2D RESISTIVITY SURVEY IN ĐỒNG THANH OLD SANITARY LANDFILLS, HỒ CHÍ MINH CITY

¹LÊ NGỌC THANH, ²NGUYỄN THÀNH VẤN, ¹NGUYỄN QUANG DŨNG
¹NGUYỄN THỤY NGỌC HÂN, ¹DƯƠNG BÁ MẪN

¹Hồ Chí Minh City Institute of Geographical Resources, VAST,
²Faculty of Physics, University of Natural Sciences, VNU-HCM

Abstract: At present, the Hồ Chí Minh City (South Việt Nam) has to cope with its sanitary landfills after closure. In practice, it has shown that in several cases the groundwater situated nearby the landfills is affected by leakage of contamination. Geophysical methods not only map the extension of old landfills, but also reveal the pollution plumes. On this basis it is possible to take actions accurately and timely.

The paper presents the results of 2D resistivity survey at Đồng Thanh old sanitary landfills in the southwest of Hồ Chí Minh City.

I. INTRODUCTION

In recent years, the increasing socio-economic development in Hồ Chí Minh City produces a big amount of waste. Although it is collected in an area of suitable processing technology, but in fact it always affects environment, especially the surrounding groundwater.

The Đồng Thanh sanitary landfill that is located in 15 km northwest of Hồ Chí Minh City centre and 32 ha in area, came into operation in 1988 and was closed in 2000. The contamination of groundwater caused by it brought serious damage to the rice-field, fruits and raised fishes in this area in 2000 year. Therefore, it is necessary to carry out groundwater monitoring so that it can be reduced in damages to the minimum.

II. DATA ACQUISITION AND PROCESSING

The ABEM Land Imaging System used for data acquisition is composed of four electrode cables, each of them has 21 take-outs, with the take-out distance of 5 m. The data acquisition is carried out by CVES method. The true resistivity structure is interpreted with the RES2DINV software (Loke and Barker, 1996).

III. GEOLOGY OF THE SURVEY AREA

From the engineering geology side, the survey area is composed of horizontal beds of Pleistocene sediments. The lithological description of borehole BH1 is as follows:

- 0 - 0.8 m: Surface soil, grey-yellow clay;

- 0.8 - 4.2 m: Firm, grey sandy clay;

- 4.2 - 7.0 m: Red-yellow laterite gravels and silty sand; coefficient of permeability k = 1.1×10^-5 cm/sec;

Table I. Resistivity of Pleistocene sediments in the Đồng Thanhsanitary landfill

Sediments	Resistivity (ohm.m)
Silty clay	5 -15
Clayish sand	20 - 60
Gravel sand	40 -100
Natural water	100 - 120
Contaminated water	< 10

- 7.0 - 7.2 m: Grey-yellow, firm clay with fine-grained sand lenses;

- 7.2 - 13.2 m: Clay mixed with grey-yellow silty sand; coefficient of permeability k = 1.7×10^-4 cm/sec;

- 13.2 - 20 m: Yellow-red medium-grained sand.

Resistivity range of Pleistocene sedimentary materials in the survey area has been estimated as follows (Tab. 1).

IV. DATA INTERPRETATION

1. Resistivity change in time

The comparison of three interpreted resistivity-depth sections in three different periods is presented as follows (Fig. 1):

1. Section T1 (dry season, Dec. 2004), the resistivity distribution is of horizon, from a few to 100 ohm.m, from 2 to 10 m in depth;

2. Section T1b (rainy season, May 2005), the resistivity distribution does almost not change, but the resistivity gradient in depth increases, i.e. over 6 months of dry season the soil from 2 to 10 m in depth becomes firmer.

3. Section T1c (rainy season, Nov. 2005), the resistivity much changes: there exists high conductivity blocks (resistivity less than 6 ohm.m). It may be the contaminated groundwater.

Figure 1. Resistivity change in time, T1, T1b , T1c.

2. Resistivity change in space

The high conductivity blocks in three interpreted resistivity-depth sections are presented as follows (Fig. 2):

4. Section T1c (rainy season, Nov. 2005), as described above.

5. Section T2 (rainy season, Nov. 2005), 18 m far from T1c.

Figure 2. Resistivity change in space, T1c, T2 , T3.

6. Section T3 (rainy season, Nov. 2005), 18 m far from T2.

Comparing these three sections, it is assumed that the velocity of pollution extension is 10 cm/day or 1.2 × 10^-4 cm/sec at least.

V. CONCLUSION

The electrical imaging method plays an important role in locating the pathways of contaminated leakage. These results provide a reliable basis for drilling and sampling works. In addition, the seismic refraction and GPR measurements should be carried out to determine the geological structure of the landfill.

REFERENCES

1. Berstone C., 1996. Electromagnetic and DC resistivity mapping of waste deposits and industrial sites: Experiences from Southern Sweden. Paper MO14 presented at EAGE, Amsterdam, The Netherlands.

2. Berstone C., 1996. 2D resistivity surveying of old landfills, Proc. 2^nd European EEGS meeting, Nantes, France.

3. Loke M.H., Barker R.D., 1996. Rapid least-squares inversion of apparent resistivity pseudosections by a quasi-Newton method. Geoph. Prosp., 44/1.

4. Vu Dinh Luu, Dang Ngoc Phan, 2001. Soil investigation data. Proj. on Đồng Thanhrubbish site treatment.