Analyzing soil characteristics at a potential grave site can provide forensic investigators with information about the evidence within before the digging commences.
Soils—typically used by forensic science to link objects and persons with crime scenes—have since been shown to help locate clandestine graves1 and estimate time since death.2 This article will discuss soil characteristics for identifying the likelihood of a burial in soil, the breakdown of a cadaver in contact with soil, and the preservation of bone in soil. If soil morphological characteristics can help determine the likelihood of a burial and differential preservation, then it may be possible to make predictions prior to excavation using soil survey information.
Soils are the physical context within which forensic evidence is found. Thus, it is important to understand some of the potential implications of different physical and chemical soil properties. Soil properties such as the depth of soil, rock fragment content, soil reaction (pH [acidity or alkalinity]), soil temperature, soil texture, and soil moisture play a significant role in bone decomposition and in locating a clandestine grave. Each of these soil properties, along with the identification of a natural versus disturbed soil, can be determined by a skilled soil scientist.
Depth of Soil and Rock Fragment Content
A number of studies have been conducted to understand cadaver decomposition following a burial in soil. Generally, the burial of a cadaver in soil results in a decreased rate of decomposition. 3,4,5 A deep burial below 1 meter [3 feet] will restrict insect and other invertebrate activity, is unlikely to attract the attention of carnivorous animals,6 and is protected from the temperature fluctuations usually experienced in an ambient environment.7 Burial at depth may also result in the material being constantly or periodically below a water table, which can restrict oxygen availability and decrease decomposition. In contrast, soils that are shallow to bedrock or restrictive material make burials unlikely or else very shallow. Rock fragments at the surface and in the soil can also interfere with the depth of burials due to the difficulty of digging. As the number and size of rock fragments increases on the surface of the soil or in the soil, digging becomes more difficult and the likelihood of a burial decreases. At the surface and in the upper soil layers, there is typically a greater level of biological activity because of the greater availability of oxygen and microbial food8 and therefore increased microbial activity and decomposition. Decomposition after one week in the air is equivalent to two weeks in water and eight weeks in soil,9 implying that the depth of a burial in soil influences the decomposition of organic materials with greater depth impeding decay.10
Soil Reaction (pH)
The soil reaction or pH of soil has the largest influence on bone preservation,11 with preservation generally advantageous in soils above pH 5.3 [moderately acid] and adverse in soils pH 5.3[strongly to extremely acid] or less. Soils containing a highly acidic pH will decompose bone rapidly due to the dissolution of the inorganic matrix of hydroxylapatite.12 If the pH of the soil is neutral or basic (pH 7 or more), a buried skeleton may persist for centuries in good condition. Soil pH can also affect adipocere formation which allows for slower decomposition. Adipocere formation is most favorable in a mildly alkaline soil and becomes more prominent with the passage of time. It has been shown that the odor of adipocere is detectable by cadaver dogs searching for clandestine burials.13
Soil Temperature Temperature is regarded as one of the most influential factors of decomposition.14,3 Soil temperature has an important influence on biological, chemical, and physical processes in the soil.15 Research shows that temperature can regulate cadaveric decomposition and associated grave soil microbial activity.10 It has been repeatedly observed that an increase in soil temperature is associated with an increase in the rate of decomposition of buried cadavers.2 This is typically due to an increase in biological activity and chemical reaction rates. Predictably, cold soil temperatures slow the cadaver decomposition as most soil microorganisms are inhibited by cold and freezing soil conditions. With cold soil temperatures, bone splintering can occur as a by-product of the freeze-thaw cycle along with wet-dry cycles of the soil. In some instances, increasing soil depth will act as a counter balance effect to such effects of the freeze-thaw cycle.