Investigating the hydrogeological behavior of Sabzevaran fault using groundwater level data, water quality parameters, and environmental isotopes (18O, 3H and 14C)

Introduction

Faults are among important geological structures, affecting groundwater flow and its quality. The faults’ function can cause the complexity of the groundwater flow in alluvial aquifers. Faults have different hydrogeological behaviors in geological environments. Fault zones can act as a conduit for groundwater flow, a barrier to groundwater flow, or a complex conduit/barrier system. The displacement of rock and sedimentary layers around the faults may lead to clear changes in the bedrock level, aquifer thickness, and groundwater depth. Faults can also channelize the groundwater flow and solute transport. Determining the hydrological behavior of fault structures can play an important role in the sustainable management of water resources. The Sabzevaran fault system is one of the most important fault systems in southern Iran. This system is a right-lateral transverse fault having a length of 150 km. Sabzevaran fault striking N-S (from the west of Jiroft city to the southwest of Kahnuj city in Kerman province, Iran) shows the evidence of Quaternary deformation. The fault scarps, cutoff ridges, and tilted rivers represent the activity of this fault. This study aims to investigate the hydrogeological behavior of the Sabzevaran fault and to evaluate its impact on the water fluctuations and groundwater quality.

Materials and Methods

Various data were used to investigate the hydrogeological behavior of the Sabzevaran fault. Geophysical surveys are one of the best indirect methods to study the subsurface geological situation. Geophysical studies were conducted by geoelectri method of Abkav- Louis Berger Company. The results were used to investigate the effect of the fault on changing the aquifer geometry and the situation of the subsurface layers. Investigating the groundwater level’ fluctuations is very important in water resources studies. The groundwater level’ fluctuations were investigated using the depth water data of the monitoring wells. The groundwater depth and isopotential maps were also plotted. Fault systems can affect the groundwater quality. To evaluate the impact of the Sabzevaran fault on the groundwater quality, 57 water samples were collected from abstraction wells and their parameters such as electrical conductivity and concentration of major cations (calcium, sodium, and magnesium) and anions (bicarbonate, sulfate, and chloride) were measured in the laboratory of Kerman Regional Water Authority. The isotopic ratio of the chemical elements of water molecules is a useful and new method in water resources studies. The isotopic contents of oxygen-18 (18O), tritium (3H) and carbon-14 (14C) were also used in this research.

Results and Discussion

The geoelectrical results confirmed the activity of the Sabzevaran fault and its role in changing the aquifer geometry. The Sabzevaran fault changed the bedrock topography, aquifer thickness and depth to water level. A significant difference in the depth of water can be observed around the Sabzevaran fault. There was a significant difference in the groundwater level due to fault action. The difference in water level on both sides of the fault ranged from 20m to more than 30m in different areas. The pattern of temporal fluctuations of groundwater was completely different on both sides of the fault. The groundwater level had a sinusoidal pattern on the west side of the fault, while the groundwater showed a completely downward trend in the east of the fault. The direction of groundwater flow was also different around the Sabzevaran fault. Groundwater flow was parallel to the fault line in the eastern part of the fault, while groundwater was channelized in the western part of the fault. The highest amount of electrical conductivity and concentrations of sulphate and chloride ions were recorded in the wells adjacent to the Sabzevaran fault. The highest concentrations of oxygen-18, tritum, and carbon-14 were observed in the middle part of the fault. Also, groundwater of the fault zone has a distinct isotopic content compared to the eastern part of the aquifer. The difference in groundwater residence time and its entrance into the aquifer through the fault were the reasons of the isotopic difference.

Conclusion

The results of this study indicated that the Sabzevaran fault caused the hydraulic disconnection of the western part of the aquifer of the Jiroft plain. This disconnectivity had affected the groundwater flow and its quality. Due to the action of the fault, clear changes in surface topography, bedrock level, aquifer thickness, groundwater level, hydraulic gradient, groundwater flow direction, and water quality had occurred. Examining the groundwater fluctuations, water quality parameters, and isotopic content of water samples also confirmed these findings. This study showed that the Sabzevaran fault behaves as a hydraulic barrier to groundwater flow in Jiroft plain and led to the channelization of the groundwater flow, especially in the western margin of the aquifer. In order to more accurately investigate the effect of the Sabzvararen fault on the groundwater system, it is necessary to carry out geoelectrical studies at the fault zone. Measuring groundwater depth, water quality and isotopic content of wells adjacent to the fault zone is very important. It is suggested to consider the hydrogeological behaviour of the Sabzvararan fault in the preparation of mathematical models of the groundwater system and the development of aquifer management strategies.