Analytical Results, Findings, and Recommendations
of the 1995 OMAFRA Sewage Biosolids Field Survey
Interpretation of Analytical Results
pH Levels
The mean pH of the treated soils (7.1) was marginally lower than
the mean pH for the non-treated soils (7.2). However, based on statistical
analysis of the analytical results, the addition of sewage biosolids
to the soils in the treated plots has had no significant effect on
soil pH. This finding is consistent with other reported findings when
sewage biosolids were applied to agricultural soils (Gestring, 1982;
Soon, 1978).
For only those sites where woodlot soils were also sampled, the mean
pH levels for the treated, non-treated and woodlots soils were 6.8,
6.9, and 6.4, respectively. None of the means were significantly different.
These findings indicate that, for those sites, agricultural use has
not had a significant effect on soil pH.
Phosphorus Levels
The mean phosphorus level for the treated soils at 32 sites (29.2
ppm) was significantly different from the mean level for the non-treated
soils (9.4 PPM). This finding indicates that the addition of sewage
biosolids to the treated soils has had a significant positive effect
on the amount of available phosphorus. The mean level of 29.2 PPM
of phosphorus in the treated soils is not excessive. Phosphorus levels
of 50-60 PPM are necessary in order to successfully grow some agricultural
crops (OMAFRA, 1995). It would appear from these findings that municipal
sewage biosolids are an excellent source of phosphorus. It would also
appear that, through controlled application rates and crop utilization,
available soil phosphorus is being maintained at acceptable levels.
At 29 of the 36 sites, the average available phosphorus level of the
treated soil was higher than the average level for the non-treated
soil.
Metal Levels
The mean arsenic and zinc levels for the treated soils were lower
than the mean levels for the non-treated soils. Excluding molybdenum,
the mean levels for the other 8 metals were all higher in the treated
soils than in the non-treated soils. The mean levels for all metals
were all well below the maximum permissible levels. Mean levels for
molybdenum for both the treated and non-treated soils were not determined
because the concentrations in all soil samples were below the method
detection limit of 2.5 PPM The mean levels for both treatments would
be less than 2.5 PPM The maximum permissible level for molybdenum
is 4 PPM
Chromium, copper and mercury levels in the treated soils were significantly
elevated, but the resulting levels were well below the maximum permissible
levels. Excluding molybdenum, the mean levels of the other 7 metals
for the treated soils were not significantly different from the mean
levels for the non-treated soils. These findings indicate that the
application of sewage biosolids to the treated soils has had a significant
positive effect on chromium, copper and mercury levels. It has had
no significant effect on the levels of the other 7 metals.
The mean cobalt, selenium and zinc levels for the non-treated soils
were higher than the mean levels for those metals in uncontaminated
Ontario soils, as given in Column 2 of Table
1, and listed in the current guidelines. However, the mean levels
were equal to or lower than the mean levels for "Rural Parkland"
soils as reported in the Ontario Typical Range of Chemical Parameters
in Soil, Vegetation, Moss Bags and Snow (MOEE, 1993).
Comparison to the Current Guidelines
Three of the 108 transect samples taken in the treated plots had
1 metal level which was higher than the maximum permissible level
shown in Table 1,
and also reported in the current guidelines. Treated site BL01 was
the only site with an average metal level (nickel, 36.33 PPM) which
was higher than the permissible level (32 PPM). However, the individual
transect sample levels for nickel were 85 PPM, 12 PPM, and 12 PPM
In light of the two low levels, the 85 PPM level was considered abnormally
high and was likely due to contamination during sampling. The sample
was not retested due to time constraints.
The soils in 31 of the 36 treated plots had at least 1 average metal
level which was lower than the maximum permissible level, but was
higher than the average level in the soils in the comparison non-treated
plot. Thirty-eight of 396 average levels determined for all metals
in the treated soils exceeded 50% of the maximum permissible level.
Twenty-three of those levels were higher than the average levels in
the comparison non-treated soils.
Excluding molybdenum, the mean levels for the other 10 metals in
the treated soils were all well below the maximum permissible levels
specified in the guidelines. Although the mean level for molybdenum
could not be determined, it would be less than 2.5 PPM (the MDL),
which is below the maximum permissible level of 4 PPM The mean levels
for all metals in the treated soils are given in Table
5.
Except for cobalt, molybdenum, selenium and zinc, the mean levels
of the other 7 metals for the non-treated soils were lower than the
mean levels for uncontaminated Ontario soils, as given in Column 2
of Table 1, and reported
in the current guidelines.
