COUNTRY REPORT (VIETNAM) for IAEA/RCA Meeting on “Exchanging of Experiences using the Cs-137 Technique for Measuring Soil Erosion/Sedimentation and Associated Pesticide Contamination” (RAS/5/039 Part II ) Kuala Lumpur, Malaysia 5-9 July 2004 by Pham Duy HIEN - Project Co-ordinator, Bui Dac DUNG- National project manager, Nguyen Hao QUANG, and Nguyen Quang LONG THE USE OF Cs-137 AND Pb-210 TECHNIQUES FOR ASSESSING SOIL REDISTRIBUTION RATES AND PATTERNS: A REVIEW OF STUDIES CONDUCTED IN VIETNAM 1. INTRODUCTION • Vietnam territory is about 350 000 km2, 3/4 of the land are in the hilly and mountainous areas. • The population (about 80 mil.) has nearly doubled from 1940s’ to 1980s’. • Natural forests have been cleared for agricultural purposes. • The per-capita arable land has reduced from 1320 to 950 m2 and deforestation occurs with a rate of about 1 Mha/yr. • Unsustainable use of the high slope lands has lead to soil quality degradation. 1. INTRODUCTION • Soil erosion rates are being estimated and soil conservation techniques have been implemented through the government agricultural projects (Ministry of Agriculture and Rural Development) • Most of the projects on soil erosion estimation have been carried out by conventional techniques at the National Institute for Soil and Fertilizer (NISF) • Advanced nuclear techniques such as Cs-137 and Pb-210 techniques for assessing soil erosion have been tried at Vietnam Atomic Energy Commission (VAEC) in some areas with promising results. • Collaboration with soil scientists as the end users needs to be improved in order to fasten the application of these techniques. 2. RECENT APPLICATIONS OF Cs-137 AND Pb-210 TECHNIQUES IN VIETNAM 2.1. Determination of Cs-137 deposition density across the territory of Vietnam • National Project on determination of Cs-137 deposition density in soil was carried out in 1997-2000 period. The Cs-137 inventory across the territory of Vietnam was determined. • The measured Cs-137 inventory varies from 129 (South) to 3294 (North) Bq m-2. • The regression model describing the Cs-137 deposition density D (Bq m-2) distribution across the territory has been found as: Ln(D)=(3.640.11)+(0.0940.004)L + (0.54 0.04)AR Where: L-latitude (oN), AR-Annual rainfall (in m) at the sampling site. LATITUDINAL DISTRIBUTION OF 137CS DEPOSITION DENSITY ACROSS THE TERRITORY OF VIETNAM inventory, Bq m-2 deposition density, Bq m-2 1200 Inventory/deposition, Bq m -2 900 600 300 0 9.25 10.25 11.25 12.25 13.25 14.25 15.25 16.25 17.25 18.25 19.25 20.25 21.25 22.25 Latitude, o N 2.2. Cs-137 technique in estimating soil erosion at Song Da afforestation field • The Song Da afforestation field lies on a mountainous area in North Vietnam where a big reservoir was constructed in 1982 for an 1800 MW hydropower plant. • Reference value for Cs-137 is 918.5 Bq m-2. • Soil samples were taken from three transects of 8 samples, in areas with bamboo, acacia and brush plant covers. • Average soil loss rates were calculated as 3.2, 4.4 and 9.8 t/ha/yr for acacia, bamboo and brush covers, respectively. 2.3. Assessing soil erosion and nutrition loss by Cs-137 and Pb-210 techniques • Aimed at combining techniques for evaluating soil erosion and nutrition loss. • The experimental area, of 200 m2 (10x20 m) on a 100 slope of a hill, lies in Doan Hung district, Phu Tho province (about 200 Km northwest of Hanoi). • Erosion rate calculated by Cs-137 technique was 5.2 t/ha/yr, by Pb-210 technique was 7.08 t/ha/yr where as the erosion rate calculated by direct weighing was 6.09 t/ha/yr. • Unfortunately, the nutrition loss has not been able to evaluate by Cs-137 and Pb-210 results due to limited number of analyzed samples. 3. ONGOING NEW NATIONAL PROJECT 3.1. Background • Although nuclear scientists have been active in the use of Cs-137 technique for soil erosion, and good laboratory facilities have been established for analyzing Cs-137 and Pb-210, our soil scientists still know little about these nuclear techniques. • Joint project (between the Institute for Nuclear Science & Technique – INST (VEAC) and the National Institute for Soil & Fertilizer - NISF) has two main objectives: One is to validate and confirm the many advantages of nuclear techniques in assessing soil degradation; The other objective is to get soil scientists involved actively in the sampling design, measurement, interpretation, and comparing nuclear techniques with conventional techniques. 3.2. Methods • Multidisciplinary team was formed with scientists from INST and NISF. • A place where plots for erosion measurements designed by the NISF has been chosen. This place is located about 100 Km from Hanoi. • The Cs-137 reference inventory value were estimated first by the regression model based on the latitude and the annual rainfall values, and found to be around 900 Bq m-2. • Research for the reference site was a very difficult task because of the dense population and unclear history of the cultivation practice in the area. After many interviews with local people we have found that there are two hilltops that have never been cultivated 3.2. Methods (Reference site) • A square of 3x3 m where five sample points were taken. • At each point, four samples from depth interval of 0-10 cm, 10-20 cm, 20-30 cm, and 30-40 cm were taken for investigating depth distribution of Cs-137 and Pb-210. • Because of the uncertainty in choosing the place, we have decided to take samples on work-and-try basic: For the first visit we took only samples from 3 points at each site. Investigated site Reference site 3.2. Methods (Reference site) • The “reference” samples were analyzed for Cs-137 and Pb- 210 by ORTEC HPGe detector at the Environmental Radioactive Monitoring Laboratory (INST). • Total Cs-137 inventory value from each point at one of the hills was around 205 Bq m-2, clearly lower than the estimated value of 900 Bq m-2. • At the other site, the Cs-137 values were 713, 937, and 1006 Bq m-2 for three first points, and are in good match with the estimated value. This hilltop, a place where the pole of the electric power transmit line is located, is considered a good choice for reference site. Reference site 3.2. Methods (Research plots) • Each research plot has been designed from the top down the hill, with observation pond at the toe to collect runoff sediments. Every plot has dimension of 7.5 m wide and more than 30 m long. There are seven plots for three different types of erosion research: • One plot for observation of bare land erosion. • One plot for observation of corn cultivated land erosion without hedgerows. • One plot for observation of tea cultivated land erosion without hedgerows. • Four plots for observation of cultivated land erosion with different hedgerows. 3.2. Methods (Research plots) • Two transects for a plot, each goes along the length of the plot and consists of 12 sample points. • One soil sample from each point is taken to be analyzed for soil property and nutrition contents at the NISF’s laboratories. • every plot has 24 samples for analyzing Cs-137 and Pb-210 and 24 samples for analyzing soil property and nutrition contents. • It is our plan to conduct all sample collection by the end of this July and have all samples analyzed by the end of October. To date, only one transect has been completed. Plot No 6 3.3. Results • 10 reference points with 40 Bq.m-2 samples have been completed. 600 • Average Cs-137 reference 400 value is 885 Bq.m-2 200 • The variation of the reference 0 inventories is about 16%. 1 2 3 4 • Depth profiles show that the Point B Depth Cs-137 distribution is typical one for an undisturbed site Cs-137 inventories at reference site (topomap unit: cm) Cs-137 inventories and conversion results for transect 1 on plot 6 Sample M1 M2 M3 M4 M5 M6 Results Cs-137 108 444 1359 300 533 983 (Bqm-2) Soil Redistribution -58.7 -33.2 36.3 -44.1 -26.3 7.8 (t/ha/yr) Sample M7 M8 M9 M10 M11 M12 Results Cs-137 237 407 1216 314 481 1146 (Bqm-2) Soil Redistribution -48.9 -35.9 25.5 -43.0 -30.3 20.2 (t/ha/yr) Cs-137 distribution along transect 1 on plot 6 Reference value: 885 Bq/m2 4. CONCLUSIONS • The Cs-137 deposition in Vietnam is rather high, particularly in the central and northern parts. The regression model established for calculating the Cs-137 deposition density in Vietnam territory is the good guide to estimate reference value in any soil erosion/sedimentation study. • Results of soil erosion studies showed promising future for the application of Cs-137 and Pb-210 techniques. A close collaboration with soil scientists is the better way to fasten the process of transfer these techniques to the end users.