Evaluation of intra-annual variation in U.S. Geological Survey National Water Quality Assessment ground water quality data.

Assessment of ground-water quality trends under the U.S. Geological Survey National Water-Quality Assessment Program (NAWQA) included the analysis of samples collected on a quarterly basis for 1 yr between 2001 and 2005. The purpose of this quarterly sampling was to test the hypothesis that variations in the concentration of water-quality parameters of selected individual wells could demonstrate that the intra-annual variation was greater or less than the decadal changes observed for a trend network. Evaluation of more than 100 wells over this period indicates that 1 yr of quarterly sampling is not adequate to address the issue of intra-annual variation because variations seem to be random and highly variable between different wells in the same networks and among networks located in different geographical areas of the USA. In addition, the data from only 1 yr makes it impossible to assess whether variations are due to univariate changes caused by land use changes, hydrologic variations due to variable recharge, or variations caused by ground-water pumping. These data indicate that funds allocated to this activity can be directed to the collection of more effective trend data, including age dating of all wells in the NAWQA network using multiple techniques. Continued evaluation of data and updating of monitoring plans of the NAWQA program is important for maintaining relevance to national goals and scientific objectives.