Field-Scale Observations of Arsenic and Boron in Soil Irrigated with Drainwater

Addition of a strong acid to irrigation water could enhance sorption of boron and arsenic in a Lahontan Valley field. Approximately 90 soil samples, taken from a field irrigated with mildly brackish drainage water (TDS ~1,250 mg L-1), were analyzed for soluble constituents. When the pH of the saturated soil paste was between 7.2 and 8.8, dissolved arsenic and boron concentrations increased, possibly indicating decreased sorption on oxyhydroxide precipitates of manganese and aluminum, and goethite, inferred from equilibrium aqueous speciation modeling. In the pH range of 7.2-8.8, linear regression models indicate a unit decrease in pH may result in a log[B] reduction of 1.18 and a log[total As] reduction of 1.58, when the concentrations are expressed in mg L-1. Geochemical modeling indicates that a reduction in pH in the observed range should not solubilize carbonates that could counteract acidity increase. Addition of HCl or H2SO4 to reduce soil water pH may be an effective way to minimize deep percolation of boron and arsenic.