ISSN:
1745-6584
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Energy, Environment Protection, Nuclear Power Engineering
,
Geosciences
Notes:
Phosphate distribution was reviewed in 10 mature, highly monitored septic system ground water plumes in central Canada. It was shown that six plumes (primarily those on calcareous sands) of enriched P concentrations (0.5 to 5 mg/L P) exceeding 10 m in length are present. In each case, phosphate migration velocity is highly retarded (retardation factor, 20 to 100) compared to the ground water velocity, hut migration rates remain sufficiently fast (−1 m/a) to the of concern when considering long-term operation and the normal setback distance of septic systems from adjacent surface water bodies (∼ 15 m). Much smaller scale phosphate plumes (〈 3 m in length) are present at the acidic sites on noncalcareous sands and on silt- and clay-rich sediments.At all of the sites, ground water concentrations are lower than effluent values by amounts ranging from 23 to 99%, suggesting that P accumulation has occurred in the vadose zone. This was confirmed by sediment analyses at four of the sites which, in each case, showed that zones of Penrichment were present within 1 m of the infiltration pipes (Wood 1993; Zanini et al. 1998). Also, observed phosphate concentrations are generally consistent with values expected based on the solubility constraints of the minerals vivianite in reducing zones (including the septic tank), and strengite and variscite in oxidizing zones, providing further evidence that min-eral precipitation reactions play a role in limiting P concentrations. Strengite and variscite have the potential to limit P to low con-centrations (co.1 mg/L) under acidic conditions, but oxidation of sewage efiluent leads to acidic conditions only in noncalcareous terrain or beneath old septic systems where calcium carbonate has been depleted. Overall, phosphate plume migration velocities in ground water appear to be controlled by sorption processes, but the phosphate concentrations that are present in the plumes appear to be strongly controlled by mineral precipitation reactions that occur in close proximity to the infiltration pipes.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1745-6584.1998.tb02107.x
