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Mr. Mesak Milintawisamai*, Ms. Variga Sawaittayotin*,Mr. Peerapong Sunthondecha*,

Mr. Manit Sonsuk**, Mr. Kiattipong Khamdee**

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** ”π—°ß“πæ≈—ßß“πª√¡“≥Ÿ‡æ◊ËÕ —𵑠∂.«‘¿“«¥’√—ß ‘µ ‡¢µ®µÿ®—°√ °√ÿ߇∑æ¡À“π§√ 10900 ‚∑√. 0-2579-0138Office of Atomic Energy for Peace, Vibhavadi Rangsit Rd., Chatuchak, Bangkok 10900 Tel. 0-2579-0138

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ABSTRACTBy 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

shal low 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. IntroductionRhonpibul 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

reached the number of 937. Japan International

Cooperation Agency (JICA) and Environmental

Research and Training Centre then launched

detai led invest igat ion 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

af fected 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 contami-

nation 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 particularly2H, 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 countermea-

sures to solve the ground water pollution

problem, objectives of the project have been

set up as followed:

1. to study ground water characteris-

tics and flow patterns

2. interconnection between surface

and ground water in Rhonpibul and

¢-8 »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 arsenictoxicity 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 f low has transpor ted t in tai l ing,

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. Theoreti-

cally arsenopyrite can cause contamination

when it is oxidized to form sulfate

compounds and the arsenic can be released

into ground water.

Geological settings and meteo-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.

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

foothi l l of the mountains. The rock is

composed of mudstone, siltstone, limestone

and their alterations. The strike of the rock

is nor th-south to nor theast-southwest

direction and dips toward east around

30 degree. In the lowland north to south

stretching black l imestone 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

2. Methods and MaterialsThe study has been initiated since

October 1998 by conduct ing var ious

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

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Table 1. Aquifer characteristics in Rhonpibul

Type of aquifer Range of transmissitivity Range of storage

(m2/min) coefficient

Shallow sand and gravel 6.59x10-3 - 1.92x10-1 -

Deep weathered rock 1.01x10-3 - 2.86x10-2 5.95x10-3 - 9.44x10-1

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 ident i fy unconsol idated

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 character ist ics 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

¢-10 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

of shallow and deep aquifers can indicate

how close the relat ion 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

par t 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 km2 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

as 8 ponds because the inf i l t rat ion

of river into ground water and outflow

of ground water to rivers is needed to be

simulated.

Conceptual modelThe 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 m2. In the center par t 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 ver tically divided into 4 layers

which are sand al luvium, impermeable

clay, weather ing rock and l imestone.

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 al luvium 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 unt i l

calculated equipotential lines came close to

the measured one and evapotranspiration

rate is not added to the model.

The values of hydraulic conductivity (K)

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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 DiscussionsResults 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 shal low 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 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

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.

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

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are in the range of - 8.55 to - 5.55 and

- 55.0 to - 35 respectively. If two isotopes

are plotted, the graph will generates a local

meteoric water l ine (LMWL) of δ2H =

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

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.

Fig 4. Local meteoric water line of rain in Rhonpibul and isotopic signaturesof surface water, shallow and deep ground water

¢-14 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

Dating of ground water by CFCsTwenty 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 Layer between

(years) aquifers

JICA-1 Shallow sand & gravel 15 siltdeep Weathering mudstone 13

JICA-2 Shallow Sand 40 mudstonedeep Weathered sandstone 31

JICA-3 Shallow Sand 31 siltdeep sand & gravel 35

JICA-4 Shallow sand & gravel 25 mudstonedeep Limestone 23

JICA-5 Shallow sand & gravel 40 sand & claydeep Weathering sandstone 35

JICA-6 Shallow sand & gravel 11 mudstonedeep Weathering mudstone 18

JICA-7 Shallow Sand 43 mudstonedeep Weathering mudstone 30

JICA-8 Shallow Sand 27 claydeep Limestone 28

JICA-9 Shallow Sand 24JICA-10 Shallow Sand 14 clay

deep Limestone 32JICA-11 Shallow Sand 15 clay

deep Limestone 23JICA-12 Shallow Sand 19 clay

deep Limestone 16JICA-13 Shallow Sand 32 clay

deep Limestone 22JICA-14 Shallow sand & gravel 22 clay

deep silty clay 24JICA-15 Shallow Sand 35 Silt

deep Limestone 12

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4. ConclusionsTo 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.

Fur thermore 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. AcknowledgementsThe 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. References1. 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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