Impact of Silvicultural Activities on Water Quality in the Thathe Vondo Forest Plantation, Limpopo Province, South Africa

Abstract

Forests are vital ecosystems that control water resources for different life forms, store carbon, and provide goods and services for living organisms. The increasing need for timber and other forest products leads to the advancement of plantation forestry as a crucial technique for managing forest resources sustainably. The plantation, managed intensively for timber production, relies heavily on glyphosate for weed control. Concerns over its persistence, possible runoff, and other contaminants in the soil and water systems have surfaced. This study aimed to evaluate the amounts of glyphosate, aminomethylphosphonic acid (AMPA), heavy metals, microbial contamination, and assess the health risks linked to their concentrations in surface waters and the surrounding soils. It also assessed seasonal and spatial variations and identified the driving factors influencing contaminant levels. Field sampling was carried out during both wet and dry seasons at 16 strategically selected sites within the Thathe Vondo Forest Plantation, located in Limpopo Province, South Africa. Surface water and surrounding soil samples were collected to assess physicochemical parameters, glyphosate and AMPA concentrations, heavy metals, and microbial contaminants. Key water quality indicators—pH, electrical conductivity, turbidity, TDS, temperature, and salinity—were measured in situ using a multiparameter water quality instrument and turbidimeter. In both water and soil samples, glyphosate and AMPA were derivatised and quantified using Gas Chromatography-Mass Spectrometry (GC-MS), while heavy metals (Cr, Mn, Fe, Co, Ni, Cu, Zn, Ba) were determined via Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Microbial indicators (total coliforms and E. coli) were enumerated using the membrane filtration technique. The sampling design enabled seasonal comparison of contamination patterns. Data analysis involved statistical methods, including descriptive statistics, Tukey pairwise comparison, principal component analysis (PCA), and Pearson correlation analysis to identify trends, associations, and significant differences across sampling periods and sites. Results showed that glyphosate concentrations in surface waters ranged from 1.55 μg/L (Mufuvhani Stream) to 1.99 μg/L (Tshipfudoni Stream), exceeding WHO's guideline of 0.1 μg/L. Glyphosate concentrations in surrounding soil ranged from 2.22 μg/g (Mukumbani Dam) to 3.94 μg/g (Tshirovha River), far above WHO and SANS thresholds. AMPA was not detected in any samples, suggesting rapid parent compound dominance or limited microbial degradation, possibly suppressed by glyphosate. Glyphosate levels were positively correlated with electrical conductivity and TDS parameters, confirming the influence on ionic balance and salinity. Heavy metals in surface water revealed elevated values in Al (up to 73.52 μg/L), Cr, Mn, Co, and Fe, with wet season samples from the northeastern plantation zones recording the highest levels. These metals likely originated from both herbicide formulations and natural lithological weathering. PCA grouped glyphosate, Fe, Mn, and Cr into a strong component, indicating co-mobilisation through seasonal hydrological processes. Microbial contamination was severe. E. coli levels exceeded 100,000 CFU/100 mL in Tshitareni Stream during the dry season, while even the lowest count (Mapengoni Stream) reached 15,833 CFU/100 mL. Total coliforms peaked at 6.18×10⁴ CFU/100 mL in the Xhaxhani Stream. Contamination persisted across seasons, highlighting inadequate barriers between anthropogenic activity and water sources. Health risk assessments revealed alarming trends, especially glyphosate in children with CDI via water ingestion reached 0.0929 μg/kg/day at Mukumbani Dam (HQ = 0.9287), approaching the risk threshold. Soil-based exposure resulted in HQ > 1.0 across all sites, with a peak of 1.8387 at Tshirovha River. Lower CDI and HQ values were recorded for adults, yet some exceeded safety margins. HQ and HI for metals such as Cr, Fe, and Mn surpassed 1.0 in wet and dry seasons in Gwadza and Lwathuwa Streams, posing a chronic risk. QMRA infection probabilities for E. coli exposure reached 1.00 annually for all sites, with no substantial seasonal improvement. These findings confirm the environmental burden introduced by glyphosate-based forestry. The absence of AMPA suggests poor microbial resilience or inhibition of degradation pathways. An increase in the levels of heavy metals, alongside an increase in the microbial population, exacerbates the risks associated with ecosystems as well as the health of the public. Spatial patterns highlight the importance of hydrological movement, timing of herbicide application, slope of the land, and livestock proximity as dominant ranching influences. The study recommends urgent environmental precautions, including the establishment of vegetative buffer zones to trap runoff, restriction of herbicide application to dry intervals, fencing off water sources from livestock, and seasonal water quality monitoring for glyphosate, metals, and pathogens. Educating local communities that rely on untreated water is also critical. Consequently, this research utilises empirical evidence to inform forest management policies and develop integrated, ecologically sensitive weed control strategies that protect human health and ecosystem function.