Hydrological convergence and surface–groundwater connectivity are associated with fluorescent groundwater DOM in the Victoria and Kyoga Basins, Uganda

Abstract

Dissolved organic matter (DOM) influences subsurface biogeochemistry yet controls on its composition in many tropical sub-Saharan aquifers remain poorly understood. Here, we integrated satellite-derived land-use/land-cover (LULC) data, terrain metrics, and inorganic tracers (NO3􀀀 /Cl􀀀 , SO42􀀀 /Cl􀀀 ) to evaluate how LULC patterns and terrain-driven hydrological convergence, expressed as topographic wetness index (TWI), influence bulk and fluorescent groundwater DOM (fDOM) across multiple spatial scales in Uganda. Groundwater DOM was dominated by weakly aromatic, highly processed material of mixed terrestrial and microbial origin (specific ultraviolet absorbance (SUVA) < 0.5 L mg􀀀 1.m􀀀 1; median humification index (HIXcorr) > 0.5, β:α < 1, and fluorescence index (FI) ~ 1.6). Total fluorescence (0.1 – 12.3 RU; median 0.3 RU) was dominated by fulvic-like (~35 %) and tryptophan-like (~29 %), followed by tyrosine-like (~18 %) and humic-like (~17 %) components. The combined influence of LULC and TWI was scale dependent, although LULC categories showed no significant associations with groundwater DOM, except surface-water cover. TWI was positively associated with dissolved organic carbon (DOC), and fDOM components consistent with potential lateral mobilisation and surface–groundwater interaction in hydrologically convergent zones. NO3􀀀 /Cl􀀀 ratios were negatively correlated with DOC (Spearman ρ ~ 􀀀 0.56), and positively with SUVA (ρ ~ 0.46), consistent with preferential depletion of relatively bioavailable DOM fractions and relative enrichment of more aromatic residual DOM under nitrate-enriched conditions. These results show that in the Victoria and Kyoga basins, hydrological convergence, rather than land use alone, likely influences groundwater DOM composition, highlighting surface waters and near-surface hyporheic zones as potential sources and transformation environments for organic matter entering aquifers.