The imbalance between groundwater recharge and exploitation has led to water scarcity. The Eastern Mitidja aquifer is facing the problem of groundwater depletion and deteriorating quality. This work aims to study the spatiotemporal evolution of the groundwater quality, based on statistical trend tests and the water quality index (WQI). The statistical analysis reveals that the WQI varies from 44.404 to 496.7 and from 28.24 to 398.666 for the wet and dry seasons, respectively. The results allow groundwater to be classified as "good" to "very poor" water quality. The prediction maps show that the highest WQI values (for both seasons) are located in the extreme east of the plain, while it deteriorates in the other regions over the years (2005-2020), especially during wet season. The WQI's temporal evolution was examined by two statistical trend tests, namely, the Seasonal Mann-Kendall (SMK) test and the Şen's Innovative Trend Analysis (ITA) method. The results revealed a significant upward trend in more than 50% of the sampled points for both seasons. For comparison purposes, the Seasonal Mann-Kendall test and the Şen's Innovative Trend Analysis method was applied to the same data. As a result, the ITA method could be successfully used for trend analysis of water quality parameters. Groundwater quality in the Mitidja East appears to be poor in general, and will deteriorate more given the upward trend in WQI revealed by the previous methods. Owing to the importance of this resource for agriculture, industry and human consumption, protecting it against various types of contamination and continuous monitoring will be essential.