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 m aximum contaminant level (MCL; 10 mg L-1), drinking water supplies are continuously sampled and analyzed. Water sampling and analytical meth ods have changed during the past 40 yr, and failure to apply the error s associated with those methods places researchers at risk of reportin g reporting invalid NO3-N changes. The objectives of this research mer e to compare analytical procedures, seasonal samplings, and storage me thods for well water NO3-N analyses using historical and recent well w ater data, to identify where changes in NO3-N concentration have taken place and possible reasons for the changes; and to determine if age o f water, well depth, and NO3-N concentration are related. Benchmark NO 3-N analyses were obtained for 46 water wells which were than sampled each season (fall, winter, spring, and summer) over a 2-yr period. For each sampling, four samples were taken from each well; two were froze n immediately (common today) and two were stored at ambient temperatur e (benchmark procedure). Nitrate-N was determined on subsamples from a ll four samples using phenoldisulfonic acid (benchmark procedure) and automated Cd reduction (common today). This work suggests that a minim um difference of 6.15 mg NO3-N L-1 is required before declaring signif icant differences between historical and current well water NO3-N leve ls.