How Do You Check a Lake Sediment Input for Heavy Metals?
Ask for an accredited lab report on the deposit and layer you will actually buy, stated on dry matter, covering cadmium, hexavalent chromium, mercury, nickel, lead, inorganic arsenic, copper and zinc. Then judge each line against the limit for your product category, not against a general claim that the lake is clean.
"Clean lake" is not a test result.
If you formulate biostimulants, soil improvers or growing media, lake sediment is attractive because it is rich in organic matter. Your auditor and your customers will still ask what else came up with it, and the answer has to be on paper.
Why metals collect in lake sediment
A lake works as a settling tank for its catchment. If you are new to the material, I have written about what sapropel is and how it forms. In Accumulation of Metals in Sapropel, Stankevica, Klavins and Rutina of the University of Latvia (2012) cite earlier work putting more than 99.9% of a water body's metals in the bottom deposit rather than dissolved in the water. Those metals have two origins: weathering rock in the catchment, and human activity such as industrial waste and atmospheric deposition.
The authors studied full sediment profiles from three Latvian lakes, found them not contaminated and called the sapropel prospective material for agriculture. That is good news about those three lakes. It tells you nothing about the next deposit.
Depth changes the answer, in both directions
Cadmium and lead rose in the upper layers of the three profiles, which the authors attribute to human pollution. In Pilveļu Lake lead grew at about 0.5 m depth and fell again at 0.1 m, so a surface sample could miss the peak.
Deeper layers are not automatically cleaner. Nickel reached 35.7 mg/kg at 1.2 to 2.4 m in Pilcines Lake, and zinc reached 198 mg/kg at about 3 m in Pilveļu Lake. The authors put those down to natural rock weathering.
So a single average for a deposit can hide both effects. Ask for results by layer, matched to the depth the supplier actually extracts.
The limits your label has to meet
Those two peaks exceeded the Latvian soil limits the authors compared them with (30 mg/kg nickel, 150 mg/kg zinc). Both sit inside the EU product limits, which shows why the yardstick matters.
Annex I of Regulation (EU) 2019/1009 sets the limits by product function category, in mg/kg of dry matter. Organic soil improvers, plant biostimulants and growing media share the same values for hexavalent chromium (2), mercury (1), nickel (50), lead (120) and inorganic arsenic (40). They differ on the rest:
Organic soil improver (PFC 3(A)): cadmium 2, copper 300, zinc 800.
Plant biostimulant (PFC 6): cadmium 1.5, copper 600, zinc 1,500.
Growing medium (PFC 4): cadmium 1.5, copper 200, zinc 500.
A raw material with 600 mg/kg zinc can pass as a biostimulant and fail as a soil improver or growing medium. Decide which label fits your product before you read the report.
Four traps in a sediment lab report
As received or dry matter. The limits are per kilo of dry matter, and raw sediment is mostly water: moisture ran from 71% to 95% in the Latvian profiles. As plain arithmetic, at 90% water a cadmium reading of 0.15 mg/kg as received is 1.5 mg/kg on dry matter, exactly the biostimulant limit.
Detection limits. A result of "below 5 mg/kg" for cadmium says nothing against a limit of 1.5. The detection limit must sit below the value you need to meet.
Total or speciated. The regulation names hexavalent chromium and inorganic arsenic. A total chromium line does not answer the first. A total arsenic result under 40 does settle the second, because inorganic arsenic cannot exceed the total, but a higher one does not.
Missing lines and small print. Count the eight lines. The Latvian study's heavy-metal comparison covered chromium, cobalt, nickel, copper, zinc, cadmium and lead, which suited its purpose, but a supplier's report must also cover mercury and arsenic. Then read the footnotes: which methods sit outside the lab's accreditation, and whether results apply only to the sample received.
What our own report shows
Our sapropel is dredged 2 to 3 m below the lake bottom under a government licence. LATSERT Test Report 97376 A (2023), on one sample of our solid product, gives on dry matter: cadmium below 1, mercury below 1, nickel below 10, lead below 5, copper 13 and zinc 36 mg/kg, with hexavalent chromium not detected. By the same rules, note that the chromium VI method is outside that lab's accreditation scope and arsenic is not on the report, so ask about both.
The takeaway
Heavy-metal safety in sediment belongs to a deposit, a layer and a batch, not to the word "lake". Ask for dry-matter results on all eight lines, by layer, with detection limits below the limit values, and check them against your own category. If you formulate organic inputs or growing media and want to see a sample with its lab analysis before you commit, let's talk.
Related questions
Does sapropel contain heavy metals? Yes, like any sediment, in amounts that vary by lake and layer; a study of three Latvian lakes found them not contaminated.
Are EU fertilising product limits per wet or dry weight? For these contaminants, per kilo of dry matter.
Which of these three EU categories is strictest on zinc? Growing media, at 500 mg/kg dry matter.
Sources
Regulation (EU) 2019/1009, Annex I, as adopted
LATSERT Laboratory, Test Report No 97376 A, HUMUSON S1, 8 June 2023 (Sapropel Organics document)
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AurimasSoil regeneration / organic fertilizer (Agronomy)28 pieces · 17 followersFollow Aurimas to see new posts in your feed, on your phone or by email.
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