U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Association of Selected Metals with Colloidal and Suspended Particulate Material in Shallow Ground Water and Surface Water at Pinal Creek, Arizona

Abstract

Trace metals are potentially transportable as solid phases in ground water and surface water in association with colloids or suspended particulates. Field data that specifies the partitioning of metals between dissolved and suspended solid phases is necessary for developing physically based models of metal transport. The purpose of the present study was to determine colloidal and suspended-particulate concentrations for selected metals in shallow ground water and in streamflow of the perennial reach of Pinal Creek during base-flow conditions. Concentrations of manganese (Mn), iron (Fe), nickel (Ni), copper (Cu), and cobalt (Co) were measured in unfiltered water samples, and compared with 0.45 µm (micrometer) and 0.001 m filtrates of the same water samples. Total metal concentrations in unfiltered water were partitioned into suspended particulate (>0.45 µm), colloidal (0.001 - 0.45 µm), and dissolved (<0.001 µm) components. Median colloidal and suspended-particulate concentrations for Mn, Ni, Cu and Co were several percent or less of total concentrations of those metals. In contrast, the sum of Fe colloidal and suspended- particulate phases was a large percentage of total Fe concentrations in some cases, which indicated that a colloidal or particulate Fe phase was significant at Pinal Creek. At present total Fe concentrations in suspension are low in shallow ground water and in streamflow at Pinal Creek during base-flow conditions. Colloids that do form do not appear to be significant sorbents of other trace metals, compared to Mn and Fe solid phases on the streambed. Colloid- facilitated metal transport will likely increase in importance if acidic ground water (which has Fe greater than 50 milligrams per liter) reaches Pinal Creek.