Fresh water resources are one of the most important resources for life on earth. As a result suitable water supply in terms of quality and quantity is of vital importance.
Ground water pollution has been one of the consequences of ignoring the environment. This type of environmental pollution has been encountered in large cities in the world (Baghvand and Givehchi, 2005).
The water for the city of Tabriz with a population of about 3 million has been supplied from Zarineh-rud, deep wells from Leeghvan basin and other underground water resources with total volume of 18x107 L (Baghvand, 1995).
Considering the sewage system, a great portion of this water is recharged into ground water. In addition a large amount of running water after passing through the industrial site of western part of the city enters the ground water resources (Mahmodian, 2002).
Although the importance of heavy metals and impact of them in human health, this research has been performed to identify which of those appear to higher than allowable value. Existence of heavy metals in drinking water in Iran has rarely been studied and reliable data exist (Baghvand and Givehchi, 2005). However some recent research work indicate that the value of heavy metals such as Cd, Pb and Cr pertinent standards (Baghvand, 1995). And other research work shows that Hg content in water of industrial zones of Tabriz (city in norteset of Iran) is higher than expected (Kavab, 1999).
This has become a major source of pollution. The city has also grown very rapidly in recent years. In this regard, ground water resources must be considered as a major source of water supply and pollution of western part of the city with heavy metal warrant investigation. The object of this research is to identify that quantity of each heavy metal in water of industrial zone of Tabriz is permissible or not. As a result a research has been carried out in industrial part of Tabriz in order to identify source of heavy metal pollution.
Area of study: Tabriz is one of the main cities in Iran that is located about 619 km from Tehran, the Capital of Iran. The study area is about 15000 hectares which is located in the north western part of Tabriz city. This area is situated between 46°8' to 46 °12' longitudes and 38°1' to 38°8' latitude.
In Fig. 1 the investigated area and sampling locations are presented.
The area of study is comprised of 100-120 m deep alluvium built on the layers of impermeable stratum of Miusen (calcareous-clayish settlements). The alluvium is comprised of fine and coarse gravel, sand, silt and clay substances.
There are two kinds of ground water in the area of study. First one is shallow ground water which is salty due to the infiltration of surface water which is salty. Adgi Chai River, the main river in the area is salty specially in the summer times.
|| The location of bore holes in the area of study
The deep ground water is inherently sweet as it originates from deep streams of water from mountains surrounding Tabriz plain. In recent years the high amount of ground water discharge, permeation of water through low-permeable clay-silt layer that separate deep ground water from shallow one and artificial connections between upper and lower aquifer had made the deep ground water salty. With initiation of the prohibition of deep ground water discharge in the region the situation is through recovery.
Yearly average temperature in the region varies between-6° to 32.9°. The range of precipitation is from 280 mm in summer to 320 mm in winter. Yearly evaporation is 1100 mm which 60% of it is during summer. The wide range of temperature is effective on solubility of the salts containing metals. So less immobility is expected in winter. More precipitation and less evaporation are also showing more dilution ability of aquifers and so less pollution by heavy metals during winter.
MATERIALS AND METHODS
For the purpose of sampling two wet (winter) and dry (summer) seasons has been chosen to observe the difference expected due to different precipitation discharge and evaporation. Twenty six wells with the depth of more than 65 m has been selected for sampling. The methods proposed by American Water Works Association have been used to determine the concentration of different metals in the ground water.
RESULTS AND DISCUSSION
Figure 2 to 7 show the concentration of
metals studied during winter and summer seasons in the area.
Currently there is no maximum contaminant level for ground water set in the national regulations but there are some limitations on discharge. As a guide the maximum concentrations which are determined for the water used in agriculture has been used as the maximum level of contamination in ground water.
For the elements Pb ,Cr, Zn, Cu and Cd concentration in ground water is less than maximum acceptable level. In some places the concentration is relatively high but the aquifer seems to have the ability to assimilate the pollution very well when we further from sources through dilution and absorption in the soil. For the element mercury, the level of contamination is high and also the ability of assimilation is low specially in summer times.
In Fig. 8 the distribution of concentration in the area using the results of samplings in summer. Minimum level of contamination in the area is 67 ppb which is not acceptable. Aquifer has attenuated the pollution from 82 ppb near the source to 67. It is not enough for 10 km of transport.
On the other hand in Fig. 9 the distribution of concentration of Lead during summer is shown. The level of contamination is much below the acceptable value. This can be a natural level of lead in the ground water. Other elements but Mercury have the same behavior as lead.
|| Chromium concentration in ground water in different wells
|| Lead concentration in ground water in different wells
|| Mercury concentration in ground water in different wells
|| Zinc concentration in ground water in different wells
|| Copper concentration in ground water in different wells
|| Cadmium concentration in ground water in different wells
Pb, Cr and Cu show more concentrations in summer and it is only explained by source variation in industries. On the other hand Hg, Cd and Zn show increase in concentration which is explainable with more dilution through more precipitation and less evaporation.
First source of ground water pollution is domestic wastewater as it is disposed in wells in Tabriz City. Although the concentration of heavy metals in domestic wastewater is not so high but regarding the high volume of discharge to ground water creates a potential of pollution.
||Iso-line for concentration of Mercury in summer, it is clear
that assimilation capacity of the aquifer is not strong enough to reduce
the concentration dramatically
||Iso-lines for Lead concentration in summer, the concentration
of lead is decreased more than 50% from maximum to minimum
Surface water discharge to ground water is another source of pollution, but according to the usual observations and with regard to the long distance to dense urban parts where the surface water may have high concentration of lead, no significant impact is visible.
Industrial wastewater which is the main concern of the article is the main
source of groundwater pollution by heavy metals. Four categories of main problematic
industries have been observed in the region that may have high emissions of
heavy metals to ground water.
||Dendrogram of heavy metal concentration in winter
||Dendrogram of heavy metal concentration in summer
||Main problematic industries in the region and corresponding
|* existence of each industry
In Table 1 the industries and potential releases are mentioned.
Looking at the release profile of industries we can see that for the six mention heavy metals in the table there is a potential to be a concern in the regional ground water.
The result of analysis has been shown in Fig. 10 and 11. Based on these results (Fig. 10 and 11) the pattern of cluster in summer is different from winter. The main reasons for this, seems to be the uneven effect of dilution in winter and different release profiles. In both summer and winter the lead and cadmium are totally bind to each other showing to have the same source in industry as expected. Mercury seems to be independent from other metals as there is little assimilation through area. Correlation between mercury, copper and zinc is observed but it is very week because the spatial distribution is not even in this case. Cu, Zn and Cr seem to be independent more or less although the Cr shows stronger correlation with Pb, Cd cluster in winter.
||Iso-lines of mercury concentration in ground water in area
map. The source of mercury seemed to be in the eastern part of the region
Concentration of mercury is above the national acceptable limit. The strong
bound of mercury with zinc show that they should have the same source. Petrochemical
industry seems to be responsible according to the place of high concentrations
of mercury at the table of sources and the correlation between Hg, Zn, Cu and
Cr at the places of high concentration. Concentration of metals increase drastically
in summers which is not favorable specially in mercury case. Although the Ban
of discharging in Ground water still exists but the dilution capacity seems
not to be enough for this element (Rand et al., 1994). Compensation measures
should consist of prohibition of any usage of ground water in the region and
even taking the advantage of some soil remediation technologies in sources with
emphasis of site investigations on petrochemical industry (National Standards
for Discharge of Wastewater, 2003).
As this is clear from the Fig. 12 there is not only one source of mercury pollution but also a main source in eastern part and another one in the south. Assimilation takes place in direction of east to west as the predicted direction of ground water and also Adgi Chai River (Azarbayjan Regional Water Office, 1990).
Source of cadmium and lead seemed to be the same but no specific industry can be appointed comparing cluster analysis and table of sources.
It is also understood through the research activity that the cluster analysis is not very efficient when dealing with uneven geographic distribution of parameters and it is more effective when the correlated parameters has been sampled from the same location.