C3 · Publication Volume 13

Sampling Media

rock, soil, stream sediment, water, vegetation and regolith

Learning goals

The learner should be able to describe what rock, soil, regolith, stream sediment, water and biological samples integrate; select a medium by the predicted dispersion process rather than convenience; define the horizon, fraction, depth, season and support needed to make samples comparable; and recognise when two media answer different questions and therefore should not be merged into one population.

There is no universally best medium. A medium is useful when it contains a detectable and interpretable response to the process of interest at a scale suited to the decision. Availability alone is insufficient. The design must also consider representativity, stability, collection safety, contamination, preservation, preparation, detection limits and the possibility of transported or externally introduced material.

Rock and mineral media

Rock samples can provide close spatial relation to exposed geology and direct information about lithology, alteration, mineralisation and mass change. Their weakness is exposure and selection. A grab of the most altered material answers a different question from a systematic channel, composite chip or mapped lithological sample. Record whether the purpose is characterisation, maximum expression, representative composition or boundary testing.

Support includes length, area, mass, particle distribution and selection rule. Weathered rind, vein, matrix and clast should not be mixed without purpose. Float is a transported medium even when lithologically diagnostic; its collection coordinate is not a bedrock contact. Mineral concentrates can enhance sparse resistant carriers but create strong density, grain-size and mineralogical selection that prevents direct comparison with bulk sediment.

Preparation must match texture. Coarse or nugget-like components require enough mass and appropriate splitting to control fundamental heterogeneity. A small pulp repeat can look precise while the original rock sample is not representative. Photograph, describe and retain enough material to test mineral residence where possible.

Soil and regolith media

“Soil” is not a single material. Horizons differ in organic matter, clay, oxides, carbonate, salts, moisture and parent material. Residual saprolite, colluvium, alluvium, wind-blown sediment and human fill can occur at similar depths. A fixed depth may cross different horizons across a landscape; a fixed horizon may be absent or difficult to recognise. The field protocol must state whether horizon, depth or material type has priority and how exceptions are coded.

Fine fractions can enrich clays, oxides and adsorbed elements; coarse fractions may retain resistant minerals or local fragments. Changing sieve fraction changes the measurand. Do not merge campaigns that use different fractions without an orientation-based equivalence test. Surface lag, subsurface fines and vegetation-root-zone material may resolve different transport histories.

Regolith mapping is part of geochemical design. Landform, erosion, deposition, drainage, weathering profile and disturbance define process domains. A threshold learned in residual material should not be applied automatically to transported cover. Where materials are mixed, field descriptions and particle or mineral indicators help estimate provenance.

Stream sediment and water media

Stream sediment integrates material delivered from an upstream area, but integration is unequal. Erodibility, tributary contribution, stream power, depositional environment, grain size, mineral density and recent events control the sample. A catchment boundary is therefore a first approximation to source support, not a uniform averaging kernel. Sample comparable active material, avoid obvious bank collapse or local contamination unless it is the target, and record flow state and geomorphic setting.

Sampling media integrate different source areas, depths, times, phases and transport pathways
Sampling media integrate different source areas, depths, times, phases and transport pathways

Water chemistry responds to dissolved, colloidal and particulate phases; acidity, redox state, temperature, conductivity, flow, filtration, preservation and holding time. A “total” water result may include suspended material that a filtered dissolved result excludes. Seasonal dilution, evaporation and episodic discharge can dominate. Field measurements and preservation events must be linked to the sample, not stored as detached notes.

Drainage sampling can rapidly screen large areas, but source location is directional and uncertain. Follow-up may move upstream through tributaries, compare sediment fractions, inspect heavy minerals or use soil and rock media to reduce the source area. A downstream high value and upstream low value are interpretable only when supports, flow conditions and analytical methods are comparable.

Biological and other selective media

Vegetation, litter, organic sediment and other biological materials can integrate bioavailable forms over root depth, species physiology, tissue and season. Uptake is selective, and species or tissue differences can exceed the geological signal. Identification, growth condition, washing, moisture basis and ash or dry-mass reporting are essential. A biological response may reveal accessible chemistry without representing total substrate concentration.

Gas, precipitate, salt, ferruginous coating and passive-collector media can target specific pathways. Their selectivity is a strength when tied to a mechanism and a weakness when treated as a generic concentration. Method blanks, substrate blanks and exposure duration may be critical. The medium definition must state what is collected and which phase the method recovers.

External material is an explicit hypothesis for every surface medium. Roads, structures, agriculture, waste, dust, sampling equipment and prior field activity can create spatial patterns. Record disturbance indicators and use exclusion or comparison samples. Never erase an inconvenient contaminated observation; retain it with a reasoned usability status.

Medium-selection matrix

Compare candidates using: predicted carrier and dispersion pathway; target depth and footprint; source displacement; spatial and temporal support; material availability; between-sample comparability; seasonal stability; preparation and detection requirements; contamination susceptibility; safety and environmental disturbance; and ability to collect a diagnostic follow-up.

A matrix should include failure predictions. If residual soil is absent, what material replaces it and how is the domain coded? If stream sediment has little fine material, is a smaller mass acceptable or is the sample indeterminate? If water is dry, does a later-season sample answer the same question? Predetermined rules prevent convenient exceptions from becoming silent changes in population.

Use an orientation survey to compare media over known or strongly constrained process contrasts, but do not optimise only for the largest amplitude. A useful medium also produces stable controls, manageable variance, interpretable transport and feasible coverage. Selection should remain conditional on the decision and domain.

Worked synthetic example

Consider a synthetic catchment where 70% of the delivered fine sediment comes from a background unit averaging 20 mg/kg of element X and 30% comes from a small source area averaging 200 mg/kg. Under complete mixing and equal recoverability, expected sediment concentration is

$0.70(20)+0.30(200)=74\ \mathrm{mg/kg}.$

If coarse source particles are preferentially trapped upstream so that only half of the source contribution reaches the site, renormalised delivered proportions are 0.70/(0.70+0.15)=0.824 background and 0.15/0.85=0.176 source. The expected concentration becomes 0.824(20)+0.176(200)=51.7 mg/kg. A change in transport, not source, reduced the signal.

A residual-soil sample directly above the source might yield 140 mg/kg but represent only a 0.5 m by 0.5 m by 0.1 m support. The drainage sample is lower yet integrates a broader upstream area. Neither is “better” without a decision: drainage may locate the anomalous tributary, while soil may refine a local source once the search area is reduced.

Media audit workflow

  1. Define the geological source, carrier phase and expected release process.
  2. Map residual, transported, drainage, biological and disturbed domains.
  3. For each candidate medium, state spatial, depth, particle, phase and temporal support.
  4. Specify material, horizon, fraction, mass, container and preservation.
  5. Identify mechanisms of dilution, enrichment, displacement and contamination.
  6. Set comparable-site criteria and exception codes before collection.
  7. Compare media in an orientation design using signal, variance and interpretability.
  8. Link collection coordinates to separate inferred source geometries.
  9. Keep media and materially different fractions as distinct populations.
  10. Choose follow-up media that can discriminate local source, transport and artefact.

Practice and review

  1. Build a medium-selection matrix for a synthetic covered source beneath transported sediment.
  2. Recalculate the catchment example if source delivery rises to 45% before trapping.
  3. Explain why two soil samples from the same depth may not share the same support or population.
  4. Design paired filtered and unfiltered water sampling, including field measurements and preservation records.
  5. List observations that would distinguish mineralised float from local bedrock.

Review questions: Which phase carries the signal? What source area and time does the sample integrate? How can the medium displace or dilute it? Are field materials genuinely comparable? Which alternative medium would test the interpretation?

Sources and further reading