RELATION BETWEEN AQUIFER COMPOSITION AND THE TENDENCY TOWARD SCALE DEPOSIT FORMATION DURING DESALINATION – EXAMPLES FROM SALINE WATERS IN NEW MEXICO

Limited freshwater resources can lead to saline ground water to be viewed as a potential resource rather than a nuisance. Use of saline ground water typically requires reduction of dissolved-solids concentrations. Reduction can be accomplished by 'reverse' osmosis desalination using a semi-permeable membrane that extracts most of the dissolved solids from saline water. Accumulation of precipitates on the semi-permeable membrane surface can decrease membrane permeability. These precipitates, called scale deposits, typically form as CaCO₃, CaSO₄^.2H₂O, and SiO_2(a).

Geochemical modeling using PHREEQC allows calculation of a scale formation factor (SFF) that quantifies the tendency toward formation of a particular scale deposit during desalination. The SFF for individual scale deposits is calculated by simulated stepwise evaporation of water to saturation with respect to CaCO₃, CaSO₄^.2H₂O, and SiO_2(a) accompanied by simulated mass transfer of precipitates out of the aqueous solution during each step. The SFF is equal to the inverse of the mass fraction of water remaining after each step and, therefore, is inversely proportional to the tendency of each potential precipitate to form scale deposits. Tendencies toward formation of scale deposits from saline ground water, as described by their SFF values, correlate well with the mineralogic composition of the host aquifer. Examples from New Mexico include: (1) elevated tendencies toward SiO_2(a) scale formation, indicated by SFF values of 2-6, in saline water from the silica-rich aquifer contained within the Rio Grande alluvium in contrast to SFF values of 5-11 for SiO_2(a) scale formation in saline water from the gypsum/anhydrite/carbonate-rich aquifers contained within the San Andres limestone and Yeso Formation and (2) elevated tendencies toward CaSO₄^.2H₂O scale formation, indicated by SFF values of 1-5 in saline water from aquifers within the San Andres limestone and the Yeso Formation in contrast to SFF values of 8-10 for CaSO₄^.2H₂O scale formation in saline water from the aquifer contained within the Rio Grande alluvium.