Marine Sediment Testing

Marine sediment testing is the analytical evaluation of seafloor deposits to characterize their physical, chemical, and biological condition. Sediments act as long-term sinks for contaminants discharged from rivers, ports, shipping, and industrial outfalls, so their quality reflects both historical and ongoing pollution pressure. A complete testing program typically covers heavy metals, organic pollutants, grain size distribution, and nutrient content, supported by rigorous sampling and quality-control procedures. The results are used to judge dredged-material disposal options, assess ecological risk, monitor remedial outcomes, and demonstrate compliance with environmental protection requirements applicable to various types of marine works.

sediment testing — scope and parameters

The scope of a marine sediment investigation is defined before fieldwork, because parameter selection determines sampling design, container type, and preservation. Core parameter groups include heavy metals such as copper, lead, zinc, cadmium, chromium, mercury, and arsenic; organic contaminants such as polycyclic aromatic hydrocarbons, organochlorine pesticides, polychlorinated biphenyls, and petroleum hydrocarbons; nutrients including total nitrogen, total phosphorus, and organic carbon; and physical descriptors such as grain size, moisture, and redox potential. Programme objectives guide the choice: dredging projects emphasize contaminant screening against disposal criteria, whereas ecological studies add biological indices. Depth-integrated and layered sampling are both used, depending on whether surface conditions or historical deposition is of interest.

Sample collection and pretreatment

Representative sampling is the controlling step of the whole workflow. Surface sediments are commonly collected with a grab sampler, while undisturbed vertical profiles require a gravity or box corer. Contamination control measures are essential: sampling tools should be of non-metallic or stainless material where metals are targeted, and samples for organic analysis must be stored in pre-cleaned glass jars with PTFE-lined caps, chilled and shielded from light. Grain-size and nutrient aliquots are kept separate. In the laboratory, samples are homogenized, sieved to remove shells and debris, and freeze-dried or air-dried according to the analyte. For metals, dried sediment is ground and digested with acid mixtures such as nitric-hydrofluoric-perchloric acid under heated or microwave conditions. Organic extracts are prepared by accelerated solvent extraction or ultrasonic extraction, followed by clean-up through silica or florisil columns.

Heavy metal analysis by ICP-MS and AAS

inductively coupled plasma mass spectrometry (ICP-MS) is the primary technique for multi-element determination because it delivers simultaneous quantification of many metals at low detection limits with wide linear range. Digested solutions are nebulized into an argon plasma, and ions are separated by mass-to-charge ratio; internal standards correct for matrix drift and carrier suppression. Interference management is a routine concern — polyatomic ions are addressed through collision-reaction cell modes or isotope selection. Atomic absorption spectrometry (AAS) remains a practical alternative for single-element work: flame AAS suits higher-concentration elements, while graphite-furnace AAS handles trace-level cadmium and lead with small sample volumes. Mercury is determined separately by cold-vapour generation coupled to AAS or direct thermal-decomposition analyzers, since volatile loss during digestion would otherwise bias results. Total metal content is reported on a dry-weight basis.

Organic

pollutant testing — GC-MS and HPLC
gas chromatography–mass spectrometry (GC-MS) dominates the determination of semi-volatile organic contaminants in sediments. Polycyclic aromatic hydrocarbons, organochlorine pesticides, and polychlorinated biphenyls are separated on non-polar or mid-polar capillary columns and quantified by selected-ion monitoring against calibration standards. Surrogate standards are added before extraction to monitor recovery through the entire preparation sequence, and confirmatory ion ratios guard against false identification. High-performance liquid chromatography (HPLC), often with fluorescence or diode-array detection, is preferred for thermolabile or high-molecular-weight PAHs and for certain surfactant residues, since these compounds degrade or adsorb under GC inlet temperatures. Petroleum hydrocarbons are frequently screened gravimetrically or by infrared spectrometry, then characterized in detail by GC-MS when source identification is needed. Extract cleanup and concentration steps strongly influence sensitivity.

Grain size and nutrient assay methods

Grain size distribution describes sediment texture and controls contaminant partitioning, as fine particles carry more adsorbed metals per unit mass. Laser diffraction is the standard approach for particles within its measurable range, offering rapid, reproducible volume-based distributions; the classical pipette and sieve methods are retained as references and for coarse fractions. Results are classified into gravel, sand, silt, and clay fractions. Nutrient assays rely on spectrophotometry: total nitrogen is determined by the alkaline potassium persulfate digestion method followed by UV or colorimetric detection, and total phosphorus by acid persulfate digestion with molybdenum-antimony colorimetry. Organic carbon is measured by elemental analysis or by the potassium dichromate oxidation titration method. Because nutrients are labile, moisture and carbon analyses should start promptly after receipt.

Detection

limits, recovery, and quality control
Data credibility rests on demonstrated method performance. Method detection limits are established through replicate analysis of low-concentration matrix blanks and must sit below the relevant assessment thresholds; reporting limits are stated on the certificate. Recovery is verified with matrix spikes, typically evaluated against the acceptance windows specified in the applied method, while certified reference sediments check trueness batch by batch. Laboratory blanks, field blanks, and duplicate samples expose contamination and heterogeneity; surrogate recoveries track organic analyte losses. Calibration curves are verified with an independent check standard at defined intervals, and control charts monitor trends over time. Holding times from collection to analysis are recorded for each fraction. Any result outside control limits triggers investigation, re-preparation, or qualified reporting, so the final dataset can be defended in regulatory review.