Overview
Son and colleagues measured Cu, Zn, and lead (Pb) in river sediments upstream of the Hapcheon-Changnyeong weir on the Nakdong River in South Korea. The study combines aqua regia extraction, Tessier five-step sequential extraction, and sediment-quality comparisons to evaluate whether the weir promotes metal accumulation. This is primary environmental sediment evidence, not food, ingredient, drinking-water, or consumer-product occurrence evidence.
Key numbers
Sampling frame and field conditions
Sediment samples were collected from two locations north of the weir: ND1 at 1 km from the weir and ND2 at 5 km from the weir. Sampling dates were 23 July 2019, 13 August 2019, and 19 September 2019. The top 2 cm of sediment from acrylic cores was dried, sieved to 0.5 mm, and analyzed in duplicate.
The Hapcheon-Changnyeong weir is 328 m long and 11.5 m high. Average retention times for the 28 days before sampling were 2.25 days in July, 2.31 days in August, and 2.75 days in September. Water-column pH ranged from 7.32 to 9.01 and dissolved oxygen stayed above 6 mg/L.
Table 2 field characteristics:
| Site/month | Depth m | Sand % | Silt % | Clay % | TOC % |
|---|---|---|---|---|---|
| ND1 July | 4.0 | 92.40 | 6.12 | 1.48 | 2.80 |
| ND1 August | 6.0 | 68.21 | 26.76 | 5.03 | 0.23 |
| ND1 September | 5.1 | 17.86 | 63.67 | 18.47 | 3.57 |
| ND2 July | 6.8 | 94.09 | 4.69 | 1.22 | 0.11 |
| ND2 August | 6.7 | 95.79 | 3.72 | 0.49 | 0.19 |
| ND2 September | 6.8 | 92.58 | 6.00 | 1.42 | 0.15 |
Aqua regia sediment concentrations
The study’s overall aqua regia concentration ranges were:
| Metal | Concentration range |
|---|---|
| Cu | 19.90-59.84 mg/kg |
| Zn | 35.85-255.6 mg/kg |
| lead (Pb) | 4.6-33.05 mg/kg |
Concentrations were higher at ND1, closer to the weir, than at ND2. ND2/ND1 concentration ratios were:
| Month | Cu ratio | Zn ratio | Pb ratio |
|---|---|---|---|
| July | 43.71% | 17.70% | 19.93% |
| August | 88.32% | 97.27% | 95.83% |
| September | 38.97% | 15.74% | 14.37% |
The authors interpret the ND1 elevation as evidence that slower flow near the weir promotes sediment and metal accumulation.
Pre-weir comparison values cited in the paper
Table 3 summarizes earlier Nakdong River sediment values before the Hapcheon-Changnyeong weir construction:
| Earlier sample frame | Cu mg/kg | Zn mg/kg | Pb mg/kg |
|---|---|---|---|
| Stagnant-flow sample, 2005 | 15.24 | 80.44 | 33.47 |
| Sampled before 2000 | 6.42 | 34.77 | 9.87 |
| Sampled in 1997 | 19.38 | not reported | 22.50 |
The PDF labels the second-row lead entry as Pd; this page treats that as an apparent table typo because the surrounding text and column are Pb.
Sequential extraction and mobility
The Tessier five-step sequential extraction separated exchangeable, carbonate-bound, iron-manganese oxide-bound, sulfide/organic-bound, and residual fractions. Pb fraction results were not presented because some fractions were below detection limits.
For Cu and Zn, total concentrations across sequential fractions were:
| Metal | Sequential-extraction total range | Comparison with aqua regia |
|---|---|---|
| Cu | 17.90-59.83 mg/kg | 110.35% of aqua regia total |
| Zn | 40.50-251.90 mg/kg | 133.12% of aqua regia total |
Linear regression between aqua regia and sequential totals had slopes of 0.92 for Cu and 1.04 for Zn, with R2 values of 0.95 and 0.96, respectively.
Cu was concentrated mainly in the residual fraction:
| Cu endpoint | Source-reported value |
|---|---|
| Residual Cu concentration | 7.93-26.1 mg/kg |
| Residual Cu share of total Cu | 33.6-63.5% |
| Exchangeable Cu share | 1.46-8.10% |
| Exchangeable + carbonate Cu share | 10.65-22.16% |
Zn was concentrated mainly in the iron-manganese oxide fraction:
| Zn endpoint | Source-reported value |
|---|---|
| Iron-manganese oxide-bound Zn concentration | 16.10-117.05 mg/kg |
| Iron-manganese oxide-bound Zn share | 38.9-56.4% |
| Exchangeable Zn share | 0.62-16.30% |
| Exchangeable + carbonate Zn share | 18.44-30.99% |
The authors caution that iron-manganese oxide-bound Zn may leach back to the water body under anaerobic conditions, especially in stagnant periods.
Risk and sediment-quality comparisons
The risk assessment code (RAC) classified Cu around the weir as medium risk. Zn was medium risk except for the August ND2 sample, which was high risk.
Against Korea NIER sediment criteria, Zn and Pb were grade 1, while Cu ranged from grade 1 to grade 2. Cu reached grade 2 in the July and September ND1 samples. Against Canadian sediment quality guidelines, all Cu concentrations exceeded the Lowest Effect Level, as did Zn in July and September at ND1 and Pb in September at ND1. Under US EPA comparisons, several Cu and Pb samples exceeded non-polluting standards.
Methods (brief)
Sediment cores were collected by an experienced diver using an acrylic core (5 cm diameter, 22 cm height), transported in an icebox at 4 degrees C, dried to constant weight (top 2 cm), and sieved through a 0.5 mm sieve (Section 2.2). In situ water-column pH and dissolved oxygen were measured with a multiparameter probe (YSI Professional Plus, USA). Total organic carbon of freeze-dried sediment was measured by elemental analyzer (EA 1110, FV Instruments, UK) after carbonate removal with 1.0 N HCl. Particle size was measured by laser diffraction (Mastersizer 2000, Malvern, UK) after organic and carbonate removal with 10% H2O2 and 0.1 N HCl (Section 2.3).
Aqua regia extraction (Section 2.4) used 1 g dried sediment with 10 mL aqua regia (HCl:HNO3, 3:1), heated first at 40 degrees C for 1 h and then at 140 degrees C for 4 h. Each sample was duplicated. Extracts were vacuum-filtered and diluted to 100 mL.
Sequential extraction used the modified Tessier method (Section 2.5): exchangeable ions in 1 M MgCl2 adjusted to pH 7 with CH3COOH for 1 h at room temperature; carbonates in 1 M CH3COONa adjusted to pH 5 with CH3COOH for 5 h at room temperature; iron-manganese oxides in 0.04 M NH2OH.HCl in 25% CH3COOH at 95 degrees C; sulfides and organics in 0.02 M HNO3 + 30% H2O2 at 85 degrees C followed by 3.2 M CH3COONH4 in 20% HNO3 at room temperature; residuals in 10 mL aqua regia at 40 degrees C for 1 h then 140 degrees C for 4 h. The fifth step used aqua regia rather than perchloric or hydrofluoric acid to enable direct comparison with the aqua regia totals. Effluents were filtered through Whatman 40 paper, diluted to 100 mL, and analyzed by the same ICP-OES instrument as the aqua regia totals.
Extracted Cu, Zn, and Pb were measured by inductively coupled plasma optical emission spectrometry (ICP-OES; ICP-5800, Agilent, USA) at the National Instrumentation Center for Environmental Management (NICEM), Seoul National University. Calibration used reference solution from Inorganic Ventures. Detection limits were 0.001 mg/L for Cu, 0.001 mg/L for Zn, and 0.003 mg/L for Pb; calibration R2 values were 0.999 for all three metals. The metals reported are total Cu, total Zn, and total Pb; no isotopic or oxidation-state speciation was performed.
The Risk Assessment Code (RAC, Section 2.6) was calculated as the ratio of (exchangeable + carbonate fractions) to total metal, expressed as percent; the five-class scheme defines no risk (RAC ⇐ 1), light risk (1 < RAC ⇐ 10), medium risk (10 < RAC ⇐ 30), high risk (30 < RAC ⇐ 50), and very high risk (RAC > 50).
Implications
Certification: Do not use this source in HMTc product or ingredient occurrence pools. It measures river sediment metals, not edible fish, foods, drinking water, or products.
App: Useful as environmental-pathway context for how weirs, retention time, sediment grain size, and organic carbon can influence metal accumulation and remobilization in river systems used for water resources.
Courses: Useful for teaching why total sediment concentration, sequential extraction fraction, RAC class, and aquatic-life sediment guideline status are different evidence types.
Related evidence
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Update history
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