Evaluation of Physical and Chemical Variables of River Water Quality in River Thanantu Sub-Catchment

Agrochemicals are widely used to boost agricultural productivity, yet their intensive and often indiscriminate application has become a major contributor to water pollution, threatening aquatic ecosystems, public health, and sustainable resource use. In Kenya, rivers traversing agricultural landscapes are increasingly vulnerable to contamination from pesticides and fertilisers. This study evaluates the physical and chemical variables of river water quality in the river Thanantu sub-catchment, in Tharaka Nithi County, Kenya. The study employed an experimental research design. Water samples were collected from three sites upstream, tributary, and confluence, identified using Geographic Information System (GIS) tools. Sampling was carried out in both wet and dry seasons between October 2024 and April 2025 to capture seasonal variations. Duplicate water samples were collected and subjected to standard laboratory procedures for analysing turbidity, total suspended solids (TSS), temperature, pH, nitrates, and total phosphorus. Agricultural practices within the sub-catchment were also assessed through questionnaires administered to farmers, key informant interviews with agricultural officers and community leaders, and transect walks for field observations. Data were analysed using descriptive statistics, one-way ANOVA, and regression analysis to test for spatial and seasonal differences. Water quality analysis showed significant seasonal variation, with higher turbidity values (14.1 NTU upstream, 22.6 NTU tributary, and 30.9 NTU downstream) and elevated TSS levels (65.72 mg/L in the dry season and 111.23 mg/L in the wet season). Nutrient concentrations also differed significantly across seasons, with total nitrogen (p = 0.008) and total phosphorus (p = 0.038) being higher during the wet season due to increased surface runoff. The study concluded that intensive agrochemical use is a key driver of water quality degradation in the Thanantu sub-catchment, with impacts exacerbated during the wet season. Current river pollution management strategies are inadequate to address the problem. It is recommended that the County Government, in collaboration with national agencies, implement integrated agrochemical management programs, strengthen enforcement of environmental regulations, and promote farmer awareness on sustainable practices. Longitudinal monitoring of pesticide residues in water, sediments, and aquatic organisms is also necessary to track cumulative impacts and inform evidence-based interventions.