## 1 Introduction  
Nowadays, landfilling is the most common practice of waste disposal all over the world [1, 2]. In 2015, a total of 10.9 million tons of municipal solid wastes were collected in Poland, of which 44.3% were landfilled in 347 operating landfills [3]. Landfill leachate is considered the main source of groundwater and surface water contamination. In recent decades, the influence of leachates on water resources has attracted a lot of attention because of its high environmental significance [4, 5]. Leachates mainly contain immense amounts of organic matter, ammonium, heavy metals, and salts [6, 7, 8]. According to [PERSON] et al. [1], the components of leachates can be subdivided into the following groups: organic matter expressed as chemical oxygen demand (COD) and total organic carbon (TOC), specific organic compounds, inorganic compounds, and heavy metals. Leachates are commonly characterized by high chemical and biochemical oxygen demand values (COD and BOD, respectively) that reflect concentrations of soluble organic matter. The organic composition of leachates is variable depending on the characteristics of the landfilled wastes, age of the landfill or climatic conditions [8, 9]. In order to protect the natural environment from leachates, each landfill must be properly secured, for example by using artificial sealing based on cohesive soil liners [8]. Municipal landfills contain a high content of organic wastes, with great impact on the biogeochemical processes in the landfill body and leachate generation with a substantial content of ammonium, dissolved organic carbon and organic compounds [9]. It is important to stress that the content of organic matter affects the level of dissolved oxygen and can be determined by measurements of the biochemical oxygen demand (BOD) and chemical oxygen demand (COD). The BOD is also used as a value to indicate the leachate \"quality\". In cases when leachate is discharged directly into a watercourse, it will absorb oxygen from the water to complete its decomposition. As a result, the level of oxygen in water may fall below a minimum supporting aquatic life [10]. Because of the decomposition of organic matter, leachate derived from landfills comprises primarily dissolved organic carbon [11]. The solubility of organic contaminants in wastes can be slightly enhanced through the presence of high levels of organic carbon in the leachate [12]. The composition of landfill leachate varies over time. leachates generated during the early stages of anaerobic degradation are characterized e.g., by high BOD values and a high BOD/COD ratio. According to [PERSON] [13], BOD values are the highest in the first five years of landfill operation and decrease with time (for example BOD equal to 2000 mg O\({}_{2}\)/L after five years and BOD equal to 70 mg O\({}_{2}\)/L after 40 years of landfill operation). Similarly, five-year leachates are characterized by a COD value at a level of 8000 mg O\({}_{2}\)/L, but, after 30 years, this value decreases to 500 mg O\({}_{2}\)/L. BOD and COD indicators have a significant meaning in environmental studies because of their ability to indicate the pollutant strength of polluted water [14,15]. In recent years, the impact of landfills on groundwater quality has enhanced numerous studies relating to the assessment of the soil-water environment near landfills and waste management facilities [6, 16, 17, 18, 19, 20, 21, 22, 23, 24]. Owing to the fact that groundwater quality within the landfill site should be controlled by a monitoring system based on a network of piezometric wells and sampling points located along surface streams near the landfills, this study focuses on detailed analyses using data from a monitoring period of almost 20 years (1998-2016). The principal aim of this paper is to evaluate the impact of the landfill and the waste management facilities on the groundwater quality within the landfill site.