Environmental impact of Karkheh Dam in the southern part of Iran on groundwater quality by intervention and trend analysis.

The main objective of this research was to investigate the impact of the construction of Karkheh Dam in 2001 (referred to as the intervention time), on groundwater quality. The time series of total dissolved solids (TDS) and other water quality data including potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), bicarbonate (HCO), sulfate (SO), and chloride (Cl) for the period between 1996 and 2012 were analyzed. The magnitude of the trend by Sen's slope estimator for HCO, SO, and TDS was 0.005, - 0.02 and - 3.04, where a decline expected for SO and TDS, whereas for HCO, an increase was expected. According to the Pettitt's test, the mean of TDS decreased from 2306.9 mg/l between 1996 and 2002 to 797.2 mg/l between 2002 and 2012. During this time, the standard deviation of TDS declined from 2187.1 to 132.0 mg/l. The results of change point detection by the Pruned Exact Linear Time (PELT) algorithm were consistent with that of Pettitt's test providing confirmation that a change point in Ca, Mg, SO, and TDS time series data occurred in 2002.The findings from intervention analysis using the Bayesian structural time series (BSTS) technique showed that TDS concentration during the post-intervention period had an average value of 1127 mg/l compared with 1972 mg/l, before the dam construction. The time series of TDS demonstrated a decrease of about 43% following the intervention time.