Soil and Sediment as Trace Evidence

Soil is a mixture of mineral particles derived from local bedrock and transported material, organic matter from plant and animal decomposition, water, air, and biological organisms. The mineral component reflects the parent rock (limestone soils have calcite; basalt soils have weathered pyroxene and feldspar; granite soils have abundant quartz and potassium feldspar). The organic component reflects the vegetation history and climate. The particle size distribution reflects the energy of the depositional environment (high-energy rivers deposit coarse material; still lakes deposit clay). Each of these parameters varies continuously across the land surface, and because they are independent of each other, their combination is often highly distinctive.

Studies by Kenneth Pye, Robin Fitzpatrick, and colleagues in Australia and the UK have demonstrated that soil samples from adjacent fields, separated by a fence line or a drainage channel, can be reliably discriminated by combined colour and mineralogical analysis. A study of New Zealand soils by ESR scientists showed similar fine-scale discrimination. The implication for casework is that a mineralogical match between a questioned sample and a reference sample from a specific location can be genuinely evidential, not just consistent with an enormous number of possible sources.

Transfer of soil to footwear occurs when the sole of a shoe or boot presses against a soil surface. The amount transferred depends on how wet the soil is (wet soil deforms plastically and infills tread grooves; dry sandy soil is easily shed and may barely transfer), the roughness of the sole, the weight and movement pattern of the wearer, and whether the wearer is standing still, walking, or running. A person who walks quickly across soft, moist soil will carry substantially more material than one who crosses dry, compacted ground.

Persistence after transfer depends on the surface. Deep Vibram-type tread grooves retain impacted soil for long periods, especially if the wearer does not walk on hard surfaces afterward. Smooth leather soles shed soil quickly. Fabric surfaces (denim, canvas) trap particles in the fibre matrix, but vigorous movement can dislodge them. Vehicle tyre treads retain soil in a pattern that preserves the sequence of terrain: recent soil is in the outer tread, older soil is deeper in the groove.

Soil collection in forensic casework follows principles that have been developed through case experience and codified in guidelines by the FBI, the Association of Chief Police Officers (ACPO, now NPCC) in England, and national forensic institutes. The core steps are well-established:

1. Scene documentation before sampling
   Photograph the area to be sampled with scale and orientation markers. Record soil surface conditions (wet, dry, disturbed, vegetated). Note any obvious anthropogenic contamination. Mark sampling points on a site plan.
2. Cross-section sampling
   Collect a column of soil to a depth of at least 10-15 cm, keeping the layers in order. Place each layer in a separate labelled container. This preserves depth information and allows comparison against specific soil horizons. At minimum, collect an A-horizon (surface organic) sample and a B-horizon (subsoil mineral) sample separately.
3. Reference (scene) samples
   Collect multiple samples across the scene, especially around footprints or contact areas, to characterise the range of soil variability at that location. A single spot sample may not represent the soil the suspect actually contacted.
4. Control samples
   Sample the suspect's home environment, known routes, and geologically adjacent areas. These allow the analyst to assess how distinctive the scene soil is compared to what the suspect could have encountered in daily life.
5. Questioned sample collection from footwear or clothing
   Footwear should be packaged in paper bags (not plastic, which promotes mould growth in wet conditions) and sealed with tamper-evident tape. Do not attempt to remove soil at the scene. The laboratory removes it under controlled conditions with appropriate documentation.

Preservation requirements differ by the analyses planned. Soil intended for mineralogical and geochemical analysis can be air-dried at room temperature, then stored at room temperature in sealed containers. Drying above about 40°C can alter the hydration state of clay minerals, which affects XRD peaks and comparative analysis. Soil intended for biological analysis (diatoms, pollen, spores, fungi) must be refrigerated (4°C) or frozen immediately after collection to prevent microbial alteration of the biological component.

Wet soil on footwear poses a packaging problem. Sealing wet soil in a plastic bag creates conditions for mould growth that will destroy the biological fraction and may alter the mineral surface chemistry. The standard approach is to place wet footwear in paper bags and allow controlled drying before sealing in a secondary evidence bag. In cold conditions, refrigerated storage (not freezing, which can crack the footwear and disturb the soil) buys time.

Chain of custody for soil evidence follows the same principles as any other forensic exhibit: each sample is labelled with a unique identifier at collection, a seal is applied that shows tampering, a form records who collected it, the conditions, the time, and every subsequent transfer of possession. At the laboratory, a receiving record is completed. The original containers are retained. Any sub-sampling for analysis is documented. Court presentation requires that every link in this chain is traceable.

A soil sample contains far more than mineral particles. Organic matter from decomposed vegetation, fungal spores, pollen grains, diatom frustules (siliceous algal shells), nematodes, protozoa, and bacteria all contribute to the soil matrix and all carry geographic information of their own. A forensic geologist working with a palynologist can combine mineralogical comparison with pollen analysis to produce a stronger provenance conclusion than either alone.

Diatom assemblages in soil are particularly valuable in catchment-scale provenance work. Different water bodies have characteristic diatom communities, and their siliceous frustules persist in sediment long after the cells die. A soil sample deposited near a specific lake or river will contain diatoms characteristic of that water body. This approach has been applied in drowning investigations and in cases involving soil moved from sites near water.

• Biological component for refrigeration (4°C): diatoms, pollen, fungal spores, nematodes. Freezing is acceptable for long-term storage but should be consistent (avoid repeated freeze-thaw cycles).
• Mineralogical component for room-temperature storage: quartz, feldspar, clay minerals, heavy minerals. Air-drying is standard, but avoid high heat.
• Geochemical component: trace elements and major oxides are stable once the soil is dry. Refrigeration is not needed unless organic acids or volatile components are of interest.
• Isotopic component: strontium, lead, and oxygen isotopes in mineral fractions are stable at room temperature. No special preservation beyond preventing contamination.

Soil evidence has been admitted in courts across common-law jurisdictions for most of the twentieth century, typically without the explicit admissibility gatekeeping that DNA evidence receives. In England and Wales, soil comparison evidence has been treated as expert opinion subject to the usual requirements: the expert must be qualified, the method must be scientifically sound, and the opinion must be explained in terms the jury can evaluate. The case of R v. Hicks and the wider body of English cases on trace evidence establish the applicable standard.

New Zealand has a longer documented history of forensic soil and mineral evidence in case reports, partly because New Zealand's variable geology makes soil comparison particularly discriminating. ESR scientists in New Zealand have developed a body of published work on soil and paint comparison cases that has been cited in New Zealand court decisions as evidence that the methodology is scientifically grounded. The New Zealand cases have also grappled with the statistical expression of soil evidence strength, with some decisions discussing the distinction between a match and a probability statement about the match.

In the United States, soil evidence typically falls under the Daubert/Kumho framework as applied to technical expert testimony. The question is whether the method is testable, has known error rates, is subject to peer review, and is generally accepted in the relevant scientific community. Published comparative studies and proficiency testing by organisations such as the ENFSI Geology Working Group provide the error-rate and validation data that Daubert analysis requires.