Artificial Intelligence and Multivariate Analysis of Sodium Flushing and Fluoride Signatures in the Merti Transboundary Aquifer: Hydrochemical Evidence of Climate Stress and Adaptation Financing Implications

Abstract

The Merti Aquifer, a confined transboundary system spanning Isiolo, Garissa, and Wajir counties in Kenya and extending into Somalia’s Jubbada Hoose region, sustains over one million people and vast livestock populations in one of the most arid environments in the Horn of Africa. Despite its critical importance, the hydrochemical evolution of the aquifer under climate stress and over-abstraction remains poorly quantified. This study integrates Principal Component Analysis (PCA) with a machine learning (ML) benchmarking framework to model Sodium dynamics and assess climate change impacts. Using a hydrochemical dataset of 31 parameters, subsequently refined to 14 key predictors, we evaluated the latent structure of groundwater chemistry and compared six base regression algorithms: Random Forest, Decision Tree, Back-Propagation Neural Network (BPNN), Ridge Regression, Lasso Regression, and k-Nearest Neighbors, alongside a Stacking Ensemble. PCA revealed two dominant orthogonal processes: PC1 (36.1% variance) represents evaporative concentration and salinization driven by Sodium, Chloride, and TDS; PC2 (18.3% variance) captures volcanic geogenic signatures including Fluoride and Sulfide. The orthogonal separation of Sodium and Fluoride confirms distinct origins—Sodium is driven by climate-induced stagnation in the lowlands, while Fluoride originates from Mt. Kenya’s volcanic highlands. The Stacking Ensemble achieved exceptional predictive accuracy for Sodium (, RMSE = 164.07), closely followed by BPNN (). Cross-validated R-scores identified Ridge () and BPNN () as the most stable predictors. We conclude that Sodium and Chloride serve as reliable conservative proxies for climate-induced salinization, and that ML-based diagnostic dashboards can inform targeted climate adaptation funding, including managed aquifer recharge and transboundary water governance.