Influence of basement rock lithology on soil hydraulic conductivity and grain size distribution in lateritic soils of western Uganda: Implications for estuarine management

The main focus of this study is to assess the control of grain size distribution, lithology, and depth on hydraulic conductivity ( K_Hazen ) within five weathered basement rocks (gneiss, gneiss-volcanics, quartzite, and granite) by means of statistical, regression, and mixed-effects modeling methods underpinned by hydrogeological interpretation. For this purpose, a total of 24 soil samples were analyzed using the GGU-SIEVE program, focusing on the determination of uniformity coefficient (Cu), coefficient of curvature (Cc), and hydraulic conductivity ( K_Hazen ). The investigation also included analysis of median-grain distribution, gradation, permeability, and soil type classifications, which are vital for sustainable soil management in regions with variable geological bedrocks, like in western Uganda. The results for Cu values range from 15 to 439, Cc ranges from 0.2 to 1.8, and K ranges from 5.8 × 10 -8 to 1.3 × 10 -4 m/s. These results demonstrate that the soils exhibit a wide range of textural variations and depth-dependent permeability trends. Gneiss-derived soils are permeable and exhibit moderate gradation. The quartzite and granite soils are more broadly graded but less porous, resulting from denser packing and finer matrix content. On the other hand, the gneiss-volcanics profile presents an intermediate characteristic due to mixed lithological influence. Based on statistical analysis, K decreases systematically with depth and is positively correlated with Cu and negatively with the percentage of fines. This is also confirmed by the model diagnostics, which suggest that K is jointly controlled by such factors as lithology, grain-size heterogeneity, and depth-dependent structure, with Cu emerging as a strong predictor, with the mixed-effects framework proving effective to model subsurface variability. These results underline the role of the controlling variables related to the parent rock and the intensity of weathering in influencing soil texture, gradation, shape (structure), and hydrologic response, which should be considered for erosion risk management, geotechnical study, and subsurface flow modeling in basement terrains.