Nested sampling and spatial analysis for reconnaissance investigations of soil: an example from agricultural land near mine tailings in Zambia

Abstract

Summary A reconnaissance survey was undertaken on soil near mine tailings to investigate variation in the content of copper, chromium and uranium. A nested sampling design was used. The data showed significant relations between the content of copper and uranium in the soil and its organic matter content, and a significant spatial trend in uranium content with distance from the tailings. Soil pH was not significantly related to any of the metals. The variance components associated with different scales of the sample design had large confidence intervals, but it was possible to show that the random variation was spatially dependent for all spatial models, whether for variation around a constant mean, or with a mean given by a linear effect of organic matter or distance to the tailings. For copper, we showed that a fractal or multifractal random model, with equal variance components for scales in a logarithmic progression, could be rejected for the model of variation around the fixed mean. The inclusion of organic matter as an explanatory factor meant that the fractal model could no longer be rejected, suggesting that the effect of organic matter results in spatial variation that is not scale invariant. It was shown, taking uranium as a case study, that further spatially nested sampling to estimate scale‐dependent variance components, or to test a non‐fractal model with adequate power, would require in the order of 200–250 samples in total. Highlights Sampling was undertaken to investigate spatial variation of metal content in soil near mine tailings. Chromium and uranium were related to soil organic matter content; uranium showed a spatial trend. Spatial variation was scale dependent, variation of copper was not scale‐invariant. Characterizing random spatial variation requires substantial sample effort.

Reconnaissance sampling and determination of hexavalent chromium in potentially-contaminated agricultural soils in Copperbelt Province, Zambia

“…The study area comprises a village within 1 km of a tailings dam used by Mopani Copper Mines in Kitwe, Zambia (-12.800346, 28.118721, Figure 1); the mine has been in operation for 28 years. Initially, reconnaissance sampling of agricultural land was undertaken; this process has been outlined by Lark et al (Lark, Hamilton, Kaninga, Maseka, Mutondo, Sakala, et al, 2017). Briefly, a nested sampling design was implemented to assess the spatial variation of chromium across the agricultural land within the village, which involved collecting samples along transects with sample main stations at loose intervals between 100 m and 200 m with substations in random directions a further 100 m, 10 m and 1 m from the initial main stations (Figure 1i).…”

Do soil amendments used to improve agricultural productivity have consequences for soils contaminated with heavy metals?

“…Four 10 Â 25-m fields assigned as B1 to B4 (Figure 1a and 1b), were selected for use, two were close to the Dam (about 100-200 m) and two further from the Dam (about 300-400 m). They were selected on the basis of previous reconnaissance sampling of the land around the Dam farmed by residents of Mugala village (Hamilton et al, 2020;Lark et al, 2017). Four 4.5 Â 10 m plots, separated by 1 m, were established in each field.…”

“…Calcium chloride (0.01 M) is commonly used but chloride ions are known to enhance dissolution of Cd by chloro-complexation (Garforth et al, 2016), thus, calcium nitrate may be a better alternative where Cd bioavailability is of interest. Therefore, this study aimed to investigate the consequences of typical agricultural amendments of lime and manure on the bioavailability of heavy metals in agriculturally productive soils contaminated to varying degrees by the close proximity of mine tailings in the Zambian Copperbelt, building on previous work by Lark et al (2017) and Hamilton et al (2020). To achieve this aim, the following objectives were: i) evaluate the bioavailability and mobility of metals in agricultural soils using DTPA-and Ca(NO 3 ) 2 extractants at locations representative of varying degrees of contamination in relation to the mine tailings; ii) determine how the bioavailable metals are affected by the application of lime and manure at typical agricultural amendment rates, and iii) assess the residual effect of the amendments after one growing season.…”

Planning spatial sampling of the soil from an uncertain reconnaissance variogram

“…A full account of the sampling undertaken in this study is provided by Lark et al (2017). The sampling was undertaken on transects with sample main stations at intervals of 100 to 200 m. At each main station a soil sample was collected.…”

“…A reconnaissance survey was undertaken on farmland in close proximity to a mine tailings dam. This survey was based on a nested sampling design (Lark et al, 2017), of which we report on 64 samples which were collected at a site of interest, the land farmed by the inhabitants of Mugala village near Kitwe. This sample size is rather fewer than the minimum of 100 suggested by Webster and Oliver (1992).…”