Organochlorine pesticide contamination in sediments from Richards Bay, South Africa: spatial trends and ecotoxicological risks

Abstract

The occurrence and distribution of organochlorine pesticides (OCPs) in aquatic systems is a matter of global concern and poses significant toxicological threats to both organisms and human health. Despite the extensive use of OCPs for pest and disease control in southern Africa, relatively few studies have examined the occurrence and toxicological risks of OCP residues in the region. This study investigates the composition, distribution, and potential sources of OCP contamination in sediments from Richards Bay, a rapidly developing industrial port on the northeast coast of South Africa. Surface sediments collected from Richards Bay Harbour and surrounding areas indicate that OCP contamination in the region is widespread. Total concentrations (∑OCP) in surface samples ranged from 135 to 1020 ng g^-1, with hexachlorocyclohexanes (∑HCH; 35-230 ng g^-1) and dichlorodiphenyltrichloroethanes (∑DDT; 12-350 ng g^-1) the dominant contaminant groups detected. Metabolite isomeric compositions indicate that the presence of aldrin and endosulfan likely result from historical agricultural usage, while recent input of γ-HCH, heptachlor, and endrin may be linked to the illegal use of old pesticide stockpiles. Total DDT concentrations were dominated by p,p'-DDT (80 ± 64 ng g^-1), which was attributed to its ongoing use in malaria vector control in the region. A ²¹⁰Pb-dated sediment core revealed that OCP input to the local environment increased dramatically from relatively low concentrations in the mid-1940s (∑OCP, 355 ng g^-1) to peak levels (∑OCP, 781 ng g^-1) in the 1980s/1990s. An overall decrease in ∑OCP concentration from the mid-2000s is likely related to restrictions on use following the Stockholm Convention in 2004. Despite current restrictions on use, OCP concentrations exceeded sediment quality guidelines in the vast majority of cases, raising concerns for protected estuarine and mangrove habitats in the area, as well as for local fishing and farming communities.

Keywords: DDT; Distribution; HCH; Malaria; Pollution; Residues; Risk assessment; Stockholm Convention.

Fig. 1

Top left: malaria endemic regions of South Africa (modified from Weiss et al. 2019) KwaZulu-Natal (KZN). Top middle: catchment area of Mhlatuze and Umlalazi Rivers. Right: sampling locations and regional overview of Goedertrouw Dam (A), Umlalazi Nature Reserve (B), and Richards Bay (C) showing dominant land use (modified from Thompson 2019). Note the characteristic change in land use from north-east (residential and industrial) to south-west (wetland/mangrove habitat) of Richards Bay. Middle: Richards Bay Harbour and adjacent industries, as well as the mangrove forest sampling location (green)

Fig. 2

Summed analyte classes and total OCP concentration (∑OCP) ordered by sampling location: a Richards Bay Harbour, b Lake Mzingazi, c Black Bass Pool, d Mangroves at Richards Bay Harbour, e Goedertrouw Dam, and f Umlalazi Nature Reserve. The whiskers of the box plot indicate the 1.5 IQR, and dots indicate outliers. See Fig. 1 for locations of sampling sites

Fig. 3

Spatial distribution maps of the metabolite sums ∑DDT, ∑HCH, and ∑chlors (left to right)

Fig. 4

The.²¹⁰Pb-derived age-depth model of sediment core SAF18-107 using the Bayesian chronological approach of Aquino-López et al. (2018)

Fig. 5

Concentrations of OCP metabolites over time from sediment core SAF18-107 recovered from Richards Bay Harbour. Historical events are modified from Coetzee et al. (2013)

Fig. 6

Average composition and concentration of individual metabolite in surface sediments from the Richards Bay region

Fig. 7

Spatial distribution patterns of p,p′-DDT/TOC (left), y-HCH/TOC (middle), and land use data (right; modified from Thompson 2019). To account for matrix effects, isomeric metabolite concentrations are normalized to TOC. At Lake Mzingazi elevated ratios occur in the northwest of the lake. In Richards Bay Harbour, increased p,p′-DDT ratios locate at the Terminal Front, in conjunction with the main industrial site of the port. y-HCH ratios locate at the mudflats and the Coal Terminal where effluents of nearby farmland might drain into the port via the canal systems

Fig. 8

Comparison of individual OCP concentrations (circles) measured in surface sediments from Richards Bay, with sediment quality guidelines proposed by NOAA (Long et al. 1995). Effect Range Low (ERL) and Effect Range Median (ERM) are depicted by colored dots connected by a line. Percentages are given for samples exceeding the ERM

Fig. 9

Regional overview of South Africa’s east coast, modified from SANLC 2018 (Thompson 2019) and additionally indicating locations of previous OCP investigations by Buah-Kwofie and Humphries (2017). In side-to-side comparison, locations match the concentrations of OCPs measured in that area. The graph indicates the effect of increased use of pesticides in Richards Bay and the Mkhuze Wetlands. Note general decreasing DDT concentrations from north to south, whereby Lake St. Lucia and Richards Bay indicate similar amounts, despite the regional difference