## 2 Geoenvironmental Setting
The salt series of the deposit consists of alternating seams and layers of pure rock salt, salt contaminated with clay and anhydrite, anhydrite and so-called zuber rocks (argillic and marly rocks with dispersed blocks of different types of salt). Thanks to the intensive drills of many boreholes, the upper part of the dome has been relatively well recognized geologically, though geological exploration reached a depth of approx. 3000 m, while the lower part in the depth range of 3000-6000 m was not explored at all [10; 11; 12; 13]. The maximum depth of the salt structure is not known (the deepest borehole of 3000 m failed to reach its bottom) but it is presumed that the dome began its development at a level of 4500-6000 m below the surface. It should be added that mining operations have penetrated the deposit only to a depth of 638 m [17; 18]. The cap rocks (gypsum and gypsum-clay rocks) covering the upper part of the salt dome are of a heterogeneous formation and appear as a disorderly, uneven mass, which may be interpreted as being a secondary formation and a residue of the underground leaching of soluble rocks [18]. It features diversity in lithological development: a gypsum cap developed over the outcrops of the salt series (mainly in the northern and western parts of the deposit), while a clay-gypsum cap with a small amount of gypsum developed over the outcrops of zuber (eastern part of the deposit). In the gypsum cap, as a result of the flow of groundwater, karst phenomena occur in the form of fissures, pits, and caverns, and these phenomena have also manifested themselves in the form of sinkholes on the terrain surface (these occurred in the past, before the era of mining activity). The localizations of these documented sinkholes reflect this geological division: most of them are placed in the western part of the town (which is the area under discussion in this study). The surface of the salt table is wavy and uneven, and it reminds us of the topography in the mountainous areas. It generally slopes to the south, but it is also more or less flat over a large area. In the northern and southern parts of the dome, the surface is more diverse, a result which should be associated with the activity of underground waters [13]. Figure 1: The map on the left: localization of the area under study and other Zechstein salt deposits in Poland. Hipometric structural map of the salt roof of the Inowroclaw dome and the Solno’s workings, and old boreholes and mining shafts on the right (after [14], modified). The legend presents the signatures used on the map and their numerical designations; their meaning is as follows: 1—boundary of the area under study; 2—sinkholes that existed before 1861, according to [PERSON] [8,13]; 3—sinkholes that occurred after 1861, according to [PERSON] [13,18]; 4—abandoned mine shafts; 5—boreholes; 6—boundary of the salt dome at the depth of 470 m; 7—workings of the shallow mines from the beginning of the twentieth century; 8—workings of the Solno mine. Another characteristic feature of the deposit is its geometry; in the horizontal projection of the upper, well-recognized and thus geometrized part, the deposit shows itself in the shape of an ellipse with a longer, meridian-oriented axis, approximately 3 km long, and a shorter one of 1 km long. Factors such as exploratory borehole drilling, followed by drilling mining and shallow underground mining and, most importantly, by intensive brine extraction, have disturbed the hydrogeological balance, causing an increased flow of water in the gypsum cap as well as in the deposit; the water circulation has greatly increased as a result [13; 14; 15]. In more recent times, more intensive leaching has taken place, which in turn has been reflected on the surface by the formation of numerous sinkholes.