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

Abstract

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 ofNO3 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).