While the meteorological temperatures differed significantly from buried temperatures in all samples, the ambient temperatures recorded on site showed no significant differences from the buried temperatures. This suggests that recordings of the ambient temperatures can be used for ADD calculations even for buried remains. Due to logistical limitations, the piglets were only buried at an approximate depth of 10–15 cm, which has been shown by Prangnell and McGowan [1] to be insufficient to noticeably alter temperatures from the surrounding environment. Furthermore, the frequent rainfall leading to a high soil water content would also ensure that the soil temperatures would match that of the environment [5]. Burial at greater depths (and in the ground, as opposed to in containers) would provide more insulation, which may in turn lead to results that differ dramatically from the present observations made regarding shallow burials.

The piglets interred in dolomite had the greatest variation in their TBS progression, with significant differences between almost all specimens interred in that soil type (Table 1A). The exact reasons for these differences are not readily apparent. It is possible that the properties of dolomite may expose interred remains to greater environmental variations than the other soil types. Forensic scientists should be cognisant of the variations of decomposing rates and patterns of remains buried in dolomite, as this will result in more variable estimated PMIs or necessitate estimating PMIs with larger ranges. In contrast, the quartzite specimens progressed relatively uniformly and displayed no significant differences in TBS progression (Table 1B). This would result in PMI estimations for remains buried in quartzite to be more reliable and narrower.

A general trend was observed in the TBS vs. ADD graphs, which all exhibited a sigmoid curve similar to previous research [22, 23]. Each curve indicated that the rate of decomposition over time exhibited three phases: an initial lagging phase, an intermediate phase of rapid decomposition, and a final plateauing phase (Figs. 2 and 3). However, the sigmoid pattern observed by Myburgh et al. [22] exhibited a rapid rate of decomposition, followed by a plateau and then a slowed rate of decomposition until skeletonization was reached. Subjectively grouping these phases of decomposition rates may be useful for obtaining a degree of error in PMI estimates, as each phase has progressively more minute changes in TBS (the observable variable). Further testing with a larger sample size is necessary to determine if these phases are repeatable and statistically relevant for use in forensic investigations. A logarithmic trend in TBS progression is widely noted in both buried [23, 26, 29] and surface remains [6, 22, 30] so there is merit in exploring this phenomenon further. The use of phases has not been explored as an option in previous literature, which provides a novel hypothesis for future studies.

Since dolomite and mine tailing specimens were the only comparison to exhibit significantly different TBS progressions from each other, this indicates that in all other soil comparisons, each soil’s unique combination of properties is not dissimilar enough to alter the rate of burial decomposition rates. Future burial studies do not need to be overly concerned with the type of soil being used in the study and can focus on variations in other burial-related factors. It is unknown what variances between dolomite and mine tailings resulted in the significant differences in TBS progression, which requires further investigation.

Since dolomite has very low water retention (and consequently low water content) (Fig. 4), the environment in which the piglets were decomposing was neither so damp and anaerobic as to promote adipocere formation, nor so dry as to cause mummification [10]. Because of this, the piglets were rapidly and continuously colonised by maggots, leading to extensive soft tissue loss. Due to the effects of gravity, all moisture was drawn towards the inferior side of the piglet, leading to much more prolific maggot activity on the underside of the piglets. This led to the superior side appearing to be largely in an articulated state with some soft tissue present, while the underside was entirely defleshed and disarticulated due to minor disturbances. It is possible that the maggot activity (as well as the lower soil water content) may have led to very little tissue left to potentially form adipocere compared to the other buried remains.

The high water retention of quartzite led to adipocere formation on both sides of the piglets buried in this soil. While the maggots present had consumed a fair amount of soft tissue, leading to bony exposure on the limbs and head, the upper side of the piglets seemed to be largely intact. However, flipping the piglets revealed extensive decomposition on their undersides, leading to the torso breaking apart. It is suspected that maggot colonisation occurred early enough to allow for the destruction of a great deal of soft tissue before the onset of saponification and the resulting preservation.

The secondary research site where the mine tailing piglets were buried was frequented by visibly different Dipteran species, whose larvae were much larger and darker than those observed at the primary research site. Perhaps partly due to this, the rate of colonisation was exceptionally high. The rate of decomposition was similarly high, likely aided by the increased temperatures of the samples caused by the large numbers of metabolically active maggots [4]. A more detailed exploration of the impact of insect activity in buried remains buried in different soil types is outside the scope of the preset study. However, this is an important variable that requires further investigation in future studies.

TBS progressions were very similar for the dolomite and quartzite piglets, resulting in no significant differences between the two plots. However, it is important to note the frequency of adipocere formation in quartzite, which may lead to significant differences in a larger sample size. Further investigation into the use of the body’s upwards facing side compared to its downward facing side may be necessary to ascertain which of them is the more accurate in determining TBS and thereby ADD. The research carried out on decomposition within mine tailings is novel and will require further research to validate and broaden.

In general, the present study shows that the relatively high temperatures of the Gauteng province of South Africa, accompanied by the prevailing storms, lead to a subterranean environment that reduces small remains buried in shallow graves to skeletons with some residual saponified tissue within 1200 ADD (approximately two months). This indicates that estimating a post-mortem interval for remains that have been buried for more than a couple of months will be challenging.

The soil pH fluctuations recorded by Haslam and Tibbett [11] are very similar to what was noted in the present study. Acidic soils (quartzite sourced from Gauteng, South Africa, and podsol sourced from Dorset county, England) both show a sharp upward pH shift soon after interment of decaying matter, followed by a prolonged decline (Figs. 6 ). The difference in pH trends between the two soils are that the total fluctuation of the quartzite pH was less than that of the podsol.

Alkaline soils (dolomite sourced from Gauteng, South Africa, and rendzina sourced from Dorset county, England) are less similar in their specific trends, but both fluctuate very little throughout the course of the experiment, barely deviating from their original pH by more than one unit. The limited pH fluctuation of dolomite may be explained by its high buffer capacity, allowing it to resist changes in pH from pollution or (in this case) decompositional fluids [17].

The more significant fluctuations of both the quartzite and the mine tailing pH indicate a conversely lower level of micronutrients, leading to a lower buffer capacity from free metal cations such as iron and magnesium [17]. This suggests that for investigations of burials in dolomite or similar soils, examining soil pH would not provide any useful information, while investigations of burials in quartzite or mine tailing might gain some information from testing soil pH. The pH trend line of the mine tailing soil does not have an analogue in the Haslam and Tibbett [11] study, as expected due to its chemically altered origin [18].

A more detailed understanding of the correlation between soil pH and the state of decomposition of a buried body will provide another data point in the estimation of PMI, which will be of great benefit to forensic investigators.

Since this was a proof-of-concept study, juvenile pigs were used for the sample according to the recommendations of Matuszewski et al. [7]. The novel results of this study prove that the soil properties do have a significant impact on the decomposition rate and patterns, and they provide a baseline for future studies to compare their results against. Future investigations require animal samples of larger size and weight and eventually validation of the results using human remains for forensic realism and application in official forensic investigations.