Evaluation of preservation methods for nutrient species collected by automatic samplers

Automatic samplers are a common method of data collection for numerous monitoring projects in the South Florida region and elsewhere. Although total phosphorus (TP) is the primary parameter of interest within this region, nitrogen species such as ammonia nitrogen (NH4-N), nitrate+nitrite nitrogen (NOx-N), and total kjeldahl nitrogen (TKN) are also collected and analyzed. Federal and state quality assurance guidelines require nutrient samples to be preserved by acidification with H2SO4 to a pH < 2 and stored immediately at 4 degrees C. However, the remoteness of many sampling locations in South Florida makes it difficult to supply electricity for the refrigeration of samples collected by autosamplers. In addition, the use of propane-powered refrigerated autosamplers is a costly and ineffective solution in the South Florida climate. Consequently, while samples collected at these remote locations are routinely pre-preserved with acid, they are not cooled to 4 degrees C for a period from one to seven days. This study evaluated if a statistically significant difference (approximately = 0.05) existed between concentrations of nitrogen species from a common source sample that was either: processed immediately; refrigerated to 4 degrees C for seven days; or not refrigerated for seven days. In all cases, the collected sample was pre-preserved by adding 1 mL of 50% H2SO4 to each 1 L discrete sample container before each 7 day testing period. Differences in concentrations of the calculated parameter total nitrogen (TN) were also investigated. Analyses using the Wilcoxon Signed-Rank Test showed no significant differences among the three treatment groups for NOx-N, TKN, TN and TP. Significant differences were observed when the NH4-N samples that were processed immediately were paired with NH4-N samples that were left unrefrigerated or refrigerated for seven days. Information from this study can be used by researchers and managers in evaluating the usefulness of nutrient water quality data that is collected when sample refrigeration is not available.