Skip Links

USGS Home Contact USGS Search USGS

Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

Comparison of Filtration Procedures and Analytical Procedures on Iron (II/III): Results from Upper Animas, Summitville, and Iron Mountain

By James W. Ball,¹ D. Kirk Nordstrom,² and Charles N. Alpers³

Although not particularly toxic, iron plays an important role in the mobility, bioavailability, and toxicity of other trace elements associated with watersheds affected by abandoned mine lands. The quantity of hydrous ferric oxide (HFO) moving in the water column strongly affects downstream transport and loading, and thus offsite toxicity, of many trace metals that may be adsorbed by HFO. Thus, realistic measurements of iron and HFO concentrations in streams are important for abandoned-mine-land studies.

Sample-processing time and filtration can have a substantial influence on analytical results for iron and associated elements. Rapid, microbially catalyzed oxidation of iron(II) to iron(III) and formation of HFO colloids can occur during sample processing. Because colloidal iron concentrations are often much higher than dissolved iron(III) concentrations, any colloidal material passing into the filtrate will result in overestimation of dissolved iron(III) concentrations. Iron oxidation and colloid formation during filtration may result in underestimates of iron(II) and total dissolved iron concentrations.

Effects of filtration apparatus and procedure on dissolved iron(II) and iron(III) concentrations were investigated at the Silver Ledge adit along Cement Creek near Silverton, Colorado; the Alamosa River below Terrace Reservoir near the Summitville Mine, Colorado; and the Sacramento River downstream of the Iron Mountain Mine, California. Water was filtered through 0.1-micrometer (µm) or 0.45-µm pore-size membranes and a Millipore Minitan ultrafiltration apparatus (effective pore size <0.001µm). To determine the volume of filtered water needed to reach steady-state concentration, ten or twelve sequential 125-milliliter (mL) subsamples were collected at two locations and analyzed for ferrous and total iron using the FerroZine colorimetric technique.

Total dissolved iron concentrations in samples from Silver Ledge adit were essentially the same for 12 sequential 0.45-µm filtrates and 12 sequential 0.1-µm membrane filtrates. Iron concentrations in initial ultrafiltrate were about 91 percent of iron concentrations in 0.1- or 0.45-µm filtrates, whereas iron concentrations in ultrafiltrate collected 4.5 hours later were about 76 percent of iron concentrations in 0.1- or 0.45-µm filtrates. These results indicate that iron colloids may form and be removed during ultrafiltration.

As much as 93 percent of the total iron in water samples from the Sacramento River is removed by filtration. Only 55 percent and 16 percent of the total iron is removed from samples from the Alamosa River and Silver Ledge adit, respectively. The combination of high particulate iron and low dissolved iron concentrations in the Sacramento River increase the potential for overestimating the actual dissolved iron concentration if filtrate is contaminated with particulate iron during sample processing.

Ferric iron concentrations frequently are 1 percent or less of iron(II) concentrations. Typically, iron(III) is determined by differencing total iron and iron(II), thereby introducing uncertainties that may approach 100 percent. A method was developed for direct determination of iron(III) using complexing reagents acetohydroxamic acid for iron(III) and FerroZine for iron(II). Onsite 0.1-µm membrane filtration with direct determination of iron(II) and iron(III) may avoid the possible processing effects inherent with ultrafiltration techniques.

¹U.S. Geological Survey, 3215 Marine, Boulder, CO 80303-1066 (jwball@usgs.gov)

²U.S. Geological Survey, 3215 arine, Boulder, CO 80303-1066 (dkn@usgs.gov)

³U.S. Geological Survey, Placer all, 6000 J. Street, Sacramento, CA 95819-6129 (cnalpers@usgs.gov)

OFR 98-297 Home | OFR 98-297 Table of Contents | Previous | Next

USGS Abandoned Mine Lands Intiative Home Page