The Effect of Hydrological and Anthropogenic Factors on the Chemical Properties of Water in the Canal Network of Southeastern Srem - page 05

Chemical properties of water in the Galovica and Petrac drainage areas

The concentrations of soluble salts were determined by measurements of the conductivity of water, ECw. According to recommendations for irrigation water quality assessment per the modified FAO classification (Ayers & Westcot, 1985), the quality of irrigation water is good if ECw is less than 0.7 mS/cm. The results for the Galovica and Petrac canals (at points 5 and 6) are shown in Fig. 9. It is apparent that none of the samples collected in the Petrac and Galovica drainage areas met the condition Ecw< 0.7 mS/cm. However, the measured ECw values do not suggest a serious problem because ECw values between 0.7 and 3.0 mS/cm pose a moderate risk. It is interesting to note that there were no major ECw fluctuations (maximum 1.59 mS/cm and minimum 0.91 mS/cm) during the three-year study period. Water salinity indicated no dependence on atmospheric precipitation. The conductivity of the Sava River was lower by a factor of nearly three (Fig. 9), compared to that of the canal network water.

Since there are no point sources of pollution in the Petrac drainage area, and given that 1200 ha of farmland is being irrigated, it was interesting to determine the spatial characteristics of soluble salts, or the water conductivity along the Petrac Canal. The monitoring points selected for these measurements, going upstream, were 5, 4, 7 and 8 (Fig. 4). Point 8 is located in the most upstream part of the Petrac Canal, in close proximity to the Novi Fenek weir. The conductivity measured at Point 8 was similar to or only slightly higher than that of the Sava (Fig. 10). This was as expected, given that at the upstream end of the canal there was no impact from diffuse sources of pollution. Going downstream (points 7 and 4), ECw increased and reached the highest value at the most downstream point 5. Consequently, as the drainage area increased, so did concentrations of soluble salts as a consequence of natural and anthropogenic drivers, which include the application of chemical/agrotechnical measures in agriculture.

Contrary to conductivity, the pH level of the drainage canal water decreases with increasing precipitation (Figs. 5 and 11). Although the test results show that all the samples were mildly alkaline, it is apparent that after September 2009 the pH level dropped significantly because the period from October 2009 through to the end of 2010 was very wet, with precipitation exceeding average levels by far (Fig. 5). Figure 11 shows the pH levels of the Galovica Canal (monitoring point 6) and the Petrac Canal (monitoring point 5). It is apparent that the water in the Petrac Canal is slightly more alkaline.

When the results of water analyses indicate elevated pH values, or alkalinity, that can be a sign of high bicarbonate ion concentrations. Figure 12 shows the results of bicarbonate ion analyses for the Petrac and Galovica canals. It is apparent that nearly all the values exceeded 8.3 meq/l, which is the cut-off value for high risk of using such water for irrigation (Ayers & Westcot, 1985). Namely, the bicarbonate ion, when in contact with calcium and magnesium in the soil, converts to calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). High calcium carbonate concentrations can considerably reduce the infiltration capacity of the soil. Additionally, the bicarbonate concentrations in the case study were generally such that the use of the water for drip or sprinkler irrigation cannot be recommended. It would therefore be preferable to inject river water into the canal network, to reduce concentrations of dissolved salts.

From an irrigation water quality perspective, salinity is usually associated with SAR (the Sodium Adsorption Ratio). Figure 13 shows SAR values at monitoring points 5 and 6 during the study period. If SAR is less than 3, the water is suitable for irrigation but salinity needs to be taken into account. If SAR is less than 3 and ECw greater than 0.7 mS/cm, there is a moderate risk (Ayers & Westcot, 1985). As such, even though SAR values were relatively low during the wet period (October 2009 to November2010), there was a moderate risk as water conductivity was greater than 0.7 mS/cm throughout the study period (Fig. 9).

 

Fig9
Figure 9: Water conductivity of the Galovica Canal (point 6), Petrac Canal (point 5) and the Sava River relative to the maximum permissible concentration (MPC).

Fig10
Figure 10: Spatial distribution of water conductivity along the Petrac Canal and Sava River.

Fig11
Figure 11: pH levels during the study period: Galovica Canal (monitoring point 6) and Petrac Canal (monitoring point 5).

Fig12
Figure 12: Bicarbonate concentrations in the Petrac Canal (monitoring point 5), the Galovica Canal (monitoring point 6) and the Sava River, relative to the maximum permissible concentration (MPC).

Fig13
Figure 13: SAR values during the study period.