Tracing and quantifying sulfur sources in karst watersheds through the stable isotope composition of dissolved sulfates

In karst ecosystems like the Gaoping River Basin (Zunyi, Guizhou Province, China), widespread evaporite deposits and exogenous acids inputs drive a shift in water chemistry from carbonate-dominated to sulfate-dominated regimes. Characterizing sulfate (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>SO</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mrow><mml:mn>2</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math>) sources and spatiotemporal dynamics is critical for managing vulnerable karst water resources. This study uses hydrochemistry, sulfur-oxygen <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>(</mml:mo><mml:msup><mml:mrow><mml:mi>δ</mml:mi></mml:mrow><mml:mrow><mml:mn>34</mml:mn></mml:mrow></mml:msup><mml:msub><mml:mrow><mml:mi>S</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>S</mml:mi></mml:mrow><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:msub><mml:mo>,</mml:mo><mml:mspace></mml:mspace><mml:msup><mml:mrow><mml:mi>δ</mml:mi></mml:mrow><mml:mrow><mml:mn>18</mml:mn></mml:mrow></mml:msup><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>S</mml:mi></mml:mrow><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:math>, and water (δD, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:mi>δ</mml:mi></mml:mrow><mml:mrow><mml:mn>18</mml:mn></mml:mrow></mml:msup><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow></mml:mrow></mml:msub></mml:math>) isotopes, combined with the SIAR model, to trace <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>SO</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mrow><mml:mn>2</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math> sources across river reaches and hydrological seasons. Results show that sulfate in the Gaoping River originates from both natural and anthropogenic sources: In the upstream, water-rock interactions during groundwater (GW)-surface water (SW) recharge drive gypsum dissolution in carbonate formations, contributing 20.9-21.7 % of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>SO</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mrow><mml:mn>2</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math> in SW and 20.7-30.4 % in GW. In midstream, where agricultural land use dominates, soil-derived sulfate becomes the primary source, accounting for 23.3-25.8 % (SW) and 24.6-28.3 % (GW), correlated with land-use intensity. In downstream, intensive human activities (sewage discharge, fertilizer application) elevate anthropogenic inputs, with sewage contributing approximately 20.1 % to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>SO</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mrow><mml:mn>2</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math> loads in both SW and GW, and fertilizer inputs accounting for 24.0 %. The SIAR model confirms a transition from geogenic (gypsum, soil sulfate) to anthropogenic dominance downstream, with overall water chemistry shifting to a HCO₃·SO₄-Ca type, indicative of carbonate-to-sulfate evolution. These findings underscore the interplay between karst geology, land use, and hydrological processes in shaping sulfate budgets. The study provides a data-driven framework for targeted management: protecting upstream gypsum outcrops, optimizing midstream agricultural practices, and improving downstream wastewater treatment, thereby advancing sustainable karst water resource management in vulnerable ecosystems.