The Rate of Decline and Trend Line Analysis of Groundwater underneath Dhaka and Gazipur City

Groundwater is reported to account for 87% of all drinking water resources in Dhaka which has suffered a decline of up to 75 m in some specific location. Over-extraction of groundwater is an extensive social problem in Dhaka and Gazipur city which needs to be investigating thoroughly. This study presents the diagnosis of groundwater depletion pattern and the yearly rate of decline over the last three decades for Dhaka and Gazipur metropolitan area. Groundwater data were collected from the relevant institutions in order to analyse the trend line and the rate of decline of groundwater levels for more than 30 years period to understand the long-time variability. Ten individual stations datasets for GWL have been analyzed for Dhaka and Gazipur within a selected reference time period (1980-2012). The highest depleted GWL were found in the Mirpur station which is now 68 m below ground. To find out the time span when the depletion rate is highest, the rate of decline of all datasets has been computed which shows that 1998-2005 is the consecutive eight years time span with the fastest depletion rate. On the other hand, the annual trend-line analysis shows rapid depletion pattern after the year 2000.


Journal of Water Resource and Protection
larly for the cultivation of Boro rice [1]. Intensive use of groundwater impacts on the groundwater table which has gradually declined in the north-west and north-central regions of Bangladesh. Specifically, in Dhaka city, where groundwater is reported to account for 87% of all drinking water resources, groundwater levels have suffered a decline of up to 75 m in some specific location [2]. In these areas, evidence indicates that recharge does not keep up with abstraction rates. To fulfill the demand of fresh drinking water for the Dhaka city dwellers, uncontrolled abstraction of groundwater has been practiced for a long time, which has created problems with sustainability with regard to the management of water resources in the city. The overall lowering of the water table over the last 38 years has been from 20 -40 m in the upper aquifer, with an average decline of more than 1.0 m/year and a maximum of 3.0 m/year in the central part of the city [2].
The purpose of this study is to find out chronological variability of GWL in Dhaka and Gazipur city because Dhaka is densely populated metro and Gazipur is the growing industrial city where water demand is increasing rapidly. The objective is to identify the trend of groundwater use in the study area and probing for an interrelation with population growth. Besides, the rate of decline analysis has been done to focus out the most vulnerable year for unsustainable abstraction.

Literature Review
Due to rapid urbanization in the city resulted in higher water demand, the water  [2]. Besides, around 65.4 m below the ground surface at Mirpur, and 20 -34 m below the ground surface in the Mohammadpur, Dhanmondi, and Sutrapur areas, which are close to the river periphery [2]. In this literature, the investigation of declining GWL is simply done by yearly trend line analysis.
According to The World Bank estimates made in June 2007, Dhaka is a highly dynamic city that attracts some 300,000 to 400,000 new migrants each year and projection for future population indicates that Dhaka is expected to grow to about 20 million in 2020, making it the world's third largest city on the basis of population [3]. So, it can be said that Dhaka is the world's fastest-growing megacity.
Dhaka is dependent mainly on the groundwater resources of the fluvio-deltaic Plio-Pleistocene Dupi Tila Aquifer, which provides about 87% of the total water supply [2]. Groundwater is the first choice for city dwellers as it is superior in quality to surface water. To meet the demand, about 2.0 Mm 3 is withdrawn daily by about 567 DTW (Deep Tube Wells) and delivered to the inhabitants by a M. Parvin Journal of Water Resource and Protection 2500-km-long pipeline network [2]. System loss is assumed to be more than 25%. It is estimated that the volume extracted from more than 900 private deep tube wells in the city area may be more than 50% of the DWASA withdrawal [2].
The renewable recharge to the aquifer is nearly negligible if it has been compared to the exploitation of groundwater [2].

Case Study Area
There are a total twenty groundwater level monitoring stations within Dhaka city from which eight stations were selected due to the continuity of the datasets [4] and matching of the available data with the reference period .
Red dots are the location of GWL station within Dhaka city. Besides this, observed data are available for Savar and Gazipur city which are 27 Km and 29 Km away from Dhaka. So, in total ten GWL stations observed data is available for a more or less 30 years period. These have been analyzed to find out if there is a pattern of depletion and the maximum depletion rate period within the selected reference period of analysis. On the other hand, Population growth datasets [5] are available for six different stations to compare with the GWL pattren. Figure   1 represents the location of GWL station in the Dhaka metropolitan area.

Groundwater Level Analysis
In order to assess the condition of the groundwater level, there are twenty wells in Dhaka district and one well in Gazipur district monitored by Bangladesh water development board (BWDB). Weekly groundwater level data were found for the various time periods where some of the data for a few months of some years was found to be missing. For assessment, mean monthly datasets for selected reference period (1980-2012) was formulated which are passed in the error check process. Finally, ten individual mean monthly groundwater datasets were selected for evaluation.

Annual Mean Variability
Annual mean groundwater level (below from ground) for ten different locations within Dhaka and Gazipur districts were plotted against time to discover variability of GWL. Besides the annual mean groundwater level variability detection, the slope of the plot has been evaluated and summarized to find out when and how slopes are changing.

Rate of Decline GWL Datasets
It is necessary to find out the maximum depletion period for the Dupi Tila aquifer from where 87% of domestic water supply for Dhaka district has been managed. In this case, the rate of decline of GWL datasets has been done by lag time and difference method using excels spreadsheets. This difference is the change of GWL for each year which is the depletion rate per year for each set of the dataset. After formulating each dataset, the rate of decline datasets has been plotted in a single graph to identify the maximum depletion years within the reference time period.

Trend Line Analysis for Eight Stations: (Mohammadpur, Sabujbagh, Mirpur, Keraniganj, Lalbagh, Sutrapur, Savar and Gazipur)
Regression analyses have been done for annual GWL datasets [4] to identify the change of the slope in the trend line with progressing time. Besides, decadal population growth has been compared for each available dataset [5] in every individual station. For the Mohammadpur station in Figure 2(a), the annual depletion rate is 0.7 m/year where the depletion rate is relatively consistent, with no sudden changes. It seems that water level is decreasing at a steady state rate over the selected time span and population growth is at a steady rate for every decade. Besides, the annual depletion rate at the Sabujbagh station in Figure   2

Analysis of the Yearly Rate of Decline
Removing a trend from the data analysis enables to focus on the fluctuations in the data about the trend. A linear trend typically indicates a systematic increase or decrease in the data. In this analysis, the rate of change of the groundwater levels is explored to find out if there are particular periods with more pronounced increases/decreases. From the rate of change pattern for ten groundwater level station, it can be seen that there is a tendency that the depletion rate increases with time. Especially after the year 1998, the depletion rate is higher.
There is a sudden increase in the year 1984-85 for three different stations though these may be outliers. Finally, it can be concluded that the years 1998-2005 is the maximum GWL depletion period for the study area. Figure 3 represents the maximum depletion year for ten GWL stations within the study area [7].

Conclusion
Ten different datasets for GWL have been analyzed for Dhaka metro and Ga-

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.