Environ Monit Assess (2015) 187: 50 DOI 10.1007/s10661-014-4211-x
Groundwater nitrate contamination and use of Cl/Br ratio for source appointment M. K. Samantara & R. K. Padhi & K. K. Satpathy & M. Sowmya & P. Kumaran
Received: 24 July 2014 / Accepted: 1 December 2014 / Published online: 1 February 2015 # Springer International Publishing Switzerland 2015
Abstract Source appointment for groundwater nitrate contamination is critical in prioritizing effective strategy for its mitigation. Here, we assessed the use of Cl/Br ratio and statistical correlation of hydro-chemical parameters to identify the nitrate source to the groundwater. A total of 228 samples from 19 domestic wells distributed throughout the study area were collected during June 2011–May 2012 and analyzed for various physicochemical parameters. Study area was divided into three spatial zones based on demographic features, viz., northern, southern, and central part. Nitrate concentration in 57 % of samples exceeded the prescribed safe limit for drinking stipulated by the World Health Organization (WHO) and Bureau of Indian standards (BIS). The central part of the study area showed elevated nitrate concentration ranging from below detection limit (BDL) to 263.5 mg/l as NO3− and demonstrated high attenuation within the immediate vicinity thereby restricting diffusion of the nitrate to the adjacent parts. Resolution of correlation matrix as statistical indicator for nitrate contamination was poor. Seventy-seven percent of samples with high nitrate concentration (>45 mg/ l as NO3−) showed strong association with high Cl/Br mass ratio (350–900), indicating mixing of sewage and septic tank effluents with groundwater as a primary source for the nitrate in the studied area. Nitrate level M. K. Samantara : R. K. Padhi : K. K. Satpathy (*) : M. Sowmya : P. Kumaran Environment & Safety Division, RSEG/EIRSG, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603 102, India e-mail: [email protected]
during monsoon (BDL, 229.9 mg/l as NO3−), postmonsoon (BDL, 263.5 mg/l as NO 3 − ), and premonsoon (0.5–223.1 mg/l as NO3−) indicated additional contribution of surface leaching to groundwater. Keywords Groundwater . Nitrate pollution . Cl/Br ratios . Parametric indicator . Source identification
Introduction Groundwater contamination with nitrate is a common problem in many parts of India and appears as a major threat to human health (Singh et al. 1995; Rengaraj et al. 1996; Mondal et al. 2008; Sankararamakrishnan et al. 2008; Kundu and Mandal 2009; Brindha et al. 2012). Although only about 5 % of ingested nitrate is converted to toxic nitrite in the digestive system by bacteria, but DNA damage due to the subsequent formation of Nnitrosamines and N-nitrosamides (Davidson et al. 2012) poses greater threat. Additionally, epidemiological studies particularly in rural agricultural areas, which use shallow groundwater for drinking, have revealed that high nitrate content leads to birth defects and cancers (Johnson et al. 2010). Some of the major sources of nitrate pollution are agricultural runoff, leakage from sewage networks, onsite sewage disposal like septic tanks and pit latrines, landfills, leaching from soil, animal waste dumping, and airborne nitrogen compounds given off by industries. Nitrate contamination of groundwater in a particular area depends on source availability and regional environmental factors. Areas
50 Page 2 of 13
with a high risk of groundwater contamination by nitrate generally have high nitrogen loading, high population density, well-drained soils, and less extensive woodland relative to cropland. In the regions of well-drained soils dominated by irrigated cropland, there is a strong propensity towards the development of large areas with groundwater nitrate content exceeding the maximum contaminant level of 45 mg/l as NO3− (Spalding and Exner 1993). Nitrate salts being easily soluble in water percolates into groundwater along with rainwater or irrigation water. Shallow groundwater with depth Cl− > NO3−/SO4−2 >PO4−3 for anions and Na+ >Ca+2 >Mg+2 > K+ for cations. Suitability of the groundwater was assessed based on the prescribed guidelines stipulated by BIS and WHO. Although most of the hydrochemical parameters of the studied groundwater were within the safe limit, observed nitrate concentrations for most of the wells were alarmingly higher than the prescribed safe limit of 45 mg/l by BIS; which is a major concern over its use as drinking water. The observed F− values were below the desired limit stipulated by BIS for drinking water. Due to relatively nontoxic nature and low environmental abundance of Br− in groundwater, its guideline value is not available. However, bromide was determined to correlate Cl/Br ratio with nitrate concentration to evaluate its use as parametric indicator for nitrate pollution in groundwater. Due to the proximity of the coast, the study area was impacted by seawaterderived NaCl, and thus, Cl− value in the studied
Environ Monit Assess (2015) 187: 50
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Table 1 Summary of groundwater hydro-chemical parameters for the studied wells Parameters
Pre-Monsoon
Monsoon
Post-Monsoon
Min
Max
Average
Min
Max
Average
Min
Max
Average
Temperature
28.7
32.4
30.3
26.6
32.6
29.6
27.3
32.7
29.9
pH
6.4
8.2
7.4
6.1
7.9
7.3
6.5
8.1
7.3
EC
0.12
2.41
1.15
0.13
2.51
1.19
0.13
2.30
1.13
Ca2+
5.0
147.4
69.6
8.0
143.2
69.1
4.8
145.4
70.9
Mg2+
0.1
124.5
26.6
1.9
58.1
27.5
0.0
83.3
24.8
Na+
15.6
298.0
130.3
1.6
256.4
119.8
1.2
429.0
135.0
K+
0.5
113.3
27.1
0.4
140.0
25.3
0.4
146.8
27.1
HCO3−
24.3
563.2
238.9
47.8
660.0
311.0
37.8
662.5
327.5
Cl−
8.5
361.0
138.3
13.4
433.6
148.3
5.9
413.2
150.1
Br
BDL
2.39
0.39
0.04
1.15
0.37
0.02
2.75
0.45
F−
BDL
0.70
0.08
BDL
0.56
0.10
BDL
0.61
0.11
NO3− (as NO3)
BDL
223.1
59.8
BDL
229.9
85.1
BDL
263.5
78.0
PO43−
BDL
8.1
3.2
BDL
7.0
3.1
BDL
7.4
3.1
SO42−
2.0
129.4
52.3
1.4
128.1
56.5
2.6
127.0
52.9
−
Unit of temperature is °C, EC is in mS/cm, and remaining ions are in mg/l BDL below detection limit
groundwater were relatively high with a strong correlation with Na+ (R2 =0.745). The presence of high chloride content in groundwater could also be attributed to chloride rich minerals or from sources like domestic effluents, fertilizers, effluents from septic tanks and pit latrines, open defecation, or from animal waste dumping (Suthar et al. 2009). The plot of sulfate with calcium plus magnesium (R2 =0.64) and chloride (R2 =0.882) showed a strong correlation (Figs. 2 and 3), which indicates that they found their way to groundwater through similar biogeochemical pathway.
Fig. 2 The relationship between sum of calcium and magnesium with sulfate
Nitrate in the groundwater Spatiotemporal variation Nitrate values in the studied area reflected extensive spatial variations. The distribution of nitrate was extremely random within this small study area. Significant variations in the nitrate concentration were also observed among very close wells, rendering the geographical distribution difficult to predict. In general, the central part of the study area consisting of two fisherman villages showed consistently high nitrate in most of the wells. This part of the study area is having high
Fig. 3 Correlation between chloride and sulfate concentration
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250
Nitrate (mg/l as NO3)
Fig. 4 Box diagram for comparison of nitrate level among the studied wells (box range: 25– 75 %; whisker range: 5–95 %, box mean)
Environ Monit Assess (2015) 187: 50
200
150
100
50
0
B IS, 2004 L im it
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19
W ell N u m ber
residential density, relatively low-moderate economic activity, not connected to public sewage system, and absence of waste water sanitation infrastructure. High nitrate level in the groundwater is generally associated with combination of above factors (Mattern et al. 2009). Yearly variation of nitrate in the studied wells is represented in Fig. 4. Its concentrations for wells W1–W4 and W18–19 located in the extreme north and south of the study area were below the drinking water guideline limit. Sampling wells W5–W17 showed nitrate concentration above the guideline limit. Out of these, 11 wells are present in the village area and two are present in the employee’s township close to Sadras backwater, which carries domestic waste from adjacent village. Highest concentration of nitrate was found in well W7 where nitrate value
Fig. 5 Contour diagram of nitrate level in the study area
varied from 135.5 to 263.5 (average 194.5) mg/l as NO3−. A previous study carried out by Mondal et al. (2010) reported nitrate concentration in the range of 10–286 mg/l as NO3− for this area, which is consistent with our observation. High nitrate contamination was strictly confined only to the middle part of the study area, and there was huge difference in its level with that of the nearest well of the adjacent part of the both sides having a mere 500-m distance apart (Fig. 5). Extremely heterogeneous spatial distribution of nitrate was indicative of the fact that immediate local sources played decisive role for its enrichment in the groundwater and also indicated that the contamination was not be old enough to get diffused into the well of nearby sampling area. Otherwise, nitrate attenuation might be associated with high level of
Environ Monit Assess (2015) 187: 50
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localized biogeochemical processes. Rivett et al. (2008) reviewed various biogeochemical controlling mechanisms responsible for nitrate attenuation in the groundwater. They have suggested that the nitrate reduction can be written as a half equation, which illustrates the role of electron (e−) transfer in the biogeochemical process (Tesoriero et al. 2000). 2 NO3 − þ 12 Hþ þ 10 e− → N2 þ 6H2 O
ð1Þ
The electron needed for the denitrification can originate from microbial oxidation of organic matter and is represented by Eq. 2 (Jørgensen et al. 2004). 5CH2 O þ 4 NO3 − →2N2 þ 4 HCO3 − þ CO2 þ 3H2 O
ð2Þ
The availability of reduced iron (Fe+2) and reduced sulfur (S−) can also provide the required electron while oxidized to their higher oxidation state by various biotic and abiotic process (Korom 1992). Although denitrification is more probable in confined aquifer, dissimilatory nitrate reduction to ammonium, assimilation of nitrate into microbial biomass, and nitrate removal via phreatophytes are some of the other processes responsible to a different degree for nitrate reduction in groundwater (Rivett et al. 2008). Seasonal trend of nitrate in the studied wells (Fig. 6) showed nitrate enrichment during monsoon period apparently due to nitrate in the overhead soil derived from various sources as discussed earlier and got percolated by rainwater enhancing its level. With the cessation of monsoon, nitrate percolation decreased and various biogeochemical attenuation mechanisms may be responsible for its subsequent reduction during the summer period. However, for wells W10 and W15, the monsoonal reduction may be
Fig. 6 Spatio-temporal variation of nitrate in studied wells
attributed to the dilution by direct rainwater in the absence of significant soil nitrogen source. Source appointment The shallow coastal aquifer of Kalpakkam is made up of sandy formation where the degree of percolation of contaminants is expected to be very high along with rainwater or irrigation water. Nitrate is soluble in water and can easily pass through soil to the groundwater table. This may be one of the reasons behind the high nitrate concentration in this region. It was noticed that, out of 228 samples collected throughout the year, 131 samples exceeded the drinking water nitrate level of 45 mg/l as NO3− stipulated by BIS, which was around 57 % of the total number of samples. The nitrate content was significantly the higher in samples collected during monsoon (BDL, 229.9; average, 85.1 mg/l as NO3−) and post-monsoon (BDL, 263.5; average, 78.0 mg/l as NO3−) as compared to pre-monsoon (BDL, 223.1; average, 59.8 mg/l as NO3−), possibly associated with leaching of applied as well as residual available nitrogen from soil to the groundwater due to monsoonal rains (Kundu and Mandal 2009). Anthropogenic wastes to subsurface soils increases the concentration of nitrate in groundwater. Leakage from onsite septic tanks, pit latrines and open defecation are the other major contributors of nitrate loading to the groundwater. In rural villages, onsite sanitation facilities are the major source of contaminants to groundwater. Use of pit latrines is a threat to the groundwater because waste is discharged without pre-treatment. It is reported that, each year, one person produces about 500 kg of urine as compared to 50 kg of feces. These feces contain about 10 kg of dry matter. Wolgast (1993) calculated and reported that one person produces approximately 5.7 kg of nitrogen, 0.6 kg of phosphorus, and 1.2 kg of potassium per year. Of the human excreta, urine contains about 90 % of the nitrogen, 50–65 % of the phosphorus, and 50–80 % of the potassium (Heinonen-Tanski and van WijkSijbesma 2005). According to Lewis et al. (1980), human excreta contain about 5 kg N year−1 per capita. Nitrate was found to be contaminated in the groundwater of middle portion of the study area where more than 80 % of the collected samples showed nitrate concentration above the safe limit throughout the year, which is largely (>90 %) inhabited by fisherman and devoid of offsite sanitation facility. Here, human excreta are released to onsite sanitation facilities (pit latrines and
50 Page 8 of 13
Fig. 7 Correlation of chloride with bromide in the groundwater samples
septic tanks), and most people prefer open defecation near the coast. Moreover, people of this area are highly dependent on the farm animals for both milk consumption and commercialization. Open surface disposal and storage of excreta generated from these household animals are major contributor for nitrate enrichment in groundwater. Animal excreta such as dung and urine produced by household animals like cows, buffaloes, and goats constitute a potential source of nitrate, phosphate, and sulfate to subsurface soils. They enter into shallow aquifer through direct discharge, storm water channels, recharge basin, or river. Somasundaram et al. (1993) reported the concentration of nitrate in Madras urban aquifer in the range of 51–382 mg/l as NO3− and attributed the high concentration to sewage generated from substantial number of farm animals available in the city. High nitrate concentration found in the subsurface water in the villages under the present study might be due to the combined effect of biological wastes released by humans and animals, supported by the geology of the soil of this area, which is sandy loam type extending up to 12 m
Fig. 8 Relationship of nitrate concentration with chloride/ bromide ratio
Environ Monit Assess (2015) 187: 50
deep having high percolation capacity. Highly localized nature of the nitrate contamination in the groundwater of one part of the studied sites and low background values in the adjacent part do not corroborate its origin to be from agricultural activities, which generally affect the nitrate level of the groundwater on a regional scale. Phosphate is an essential ingredient of commonly applied agricultural fertilizer along with nitrate. Observed low phosphate level in the entire study area also indicative of the fact that the study area was not impacted by agricultural pollution and agricultural activities could not be appointed as source for nitrate pollution in this area. According to Fetter (1999) and Hudak (1999), non-agricultural sources of nitrogen such as leakage from septic systems and municipal sewers significantly affect the groundwater quality locally though less significant regionally, which was also observed in our present study. Cl− to Br− ratio as a parametric indicator Groundwater samples showed a strong linear relationship of chloride with bromide for the wide range of observed chloride values (R2 =0.89, n=216), indicating their stable co-occurrence (Fig. 7). Samples pertaining to well W2 (n=12), exhibited significantly high Br− with Cl/Br ratio distinctly different than observed value for remaining samples. The elevated Br− in W2 might have originated from contamination source not associated with chloride and were not taken into account for chloride–bromide correlation. Because Cl- and Br− remain essentially constant from source to receptor, Cl/Br ratio is used as a parametric indicator for identifying various sources of anthropogenic and naturally occurring contaminants in the groundwater. Both ions move
Environ Monit Assess (2015) 187: 50
(a)
Pre-Monsoon
250
NO3 Cl/Br
700 600 500
150 400 300
100
Cl/Br Mass Ratio
Nitrate (mg/l as NO3)
200
200 50 100 0
0 W1
W3
W5
W7
W9
W11 W13 W15 W17 W19
--
Well Number
(b)
Monsoon
250
NO3 Cl/Br
700 600 500
150 400 100
300
Cl/Br Mass Ratio
Nitrate (mg/l as NO3)
200
200 50 100 0
0 W1
W3
W5
W7
W9
W11 W13 W15 W17 W19
--
Well Number
(c)
Post-Monsoon 250
NO3 Cl/Br
700 600
200
500 150 400 100
300 200
50 100 0
0 W1
W3
W5
W7
W9
W11 W13 W15 W17 W19
Well Number
--
Cl/Br Mass Ratio
Nitrate (mg/l as NO3)
Fig. 9 Comparative plot for wellwise variation in nitrate concentration with variation in Cl/ Br mass ratio: a pre-monsoon, b monsoon, and c post-monsoon
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50 Page 10 of 13
Environ Monit Assess (2015) 187: 50
Table 2 Pearson’s correlation matrix for groundwater parameters (n=228) Variables
pH
EC
Na+
K+
Ca+2
Mg+2
HCO3−
Cl−
pH
1
EC
0.006
1
Na+
0.050
0.805
1
K+
−0.172
0.695
0.504
1
Ca+2
−0.064
0.758
0.488
0.675
1
Mg
−0.004
0.551
0.455
0.248
0.413
1
HCO3−
−0.021
0.760
0.712
0.586
0.524
0.414
1
Cl−
−0.059
0.907
0.779
0.556
0.648
0.624
0.659
1
Br−
0.237
0.464
0.608
0.191
0.017
0.162
0.560
0.497
+2
−
Br−
F−
NO3−
PO4−3
1
0.314
−0.026
0.269
−0.258
−0.469
0.031
0.213
0.076
0.707
NO3−
−0.059
0.400
0.197
0.346
0.576
0.118
0.123
0.262
−0.170
−0.472
PO4−3
−0.117
0.394
0.179
0.571
0.619
0.067
0.329
0.228
−0.160
−0.521
0.344
1
SO4−2
−0.038
0.924
0.794
0.562
0.698
0.575
0.672
0.919
0.456
0.006
0.426
0.311
F
SO4−2
1 1 1
(Values in italics are significantly different from 0 with a significance level alpha=0.01)
conservatively in water and have different abundances in natural fluids and solids. As a result, the mass (or molar) ratio of chloride to bromide has been used as a technique to distinguish pristine groundwater from others such as wastewater sources and other anthropogenic and natural salinity sources. Several studies reported distinct ranges of Cl/Br ratios for various end members that control salinity processes in groundwater (Hudak 2003; Davis et al. 2004; Alcalá and Custodio 2008). Variations in Cl/Br ratios in rainfall and recharge water have been extensively studied and are close to or below Cl/Br ratio in seawater (288–292). Cl/Br ratios in wastewater such as effluent from pit latrines and septic tank tend to have larger range due to variation in source water, salt intake, and other anthropogenic factors. Ranges of Cl/Br ratio for various sources in groundwater is provided in detail by Katz et al. 2011. According to Thomas (2000), Cl/Br ratio above 400 were correlated with chemical constituents associated with human activities resulting in high nitrate concentration and higher number of volatile organic compounds and pesticides. In the study, correlation between Cl/Br ratio and nitrate concentration provided a fair resolution in appointing nitrate source. Results indicate that 77 % samples with Cl/Br ratio higher than 350 had nitrate level more than the BIS (2004) drinking water guideline limit (Fig. 8). Analogous seasonal trends were obtained between Cl/ Br with NO3− depicting similar factor controlling the fate of these ions (Fig. 9a–c). The primary source of
chloride and bromide in pristine groundwater is atmospheric deposition, which has Cl/Br ratio in the range of 50–100; near the coast, the Cl/Br ratio is closer to that for seawater, i.e., about 290 (Panno et al. 2006). Davis et al. (1998) reviewed Cl/Br ratio from studies conducted at several places worldwide. They concluded that, in general, chloride/bromide ratio ranged from 50 to 150 in atmospheric precipitation, 300–600 in domestic sewage, 1000–10,000 in dissolved evaporates, and 10–100 in urban runoff. Cl/Br ratio observed in the present groundwater study ranged from 50 to 1000, i.e., bromide enrichment exists for well W2, thereby observation of low Cl/Br ratio and for other wells mixing of high Cl/Br water was predominant, leading to enhanced Cl/Br ratio in the groundwater. NaCl is widely used in different anthropogenic applications, and thus, anthropogenic high Cl/Br signal is considered a universal phenomenon for the impact of domestic waste water. High nitrate level confined to the middle part of the study area corresponds to high Cl/Br ratio, consistent with that originates from domestic activities, which was predominant in this particular part of study as described earlier. High bromide content and hence low Cl/Br ratio may originate from decomposition of organic matter (Gerritse and George 1988). It also can be attributed to the impact of street runoff (Cl/Br ratio varying from 9 to 165; average, 34), which is enriched in bromide due to the addition of bromide from the oxidation of organobromines, viz., triethylbromide, ethylenedibromide used
Environ Monit Assess (2015) 187: 50
as additive with anti-knocking agent used in vehicular fuels (Flury and Papritz 1993). These sources are generally not self-associated with nitrate, and the groundwater is not expected to have high nitrate unless leaching of nitrate salts disposed on the surface. This process was distinctly observed and groundwater having low Cl/Br ratio in general were associated with comparatively low nitrate level. Correlation matrix as statistical indicator Correlation matrix of 13 groundwater parameters at 99 % confidence level is presented in Table 2. A significance level of p≤0.01 was considered as strongly correlated. Strong positive co-relation of nitrate with Na+ and Cl− and negative correlation with Br− is expected if domestic waste is assumed as nitrate source in the corresponding groundwater. Such correlations were unfounded in the present correlation matrix [NO3− and Na+ (r = 0.197, p = 0.013), NO3− and Cl− (r = 0.262, p = 0.001), and NO3− and Br− (r=−0.170, p=0.034)]. Instead, nitrate showed fair correlation with phosphate (r= 0.344, p=
Groundwater nitrate contamination and use of Cl/Br ratio for source appointment M. K. Samantara & R. K. Padhi & K. K. Satpathy & M. Sowmya & P. Kumaran
Received: 24 July 2014 / Accepted: 1 December 2014 / Published online: 1 February 2015 # Springer International Publishing Switzerland 2015
Abstract Source appointment for groundwater nitrate contamination is critical in prioritizing effective strategy for its mitigation. Here, we assessed the use of Cl/Br ratio and statistical correlation of hydro-chemical parameters to identify the nitrate source to the groundwater. A total of 228 samples from 19 domestic wells distributed throughout the study area were collected during June 2011–May 2012 and analyzed for various physicochemical parameters. Study area was divided into three spatial zones based on demographic features, viz., northern, southern, and central part. Nitrate concentration in 57 % of samples exceeded the prescribed safe limit for drinking stipulated by the World Health Organization (WHO) and Bureau of Indian standards (BIS). The central part of the study area showed elevated nitrate concentration ranging from below detection limit (BDL) to 263.5 mg/l as NO3− and demonstrated high attenuation within the immediate vicinity thereby restricting diffusion of the nitrate to the adjacent parts. Resolution of correlation matrix as statistical indicator for nitrate contamination was poor. Seventy-seven percent of samples with high nitrate concentration (>45 mg/ l as NO3−) showed strong association with high Cl/Br mass ratio (350–900), indicating mixing of sewage and septic tank effluents with groundwater as a primary source for the nitrate in the studied area. Nitrate level M. K. Samantara : R. K. Padhi : K. K. Satpathy (*) : M. Sowmya : P. Kumaran Environment & Safety Division, RSEG/EIRSG, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603 102, India e-mail: [email protected]
during monsoon (BDL, 229.9 mg/l as NO3−), postmonsoon (BDL, 263.5 mg/l as NO 3 − ), and premonsoon (0.5–223.1 mg/l as NO3−) indicated additional contribution of surface leaching to groundwater. Keywords Groundwater . Nitrate pollution . Cl/Br ratios . Parametric indicator . Source identification
Introduction Groundwater contamination with nitrate is a common problem in many parts of India and appears as a major threat to human health (Singh et al. 1995; Rengaraj et al. 1996; Mondal et al. 2008; Sankararamakrishnan et al. 2008; Kundu and Mandal 2009; Brindha et al. 2012). Although only about 5 % of ingested nitrate is converted to toxic nitrite in the digestive system by bacteria, but DNA damage due to the subsequent formation of Nnitrosamines and N-nitrosamides (Davidson et al. 2012) poses greater threat. Additionally, epidemiological studies particularly in rural agricultural areas, which use shallow groundwater for drinking, have revealed that high nitrate content leads to birth defects and cancers (Johnson et al. 2010). Some of the major sources of nitrate pollution are agricultural runoff, leakage from sewage networks, onsite sewage disposal like septic tanks and pit latrines, landfills, leaching from soil, animal waste dumping, and airborne nitrogen compounds given off by industries. Nitrate contamination of groundwater in a particular area depends on source availability and regional environmental factors. Areas
50 Page 2 of 13
with a high risk of groundwater contamination by nitrate generally have high nitrogen loading, high population density, well-drained soils, and less extensive woodland relative to cropland. In the regions of well-drained soils dominated by irrigated cropland, there is a strong propensity towards the development of large areas with groundwater nitrate content exceeding the maximum contaminant level of 45 mg/l as NO3− (Spalding and Exner 1993). Nitrate salts being easily soluble in water percolates into groundwater along with rainwater or irrigation water. Shallow groundwater with depth Cl− > NO3−/SO4−2 >PO4−3 for anions and Na+ >Ca+2 >Mg+2 > K+ for cations. Suitability of the groundwater was assessed based on the prescribed guidelines stipulated by BIS and WHO. Although most of the hydrochemical parameters of the studied groundwater were within the safe limit, observed nitrate concentrations for most of the wells were alarmingly higher than the prescribed safe limit of 45 mg/l by BIS; which is a major concern over its use as drinking water. The observed F− values were below the desired limit stipulated by BIS for drinking water. Due to relatively nontoxic nature and low environmental abundance of Br− in groundwater, its guideline value is not available. However, bromide was determined to correlate Cl/Br ratio with nitrate concentration to evaluate its use as parametric indicator for nitrate pollution in groundwater. Due to the proximity of the coast, the study area was impacted by seawaterderived NaCl, and thus, Cl− value in the studied
Environ Monit Assess (2015) 187: 50
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Table 1 Summary of groundwater hydro-chemical parameters for the studied wells Parameters
Pre-Monsoon
Monsoon
Post-Monsoon
Min
Max
Average
Min
Max
Average
Min
Max
Average
Temperature
28.7
32.4
30.3
26.6
32.6
29.6
27.3
32.7
29.9
pH
6.4
8.2
7.4
6.1
7.9
7.3
6.5
8.1
7.3
EC
0.12
2.41
1.15
0.13
2.51
1.19
0.13
2.30
1.13
Ca2+
5.0
147.4
69.6
8.0
143.2
69.1
4.8
145.4
70.9
Mg2+
0.1
124.5
26.6
1.9
58.1
27.5
0.0
83.3
24.8
Na+
15.6
298.0
130.3
1.6
256.4
119.8
1.2
429.0
135.0
K+
0.5
113.3
27.1
0.4
140.0
25.3
0.4
146.8
27.1
HCO3−
24.3
563.2
238.9
47.8
660.0
311.0
37.8
662.5
327.5
Cl−
8.5
361.0
138.3
13.4
433.6
148.3
5.9
413.2
150.1
Br
BDL
2.39
0.39
0.04
1.15
0.37
0.02
2.75
0.45
F−
BDL
0.70
0.08
BDL
0.56
0.10
BDL
0.61
0.11
NO3− (as NO3)
BDL
223.1
59.8
BDL
229.9
85.1
BDL
263.5
78.0
PO43−
BDL
8.1
3.2
BDL
7.0
3.1
BDL
7.4
3.1
SO42−
2.0
129.4
52.3
1.4
128.1
56.5
2.6
127.0
52.9
−
Unit of temperature is °C, EC is in mS/cm, and remaining ions are in mg/l BDL below detection limit
groundwater were relatively high with a strong correlation with Na+ (R2 =0.745). The presence of high chloride content in groundwater could also be attributed to chloride rich minerals or from sources like domestic effluents, fertilizers, effluents from septic tanks and pit latrines, open defecation, or from animal waste dumping (Suthar et al. 2009). The plot of sulfate with calcium plus magnesium (R2 =0.64) and chloride (R2 =0.882) showed a strong correlation (Figs. 2 and 3), which indicates that they found their way to groundwater through similar biogeochemical pathway.
Fig. 2 The relationship between sum of calcium and magnesium with sulfate
Nitrate in the groundwater Spatiotemporal variation Nitrate values in the studied area reflected extensive spatial variations. The distribution of nitrate was extremely random within this small study area. Significant variations in the nitrate concentration were also observed among very close wells, rendering the geographical distribution difficult to predict. In general, the central part of the study area consisting of two fisherman villages showed consistently high nitrate in most of the wells. This part of the study area is having high
Fig. 3 Correlation between chloride and sulfate concentration
50 Page 6 of 13 300
250
Nitrate (mg/l as NO3)
Fig. 4 Box diagram for comparison of nitrate level among the studied wells (box range: 25– 75 %; whisker range: 5–95 %, box mean)
Environ Monit Assess (2015) 187: 50
200
150
100
50
0
B IS, 2004 L im it
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19
W ell N u m ber
residential density, relatively low-moderate economic activity, not connected to public sewage system, and absence of waste water sanitation infrastructure. High nitrate level in the groundwater is generally associated with combination of above factors (Mattern et al. 2009). Yearly variation of nitrate in the studied wells is represented in Fig. 4. Its concentrations for wells W1–W4 and W18–19 located in the extreme north and south of the study area were below the drinking water guideline limit. Sampling wells W5–W17 showed nitrate concentration above the guideline limit. Out of these, 11 wells are present in the village area and two are present in the employee’s township close to Sadras backwater, which carries domestic waste from adjacent village. Highest concentration of nitrate was found in well W7 where nitrate value
Fig. 5 Contour diagram of nitrate level in the study area
varied from 135.5 to 263.5 (average 194.5) mg/l as NO3−. A previous study carried out by Mondal et al. (2010) reported nitrate concentration in the range of 10–286 mg/l as NO3− for this area, which is consistent with our observation. High nitrate contamination was strictly confined only to the middle part of the study area, and there was huge difference in its level with that of the nearest well of the adjacent part of the both sides having a mere 500-m distance apart (Fig. 5). Extremely heterogeneous spatial distribution of nitrate was indicative of the fact that immediate local sources played decisive role for its enrichment in the groundwater and also indicated that the contamination was not be old enough to get diffused into the well of nearby sampling area. Otherwise, nitrate attenuation might be associated with high level of
Environ Monit Assess (2015) 187: 50
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localized biogeochemical processes. Rivett et al. (2008) reviewed various biogeochemical controlling mechanisms responsible for nitrate attenuation in the groundwater. They have suggested that the nitrate reduction can be written as a half equation, which illustrates the role of electron (e−) transfer in the biogeochemical process (Tesoriero et al. 2000). 2 NO3 − þ 12 Hþ þ 10 e− → N2 þ 6H2 O
ð1Þ
The electron needed for the denitrification can originate from microbial oxidation of organic matter and is represented by Eq. 2 (Jørgensen et al. 2004). 5CH2 O þ 4 NO3 − →2N2 þ 4 HCO3 − þ CO2 þ 3H2 O
ð2Þ
The availability of reduced iron (Fe+2) and reduced sulfur (S−) can also provide the required electron while oxidized to their higher oxidation state by various biotic and abiotic process (Korom 1992). Although denitrification is more probable in confined aquifer, dissimilatory nitrate reduction to ammonium, assimilation of nitrate into microbial biomass, and nitrate removal via phreatophytes are some of the other processes responsible to a different degree for nitrate reduction in groundwater (Rivett et al. 2008). Seasonal trend of nitrate in the studied wells (Fig. 6) showed nitrate enrichment during monsoon period apparently due to nitrate in the overhead soil derived from various sources as discussed earlier and got percolated by rainwater enhancing its level. With the cessation of monsoon, nitrate percolation decreased and various biogeochemical attenuation mechanisms may be responsible for its subsequent reduction during the summer period. However, for wells W10 and W15, the monsoonal reduction may be
Fig. 6 Spatio-temporal variation of nitrate in studied wells
attributed to the dilution by direct rainwater in the absence of significant soil nitrogen source. Source appointment The shallow coastal aquifer of Kalpakkam is made up of sandy formation where the degree of percolation of contaminants is expected to be very high along with rainwater or irrigation water. Nitrate is soluble in water and can easily pass through soil to the groundwater table. This may be one of the reasons behind the high nitrate concentration in this region. It was noticed that, out of 228 samples collected throughout the year, 131 samples exceeded the drinking water nitrate level of 45 mg/l as NO3− stipulated by BIS, which was around 57 % of the total number of samples. The nitrate content was significantly the higher in samples collected during monsoon (BDL, 229.9; average, 85.1 mg/l as NO3−) and post-monsoon (BDL, 263.5; average, 78.0 mg/l as NO3−) as compared to pre-monsoon (BDL, 223.1; average, 59.8 mg/l as NO3−), possibly associated with leaching of applied as well as residual available nitrogen from soil to the groundwater due to monsoonal rains (Kundu and Mandal 2009). Anthropogenic wastes to subsurface soils increases the concentration of nitrate in groundwater. Leakage from onsite septic tanks, pit latrines and open defecation are the other major contributors of nitrate loading to the groundwater. In rural villages, onsite sanitation facilities are the major source of contaminants to groundwater. Use of pit latrines is a threat to the groundwater because waste is discharged without pre-treatment. It is reported that, each year, one person produces about 500 kg of urine as compared to 50 kg of feces. These feces contain about 10 kg of dry matter. Wolgast (1993) calculated and reported that one person produces approximately 5.7 kg of nitrogen, 0.6 kg of phosphorus, and 1.2 kg of potassium per year. Of the human excreta, urine contains about 90 % of the nitrogen, 50–65 % of the phosphorus, and 50–80 % of the potassium (Heinonen-Tanski and van WijkSijbesma 2005). According to Lewis et al. (1980), human excreta contain about 5 kg N year−1 per capita. Nitrate was found to be contaminated in the groundwater of middle portion of the study area where more than 80 % of the collected samples showed nitrate concentration above the safe limit throughout the year, which is largely (>90 %) inhabited by fisherman and devoid of offsite sanitation facility. Here, human excreta are released to onsite sanitation facilities (pit latrines and
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Fig. 7 Correlation of chloride with bromide in the groundwater samples
septic tanks), and most people prefer open defecation near the coast. Moreover, people of this area are highly dependent on the farm animals for both milk consumption and commercialization. Open surface disposal and storage of excreta generated from these household animals are major contributor for nitrate enrichment in groundwater. Animal excreta such as dung and urine produced by household animals like cows, buffaloes, and goats constitute a potential source of nitrate, phosphate, and sulfate to subsurface soils. They enter into shallow aquifer through direct discharge, storm water channels, recharge basin, or river. Somasundaram et al. (1993) reported the concentration of nitrate in Madras urban aquifer in the range of 51–382 mg/l as NO3− and attributed the high concentration to sewage generated from substantial number of farm animals available in the city. High nitrate concentration found in the subsurface water in the villages under the present study might be due to the combined effect of biological wastes released by humans and animals, supported by the geology of the soil of this area, which is sandy loam type extending up to 12 m
Fig. 8 Relationship of nitrate concentration with chloride/ bromide ratio
Environ Monit Assess (2015) 187: 50
deep having high percolation capacity. Highly localized nature of the nitrate contamination in the groundwater of one part of the studied sites and low background values in the adjacent part do not corroborate its origin to be from agricultural activities, which generally affect the nitrate level of the groundwater on a regional scale. Phosphate is an essential ingredient of commonly applied agricultural fertilizer along with nitrate. Observed low phosphate level in the entire study area also indicative of the fact that the study area was not impacted by agricultural pollution and agricultural activities could not be appointed as source for nitrate pollution in this area. According to Fetter (1999) and Hudak (1999), non-agricultural sources of nitrogen such as leakage from septic systems and municipal sewers significantly affect the groundwater quality locally though less significant regionally, which was also observed in our present study. Cl− to Br− ratio as a parametric indicator Groundwater samples showed a strong linear relationship of chloride with bromide for the wide range of observed chloride values (R2 =0.89, n=216), indicating their stable co-occurrence (Fig. 7). Samples pertaining to well W2 (n=12), exhibited significantly high Br− with Cl/Br ratio distinctly different than observed value for remaining samples. The elevated Br− in W2 might have originated from contamination source not associated with chloride and were not taken into account for chloride–bromide correlation. Because Cl- and Br− remain essentially constant from source to receptor, Cl/Br ratio is used as a parametric indicator for identifying various sources of anthropogenic and naturally occurring contaminants in the groundwater. Both ions move
Environ Monit Assess (2015) 187: 50
(a)
Pre-Monsoon
250
NO3 Cl/Br
700 600 500
150 400 300
100
Cl/Br Mass Ratio
Nitrate (mg/l as NO3)
200
200 50 100 0
0 W1
W3
W5
W7
W9
W11 W13 W15 W17 W19
--
Well Number
(b)
Monsoon
250
NO3 Cl/Br
700 600 500
150 400 100
300
Cl/Br Mass Ratio
Nitrate (mg/l as NO3)
200
200 50 100 0
0 W1
W3
W5
W7
W9
W11 W13 W15 W17 W19
--
Well Number
(c)
Post-Monsoon 250
NO3 Cl/Br
700 600
200
500 150 400 100
300 200
50 100 0
0 W1
W3
W5
W7
W9
W11 W13 W15 W17 W19
Well Number
--
Cl/Br Mass Ratio
Nitrate (mg/l as NO3)
Fig. 9 Comparative plot for wellwise variation in nitrate concentration with variation in Cl/ Br mass ratio: a pre-monsoon, b monsoon, and c post-monsoon
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50 Page 10 of 13
Environ Monit Assess (2015) 187: 50
Table 2 Pearson’s correlation matrix for groundwater parameters (n=228) Variables
pH
EC
Na+
K+
Ca+2
Mg+2
HCO3−
Cl−
pH
1
EC
0.006
1
Na+
0.050
0.805
1
K+
−0.172
0.695
0.504
1
Ca+2
−0.064
0.758
0.488
0.675
1
Mg
−0.004
0.551
0.455
0.248
0.413
1
HCO3−
−0.021
0.760
0.712
0.586
0.524
0.414
1
Cl−
−0.059
0.907
0.779
0.556
0.648
0.624
0.659
1
Br−
0.237
0.464
0.608
0.191
0.017
0.162
0.560
0.497
+2
−
Br−
F−
NO3−
PO4−3
1
0.314
−0.026
0.269
−0.258
−0.469
0.031
0.213
0.076
0.707
NO3−
−0.059
0.400
0.197
0.346
0.576
0.118
0.123
0.262
−0.170
−0.472
PO4−3
−0.117
0.394
0.179
0.571
0.619
0.067
0.329
0.228
−0.160
−0.521
0.344
1
SO4−2
−0.038
0.924
0.794
0.562
0.698
0.575
0.672
0.919
0.456
0.006
0.426
0.311
F
SO4−2
1 1 1
(Values in italics are significantly different from 0 with a significance level alpha=0.01)
conservatively in water and have different abundances in natural fluids and solids. As a result, the mass (or molar) ratio of chloride to bromide has been used as a technique to distinguish pristine groundwater from others such as wastewater sources and other anthropogenic and natural salinity sources. Several studies reported distinct ranges of Cl/Br ratios for various end members that control salinity processes in groundwater (Hudak 2003; Davis et al. 2004; Alcalá and Custodio 2008). Variations in Cl/Br ratios in rainfall and recharge water have been extensively studied and are close to or below Cl/Br ratio in seawater (288–292). Cl/Br ratios in wastewater such as effluent from pit latrines and septic tank tend to have larger range due to variation in source water, salt intake, and other anthropogenic factors. Ranges of Cl/Br ratio for various sources in groundwater is provided in detail by Katz et al. 2011. According to Thomas (2000), Cl/Br ratio above 400 were correlated with chemical constituents associated with human activities resulting in high nitrate concentration and higher number of volatile organic compounds and pesticides. In the study, correlation between Cl/Br ratio and nitrate concentration provided a fair resolution in appointing nitrate source. Results indicate that 77 % samples with Cl/Br ratio higher than 350 had nitrate level more than the BIS (2004) drinking water guideline limit (Fig. 8). Analogous seasonal trends were obtained between Cl/ Br with NO3− depicting similar factor controlling the fate of these ions (Fig. 9a–c). The primary source of
chloride and bromide in pristine groundwater is atmospheric deposition, which has Cl/Br ratio in the range of 50–100; near the coast, the Cl/Br ratio is closer to that for seawater, i.e., about 290 (Panno et al. 2006). Davis et al. (1998) reviewed Cl/Br ratio from studies conducted at several places worldwide. They concluded that, in general, chloride/bromide ratio ranged from 50 to 150 in atmospheric precipitation, 300–600 in domestic sewage, 1000–10,000 in dissolved evaporates, and 10–100 in urban runoff. Cl/Br ratio observed in the present groundwater study ranged from 50 to 1000, i.e., bromide enrichment exists for well W2, thereby observation of low Cl/Br ratio and for other wells mixing of high Cl/Br water was predominant, leading to enhanced Cl/Br ratio in the groundwater. NaCl is widely used in different anthropogenic applications, and thus, anthropogenic high Cl/Br signal is considered a universal phenomenon for the impact of domestic waste water. High nitrate level confined to the middle part of the study area corresponds to high Cl/Br ratio, consistent with that originates from domestic activities, which was predominant in this particular part of study as described earlier. High bromide content and hence low Cl/Br ratio may originate from decomposition of organic matter (Gerritse and George 1988). It also can be attributed to the impact of street runoff (Cl/Br ratio varying from 9 to 165; average, 34), which is enriched in bromide due to the addition of bromide from the oxidation of organobromines, viz., triethylbromide, ethylenedibromide used
Environ Monit Assess (2015) 187: 50
as additive with anti-knocking agent used in vehicular fuels (Flury and Papritz 1993). These sources are generally not self-associated with nitrate, and the groundwater is not expected to have high nitrate unless leaching of nitrate salts disposed on the surface. This process was distinctly observed and groundwater having low Cl/Br ratio in general were associated with comparatively low nitrate level. Correlation matrix as statistical indicator Correlation matrix of 13 groundwater parameters at 99 % confidence level is presented in Table 2. A significance level of p≤0.01 was considered as strongly correlated. Strong positive co-relation of nitrate with Na+ and Cl− and negative correlation with Br− is expected if domestic waste is assumed as nitrate source in the corresponding groundwater. Such correlations were unfounded in the present correlation matrix [NO3− and Na+ (r = 0.197, p = 0.013), NO3− and Cl− (r = 0.262, p = 0.001), and NO3− and Br− (r=−0.170, p=0.034)]. Instead, nitrate showed fair correlation with phosphate (r= 0.344, p=
