This is the question! And the answer is (almost) anywhere!
Where is natural soil salinity on my site, and what is the magnitude of concentrations? These questions are asked all the time. In general, consultants go to an area upgradient of the site, away from known impacts, poke a hole in the ground and call it background. Although this is true, that location represents background soil salinity. However, it is not the ONLY background location available. Background samples with natural soil salinity can be in low lying areas, elevated hilltops or sometimes… right in the middle of the site!
With the right samples collected and the right approach to analyzing salinity data, natural soil salinity can be found anywhere and properly distinguished from anthropogenic salinity. The analysis can be completed with low natural salinity conditions and elevated natural salinity conditions that may even exceed onsite salinity impacts. The prairies are known to have high naturally saline/sodic soils; one just requires the right tools to find it.
What are the right tools?1. Starting tool - Collecting the right samples in all possible locations.
Samples should be collected from multiple areas around the site that are thought to be free from impacts. These can be upstream of the site if confident anthropogenic salinity hasn’t migrated there. They can be in low lying, unvegetated areas or lush green, healthy areas. They can be collected from highly sandy stratigraphy's to pure clay and anywhere in between. The point is to collect salinity samples from all sorts of various spots and not only one from each, but multiple. If only one sample is collected from sand, and one from clay, and one from the hilltop and one from the low-lying area, it is impossible to find trends. Multiple soil samples should be collected from each unique location.
2. The next tool - Laboratory analysis.
The full suite of salinity should be analyzed at an accredited laboratory. This is important. The right tools require chloride concentrations and all of the anions and cations (including bicarbonate and carbonate), EC, and SAR. When only partial salinity analysis is completed, it leaves a hole in the dataset and potentially a lot of unanswered questions. There have been times when only chloride, sulphate, calcium, potassium and magnesium were analyzed; however, the ion balance wasn’t adding up. These times indicate that bicarbonate might play an essential role in the soil salinity profile. It’s always easier, especially when sampling at a site the first time, to analyze the samples for the full salinity, instead of encountering questions and having to go back and sample more or reanalyze old samples.
3. The last tool - Statistical analysis.
This is where everything comes together. Simple correlation graphs and guideline comparisons are insufficient to prove that the well-positioned and thoroughly analyzed samples are natural—especially when dealing with concentrations elevated above guidelines. To prove elevated concentrations are natural, there must be multiple lines of evidence to show how the natural soils are different from the impacted soils. Some of the statistical analysis toolbox include multivariate analysis, like principal component analysis and hierarchical cluster analysis, and simple fingerprinting radar plots and correlation graphs. Altogether Chemistry Matters uses a mix of four analyses to complete the process. It is expected that all four analyses agree with each other to call samples natural or anthropogenic; otherwise, samples are flagged and investigated further. Additional investigation might include more drilling for additional samples or reanalysis of the sample to rule out laboratory error. At the end of the process, and the site is thoroughly analyzed, everyone can rest easy knowing what areas onsite represent background salinity and the natural range of concentrations.
Make the Call
Breaking the traditional ‘upgradient = background’ thought process, this investigative procedure can help create site-specific guidelines and reduce the perceived salinity impacts on site by finding background samples all over the site! This process has been used on many sites with success showing salinity concentrations elevated well above suggested guidelines. If you have any sites that have elevated salinity parameters, don’t hesitate to call Chemistry Matters.