Release Behavior of Copper and Zinc from Sandy Soils

Copper (Cu) and zinc (Zn) transport after agricultural applications have accumulated in the sediments and are suspected to affect fish health in the Indian River Lagoon and St. Lucie Estuary, South Florida. Minimal information is available on the release of soil Cu and Zn to water and its relation to their concentrations and physicochemical forms in the soils. Sandy soil samples (n = 13) with a wide range of total Cu and Zn content were collected from forestland and commercial citrus groves in the Indian River area, Florida. The soils were subjected to column leaching and batch extractions to understand the release behavior of Cu and Zn as affected by soil-water contact time, soil/water ratio, pH, and electrolyte concentration and cations. Copper released in batch extractions that simulated long-term leaching was primarily from exchangeable and carbonate-bound fractions, whereas Zn was primarily from the carbonate-bound fraction. The Cu and Zn released from the column represented short-term leaching and were primarily from their exchangeable fractions. Leached Zn decreased linearly as the solution pH was raised from 3.0 to 9.0. Leached Cu was at a minimum at pH 5 to 7 and increased at higher or lower pH beyond that range. These results indicate that long-term saturated conditions after precipitation enhanced Cu and Zn release to water from the sandy soils. The released Cu decreased with increasing Ca concentration but increased with sodium (Na) concentration in the soil solution because of their differential effects on soil colloids, especially organic matter (flocculated by Ca²⁺ and dispersed by Na⁺) because organically bound Cu is the dominant fraction in the soils. Increasing Ca, K, Na, or NH₄ ⁺ concentration generally increased Zn release through cation exchange. These findings merit attention in the development of best management practices to reduce transport of heavy metals from land to water.