The sedimentation ponds of the former Solvay soda plant, revegetated 30 years ago, as well as changes following road construction, were investigated regarding their current influence on the chemistry of the adjacent Wilga River (Southern Poland). Eight monitoring points, directly adjacent to the sedimentation ponds, were established along the Wilga River. Samples were collected four times per season, i.e., preinvestment in 2018/19 (T1) and in 2020/2021 (T2) after earthwork during road construction. The obtained results of river water quality significantly exceeded the environmental quality standards of the Polish Regulation of the Ministry of Environment (2016/1187) and Directive 2008/105/EC. The water had high electrical conductivity (EC 1285–1833 µS cm−1) and total mineralization (up to 2433 mg L−1). After the road construction, significant increases in the concentrations of inorganic carbon (IC), hydrogen carbonate ions (HCO3−), and ammonium ions (NH4+) were observed. These phenomena were caused mainly by initial organic layer disturbance and sediment movement during road construction work. We conclude that soda production wastes retain a long-term influence on the river ecosystem and increase EC, Na+, Cl−, and Ca2+ concentration. Even though revitalization and revegetation processes had been conducted for these sedimentation ponds, ensuing earthworks on the stabilized object can generate a second effect on it, expressed by the increasing IC, HCO3−, and NH4+ concentration. The high salinity of the Wilga River, as a Vistula River tributary, is one of the most important negative environmental effects. Additionally, the localization of the studied objects in a highly populated region is a dangerous effect.