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ffydrological Sciences—Journal—des Sciences Hydrologiques, 43(5) October 1998 701

Impact of clayey soils on nitrate pollution in the groundwater of the lower Vanisadhara River basin, India

N. SRINIVASA RAO Department of Geography, Andhra University, Visakhapatnam 530003, India

Abstract The main contributors of high nitrate concentrations in the groundwater of the Vanisadhara River basin are the point sources associated with livestock barns while the seasonal variations in nitrate (N03) concentrations are attributed to the prevailing hydrogeological conditions. The observed increase in nitrate under rising water level conditions, contradicting an expected decrease due to dilution by rainwater, is believed to be caused by the contribution of N03 from the clayey soil. The processes of sorption, retention and slow denitrification in clay are probably responsible for the contribution of nitrate to the groundwater in the post-monsoon season. The positive linear relationship between chloride and nitrate is in agreement with the similarities in behaviour of these ions in the clayey formations.

Influence des sols argileux sur la pollution par les nitrates des eaux souterraines de l'aval du bassin de la Rivière Vanisadhara (Inde) Résumé La principale origine des importantes concentrations de nitrates observées dans les eaux souterraines du bassin de la Rivière Vamsadhara est constituée de rejets ponctuels provenant d'élevages tandis que les variations saisonnières des concentrations en nitrates peuvent être attribuées au contexte hydrogéologique. Nous pensons que l'accroissement anormal des teneurs en nitrates associé à l'élévation du niveau des eaux peut être attribué à un apport de nitrates provenant des sols argileux. Les processus d'adsorption, de rétention et de dénitrification lente dans les argiles sont probablement à l'origine d'un apport de nitrates dans les eaux souterraines dans la période suivant la mousson. La corrélation linéaire positive entre chlorures et nitrates est cohérente avec le comportement de ces ions dans les formations argileuses.

INTRODUCTION

In recent years, groundwater has become the major source of water supply for the domestic, industrial and irrigation sectors of many countries. Therefore, water quality and its management strategies have become increasingly important in the developing nations for the past two decades. The water quality management mainly involves the identification and analysis of the contaminants, identification of their sources and the possible implementation of remedial measures. Nitrate (N03) contamination of the groundwater, due to the intensive use of fertilizers, has become a serious ecological problem in many rural areas of India and in many developing nations worldwide.

It is well known that serious and occasionally fatal poisonings in infants have occurred following ingestion of well waters which contain more than 45 mg l"1 N03.

Open for discussion until 1 April 1999

702 N. Srinivasa Rao

In USA and Europe, approximately 2000 cases of methaemoglobinaemia were reported during the period 1945-1960 and about 7-8% of the infants died (Handa, 1989). Elevated levels of methaemoglobin concentrations were found in Russia, in children who drank water containing as much as 182 mg l"1 N03 (Handa, 1989). In Colombia and Italy, high levels of nitrate in well waters were associated with an increased risk of gastric cancer (Cuello et al., 1976; Gilli et al., 1984). In a cross-sectional study in an area with a high incidence of gastric cancer in northeastern China, an association was observed between high levels of nitrate in drinking water supplies and neoplastic changes in the stomach (Xu et al., 1992). In addition to these toxic effects, high nitrate concentrations are undesirable in the fermenting and dyeing industries (Lunkad, 1994).

In this context, the lower Vamsadhara River basin in India, representing a typical rural location, has been selected for a systematic study to establish the baseline characteristics of the groundwater and the sources of contamination. The study has considerable significance as 80% of the drinking water needs of the area are met from the groundwater resources.

A number of workers from India and abroad have reported the presence of high concentrations of nitrate in groundwaters (Malik & Banerji, 1981; Handa, 1983; Sankaranarayana et al, 1989; Sehgal et al, 1989; Bulusu & Pande, 1990; Mehta étal, 1990; Kondratas & Mikalauskas, 1973; Klimas & Paukstys, 1993; Hamilton & Shedlock, 1992; Kolpin et al., 1994) and identified their probable sources. However, the behaviour of nitrate in different hydrogeological conditions and its seasonal response in a river basin have not been well documented. A critical evaluation of the existing status suggests the need for a systematic investigation of the occurrence and behaviour of nitrate to design effective mitigation and management techniques. In view of this background, a detailed study of nitrate in groundwaters of the lower Vamsadhara River basin enclosed between 83°50'-84°10'E longitude and 18°15'-18°38'N latitude was taken up during post-monsoon (November 1992) and pre-monsoon (April 1993) seasons.

The Vamsadhara River basin is a medium size, narrow elongated mature basin with its basin order reaching "six" according to Strahler ordering (Chow, 1964). The study area (Fig. 1), spreading over 817 km2, is mostly covered by recent alluvium. The alluvial cover is underlain by rocks of varied petrological characteristics. Garnetiferous granite gneiss is the most abundant rock type (Padmanabhayya, 1958; Suryanarayana, 1957) while pegmatites, granite gneiss and khondalites (Prasadarao, personal communication) follow the sequence. The alluvium thickness varies from a few metres to about 30 m. In a year, the river can be seen with full capacity for only a few days up to a month. During the rainy season, most of the area surrounding the river and some other relatively plain regions are waterlogged. The rest of the days, only a small amount of water flow can be seen in the river along one of its banks. Though the quantity of flow is less, it is perennial. Eighty percent of the domestic and agricultural needs of the catchment area are met from groundwater. The rich alluvial soils have facilitated agricultural activity. Apart from paddy crops, commercial crops like jowar, black gram, green gram, sugar cane, groundnut and gingelly are grown in the region. In recent years indiscriminate use of fertilizers like

Impact of clayey soils on nitrate pollution in the groundwater 703

Fig. 1 Map showing the well locations in the study area.

superphosphate, urea, NPK 15 15 15 and potash is prevalent in the area. The average annual rainfall is 910 mm. Temperatures range from 18°C in winter to 40°C in summer.

METHODS

One hundred and twenty-five villages were selected for the present study (Fig. 1). From each village, one groundwater sample was collected from a selected dug well


during the post-monsoon period (November 1992). Based on the distribution pattern of the nitrate in the groundwater during the post-monsoon period (November 1992), only 60 groundwater samples were analysed for nitrate during the pre-monsoon period (April 1993). The wells chosen for the study are common to both the periods. All wells, except a very few, have parapet walls. The groundwater from each well was sampled at 0.5 m below the water table. The preservation of the samples was done according to the methods suggested by APHA (1985). The Ultraviolet Spectrophotometric Screening method (APHA, 1985) was used to determine the nitrate content. Other major ionic concentrations, viz. chloride, sulphate, bicarbonate, calcium, magnesium, sodium, potassium, fluoride, iron, phosphate were also determined in the laboratory using standard methods (APHA, 1985), apart from the in situ measurements of electrical conductivity, temperature, pH and dissolved oxygen. The water level conditions and their seasonal variations were recorded. The water level data was reduced to mean sea level.

RESULTS

Water level conditions

The groundwater in the area occurs mostly under semi-confined and unconfined conditions. A considerable variation was observed in water levels between pre- and post-monsoon periods. The water level varied from 1.8 to 12.3 m below the ground level (Table 1) during the pre-monsoon period (April) while it rose to 0.30-4.8 m during the post-monsoon period (November). The distribution pattern of water levels, which is related to the general topography, was observed to be similar in the pre- and post-monsoon periods (Fig. 2(a) and (b)). The hydrographs for some typical wells are shown in Fig. 3. The water level fluctuation in each well was calculated and the statistics of these fluctuations are presented in Table 2. The hydraulic gradient computed from the maps (Fig. 2(a) and (b)) was observed to vary between 0.34 and 3.4 m km4.

Table 1 Water levels during pre- and post-monsoon periods.

Range of water levels (mb.g.l.)

1-2 2-4 4-6 6-8 8-10 10-14 Minimum Maximum Average Median

Percentage of wells Pre-monsoon

6.4 42.7 31.2 15.0 2.9 1.8 1.80 mb.g.l.

12.30 mb.g.l. 5.45 mb.g.l. 5.13

Post-monsoon

73.3 23.5 3.2

---

0.21 mb.g.l. 4.85 mb.g.l. 1.65 m b.g.l. 1.35

b.g.l.: below ground level.


706 A'. Srinivasa Rao

Fig. 3 Hydrographs of some typical wells in the area.

Distribution of nitrate

The nitrate concentrations in groundwater were observed to vary from 2 to 146 mg T1 during the pre-monsoon period (April) and 2-324 mg T1 during the post-monsoon period (November). The frequency distribution of nitrate during the post-monsoon period (Fig. 4) indicates that more than 75% of the groundwater


Table 2 Fluctuations in water levels between pre- and post-monsoon periods.

Fluctuation (m) Percentage of wells

<2 2-4 4-6 6-8 8-11 Minimum Maximum Average Median

13.9 39.8 34.4 10.8 1.1 0.31m

10.27 m 3.93 m 3.88

samples contain nitrate concentrations within the safe limit of 45 mg l"1 prescribed by the World Health Organisation (WHO, 1984). To understand the overall quality of groundwater, the statistics of other chemical constituents in the groundwater are presented in Table 3 along with their drinking water standards. The distribution of nitrate during the post-monsoon period, as shown in Fig. 5, is represented by 104 samples. The distribution diagram for nitrate concentrations during the pre-monsoon period (April) is not presented as there are only 60 samples and they do not represent the entire area. The nitrate concentrations in these 60 wells were used to study the seasonal variations between the pre- and post-monsoon periods since the samples were collected from the same wells during both the periods.

Table 3 Statistics of chemical constituents in the groundwater.

Constituent

Calcium

Magnesium

Sodium

Potassium

Chloride

Sulphate

Fluoride

Iron

Phosphate

Season

Pre Post Pre Post Pre Post Pre Post Pre Post Pre Post Pre Post Pre

Pre

Range (mg 1-')

11-196 10-159 29-183 46-298 14-1245 12-870 1-455 1-486 10-2170 10-1487 BDL-625 BDL-483 BDL-3.4 BDL-3.2 BDL-7.0

0.7-7.0

Average (mg I"1)

70 61 64

105 163 157 41 42

269 247

69 57 0.54 0.47 0.85

3.17

Median

56 44 60

107 96

116 14 10

130 157 35 38 0.40 0.35 0.48

3.16

Tolerance limit (mg I"1)

75-200 (ICMR) 50-100 (ICMR) 200 (WHO) 12 (EC) 250 (WHO) 400 (WHO) 1.5 (WHO) 0.3 (WHO) 0.54 (WHO)

No. of wells < tolerance limit

123 114 112 48 89 88 54 56 86 81

112 110 111 113

17

Nil

> tolerance limit

Nil 2

11 69 27 24 60 61 37 36 11 7 7 4

95

113

WHO: World Health Organisation (WHO, 1984); ICMR: Indian Council for Medical Research (ICMR, 1975); EC: European Community Directives (Smeats & Amavis, 1981). Pre: pre-monsoon period; Post: post-monsoon period.

708 TV. Srinivasa Rao

50

10 ~i-n < 10 11-20 21-30 31-45 46-100 101-250 251-324

N03 (mg/l)

Fig. 4 Frequency distribution of nitrate for the post-monsoon period (November).

DISCUSSION

The distribution pattern of nitrate (Fig. 5) does not seem to have any relationship with water level conditions or the physiography. An analysis of the agricultural practices of this area indicates that nitrate fertilizers as well as farmyard manures are applied more or less uniformly in the entire region. Walter et al. (1975) reported that the nitrogenous fertilizers that are applied are converted into mobile nitrates by natural processes irrespective of their form and composition. Many investigators (Malik & Banerji, 1981; Handa, 1983; Sankaranarayana et al, 1989; Sehgal et al., 1989; Bulusu & Pande, 1990; Mehta et al., 1990; Kondratas & Mikalauskas, 1973; Klimas & Paukstys, 1993; Hamilton & Shedlock, 1992; Kolpin et al., 1994) have reported that the contribution of nitrate from the fertilizer to the groundwater can vary from as little as 3 mg l"1 to as much as 1800 mg l1.

Generally the farmers in this area used to apply some 150 kg of fertilizer (mostly nitrogenous) per acre per year. The groundwater in most of the area, as shown in Figs 4 and 5, contains less than 20 mg l"1 of nitrate. This indicates that the pollution due to the application of fertilizers is within the safe limit. The practice of fertilizer application is quite recent in most of the area. This may be one of the reasons for such low concentrations of nitrate in the groundwater of the majority of the locations.

However, the high concentrations of nitrate, reaching a maximum of 324 mg l"1, are associated with the cattle barns where a large number of animals, mostly buffaloes and cows, are maintained in relatively small areas. These places can be treated as point sources as the nitrate content in the groundwater is 10-45 times more than that in the surrounding villages. The observed point sources are shown in Fig. 5 as PI, P2, ..., PI3. The sizes of the cattle bams in these places are about 200-500 m2. The stretch of nitrate pollution surrounding the point sources could not be traced out as there are no wells nearby. Hence a small circle is drawn in Fig. 5 at each point source location. The size of the circle is not related to its pollution spread. However, the point sources at P6 and Pll are not shown as circles because two to three adjacent cattle barns were observed there. In addition to the cattle barn at P9, there is a weekly cattle market at about 0.5 km from P9. This market spreads over an


Legend for Nitrate (NOa) concentrations

< 20 mg/l 21-45 mg/l 46-100 mg/l 101-200 mg/l 201-300 mg/l 301-324 mg/1

Fig. 5 Distribution of nitrate in the groundwater of lower Vamsadhara River basin (November).

extent of 500 m2 in open land. Though nitrogenous fertilizers are causing nitrate pollution in the groundwater,

the point sources are the dominant/major contributors of nitrate in this region. Further, the nitrate pollution from animal excrement and urine can be observed daily at the point sources, while the nitrate from fertilizers is only seasonal. Hence the accumulation of nitrate at the point sources over several years has caused the deterioration of groundwater quality more severely compared to the nitrate pollution from fertilizers.

In general a reduction in ionic concentrations will be observed in groundwater during the post-monsoon period (November) in regions where the recharge is mainly due to rainfall. The rainwater creates dilution in ionic concentrations due to a rise in the water level. However, the nitrate in the groundwater exhibits two distinctly


different behaviours in the study area: Situation I: the nitrate concentration is reduced with a rise in water level; and Situation II: the nitrate concentration increases with a rise in water level.

It is expected that the prevailing hydrogeological conditions may be responsible for such behaviours of nitrate in the groundwater. To understand the hydrogeological conditions, the lithological information was collected from the local groundwater department and geo-electrical surveys were carried out at the selected sites. From the lithological information and the interpretation of geo-electrical soundings, it was observed that two major hydrogeological conditions are prevalent in this area (Fig. 6). In situation I the surface layer is composed of sandy soil with very little silt and clay contents resting over a thin layer of clayey soil and a weathered/fractured rock, while in situation II the clay content in the surface layer is more dominant than silt and sand. The thickness of this layer varies from 1.0 to 4.8 m. The water level below ground level rises between 0.8 and 4.1 m in the post-monsoon period (November) while it falls to between 5.1 and 8.0 m during the pre-monsoon period (April). This essentially creates a situation wherein the groundwater mostly fluctuates within the clayey zone in situation II and within the sandy zone in situation I during the post-monsoon period.

In the regions covered by a surface clay layer, the water level drops down into the sand horizon during pre-monsoon. Though the variation of nitrate with respect to water level fluctuation in different hydrogeological situations cannot be projected along specific profiles due to their point source characteristics, the variations of nitrate and the associated water levels in selected wells in two different hydro-geological conditions (situations I and II) are shown in Fig. 7(a) and (b) to give a clear idea of this phenomenon. The point sources are observed incidentally in clayey plain regions only. Hence the distinction between nitrates due to point and non-point sources could not be presented for Fig. 7(a). There may be a chance of tracing point

Ground surface

Water Level

SANDY SOIL

Water Level

CLAYEY SOIL

WEATHERED/FRACTURED ROCK FORMATIONS

Fo rma t i on

C layey

S a n d y

W e a t h e r e d rock

F rac tu red rock

ç- Post-monsoon -^ (November)

<r Pre-monsoon -> (April)

Ground surface

Water Level

CLAYEY SOIL

Water Level

SANDYSOIL

WEATHERED/FRACTURED ROCK FORMATIONS

Electr ical res is t i v i t y Th i ckness (Q m) (m)

2 . 3 - 7 . 2 1 . 0 - 1 6 . 8

1 0 . 1 - 2 5 . 2 1 . 0 - 1 8 . 6

2 8 . 0 - 4 6 . 1

5 9 . 0 - 1 1 1 . 0

—

—

Fig. 6 Typical hydrogeological sections of the study area and their geo-electrical parameters.


sources in sandy regions also, if all the villages, other than those chosen for the present study, are also monitored.

As animal excrement and urine also contain chloride, an attempt has been made to establish a relationship between nitrate and chloride. The samples collected from point source regions (the bold circles as shown in Fig. 8) are followed by a straight line

chloride = 2.257(nitrate) + 95 0.95

This line is represented by only 12 points close to it. To show the relationship between nitrate and chloride in the groundwater of non-point source regions also, the samples are plotted as small dots on the same figure. No significant relationships between nitrate and other chemical constituents in the groundwater are observed.

The decrease in nitrate concentrations with rising water levels in situation I (Fig. 7(a)) can be attributed to the dilution effects of the groundwater recharge by rainfall. Contrary to this, in situation II (Fig. 7 (b)), the clay layer is responsible for

E

O 20

219 237 238

2 0 0 -

e 150 E.

O" 100-

z 50-

POINT SOURCE NON POINT SOURCE-

HUE I—i rm_ 210 216 227

WELL NUMBER

241

N03 during Posî-monsoon Q N03 during Pre-monsoon

I Wat. level during Post-monsoon | Wat. level during Pre-monsoon

Fig. 7 Nitrate concentrations and groundwater levels in the selected wells of (a) situation I, and (b) situation II.


700

6 0 0 -

"& 5 0 0 -

g 400-Ë 3 300 X o

200-

100

CHLORIDE = (2.257) NITRATE + 95 (Based on 12 bold markers close to the line)

Sample collected from

• Point Source region

• Non-point Source region

50 100 250 150 200

NITRATE (mg/l)

Fig. 8 Relationship between nitrate and chloride in the groundwater.

300 350

the increase in nitrate concentration under raised water level conditions. The perennial pollution due to animals in situation II and the mechanism influencing the nitrate concentrations in the clayey soils are responsible for the high nitrate concentration during the post-monsoon period (November).

The main mechanism influencing high nitrate concentrations in clayey soils may be that ammonium (released from animal excrement and urine) is sorbed on clay during dry periods and this sorbed ammonium creates and releases nitrate into the groundwater when it comes into contact with groundwater. Hence, high nitrate concentrations are observed during the post-monsoon period. In sandy soils, the ammonium volatilizes to a great extent (no sorption) during dry periods. When the soils become saturated, relatively little nitrate is generated. However the other possible mechanisms which may be responsible for high nitrate concentrations in clayey soils, are discussed in the following: (a) Hem (1986) attributed the retention of chloride in clayey soils to its large ionic

size. The author feels that having a relatively large ionic size compared to chloride, nitrate is more likely to be retained in clayey soils.

(b) The denitrification process will be slower in clayey soils (Klimas & Paukstys, 1993) than in the sandy formations. Klimas & Paukstys (1993) reported higher nitrate concentrations in the groundwater of clayey plain areas than sandy plain areas. They suggested that the slow denitrification process in clayey soils was responsible for such high nitrate concentrations. However, studies on the vertical distribution of organic matter and manganese (which were not determined in the present study) are needed to understand the status of the denitrification process in the study area.

(c) The high concentrations of nitrate in the clayey formation and the reduction of nitrate in the following sandy formation (i.e. when the water level drops down from the clayey formation to the sandy formation) may be explained by the filtration effect of the clay. The clay acts as an ideal membrane electrode. When


a three-phase system of two solutions of electrolytes is separated by a clay membrane, the effect can be described by the equation of a perfect electrode (Degens & Chillingar, 1967):

E = (RT/F) ln(0/C")

in which E is the electrochemical potential, R the gas constant, T the temperature, F the Faraday equivalent and C and C" the concentrations of ions in the respective phases. Hence it follows that the salts can be retained in clay beds so that the concentrations in pore solutions can rise. The retention of ions depends on the large excessive charge that is bound to the clay membrane which hinders the passage of ions bearing the same charge, positive or negative. The separation is therefore brought about through the electrical properties rather than the size of the ions. Water molecules can pass through the membrane layer so that the filtrate shows a lower salt content than that of the original solution (Degens & Chillingar, 1967; Engelhardt, 1960). Further, Matthess (1982) reported that filtration by clay membranes must be considered when explanations of raised salt content in groundwater are sought.

CONCLUSIONS

The nitrate pollution due to the recent practice of fertilizer application in the study area is within the safe limit (45 mg l"1). The nitrate content in the groundwater in most of the areas is less than 20 mgi"1. In a situation where the groundwater level mostly fluctuates within the sandy soil, the post-monsoon (November) nitrate concentrations in the groundwater are less than the pre-monsoon (April) concentrations. The low concentration of nitrate during post-monsoon is due to the dilution effect of groundwater recharge by rainfall. In the other situation, where the groundwater level fluctuates between the top clayey formation and the following sandy formation, higher nitrate concentrations in the groundwater were observed during the post-monsoon than the pre-monsoon period. The possible mechanisms which may influence high nitrate concentrations in clayey soils are: (a) sorption of ammonium (released from animal excrement and urine) on clayey soils during dry periods and the release of nitrate into the water when the soil comes into contact with the water; (b) retention of nitrate in clayey soils by the membrane effect of clay or due to its large ionic size; and (c) the slower denitrification process in clayey soils than in other soils. The high nitrate concentrations reaching a maximum of 324 mg l"1 are believed to be due to the point-sources of nitrogen associated with 13 isolated livestock barns. These barns are incidentally located in clayey plain regions only. The accumulation of perennial pollution at these barns over years along with one or more of the possible mechanisms occurring in clayey soils is responsible for such high nitrate concentrations in the groundwater.

Acknowledgements The author is grateful to Dr B. K. Handa and Prof. M. S. Prasada Rao for critical evaluation of the paper. He is also grateful to Mrs D. Suryakumari, Mr D. V. Ramana Murthy, Mrs N. Lalitha and Mr N. Ravi


Kumar for their kind hospitality and encouragement during the field work period. The financial support received from the Department of Science and Technology, New Delhi is gratefully acknowledged. The useful comments of anonymous reviewers are appreciated.

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Received 3 June 1997; accepted 11 January 1998

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