Nitrogen compounds and iron contamination in ground water of Holocene aquifer in Nam Dinh region

I. INTRODUCTION

Nam Định, together with Hà Nội and Hải Phòng cities, forms the most important geo-economical triangle of the northern part of Việt Nam. This is why the area draws nationwide attention in many different aspects: financial investment, social development, human health protection and last but not least, natural resource management.

The usage of groundwater in the region has gradually increased in the last 10 years because of the increase of water demand during the growth of population and rapid development of economy. Up to now, only about 89% of inhabitants have access to fresh water and only about 38 % of population can get safe drinking water [3].

With the support from the National Fundamental Research Program by MOST and by the Department of Science and Technology of Nam Định, an overview study on the groundwater resources in Nam Định areas has been implemented by the Institute of Geological Sciences (VAST, Hà Nội) and the Hà Nội University for Mining and Geology from 2002 to 2003. The study was focused mainly on the investigation of the hydrogeological characters, the evaluation of the groundwater potential and the identification of contaminants, if any, in the two important aquifers, the Holocene and the Pleistocene, in the area.

This paper presents some results of the above mentioned study on the groundwater contamination in the Holocene aquifer.

II. STUDY AREAS

The study areas are located on the right side of the Red River. It is bounded by Đào River in northwest, by Ninh Cơ River in southeast and Bắc Bộ in the East. The Nam Định city is located in the northern part of this area (Fig. 1).

The main meteorological data of Nam Định are shown in the Figure 2. The climate of the region is a humid tropical type. Usually, 70-80 % of the total annual rainfall occurs during July, August and September (National statistic data book, 2003), the driest month of the year is November.

The Red River Delta, which covers an area of 17,000 km², is a part of the Hà Nội neotectonic depression. The tectonic movement of the Red River deep-seated fault system is one of the most dominant geological process in the area [12]. The basin is filled up mainly with deltaic sediments of Red River System and marine transgressional sediments of from coarse- to fine-grained materials, such as pebble, gravel, sand and silty, clay, silty clay [13].

The Nam Đinh area lies in the southeastern most part of this basin and is a typical agricultural area with small industries. The area is mainly composed of farmland, residential quarters, many mechanical craft settlements and, in the coastal zone, aquacultural farms.

There are five available aquifers in the study area [5], but only two of them play an economical role as groundwater sources: the Holocene aquifer in the Hải Hưng Formation (Q₂hh) and the Pleistocene aquifer in the Hà Nội Formation (Q₁^2-3hn). The main hydrogeological feature of the region is briefly summarized according to data published in [4, 5, 9] and shown in the Table 1.

Table 1. Hydrogeological character of water-bearing formations in the Nam Định area

Stratigraphy	Fm	Thickness (m)	Lithology	Water table	Discharge (l/s)	Water type
Holocene	Thái Bình (Q₂tb)	2-28	Sand, silty clay, clay containing fauna and plant remains	0.5-3	0.05-1.45	Na-Mg-HCO₃-Cl
Holocene	Hải Hưng (Q₂hh)	1.3-27.5	Fine sand, silty sand, silty clay, interbedded with sand lenses	0.5-3.4	0.5- 5	Na-HCO₃-Cl
Pleistocene	Hà Nội (Q₁^2-3hn)	10-50	Quartz sand, grit and pebbles containing polymictic pebbles	0-2.5	> 5	Na-HCO₃-Cl
Pliocene	Vĩnh Bảo (N₂vb)	35-85	Fine and medium sandstone contains pebbles, siltstone and clay shale	0.6-1.2	< 12	HCO₃
Triassic	Đồng Giao (T₂a dg)	100	Gray and dark gray massive and thick-bedded limestone	0.2	5.78	Na-Ca-HCO₃-Cl

III. SAMPLING AND ANALYSIS

In total 106 groundwater samples were collected during the rainy period (from May to August of 2002 and 2003) and during the dry period (from October to December of 2002 and 2003) according to the VN-groundwater sampling guide (TCVN 6000 - 1995). The sampling sites from these two campaigns including factories, residential quarters and settlements are mostly the same. The samples were taken by hand pumps from house-wells and so-called UNICEF-wells, as well as from small water supplies in the rural area after at least 10 minutes of pumping.

The samples were splitted into acidified (using 15% HCl) and non-acidified subsamples. The non-acidified subsamples are used for measuring in situ the pH value, specific conductance, temperature and oxidation-reduction potential using the portable multilane P4 and for estimating the nitrogen compounds and other environment data. The acidified subsamples are used for analyzing iron and other metals.

The analysis of water samples for nitrogen compounds, iron and other metals have been carried out in laboratories of the Institute of Chemistry, Hà Nội and of the University of Mining and Geology, Hà Nội.

IV. RESULTS AND DISCUSSION

In this paper the Vietnamese quality standard of groundwater (TCVN - 1995, QCXDVN I - 1997) and the Guideline for quality of drinking water supply-sources from Vietnamese Ministry of Health (TCBYT 2002) were used for the discussion. The World Health Organization (WHO) Guideline was used for the comparison.

BOD and COD and pH value

The average pH-values of groundwater from Holocene aquifer lie within neutral or weakly alkaline range. The content of BOD and COD in all analyzed water samples exceed the Vietnamese permitted values for groundwater (Tab. 2).

Table 2. Concentration of selected element and compounds in water samples
of the Holocene aquifer in Nam Định area (mg/l)

	Elements
	BOD5	COD	pH	NO₂	NO₃	NH₄	Fe
Analyzed in 2002 (rainy and dry period)
Max	24.0	78.6	8.03	13.5	12.4	176.4	26.3
Min	2.6	5.1	6.55	0.01	<0.1	0.01	0.02
Mean	9.0	26.5	7.11	1.4	1.2	51.3	6.18
Analyzed in 2003 (rainy and dry period)
Max	36.0	78.4	7.85	7.0	9.1	76.0	31.02
Min	0.8	2.4	6.2	0.01	<0.1	0.03	0.02
Mean	13.6	30.1	6.8	2.7	1.6	13.5	6.83
TCVN 1995*	<4	10	6.5-8.0	0.1	10.0		5.0
TCBYT-1329- 2002**		2.0		3.0	50.0	1.5	0.50
WHO***				0.1	10.0		0.30

Nitrogen compounds

Nitrogen in groundwater is drawing increasingly attention, because of its serious impact on the environment and human health. Nitrogen can exist in different forms of various oxidation states, and it can readily change from one to another depending on the oxidation state. The principal forms of nitrogen are ammonia (NH³⁺), nitrite (NO^2-) and nitrate (NO^3-).

Ammonia is very soluble in water and is extremely toxic for the aquatic ecosystem, especially for fishes, at concentration ranging from 0.53 to 22.8 mg/l. Its toxicity increases by decreasing of pH value and temperature [1]. It is also an oxygen-consuming compound, so that dissolved oxygen in water can easily be depleted.

Nitrate itself is not toxic, but if the conversion to nitrite in the environment is available, it becomes dangerous. Nitrate is a typical pollutant introduced from sewage waste water and from livestock farms.

Of the three, nitrite is the most toxic to animals and human. In the body, nitrite can cause many serious diseases, such as shortness of breath, diarrhea etc. [1, 8,10].

Ammonia

The discussion of ammonia content in water is based on the guidelines of Vietnamese Health Ministry TCBYT 2002, in which it is prescribed that the ammonia concentration in groundwater used as water source for human use must not exceeded the level 1.5 mg/l and the Vietnamese guideline for groundwater TCVN 1995.

As mentioned, ammonia is a toxic gas. Many authors have demonstrated that farmyards, septic tanks, cesspools and livestock are potential sources of this gas. In the Nam Định area it could be considered that the disintegration of organic substance existing in aquifer is another sources of ammonia in groundwater.

By comparing the content of NH₄ in water to the acceptable value (TCBYT 2002), the contamination level by ammonia and other nitrogenous compounds in groundwater of the study area is estimated. The result is shown in Table 2 and Figure 3.

The ammonia concentration in analyzed samples (2002) ranges from 0.01 to 176.35 mg/l, and during the rainy season it ranges from 0.01 to 120.8 mg/l. The water samples analyzed in 2003 show an ammonia concentration from 0.03 to 75.95 mg/l, during the rainy season from 0.03 to 64.7 mg/l; in the dry season, the concentration ranges from 0.11 to 75.95 mg/l. Among 106 analyzed samples, 75% (2002) and 65% (2003) of the samples exceed the TCBYT 2002 guideline. The generally tendency of higher ammonia content in water samples taken during the dry season in comparing to this one during the rainy season has previous estimating study on groundwater of Red River Delta, too [7]. This tendency is considered to be controlled mostly by the geochemical behaviour of NH₄, decreasing of content from reduction condition (dry season) to the oxidation condition (rainy season).

In Figure 3 it is clear that the shallow groundwater of Holocene aquifer in Nam Định arrea is strongly polluted by ammonia. The pollution level ranges from 10 to more than 30 times higher than the permitted value.

It could also be the result from an ammonia pollution in groundwater of the Red River Delta, however, in much lower level [7]. This is a noteworthy sign of the decline of groundwater quality in this area. The shallow water quality concerning ammonia is zoned and shown in Figure 4 after QCXDVN 1997. It could be zoned into areas: an area with an ammonia concentration of > 3.0 mg/l (not permitted as water sources for domestic use) in the north and an area with an ammonia concentration of < 3.0 mg/l in the south (permitted as water sources for domestic use). Only in a small area of about 130 km² in the southern part of Hải Hậu and Nghĩa Hưng districts, the water has an ammonia content of < 1.0 mg/l..

Nitrite

The permitted value for nitrite concentration in water for human use is <3 mg/l after the guideline TCBYT 2002.

The nitrite concentration in analyzed samples (2002) ranges from 0.01 to 13.5 mg/l; during the dry season it ranges from 0.01 to 12.1 mg/l; but a concentration of over 3 mg/l NO₂ is estimated only in some water samples. The mean nitrite concentration for the total area is 1.23 mg/l. The water samples analyzed in 2003 show a nitrite concentration from 0.001 to 7.0 mg/l; during rainy season it ranges from 0.01 to 7.0 mg/l; during the dry season it ranges from 0.001 to 4.0 mg/l. And a nitrite content of over 3 mg/l is determined only in some locations. A nitrite contamination in water of Holocene aquifer after TCBYT 2002 is not observed (Fig. 3). However, it must be emphasized that among 106 analyzed water samples about 45% have a nitrite content exceeding the WHO’s guideline of >0.1 mg/l.

The water samples with Fe concentration of > 5.0 mg/l occur in almost all areas of Nam Định, with about 1200 km². Only samples from a small area of 250 km² in Nghĩa Hưng and a small arc in the NW region have an iron concentration of < 5.0 mg/l. The highest iron concentration is estimated in Giao Thuỷ (26.5 mg/l) and Nam Trực (25.0 mg/l).

The sediments of Holocene aquifer are composed of sand, silty clay and clay containing fauna and plant remains, so that a reduction condition could be considered here. Stumm and Morgan [11] suggested that the Fe concentration in pore water could be controlled by the upward diffusion of irons out of the reduced zone, leading to reprecipitation as Fe (III) at or near the water/sediment interface. These facts have demonstrated the high Fe content in the groundwater of Holocene aquifer in Nam Định.

Generally in Việt Nam, even an Fe concentration of from 0.5 to 5.0 mg/l is considered to be acceptable as water supply for domestic use. However, it needs a pretreatment of iron, if its concentration is higher than 0.5 mg/l.

In the fact that the occurrence of > 5.0mg/l is very common in the area, so that with respect to the iron concentration, the shallow groundwater would be unacceptable for public use, aquaculture and even for some industrial branches in almost all locations.

It is clear that iron contaminant in the shallow groundwater in Nam Định area is a serious problem for a safe drinking water supply.

V. CONCLUSIONS

The groundwater of the Holocene aquifer in Nam Định is widely contaminated by COD and BOD.

The high iron and ammonia content in this shallow groundwater source is a serious problem for a safe water supply for domestic use.

After TCBYT 2002 the nitrite content is not a dangerous contamination. However, the fact that nearly a half of the analyzed samples showing a nitrite concentration exceeding the WHO’s guideline is a real reason for finding a solution in the near future in order to avoid the implicit dangers for the human use of this “unsafe” water. Following measures could be suggested:

1. The abundance of available surface water, if managed appropriately, would obviate the need for utilization of this shallow groundwater in the first place. Installing an infrastructure of a region-wide basis for treatment and delivery of surface water is therefore a major undertaking in recent time and in the near future.

However it is very important not to outright reject the shallow groundwater concerning its high iron and nitrogen content as a fresh-water resource. It is possible that this kind of water resources could be used effectively by a number of potential industries.

2. It is important to have a fully understanding for this problem and to develop a cohesive strategy about ‘safe’ water supply in regional and local basis. Implementing a decentralized water management system in Nam Định area is therefore essential for a sustainable water supply in the region.

3. It needs furthermore to emphasize, the raising public awareness on the water pollution and related health problems is largely a social issue.

ACKNOWLEDGEMENT

The authors express their thanks to the National Fundamental Research Program MOST and the Department of Science and Technology of Nam Định Province for the financial and logistical support.

Ing. Dao Dinh Thuan, Ing. Do Van Binh, Ing. Pham Khanh Huy and MSc. Pham Thai Nam are greatly appreciated for ther helpful works.

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