Halls Brook (eastern Massachusetts, USA) is a significant source of total dissolved ammonia (sum of NH3 and ; (NH3)T) to the Aberjona River, a water body listed for NH3 impairment on the Clean Water Act §303(d) list. We hypothesized (1) that (NH3)T in Halls Brook derived from a hazardous waste site via groundwater discharging to a two-basin pond that feeds the brook; and (2) that transport of (NH3)T to the Aberjona River was controlled by lacustrine and wetland processes. To test these hypotheses we measured (NH3)T levels in the brook, the pond, and a wetlands directly downstream of the pond during both dry and wet weather over a ten month period. In addition, we analyzed sediment cores and nitrogen isotopes, and performed mass balance calculations. Groundwater discharge from beneath the hazardous waste site was the major source of (NH3)T (20–67 kg d?1) and salinity to the north basin of the pond. The salty bottom waters of the north basin were anoxic on all sampling dates, and exhibited relatively stable (NH3)T concentrations between 200 and 600 mg N l?1. These levels were >100-times higher than typical background levels, and 8–24-times above the acute effects level for (NH3)T toxicity. Bottom waters from the north basin continuously spill over into the south basin contributing 50% of the (NH3)T load entering this basin. The remainder comes from Halls Brook, which receives (NH3)T loadings from as yet unknown sources upstream. During storm events up to 50% of the mass of (NH3)T was flushed from the south basin and into the wetlands. The wetlands acted as a (NH3)T sink in dry weather in the growing season and a discharge-dependent (NH3)T source to the Aberjona River during rainstorms.
