Sources of uncertainty in nutrient sampling below a point source

Nutrient samples are often collected below point sources, such as wastewater treatment plants, to ascertain if nutrient quantities exceed water quality management standards. When analyzing these samples, it is imperative that the samples, at the time of analysis, are representative of the samples at the original time of sampling. This study, funded by the Utah Department of Environmental Quality and the iUTAH project, funded by the National Science Foundation, determined what aspects of sampling and sample storage could lead to uncertainty. Sources of uncertainty studied were the locations where the samples were taken to assess if nutrients were adequately mixed within a cross-section, filtration techniques, dilution errors, analytical uncertainty, and storage time. Results obtained using statistical tests such as bootstrapping analyses, one-way ANOVAs, and sample spikes and repeat sample analyses indicate that (a) nutrient mixing patterns appeared to be different for each cross-section tested, (b) filtration with a syringe and a pump differed only in ammonium concentrations, (c) dilutions had the highest uncertainty relative to other treatments, (d) analytical uncertainty proved to be less than uncertainty observed during sample collection and storage, and (e) freezing appeared to be an adequate storage technique