EFFECT OF SALINE GROUNDWATER ON THE AGGREGATION AND SETTLING OF SUSPENDED PARTICLES IN A TURBID AUSTRALIAN RIVER

Laboratory studies have modelled the interaction of Darling River wate r and a saline groundwater intrusion. Kinetic measurements have shown that the rate determining step in water column clarification is the ag gregation of small colloidal particles which then settle rapidly after reaching a critical diameter. Divalent cations (Ca2+ and Mg2+) are ex tremely effective in enhancing the rate of clarification by increasing the colloid stability factor. Three different phases have been observ ed in the cation-mediated removal of iron from solution: (i) rapid coa gulation induced by the initial velocity shear resulting from solution mixing; (ii) a slower second-order iron removal, consistent with conv entional aggregation kinetics; and (iii) a decrease in rate after 90% iron removal which is attributed to a lower iron content in the ultraf ine colloid fraction. Specific interactions between the divalent catio ns and the organic coatings on the particles are proposed in order to explain the much higher rate of coagulation than expected on electrost atic grounds when compared with the monovalent Na+ and K+. Water colum n clarification occurred more rapidly in these model laboratory studie s than was observed in the Darling River weir pool. Iron removal rates in the river tend to be inhibited by hydrological effects. Turbulence inducing processes are required to mix the dense saline groundwater w ith the overlying water column and thus may ultimately limit the rate of turbidity reduction.