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 month- study 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 1.41- 6.77 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.

Tropical Agricultural Research Vol. 23 (3): 277-282 (2012)