POSITION OF THE HILLSLOPE
Sample
Topography and Redistribution of Transect in deforested area using PDM
-60 -30 0
0 40 80 120 160 200
Distance (m)
Soil Redist. (t/ha/yr)
850 870 890 910 930 950
RL (m) a.m.s.l.
FIG. 8. Redistribution along the transect in deforested area.
The high net soil loss of 31.9, 21.4 and 37.7 t ha-1 a-1 can be attributed to land use and slope gradient in the areas outside the managed sub-catchment. The largest soil losses appear to be associated with the areas where bulldozing and land levelling have been carried out for the construction of civil structures and roads. The sediment flux values of between 40 and 90 t ha
-1 a-1 have been reported in Sutherland in Bulldozing areas (18). The same trend is observed in the study area. The management practices in the NARC managed area seem very effective in controlling the erosion rates. However, soil loss rates of 10.2 t ha-1 a-1 suggest that much more efforts are needed to control the erosion from the slopes.
4. CONCLUSIONS
The use of 137Cs in studies of soil erosion/redistribution provides useful information in the study area. Although, it does not discriminate different erosion processes, the Cs137 inventories at the reference site, eroded slope (managed and unmanaged sites) and valley floor are still sufficient for use in soil erosion and sedimentation investigations in the sub-catchment of Rawal Lake. The erosion rates obtained by the 137Cs technique (10.2 t ha-1 a-1) are in good agreement with the results obtained by conventional techniques (11.3 t ha-1 a-1).
However, the results by conventional techniques took a long time to establish the soil loss (approximately 11 years), whereas by using 137Cs technique, we were able to get the data within three years of study. The 137Cs technique also provides information on the spatial soil redistribution within the catchment which is very difficult to be obtained by conventional methods.
The erosion rates from two deforested areas outside the NARC managed site are 31.9 and 21.4 t ha-1 a-1 respectively, whereas the value from area under civil work is around 37.7 t ha-1 a-1. The low values of redistribution (10.2 t ha-1 a-1) in the NARC managed sub-catchment indicate the potential of conservation/management practices in controlling the erosion. It also indicates that impacts of human activities are major contributors for soil erosion.
The 137Cs technique provides an opportunity for quantifying the erosion/redistribution rates.
Estimates of soil loss from the catchment derived by 137Cs measurement provide a clear confirmation of the validity of the technique for estimating erosion and deposition processes within the sub-catchment. However, the determination of a reference inventory is very crucial.
There is a need for more detailed studies of this type in order to understand the response of sediment redistribution and mobilization processes from sub-catchment to catchment scale.
ACKNOWLEDGEMENTS
The authors wish to thank the International Atomic Energy Agency for providing the financial and technical support under research contract N°. PAK-12392. Sincere thanks are due to Technical Officers and other IAEA Staff for their support in implementing the project. The authors are also thankful to the Pakistan Atomic Energy Commission Authorities, especially Director General, Pakistan Institute of Nuclear Science and Technology and Head, Isotope Application Division for facilitating the project. Cooperation of NARC (end-user department) is acknowledged with gratitude. The authors are also grateful Mr. Muhammad Zamir, Mr.
Muhammad Gulistan and Mr. Amjad Ali for their help in collection and analysis of samples.
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ASSESSMENT OF SOIL EROSION SEVERITIES AND CONSERVATION