Seasonal variation of Nitrate-N in groundwater: A case study from Chunnakam aquifer, Jaffna peninsula

Abstract

The Jaffna Peninsula has four main aquifer systems. The largest of these is the Chunnakam aquifer in the Valikamam area. Intensive cultivation in this area has led to groundwater pollution, which is due to the excessive use of inorganic and organic ni-trogen fertilisers. The situation is aggravated by excessive irrigation applications, which carries the fertiliser applied to the groundwater almost on a daily basis. In addi-tion, the heavy rainfall experienced during the wet season not only carries the nitrogen to groundwater but also brings the groundwater level close to the surface. As a result, the fertiliser residues remain in the upper layers and also get dissolved in the ground-water. The aim of this study was to assess the spatial and temporal variation of ni-trate-N contamination in drinking water in the Chunnakam aquifer, which was a sub-objective of a research project carried out by the International Water Management In-stitute (IWMI). Forty-four wells representing different uses and land-use patterns were monitored on a monthly basis over a period of one year from January to Decem-ber, 2011. Nitrate-N concentrations in the water samples were determined using a colorimetric spectrophotometer. The spatial variations of water quality were mapped using ArcGIS 10.1. The Nitrate-N values from domestic, domestic with home garden, and public wells ranged from below 0.1 to 12.1 mg/l. During the rainy season, 38% of the agro-wells exceeded the nitrate-N limit of the World Health Organization (WHO) drinking water guidelines (10 mg/l) and became unsuitable for drinking purposes. However, only 15% of the agro-wells exceeded this limit by the end of the dry season. A decreasing trend in nitrate-N concentrations was observed from January to March. Nitrate-N was found in most of the wells surrounded by areas with highland crops (onions, chilies, tobacco and eggplant), and the concentrations frequently exceeded the acceptable lev-el (10 mg/l). Even though these agro-wells are used for agricultural purposes, people who work in the field also rely on them for their drinking water supplies. The study clearly indicates that the groundwater quality in some areas of the Chunnakam aquifer is becoming unsuitable for drinking purposes during the wet season while groundwa-ter quality in others remain unsafe throughout the year. Therefore, effective manage-ment of groundwater quality in the region is vital. Creating awareness among the peo-ple about the possible contamination of groundwater would possibly reduce the ex-cessive use of irrigation water and chemical fertilisers in agriculture.