Effects of Catchment Land Use on Nutrients and Heavy Metals Inflows into Maragua and Mathioya Wetlands in Murang’a County, Kenya

Abstract

Wetland ecosystems in Murang’a County are diminishing due to increased catchment land use practices. Part of wetlands have been converted into farmlands where various agricultural activities are carried out while some parts have been converted into settlement points. Agricultural practices carried out along wetland ecosystems involve the use of excessive agrochemicals during crop production which later contribute to wetland pollution through nutrients and heavy metals inflows. This study aimed at assessing the effects of catchment land use on nutrient and heavy metals inflows into Maragua and Mathioya river basins in Murang’a County. Water samples were collected using the Grab technique, packed in plastic containers, kept in cool boxes, and transported to the research laboratory for analysis. Phosphate and nitrate concentrations were analyzed across different seasons, sampling stations, and sampling levels. During dry season, the mean phosphate concentration was 0.0259 ± 0.0051 mg/L with a standard deviation of 0.0124 mg/L. In wet conditions, the mean phosphate concentration increased to 0.1631 ± 0.1509 mg/L with a standard deviation of 0.3697 mg/L. For nitrate, the mean concentration during dry conditions was 9223.37 ± 2672.33 mg/L with a standard deviation of 6545.84 mg/L, and during wet conditions, it remained the same at 9223.37 ± 2008.17 mg/L with a standard deviation of 4919.00 mg/L. Elemental analysis was performed using Microwave Plasma-Atomic Emission Spectroscopy (MP-AES) instrument. Mean concentrations of lead during dry seasons were 0.005 ± 0.002 mg/L and during wet season, they increased to 0.012 ± 0.004 mg/L. Zinc concentrations were 0.01 ± 0.003 mg/L during dry season and increased to 0.015 ± 0.004 mg/L during the wet season. The mean concentrations of zinc in the water samples were significantly lower than the maximum residue concentrations set for drinking water by the World Health Organization (WHO). However, lead (Pb) concentrations were above the WHO recommended maximum residue level. ANOVA analysis indicated no significant seasonal differences in phosphate (F=0.825, p=0.385) and nitrate (F=3.090, p=0.109) levels. Similarly, no significant differences were found between different sampling stations for phosphate (F=1.081, p=0.323) and nitrate (F=0.478, p=0.505). Analysis by sampling levels showed no significant differences noted for phosphate (F=0.979, p=0.412) and nitrate (F=1.949, p=0.198). For heavy metals, no significant differences were found for lead (F=1.234, p=0.271), cadmium (F=0.893, p=0.348), and zinc (F=1.567, p=0.223) across different seasons and sampling stations. These findings suggest that nutrient and heavy metal inflows into the wetlands are relatively stable and uniform across different spatial and temporal scales. The study highlights the importance of consistent land use practices and the effective buffering capacity of wetlands in maintaining ecological balance.