Seasonal and Long-Term Changes in Nitrate-Nitrogen Content of Well Water in Oklahoma

To ensure that NO3-N concentrations in groundwater do not exceed the maximum contaminant level (MCL; 10 mg L-~), drinking water supplies are continuously sampled and analyzed. Water sampling and analytical methods have changed during the past 40 yr, and failure to apply the errors associated with those methods places researchers at risk of reporting invalid NO3-N changes. The objectives of this research were to compare analytical procedures, seasonal samplings, and storage methods for well water NO~-N analyses using historical and recent well water data, to identify where changes in NO~N concentration have taken place and possible reasons for the changes; and to determine if age of water, well depth, and NO~-N concentration are related. Benchmark NO~-N analyses were obtained for 46 water wells which were then sampled each season (fail, winter, spring, and summer) over a 2-yr period. For each sampling, four samples were taken from each well; two were frozen immediately (common today) and two were stored at ambient temperature (benchmark procedure). Nitrate-N was determined on subsamples from all four samples using phenoldisulfonic acid (benchmark procedure) and automated Cd reduction (common today). This work suggests that a minimum difference of 6.15 mg NO~-N L-~ is required before declaring significant differences between historical and current well water