Water quality degradation in the sources of drinking water: an assessment based on 18 years of data from 441 water supply systems.

Degradation of source water quality complicates water treatment processes, resulting in additional treatment cost and tap water quality deterioration. In this study, source water quality was investigated for 441 water supply systems (WSSs) during the period of 18 years (1999-2016). The investigation was performed on 21 water quality parameters (WQPs) for groundwater (GWS) and surface water (SWS) sources. The averages of dissolved organic carbon (DOC), color, and Kjeldahl nitrogen (N) were much higher in SWS than GWS while other 18 WQPs (e.g., alkalinity, conductivity, and pH) were higher in GWS. In SWS, averages of DOC during 2000-2005, 2006-2010, and 2011-2015 were 6.08, 6.74, and 6.78 mg/L, respectively. In these periods, pH were 6.39, 6.62, and 6.77, respectively. In GWS, averages of DOC in these periods were 1.43, 1.36, and 1.81 mg/L, respectively, while pH were 7.50, 7.69, and 7.89, respectively. The DOC in SWS and GWS were increasing at the rates of 0.0722 and 0.0491 mg/L/year, respectively, while pH were increasing at the rates of 0.0375 and 0.0441 units/year, respectively. Trihalomethanes showed increasing trends in drinking water from SWS and GWS while haloacetic acids showed no trend. In SWS, DOC and its rate of increase were higher while in GWS, pH and its rate of increase were higher. The higher DOC and pH, and their increasing rates could increase disinfection byproducts (DBPs) in drinking water. Many DBPs are known as possible or probable human carcinogens and some DBPs are regulated. The other WQP and their increasing patterns can also impart new challenges, which are likely to increase the treatment cost and/or deteriorate drinking water quality.