All burial grounds are unique both in their natural environment (including soil type, parent material, vegetation, topography and climate) as well as anthropogenic burial numbers/styles/depths (Fig. 1), body distributions and above-ground placement of memorials and installation of pathways and roads to access the site. The mechanical disturbance via re-excavation and re-infilling of burial sites, alongside varying above ground vegetation types, as well as the presence of human remains, make graveyard soils unique and have their own soil type category; necrosols (Amuno and Amuno, 2014; Asare et al., 2020). Necrosols, despite increasing numbers of cemeteries and burial sites both in rural and urban environments, are poorly understood, especially regarding potential contamination and ecological risks (Jonker and Oliver, 2012). This is largely due to the complex biological and chemical processes occurring in these soils, resulting in both spatial and temporal heterogeneity of necrosols (Amuno and Amuno, 2014).
Reuse of graveyard and cemetery sites for burying human remains has been happening for at least 10,000 years since Early Mesolithic times (Schulting et al., 2019). The practice of reusing existing graveyards differs by country, region and timescales of clearing old graves before new ones are emplaced. For example, the United States generally leave human remains untouched in situ in perpetuity, whereas in the United Kingdom it is common to have a 100-year period, by which time any direct relatives should have died before an existing graveyard can be reused (Mytum, 2000), and in Germany remains can be moved when only buried for 25 years and a fresh grave emplaced (Fiedler et al., 2009).
Soil from graveyard sites differ from the natural soil profile largely through disturbance and due to the nature of the material buried. Previous research have likened graveyards to landfills (Fiedler et al., 2012), with elevated levels of organic matter (Kim et al., 2008), embalming fluids (Chiappelli and Chiappelli, 2008; Uslu et al., 2009), creosote from coffins (Mininni et al., 2007), as well as materials from the bodies themselves (Fiedler et al., 2012). In a few cases, cemetery materials carried in soil water have also been found to have contaminated local groundwater supplies with pathogens, viruses and heavy metals (Knoefes and McGee, 2002; Matias et al., 2004; Kim et al., 2008).
Non-invasive geophysical studies in such burial grounds indicate elevated conductivity levels in grave soil (Hansen et al., 2014), with individual grave geophysical anomalies decreasing with increasing burial age, when compared to background values. However, soil texture and moisture content have been shown to be major variables with sandy soils causing leaching of grave contents well beyond the grave-cut, whereas clay-rich soils tend to retain these fluids within the grave-cut itself (Pringle et al., 2012, 2016; Dick et al., 2017).
Archaeological studies have shown ancient burial ground soils to have elevated levels of heavy-metal elements such as Iron, Lead, Manganese and Copper Amuno and Amuno, 2012, Jonker and Oliver, 2012), as well as other elements such as Phosphorus and Nitrogen (Bethell and Carver, 1987; Asare et al., 2020), with human exposure to such toxic metals causing kidney damage (Khan et al., 2011) and links to Parkinson’s and Alzheimer’s disease (Mohod and Dhote, 2013). This may have important health implications for local graveyard workers, residents in the surrounding areas as well as potential risks to the local environment via leaching into surrounding soils and groundwater (Jonker and Oliver, 2012).. However, there has been, to-date, limited research on the soil contamination potential of cemeteries and graveyards
X-ray fluorescence spectrometry (XRF) is an analytical technique for the chemical characterisation of environmental materials. Although traditional, laboratory-based, wavelength dispersive XRF spectrometry will undoubtedly remain the analytical method of choice for studies in which data must be of the highest possible quality, such analyses inevitably involve greater costs, lengthier sample preparation and data processing procedures, and longer analysis times.
Portable XRF (pXRF) field surveys have been shown to be effective for rapid evaluation of heavy metal soil contamination (Radu and Diamond, 2009; Brent et al., 2016; Rouillon et al., 2017; Liang et al., 2018), biogeochemical mapping over mine tailings (Rincheval et al., 2019), archaeological object studies (Kasztovsky et al., 2018; Michalowski et al., 2020), marine microplastics (Turner, 2017), species profiling (Nganvongpanit et al., 2016), and even lead levels in living human bones (Zhang et al., 2021). The ability to carry out in-situ analysis for many other elements means that the methodology has considerable application as a field-based analysis tool (Frahm and Doonan, 2013).
However, soil water content is highly naturally highly variable across different land uses, soil types and habitats, which can be somewhat challenging when conducting physical measurements for site comparisons, for example, electrical resistivity surveys (Jervis and Pringle, 2014). This holds true for pXRF analysis, with the additional complication that the attenuation of X-rays by water is a function of the energy used to characterize the elements of interest. Low-energy X-rays are more strongly attenuated than high-energy ones, thus for pXRF analysis, elements with lower atomic masses are more strongly impacted. Studies have shown that for every 1% increase in soil water content, there is a 1.15% – 1.75% decrease in reported elemental concentration for Mn through to As, while elements lighter than Mn are even more greatly attenuated (Parsons et al., 2013; Imanishi et al., 2010). This means that sample water content must be measured and corrected for, as light element values determined in situ will likely not be directly comparable to results of a traditional laboratory analysis (Padilla et al., 2019).
The two aims of this study were therefore: (1) to determine the utility of pXRF measurements by comparing field- and lab--based pXRF measurements both spatially and temporally (down the soil profile) and; (2) to evaluate the potential heavy metal contamination of long-used (500+ years) church necrosol graveyards with different soil textures. Study objectives were therefore to: (1) collect pXRF field surface graveyard soil measurements and then take samples for subsequent laboratory soil pellet analysis, (2) collect surface samples and shallow surface soil auger profiles in long-established church graveyards for pXRF laboratory sample analysis in sandy and clay soil types respectively, (3) assess the respective known burial population and areal extent to quantify the respective graveyard burial histories and, (4) analyse pXRF results, compare to other studies and suggest implications for those living adjacent to burial grounds, adjacent to water supplies and re-use of ex-burial grounds.