The trend of groundwater quality parameters in Zarringol Plain using nonparametric Mann-Kendall and Sen̕s Methods

Groundwater is the most important water resources for agriculture, industry and drinking and therefore qualitative changes in program planning and sustainable management of water resources is necessary. The aim of this study was to evaluate the trend of groundwater quality parameters of some selected stations in Golestan province by using non-parametric Mann-Kendall and Sen̕s Methods. For this purpose, the water quality data were gathered in 8 stations of Zarringol plain during 1996-2011. The quality parameters used in this study were TH, Cl-, K+, Mg2+, Ca2+, Na%, TDS, SAR, Na+, HCO3 -, SO4 2-, EC, pH, the total sum of cation and anion. Before analyzing the data, the effect of significant first order autocorrelation was removed from the data set. The results showed most water quality parameters tends to rise. 41% and 20% of groundwater quality time series show meaningful positive trend during wet and dry season, respectively. Also 20% and 10% of groundwater quality time series show meaningful negative trend during wet and dry season, respectively. Due to the positive slope of the trend line for most quality parameters, water quality in this region tends to decreases. The maximum positive slope of the trend line is belong to EC, where observed in downstream of plain. The loss of groundwater level due to increase of consumption can be one of the main reasons for decreasing the ground water quality and salinity.