REVERSE OSMOSIS WASTE WATER APPLIED TO AGRICULTURAL LAND: SOIL IMPICATIONS IN THE SEMI-ARID SOUTHWESTERN UNITED STATES

Large-scale reverse osmosis (R.O.) plants are used to desalinize water used in oil and gas pumping equipment.  Several thousand liters of saline reject water also is generated in the process.  In Northwest NM, where water is scarce, an R.O. plant irrigated an adjacent 1.7 hectares (4.1 acres) field of pasture grass with R.O. reject water (conductivity 2.7 mS/cm; sodium adsorption ratio 9.1).  After two growing seasons, they observed symptoms of salt impacted soils (e.g. reduced vegetation cover).  New Mexico State University was contacted to develop recommendations for remediating the site and determine if the R.O. reject water could be put to future beneficial agriculture use.  San Juan College horticulture students took 40 soil samples from the irrigated site at a depth of 20 cm on a 12.5 m x 40 m grid marked with a global positioning (GPS) device.  An adjoining un-irrigated site comprised of native vegetation was sampled for base-line purposes.  Aqueous soil extracts were analyzed for electrical conductivity (EC) and pH using the saturated paste extract method.  Soils irrigated with R.O. reject water averaged higher for EC than the untreated soils. Some individual soil sample points in the treated field exceeded 4 mS/cm, the threshold level for most agricultural crops where yield declines occur.  Reverse osmosis reject water applied to agricultural lands in the semi-arid southwestern U.S. has serious negative implications to soils because soils are already prone to “salting out”.  Reverse osmosis reject water may be better suited for aquaponics (shrimp culture) or greenhouse hydroponics.