Factors Affecting the Concentration of Nitrates in Groundwater in the Rice-Wheat Cropping System of India: A Geo-Spatial Approach.

Increased use of nitrogenous fertilizers in the intensively cultivated rice-wheat cropping system (covers a 13.5-mha area in South Asia) has led to the concentration of nitrates (NO3-N) in the groundwater in Haryana State of India. Six districts from the freshwater zone were selected to demarcate the risk categories in terms of NO3-N and to identify major causal factors. Water and soil samples were collected from 1580 locations during 2006 and analyzed for their chemical properties. ARCGIS 9.2 was used for spatial and proximity analysis. Kriging interpolation was used for the creation of grid maps followed by cross validation (r2 = 0.84). A weighted score and ranking method was developed to classify different NO3 risk categories. In this method the weightage was given according to the intensity of contamination and area. About three percent (26796 and 10588 ha) of the area was estimated to be under moderately high (7.5-10 mg l-1) and high (>10 mg l-1) risk categories, respectively. The other 15 percent (153406 ha) of the area was found under medium risk (5-7.5 mg l-1) category. The Results revealed that NO3-N was 10-50% higher during the pre-monsoon season than in the monsoon season. Nitrate-N decreased with the increase in aquifer depth (r2 = 0.99). Overlaid, geomorphologic and proximity analyses showed that (1) clay material in surface and sub-surface texture restricts N leaching, (2) piedmont and rolling plains act as an N sink, and (3) perennial rivers bring a dilution effect whereas seasonal rivers provide favorable conditions for NO3- enrichment, respectively. To understand the process of nitrate formation and leaching in piedmont plains and its transportation to the mid-plain zone, a conceptual framework was developed and explained. The study concludes that chemical N fertilizers applied in agro-ecosystems are not the sole factor determining the NO3 in groundwater; rather, it is an integrated process governed by several factors. Therefore, future studies should cover a larger area (at least watershed scale) to understand the mechanistic pathways of NO3 enrichment in groundwater.