The pH-Salinity Divergence Index (PSDI): a depth-resolved metric for estuarine mixing dynamics

In estuaries, pH and salinity exhibit interdependent gradients due to the mixing of acidic freshwater and alkaline seawater, yet existing metrics often overlook this chemical divergence, creating a knowledge gap in quantifying depth-resolved mixing dynamics. This study introduces the pH-Salinity Divergence Index (PSDI), a chemical metric designed to quantify estuarine mixing dynamics using depth-resolved pH and salinity profiles collected at three stations (S1: 1 km, S2: 4 km, S3: 7 km from the estuary mouth) during high tide (HW) and low tide (LW) tides on 11 October 2021, 20 March 2022, and 2 July 2022. The PSDI, defined as the normalized divergence between pH and salinity gradients, identifies Station 3 as a dynamic mixing zone with elevated PSDI values (up to 3.5), driven by steep salinity (e.g., 19.3–20.99 ppt, 20 March 2022) and pH (7.10–7.19) gradients. Station 1 exhibits stable conditions with low PSDI (up to 1.5), high salinity (∼29–35 ppt), and pH (∼8.3–8.4), while Station 2 shows intermediate PSDI (up to 2.7) with salinity (∼24–31 ppt) and pH (∼7.3–8.1). Temperature (17–23 °C) had minimal influence (R2 = 0.01, p = 0.32). Validated against salinity gradients (r = 0.87, p < 0.001 at S3), the PSDI demonstrates high sensitivity (0.16 PSDI per ppt/m), precision (coefficient of variation, CV <3.8%), and robustness across sampling dates. The metric offers a potential tool for estuarine monitoring and identifying zones of potential vulnerability to acidification based on pH trends, though confirmation requires carbonate chemistry measurements. Furthermore, the PSDI may help monitor climate change impacts, such as amplified saltwater intrusion and acidification risks in estuaries. As the present validation is based on three sampling periods only, broader temporal datasets are needed to confirm the PSDI’s long-term applicability and generalizability.