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A new alternative septic‐system design is presented utilizing reactive porous media barriers for passive in situ attenuation of NO3−. The reactive material consists of solid organic carbon (sawdust) which promotes NO3‐ attenuation by heterotrophic denitrification. Four field trials are discussed demonstrating two barrier configurations: as a horizontal layer positioned in the vadose zone below a conventional septic‐system infiltration bed and as a vertical wall intercepting a horizontally flowing downgradient plume. During one year of operation both barrier configurations have been successful in substantial attenuation (60 to 100%) of input NO3‐ levels of up to 125 mg/1 as N. The horizontal layer configuration can be readily installed during the construction of new infiltration beds, whereas the vertical wall configuration may be more appropriate for retrofitting existing septic systems where NO3‐ contamination has already occurred. The layer configuration allows the flexibility of constructing the barrier in the vadose zone by using coarse silt or fine sand matrix material that has the ability to remain tension‐saturated, and thus anaerobic, even when positioned above the water table. Advantages of the barrier system are that it is simple to construct, no surface structures or additional plumbing are necessary, and treatment is passive requiring no energy consumption and little or no maintenance. Mass balance calculations and preliminary results suggest that conveniently sized barriers have the potential to last for decades without replenishment of the reactive material.

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On-site disposal of sewage in septic systems can lead to groundwater plumes with NO(3)(-)-N concentrations exceeding the common drinking water limit of 10 mg/L. Currently, denitrification is considered as the principal natural attenuation process. However, at a large seasonal-use septic system in Ontario (256 campsites), a suboxic zone exists where nitrogen removal of up to 80% occurs including removal of NH(4)(+)-N. This zone has both NO(3)(-)-N and NH(4)(+)-N at >5 mg/L each. In the distal NH(4)(+)-rich zone, NH(4)(+)-N concentrations (8.1 ± 8.0 mg/L) are lower than in the proximal zone (48 ± 36 mg/L) and NH(4)(+)-N is isotopically enriched (concentration-weighted mean δ(15)N of +15.7‰) compared to the proximal zone (+7.8‰). Furthermore, δ(15)N-NH(4)(+) isotopic enrichment increases with depth in the distal zone, which is opposite to what would result if nitrification along the water table zone was the mechanism causing NH(4)(+) depletion. Bacterial community composition was assessed with molecular (DNA-based) analysis and demonstrated that groundwater bacterial populations were predominantly composed of bacteria from two Candidatus genera of the Planctomycetales (Brocadia and Jettenia). Together, these data provide strong evidence that anaerobic ammonium oxidation (anammox) plays an important role in nitrogen attenuation at this site.

The detailed distribution of tritium (3H) in the recharge area of a shallow unconfined sand aquifer near Sturgeon Falls, Ontario, is described. At this forested, shallow water table site, bomb tritium has penetrated uniformly to a depth of 8–12 m indicating recharge of 15 cm/year, which is 16% of precipitation. The zone of bomb tritium contains 3H concentrations of from 16 to 269 tritium unit (TU), whereas much lower values (<1 TU) are observed in groundwaters recharged only a few years prior to 1953, the year when significant fallout of bomb tritium began. Tritium distribution is accurately simulated, using one‐ and two‐dimensional models, when the Ottawa record is used for the post‐1953 tritium input function, when dispersion is low and when a prebomb tritium input value of 3 TU is used. This prebomb value is slightly lower than that suggested by Kaufman and Libby (1954), based on precipitation and surface water samples from the early 1950s. The simulations indicate and field data corroborate, that in 1986, 3H levels of more than 100 TU occur only in groundwaters recharged between 1957 and 1971, while levels in excess of 200 TU occur only in groundwaters recharged during 1961–1967. The 1960s tritium peak is observed within a narrow distinct depth zone at all locations along the flow section investigated.