W6 AM Evaluation of Groundwater Communication with Surface Water and Contributions to Sediment Toxicity
Wednesday, 16 November 2005: 8:00 AM - 11:40 AM in 324-326

(MCL-1122-328641) Application of Diffusion Samplers to Assess Changes in Ammonia Concentrations in Groundwater Discharging to a River.

McLinn, E¹, ¹ RMT, Inc.

ABSTRACT- Ammonia is common in groundwater affected by municipal landfills. Ammonia can be of concern in settings where affected groundwater discharges to surface water bodies, because it is toxic to aquatic animals. A Midwestern municipal landfill occupies 1,300 m of river frontage. We monitored groundwater with wells installed near the landfill's edge, about 30 m from the river, and with diffusion samplers in the bed of the river. The diffusion samplers were designed to collect pore water samples at four discrete vertical intervals along the last 50 cm of the groundwater flow path, just prior to discharging to the river. Geometric mean ammonia concentrations in groundwater affected by the landfill decreased from 9 mg/L, 35 cm below the sediment-water interface, to 1 mg/L, 5 cm below the interface. Ammonia concentrations in the wells (14 mg/L) were similar to those in the deep interval of the diffusion samplers. We expected the ammonia to be converted to nitrate as the groundwater migrated through the hyporheic zone, and nitrate concentrations measured in the pore water increased by an order of magnitude along this same flow path; however, the nitrate concentration increase was proportionately much less than the ammonia concentration decrease. This suggests that only a small portion of the ammonia was converted to nitrate; the remainder of the ammonia may have formed biomass in the river sediments, or may have transformed to nitrous oxide or nitrogen gas. Monitoring wells located a few tens of meters upgradient from the discharge zone yielded substantially different ammonia results than diffusion samplers installed in the riverbed at this site. For risk characterization of groundwater discharging to surface water, sampling approaches should allow for collection of minimally disturbed, high-resolution samples over the last portion of the groundwater flow path prior to discharge to the surface water body.

Key words: Groundwater-surface water interactions; ammonia; diffusion samp