Temporal Variation of Nitrate and Phosphate in Selected Six Small Tanks of Dry Zone in Sri Lanka

A tank cascade or a chain of tanks is a series of small reservoirs which is a unique irrigation system that flourished in the dry zone of Sri Lanka. The equilibrium state of a tank in relation to the water quality could be disrupted by intensive agriculture practices and human activities with growing population, resulting in artificial enriching these water bodies with various elements such as nitrogen, phosphorus and organic matter. Even though many hydrological studies have been conducted on dry zone tank cascade system, no systematic work, however, has been carried out on water quality aspects, particularly on the dynamics of nitrate (NO3 -N) and phosphate (PO4 3-P) in tank water of the dry zone. The present study was conducted to evaluate the temporal variation of NO3 -N and PO4 3-P in six selected tanks of two tank cascade systems viz. Thirappane and Mahakanumulla of the dry zone of Sri Lanka. NO3 -N and PO4 3-P of the tank water were analyzed in monthly intervals for one year starting from February 2010 to January 2011 using standard analytical techniques. According to the results, water in the six tanks of the two cascades exhibited a significant temporal variation during the twelve monthstudy period. Both cascades showed a bimodal pattern of nutrient fluctuation similar to the bimodal pattern of rainfall. NO3 -N concentration of the six tanks varied from 1.416.77 mg/l. In the Maha season “dilution effect” caused by the large volume of water in the tanks due to heavy rainfall which may reduce the concentration of the soluble ionic species in tank water. Concentration of PO4 3-P of the tank water of the six tanks of the two cascades also showed a significant temporal variation during the study period. Similar to the NO3 -N, concentration of PO4 3-P in tank water was high in the month of May in the Yala season while the lowest was recorded in the Maha season. Five tanks of the two cascade systems, especially, all three tanks of Mahakanumulla cascade showed high amounts of PO4 3-P over the EPA suggested value for eutrophication, in several months of the two seasons. In relation to the two nutrients studied, it can be recommended that waters of all tanks can be used for domestic needs and for irrigation as nutrient levels are less than the permissible level.


INTRODUCTION
A tank cascade or a chain of tanks is a series of small reservoirs that are unique irrigation system flourished in the dry zone of Sri Lanka and some parts of Southern India and Thailand (Sakthvadivel et al., 1997;Shinogi, 2001).Undulating landform and the hard rock formation with shallow or moderately deep soil create ideal situation to construct small water storage tanks in the region.In this physical setting a large number of small tanks had been constructed along the valleys.Annual variable low rainfall (75 % expectancy rainfall ranges 900-1400 mm) combined with high annual evaporation rates create water scarcity affecting the livelihood of the people in the dry zone of Sri Lanka.Therefore, tanks are an important source of fresh water and help in replenishing ground water supplies (Dharmasena, 2005).It also provides habitat for numerous plants and animal species and helps to ease impacts of floods by storing large amounts of water, and droughts by releasing water during shortages.
The equilibrium state of tank in relation to the water quality could be disrupted by the intensive agriculture practices and human activities with growing population, artificially enriching the water bodies with elements such as nitrogen, phosphorus and organic matter.Accumulation of nutrient elements and organic matter which degrade the quality of water is called as eutrophication and is the principal threat to tank water quality resulting potential health hazard to humans (Young et al., 2010).
Even though many hydrological studies have been conducted by several researchers (Perera & Piyasiri, 1999;Young et al., 2010) on dry zone tanks cascade system, no systematic work, however, has been carried out on water quality aspects particularly on the dynamics of NO 3 --N and PO 4 3--P in tank water of dry zone.Further, the data available on the NO 3 --N and PO 4 3--P accumulation in the water of the tank cascade system due to the intensive agriculture practices and human activities is very limited (Amarasri, 1973;Silva, 2004).Therefore, the objective of the present study was to evaluate the temporal variation of NO 3 --N and PO 4 3--P in selected six tanks of two tank cascade systems of the dry zone of Sri Lanka.

MATERIALS AND METHODS
Three tanks from Thirappane tank cascade (Meegasagama, Alisthana and Thirappane) and three tanks from Mahakanumulla cascade (Siwalagala, Amanakkatuwa and Maha kanumulla) located in Anuradhapura district, North Central Province were selected for this study.
Sampling had been carried out for one year from February 2010 to January 2011.Water samples were collected monthly intervals from three locations.There were three replicates for each analysis.Water samples were collected in clean plastic bottles and few drops of chloroform was added to prevent microbial growth.All the water bottles were brought to the Department of Soil Science, University of Peradeniya for analysis.All water samples were subjected to NO 3 --N and PO 4 3--P analysis.NO 3 --N and PO 4 3--P, of the water samples were measured using Sodium Salicylate (Markus et al., 1985) and Molybdate blue colorimetric procedures (Murphy and Riley, 1962) respectively.The rainfall, temperature and evaporation data were obtained from Maha Illuppalama meteorological station which is located 8 km from the experimental site.The data were analyzed statistically using SAS software package.

RESULTS AND DISCUSSION NO 3 --N in the two tank cascade systems
The results pertaining to NO 3 --N concentrations of water samples are given in Table 1.NO 3 --N concentration of water in the six tanks of two cascades showed a significant temporal variation (1.41-6.77mg/l) during the twelve month study period.Maximum value of NO 3 --N (6.77 mg/l) was recorded in Thirappane tank of the Thirappane cascade in the month of May 2010.Lowest value (1.41 mg/l) was observed in the water of Siwalagala tank of the Mahakanumulla cascade during the month of September 2010.Both cascades showed a bimodal pattern of nutrient fluctuation similar to the bimodal pattern of monsoonal rainfall (Fig. 1, 2 and 3).Rice is the main crop cultivated in the commanding areas of the tanks in both seasons.
Fertilizers are applied at the months of April to May and October to November in yala and maha seasons, respectively.In the yala season, land preparation for the paddy cultivation is done in late April to early May and planting begins early weeks of May.In this period fertilizers are applied as basal and top dressings to the farm fields in the catchment area.
With the intense rainfall in both seasons applied fertilizers, plant residues, cow dung and other animal excreta along with the soil sediments are being transported to the tanks through surface runoff and seepage of water (Bandara, 2007).As a result, the NO 3 --N and PO 4 3--P concentrations had been increased in tank waters corresponding to the two cultivation seasons (Fig 2, 3, 4 and 5).In this study, all the tanks showed highest amounts of NO 3 --N during the period of April to June which falls within the yala season of the dry zone.The high concentration of NO 3 --N in yala season could be further aggravated by the low water storage of the tanks due to the low rainfall (900 mm/yr) and the high evaporation rate (6 mm/day) of the dry zone.
An increasing trend of NO 3 --N was observed in waters of Thirappane and Mahakanumulla tanks which are located in the lowest part of the two cascades (Table 1).Out of that, the highest accumulation was recorded in Thirappane tank.Immediate catchment of Thirappane tank is used as a dumping ground for garbage from a nearby town and also subject to frequent grazing by relatively large cattle population in large extent of the thaulla (land just above the tank).

Fig. 3. NO 3 --N concentration in Thirappane cascade
Accumulation of cow dung and other animal excreta in large quantities may partly contribute to the high value of NO 3 --N.However, the NO 3 --N concentration of six tanks of the two cascades did not exceed the permissible level which is 10 mg/l (WHO, 1984) for irrigation and drinking water quality.In the maha season "dilution effect" caused by the large volume of water in the tanks due to heavy rainfall may bring down the soluble ionic species concentration in tank water compared to the yala season.Therefore, it is certain that the NO 3 --N content of the water goes down due to the high volume of tank water.

PO 4 3--P in the two cascade systems
Concentration of PO 4 3--P of the tank water of six tanks of the two cascades was also showed a significant temporal variation during the study period (Table 2).Similar to the NO 3 --N, concentration of PO 4 3--P in tank water was high in the month of April in yala season while lowest was recorded in the maha season.The increasing trend of PO 4 3--P in tank water was observed as two peaks corresponding to the two rainy seasons (Fig. 1, 4 and 5).

Table 2. Temporal variation of PO 4
3--Pconcentration (mg/l) in six tanks of Mahakanumulla 1 and Thirappanecascades 2 from February 2010-January 2011 Feb March April May June July Aug Sep Oct Nov Dec Jan Siwalagala 1 0.01 cd 0.01 cd 0.22 a 0.10 b 0.03 c 0.03 cd 0.01 cd 0.01 cd 0.10 b 0.11 b 0.01 cd 0.002 d Amanakkatuwa 1 0.01 c 0.02 c 0.35 a 0.23 b 0.05 c 0.03 c 0.02 c 0.02 c 0.18 b 0.18 b 0.07 c 0.003 c MahaKanumulla 1 0.02 ef 0.38 a 0.39 a 0.30 b 0.07 d 0.06 de 0.07 ef 0.02 ef 0.23 c 0.21 c 0.03 ef 0.002 f Meegasagama 2 0.03 abc 0.03 abc 0.04 ab 0.04 a 0.01 dc 0.01 d 0.01 dc 0.01 dc 0.02 bdc 0.02 bdc 0.01 d 0.01 d Alisthana 2 0.03 bc 0.04 b 0.08 a 0.05 b 0.04 b 0.01 de 0.03 bcd 0.02 cde 0.04 b 0.03 bcd 0.02 cde 0.006 e Thirappane 2 0.05 cde 0.08 b 0.11 a 0.07 bc 0.06 bcd 0.05 cde 0.02 fg 0.02 efg 0.07 b 0.04 def 0.03 def 0.007 g *Means with same letters aren't significantly different within the months An increase of PO 4 3--P was observed from the beginning of the two seasons.This was certainly due to the surface runoff of water to the tanks with the soil sediments enriched with the applied phosphate fertilizers in the recent catchment areas.The PO 4 3--P is one of the major macro nutrients which are responsible for the eutrophication of tank water.During the study period, similar to NO 3 --N, the highest PO 4 3--P values were observed during yala season.The PO 4 3--P concentration of six tanks ranged between 0.007-0.39mg/l, where the highest value was recorded in April 2010 in Mahakanumulla tank and the lowest value was observed in January 2011, in Thirappane tank.PO 4 3--P concentration in tank water generally showed a little variation.The critical level for PO 4 3--P for occurrence of eutrophication in tanks is 0.08 ppm (EPA, 1988).Higher PO 4 3--P concentrations than the EPA standard were observed in water of tanks of Mahakanumulla cascade during some months (March, April, May, October and November) in both seasons.This may be due to the specific physical setting of the Mahakanumulla cascade which is a branch type and has a large extent of catchment area compared to the Thirappane cascade.Therefore, there is a possibility for occurrence of eutrophication in the future due to the enrichment of PO 4 3--P in tank waters of the Mahakanumulla cascade.

CONCLUSIONS
Both cascades showed a bimodal pattern of nutrient fluctuation similar to the bimodal pattern of monsoonal rainfall.NO 3 --N concentration of water in the six tanks of two cascades showed a significant temporal variation during the twelve month study period.It was varied from 1.41-6.77mg/l.With the intense rainfall, applied fertilizers, crop residues, cow dung and other animal excreta along with the soil sediments are being transported through the surface run off and accumulated in the tanks resulting in increased concentrations of NO 3 --N and PO 4 3--P in tank water.The high concentrations of NO 3 --N and PO 4 3--P in yala season could be further aggravated by the low water storage of the tanks due to the low rainfall (<900 mm/yr) and the high evaporation rate (6 mm/day) during months of the dry zone.In the maha season "dilution effect" caused by large volume of water in the tanks due to heavy rainfall may bring down the soluble ionic species concentration in tank water.Concentration of PO 4 3--P of the tank water of six tanks of the two cascades was also showed a significant temporal variation during the study period.Similar to the NO 3 --N, concentration of PO 4 3--P in tank water was high in the month of May in yala season while the lowest was recorded in the maha season.Five tanks of the two cascade system, especially all three tanks of Mahakanumulla cascade, showed high mounts of PO 4 3--P over the EPA suggested value for eutrophication in several months of the two seasons.In relation to two nutrients studied, it can be recommended that waters of all tanks can be used for domestic needs and for irrigation as nutrient levels are less than the permissible level.

Fig. 1 .
Fig. 1.Rainfall distribution pattern during February 2010-January 2011 In dry zone, there are two distinct seasons namely yala (low rainfall season) and maha (high rainfall season) based on the rainfall distribution.Yala and maha seasons fall in the months of April to September and October to March, respectively.