Influence of Quarry Rehabilitation on Soil Physico-chemical Properties in Bofa, Kilifi County, Kenya

Abstract

Quarrying activities create significant health, environmental, and visual impacts that must be addressed through the implementation of rehabilitation practices. This process usually begins and proceeds through key steps determined by the targeted final ecosystem and its successional phases. During rehabilitation, soil quality is among the most important factors affecting ecosystem integrity and is determined by physicochemical and biological characteristics that influence ecosystem structure and function. Many rehabilitation efforts have lacked clear objectives and meaningful monitoring of rehabilitation progress. Additionally, the literature indicates that many previous ecosystem rehabilitation studies have used inaccurate, time-consuming, and costly approaches. This study assessed changes in selected soil physicochemical properties and how they were influenced by quarry rehabilitation at different stages. Soil bulk density (BD), organic matter (OM), pH, nitrate (NO₃⁻–N), nitrite (NO₂⁻–N), ammonium (NH₄⁺), phosphate (PO₄³⁻), and cation exchange capacity (CEC) were analysed in rehabilitated quarries. The plots from which samples were collected were categorised as early (<10 years), intermediate (11–20 years), and mature (>20 years). Control samples were also collected from undisturbed virgin sites and abandoned, naturally recovering sites. BD, OM, nitrate, nitrite, ammonium, and phosphate differed significantly (P < .0001) at α = 0.05 across all study plots. As expected, BD decreased as soil OM increased: early plots had almost twice the BD of mature plots, whereas OM was highest in virgin and mature plots. Nitrate and nitrite concentrations were highest in intermediate plots, while ammonium and phosphate concentrations were highest in mature plots; in contrast, CEC was highest in virgin plots. Soil nutrient ions, particularly nitrate, are affected by differences in soil management, leaching, and denitrification, while changes in phosphate concentrations may reflect differences in weathering and sorption processes over time. As rehabilitation plots matured, soil OM increased, thereby increasing the availability of cation-exchange sites. Soil pH, which affects nutrient solubility, ranged from near-neutral to weakly alkaline values (7.7–8.3) and did not show abrupt changes with rehabilitation. The study concluded that OM, ammonium, and phosphate showed the clearest rehabilitation-related patterns, while CEC remained highest in virgin plots. These physicochemical parameters were therefore the most responsive to quarry rehabilitation and may have potential as indicators for monitoring rehabilitation status.