Abstract
The agricultural sector employs about 57% of the population in Africa, which makes
this sector one of the main contributors to the economy of the continent and also the
main important food source. However, the extensive use of pesticides as means to
improve crop quality and as a protection mechanism, has indirect consequences on
the environment, particularly, water quality. Recently, a class of pesticides,
neonicotinoids, have sparked the interest of researchers globally. These are
systematic agricultural insecticides that include compounds such as imidacloprid,
acetamiprid, clothianidin, thiamethoxam and thiacloprid. The interplay of climatic
factors and the physicochemical properties of neonicotinoids facilitates their
movement from their initial sources into water bodies, indicating a potential for
environmental persistence. However the identification and distribution of
neonicotinoids in South African water bodies is not yet prominently investigated,
especially in areas with a high concentration of agricultural activity, such as the
province of Limpopo, where agriculture is the main industry. The objective of this study
was to assess the spatial and temporal patterns of neonicotinoids and their
metabolites in both water and sediment within the Letaba River Catchment and its
tributaries. The Letaba River forms part of the Limpopo River basin and is in the vicinity
of intense agricultural activities including vegetable and fruits farming (soy beans,
maize, peppers and citrus fruits), where neonicotinoids are used. Solid phase
extraction (SPE) and Liquid-liquid extraction (LLE) were used to extract neonicotinoids
from seasonal water samples, while microwave-assisted extraction (MAE) and
centrifugation were used to extract neonicotinoids from sediment. Ultra-performance
liquid chromatography-high resolution mass spectrometry (UPLC-QTOF) was used to
examine the neonicotinoids in water and sediment. Methods of descriptive statistical
analysis were employed to assess the neonicotinoids' distribution trends and the main
contributing elements. Results for water quality parameters (WQP) showed, pH
ranging between 6.1 – 8.9, temperature ranged between 17.0 – 32°C, dissolved
oxygen (DO) ranged between 6.2 – 11.8 mg/L, electrical conductivity (EC) between
50.2 – 1483 μS/cm and total dissolved solids (TDS) ranged from 52.0 – 200.0 mg/L.
Higher values for electrical conductivity (EC) for were obtained low flow season as
compared to high flow season, while pH and dissolved oxygen (DO) were
low/moderate for all sampling seasons. For water and sediment, the corresponding
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limit of detection (LOD) and limit of quantification (LOQ) ranged from 0.2365 to 0.7166
ng/mL and 0.3729 to 0.9466 ng/g, respectively. The SPE and MAE methods gave
recoveries of the target compounds varying from 0.02 to 92% and 0.04 to 89% and
standard deviations ranging from 0.01 – 2.79 ng/mL, the highest concentrations were
detected for imidacloprid while the lowest concentrations were detected for
clothianidin. The concentrations of neonicotinoids ranged from <LOD to 6.1007 ng/mL
in water, and from <LOD to 9.6633 ng/g for sediment samples across all the seasons.
For overall results the standard deviations for water and sediment were between
0.0203 to 1.3027 ng/mL and 0.0014 to 3.7249 ng/g respectively. The spatiotemporal
variations of the target analytes, highlight the key events resulting in varying
contaminant profiles. These findings were further supported by the non-metric
multidimensional scaling (NMDS) results which showed that there was significant
differences for the neonicotinoids detected for low flow and high flow seasons. This
work is important because it contributes to our understanding of the composition of
neonicotinoid species in aquatic ecosystems in South Africa and evaluates the effects
of human activity on spatiotemporal variations and water quality.
Keywords: Pesticides, Neonicotinoids, Letaba River, Water quality, Spatio-temporal
variations, Chromatographic analysis