Chemical composition, source apportionment of rainwater, and its contribution to nutrient deposition at an urban site of the middle Indo-Gangetic Plain region

Abstract

The study of rainwater chemistry plays an important role in understanding scavenging processes, sources of atmospheric pollutants, and impacts on ecosystems. This study investigated the chemical composition, variations, and potential sources influencing rainwater chemistry in Varanasi, India from 2018 to 2022. A total of 158 event-based monsoonal rainwater samples were analyzed for physical (pH, EC and TDS) and major ionic species (Cl^ˉ, F^ˉ, NO₃^ˉ, SO₄²⁻, PO₄³⁻, Mg²⁺, Ca²⁺, NH₄⁺, Na⁺ and K⁺). The average rainwater pH was 6.22 ± 0.45 (n = 158) with ∼92 % of the samples being alkaline with the dominance of Ca²⁺ and NH₄⁺ ions, whereas the rest of the samples were acidic in nature with high SO₄^2ˉ and NO₃^ˉ levels. NH₄⁺ concentrations increased significantly until 2020, while those of Ca²⁺, K⁺, and Mg²⁺ initially decreased and rose after 2020. The study highlighted significant deposition of dissolved inorganic nitrogen (in the form of NO₃⁻, NO₂⁻, and NH₄⁺). The average monsoonal nitrogen deposition flux was 8.04 kg ha⁻¹ with significant contributions from NO₃⁻ (3.36) and NH₄⁺ (4.67). In contrast, the deposition of inorganic phosphorus was significantly lower (∼0.72 kg ha⁻¹). Thus, the rainwater deposition contributed to overall nutrient deposition, specially N and P which could significantly impact the ecosystem. Neutralization and enrichment factors indicated influences from crustal and anthropogenic sources. This is also evident from the study as ∼99 % of Ca²⁺ and ∼98 % of SO₄²⁻ fractions were determined to be of non-marine origin. Over 800 brick kilns were identified around Varanasi and contributing to an increased NO₃⁻, SO₄²⁻ and particulate matter. The Positive Matrix Factorization (PMF) technique identified sea-salt, crustal dust, fossil fuel and biomass combustion, and agricultural emissions as potential sources of major ionic constituents over Varanasi.