Hydrogeochemical features of groundwater of semi-confined coastal aquifer in Amol–Ghaemshahr plain, Mazandaran Province, Northern Iran

Abstract

Hydrogeochemical data of groundwater from the semi-confined aquifer of a coastal two-tier aquifer in Amol-Ghaemshahr plain, Mazandaran Province, Northern Iran reveal salinization of the fresh groundwater (FGW). The saline groundwater zone is oriented at an angle to both Caspian Sea coastline and groundwater flow direction and extends inland from the coastline for more than 40 km. Spearman's rank correlation coefficient matrices, factor analysis data, and values of C ratio, chloro-alkaline indices, and Na(+)/Cl(-) molar ratio indicate that the ionic load in the FGW is derived essentially from carbonic acid-aided weathering of carbonates and aluminosilicate minerals, relict connate saline water, and ion exchange reactions. Saline groundwater samples (SGWS) (n = 20) can be classified into two groups. SGWS of group 1 (n = 17) represent the saline groundwater zone below the Caspian Sea level, and salinization is attributed essentially to (1) lateral intrusion of Caspian seawater as a consequence of (a) excessive withdrawal of groundwater from closely spaced bore wells located in the eastern part of the coastal zone and (b) imbalance between recharge and discharge of the two-tier aquifer and (2) upconing of paleobrine (interfaced with FGW) along deep wells. SGWS of this group contain, on average, 7.9% of saltwater, the composition of which is similar to that of Caspian seawater. SGWS of group 2 (n = 3) belong to the saline groundwater zone encountered above the Caspian Sea level, and salinization of the groundwater representing these samples is attributed to irrigation return flow (n = 2) and inflow of saline river water (n = 1).

Assessing Aquifer Salinization with Multiple Techniques along the Southern Caspian Sea Shore (Iran)

“…As reported by other authors [27,41], the aquifer system of the Amol-Ghaemshahr plain is characterized, especially in its eastern portion, by residual connate saline waters due to the eustatic variations in the Caspian Sea over the last 10,000 years [42]. Holding this, while Sites A, B, and C may essentially be affected by actual sweater intrusion, Site D may also be affected by autonomous salinization processes, exacerbated by the upconing of saline groundwater due to aquifer overexploitation.…”

“…This is due either to a large fresh groundwater discharge toward the Caspian Sea in the eastern zone, induced by the proximity to the coast of the Elburz Mountain's recharge area, or to the upconing of the paleo brines in the western zone [27,41], which adds its effect to the actual seawater intrusion from the Caspian Sea. These natural phenomena are then exacerbated by the excessive withdrawal of groundwater from a dense network of wells used for domestic purposes and for irrigation, which in turn may worsen the picture by adding a polluted returned flow to the system.…”

“…An average of 0.12 Mm 3 /km 2 of groundwater is pumped from this aquifer to supply the water demand for cultivation [28], providing also drinking water for 75% of the local population [27]. The drilling and operation of many wells in this area have provoked a progressive lowering of the water table in recent years [27]. Therefore, in the east of the Mazandaran Province, farmers often find saline waters during the midsummer season, which provoke many problems for agriculture and residents.…”

“…The mean annual salinity increases from the surface to the bottom of the sea, reaching values of approximately 13.3 g/L [26]. Despite this low salinity boundary, the coastal aquifer system hosts saline groundwater due to the presence of relict connate seawater within the permeable marine-sandy and sandy-loam Holocene sediments of the semi-confined aquifers of the Amol-Ghaemshahr plain [27]. In addition, as far as the authors are aware, in this area, there are no studies that have used multi-level samplers to precisely locate the transition zone, while a few studies have used integrated depth sampling in long screen wells to characterize groundwater salinity and geochemistry [27,28].…”

“…Despite this low salinity boundary, the coastal aquifer system hosts saline groundwater due to the presence of relict connate seawater within the permeable marine-sandy and sandy-loam Holocene sediments of the semi-confined aquifers of the Amol-Ghaemshahr plain [27]. In addition, as far as the authors are aware, in this area, there are no studies that have used multi-level samplers to precisely locate the transition zone, while a few studies have used integrated depth sampling in long screen wells to characterize groundwater salinity and geochemistry [27,28]. Unfortunately, the integrated depth sampling technique is known to produce misleading results when used to interpret the origin of different saline sources [29] due to artificial mixing within the long screen wells.…”