Spatial and Temporal Variability of Groundwater Nitrate Concentrations in Irrigated Mediterranean Agriculture

As nitrogen (N) fertilizer-use efficiency rarely exceeds 50% in most agricultural systems, the potential leaching of N to the groundwater, particularly under irrigated conditions, has economic, health, and environmental implications. Research in the Akarsu irrigation district in the Lower Seyhan Plain in southern Turkey sought to determine spatial and temporal variability of groundwater (GW) nitrate (NO 3 ) concentrations in 2007-2008. Shallow groundwater observation wells 3 m deep were installed at different locations to represent the whole irrigation district. Groundwater samples were collected (February, April, July, October) and analyzed for ammonium (NH 4 ) and NO 3 concentrations. Because NH 4 values were negligible, only NO 3 data were processed to determine spatial and temporal variability and then used to develop regional NO 3 maps using geographic information systems. Groundwater NO 3 concentrations ranged between 0.17 and 55.96 mg L -1 during the 2 years, only exceeding the critical 50 mg L -1 concentration in 1% of the area sampled. The areal mean of NO 3 concentration was greatest in February, indicating a potential N leaching of unused N from the early season with intensive rainfall, especially in wheat-growing areas. Groundwater NO 3 concentrations decreased after February; however, during the peak irrigation season in July, NO 3 was relatively low because of crop uptake during spring and summer. In about half the area, groundwater NO 3 concentrations ranged between 20 and 50 mg L -1 and were thus marginal relative to the critical pollution level. As N fertilizer use will continue to increase, especially with the expanded irrigation that is now occurring in the Mediterranean region, regular monitoring of groundwater NO 3 is advisable under such conditions.