Nitrate removal by laboratory and field continuous-flow zero-valent iron (Fe(0)) packed bed columns was evaluated for different influent water qualities (pH, dissolved oxygen (DO), nitrate concentration) and several months of operation (600-1500 bed volumes (BVs)). In contrast to previous batch experiments with Fe(0) where nitrate was stoichiometrically converted to ammonium, only 70% of the applied nitrogen was recovered as nitrate, ammonium, or nitrite (<0.1mg/L) during shorter-term column tests (2-20 BVs) and less than 25% of the applied nitrogen was recovered during longer-term field testing (500-1000 BVs) at elevated nitrate levels (approximately 25mg N/L). Nitrate removal was accompanied by a pH increase, DO decrease, and soluble iron increase. During longer-term operation (500-1500 BVs) iron and calcium precipitates were observed, by SEM and EDX analyses, to form in the packed columns. Precipitation led to cementation and reduction in permeability for the Fe(0)/sand media in the packed column. Different abiotic and microbial-mediated mechanisms may be involved during shorter- and longer-term operation of Fe(0) systems and the role of iron precipitates should be further evaluated.