Desalinated water poses problem for agriculture
countries desalinized brackish water, the price of which is typically a third of desalinated seawater, is already widely used by farmers. For instance, about 22 percent of water desalinated in Spain goes to agricultural irrigation. An Australian survey found that 53 percent of the population envisioned desalinated water usage for irrigation of vegetables as highly likely. In Israel, the promise of new, profitable crop options has inspired farmers to request allocations of relatively higher priced desalinized waters.
The four Israeli researchers find, though, that the very quality of desalinized water — the water’s lower salinity — is a problem for agriculture. As a rule, the lower salinity of desalinized water is perceived as an advantage because the salts, especially Na+ and Cl-, damage soils, stunt plant growth, and harm the environment. The trouble, the researchers write, is that “desalination not only separates the undesirable salts from the water, but also removes ions that are essential to plant growth.” Irrigation water from natural sources provide basic nutrients such as calcium (Ca2+), magnesium (Mg2+), and sulfate (SO42I-) at levels sufficient to obviate the need for additional fertilization, but desalinized water either does not contain these nutrients or contains insufficient levels of them. What complicates the problem is the fact that the “purer” desalinized water is suitable for human consumption.
The authors conclude: “If desalinized water was destined for agricultural use alone, simple blending [that is, adding missing nutrients] strategies would be the most probable economical strategy, providing stable and high water quality. Yet, in more typical cases, where water supplies both municipal and agricultural uses, economic efficiency requires a balancing of treatment costs, drinking-water quality, and agricultural benefits.” Would adding nutrients be harmful to humans? No: Adding nutrients so desalinized water is more suitable for agriculture does not “contradict nor compromise the quality of the water for human consumption as defined by WHO [World Health Organization] standards.” In fact, the opposite is the case: “Increased buffering capacity [see below] and higher Ca2+ and Mg2+ concentrations make the water more chemically and biologically stable and provide a higher amount of essential elements, which contribute to public health. Desalination facilities built today will be in place for decades, making planning now essential for long-term increased economic prosperity and agricultural productivity.”
Low buffering capacity increases risks of corrosion to metal distribution pipes. It also can have a profound impact on pH (and agricultural productivity) when the water is mixed with other sources.
Water chemistry
Water has four measurable properties which are commonly used to characterize its chemistry. They are pH, buffering capacity, general hardness, and salinity. In addition, there are several nutrients and trace elements. pH refers to water being either an acid, base, or neither (neutral). A pH of 7 is said to be neutral, a pH below 7 is “acidic”’ and a pH above 7 is “basic”’ or “alkaline.” Note that as is the case with the Richter scale used to measure earthquakes, the pH scale is logarithmic. A pH of 5.5 is 10 times more acidic than water at a pH of 6.5.
Buffering capacity refers to water’s ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined: It may appear that adding equal volumes of an acid and neutral water would result in a pH halfway in between, but this rarely happens in practice. If the water has sufficient buffering capacity, that buffering capacity can absorb and neutralize the added acid without significantly changing the pH. Think of buffering capacity as a large sponge: As more acid is added, the “sponge”’ absorbs the acid without changing the pH much. The sponge’s capacity is limited however; once the buffering capacity is used up, the pH changes more rapidly as acids are added. Read more about water chemistry here.
-read more in U. Yermiyahu et al., “Rethinking Desalinated Water Quality and Agriculture,” Science 318, no. 5852 (9 November 2007): 920-21 (DOI: 10.1126/science.1146339) (sub. req.)
