Explanation for Anomalous Readings during Monitoring of a Best Management Practice

The USEPA’s Urban Watershed Management Branch (UWMB) has monitored storm-water drainage and best management practices (BMPs) as part of its overall research program. As part of this effort, continuous monitoring equipment was deployed to measure both storm events and periods between storms in a BMP. Of particular concern were electrodes used to measure ammonium $\left({\text{NH}}_4^{+}\right)$ concentrations. During the third deployment of the electrode there were readings of concern for ${\text{NH}}_4^{+}$ which exceeded expectations for drift or storm event response. The electrode indicated concentrations of ${\text{NH}}_4^{+}$ exceeded 2 mg/L, with a sustained maximum concentration of 8 mg/L recorded. These observations occurred between December 5, 2003 and December 10, 2003 after snow events. This period was marked by increased measures of conductivity measured which were contrary to typically low conductivity readings especially after precipitation events. Deicers (sodium chloride) spread on the roadway most likely caused increased ${\text{NH}}_4^{+}$ readings, as sodium ions are a known interference for ${\text{NH}}_4^{+}$ . To back up this claim, controlled laboratory experiments were performed on the ${\text{NH}}_4^{+}$ electrode to test the response. Two tests were performed where salt was introduced into a bucket containing a probe. A determination was also made as to whether decaying leaves may have contributed to these elevated ${\text{NH}}_4^{+}$ readings. Results of the tests indicated that at low ammonia concentrations, salt is a measurable interference. Decaying leaves did increase ${\text{NH}}_4^{+}$ concentrations as measured by the electrodes, but only to 1 mg/L which coincided with both electrode readings and sample concentrations from the field.