Nitrates and phosphates in cave waters of Kraków-Częstochowa Upland, southern Poland

The paper presents the varied presence of nitrates and phosphates in water from caves located in Częstochowa and Kraków, in urban, strongly anthropogenic conditions, representing the vadose zone of the fissure-karstic-porous massif of Upper Jurassic limestones. Hydrochemical research was carried out by the authors in the Cave on the Stone in Częstochowa in 2012–2015, in caves of the Zakrzówek horst from 1996 to 2002, and in the Dragon’s Cave by the research team of J. Motyka in 1995–1998. A number of NO₃ and PO₄ measurements were performed in waters sampled at these research sites: 20 measurements each of NO₃ and PO₄ at the Cave on the Stone, 228 of NO₃ and 422 of PO₄ at Zakrzówek, and 19 each of NO₃ and PO₄ at the Dragon’s Cave. To assess the quality aspect of N and P compounds in waters from the Cave on the Stone, the results of geochemical modelling were processed using PHREEQC software. In cave waters, the oxidised form of nitrogen NO₃ ⁻ predominates; in surface waters in the vicinity, unoxidised forms prevail: NH₄₊, NH₃, and NH₄SO₄ ⁻. Among phosphorus speciations, dissolved forms are dominant: HPO₄ ²⁻, H₂PO₄ ⁻, and the insoluble form CaHPO₄; in surface waters, these forms are practically absent. Transformations of water chemistry in ‘urban’ caves, often centuries old, manifest themselves in, inter alia, the occurrence of multi-ionic waters, including seasonal variations and extremely diversified concentrations, with very high concentrations in subpopulations of NO₃ (0.2–485 mg dm⁻³) and P (0.02–6.87 mg dm⁻³). The common presence of NO₃ in waters of the phreatic zone of the Częstochowa Upland, an area developed in an agricultural direction, is documented by, inter alia, the exploitation of intakes supplying the city of Częstochowa (10–57 mg dm⁻³, 2011) and crenological studies from 2008 to 2015 (NO₃, 2–58 mg dm⁻³), at simultaneously low phosphate concentrations (PO₄, 0.02–0.24 mg dm⁻³).