Seasonal water samples were collected from three important stations which have been under different conditions (Table 1 and Figure 1). The samples were selected along the Shatt Al-Arab River from January to November during 2013. Water samples were taken from the subsurface in the middle of the river for each station, in order to investigate pH values trends. In addition, eight daily water samples were collected from Basrah station during May for physical parameters such as water temperatures and TDS as well as pH. In this study, pH values were measured in situ by WTW Multi-meter model 4430. It should be noted that pH values were measured by portable tools.

4 Results and Discussion

4.1 Factors effecting pH

The explanation of the pH concept is important to investigate the factors affecting pH.

A water molecule consists of two hydrogen atoms bonded to a single oxygen atom (H2O). A small number of water molecules dissociate into separate: hydrogen (H+) and hydroxyl (OH-) ions by ionization process. If the hydrogen ions are greater than the hydroxyl ions water is acidic while the hydroxyl ions dominate, then the water is basic (also called alkaline) (UNEP, 2006).

Quite literally pH is the power of hydrogen (power from German potenz) in the water (Roberge, 2008). The pH of a water is a measure of its acidity, and it is known as the acidity of water which is given by the hydrogen ion. Therefore the pH is defined as the negative logarithm of the hydrogen ion concentration in water, given by the equation that (Hach Company, 2010; Davie, 2008):

pH = -log10 [H+]                                                                                      (1)

Or pH= log〖1/[H⁺] 〗

Where [H+] is the hydrogen ion concentration in mol/L.

The pH scale is logarithmic which means that, when the hydrogen ion concentration increases or decreases by a factor of 10, the pH changes by one unit. pH scale ranges from 0 to 14. Water with a pH less than 7 is acidic, while at pH greater than 7 water is basic or alkaline. Water is considered neutral when the pH is approximately 7.This scale ranges product for water temperature at 25 °C.

The pH value of the Shatt Al-Arab River is affected by several factors including:

(1) Aquatic organisms

The aquatic organisms have a significant natural impact on the pH value of rivers. pH and Carbon dioxide (CO2) are conversely related. Although bacterial and algal respiration provide a source of carbon dioxide in water, however algal activity leads to decrease of concentration of carbon dioxide, due to the utilization of carbon dioxide by algae during active photosynthesis. Thus a pH value increases. For example, a pH value increasing to 10 during an algal bloom on the Potomac River estuary (NERR, 1997).

The phytoplankton were common in Shatt Al-Arab River, the majority is diatoms 75.77%, the blue-green algae came in the second place 11.42%, and followed by green algae which comprise 7.14% (Al-Mousawi et al., 1990).

(2) Climatic conditions

Photosynthesis increases during the summer season and the middle of the day where sunlight is at the maximum (Long et al., 2006). This increase is indicated by the highest pH values during the summer and lower values during the winter. Rainfall will naturally have a lower pH, due to the absorption of gases such as carbon dioxide by Rainwater and increasing the concentration of hydrogen ions in solution, therefore the pH value of rainfall was between 5 and 6 but may drop as low as 4, in air pollution nearby (Davie, 2008). The temperature affected neutral value of pH, where the neutral pH at 30 °C is 6.92, while it would be 7.48 at 0 °C (Hem, 1989). 

The climate in the Shatt Al-Arab region is drought, extremely hot summer temperatures and cooler with some rain, medium humidity in the winter. Average temperatures vary from 9°C to 41°C in the winter and summer respectively. Annual rainfall rates range from 100 mm to about 200 mm (UN-ESCWA, 2013).

(3) Soil and Rocks

Some types of rock which contain carbonate and bicarbonate ions lead to variations of pH values in the waters flowing through this rock. Limestone is rich in carbonates, so that it contributes to increase pH values. Conversely, granite does not have minerals that contribute to alkalinity (Al-Imara et al., 2013). Therefore the rivers that drain limestone have a higher pH due to the dissolved bicarbonate ions. The limestone is a dominant rocks on the Shatt Al-Arab drainage basin.

(4) Human activities

A number of human activities have a harmful effect on the pH of nearby water sources. When sulfur dioxide and nitrogen oxides are emitted, through industrial operations and vehicles. Chemical pollution, from industrial operations, individuals and communities, can cause a water body to become acidic (UNEP 2010). These chemicals can enter the water through illegal discharges or after inadequate wastewater treatment. Humans contribute to lower the pH values of the water, due to contents wastewater discharges on the organic matters and heavy metals, which affect all organisms and lead to biodiversity decrease with decreasing pH values.

Shatt Al-Arab River suffered from influx of domestic sewage and agricultural irrigation water as well as the industrial wastes discharged. There are more than 600 side channels connected with Shatt Al- Arab River (Hussain et al., 1991). Most of those channels are affected heavily by organic pollution due to elevated nutrients concentrations, especially nitrates and phosphates (Eassa et al., 2015; Hassan, 2013) which contribute to relatively low pH values. For wastewater the pH ranged from 6.14 to7.86 (Basrah water directorate, 2013).

(5) Tidal phenomenon

The behavior of marine water is mainly controlled by the river runoff nearby the estuary region . When the tidal water mass in the continental shelf flows into the estuary during the flood tidal period, the sea water diffuses and mixes with the freshwater coming from upstream, which makes the water in upstream rivers salty (Xinfeng & Jiaquan, 2010). Seawater is slightly alkaline with a pH between 7.5 and 8.5, and its pH value of seawater remains relatively constant because of the buffering action of carbon dioxide in the water (Sverdrup et al., 2006). While pH values in freshwater may fluctuate dramatically, due to weaker carbonate buffering system in comparison with marine systems (Chenl & Durbin, 1994).

Shatt Al-Arab River is influenced by the seawater of Arabian Gulf, whereas the Gulf tidal wave advances along the Shatt Al- Arab River. Similar tidal regime of Arabian Gulf and Shatt Al- Arab River is mixed type, however the semi-diurnal is the dominant (Al-Badran et al., 2001). The pH values in water of Arabian Gulf ranged between 7.8 and 8.4 (Zyadah & Almoteiry, 2012) are relatively high into 8.2 as an average in Marine water (Chenl & Durbin, 1994).

4.2 Spatial and temporal variation for pH values:

The spatial distribution of pH average in the Shatt Al-Arab River varies from 7.71 at Basrah to 7.96 at AbulKhaseeb to 8.09 at Fao (Table 2 and Figure 2), due to the difference of environmental conditions between the three stations, where there is likely a large difference impacted by the urban activity, agriculture and tides respectively. Therefore, the surface water quality at the three stations is slightly Alkaline. These values demonstrated that the spatial pH changes were small about 0.38 unit. 

The seasonal variation of pH values between 7.38- 7.98 at spring season and 8.00- 8.24 at summer season. The lower pH values in spring may be due to the increase of freshwater discharge in the river and rainfall events, the highest pH values during the summer season may be due to peak algal activity and low flow.

Table 2 Statistical parameters of pH values in Shatt Al- Arab River at three sites during 2013

Figure 2 pH values in Shatt Al- Arab River at three sites during 2013

The maximum pH value occurred at Fao in Summer 8.24, which happened during a period of decrease of freshwater discharge and increase the progress of tidal brackish water in addition to increase of photosynthesis. The minimum pH value occurred at Basrah in spring 7.38, which happened during a period of high flow, rainfall and relatively decrease of water temperature. These values indicated that pH changes in the seasons were moderate about 0.86 unit, and they also tend to drop toward Fao station (Figure 2). The seasonal variation of pH also confirmed by calculation of coefficient of variation statistical parameter (C_v), (Table 2). The C_v is computed as:

                                                                                 (2)

where, s is the standard deviation and is computed as:

                                                                                (3)

                                                                                     (4)

N is the total number of values.    

The highest value of C_v is observed at Basrah station 3.945 may be due to the strong variation of influenced factors, while the lowest value is recorded at Fao station 1.352 due to the dominant of the sea water.   

The relationship between pH, discharge, and TDS values for the period (1977- 2013) is presented in (Table 3). It is obvious that there is no temporal relevant association between these parameters. Although, the average of values of discharge decrease significantly from 919 m³/s to 41 m³/s, and the average values of TDS increased from 1.13 g/L to 17.30 g/L, the variation in pH values is still very small. This relatedness is attributed to the same ionic structure of the river water, may be due to the balance between the sea water which contributes to the increase of pH values and sewage effluent of human activities that contributes to the decrease of pH values (Chenl & Durbin, 1994), when the discharge of freshwater in the Shatt Al-Arab River decreases the impact of sea water and sewage effluent increasing, in addition to impact increased of TDS on height internal resistance to pH change in water and height buffering capacity, where the waters with low dissolved solids, which consequently have a low buffering capacity (EPA, 2001).

Table 3 Variations in pH values of Shatt Al-Arab River during 1977–2013

The observation of daily fluctuations of pH values in Shatt Al- Arab River showed small pH change (0.56 unit), the pH value may fall to 6.96 in the early morning at 6:00 AM and increase to 7.52 in the afternoon at 3:00 PM (Table 4 and Figure 3). This daily pH change gives an indication of the limited impact of aquatic organisms on pH change which may be due to the insignificant change of daily cycles in the river flow, and large flow from 741- m³/s at flood to 882 m³/s at ebb (Al-Asadi, 2012). Also daily fluctuations in river water temperature are not clear, it ranges from 27.1 to 31.4 °C.

Table 4 Daily variations in pH values of Shatt Al-Arab River during May (28-29/5) at Basrah station during 2013

Figure 3 Daily variations in pH values of Shatt Al-Arab River during May (28-29/5) at Basrah station during 2013

5 Evaluation of a pH Value for Different uses

The pH values of water in the Shatt Al-Arab River in this study ranged between 7.38- 8.24, effecting water suitability for usage. A comparison with WHO, FAO standards indicates that all the station showed a pH value within the permissible limit of 6.5- 8.5 (Table 5). So, the river water of these stations are suitable for drinking due to no significant effects on health, and suitable for irrigation, livestock and poultry. The pH values are in acceptable limits 5.0- 9.0 for aquatic organisms.

Table 5 Water quality parameters and comparative guidelines

6 Conclusions

1- The range in pH values was moderate in Shatt Al-Arab, about (0.86 unit). The minimum value occurred at Basrah Station on Spring (7.38), while the maximum value occurred at Fao Station on August (8.24).

2- The change in pH values resulted from spatial and temporal variations in environmental conditions such as the tidal phenomenon, human activities, aquatic organisms and climatic conditions.

3- The pH values during the study period tend to be slightly alkaline and does not fluctuate widely, which is consistent with previous studies.

4- There is no variation in pH values among the studied years during 1977 to 2013.

5- The Shatt Al- Arab water has a good buffering for changing the pH values.

6- The observance of daily fluctuations of pH values showed that small pH change 0.56 unit.

7- The pH values in the Shatt Al-Arab River show seasonal and spatial differences but for all Stations fall within the acceptable range of 6.5 to 8.5.

Acknowledgements

We would like to thank Prof. Dr. Abdul Zahra A. AL- Hello, Department of Marine Chemistry, Marine Science Center, University of Basrah, for reading the manuscript and for his critical advice. And Asst. prof. Mahdi Alasadi, chairman of the department of English, College of Education for Human Sciences, University of Basrah.

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