A comparative air quality and spatial planning study between two sites in a legacy mining town, Welkom, South Africa 2022-2023

Introduction

Historically, intense underground mining for precious metals was the backbone of the South African economy for many decades. However, due to changing rent value of the targeted material and a shift in focus of the core economic activities, a number of small legacy mining towns are scattered across South Africa. 

In this project, PM_2.5 particulates were sampled at two points in Welkom. A residential area and the industrial area, 5.8 Km South of the residential site. A comparison for their total PM_2.5, BC, UV-PM and trace elemental composition, sources and geographical origins was performed. The main objective of the study was to determine the air quality in a legacy mining town where past activities, more recent developments and spatial priorities contribute to the sources and matrix of the air pollutants.

Methods

Sampling was performed by gravimetric methods, with PM_2.5 and the constituent elements analysed by XRF. The source apportionment and back trajectory transport was performed on EPA-PMF and HYSPLIT models respectively. A total of 75 samples were collected with 5 duplicates per site. Statistical analysis included descriptive statistics, Spearman’s Rank correlation and Kruskal-Wallis test for seasonality.

Results

The annual mean PM_2.5 level for the 12-month study period was 14.7 µg.m^-3 (11.6 – 66) at the Industrial area and 6.34 µg.m^-3 (6.34 – 23.4) at the Residential Site. The daily PM_2.5 WHO guidelines (15 µg.m^-3) and the daily PM_2.5 South African NAAQS (40 µg.m^-3) were exceeded on 27 and 5 days of the 75 days, respectively, at the industrial and residential sites. The sum of the trace elements per 12-month period was 2.3 µg.m^-3 and 1.5 µg.m^-3 and these constitute 12.4% and 10% of the total PM_2.5 per year. Of interest is that Cl and K were present at both sites, Mn was present at the residential site and P was present at the industrial only. Although 26% of wind trajectories are from the Westerly direction, the highest recorded particulates are from the Northerly direction with 19% of wind trajectories. The highest three recordings for PM_2.5 for the industrial site was 66.1, 61 and 56 µg.m^-3 on the 23 August, 6 June and 12 July 2022 respectively.  From the NNE and Easterly directions. At the Industrial site, the highest average concentration (19.9 µg.m^-3) recorded for a cluster was from the Northern direction over 13 days despite the cluster from the easterly direction recording 30 days (15.2 µg.m^-3).

Conclusion

New spatial development priorities such as rezoning for industrial activities that are currently upwind of the residential sites present new air quality challenges to legacy ‘extractional’ towns such as Welkom in South Africa.