Monitoring Groundwater Nitrate Concentrations under Irrigation in the Cukurova Region of Southern Turkey

Abstract

Agricultural output globally is largely dependent on fertilizer use, particularly nitrogen (N), especially in irrigated cropping. While a substantial amount of the applied N is taken up by the crop to stimulate growth and yield, much of it does not positively affect crop growth due to losses by leaching, volatilization, denitrification, and immobilization. Thus, N fertilizer-use efficiency is less than optimum in most production systems. Loss of N by leaching to the groundwater has economic, health and environmental implications. In a study area with poor drainage outlet conditions due to the low-lying drainage basin, Yemisli Irrigation District in the Cukurova region of southern Turkey, we determined the variability over space and time of ground-water nitrate (NO3) concentrations during years 2007 and 2008. Relatively shallow groundwater observation wells (56), up to 3 meters deep, were dug at various locations to represent the most common crops and soils in the irrigation district. Groundwater depth was measured, and water samples were collected (five times in 2007 and four times in 2008) and analyzed for NO3 concentration. Regional maps of groundwater depth and NO3 concentration were developed from the point data by using the inverse distance- squared interpolation technique. Groundwater NO3 concentrations ranged between 0.78 and 56.38 mg L-1 in 2007 and between 1.48 and 52.79 mg L-1 in 2008, only exceeding the critical 50 mg L-1 concentration in 0.4 to 5.4 % of the wells, depending on the sampling dates The NO3 concentrations were highest in February and in June-July. The peaks suggest that there is a likelihood that in the early part of the wheat cropping season (February), NO3 could be leached by the high rainfall in winter; similarly, N losses can occur by irrigation in June-July during the growth of cotton (Gossypium hirsutum) and corn (Zea mays). In most of the area (>60% of total), groundwater NO3 concentrations ranged from 20 to 50 mg L-1, and were thus marginal relative to the threshold pollution level (30 mg L-1). As both N fertilizer use and the extent of irrigation are likely to increase in this region of Turkey, so too will the poor drainage conditions inevitably continue. Consequently, regular groundwater NO3 monitoring, as adopted in this study, is advisable, and is also applicable to irrigated areas in other regions of the world.