Behavior of nitrate-nitrogen movement around a pumping high-capacity well: A field example

This study examines the near-field flow regime influencing the chemical composition of water samples collected from an irrigation well during short pumping periods. Data on the radial and vertical distribution of nitrate-nitrogen (NO₃-N) and on drawdown were collected from a closely spaced multi-level monitoring well network installed around an irrigation well. Aquifer properties were determined from grain-size analyses performed on samples collected from test holes drilled around the irrigation well and from drawdown data using the method of Neuman (1974). Grain-size characteristics were determined from cumulative distribution curves and used to construct vertical hydraulic conductivity (K) profiles based on frequently used empirical formulas applied to grain-size data for the determination of aquifer properties. Resultant vertical profiles show a general increase in K with depth. Results from the analysis of drawdown curves support the general trend of K determined from the grain-size data, drawdown-based K values, however, were 1.5 to 2 times greater than those determined from the empirical methods. Results from four pump-and-sample experiments at different times during the irrigation season indicate: (1) the distribution of NO₃-N around the irrigation well is complex and variable over time and space; (2) shapes of concentration-time curves for individual sample points are governed by the initial contaminant distribution, and shapes of concentration-time curves for the pumping well are governed by variables such as well screen position and hydrogeologic properties; (3) irrigation well samples underestimate the highest, as well as the average NO₃-N concentrations within the aquifer; and (4) a sampling strategy based on the behavioral characteristics of the well as reflected in concentration-time curves can improve the interpretation of water quality data collected from high-capacity wells.