Burial Interval Estimation

The ADD concept was developed in forensic entomology in the 1980s to move beyond simple elapsed-day estimates for post-mortem interval. The key insight is that the metabolic processes driving decomposition (bacterial enzyme activity, insect development) are temperature-dependent. A day at 25 °C does more decomposition work than a day at 5 °C, so a calendar-day count is a poor proxy for biological progress.

To calculate ADD: record mean daily temperature (at the burial depth) for each day of the suspected burial interval, subtract the baseline temperature (0 °C by convention, though some models use a higher threshold for insect-based applications), and sum the results. The accumulated value can then be compared to reference datasets of decomposition progression under known conditions.

Retrospective temperature reconstruction uses several sources when a logger was not placed at excavation. Nearby weather stations provide air temperature. Historical ground-temperature records from meteorological networks provide soil temperature for standard depths. The further the burial is from a monitored site, and the deeper the grave, the less reliable the retrospective estimate becomes. Courts should be told the uncertainty this introduces.

Standard practice in UK and many European forensic archaeology teams now includes placement of calibrated thermistor dataloggers at the grave depth (and, for comparison, at a control position in undisturbed soil nearby) at the time of excavation. The loggers record temperature at regular intervals (typically 15 or 30 minutes) and are recovered after an agreed monitoring period, usually one full year to capture the annual seasonal cycle.

• Matching a full year of current soil temperature to published regional climate data allows the team to reconstruct what the soil temperature was likely doing at the burial depth in the years before discovery.
• The difference between the grave-fill soil temperature and the control-position temperature quantifies the microclimate effect of the burial itself (decomposing organic matter can generate heat in active-decay phases).
• Temperature records form part of the evidential archive and must be chain-of-custody compliant: the logger ID, calibration certificate, and download records are all relevant exhibits.

Root growth into a grave fill provides evidence of burial duration for two related reasons: roots cannot be present before burial (since the grave did not exist), and root penetration proceeds at broadly predictable rates under known conditions. The evidence takes several forms.

• Root penetration depth: the maximum depth to which the root system of identified species has descended into the fill, compared against published growth-rate data for that species and soil type. This provides an approximate minimum burial duration.
• Root tip injury dating: roots show growth rings analogous to tree rings at the cellular level; measuring the distance from the tip to the first node gives a within-season precision for when a root entered a particular context.
• Root cross-sections: some woody roots produce readable annual rings. Counting rings on roots that penetrate into the body zone sets a minimum burial duration in whole years.

Pollen stratigraphy in the grave fill is a more specialist technique but can add seasonal precision. Pollen from wind-dispersed species is deposited continuously into open soil and is then sealed into the fill as the grave is backfilled or as the profile develops. If a species with a known short pollen season (such as a wind-pollinated tree with a spring flowering peak) is found at a discrete depth within the fill, that depth layer was deposited during or after that season in that year.

Artefacts with datable manufacture or issue dates provide minimum age constraints on a burial. The reasoning is simple: a body cannot have been buried before the most recently manufactured item found associated with it. This approach does not give a maximum date, but it sets a hard lower bound that is typically more reliable than any decomposition model.

In practice, the most useful artefacts in recent clandestine graves are mobile phones. A recovered handset's IMEI can be queried against the manufacturer's records to establish the earliest possible date that model was released for sale; network data may establish the last date of active use. Combined, these constrain both the minimum burial date (the phone existed) and provide a probable temporal envelope for the burial (the phone was active until approximately this date, then disappeared from network records).

One of the persistent problems in burial interval estimation has been that decomposition observations are described in subjective language that varies between practitioners and laboratories. Terms like extensive soft-tissue loss or advanced skeletonisation mean different things in different reports, making it impossible to build statistical models from pooled case data.

The PFIDAC protocol, developed by researchers at the UK Forensic Archaeology Research Group, addresses this by providing standardised scoring criteria for decomposition stage (analogous to the Universal Decomposition Scale for surface remains), tissue preservation, and bone condition across a defined set of anatomical regions. Each observation receives a numerical score, and the total can be entered into regression models that relate decomposition scores to ADD ranges derived from reference datasets.

The protocol is not a closed system. Practitioners can use it to record their findings in a format that is transparent and reproducible, allowing peer review of their observations independent of their interpretive conclusions. For international tribunal cases, where independent verification of methodology is expected by the court, this kind of standardised documentation is particularly valuable.

No single burial interval method delivers a precise date with small error margins. The way practitioners build a credible estimate is to assemble multiple independent lines of evidence, check them for consistency, and report a range that all the evidence can accommodate. When lines contradict each other, that contradiction must be explained rather than ignored.

The language of uncertainty in court is important. A burial interval opinion should be framed as consistent with a range of months or years, not as the burial occurred in a specific month. The report should identify which method provides the most constraint (often the artefact minimum date), which provides corroboration (often ADD), and which is the weakest (often botanical, due to species identification uncertainty and root-rate variability). Giving each method its due weight, rather than averaging or discarding outliers, is what makes the expert's evidence defensible under cross-examination.