Estimation of Measurement Uncertainty in Sampling of Cultivated Fields at Former Tree Nurseries

Under a framework agreement with the Geological Survey of Sweden (SGU), Kemakta conducted an environmental soil investigation at a former tree nursery. The primary objective was to propose a sampling strategy that could be generally applicable for investigating former cultivation fields within the closed tree nurseries being examined by SGU.

 

Background and Assignment

 

The investigated field is one of approximately 40 identified former tree nurseries that SGU is responsible for inventorying, investigating, and, if necessary, remediating. Previous investigations have shown that large areas of low to moderately contaminated soil exist at these former cultivation fields due to pesticide use. Since DDT was likely applied uniformly across a field, a former cultivation field can be viewed as a single sampling unit for which a representative concentration needs to be determined. In this assignment, three different sampling strategies were used to collect soil samples from the fields: the ISM strategy, the postage stamp strategy, and the test pit strategy. The latter two strategies have been used in previous investigations but never on the same site. Therefore, the sampling method that provides the most representative concentration of DDT in the soil has not been previously evaluated. Obtaining a representative concentration of contamination in the soil is crucial for making correct assessments regarding the risks associated with the contamination and how any future remediation measures should be monitored. Uncertainties exist throughout the process—from sampling to the laboratory analysis report.

 

The ISM strategy involves systematic, random sampling with the collection of 30 increments (small soil samples) across the entire cultivation field using a soil auger. In the postage stamp strategy, these 30 increments were collected within a square area of 1,000 m² (a “postage stamp”), randomly placed on the field. Sampling was also conducted with a soil auger in the postage stamp strategy.

 

The test pit strategy aimed to dig 4 test pits on the field and collect 10 increments per pit, which were then combined into a composite sample for laboratory analysis.

 

The work involved two stages of fieldwork in the form of customized certified sampling (according to Nordtest Envir 08), compilation and data analysis, as well as statistical simulation.

 

Results

 

The ISM strategy (using a soil auger) was deemed the most suitable sampling strategy, as it resulted in the lowest variability in contamination levels between three repeated samplings and provided the best coverage across the entire cultivation field. The statistical simulation also showed that the test pit strategy was a reasonable alternative. The test pit strategy was also the only sampling method where the plowing depth was clearly visible at all points, thereby ensuring the correct sampling depth (see figure below).

 

The postage stamp strategy was excluded due to high variability in contamination levels between the three repeated samplings, likely due to significant heterogeneity.

 

Analytical uncertainties were considered minor compared to other measurement uncertainties, i.e., uncertainties due to heterogeneity in contamination levels and sampling uncertainties.

 

The work has been presented in a report (Konsultrapport 10, SGU). The chosen sampling strategy and proposed quality controls will be used as the basis for future investigations of former cultivation fields at tree nurseries conducted under SGU’s direction.

 

Link to the published report (in Swedish):

Undersökningar (sgu.se)

Uppskattning av mätosäkerhet vid provtagning av odlingsfält vid f.d. skogsplantskolor (sgu.se)

 

Schematic illustration of how soil sampling at the former cultivation fields is conducted using two different sampling methods: test pit digging (4 test pits, 10 increments per pit) and the use of a soil auger (30 increments within the entire cultivation field or a smaller area). The sum of concentrations of DDT, DDD, and DDE is significantly higher above the plough depth. When using the test pit method, the plough depth is noted in the pit, ensuring that the increments are taken from above the plough depth. When using the soil auger, this cannot be guaranteed, as the plough depth cannot be visually observed in all samples taken.

Contact

Rickard Wennström

rickard@kemakta.se
+46-8-617 67 18

Provtagning mätosäkerheter