Peri-urban karst pond cascades act as reactive biogeochemical filters coupling non-point-source pollutant fate with carbonate weathering

Small cascade ponds in peri‑urban karst areas are key reaction zones where nonpoint source inputs, solute transport and transformation, and carbonate weathering interact, yet their biogeochemical functions remain poorly understood. Here, hydrochemistry, multiple stable isotopes, principal component analysis-absolute principal component scores-multiple linear regression (PCA-APCS-MLR), and MixSIAR were integrated to determine how anthropogenic solute inputs affect pollutant fate and carbonate-weathering mechanisms in a peri‑urban karst cascade pond system. The Ca-HCO₃ hydrochemical facies reflected a background dominated by carbonate dissolution, whereas the mixed Ca·Na-HCO₃ signature, ionic ratios, and modified Gibbs diagrams jointly indicated substantial restructuring of solute composition by livestock farming, domestic sewage, and agricultural runoff. Combined constraints from δ¹³C_DIC, δ¹⁵N/δ¹⁸O-NO₃^-, and δ³⁴S-SO₄^2- showed that nitrate and sulfate were jointly controlled by sewage/manure, fertilisers, soil organic nitrogen, atmospheric deposition, sulfide oxidation, and sulfate reduction. Along the flow path, the cascade ponds exhibit a non-linear process characterized by a sequence of "input superposition, mixed dilution, filtration and self-purification, pollutant reloading, and reattenuation", with anthropogenic source contributions decreasing by 13.5 and 1.9 percentage points before and after the secondary pollutant loading, respectively (equivalent to relative reductions of 21.43 % and 3.35 %) Nitric and sulfuric acids generated by nitrification and sulfide oxidation promoted carbonate dissolution, causing HCO₃^-accumulation and enriched δ¹³C_DIC values. These findings indicate that anthropogenic pollution altered both the acid drivers of weathering and the sources of dissolved inorganic carbon (DIC), thereby inducing a decoupling between enhanced carbonate dissolution and reduced karst carbon sink efficiency. We identify peri‑urban karst cascade ponds as reactive biogeochemical filters that regulate C-N-S cycling, attenuate non-point-source pollution, and mediate feedbacks on carbonate weathering, providing new process-based insights for targeted control of peri‑urban diffuse pollution, ecological management of small water bodies, and evaluation of karst carbon sink effects.