Assessment of co-occurrence of cyanotoxins, toxic metals and anionic surfactants in irrigation water, agricultural soils and food crops

Department of Geography and Environmental Sciences

Department of Geography and Environmental Sciences

Globally, the occurrence of cyanobacterial blooms in freshwater ecosystems has become a concern. Cyanobacteria produces secondary metabolites, known as cyanotoxins that cause acute and chronic poisoning in animals and humans. History of mining, industrial activities and poor maintenance of wastewater treatment infrastructure are the main causes of the hyper-eutrophic conditions affecting most dams in South Africa. The co-occurrence of multiple stressors in agricultural waters and soils potentially pose a human and animal risk if contaminated water and plants are ingested. The study investigated the co-existence of cyanotoxins, anionic surfactants and metal species in irrigation water, agricultural soils and food crops and determine the health risks associated with consuming cyanotoxins contaminated plants in the Crocodile (West) Marico Water Management Area, which covers parts of Gauteng and Northwest Provinces. Lastly, the study assessed the applicability of passive sampling technology in monitoring of cyanotoxins using DIAON HP20 resins as an adsorbent. Water, food crops and soil samples were collected from Roodeplaat and Hartbeespoort dam sites in irrigation canals and cropping fields in June 2019, September 2019, February 2020, and March 2021. Seven sites were selected for sampling of water for cyanotoxins, anionic surfactants and toxic metals, while 4 farmland sites were selected for agricultural soils and food crops in Roodeplaat and Hartbeespoort sites. Physicochemical parameters of the irrigation water (pH, temperature, EC, TDS, DO), chlorophyll-a and dissolved nutrients were also monitored using Spectrophotometer and Spectro-Quant® Merck Pharo 100 with the photo-metric test kits from Merck, respectively. The levels of Microcystins (MCs), anionic surfactants, and metals were detected and quantified using the ELISA method, anionic surfactant portable photometer and inductively coupled plasma mass spectrometry (ICP- MS), respectively. The results are presented for each chapters below. The results for chapter 1 revealed the co-existence of cyanotoxins, metal species and anionic surfactants in the irrigation water, and agricultural soils, across sampling sites, throughout sampling period. The microcystins in irrigation water ranged from 0.00 to 15.57 μg/L. Total anionic surfactants in irrigation water and agricultural soil ranged from 0.01 to 3.49 mg/L and 1.81 to 5.46 mg/kg, respectively. Among all the physicochemical parameters only pH (p = 0.624), TDS (p = - 0.466), EC (p = - 0.445), and turbidity (p = 0.521) correlated with MCs. Moreover, total anionic surfactant showed to have positive moderate relationship with levels of MCs in irrigation water (p = 0.342). Metal species in irrigation water were decreased in the following order: Al > Mn > Fe > B > Zn > Ni > Cu > Pb > Cr > As and were all below the maximum DWAF acceptable limit, implying that the water was safe for irrigation use. Metal species in other soil sampling sites such as 16534.61 – 33285 mg/kg (Fe), 111.25 – 723.4 mg/kg (Cr),4.44 – 23.93 mg/kg (Pb), 0.80 – 9.70 mg/kg (As), 22.11 – 33.95 mg/kg (Cu), and 33.70 – 85.885 mg/kg (Ni) were above the maximum limit set by DEA, USEPA, and FAO/WHO for agricultural use. Thus, soils from Roodeplaat and Hartbeespoort farmland sites are contaminated by the mentioned metals. The findings from the second chapter of results revealed the bio-accumulation of microcystins and metals in food crops. The estimated daily intake (EDI) for MCs in all food crops for both adults and children were below 0.04 μg/kg DW acceptable value set by World Health Organisation, implying that the crops were safe for human consumption by adult and children population. Metal species levels accumulated in plant samples collected from different sampling sites, showed that 0.21 to 10.80 mg/kg (Cr), 19.64 to 734.00 mg/kg (Fe), 5.45 to 76.80 mg/kg (Zn), 0.01 to 0.20 mg/kg (As), 0.96 to 60.40 mg/kg (Cu), and 0.10 to 0.70 mg/kg (Pb) were above the EU and FAO/WHO guideline standards. Spearman correlation between metals in plants and water showed that only Pb (p = 0.874) and As (p = 0.809) in irrigation water had a positive moderate association with metals in plants collected from the sampling sites. The estimated daily intake (EDI) of metals via consumption of the crops were found to be below the maximum tolerable daily intake (MTDI) proposed for each metal. The translocation factors (TF) showed that only Cu and Cd were rapidly transported to the plant’s edible parts from the soil. Moreover, target hazard quotient (THQ) for each metal were below 1, indicating that consuming the food crops wont cause carcinogenic effect to the adult population, while hazard index (HI) for other sites was found to be >1 for crop plants, thus plants from these sites pose a health hazards to adult population. In addition, the target cancer risk (TCR) value for Cr and Ni in crops from other sampling sites were above the maximum threshold implying that there is a potential cancer risk to adult population over a long-term. In addition, findings from the third chapter showed that SPATT was applicable in monitoring and detecting MCs across all sampling sites and sampling months. The MCs levels in grab and SPATT bags ranged from 0.14 to 13.03 μg/L and 0.99 to 2.28 ng/g resin throughout the sampling sites and months, respectively. Thus, showing the persistence of MCs in canals and farm dams of Roodeplaat and Hartbeespoort. A spearman correlation revealed that pH (p = 0.776), Turbidity (p = 0.699) and DO (p = 0.829) had a significant positive association with total toxins in grab samples, while total dissolved MCs in SPATT samples showed negative moderate relationship with TDS (p = - 0.615) and EC (p = - 0.602). Total toxin concentrations in SPATT bags and Grab samples did not show any correlation this is because SPATT bags detect and collect microcystins within water column overtime, unlike point (Grab sampling), hence, there is no relationship between the two-sampling method. Overall results showed that SPATT bags with DIAON HP20 resin as an adsorbent proved to be applicable in monitoring and detecting microcystins in the irrigation water of Roodeplaat and Hartbeespoort sites.

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