Environmental effects of copper mine tailings reclamation with biosolids : Field and laboratory experiments

Anaerobically digested biosolids (treated sewage sludge) were applied to copper mine tailings (pH 8.0) in Princeton, B.C. to determine how well biosolids could achieve land reclamation on a site prone to wind erosion in a semi-arid climate (350 mm mean annual precipitation). In October 1992, biosolids at 62, 77 (two plots), and 179 dry tonnes/ha (dt/ha) were applied to 0.5 ha plots. In the first growing season, vegetation established on all plots without irrigation, and the 77 dt/ha treatment led to the best vegetation quality and yield (5500 kg/ha). Trends in the first growing season included: lower foliar Mo concentrations, higher foliar Cu:Mo ratios, decreased soil pH, and increased concentrations of TKN, NH4-N, NO3-N, Total P, Bray P-1, Total Fe, and Total Hg with increasing application rates. Nitrate in the tailings below 60 cm was negligible. Metal concentrations were below the CCME criteria (1991) for agricultural and residential soils except for Cu. Associated with the field trial were laboratory leaching experiments consisting of two runs of 26 columns and one run of 10 pots testing application rates of 0, 30, 100, and 300 dt/ha biosolids. Leaching experiments primarily estimated the magnitude of nitrate leaching, the mineralization rate of biosolids, and the behaviour of metals. The first column run was conducted under wetter conditions than the other trials. Under wetter conditions, the leaching of nitrate, TKN, and TP was minimal. Under dryer conditions, TKN leaching was below 0.6 kg/ha for all columns except col. H (103 kg/ha), and nitrate leaching was less than 0.4 kg/ha for all columns except col. G (123 kg/ha) and col. H (79 kg/ha). The high nitrate concentrations were probably due to a preferential flow. Mineralization rates ranged from 17 to 31 % for the wetter run (10 weeks) and from 29 to 43% for the dryer run (13 weeks). Mineralization was highest for 30 dt/ha treatments. For the 300 dt/ha treatments, soil mineral N ranged from 200 to 745 kg/ha under wetter conditions and from 1200 to 3000 kg N/ha under dryer conditions. Nitrogen losses increased with application rate (30-34% of added N was lost for 300 dt/ha biosolids). Metal concentrations were below the CCME criterion for residential use except for Cu.