Water Air Soil Pollut (2011) 219:563–577 DOI 10.1007/s11270-010-0728-y

Vulnerability of Coastal Aquifers Due to Nutrient Pollution from Agriculture: Kalpitiya, Sri Lanka Pathmakumara Jayasingha & A. Pitawala & H. A. Dharmagunawardhane

Received: 9 June 2010 / Accepted: 20 December 2010 / Published online: 12 January 2011 # Springer Science+Business Media B.V. 2011

Abstract This study focuses on spatial and temporal nutrient pollution of groundwater in the unconfined sandy aquifers of Kalpitiya peninsula, Sri Lanka, where agricultural activities are intense. The study covers two consecutive dry and rainy seasons during the period from 2008 to 2010. Nitrate is the dominant nutrient pollutant in groundwater. The values of Nitrate-N contents ranged from 0.60 to 212.40 mg/L in the dry seasons and 0.20–148.50 mg/L in rainy seasons. Phosphate in groundwater ranged from 0.20 to 5.70 mg/L in dry seasons and 0.04–10.35 mg/L with few exceptions in rainy seasons. About 50% of the studied water samples had Nitrate-N concentrations above WHO drinking water guideline values both in dry and rainy periods. These high concentrations were recorded from wells in agricultural lands. Although there is a slight decrease in the Nitrate-N concentrations at random in rainy seasons, an increasing trend of average concentrations became evident over the study period as a whole, probably indicating building up of Nitrate-N in groundwater in the vegetable growing areas. The spatial distribution P. Jayasingha (*) : A. Pitawala : H. A. Dharmagunawardhane Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka e-mail: [email protected] P. Jayasingha Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Sri Lanka

of Nitrate-N too shows a good match of high NitrateN bearing zones with vegetable cultivated areas indicating intensive leaching from application of excessive chemical fertilizers. High Nitrate-N zones also showed fairly steady lateral distribution indicating slow lateral mobility of Nitrate-rich groundwater probably due to low hydraulic gradients. Low phosphate concentrations in both groundwater and surface soils either indicates their less use in the area or that the available phosphate is leached and removed from the aquifer water and (sandy) soil solutions and probably adsorbed in clayey deeper horizons. Low concentrations of major cations (especially K, Ca, and Na) indicate less impact on cation concentrations in groundwater by the fertilizer application or sea water intrusions/up-coning. Keywords Nutrient pollution . Coastal aquifers . Agriculture . Fertilizer

1 Introduction 1.1 Groundwater Pollution The vulnerability of aquifers depends on their intrinsic properties such as aquifer media and rate of recovery (Gemitzi et al. 2006).Vulnerability is also increased if the anthropogenic activities are intense (Tesorieroa et al. 2004). Among the different types of aquifers, sandy aquifers are conducive to water

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quality deterioration by surface and/or subsurface contaminants derived from point and/or nonpoint sources (Simsek et al. 2008). Coastal shallow sandy aquifers are highly studied in worldwide since they are very good fresh water storages, and the use of groundwater of coastal aquifers has rapidly increased during the past few decades due to urbanization, industrialization, and agricultural activities (Sikdar et al. 2001; Tesorieroa et al. 2004; Zilberbrand et al. 2001, Voudouris et al. 2004). With its high permeability, shallow sandy coastal aquifers are more susceptible to contamination from natural and anthropogenic activities (Park et al. 2005; Milnes and Renard 2004). The application of excessive amounts of chemicals (fertilizers, agrochemicals, industrial chemicals, waste effluents, etc.) and intense ground water extractions have placed those aquifers at a greater risk to contamination (Tesorieroa et al. 2004; Canora et al. 2008). Nitrate and phosphate as well as some organic compounds are commonly found in the anthropogenic pollutants in aquifer water (Chen et al. 2005; Suzumura et al. 2000; Zebarth et al. 1998). Increase of nutrient contents in groundwater has caused many environmental problems (Spalding and Exner 1993) such as eutrophication and algal blooms and health hazards (Fan and Steinberg 1996; Gelberg et al. 1999; Gulis et al. 2002). Nitrate which is the most widespread groundwater contaminant in the world (Liu et al. 2005; Spalding and Exner 1993) can be accumulated mainly due to agricultural practices, animal farming, and septic tank uses (Aelion and Conte 2004; Fields 2004; Addiscott et al. 1991, Mikkelsen 1992). Nitrate derived from the above-mentioned sources can be introduced into surface water and groundwater systems via runoff and infiltration (Limbrick 2003). Nitrate leaching to the groundwater due to agricultural activities may depend on the types of crops whether short- or longterm, irrigation practices, type of soil, and climate of the area. With the application of heavy loads of urea mixed fertilizers, nitrate is easily leached into the groundwater through the infertile sandy soil aided by the improper irrigation practices and high rain fall intensity. Coastal sandy aquifers are also vulnerable to salinization due to many reasons such as seawater intrusion, extensive extraction of groundwater due to agricultural practices, leaching of deicing salts, and high rate of evapotranspiration and contamination of natural salts in aquifer media (Park et al. 2005; Milnes

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et al. 2006; Sherif and Singh 2002; Kim and Yun 2005, Rosenthal et al. 1992). Salinization of groundwater of coastal aquifers due to agricultural activities is twofold. High rates and continuous groundwater extraction cause lowered water tables, hence facilitate the sea water intrusion and intense irrigation leads to salt recycling within the aquifer instigating salinization (Milnes and Renard 2004; Penny and Lee 2003). Since groundwater has been intensely used for irrigation purposes in Sri Lanka where agricultural practices are common, groundwater has been threatened by contamination of nutrients, organic pollutants, and sea water intrusion. Among the different types of aquifers, groundwater in coastal aquifers is vulnerable to pollutants as recorded from investigations. 1.2 Study Area Kalpitiya peninsula in the Northwestern coast of Sri Lanka (Fig. 1) is one of the highly productive unconfined aquifers, and it is characterized by its shallow nature and sandy aquifer media derived from marine origin during the Quaternary age (Cooray 1984). Except at the southern end, the peninsula is surrounded by Indian Ocean and Puttalam lagoon (Fig. 1). Generally, the area is a flat land with slightly higher elevation in the central part due to paleo dunes and some places along western margin due to recently developed dunes by monsoon winds. The southern end is connected to the main land at Palavi area where the lagoon ends. The entire peninsula is composed of fine to coarse coastal sands from successive aggradations processes of beach formation. 1.3 Land Use Pattern and Hydrogeological Setting Most of the lands of the Kalpitiya area are agricultural lands mainly in western and middle part of the peninsula. Along the coast of the lagoon and some scattered areas in the middle, coconut cultivations are found. In addition, the Kalpitiya peninsula is characterized by densely populated areas especially along the Kalpitiya–Puttalam main road, some abandoned lands, small areas of shrubs and bushes, and prawn farms along the stretch of lagoon coast. Farmers use lands for cultivations throughout the year without adopting any seasonal cultivation practices. Almost all the cultivations are carried out on fine- to coarse-textured sandy soil which is used as

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565

Fig. 1 Study area and sampling locations

only a substrate for the crops. Excessive amounts of chemical fertilizers are applied due to the infertility of sandy soil and crops are intensively watered due to high evaporation and less retention of water in the soil. Therefore, these agricultural practices can be considered as unique to the study area. The short-term crops such as vegetables, fruits such as papaya and guava, and tobacco have been the main cultivations in

the area for decades (Lawrence and Kuruppuarachi 1986). Coconut cultivation which is the principal long-term crop type covering about 25% of the area. Resting on the Miocene limestone, the overlying quaternary sandy regosol has the thickness of 15– 20 m. According to Lawrence and Kuruppuarachi (1986), the ground water system of the Kalpititya peninsular has been described as a series of cells

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present at depths of 1–3 m over most of the peninsula. Shallow aquifers in the area are recharged by the only monsoonal rainfall that occurs from October to January. The annual rainfall is 800–900 mm. The semiarid condition promotes the high evaporation over the entire peninsula. Due to high permeability, almost all rainwater infiltrates and recharges the ground water cells. Since the area has only a low rainfall throughout the year, the chief source of water for drinking purposes as well as for agricultural practices is the groundwater of the shallow sandy aquifers of the area. The ground water is extracted mostly twice a day for agricultural purposes. Depending on the type of vegetable, the rate of watering is highly variable. However, it needs at least 10,000 L per hectare per day to compensate losses from infiltration and evapotranspiration. Nitrate concentrations of groundwater of the area were measured during the last decade (Liyanage et al. 2000; Lawrence and Kuruppuarachi 1986; Panabokke and Pathirana 2002), and it was revealed that the groundwater was highly vulnerable to nitrate contaminations. However, the farmlands of the area are being expanded and heavily cultivated during last two decades. Hence, the groundwater of the coastal sandy aquifers may have also been contaminated simultaneously. Therefore, our study was focused to understand the present state of the groundwater quality of the Kalpitiya area.

2 Materials and Methods This study was carried out during the period from 2008 to 2010. Four sampling campaigns were carried out covering two dry periods and two rainy periods. Dry season samplings were carried out during the month of June to July 2008 and 2009, while rainy season samplings were carried out from January to February 2009 and 2010. Water samples were collected from 58 locations (Fig. 1) covering 46 dug wells and 12 shallow tube wells. Sampling locations represent the lands of short-term cultivations, home gardens, coconut cultivations, and barren lands. Physiochemical parameters such as salinity, conductivity, pH, total dissolved solid (TDS), and turbidity were measured in the field at the time of sampling. Nitrate as Nitrate-N and phosphate content of water were measured in the laboratory within 24 h from

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sampling using a HACH DR/2400 spectrophotometer according to its manual. Dissolved cations were measured using a Perkin Elmer Atomic Absorption Spectrophotometer using standard methods for the examination of water (APHA 1992). Water samples for measuring the cations were preserved by acidifying with analytical-grade concentrated HNO3 during sampling and kept below 4°C in the laboratory. During water sampling, surface soil samples were also collected, and the total soil phosphate (P tot) was measured after extraction by simple ignition followed by acid digestion (Anderson 1976; Aspila et al. 1976). Standard methods and procedures were followed at all times during sampling and analyses.

3 Results Table 1 shows the minimum, maximum, and mean values of measured physicochemical parameters: Nitrate-N, phosphates, and dissolved cations of groundwater samples collected for each sampling campaign. 3.1 Physicochemical Parameters Most of the measured pH values of groundwater were clustered around 7 (Table 1) and median (Md) and standard deviation (SD) of pH values during the dry seasons in 2008 and 2009 are 7.26, 1.07 and 7.23, 1.02 and that of during the rainy seasons in 2009 and 2010 are 7.38, 0.70 and 7.41, 0.76, respectively. Groundwater of some wells in highly populated areas shows somewhat low pH values. The conductivity of groundwater during the study period ranged from 0.10 to 101.20 μs/cm. Md and SD of conductivity are 15.74, 56.32, and 18.35, 65.21 μs/cm for 2008 and 2009 dry seasons, and those are recorded as 57.74, 37.32, and 47.39, 36.21 μs/cm in successive rainy seasons of 2009 and 2010. Recorded salinity values throughout the study period show the least significant variation when compared with other physicochemical parameters. Md and SD of successive dry seasons of 2008 and 2009 are 0.05 and 0.07. In the rainy season of 2009 and 2010, the values were 0.04, 0.03 and 0.03, 0.03. TDS of groundwater of the study area have Md and SD of 0.84 and 1.64 for 2008 and 0.75 and 1.53 for 2009 dry seasons. Md and SD of TDS

8.40 1025.00 140.70 67.24 218.69 5.20 560.00 74.00 43.92 107.89

3.10

0.01

4.80

0.10

0.02

0.10

0.10

Fe

pH

Conductivity

Salinity

TDS

Turbidity

54.60

4.20

0.22

97.30

8.50

1.40

81.80

85.40

7.26

0.05

3.27

0.95

0.06 0.10

0.80

0.05

33.65 15.74

7.00

0.13

28.20 23.55

23.50 19.60

7.20

0.32 7.38

0.14

2.80

0.17 6.63

0.75

0.04 0.10

0.61

0.04

97.50 45.01 57.74

8.70

4.00

60.60 20.20 18.71

63.30 15.80 11.90

10.61 0.10 190.70

1.64 0.10

0.07 0.01

56.32 0.10

1.07 4.50

0.31 0.01

21.13 1.30

17.53 2.70

0.04

0.80

Min

27.13

0.66

0.04

37.32

0.70

0.62

15.49

12.39

91.40

28.30

0.13

0.10

0.03

0.13

4.70

0.01

1.34

53.12

4.60

0.25

101.20

8.60

1.56

132.20

3.14

0.98

0.06

48.80

7.10

0.14

32.60

9.30 1100.30 163.70

4.10

0.55

37.50

Mean

0.33

21.95

Md

Min Max

Mean Md

0.79 0.10

21.50

1.30

0.29

53.32 0.20 148.50 23.40 10.57

SD

January–February 2010

Rainy period

18.74 2.10

51.30 13.50 13.28

12.92

56.74

2.43

31.34

SD

0.10

0.75

0.05

18.35

7.23

0.05

28.30

7.10

0.33

7.41

0.13

3.30

0.14

7.23

0.85

0.03

0.10

0.63

0.03

98.40 44.11 47.39

8.40

4.30

62.60 22.50 19.21

9.21 0.10 201.40

1.53 0.10

0.07 0.01

65.21 0.10

1.02 4.60

0.29 0.01

25.29 1.70

19.24

0.59

0.03

36.21

0.76

0.59

16.72

78.34 238.31 4.90 540.00 67.00 45.49 114.23

24.90

408.30 130.10 112.20 103.98 9.40 431.00 60.60 53.18

4.40

212.40

Max

June–July 2009

Dry period

53.76 14.90

4.35

29.99

K

0.37

9.15

Na

1.40

3.70

30.50

Mg

0.96 0.10

43.15 0.40 132.50 20.10

368.00 120.70 90.75 101.22 8.50 424.00 57.60 51.50

0.27

11.80

0.40

28.10 13.35

Ca

5.70

0.20

151.00

0.60

SD

P04-

Mean Md

No3-N

SD

Min Max

Md

Min

Mean

January–February 2009

June–July 2008

Max

Rainy period

Dry period

Table 1 Chemical parameters in groundwater before and after the rainy season

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values for the rainy season are 0.61 and 0.66 for 2009 and 0.63 and 0.59 for 2010. Turbidity also shows very low values for groundwater samples collected during each sampling campaigns. According to the values of each physicochemical parameter, they do not show any significant temporal or/and spatial variation. All of these parameters were within the WHO (2004) guideline values. 3.2 Nitrate-N in Groundwater 3.2.1 Temporal Variation Recorded Nitrate-N concentrations in groundwater during the study period range from 0.20 to 212.40 mg/L (Table 1). Md and SD of Nitrate-N of groundwaters of dry periods are 13.35 and 43.15 for 2008 and 21.95 and 53.32 for 2009, and the means are 28.10 and 37.50 mg/L, respectively. The mean values have been decreased to 20.10 and 23.40 mg/L for the following rainy periods in 2009 and 2010 with Md and SD values of 9.15 and 29.99 for 2009 and 10.57 and 31.34 for 2010. These changes may have been caused by the recharge of aquifers by infiltration of rain water through the highly permeable aquifer media. However, all mean values of each period have exceeded WHO (2004) guideline values (Nitrate-N, 10 mg/L) for safe drinking water. According to the recorded values of Nitrate-N, groundwater of the study area can be categorized into four groups as safe (<10 mg/L), moderate (11–40 mg/ L), high (41–100 mg/L), and unsuitable (>100 mg/L) on the basis of impact on human health (modified after Mahler et al. 1990). The percentages of each category for study area and the vegetable cultivated area are given in Table 2. It is noted that nearly half of the water samples collected from the study area during the study period have exceeded the WHO guideline values. According to the percentages of unsuitable category for the dry periods, more than 10% is unsuitable for human consumption. This category shows a decrease during the rainy periods whereas an increase is in the moderate- and high-risk categories. Among the one third of collected water samples that represent the wells from vegetable cultivations, more than 84% show higher Nitrate-N values that exceed WHO guideline values during the study period irrespective of seasonal variations. However, more than 30% of them collected during

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the dry seasons have fallen into the unsuitable category, and the percentage has decreased down to 10 during the rainy season. 3.2.2 Spatial Variation Spatial variation of Nitrate-N shown in Fig. 2 for the dry and rainy periods is remarkably significant. Two distinct zones with elevated Nitrate-N in groundwater can be identified in the southern and northwestern parts of the area. These areas coincide well with the vegetable cultivated lands (Fig. 1). It is also clear that high Nitrate-N zones have remained unchanged over different seasonal conditions even though the general trend shows an increasing pattern. Lower levels of Nitrate-N were recorded from newly started vegetable cultivated areas (the area between zones of elevated concentrations, see Fig. 2), coconut cultivations, barren lands, and settlement areas. 3.3 Phosphate in Groundwater 3.3.1 Temporal Variation The mean, Md, and SD of phosphate concentrations in dry seasons are 0.40, 0.27, and 0.96 for 2008 and 0.55, 0.33 and 0.79 for 2009, respectively. These values for successive rainy seasons are 1.40, 0.37 and 4.35 for 2009 and 1.30, 0.29 and 2.43 for 2010. It is noted that the content of phosphate in groundwater samples collected during the study period lies within the WHO guideline limits (Table 1). Fifteen percent of the water samples have shown higher phosphate contents than 1.00 mg/L during dry seasons of 2008 and 2009. A substantial increase of phosphate contents in ground water in the rainy periods of 2009 and 2010 was observed. The recorded higher phosphate levels were from the unprotected dug wells in home gardens. 3.3.2 Spatial Variation Spatial distribution of phosphates in groundwater is shown in Fig. 3. Unlike the Nitrate-N variations, phosphate values do not show significant correlations with the land use pattern of the study area. In addition, the spatial variation too does not significantly vary with time except for a slight increment during rainy seasons.

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Table 2 Categories of water based on nitrate-N content Category

Percentage of samples

Safe <10 mg/l

Dry period 2008 Rainy period 2009 Dry period 2009

Unsuitable >100 mg/l

Entire study area

51.7

27.6

8.6

12.1

15.7

31.6

21.1

31.6

Entire study area

56.9

29.3

10.3

3.4

Vegetable cultivated area

15.8

47.4

26.3

10.5

Entire study area

41.5

37.9

10.3

10.3

5.2

36.9

26.3

31.6

Entire study area

56.9

29.3

10.3

3.4

Vegetable cultivated area

15.8

47.4

26.3

10.5

3.4 Soil Phosphate As also recorded in sandy soils of other areas of the world (Carlyle and Hill 2001), the sandy soils of the study area contains remarkably low amounts of total phosphate where all values lie below 170 mg/kg with a mean value of 88.23 mg/kg. Similar results have been reported by Loganathan et al. (1986) from sandy Fig. 2 Spatial distribution of nitrate-N in groundwater a Dry periods. b Rainy periods

High 41–100 mg/l

Vegetable cultivated area

Vegetable cultivated area Rainy period 2010

Moderate 11–40 mg/l

soils in some coconut cultivations of Sri Lanka. The spatial variation of soil phosphate (total) of the study area does not show any correlation with land use pattern (Fig. 4). The soil gradation analysis revealed that the soil of the study area contains more than 99% sand and a less than 1% clay/silt fraction. The positive correlation of total soil phosphate with fine fraction of the soils (Fig. 5) indicates that most soil phosphate is

570

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Fig. 3 Spatial distribution of phosphate in groundwater a Dry periods. b Rainy periods

bound to clay and silt fraction of the sandy soils of the area. 3.5 Dissolved Major Cations Sodium and calcium were found to be the dominant cations in groundwater though varies over a wide range (Table 1). The mean concentration of sodium was below 200 mg/L (WHO Max. Guideline value) for the whole period whereas mean calcium content exceeds the WHO max guideline value of 75 mg/L. The magnesium concentration varied from 2.10 to 91.40 mg/L with a mean value exceeding the WHO standards of 30 mg/L specially in dry seasons. The higher concentrations of both calcium and magnesium were found in vegetable cultivated areas. Concentrations of all major cations decreased during the rainy season, most probably due to the dilution. One tenth of the values of the iron contents in the dry seasons were beyond the WHO guide line values, and a marked increase of iron content was reported during the rainy periods.

4 Discussion Concentration of Nitrate-N in groundwater (Table 2) clearly indicates that the groundwater in the area is contaminated with nitrate in almost all parts of vegetable cultivating areas (11–212.40 mg/L). Comparatively low Nitrate-N concentrations were observed in the newly started vegetable cultivations where cumulative fertilizer application is still less than that of the continuously cultivated areas. The barren lands, coconut-cultivated lands, and settlement areas have no or low fertilizer application and have also shown low Nitrate-N concentrations in groundwater. This situation confirms that the main cause of high Nitrate-N in groundwater in the study area is the intense agricultural practices where application of excessive amounts of nitrate fertilizers is accompanied by intense watering to maintain moisture in the dry sandy soil. The combined effect of these two practices is rapid leaching of nitrate through highly permeable soil and ending up in groundwater with gradual increasing concentrations (Strebel et al. 1989).

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Fig. 4 Spatial distribution of soil phosphate

The Nitrate-N in groundwater shows a seasonal variation and also slender cyclic change of the percentages of affected wells (exceeding WHO guideline values). However, the effect of monsoonal precipitation in dilution of Nitrate-N in groundwater Fig. 5 Correlation between soil phosphate (tot) and percentage of fine fraction of soil of study area

of vegetable cultivated areas appeared to be insignificant because migration of nitrate from soil zone to groundwater table has continued with infiltration of precipitation. This situation is facilitated by the consecutive cultivations throughout the year. In contrast, seasonal cultivations are carried out in the other areas of the country where the lower nutrient values are recorded (Young et al. 2010; Gunatilake and Gunatilake 2004). Steady or even higher fluxes of nitrate can be expected into ground water during rainy season because farmers tend to almost double the fertilizer application during this period to supplement the infiltration losses. Therefore, it is well noticed that the percentages of affected wells remained unchanged, although the mean values were decreased during the rainy seasons (Table 2). The mean Nitrate-N value was increased during the study period, and it was 33.4% increment at the final stage. The calculated rate of increase is about 4.7 mg/L per year. Compared with the present higher mean Nitrate-N value (up to 37.5 mg/L), it was only about 14.8 mg/L in the year 2000 (calculated based on the data of Liyanage et al. 2000, see Table 3), and the value was increased by 153% over the last 9 years. The rate of increase (2.3 mg/L) for last 9 years is also less compared with the accumulation rate during last 2 years. Despite the nitrate contaminated water causes several health problems (Gulis et al. 2002; Weyer et al. 2001) in humans, there are no any recorded health issues from the study area. According to the field investigations, the average annual inputs of nitrogen as urea and phosphate fertilizers are around 1.5–2.0 and 0.5–0.7 t/ha, respec-

1.20

1.00

Fine fraction, %

R2 = 0.6475 0.80

0.60

0.40

0.20

0.00 0.00

20.00

40.00

60.00

80.00

100.00

120.00

Total phosphate, mg/l

140.00

160.00

180.00

572

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Table 3 Nitrate-N concentrations of groundwater of the study area in the year 2000 (after Liyanage et al. 2000)

Range of concentrations, mg/L

No. of wells

0–11.1

The measured nitrate concentrations were converted into Nitrate-N values

Mean Nitrate-N content, mg/L

116

6.9

11.1–22.2

68

16.1

22.2–33.3

22

27.0

33.3–44.4

11

39.1

44.4–55.5

4

50.7

55.5–66.6

3

62.9

tively. Therefore, it can be inferred that increase of Nitrate-N in groundwater can be a result of conversion of urea in to nitrate or nitrification process. This process is mediated by microbes in the soil (Fig. 6). Once urea is applied to the soil, it breaks down rapidly into ammonia and carbon dioxide with the presence of urease enzyme (Francis et al. 2007). Ammonia can be volatilized if the soil conditions are dry (Sloan & Anderson 1995). But, the surface soil of the area is mostly wet due to consecutive watering. Ammonia or hydrated form is converted or oxidized into nitrite followed by to nitrate by autotropic bacteria in the soil. This process is facilitated by high temperature. Plant uptake and de-nitrification process (see Fig. 6) are the only expected nitrate removal processes in the area. Since the most of the plants are vegetable, their root systems are not deep. Hence, these plants are only capable of absorbing dissolved nitrate in the uppermost soil solution. With the watering and transpiration of soil moisture, the upper most soil solution is diminished, and very soon, plant uptake is restricted. Most of the dissolved nitrogen fertilizers are infiltrated into groundwater due to the high permeability of the sandy soil. However, soil saturation with water generates anaerobic conditions promoting the biological denitrification process which is the predominant nitrate

removal mechanism from groundwater (Desimone and Howes 1996). The anaerobic condition causes soil microbes to use nitrate as an electron donor in respiration process. After depletion of nitrate, the next acceptor use manganese. When the availability of manganese is low, iron becomes the next available ion for reduction (Desimone and Howes 1996). Higher concentrations indicate the sufficient reduction of iron in groundwater. Therefore, a negative correlation of iron with nitrate is shown (Vendrell et al. 2001; Thayalakumaran et al. 2004). Figure 7 shows the correlation between Nitrate-N and iron in ground water in the study area. Most of the higher Nitrate-N content falls within the range of 0–0.3 mg/L of iron in groundwater. However, compared with the higher input of nitrate in to the groundwater low iron content indicates the lower reduction rates of iron in the aquifer due to the higher oxygen content supplied into the highly permeable sand by intense watering. In contrast, reducing condition which is observed in clayey soil areas control the input of nitrate into the groundwater by conversion them into nitrogen and therefore recorded nitrate values show lower figures, although the nitrate fertilizers are applied intensively (Young et al. 2010; Gunatilake and Gunatilake 2004, Pitawala et al. 2007).

Table 4 Comparison of cations in different water sources Surface watera

Groundwaterb

Sea water

c

Groundwater in study area Dry period 2008

Rainy period 2009

Dry period 2009

Rainy period 2009

Na

5.2

30

10780

140.75

74.08

163.7

67.0

K

1.3

3

399

28.26

20.24

32.6

22.5

Ca

13.4

50

412

120.77

57.67

130.1

60.6

Mg

3.4

7

1280

23.52

15.89

28.3

13.5

a

Meybeck, 1979

b

Turekian, 1977

c

Millero, 1996

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Fig. 6 Schematic diagram of nutrient migration of the sandy aquifers of Kalpitiya area

The spatial variation of Nitrate-N content (Fig. 2) demarcates the highly affected zones and also outlines the extent of distribution of zones of nitrate inputs and contamination of groundwater in the area. The two Fig. 7 Correlation between Nitrate-N and iron in groundwater of the study area

distinct zones of elevated levels of Nitrate-N in groundwater (>10 mg/L) is characterized by the areas of intense agricultural practices taken place for several decades. Coincidence of zones of high Nitrate-N with

574

the vegetable cultivated areas indicates that the input of nitrate fertilizers, contamination and accumulation have been taken place in the same area. It is indicated that the extent of spatial distribution of affected area has shown a seasonal variation caused by climatic changes, but did not show as any lateral shift. Since aquifers of the area occur as a series of cells (Lawrence and Kuruppuarachi 1986), the lateral distribution of nutrients should be restricted within each cell and the contaminated groundwater cells can remain high in concentrations of Nitrate-N for decades without mixing and limiting the mobility of it with the adjacent water. This situation is clearly shown by constant spatial distribution of the two zones of high Nitrate-N (Fig. 2). Hence, identification and delineation of contaminated and fresh water lenses is important in groundwater development for water community supplies for the area. As a nutrient pollutant, phosphate appears to make less impact on ground water in the area. That can be due to two reasons: low inputs and/or removal from the soil system before reaching to the groundwater. Application of phosphate fertilizer: triple super phosphate is also limited to once a year or once in 2 years. Application of organic manure is also limited despite of low organic matter content in sandy soils of the area. Therefore, addition of phosphate into groundwater is low from agricultural activities in the area. However, the slight increasing of phosphate Fig. 8 Correlation between calcium and phosphate in groundwater

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concentration in groundwater during the rainy season can be a result of percolation of surface runoff that carries available phosphate. Spatial distribution of phosphate (Fig. 3) in groundwater of the area does not appear to correlate with agricultural areas, hence, non-indicative of the impact of fertilizer application and other anthropogenic activities though significant in some other parts of the world (Schoumans and Groenendijk 2000). Also, the present study suggests that input of phosphate to the groundwater from agricultural activities is lower than that from non agricultural lands. Since sandy soils have a low binding capacity of phosphate (Koopmans et al. 2004; Loganathan et al. 1986) due to low amount of clay and silt fraction, phosphate leaching into the groundwater from sandy soils should be high. Low contents of phosphate and fine particles of the soil of the Kalpitiya area imply that added phosphate leach rapidly along the soil profile. And also, the neutral pH of groundwater of the aquifers gives favorable conditions for dissolution of accumulated phosphate in soil (Holliday and Gartner 2007). But, it seems that the effect of these favorable conditions for the availability of phosphate in the aquifer water is inconsequential and another process that removes the available phosphate before reaching the aquifer has taken place. The correlation of phosphate with calcium in groundwater (Fig. 8) also shows that

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most of the phosphate values are clustered around the low concentrations of calcium. This can be due to the removal of phosphate with the calcium from the system. But, the low concentration of phosphate (P tot) of surface soil has shown that such a removal process has not taken place at the soil surface. It may be that this is due to intensified irrigation with the neutral pH groundwater. Therefore, the precipitation of phosphate can be expected at the deeper part of the soil profile (Koopmans et al. 2004) before reaching to the deep water table of the area (Fig. 6). Dissolved cations in groundwater have been mainly derived from dissolution of aquifer materials. Carbonate materials in marine origin sands could be the main sources of dissolved calcium, sodium, and magnesium contents of aquifer water in the area. Also, elevated levels of dissolved calcium and magnesium of groundwater in vegetable cultivated lands indicates the anthropogenic input on geochemical evolution of groundwater of the area. The sea water sprays associated with high monsoon winds can be expected to contribute to sodium contents in groundwater. Iron in groundwater can derive from weathering of iron-bearing minerals such as magnetite and ilmenite. Table 4 compares the situation of the study area with global average concentrations of major cations in surface water, groundwater, and seawater. However, with regard to the dissolved cations, the groundwater in the study area is safe from deterioration especially from sea water intrusion. Although the pumping rates are high in the area, the highly permeable sandy formation with high transmissivity makes only small draw downs in the groundwater table. Lowering of water table, which causes the up-coning or intrusion of saline water, (Zilberbrand et al. 2001; Aris et al. 2005), was not recorded as a critical factor of groundwater contamination in the area. The water quality parameters such as salinity, electrical conductivity, and low cation concentration, too, indicate the insignificant effect of fresh water seawater interaction. Extracted groundwater from the aquifer is used to irrigate the crops of the same area, and a large amount of water is available for the aquifer as irrigation return flows through permeable sand and as a result fast recovery of water table does not favor up-coning or intrusion of saline water.

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5 Conclusions Nitrate is the dominant nutrient pollutant in groundwater of the Kalpitiya peninsula, and most of the contamination is present in the vegetable cultivated areas where intensive agricultural activities are taking place for decades. The elevated levels of Nitrate-N indicate that the aquifers are at risk and already contaminated, therefore people of the area are under threat of health hazards. Nitrate-N levels have increased gradually with continuous application of fertilizer in agricultural areas without significant areawise expansion of the contaminated zones. This restricted mobility of nutrients can be used in planning for safe source areas for future water supply schemes. Other than agricultural activities, anthropogenic nutrient pollutions, and seawater intrusion or up-coning appear to be in significant in the area Acknowledgments We gratefully acknowledge the National Science Foundation of Sri Lanka for financial assistance through grant NSF/RG/2007/AG/02. We are also very grateful to Prof. C.B. Dissanayake for his advice and encouragements and Mr. Asanaka Sanjeewa for his valuable support during the field works.

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