CURRENT STATUS OF GROUNDWATER POLLUTION IN HÀ NỘI AREA*

¹NGUYỄN VĂN ĐẢN, ²NGUYỄN THỊ DUNG

¹Division of Hydrogeology - Geological Engineering of the North, Nghĩa Tân, Hà Nội
²Department of Geology and Minerals of Việt Nam, 6 Phạm Ngũ Lão, Hà Nội

Abstract: The water supply in Hà Nội is based mainly on groundwater. The exploitation of groundwater for living and industrial use in Hà Nội has been beginning since 1909 with increasing amount. The abstracted water amount is 100,000m³/day in 60’s, 200,000 m³/day in 70’s, 300,000 m³/day in 80’s, 400,000 m³/day in 90’s, and over 500,000 m³/day in present time. It is estimated that in 2010 the abstracted amount will be 1,000,000 m³/day. The abstraction with large amount of groundwater causes negative impacts, such as lowering of water level, expansion of area of depression cone, land subsidence and groundwater pollution.

The groundwater pollution occurs both in the Holocene aquifer (Upper aquifer) and Pleistocene aquifer (Lower aquifer - production aquifer). The pollutant elements are detected like nitrogen component, microorganisms, organic matter, toxic elements: arsenic, mercury… The pollution by nitrogen components has been studied since 90’s. The studied results showed that the main contaminant is ammoni (NH₄⁺), contributing in large area with high concentration. The polluted area and concentration of pollutants are increasing with time.

I. GENERAL OUTLINES

The Hà Nội City extends from near the centre of the Bắc Bộ Plain to its northeast margin. With an area of about 900 km² it includes 7 prefectures and 5 suburban districts with the population of about 2.7 mill. persons

The major part of the Hà Nội City consists of plain land with the surface elevation of 5-20 m, a small part in the north consists of hilly area belonging to the southeast end of the Tam Đảo Range with the highest summit of 462 m in height.

The mean annual rainfall reaches about 1600 mm, divided into two seasons. The rainy season extends from May to October, occupying 85% of annual rainfall. The remaining months belong to the dry season having an inconsiderable rainfall.

The Red River, the greatest one in North Việt Nam, crosscuts the city and at the centre there is the Đuống tributary. The mean discharge observed in many years in the two Red and Đuống rivers is about 3500 m³/s. The discharge of these rivers is concentrated in the flood season, coinciding with the rainy season, with about 73% of the water amount of a year.

On the hydrogeological side, Quaternary sediments have the greatest importance. They are distributed in the plain area, and due to the rhythmic character of sediments they form two main aquifers in the geological section.

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* This paper was presented in the International Symposium on Environment and Injure for Community Health caused

by Pollution during the Urbanization and Industrialization in , Hà Nội, 12/2002.

Holocene aquifer (qh), called also as Upper aquifer, is exposed on the surface and largely distributed continuously from the Red and Đuống rivers southward with an area of about 530 km². The lithological composition of the aquifer consists of two members. The upper member includes weakly impermeable silty clay, silty sand with the thickness from very small to about 10 m. The lower member is composed of sand of different sizes, sometimes mixed with gravel and grit at the base, with the average thickness of about 9.2 m in the north of the Red River, 10 m at Gia Lâm and 13.3 m in the south of the Red River. The conductivity coefficient (T) of water-bearing sediments is about 20-800 m³/day. The gravitational water-release coefficient (m) changes from 0.01 to 0.17. The depth of groundwater level is usually about 3-4 m under the ground surface; particularly in the south of the Red River this depth is greater due to the water exploitation. The output quantity (q) of experimental boreholes is from very small to 4.5 l/s.m. The supplying source of aquifers consists of rainy water, drainage water and river water (in the rainy season); as for the water escaping from the river (in dry season) it is due to evaporation and supply to lower aquifers.

Water of the qh aquifer is the fresh one with the total dissolved solids usually under 0.5 g/l, mainly of calcium bicarbonate type. The iron content of water in the most part of area is from 0.4 to 10 mg/l, locally greater; the manganese content – from 0.2 to 2.0 mg/l; ammonium content from very small to very great, such as in Thanh Trì District situated the south of the city it reaches some tens up to 100 mg/l. This aquifer is of small scale in water-supply significance. People of rural areas usually dig wells or drill small boreholes for exploiting water from this aquifer.

Pleistocene aquifer (qp), also called as Lower aquifer, occurs from the south of Sóc Sơn District and is continuously developed southward, but completely covered, therefore, met only through boreholes. The depth of water meeting is 2-10 m in the north of the Red and Đuống rivers, 5-22 m at Gia Lâm and 10-35 m in the south of the Red River. The qp aquifer is separated from the qh one by weakly permeable sediments of the upper part of Vĩnh Phúc Formation (Q₂³ vp₂), particularly in the band extending along the Red and Đuống rivers, due to the abrasion, the two aquifers lie directly upon one another forming the geohydrological window. There is a close hydrodynamic relation between the aquifers and river water.

The water-bearing sediments consist of sand mixed with gravel and pebble with a thickness of from 10 to 35 m. The conductivity coefficient (T) is 260-1600 m³/day, The elastic gravitational water-release coefficient (m*) changes from 0.00004 to 0.066. The output quantity (q) of experimental boreholes in most cases is over 1 l/s.m, therefore, this aquifer is ranked to very rich in water type.

Water of the qp aquifer is the fresh one with the total dissolved solids of from very small to 0.78 g/l, the main composition of calcium bicarbonate, locally calcium-natrium bicarbonate-chloride. The iron content in water is high, changing from 0.4 to 20 mg/l, locally up to 50 mg/l; manganese content from very small to 0.33 mg/l, ammonium content in the south of Thanh Trì District is very high; on an area of about 80 km² this content reaches over 10 mg/l.

Due to its richness in water, the qp aquifer is being strongly exploited, and is the main water supply source of the Hà Nội City.

The Hà Nội City began to use groundwater as living water since 1909 by drilling first exploiting boreholes at Yên Phụ. The exploiting water quantity increases day by day.

In 1954 the quantity of exploited water was, in average, 20,000 m³/day. During the 60’ it was 100,000 m³/day; during the 70’ - 200,000 m³/day; during the 80’ - 300,000 m³/day and; during the 90’- 400,000 m³/day.

At present, water used in living purposes and agricultural production is taken mainly from groundwater source with three main forms:

1) Concentrated water-exploitation realized by special institutions (companies of water business); at present they are exploiting water in 10 large well fields and many small well fields with about 150 drill wells. The average exploited water quantity is about 420,000 m³/day from the qp aquifer.

2) Solitary water-exploitation realized by companies, economic units, schools, hospitals, institutions, because they do not receive the water supply from the city. According to incomplete statistic data, this form of exploitation has about 500 drill wells exploiting the qp aquifer with the total output of about 150,000 m³/day.

3) Water exploitation in the rural areas, carried out by families with shallow boreholes of small diameter or digged wells, exploiting mainly the qh aquifer, locally the upper part of the qp aquifer. The exploiting equipment is rudimentary. The quantity of these wells increases in time, and it is difficult to make the statistics. The investigation in 1999 year has been showing that in the whole city there are about 115,000 wells exploiting water with the total discharge of about 115,000 m³/day.

The water used in living demands will increase in time. According to the guiding planning in water for the city, in 2010 year the water for living purpose in the south of Red River will be 700,000 m³/day, and to the 2020 year in the whole city will be 1,400,000 m³/day. Therefore, the corresponding water quantity exploited from groundwater source will also increase.

II. NEGATIVE INFLUENCES CAUSED BY GROUNDWATER EXPLOITATION

The major part of great exploiting works (well fields) has been realized after the results of exploration and evaluation of groundwater reserves. Only some, especially solitary exploiting works and those in rural areas were realized without any hydrogeological investigation. The exploitation of groundwater usually leads to negative influences, such as land subsidence, lowering of water level and pollution of water sources.

1. Land subsidence is the phenomenon of gradual or sudden lowering of the ground surface caused by natural or artificial process. One of the causes of land subsidence is the groundwater exploitation. When the water level is lowered the volume of loose and weak soil decreases leading to the land subsidence. The land subsidence of large scale related to groundwater exploitation has been determined in many places in the world, such as Texas, California states of USA, Bangkok City of Thailand, Osaka City of Japan, Mexico, etc.. In Hà Nội, the study is still rare, but the land subsidence has been recognized.

In 1988 year, during the exploration of groundwater the Division of Hydrogeology - Geological Engineering of the North (DHGEN) constructed 32 landmarks for measuring the subsidence in the territory of the city in the south of Red River. From 1991 to 1995, when carrying out the Hà Nội Water Program the Survey of Communication and Public Works received this network and complemented 13 landmarks. The study on subsidence was realized by one time/year water level measurement from the two National Landmarks of 1^st class at Do Lộ and Đông Ngạc lying in the stable area. The difference of landmark height according to the result of measurement in two successive years is considered as subsidence quantity.

The result of subsidence study in the 1988-1995 period showed that almost all the city interior (in except of the Red River bank area) and adjacent areas were subsided. The strongly subsided areas (average rate >10 mm/year) are the central and the south parts of the city. The strongest subsided areas (average rate of 20-44 mm/year) are Giảng Võ - Thành Công in the centre and Pháp Vân in the south. Since 1997 the Institute for Sciences and Engineering of the Hà Nội Survey of Construction carried out the study on subsidence in 6 stations: Ngọc Hà, Mai Dịch, Lương Yên, Hạ Đình, Pháp Vân and Thành Công, that has been giving similar results.

The above study on subsidence has been allowing to set forth following remarks:

1) The strong exploitation of groundwater in the areas having weak soil foundation (mud, peat, sediments bearing organic matter) in the geological section causes the strongest subsidence;

2) The areas not influenced or influenced by groundwater exploitation, but having not weak soil foundation (such as the band along the Red River bank) are not subsided;

3) The subsiding rate tends to decrease in time, among this in the strongest subsided Pháp Vân area the subsiding rate has been changing from about 60 mm/year at the beginning of the studying period to about 40 mm/year recently, in the strongly subsided Thành Công area – from 40 to 25 mm/year, in the weakly subsided Mai Dịch area – from 12 to 5 mm/year.

4) The cause of land subsidence can be numerous, but the groundwater exploitation is the main one.

2. The lowering and enlargement of lowering funnel of water level

The strong groundwater exploitation has been having strong influence to the groundwater regime, especially in the south of the Red River the groundwater level of the exploited aquifer (qp) deeply lowers forming a large lowering funnel in the whole city and adjacent areas. It has the ellipsoid form with the great axis parallel with the Red River extending from Cổ Nhuế to Ngọc Hồi, and the small axis perpendicular to the Red River extending from Hà Đông to Yên Phụ. In this funnel there are many small funnels related to well fields. For the convenient watch on the development of these funnels in time one can divide them into 3 areas after the level of exploiting influence as follows:

1) Areas influenced by exploitation having the water level under 0 m;

2) Areas of strong influence having the water level under -8 m;

3) Areas of very strong influence having the water level under -14 m.

The funnels of water lowering have been continuously mapped on the basis of data on average water level per month from 1992 to present time of the qp aquifer; this has been showing the change of their area as follows:

1) Surface of exploitation-influenced areas during 1992-1997 is annually restricted during the rainy season (September) and enlarged during the dry season (April) from 180 to 220 km² but does not tend to change. From the 1997 to present time it tends to increase, and at the end of 2001 reaching 250 km², that means the increasing rate is 7 km²/year.

2) Surface of strongly exploitation-influenced areas continuously increases from 55 km² (in 1992) to 75 km² (in 1997) with the rate of 4 km²/year. Then it stronger increases and to the end of 2001 reaching 95 km², it means with the rate of 5 km²/year.

3) Surface of very strongly exploitation-influenced areas increases also continuously from 3 km² (in 1992) to 14 km² (in 1997) with the rate of 2 km²/year. To the end of 2001 it reaches 27 km², it means with the rate of 3 km²/year.

The above watching results have been showing that the lowering funnels of groundwater level have been continuously enlarged both in space and in depth.

Together with the enlargement of lowering funnels, the groundwater level has been decreasing in time. The analysis of features of water level lowering in exploiting well fields allows to divide them into 2 groups: group of well fields in the band of Red River bank (Yên Phụ, Ngô Sỹ Liên, Lương Yên, etc.) and group of far from Red River well fields (Mai Dịch, Ngọc Hà, Hạ Đình, etc.).

Along the Red River bank, in general, the groundwater level is not so deep, due to the supply of the river the annual oscillation of water level is not great. In all monitoring boreholes the water level tends to gradually decrease in time, but the decreasing rate is not great.

In the far from the Red River areas, the groundwater level lies very deeply, such as in the borehole P.41^a situated in the centre of the Hạ Đình well field the groundwater level is 32 m deep from the surface, corresponding to the elevation of -25 m. Due to the small influence of meteo-hydrological factors, the water level strongly decreases in time with the rate of 0.3 m (Q.63^a – Mai Dịch well field) to 0.4 m (P.41^a - Hạ Đình well field).

The decrease of water level leads to the resource exhaust, that entrains a lot of disadvantageous problems, such as water source pollution, land subsidence, etc..

III. WATER SOURCE POLLUTION

The water resource pollution is understood as a process changing the quality of water caused by human, that makes water having restricted use or impossible for using in intended purposes.

1. Current status of groundwater pollution

The pollution of groundwater in the Hà Nội City area has been paid attention long ago, although the study is still not sufficient, but it has been setting forth some remarks and alarms on the groundwater pollution. It comprises nitrogen pollution, microbiologic pollution, organic compound pollution, metallic element pollution, etc.. They have been studied in different levels.

a. Pollution by nitrogen compounds

The study on pollution of groundwater by nitrogen compounds has been carried out by the DHGEN since the beginning of 90’ of the past century in the south of Red River. Samples were taken for analysis 2 times a year in the dry and rainy seasons in the monitoring network with studied compounds as NH₄⁺, NO₂^-, NO₃^-.

Standards for classification of levels of pollution by nitrogen compounds have been set forth for serving the study as seen in the Table 1.

Table 1. Classification of levels of pollution by nitrogen compounds

Pollution matter	Drinking water standard, mg/l	Limit of content after pollution levels, mg/l
		Clean	Slight-dirty	Medium-dirty	Heavy-dirty
NH4^+ NO2^- NO3^-	< 3 < 0.1 < 5	<0.5 <0.1 <5	0.5-3 0.1-0.5 5-10	3-10 0.5-1 10-50	>10 >1 >50

The study results have been showing that the groundwater is polluted mainly by ammonium of high content, that continuously increases in time and occurs on large area. The nitrate and nitrite pollution is inconsiderable, because the localities having NO₂^- and NO₃^- are a little and in restricted area.

The results of analysis of NH₄ content from 1992 to present time in the south of Red River synthesized for calculating the average values for the dry and rainy seasons on the total of studied samples have been presented in the Table 2 and Figure 1.

The results have been showing that:

1) Average content of ammonium in groundwater always higher than permitted standards, among this it is higher in the qh aquifer than in the qp aquifer; this proves that the pollution is developed in downward direction;

Table 2. NH₄⁺ content in groundwater in the south of Red River during the 1992-2002 period

Year	Season	Holocene aquifer (qh)			Pleistocene aquifer (qp)
		Number of studied samples	Content, mg/l		Number of studied samples	Content, mg/l
			Max	Average		Max	Average
1992	Dry	41	58.1	7.1	43	58.1	4.2
	Rainy	42	64.5	8.7	46	51.6	4.7
1993	Dry	42	34.6	5.2	43	24.2	4.4
	Rainy	45	48.4	5.1	48	19.3	4.1
1994	Dry	43	84.7	7.6	48	33.6	5.1
	Rainy	49	51.7	4.3	51	17.4	3.7
1995	Dry
	Rainy	50	100.0	11.9	52	80.0	7.4
1996	Dry
	Rainy	40	128.0	16.8	47	128.0	7.6
1998	Dry	30	144.0	11.1	42	100.0	8.9
	Rainy	27	151.5	11.2	42	42.0	8.5
1999	Dry	31	168.0	16.1	42	44.0	8.7
	Rainy	29	157.2	16.2	40	45.5	9.3
2000	Dry	31	178.0	18.7	41	50.4	7.5
	Rainy	27	118.0	14.7	39	30.5	6.9
2001	Dry	36	204.0	14.6	18	32.0	7.5
	Rainy	38	56.4	8.1	37	39.2	5.2
2002	Dry	34	92.4	16.9	46	72.4	11.1
	Rainy	32	135.2	17.5	47	64.0	9.54

2) The ammonium content of the two aquifers tends to gradually increase in time; this affirm that the groundwater was and is being polluted;

3) The NH₄⁺ content of the qh aquifer strongly changes in time due to the direct dependence to the polluting sources and meteo-hydrological factors; as for in the qp aquifer the change in season is inconsiderable.

For studying the polluted area, during the 2000 year the sampling was carried out regularly on the surface of the whole city. The results have been showing that both the qh and qp aquifers are polluted by ammonium. The polluted and heavily polluted areas occur mainly in the south of the city belonging to the Thanh Xuân, Đống Đa, Hai Bà Trưng prefectures and the Thanh Trì District, a part of the Từ Liêm District and a part of the southwest of the Gia Lâm District, with the surface of about 150 km².

In the north part of the city the major part of the surface of the aquifers is not polluted yet.

b. Pollution by organic compounds

The amount of organic compounds has been studied at the same time with the study on nitrogen compounds in the south of Red River by means of the degree of oxidation. The results are presented in the Table 3 and Figure 2.

The study has been giving similar results as in the study on ammonium content.

1) The average degree of oxidation of the groundwater is always higher than the permitted standards (a mg/l), among this that of the qh aquifer is higher than that of the qp aquifer and both tend to increase in time, proving that the groundwater was and is being polluted, and polluted in downward direction;

Figure 1. Diagram of the change of average content of ammonium in groundwater
in the south of Red River

Table 3. Degree of oxidation of groundwater in the south of Red River
during the 1992-2002 period

Year	Season	Holocene aquifer (qh)			Pleistocene aquifer (qp)
		Number of studied samples	Content, mg/l		Number of studied samples
			Max	Average		Max	Average
1992	Dry	41	16	3.79	43	11.36	2.7
	Rainy	40	25.6	5.99	46	67.2	4.14
1993	Dry	42	13.44	3.98	43	10.88	2.56
	Rainy	45	21.12	4.46	48	12.8	3.71
1994	Dry	43	21.76	3.52	48	11.2	2.77
	Rainy	49	16.2	3.10	51	64	4.79
1995	Dry
	Rainy	50	132	12.05	52	14.72	4.16
1996	Dry
	Rainy	42	46.4	9.28	46	17.6	5.83
1998	Dry	30	26.6	9.10	41	20.8	7.22
	Rainy	26	31.6	8.91	41	24	7.89
1999	Dry	31	28.8	10.52	41	22.8	8.92
	Rainy	29	31.2	10.94	39	68	10.31
2000	Dry	31	33.2	9.16	41	31.2	8.32
	Rainy	27	31.6	10.05	39	34	9.07
2001	Dry	35	38.4	9.87	18	19.2	6.24
	Rainy	38	19	6.32	37	13.7	4.62
2002	Dry	34	21.84	7.24	46	14.32	5.49
	Rainy	32	23.2	7.26	47	13.2	4.76

Figure 2. Diagram of the change of average degree of oxidation in groundwater
in the south of Red River

2) The polluted area is situated also in the south of the city.

The results of preliminary study by Phạm Hùng Việt (Centre of Environmental Chemistry, Hà Nội University of Natural Sciences, 2000) have been showing that the content of easily evaporated organic compounds (benzene, toluene, xylene, chloric derivatives of hydrocarbon containing 1-3 atoms C) in groundwater is inconsiderable and under the permitted standards. The content of polychlorbiphenyl and aromatic hydrocarbons has not been studied yet.

c. Microbiological pollution

The microbiological pollution, consisting of the content of total Coliform (permitted standard: 0/100 ml) and Fecal coliform (permitted standard: 0), has been studied by Đỗ Trọng Sự, Research Institute of Geology and Mineral Resources, giving the results presented in the Table 4.

Table 4. Result of determining microbiological content in groundwater in 1993 year

Season	Holocene aquifer (qh)			Pleistocene aquifer (qp)
	Number of studied samples	Samples not meeting the standard	Ratio %	Number of studied samples	Samples not meeting the standard	Ratio %
Dry	36	28	77	31	15	48
Rainy	14	7	50	20	9	45

The results of study have been showing that:

1) In both aquifers the microbiological content are greater than the permitted standard, proving that the groundwater is polluted, among this the pollution in qh aquifer is heavier than in qp aquifer, and the pollution in dry season is heavier than in rainy season;

2) Groundwater is polluted mainly by Fecal coliform; in most cases the content is from 3-10 to 100/100 ml.

d. Pollution by heavy metals

The pollution by heavy metals has been studied desultorily and unsystematically, therefore, we cannot have a precise conclusion on the pollution, but only have the knowledge on

Table 5. Results of the study on arsenic content in groundwater
during the dry season of 2000 and rainy season of 2001

Studied areas	Holocene aquifer		Pleistocene aquifer			Max values, mg/l
	Dry season of 2000	Rainy season of 2001		Dry season of 2000	Rainy season of 2001
Đông Anh District
-Total of studied samples - Number of samples higher than the permitted standard - Ratio (%)				78 6 7.7		0.105
Sóc Sơn District
-Total of studied samples - Number of samples higher than the permitted standard - Ratio (%)				37 1 2.7		0.196
Gia Lâm District
-Total of studied samples - Number of samples higher than the permitted standard - Ratio (%)	20 8 40	19 2 10.5		72 13 18.1	72 2 2.8	0.274
Thanh Trì District
-Total of studied samples - Number of samples higher than the permitted standard - Ratio (%)	72 43 59.5	72 29 40.3		24 13 54.2	23 9 39.1	0.292
Từ Liêm District
-Total of studied samples - Number of samples higher than the permitted standard - Ratio (%)	55 8 14.5	55 1 1.8		25 9 36	25 3 12	0.216
Urban area
-Total of studied samples - Number of samples higher than the permitted standard - Ratio (%)	47 18 38.3	46 12 26.1		42 17 39.5	43 8 19	0.331

the current status of their content in groundwater. The most significant is the recent finding and study on arsenic content. The main research works include:

In 1994, when studying on the pollution of groundwater in Hà Nội, Đỗ Trọng Sự informed on some samples having the arsenic content greater than the permitted standard (PS = 0.05 mg/l).

In 1999, basing on the discovery by some scientists the mass media informed on the arsenic pollution of some water source at Quỳnh Lôi, prefecture Hai Bà Trưng. Then, the UNICEF in Việt Nam organized the sampling in series of wells of this locality, and 25% of studied samples contain the arsenic content higher than the permitted standard. Other hydrogeologists, such as Ngô Ngọc Cát, Hồ Vương Bính, Nguyễn Anh, Nguyễn Thị Phương Thảo, etc. carried out the study and obtained similar results.

The study on arsenic content in groundwater has been realized rather sufficiently by the DHGEN. Samples were collected with great quantity with 800 samples on the whole city and in both dry (Dec., 2000) and rainy (Aug., 2001) seasons. The study was carried out under the order of the UNICEF in Việt Nam. The study results are presented in the Table 4.

The analysis of data of this Table shows that:

1) In the north of Red and Đuống rivers (Sóc Sơn and Đông Anh Districts) only the qp aquifer is studied; the number of samples having the arsenic content higher than the permitted standard is inconsiderable;

2) In the south of the Red and Đuống rivers the number of samples having the arsenic content higher than the permitted standard is more numerous, occupying from 1.8 to 59.7% of the studied sample number;

3) In the Gia Lâm area the number of samples having the arsenic content higher than the permitted standard of the qh aquifer is higher than that of the qp aquifer. In the Từ Liêm area the arsenic content in the qp aquifer is higher than that of the qh one; in the Thanh Trì and city interior areas the two aquifers have the same degree of pollution;

4) In all the studied areas, during the dry season water is more polluted by arsenic than in the rainy season.

2. Some problems of cause of pollution

The cause of groundwater pollution has not been studied in detail yet, however, we can set forth the problems associated with each pollution type as follows:

1) At present, the problem of environment pollution in Hà Nội is very serious. Almost not processed living and industrial waste waters follow natural rivers (Tô Lịch, Kim Ngưu, Lừ and Sét) and artificial canals flowing to the south of the city, that is the Thanh Trì District where the relief height is lowest (about 3-5 m). The rubbish dumps (Tam Hiệp, Mễ Trì) and large cemeteries are concentrated there; this is the large pollution source, the cause of microbiological, nitrogen and organic compound pollution and the cause for explaining the ammonium and organic compound heavy pollution of the south of the city.

2) The studied area, especially the city interior, has very many boreholes (water exploiting boreholes, exploration boreholes), besides the exploitation of brick and tile clay, the excavation and stake driving for construction, etc. create good conditions to the pollution, especially the downward one.

3) The composition and origin of sediments can be the source creating pollution factors, among them there are organic matters in the rocks (mud, peat, etc.) that are the source of nitrogen compound and arsenic pollution.

4) The strong exploitation of groundwater can be the direct cause of pollution. It makes the water level deeply lowering, forming large water lowering funnels accompanied by changes in hydrodynamic conditions, such as current rate, hydrodynamic inclination, permeability, that can lead to changes in physico-chemical conditions, strengthening the rate of groundwater pollution.

REFERENCES

1. Đỗ Trọng Sự, 1997. Hiện trạng ô nhiễm nước dưới đất ở một số khu dân cư kinh tế quan trọng thuộc đồng bằng Bắc Bộ. (Current status of groundwater pollution in some important population and industrial areas in the Bắc Bộ Plain). Báo cáo Hội thảo Tài nguyên nước dưới đất: 99-112. Hà Nội.

2. Phạm Hùng Việt, 2002. Chất lượng nước ngầm vùng Hà Nội. (Quality of groundwater in the Hà Nội area). Báo cáo Hội thảo về nước ngầm Hà Nội. Bộ Kế hoạch và Đầu tư, Hà Nội. .

3. UNICEF Việt Nam, 2002. Hướng tới giảm nhẹ sự ô nhiễm arsen ở Việt Nam. (To the mitigation of arsenic pollution in Việt Nam). Báo cáo Hội thảo xây dựng chương trình hành động về arsen, Bộ Nông nghiệp và Phát triển nông thôn, Hà Nội.

4. Vũ Nhật Thắng, Nguyễn Văn Đản, 2004. Địa chất và tài nguyên khoáng sản thành phố Hà Nội. (Geology and mineral resources of Hà Nội City). Cục Địa chất và Khoáng sản Việt Nam. Hà Nội.