Introduction

Knowledge of groundwater quality and quantity changes is needed for an appropriate management and optimum utilization of groundwater resources. Many different statistical methods are available for detecting and estimating trends present in water quality parameters. These methods range from the simple correlation and regression analyses to time-series analyses and nonparametric approaches. Analysis of the time series is the most common technique used to detect changes in climate and hydrologic phenomena. Trend analysis can be defined as the use of an experimental approach to determine and explain the changes in a system over a period of time in statistical terms. In nonparametric methods, unlike parametric approaches, fewer assumptions about the data frequency distribution (e.g. normality of the data distribution) need to be made. Besides, the knowledge about spatial distribution pattern of groundwater quality is required for a sustainable and appropriate groundwater management. The nonparametric tests of Mann-Kendall (MK) and Spearman’s rho (SR) are commonly used to evaluate the trend of changes in climate and hydrology data.

The aim of this study is to use Mann-Kendall and Spearman methods for investigating the changes in groundwater quality in Khash region during 2004-2013, according to the data of 17 stations. The slope of the trend line for all variables is calculated using Sen's estimator. Moreover, ordinary kriging is used to predict the spatial distribution pattern of groundwater quality parameters.

Matherials and Methods

The study area is Khash region located in Sistan and Baluchestan Province. The study area is located between 61-00 to 61-45 latitudes and 27-45 to 28-40 longitudes. Groundwater quality parameters studied include acidity (pH), electrical conductivity (EC), sodium (Na⁺), sodium percentage (Na%), sodium adsorption ratio (SAR), sulfate (SO₄^2-), the bicarbonate ion (HCO₃^-), chlorine (Cl^-) , magnesium (Mg²⁺), calcium (Ca²⁺), potassium (K⁺), total cations, total anions, total hardness (TH) and dissolved solids (TDS). The data were collected from 17 stations during 1383-1392. The Spearman's rho, Mann-Kendall and Sen's estimator tests were used for detecting trend in quality parameters of groundwater. A brief representation of the methods used is provided as following:

In this study Mann-Kendall (MK) and Spearman’s rho (SR) tests are used to analysis the trend of changes in groundwater quality data over Khash plain. The slopes of the trend lines can be estimated by using a simple non- parametric procedure called the sen’s slope estimator.

To better investigate the groundwater quality changes over 2004 to 2013 across the whole area, the spatial distribution patterns of quality parameters are mapped using ordinary kriging (OK) interpolation method. OK estimator estimates the unknown values of a groundwater quality parameter through a linear weighted moving average of surrounding data values.

Discussion of Results

In this study, the annual changes in groundwater quality in Khash region during the period 2004-2013 with the Mann-Kendall and Spearman nonparametric methods were studied. Sen's estimator slope was used to estimate the slope of the trend line for qualitative variables. The results showed that the majority of water quality variables have positive trend at most stations. The strongest positive trend (Mann-Kendall ’Z=3.93 and Spearman ’Z=3) was seen for EC at station number 4.

Based on the results of the Mann-Kendall test, stations numbers 4, 10, 16 and 17 had the most significant positive collections. Also EC, TDS, K⁺ and Cl^- had the most significant collections are among other variables. The results of Spearman method showed that the stations 4, 10, 15 and 16 have more significant positive trends. Ca²⁺ and SO₄^2- had more negative trends at most stations. The most significant positive trends have seen for EC and TDS, followed by K⁺, Na%, Cl^-, TH and HCO₃^- in the majority of the stations. According to this method, SO₄^2- tends to decrease in the most of the stations. Sen's estimator results showed that the stations 10, 8, 1, 14, 15 and 16 had more positive slopes. According to these results, EC, TDS, Na%, Na⁺, SAR, HCO₃^-, Cl^-, pH and total cations have increased in most stations.

According to the results of all three methods (Mann-Kendall, Spearman and Sen's estimator), stations 10 and 16 are critical situation. The results of the Mann-Kendall and Spearman showed that EC and TDS have increased at most stations. Also, the slope of these two variables is positive at most stations. The results of all three methods showed that SO₄^2- tends to decrease in the most of stations. According to the maps generated using ordinary kriging, the amounts of %Na, SAR, pH and HCO₃^- increased from 2005 to 2013.

To investigate the factors affecting the quality of groundwater in the area, trend analysis of rainfall was also performed. Mann-Kendall and Sen's estimator test results showed that both stations had negative trends of annual rainfall. The results showed a decrease in rainfall, which reduces the volume of water entering the aquifer. Excessive groundwater withdrawals, population growth and development of industrial life are among the factors decrease the water quality in the study area.

Conclusions

The results of this study showed that the Mann-Kendall and Spearman methods have similar performance in detecting stations with degrading water quality. According to the results, EC and TDS tend to increase and SO₄^2- tends to decrease in most stations. Groundwater quality in stations No. 10 and 16 showed the most meaningful positive trends among others. Middle slope of the trend line was positive for 70% of the plotted curves. In general, the results showed a positive trend of groundwater quality parameters in most stations meaning that groundwater quality tends to decrease in this region over time.

Low amounts of rainfall, excessive groundwater withdrawal, population growth and development of industrial life are the main factors threaten groundwater quality in the area. To prevent further groundwater quality degradation and protect the water resources for future generation, appropriate management strategies should be considered.