The Impact of Climate Change on the Content and Spatial Distribution of the Total Dissolved Solids in Bandar-e-Gaz Coastal Aquifer

Groundwater resources are the most important sources of agriculture, industry and drinking water supply in many areas. Water quality management for groundwater resources are very important due to the wide use of these resources and there is a need for an adequate understanding of the groundwater quality in the present and future. Coastal aquifers are in contact with the sea which are exposed to saltwater intrusion into fresh groundwater. Therefore, they have a high risk of groundwater water quality degradation. On the other hand, the presence of residential areas in coastal aquifers is accompanied by the entry of drinking water’s wastewater by absorbing wells and leakage from the sewage collection network, which can increase the risk of quality pollution in these areas. Furthermore, the climate change can be very effective in changing water quality in this area due to the direct impact on recharge, and discharge. The Bandar-e-Gaz coastal aquifer is located in the northern part of Iran which is the most important source of fresh water for the inhabitants of this area. The present study aims to assess the impact of climate change on the content and spatial distribution of total dissolved solids, as a water quality measure, in the coastal aquifer level using numerical simulation.

Bandar-e-Gaz coastal aquifer is located to the west of Golestan province in the north of Iran with an area of 173 square kilometers. This aquifer is limited to the Gorgan Gulf from the north, and to the highlands of the Alborz Mountains from the south, and also to the adjacent aquifers from the east and west. In the northern part of the aquifer, groundwater levels have low depth and, on the other hand, significant areas of the Bandar-e-Gaz coastal aquifer area are in the residential area. The drinking water’s wastewater in residential areas is mainly evacuated by absorbing wells, which can significantly reduce the quality of groundwater resources in this aquifer.
The MODFLOW and MT3DMS groundwater models in the form of an integrated model were used to simulate the groundwater level and content and spatial distribution of total dissolved solids. These models solve the driving partial differential equations numerically using a finite difference method. To implement this model, determination of boundary conditions, quantitative and qualitative observation wells, hydrodynamic coefficients of the aquifer, recharge, and discharge from the aquifer, porosity, and longitudinal dispersion coefficient are necessary. The models used for a 12-month calibration period were verified in next 12 months. Besides, the mean of absolute error or MAPE was used to determine the precision of the model.
In this study, the 5th report of International Panel on Climate Change (IPCC) and new emission scenarios were used to investigate the climate change impacts on future changes in content and spatial distribution of total dissolved solids in the Bandar-e-Gaz coastal aquifer. The impacts of climate change on groundwater quality were studied under two optimistic and pessimistic scenarios. These are as follows:Optimistic climate scenario (RCP2.6) + extraction of groundwater at a constant rate until 2100+ no change in Caspian Sea level up to 2100.
Pessimistic climate scenario (RCP8.5) + extraction of groundwater with an increase equal to 2% per year by the year 2100 + 5 meters decrease of Caspian Sea level at the end of the period 2100.

Considering the model's precision criterion (mean absolute percentage error) indicates that this criterion in the calibration and verification period is in the range of (0.0008-0.029) and (0.032-0.055) which indicates the precision and also the perfect reliability of the model to estimate the content and spatial distribution of the total dissolved solids in Bandar-e-Gaz coastal aquifer. The results of the emission scenarios show that both scenarios indicate an increase in future precipitation and temperatures, but the intensity of the changes in these two scenarios are different. The simulation of total dissolved solids based on the optimistic scenario shows that the average content of this pollutant will have a seasonal periodic behavior in the long term and will experience a very limited increase in the aquifer level. The use of a pessimistic scenario represents a periodic and increasing behavior in the average content of total dissolved solids at the aquifer. Moreover, there will be a different intensity of increases in different periods, with the greatest increase in near future. Furthermore, the prediction of spatial distribution of total dissolved solids indicates that pollution in the north of the aquifer will be significantly increased, and the increase in pollution at the aquifer level will not be uniform in the spatial pattern, which can be due to the spatial distribution of residential areas, as well as water extraction wells.

The prediction of the response of coastal aquifers to pollutants from the sea and by humans is very important. In this study, numerical models were used to predict the quality response of Bandar-e-Gaz coastal aquifer to climate change as well as human activities. The results of this study indicate that an increase in the total dissolved solids content in the aquifer will occur in future. The numerical simulation also leads to the estimation of spatial distribution and changes of total dissolved solids c in Bandar-e-Gaz coastal aquifer. The predicted values for increasing the total dissolved solids content, as well as the expansion of spatial distribution maps of this pollutant, can be used as a suitable tool for managing groundwater quality in the area.