Groundwater Nitrate Spatial and Temporal Patterns and Correlations: Influence of Natural Controls and Nitrogen Management

To use shallow groundwater NO 3 –N concentration as an indicator of groundwater quality requires understanding its patterns, correlations, and controls across space and time. Within a study comparing variable‐rate and uniform N management, our objectives were to determine groundwater NO 3 –N patterns and correlations at various spatial and temporal scales and their association with natural controls and N management. Experiments in a random, complete block design were conducted in a 2‐yr crop rotation in North Carolina that included one variable‐rate and two uniform N management treatments to wheat ( Triticum aestivum L.) and corn ( Zea mays L.). We measured groundwater NO 3 –N and depth every 2 wk at 60 well nests, sampling the 0.9‐ to 3.7‐m depth. Field‐mean NO 3 –N varied with time from 5.5 to 15.3 mg NO 3 –N L −1 These variations were correlated primarily with concurrent changes in water table elevation and depth. Mean NO 3 –N exhibited two preferred states: high when the water table was shallow and low when the water table was deep. Temporal NO 3 –N fluctuations greatly exceeded treatment effects. Treatments appeared to affect NO 3 –N temporal covariance structure. Groundwater NO 3 –N spatial patterns and correlations were associated mostly with saturated hydraulic conductivity and water table fluctuations and appeared influenced by subsurface lateral flow. When treatment effects became consistently significant later in the study, they overrode natural controls, and NO 3 –N was spatially uncorrelated or exhibited shorter spatial correlation ranges and patterns associated predominantly with treatments.