A multi-year survey of organic disinfection by-products in drinking waters of Castilla y León, Spain. The need and difficulty to comply with the legal limit of 2009.

This article explains the general difficulties of the Spanish source waters and climatic conditions regarding a control of trihalomethanes (THMs), as reflected by the case of Castilla y León, and how the median values of 75 and 163 microg L(-1) of years 1999 and 2002 gave way to the more moderate of 31 and 47 microg L(-1) of years 2006 and 2007, respectively--both the latter being measured during the warmer season. Particular circumstances such as raw surface water--with frequently high total organic carbon (T.O.C) values--being the source for 80% of population served, the moderate-to-high water temperatures during the warm seasons and the high chlorine dosages frequently applied account for such as high levels. The median global value (n = 98) for raw water T.O.C. was 4.26 mg L(-1) (90th percentile of 9.81 mg L(-1)) and a median T.O.C. removal of 30% was observed during the treatment, but with an enormous variety (a 90th percentile in the order of 70%). Regression analysis associated the variables raw water temperature, prechlorination dosage and raw and finished water T.O.C. with the THMs measured in the finished waters and in the distribution systems. A certain linear correlation exists between THMs and haloacetic acids (HAAs) contents. However, a shift on their profile is noticeable with the temperature of the water, so that above 11.12 degrees C, THMs concentration tends to be higher than that of HAAs, and vice versa.