Toward a methodology to explore historical groundwater level trends and their origin: the case of Quebec, Canada

In this study, a methodology combining some state-of-the-art methods for the analysis of groundwater monitoring networks is proposed to test the hypothesis that it can be successfully used to assess trends in groundwater levels and their origin with minimal hydrogeological data requirements. The methodology first employs trend detection methods, namely the trend-free prewhitening Mann–Kendall test and a bootstrap test, to explore trends; then it uses the reference hydrograph method, land-use change maps, and consultation with groundwater resource managers to understand the origin of the trends. The methodology was tested on the groundwater monitoring network of the province of Quebec, Canada. This study focuses on short-term trends, given the length of the data available. The results showed that all but one of the observation wells in the monitoring network exhibited significant upward (38%) and downward (62%) trends at the 5% statistical significance level, but that the majority of observation wells (77%) exhibited a trend amplitude of less than 3 cm/year in absolute value, the threshold below which the rate of upward or downward change is considered stable. Application of the bootstrap test validated the representativeness of the trends calculated at well scale. For 33% of the observation wells with moderate to large trends (amplitude greater than or equal to 3 cm/year in absolute value), changes in the field around the wells were identified and could explain the observed trends for half of them. For the remaining 67% of wells, no changes in the field were reported.