Salinity Management in Arid Regions - Restorative Redevelopment of Devastated Ecocultural Landscapes - page 424

however, this is not enough to grow a crop and so groundwater must be extracted
in an amount of about eighty-five inches per year. Although this groundwater has a
relatively low TDS concentration of 997 mg/L, it will nevertheless generate about
a million and a half pounds of salts per year applied to this eighty-acre field. The
plants transpire sixty-five inches per year, consuming freshwater and leaving behind
all the salts. The salts concentrate in the root zone, and twenty-five inches of water
have to be applied as overirrigation to keep this upper five feet of soil fresh so crops
can continue to be grown by moving the salt to below the root zone.
That salt—the million and a half pounds that came out of the groundwater—gets
moved with that deep percolation water back to the groundwater. But now it is at
3,600 mg/L due to being concentrated by plant extraction of freshwater that exceeds
the rainfall. Groundwater concentrations have consequently increased over time,
going from 1,000 mg/L to 1,400 mg/L in eight years. This farm needs to develop a
more efficient form of irrigation so that sixty-five inches per year of supplemental
irrigation are not required. Ways need to be explored either to use less water to grow
almost the same crops in a sustainable water balance or to develop an alternative,
supplemental water source from somewhere other than the groundwater (Dickey and
Madison 2004).
One potential solution occurs through seasonally reclaiming soils and exporting
salts. Seasonal drainage occurs in many agricultural areas where groundwater exists
close to the surface or when a low rate of irrigation during the summer accumulates
salts in the root zone. The plants are still extracting freshwater and leaving behind
salt, so whatever salt is in the water source can be stored in the soil during the
growing season. If the region receives winter rainfall, this can be used with some
additional irrigation to leach the salts into the drainage system so that they can be
discharged with the winter floods when the system is full of freshwater from rainfall
and it thus can tolerate the salt present without causing downstream damage. This
is a valuable strategy if one is concerned about upstream drainage water affect-
ing the quality of the downstream water resource. Therefore, controlled irrigation
upstream is required so salts can be stored in the system during the growing season
until such time during the winter storms when they can be safely flushed out. At this
time, tolerance is increased because the plants are growing more slowly and are not
affected as seriously by the salt, and of course there is more water in the basin so salt
concentrations are diluted.
An example of applying such a seasonal salt management strategy is the Oremet
titanium mine in Albany, Oregon (Dickey and Madison 2004). Titanium is an ore
found in titanium tetrachloride liquid and whose extraction leaves behind a high
amount of chloride. The chloride is leached with nitric, sulfuric, and phosphoric
acids in a very polluted waste stream which, however, can be neutralized and put
into the soil to grow trees and grasses. About 2-3 tons per ha per year of waterborne
salt is applied to these crops in this way. To make the process sustainable, salts are
accumulated during the growing season (to about 1,500 mg/L in the surface-applied
irrigation water) until the winter, when rainfall and rising streams are used to flush
the salts out of the system and into the drains to collect the salinity and discharge it
downstream at a very high flow rate so it will not impact biota (i.e., concentrations
of below 500 mg/L).