Application of passive sampling for soluble nitrogen and phosphorous in agricultural watersheds

Monitoring nitrogen and phosphorous is crucial to sustainable planning and management of water resources from both agro-economic and environmental perspectives. Grab sampling, a typical nutrient monitoring method, provides a concentration of pollutant at a certain point of time and may not be representative of the average concentration despite their use as averages to compute loadings in water resources modeling and planning. Conversely, continuous monitoring requires extensive labor, and higher capital, operation and maintenance cost. An alternative method that is low in maintenance, cost and labor, potentially more accurate than grab sampling, and less labor-intensive than continuous monitoring is passive sampling. The relatively new passive sampler, "sorbi cells," measures average concentration of pollutants over a period of time (weeks to months). We evaluated the performance of a passive sampler for nitrogen and phosphorous in field conditions and compared the results to grab sampling and continuous monitoring results. Sorbi cells were deployed to monitor nitrate and phosphate in drainage streams in the Lower St-Francis Basin in Poinsett County and Little River Ditches Basin in Mississippi County in NE Arkansas. Sorbi cells were sampled weekly during May-September 2015. Soluble nitrate and phosphate were analyzed in the laboratory. The results were compared to that from weekly grab sampling and continuous measurements for nitrate, and grab sampling for phosphate at the same locations. The results were promising for phosphate as it recorded concentrations in the same order despite slightly higher readings than grab sampling and showed temporal variation in phosphate concentration that was missed by grab sampling. However, for soluble nitrate and nitrite, the concentrations were either undetected or too low as compared to grab sampling and continuous sampling. Research in progress is expected to further the application of the cells to monitor nutrient loads in the water resources.