Investigation of nitrate concentrations in groundwater resources of Marand plain and groundwater vulnerability assessment using AVI and GODS methods

Introduction
More than 90% of drinking water in cities around the world and about 40% of the agricultural water supply from groundwater resources.,Thus, groundwater quality consideration issue is inevitable . One of the most important parameter which can show the quality of drinking water is nitrate concentration. Nitrate enters the groundwater and surface water through the decomposition of human and livestock wastes, industrial outputs and agricultural fertilizers leaching. Typically, the concentration of nitrate is higher in shallow groundwater and decreases with increasing depth and toward downstream due to the diffusion process, mixing and dilution with low nitrate groundwater.
 
Materials and methods
Marand plain is located in East Azarbaijan Province in the northwest of Iran, with an area of approximately 826 square kilometers. The plain is a part of Caspian basin. Groundwater resources of the plain have been formed in Quaternary alluvial sediments. The sediments formed in the mountain range pediments are coarse and gradually the grain sizes decreases towards the central parts of the plain and turned into clay and silt at the and end parts. Zilbir Chay and Zonouz Chay are important rivers in the study area. Based on the results of geophysical investigations and geological logs, there are  three types aquifer including; unconfined, confined and semi-confined aquifers in the plain (Fig 1). Unconfined aquifer is formed in ancient terraces, recent terraces, alluvial fans and fluvial sediments and the main materials of deposits are gravel, sand, silt and clay. The thickness of the unconfined aquifer varies in different parts of the plain. The southern part of the plain is made ​​of semi-hard conglomerate with Plio-Pliostecene debris and it must consider as low permeable and semi-permeable layers because of clay and marl layers. Confined aquifer, mostly in the form of ancient alluvial deposits, is covered by clay and marl layers with thicknesses varying from 10 to 30 m. The maximum thickness of the confined aquifer reaches to 170 meters in some parts of the plain. The confined aquifer has expanded in the central and western parts of the plain and even in some parts of the Zilbir Chay and Zonouz Chay Rivers terraces. The semi-confined aquifer is placed in western part of the plain.
To evaluate nitrate contamination, sampling from 48 shallow and deep wells of the aquifers was carried out in July 2012. Parameters and analyzed ions include pH, EC, major cations, anions and nitrates. Nitrate was analyzed by spectrophotometer and other cations and anions by standard methods at hydrogeology laboratory of Tabriz University. Then, the spatial distribution map of nitrate concentration was plotted using the ArcGIS10 software. Groundwater vulnerability assessment performed by two methods including AVI (Aquifer Vulnerability Index) and GODS (Groundwater Occurrence, Overlying lithology, Depth of groundwater and Soil rates) methods.
 
Results and Discussions
Nitrate contamination
Evaluation of nitrate concentrations in groundwater of the Marand plain indicated that nitrate concentrations are over the allowable concentration in drinking water (45 mg/L) in 12 analyzed water samples (Fig 1).  an Based on nitrate concentrations and land use map, it can be certainly stated that the nitrate concentration are well defined with land use map however,  the particle size distribution of sediments and hydrogeological conditions have a determinant role in the distribution of nitrate concentrations. Nitrate concentration in the central and eastern parts of the plain is more than other places because of the active ​​agricultural area, entrance of wastewater to groundwater, unconfined aquifer type and coarse sediments are located in this part of the plain. The lowest nitrate concentrations collected from west and northwest parts of the plaindue to the low agricultural activity in comparison with other parts of the plain, the confined aquifer condition and fine-grained sediments in this part of the plain.
The nitrate concentrations decrease with increasing the depth of wells. It can be mentioned that the concentrations of dissolved oxygen in water decrease with increasing depth, hence it is possible to enhance denitrification process and remove some amount of nitrate.
 
 
Fig 1. Spatial distribution of nitrate concentration and the maximum allowance concentration
 
Figure 2 shows the spatial distribution of nitrate concentration and land use map of the study area. This figure shows, that the high concentrations of nitrate in the east and southeast part of the plain reflects extent and intensity of agricultural activity and therefore increase agricultural fertilizers, including phosphate fertilizers, nitrogenous and potash in this regions. Moreover, unconfined aquifer type and coarse sediments cause to increase the permeability of the aquifer and the rapid nitrate leaching from the unsaturated zone and lead to increasing nitrate concentrations in this part of the plain. The reasons for the lower nitrate concentrations in the central and western parts of the plain are type of the aquifer (confined aquifer), fine-grained sediments and undesirable quality of water for agricultural purposes.
 
 
 
Fig 2. Comparison of variation in nitrate concentration with land use map
 
Vulnerability of Marand plain groundwater
To determine the contamination potential of the plain, two Vulnerability assessment methods named as AVI and GODS methods were used. Vulnerability mapping with AVI and GODS methods (Fig 3a,b) show that those  parts of the plain, which contain unconfined aquifer type have the highest contamination potential whereas those  parts of the plain containing confined aquifer  condition have lowest contamination potential.
 
 





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Fig 3. Vulnerability map of a) AVI and b) GODS methods