Measuring contaminants in biosolids – How much is one part per billion?

Written by: Christine Brown, Field Crops Sustainability Specialist – OMAFRA

Data is thrown at people every day, whether in research results, marketing, or sometimes to manipulate or confuse a message.

Often we hear about contaminants in food or soil or other products such as biosolids measured in percent but also in parts per million (ppm), or in the past decade as testing becomes more advanced, parts per billion (ppb) or even parts per trillion (ppt). Most people can’t visualize what one million or one billion of anything looks like. To put these measurements into perspective these units can be compared as a measure of time.

  • One part per million (ppm) would be equivalent to 1 second in 11.6 days
  • One part per billion (ppb) would be equivalent to 1 second in 31.7 years
  • One part per trillion (ppt) would be equivalent to 1 second in 31,728 years

It is difficult to comprehend one second relative to more than 3 decades when discussing parts per billion, but it does put into perspective the difference in potential impact of materials measured in these units.

Soil composition is complex and when measured the quantity of various components will be reported in a variety of measurements from percent to ppb

When it comes to determining the potential impact of chemicals in biosolids, the Ministry of Environment, Conservation and Parks (MECP) and other institutions conducting similar studies, focus on researching the impacts of application more than setting limits (ppb/ppt) for each specific contaminant. Testing each chemical separately would be costly and time consuming and would not consider chemical interactions. Instead, testing is more often focused on the plant uptake, microbial or other changes to the soils where biosolids are applied and to the crops grown subsequent to application. Nearby water sources are also monitored for impact.


Biosolids have contributed significant nutrients and organic matter to Ontario soils. Considering the decades of research on substances commonly found in our environment, current Ontario Nutrient Management Act standards and regulations for Non-Agricultural Sourced Materials (NASM) provide the best approach for safe application of biosolids.

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