Water sample holding times at appropriate temperature, filtering, and storage conditions for accurate analysis of soluble reactive phosphorus

Abstract

<b><sc>Abstract.</sc></b> Phosphorous (P) loss to surface water contributes to eutrophication and harmful algal blooms and restricts water use for fisheries, recreation, industry, and human consumption. When P becomes soluble it can include small amounts of organic particulates and purely inorganic forms of orthophosphate anion (PO₄^3-), H₂P0₄^-, and HP0₄^2-, depending on soil pH. Sorption-desorption is arguably the most important chemical process governing the retention-release of the orthophosphate, PO₄^3-in the soil solution. Our objective was to investigate the temporal impact of two combinations of sampling-filtering analysis and storage-filtering analysis on the soluble reactive phosphorus (SRP) concentration. Determined SRP concentrations of the temporal sampling-filtering revealed a significant concentration reduction of SRP in water samples filtered and analyzed immediately compared with other water samples taken and analyzed after 24 hours, 7 days, and 14 days. The temporal combination of filtering- storage of water samples taken and analyzed within 24 hours at four successive weeks and re-analyzed after filtering for 28 days revealed a constant concentration of SRP stored in the refrigerator compared to those stored in the freezer, which lost up to 40-60% of their SRP concentrations, although they were filtered. Significant loss, up to 90% of SRP concentration, was recorded in the filtered and non-filtered samples stored in the lab at room ambient temperature. Freezing a water sample will rupture any cell material in the sample releasing P. It can also affect particulate P by the same process. For accurate SRP analysis, we recommend filtering the water sample immediately after collection. Then, store in the refrigerator and analyze within 48 hours after collection. If the water sample is not filtered immediately, SRP concentration will reduce over time due to sorption. When immediate filtering is impractical with automated samplers, retrieve the unfiltered water samples and store them in a refrigerator in the lab. Then, filter and analyze within 48 hours after retrieval, as detailed in the SERA-17 publication.