Trend is another state indicator to show the temporal changes in water quality at a long-term scale (e.g. decades). Significant upward or downward trends can be determined either with statistical, linear methods (e.g. linear regression) or with non-parametric methods (e.g. based on Mann-Kendall) (Visser et al., 2007; Aguilar et al., 2007; Hansen et al., 2011).
A trend in groundwater quality is defined as a change over a specific period in time within a given region that is related to land use or water quality management (Schlosser et al, 1988; Loftis, 1996). Both temporal variations due to climatic and meteorological factors and spatial variability may complicate trend detection. Important is the period of time under consideration. Periods of 8 to 30 years have been recommended, depending on the sampling frequency for water analysis (Grath et al., 2001).