Agriculture and Environment, 3 (1976)57--67 57 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

N I T R A T E P O L L U T I O N O F G R O U N D W A T E R F R O M N I T R O G E N F E R T I L I Z E R S A N D A N I M A L W A S T E S IN T H E P U N J A B , I N D I A

BIJAY SINGH and G.S. SEKHON

Department of Soils, Punjab Agricultural University, Ludhiana (India)

(Received 20 April 1976)

ABSTRACT

Singh, B. and Sekhon, G.S., 1976. Nitrate pollution of groundwater from nitrogen fertilizers and animal wastes in the Punjab, India. Agric. Environm., 3: 57--67.

Fifty-seven water samples were obtained from wells situated on cultivated farms and near village settlements in the Ludhiana and Hoshiarpur districts of Punjab (India). Water samples collected in June and September, 1975 were analysed for nitrate. Probability plots indicated that 90% of the well-water samples may contain less than 45 mg NO~/l, the upper safe limit. The nitrate content of well water near villages was significantly higher than in the cultivated area. Animal wastes appear to be the major contributor in the village environ- ment. The nitrate concentration of well water decreased significantly with depth to water table, but correlated positively with amount of nitrogenous fertilizers added per unit area per year. Soil profile samples taken in June, 1975 from farms in the immediate vicinity of the sampled wells were analysed for nitrate and water content. The amount of NO~ con- tained in the soil profile down to 2.10 m depth correlated significantly with the nitrate concentration of well water in September, thus confirming that nitrates tend to reach the water table during the rainy season (July--September). The projected geometric mean nitrate concentration of ground water (computed on the expectation that the nitrate content of the saturated zone would equal that of the 1.50--2.10 m soil layer) was found to be 35.6 mg NO~/1, many times higher than the observed mean of 1,88 mg NO~/1.

INTRODUCTION

The a p p r e h e n s i o n s t h a t fe r t i l i zer n i t r o g e n m a y be a poss ib le c o n t a m i n a n t of g r o u n d w a t e r have inc reased in the pas t few years . Inc rease in t he n i t r a t e c o n c e n t r a t i o n o f g r o u n d w a t e r has been o f spec ia l c o n c e r n because o f the s u s p e c t e d h a r m t h a t t he NO~ ion can do t o ch i ld ren if its c o n c e n t r a t i o n in d r i n k i n g w a t e r exceeds 45 p p m . S o m e s tud ies on c o n t a m i n a t i o n o f g round- w a t e r w i th n i t r a t e o r ig ina t ing f r o m fer t i l izers have shown l i t t l e ev idence fo r such an o c c u r r e n c e (Smi th , 1967; Hed l in , 1971; Olsen e t al., 1973} whi le o the r s have i n d i c a t e d an increase in the NO~ c o n t e n t in g r o u n d w a t e r due t o fe r t i l i ze r a p p l i c a t i o n ( S t o u t and Burau , 1967 ; Walker , 1969}.

In t he Pun jab s t a t e of Ind ia , whe re agr icu l tu ra l p r o d u c t i o n and fe r t i l i ze r a p p l i c a t i o n ra tes are t he h ighes t in t he c o u n t r y , soils are in t ens ive ly i r r iga ted and are genera l ly l ight in t e x t u r e ( l o a m y sand t o s a n d y loam) . In some o f

58

these soils, a considerable amount of NO3 has been found to leach below the root zone of most crops (Singh and Sekhon, 1976a, b). The present investiga- tion was undertaken to s tudy the effect of N-fertilization on nitrate concen- tration in soil and groundwater under cultivated areas of the Ludhiana district where N-application rates are the highest in the Punjab.

METHODS

Study area and sites

The study area was selected to represent a wide range of fertilizer applica- tion rates used in the vicinity of wells. Fifty-one wells were located in the central part of the Ludhiana district (30031 ' to 31001 ' North and 75025 ' to 76027 ' East), covering most of the Delhon, Pakhowal and some parts of the Ludhiana and Sudhar blocks. The study area was in the upland plain of the district, at a mean elevation of about 250 meters. I t was situated entirely on alluvium deposits which consist of silt and clay with occasional beds of gravel and cemented sands (Suri, 1970). The soils were predominantly light in texture, with sandy loam and loamy sand as the dominant textural classes. Average rainfall during the periods July--September, October--March and April--June was about 47, 12 and 7 cm, respectively. Of the total cropped area in the district (which was 83% of the total area), 75% was irrigated by tubewells or canal (Bajwa and Randhawa, 1974).

Wheat (Triticum aestivum ), maize (Zea mays), groundnut (Arachis hypogaea), cotton (Gossypium hirsutum and Gossypium arboreum ) and sugarcane (Saccharum officinarum) were the major crops grown with a cropping intensity of more than 150%. According to an estimate (Bajwa and Randhawa, 1974) the average consumption of N and P2Os in the district was 157 kg/ha in 1972-- 1973, with an N and P2Os ratio of 1 : 0.5.

Another six wells were examined in the Hoshiarpur district (30 ° 58' to 32o05 ' North and 75028 ' to 76038 ' East) where the rate of fertilizer applica- tion is extremely low (consumption of N and P2Os in 1972--1973 was 54 kg/ ha). Rainfall was more than 75 cm, about 25 cm more than in Ludhiana.

A total of 46 wells were located on the farms (cultivated area) and 11 in the villages. Water drawn from the village wells is regularly used for drinking.

Sampling and analysis o f soil profiles and well water

Samples of water from all 57 wells were collected during the last week of June, 1975. Depth to water table was measured. The past fertilization history of the farms situated in the immediate vicinity of the wells was recorded from conversations with the farmers concerned. Samples of soil from a field closest to each of the 46 wells were taken by a screw-type auger to a depth of 210 cm. From every soil profile 9 samples were collected; the first four at 15-cm inter- vals and the last five at 30-cm intervals.

59

During the last week of September, 1975, when the monsoon season was just over, water samples from wells located in the Ludhiana district were again collected. After transportation to the laboratory, the samples were immediately refrigerated to avoid loss of nitrate. The samples were analysed for nitrate and electrical conductivity within 2--3 days of their removal from the wells. Nitrate was estimated by the phenoldisulphonic acid method as described by Bremner (1965). Electrical conductivity was determined on a Wheatstone bridge conductivity meter.

Soil samples were weighed immediately after they were brought to the laboratory, and dried at 60°C in the oven. Dried samples were again weighed and processed for nitrate determination. Nitrate was extracted by shaking soil samples with water using a soil : water ratio of I : 5. Water extracts were analysed for nitrate by using the method described for the water samples.

Statistical analysis

Probability analysis was applied to data pertaining to electrical conductivity and nitrate content of well water in the cultivated area. Nitrate content of different soil layers in the profiles sampled in the vicinity of the wells was treated in a similar manner to obtain frequency of occurrence and cumulative frequency distribution curves (probability curves).

RESULTS AND DISCUSSION

Electrical conductivity and nitrate content o f well water

Fig.1 shows frequency distribution of nitrate content in samples of well water collected in the months of June and September, 1975 from the cul- tivated area. The curve A shows a plot of the class interval values versus normal probability for sampling in June, and line B indicates the same relation- ship with logarithmic transformation to the class interval values. Lack of normal distribution with untransformed sample population has been reported by Davis (1966). Le Clerg et al. (1962) suggested the transformation of popula- tion to another ,scale to normalize the distribution. The distribution of points around a straight line indicates the close fit of the sample population to a log-normal distribution or square root-normai distribution. The intersection of these lines with 50% frequency is an approximation of the geometric mean, and the slope is a measure of the dispersion of the sample population.

The use of arithmetic mean and standard deviation of the mean in describing a population assumes that the variate is normally distributed. This assumption is not true for data pertaining to the electrical conductivity and nitrate content of groundwater in the study area. Arithmetic means are greater than the geometric means, as shown in Table I which summarizes various statistical parameters. This is because of the positive skewness of the data. Thus arith- metic means, even in combination with standard deviation, fail to evaluate at

60

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Fig. 1. Probabi l i ty plots o f nitrate con ten t o f wel l water from cult ivated area and projected ground water nitrate concentrat ion . X - - normal probabi l i ty p lo t for nitrate concentra t ion in June; • - - log-normal probabi l i ty p lo t for nitrate concentra t ion in June; ~ - - log-normal probabi l i ty p lo t for nitrate concentra t ion in September; o - - log-normal probabi l i ty p lot for projected nitrate concentrat ion .

the extremes of the distribution. A change in the standard deviation of arith- metic mean does not suggest the point in the spectrum of values where the change occurred (Nightingale, 1972).

Fig. 1 indicates the probability of the occurrence of a given nitrate concen- tration in well water under the study area. The scatter of points around line B, which combines the data of both June and September samplings, indicates that no change occurred during this period in the distribution of nitrate according to its concentration in well water under the cultivated area. This is also confirmed by comparing arithmetic and geometric means (Table I) at two sampling dates. The line B indicates that 90% of the ground water samples can be expected to contain less than 45 mg NO~/1, the upper safe limit.

In comparison to that in the cultivated area, the nitrate content of well water near villages is significantly higher (Table I). The magnitudes of both geometric and arithmetic means are higher in September (just after the rainy season) than in June. Animal wastes appear to be the major contributor to high nitrate levels in the groundwater under villages. Although farmyard manure is being added along with chemical fertilizers in the cultivated area, crop plants absorb most of the applied nutrients from both of these sources, leaving only a negligible amount which could be leached beyond the potential rooting zone.

Data pertaining to electrical conductivity of groundwater (Fig.2 and Table I) in the cultivated area and the village area show trends similar to those for nitrate. However, while the nitrate content was markedly higher in ground-

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water near villages, the mean electrical conductivity of the groundwater was only slightly higher than under cultivation. Both arithmetic and geometric means increase slightly in the villages in September (just after the monsoon season), but no significant increase is observed in the cultivated area. These observations indicate that groundwater in the s tudy area is recharged during the rainy season and that salts are being leached to the water table. These data also confirm indirectly the role of animal wastes in increasing the nitrate concentrat ion of well water near villages.

Correlation o f nitrate concentration o f well water with fertilizer use

As expected, the nitrate content of well water in both June and September correlated significantly with depth to water table. Correlation coefficients we re - -0 .291" and - -0 .414" for June and September, respectively. The higher value for September indicates increased leaching of nitrate from the soil profile during the rainy season.

The correlation coefficient between the amount of fertilizer-N applied per unit area per year and nitrate concentrat ion of well water in September was 0.505*. Multiple correlation between well water nitrate concentrat ion and the amount of fertilizer-N applied (X1 in kg N/acre/year) in the vicinity of a well and depth to water table (X2 in meters) was 0.529**. The corresponding multiple regression equation is:

Y, = 7.11 + 0.081X1 -- 1.72X2

The multiple correlation coefficient and multiple regression of nitrate con- centration of well water in June, 1975 on X1 and X2 were:

I12 = 20.86 + 0.0039X1 -- 2.17)(2, R = 0.391"

From the data on nitrate concentration in dry soil at various depths in the

63

vicinity of wells, the amount of NO~ per unit area to a depth of 2.10 m was computed for each of the 46 sampling sites. When soil sampling was done in June, the correlation coefficient between nitrate concentration of well water and nitrate content in the soil was 0.0768. The most likely explanation here is that a wheat crop was raised in the study area during November--April and there was little rainfall during that period. From June to September a total of 50 cm of rain was recorded and a large part of the NO~ present in the soil profile probably leached down to the grouridwater. Such intensive leaching of NO~ after excessive precipitation has been observed in Illinois by Harmeson et al. (1971). The hypothesis is also verified by a multiple correlation coefficient of 0.448* between nitrate concentration of well water in September (Y3, mg NO~/1) on the one hand and amount of NO~ in soil profiles (X1, kg NO~/ha) in June and depth to water table (X2, meters) on the other. The corresponding regression equation is:

Y3 = 26.74 + 0.014X1 -- 2.66X2

Correlation between various soil and water chemical parameters thus indicates a marked influence of fertilizers and rainy season on the enrichment of groundwater with nitrate.

Nitrate in soil profiles adjacent to sampled wells

The nitrate concentrations observed in dry soil (mg NO~/I) at different depths are expressed as cumulative percentage frequency distribution curves in Fig.3 and 4. Both these parameters follow square root-normal distribution. The data are presented for soil depths of 0--0.30 m, 0.30--1.50 m and 1.50-- 2.10 m, since it was evident from a preliminary examination of the data for soil water content and soil nitrate concentration that these can be combined into three intervals wi thout distorting the trends of the parameters with depth. The lines shown in Fig.3 and 4 can be interpreted in terms of probability of occurrence of a particular concentration of NO~ at various depths. For example, one can expect 35, 82 and 91% of soil samples to con- tain less than 25 mg NO~/kg of dry soil at depths of 0--0.30 m, 0.30--1.50 m and 1.50--2.10 m, respectively. A summary of various statistical parameters for nitrate concentration in soil and soil solution is given in Table II. The arithmetic mean is greater than the geometric mean, indicating positive skew- ness in the data. Both the means decrease significantly with depth.

The geometric mean of soil water content increased with depth. Values for three soil layers are 8.23, 11.40 and 13.40 g H20/100 g soil. In order to calculate projected groundwater nitrate concentration, a most probable soil water content for soil at saturation in the 1.50--2.10 m layer was determined. Out of 46 soil samples taken at this depth, 27 were coarse textured, 13 were medium textured and 6 were fine textured. Average saturation percentages for these categories were 20.8 -+ 1.2, 24.2 + 0.8 and 28.0 + 1.1 with a most probable mean value of 22.7%.

64

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From the foregoing, it is reasonable to state that variability in nitrate con- centration in the upper region of the groundwater body should correspond to the variability in soil nitrate content. Using the assumptions that (a) the soil nitrate concentration distribution curve (Fig.3) reasonably describes the variability in the soil nitrate concentration at 1.50--2.10 m, (b) a reason- able estimate o f the average water content required to saturate the soil is 22.7% for the particle size distribution at this depth, and (c) below 2.10 m, any change in the nitrate content of the soil is independent of microbial activity, one can project that the groundwater would have a geometric mean nitrate concentration of 35.6 mg NO~/1 and a probable distribution as shown in Fig. l , line C. The projected geometric mean of groundwater nitrate con-

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66

centration is far greater than the geometric mean of 1.88 mg NO~/1 observed in the study area's well water in June or September, 1975. There may be several reasons for this difference. Root activity may extend beyond a depth of 2.10 m so that nitrate in the soil solution up to and beyond this depth is susceptible to absorption by crop plants (Stewart et al., 1968). Nitrate leached beyond 2.10 m may indeed be lost through denitrification. Since depth to water table in the wells ra~ged from 4 to 10 m, most of the nitrate that leached beyond 2.10 m may still be in the unsaturated zone and has not yet reached the water table. If this is the case, then there is a significant potential for groundwater pollution from present day fertilization practices. In other words, if nitrate concentration of well water now is not in equilibrium with present day agriculture, a mean (geometric) nitrate concentration as high as 35 ppm in groundwater can be expected in the coming years. These hypotheses, however, can only be confirmed by further investigations. A large-scale soil sampling programme should be initiated to determine the depth- concentration profiles for nitrate down to the water table so that questions concerning safe levels of nitrogen fertilization can be resolved.

ACKNOWLEDGEMENTS

The first author thanks the Indian Council of Agricultural Research, New Delhi, for financial help in the form of a Senior Fellowship.

REFERENCES

Bajwa, M.S. and Randhawa, N.S., 1974. Soil fertility and fertilizer use in Ludhiana district. Punjab Agricultural University, Ludhiana, 59 pp.

Bremner, J.M., 1965. Inorganic forms of nitrogen. In: C.A. Black (Editor), Methods of Soil Analysis 2. Agronomy, 9: 1179--1237.

Davis, G.H., 1966. Frequency distribution of dissolved solids in ground water. Ground Water, 4: 5--12.

Harmeson, R.H., Sollo, F.W. and Larson, T.E., 1971. The nitrate situation in Illinois. J. Am. Water Works Assoc., 63: 303--310.

Hedlin, R.A., 1971. Nitrate contamination of ground water in the Neepawa-Langruth area of Manitoba. Can. J. Soil Sci., 51: 75--84.

Le Clerg, E.L., Leonard, W.H. and Clark, A.G., 1962. Field Plot Technique. Burgess Publishing Co., Minneapolis, Minnesota, 373 pp.

Nightingale, H.I., 1972. Nitrates in soil and ground water beneath irrigated and fertilized crops. Soil Sci., 114: 300--311.

Olsen, R.A., Edwin, C.S. and Muir, J., 1973. Influence of agricultural practices on water quality in Nebraska: a survey of streams, ground water and precipitation. Water Resour. Bull., 9: 301--311.

Singh, B. and Sekhon, G.S., 1976a. Some measures of reducing leaching loss of nitrates beyond potential rooting zone. I. Proper coordination of nitrogen splitting with water management. Plant Soil, 44: ' 193--200.

Singh, B. and Sekhon, G.S., 1976b. Some measures of reducing leaching loss of nitrates beyond potential rooting zone. II. Balanced fertilization. Plant Soil, 44: 391--395.

Smith, G.E., 1967. Fertilizer nutrients as contaminants in water supplies. In: N.C. Brady (Editor), Agriculture and the Quality of our Environment. Am. Assoc. Adv. Sci., Washington, D.C., pp. 173--186.

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Stewart, B.A., Viets, F.G. and Hutchinson, G.L., 1968. Agriculture's effect on nitrate pollution of ground water. J. Soil Water Conserv., 23: 13--15.

Stout, P.R. and Burau, R.G., 1967. The extent and significance of fertilizer building in soils as revealed by vertical distribution of nitrogenous matter between soils and underlying water reservoirs. In: N.C. Brady (Editor), Agriculture and the Quality of our Environment. Am. Assoc. Adv. Sci., Washington, D.C., pp. 283--310.

Suri, V.S., 1970. Punjab District Gaze t tee rs - - Ludhiana. Controller of Printing and Stationery, Government of Punjab, Chandigarh, 694 pp.

Walker, W.H., 1969. Illinois ground water pollution. J. Am. Water Works Assoc., 61: 31--40.