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ABSTRACT By tin mining activity in Rhonpibul district, arsenic was spreaded and contaminated ground water in the area. This research was aimed to simulate ground water flow in both shallow and deep aquifers by using MODFLOW code. In running MODFLOW, it is necessary to construct correct conceptual model. Data of stable isotope of water collected in Rhonpibul can be used to assist the construction of correct conceptual model. Consequently this will generate valid and reliable model outputs. The research also date ground water by measuring contents of chlorofluorocarbons (CFCs) in 29 tube wells to obtain ages of ground water. The data of CFCs will be useful for adjusting input parameters of the model which finally will make the ground water flow model in Rhonpibul reliable and valid.
1. Introduction Rhonpibul is a district in Nakorn Si Thammarat province locating on southern peninsular of Thailand. District economy has grown up for 100 years from mining activity on tin and tungsten deposit on granite mountain ranges and mineral placer in Rhonpibul valley. In 1987 chronic arsenic disease was found in about 1,000 patients living in the area. Oshikawa (1) has studied the problem by comparing arsenic affected patients of present and past. She reported that in 1987 number of patients with different stage of arsenic skin lesions diagnosed by a physical examination at Rhonpibul hospital ¢-8
reached the number of 937. Japan International Cooperation Agency (JICA) and Environmental Research and Training Centre then launched detailed investigation in the area by conducting 450 auger drills and 30 deep core drills at 15 locations to analyze arsenic content in ground water. The result of the study showed that patients in the areas affected with arsenic skin lesions in Rhonpibul coincides with the areas of high arsenic contamination in ground water. Therefore the main cause of arsenic poisoning in the population is suspected to be from drinking arsenic-contaminated ground water. By this investigation 10 polluted areas in surface soil were identified and the soil of these areas are the main sources supplying arsenic to ground water. However contamination mechanisms and movement of arsenic in subsurface are not clearly known. In order to find appropriate countermeasures to solve the problem, this project is therefore initiated to study ground water flow and mechanisms of arsenic transport in subsurface in more detail. The project is aimed to simulate ground water flow in both shallow and deep aquifers and transport of arsenic by applying the finite different model MODFLOW. Furthermore the model will be validated by applying isotope characteristics particularly 2 H, 18O, 3H and chlorofluorocarbons (CFCs) of rain and groundwater collected in the area. The results of modeling will be used for planning the appropriate countermeasures for rehabilitate ground water and soil in Ronpibul. In order to find effective countermeasures to solve the ground water pollution problem, objectives of the project have been set up as followed: 1. to study ground water characteristics and flow patterns 2. interconnection between surface and ground water in Rhonpibul and »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡
3. to simulate ground water flow and validate it by applying isotopic signatures of water in Rhonpibul
History of mining and arsenic toxicity in Rhonpibul Rhonpibul town has been prospered from tin mining activity. In the mountainous area there are several tin deposits attached with arsenopyrite mineral. Primary deposit lying in the mountain ranges is a high-grade vein typed tin mineral. However in the lowland mineral placers occurs in many locations. Since arsenic pollution problem occurred, mining and ore dressing in the area is prohibited but some small activities of placer mining are still in operation. In 1977 an avalanche of mud and rocks was reported to cause major disaster in the area. It is believed that the flood and mud flow has transported tin tailing, arsenopyrite piled up at the foot hill of Khao Ronna to lowland. Since then the arsenopyrite is widely distributed in the area and arsenic concentration in soil is high. These hot spots of arsenopyrite are expected to be arsenic sources of ground water and the main causes of arsenic toxicity in the area. However arsenopyrite is very stable mineral, it is impossible to believe that only existence of arsenopyrite can cause serious ground water contamination. Theoretically arsenopyrite can cause contamination when it is oxidized to form sulfate compounds and the arsenic can be released into ground water.
Drainage patterns of the area are tree branch shape developed in homogeneous rock. In the valley, Huai Hua Meung and Huai Ronna streams joining in the north part of Rhonpibul town forms Klong Nam Khun river flowing eastward. Geology of the area can be divided into 3 units; old sedimentary rock, granite massive and young alluvium deposit. The old sedimentary rock was formed in Paleozoic era and occupies the lower elevation and foothill of the mountains. The rock is composed of mudstone, siltstone, limestone and their alterations. The strike of the rock is north-south to northeast-southwest direction and dips toward east around 30 degree. In the lowland north to south stretching black limestone appears as monadnock. Granite intrusion aged 187-215 million years covers most of Khoa Luang mountain. Old sedimentary rock is often found as a roofpandant of granite massive. Young alluvium deposit developing in the eastern part of the area fills a flat area of less than 40-50 meters in elevation. Weather of the area is tropical monsoon. There are 2 seasons; summer starting from February and rainy season starting from June. Average annual rainfall is 2381 mm. Wind direction dominates from southwest from May to October and from northeast from November to January. Average monthly temperature is in the range of 25.8-28.5oC
Geological settings and meteo- 2. Methods and Materials The study has been initiated rology of the study area Rhonpibul area is situated in the valley of Khao Ronna and Khao Suangchan mountains. The mountains are part of Khao Luang mountain having the highest peak of 925 meters above mean sea level. The area having elevation higher than 50 meters above MSL is steep mountainous topoghraphy. »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡
since October 1998 by conducting various types of surveys which are: 2.1 Auger locations was placed at 450 points in the area of 3 X 4 square kilometers. At each point soil samples at 0.3 and 1 meter depth were collected and analyzed for arsenic content and conducted ¢-9
elution test to study the solubility of arsenic from soil into water. After soil coring, piezometers were installed to collect shallow groundwater for analyzing arsenic content and arsenic speciations; As+3 and As+5 in water. 2.2 Deep core drills at 15 locations were done to identify unconsolidated layer and base rock in the area and 28 wells of diameter 3-4 inches were installed to intercept water in each aquifer. Deep and shallow groundwater was sampled to analyze arsenic content, arsenic speciation, cations and anions and 2H, 3H, 18O and 14C. The wells were also used to conduct pumping test to obtain aquifer characteristics such as transmissitivity and storage coefficients. 2.3 Surface water from streams and ponds were collected and analyzed for arsenic content and stable isotope of hydrogen and oxygen in water. Furthermore rain at different elevation from 25 - 700 meters above sea level, and tube wells have been collected every month and every quarter to analyze isotope characteristics of the water to study the sources of water as well as relationship between deep and shallow aquifer. 2.4 Twenty nine samples of ground water were collected in May 2000 from tube wells intercepting shallow and deep aquifers are sent to IAEA to date the ages of the water by CFC contents in ground water. This data will be used to calibrate the numerical flow model.
Description of the model area, model parameters and boundaries By the results of geological and hydrogeological survey in October 1998, it was found that aquifers in Rhonpibul can be divided into 3 units; shallow gravel and sand aquifer, deep aquifer at weathered rock zone and fractures aquifer in limestone basement rock. Between sand and weathered rock aquifers there is a clay layer with thickness of 5 - 25 meters acts as an impermeable layer. Pumping test conducted at 15 locations of shallow and deep aquifers are used to estimate aquifer characteristics which can be shown in the table 1. The parameters of aquifer characteristics such as transmissivities and storage coefficients obtained will be used in ground water flow simulation. By the result of water sample analysis, it is shown that contamination in shallow aquifer is quite extensive. Water samples collected from dug wells and auger wells were detected to have high arsenic up to 12 mg/l at some locations. The contamination is mainly detected in the belted zone along Huai Hua Meung river. In contrast to shallow aquifer, deep aquifer is not contaminated so high as of the shallow one. Five of monitoring deep wells contain high arsenic concentration of 6 mg/l. Arsenic contaminated ground water does not uniformly spread over the survey area but it distributes in several areas along Huai Hua Meung river. Additionally it was found that the consistency of the contaminated areas
Table 1. Aquifer characteristics in Rhonpibul Type of aquifer Shallow sand and gravel Deep weathered rock
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Range of transmissitivity (m2/min)
Range of storage coefficient
6.59x10-3 - 1.92x10-1 1.01x10-3 - 2.86x10-2
5.95x10 - 9.44x10-1 -3
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of shallow and deep aquifers can indicate how close the relation between the contamination in shallow and deep aquifers is. Nevertheless ground water flow model will be constructed to simulate the movement of ground water in both shallow and deep aquifers and the model will be validated by isotope technique in the next part of the study. By measuring ground water table at constructed piezometers, dug wells and rivers monthly, it is shown that ground water level flows by topography of the area. Generally shallow ground water flow from surrounding mountains to the central part of Rhonpibul basin before moving eastward to the plain. In the plain ground water flows eastward although scattered flow patterns were observed due to minute topography variations. The ground water flow pattern of deep aquifer is same as of the shallow one but the pattern of vertical flow is difficult to obtain. Three dimension flow simulation will be needed to predict this flow. By data of geological loggings and pattern of ground water flow, the conceptual model for the area of 3 X 4 km 2 was constructed for simulation with MODFLOW by dividing model layers into 5 of which layers 1 - 3 represents shallow alluvium sand aquifer and layer 4 represents clay impermeable layer. Because of the shapes and arrangements of limestone aquifer is not known clearly, the weathered and fractured rocks are lumped together and represented by layer 5. Three types of boundaries are set for the model which are recharge, river and constant head boundaries. Since the data on recharge of the area has never been measured, recharge rate was arbitrarily set from 200 - 30,000 mm/year. River boundaries were set on Rhonna, Hua Muang and Klong Nam Khun river as well »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡
as 8 ponds because the infiltration of river into ground water and outflow of ground water to rivers is needed to be simulated.
Conceptual model The model covers the entire area defined by the UTM coordinates of 591500 596000 East and 903000 -906000 North. The area of 3 X 4 km2 was divided in to coarse grid of 30 rows and 40 columns which makes each cell covers the area of 100 X 100 m 2 . In the center part of the modeled area where arsenic sources are located the grid was refined by 2. As indicated in geological profiles, the hydrogeological structure of Rhonphibul basin is vertically divided into 4 layers which are sand alluvium, impermeable clay, weathering rock and limestone. Because of the characteristics of weathering rock and limestone are not clearly known and the contamination level in these aquifers are not high compared with of the alluvium aquifer, these two geological settings are then combined and represented by layer 5 in the model. Since arsenic contents in ground water are very high at many locations in alluvium aquifer, the aquifer was represented by layer 1 - 3 for observing vertical movements of ground water. The impermeable clay layer is represented by layer 4. Because there are no shallow aquifer in the mountainous area, it is specified as inactive cells. Rivers that flow within the basin and ponds are simulated by river package. Because there is no data on the recharge and evapotranspiration in the study area, recharge is therefore arbitrarily assumed for running the model until calculated equipotential lines came close to the measured one and evapotranspiration rate is not added to the model. The values of hydraulic conductivity (K) ¢-11
shown were derived from data of 30 pumping tests conducted on the JICA test wells. Fifteen data of hydraulic conductivity of shallow and 15 of deep aquifer were plot on the base map and the conductivity of the whole area were interpolated from these data by Krigging method. The distribution of K(x,y) of shallow and deep aquifers were then assigned each cell in the model. The model layer 1 - 3 was assigned by the same K(x,y) of shallow aquifer while the forth layer was assigned with K (x,y) of clay specified in hydrogeological textbook and the fifth layer was assigned with K (x,y) of deep aquifer. Three types of boundaries were applied to the model which are no flow boundary to the mountainous area, river boundary to rivers and ponds and constant head to east of plain area of modeled area. Since the abstraction of ground water is not extensive the simulation was therefore done in steady state mode for 10 years period.
3. Results and Discussions Results of steady state simulation shows that general direction of ground water flow in both shallow and deep aquifers is from the west mountainous area to the eastern plain as shown in Fig. 1 and 2 respectively. Furthermore shallow ground water flows to the middle of Rhonpibul valley to discharge water to Hua Muang, Rhonna and Klong Nam Khun rivers. The calculated equipotential lines of ground water table of shallow aquifer are matched with the measured one very well as shown in Fig. 3. Results of deep ground water simulation shows flow direction of ground water from the west to the east as same as the shallow one. However in the west mountainous area the simulated ground water table is lower than the measured one by 3 - 4 meters but in the east plain area the simulated ground water table is higher than the measured one by 2 meters. The discrepancy of
Fig 1. Simulated ground water table of shallow aquifer in Rhonpibul valley ¢-12
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Fig 2. Simulated ground water table of deep aquifer in Rhonpibul valley
Fig 3. Correlation between observed and calculated ground water table of shallow aquifer ground water table might come from either incorrect applied recharge rate or incorrect arrangement of impermeable clay of the forth layer. The two parameters should be reevaluated. »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡
Use of stable isotope data of water to design conceptual model of ground water flow Isotope ratio of 18O and 2H of 10 summer rain samples collected at 5 elevations in Rhonpibul valley in the period of 2 months ¢-13
Fig 4. Local meteoric water line of rain in Rhonpibul and isotopic signatures of surface water, shallow and deep ground water are in the range of - 8.55 to - 5.55 and respectively. If two isotopes - 55.0 to - 35 are plotted, the graph will generates a local meteoric water line (LMWL) of δ 2 H = 7.88δ 018 + 7.83 which is closed to the global meteoric water line (GMWL). In the first sampling campaign 30 ground water samples (15 from shallow wells and 15 from deep wells) and 5 surface water collected from rivers and ponds are compared with LMWL as shown in Fig 4. . It can be seen that plots of deep ground water are located around LMWL while plots of shallow ground water have tendency to fall on one evaporation line and surface water falls on another evaporation line. This indicates that deep ground water is recharged by rain directly but recharge of shallow ground water has passed the process of slight evaporation and certainly surface water has passed higher evaporation rate. However the plots of each group of water are not clearly separated from each other. This may indicates that three types of water, deep, shallow ground water and ¢-14
surface water might have close relationship. In the other word they can be connected to each other or recharge water of shallow and deep aquifers comes from rain forming at the same latitude. Nevertheless more sampling campaigns are needed to obtain more information of stable isotope of water in the area particularly for the isotopic signature of rain and ground water in rainy season By the data of stable isotopes of rain, shallow and deep ground water in Rhonpibul, the improved conceptual model of ground water flow can be designed. Since the isotopic data of each water group are not clearly separated, this means that both shallow and deep aquifers are recharged mainly by water discharged from stream flowing from Hua Muang and Rhonna mountains. Therefore at the area close to the mountains, the hydraulic conductivity of the 4 model layer was assigned to represent sand instead of clay to let water recharges deep aquifer in that area.
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Dating of ground water by CFCs Twenty nine shallow and deep ground water samples were collected and shipped in May 2000 to International Atomic Energy Agency for conducting CFC-analysis. The ages of water are shown in Table 2. In general the range of 11-43 years and 13-35 years for shallow and deep ground water were found respectively. By average water in the shallow aquifer is slightly older than the deeper. However it can be concluded that water of both shallow and deep aquifers
have the same ages if the error of analysis is taken into account. This can confirm the conclusion of stable isotope data of the water that water in both shallow and deep aquifer are recharged from the same water source and the data of CFCs show that ground water in both aquifer flow from the recharge area to the lower plane with the same flow velocity. This findings will be used to validate the flow velocity of ground water in Rhonpibul in following part of the research.
Table 2. CFC-12 apparent age (years) of ground water collected from different aquifers Well
Type
Type of aquifer
CFC-12 age (years)
Layer between aquifers
JICA-1
Shallow deep Shallow deep Shallow deep Shallow deep Shallow deep Shallow deep Shallow deep Shallow deep Shallow Shallow deep Shallow deep Shallow deep Shallow deep Shallow deep Shallow deep
sand & gravel Weathering mudstone Sand Weathered sandstone Sand sand & gravel sand & gravel Limestone sand & gravel Weathering sandstone sand & gravel Weathering mudstone Sand Weathering mudstone Sand Limestone Sand Sand Limestone Sand Limestone Sand Limestone Sand Limestone sand & gravel silty clay Sand Limestone
15 13 40 31 31 35 25 23 40 35 11 18 43 30 27 28 24 14 32 15 23 19 16 32 22 22 24 35 12
silt
JICA-2 JICA-3 JICA-4 JICA-5 JICA-6 JICA-7 JICA-8 JICA-9 JICA-10 JICA-11 JICA-12 JICA-13 JICA-14 JICA-15
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mudstone silt mudstone sand & clay mudstone mudstone clay
clay clay clay clay clay Silt
¢-15
4. Conclusions To make a valid numerical ground water flow model, both correct conceptual model and true input parameters are needed. This research demonstrates that by applying isotopic data of water, the correct conceptual model can be designed. Furthermore tracer technique like CFCs is not only useful in designing the correct conceptual model but also useful in validating the outputs of the ground water flow model. However to use the techniques successfully, experienced and skillful persons are needed.
5. Acknowledgements The authors would like to thank International Atomic Energy Agency (IAEA)
for supporting this project by providing modeling softwares as well as conducting stable isotope and CFCs analyses. Particularly the encouragement of Dr. Yucel Yutserver, IAEA expert in proposing the research to be included in IAEA Regional Cooperation Agreement program made this research project possible.
6. References 1. Shoko Oshikawa, Personal communication. 2. Japan International Cooperation Agency, Report on Environmental Management Planning Survey for Arsenic Contaminated Area of the Nakhon Si Thammarat Province in the Kingdom of Thailand. December 1999.
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