Comparison of high frequency, in-situ water quality analysers and sensors with conventional water sample collection and laboratory analyses: phosphorus and nitrogen species

Abstract. The long-term collection of water samples for water quality analysis with high precision laboratory instrumentation is routine in monitoring programmes however, such sampling is labour intensive, expensive, and therefore undertaken at a low temporal frequency. Advances in environmental monitoring technology however, mean that it is now possible to collect high temporal frequency measurements for a wide range of water quality parameters without the need for the physical collection of a sample. The downside to this approach is that the data can be subject to more noise, due to environmental and instrument variables. This raises the question of whether high frequency, lower precision data are better than low frequency, higher precision data. This study reports the findings of an investigation of agricultural land drainage comparing measurements of total phosphorus (TP), total reactive phosphorus (TRP), ammonium (NH₄-N) and total oxidised inorganic nitrogen (NO_x-N) collected using both equipment in situ and concurrent water samples analysed in the laboratory. Results show that both in situ PHOSPHAX TP and NITRATAX NO_x-N instruments can provide comparable data to that measured using samples analysed in the laboratory; however, at high discharge and low NO_x-N concentrations, the NITRATAX can under report the concentration. In contrast, PHOSPHAX TRP and YSI sonde NH₄-N data were both found to be incomparable to the laboratory data. This was because concentrations of both parameters were well below the instruments accurately determinable level, and because the laboratory data at low concentrations were noisy.