Water Quality for Irrigating Vineyards
Alkalinity
Alkalinity is a measure of the total carbonates (CO3 2-), bicarbonates (HCO3¯), and hydroxyl ions (OH¯). Water alkalinity also is referred to as the buffering capacity of water. Alkalinity increases as the amount of dissolved carbonates and bicarbonates rises. Hydroxide ions are a minor contributor in most cases. The dissolved carbonates and bicarbonates increase the media pH over time by neutralizing hydrogen (H?) ions in the media solution. Irrigation water containing high levels of bicarbonates and carbonates can worsen the soil sodium (Na?) hazard by causing calcium (Ca2?) and magnesium (Mg2?) to precipitate, thereby becoming unavailable to counteract the negative effects of sodium (Na?).
Measuring Alkalinity
Alkalinity can be expressed as equivalents of alkalinity or concentration (ppm or mg/L) of carbonates (Table 15.6). Since bicarbonates and carbonates are the major components of water alkalinity, most laboratories assume that Total Carbonates (TC = carbonates + bicarbonates) equals alkalinity. Other laboratories assume that bicarbonates are the sole contributors to alkalinit
Managing Irrigation Water High in Carbonates
Acids are injected into irrigation water to treat plugging caused by calcium carbonate (lime) and magnesium precipitation. Water with a pH of 7.5 or higher and a bicarbonate level higher than 100 ppm has a risk of mineral precipitation, depending on the hardness of the water. The most effective way to deal with high carbonates is by injecting acid into the irrigation water. The bicarbonates in the water react with the hydrogen ions from the acid without removing hydrogen from the media solution. The common acids used for reduction of alkalinity in irrigation water are: phosphoric (75 and 85%), sulfuric (35 and 93%), and nitric (61.4 and 67%).
Acid Requirements to Reduce Carbonates
As mentioned, the best way to determine if irrigation water contains an excessive carbonate concentration is to have the water tested for liming potential by titrating it with an acid. However, if titration is not available, the next best way is based on the concentration calcium and magnesium.
Nutrients Added by Acidification
Growers who acidify their water should adjust their fertilizer program to take into account the nutrients added in the acid. When alkalinity is very high, it is often desirable to use a combination of two acids to prevent excessive levels of individual elements.
Synthetic Scale Inhibitors
The most common alternative chemicals to chlorine and acids are scale inhibitors, which lessen scale formation by preventing precipitation reactions from occurring long enough for problem ions to clear the irrigation system.
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