Fig. 1. Diel variations in dissolved Zn concentration from several streams in SW Montana: HOC, High Ore Creek; PPC, Prickly Pear Creek; IC, Indian Creek; CFR, Clark Fork River; SR, Stillwater River. All data are from Nimick et al. (2003) with the exception of CFR-1994 (Brick and Moore, 1996), CFR-2003 and IC-2003 (C. Gammons, unpublished data). Shaded regions denote approximate nighttime hours. All of the data were collected under baseflow conditions in late June, July, August, or early September.

Fig. 2. (a) XRD pattern of three samples of HMZO crust from High Ore Creek (all three scans are superimposed on the same scale); (b) SEM-EDX spectrum of HMZO crust.

Fig. 3. (a) Scanning electron image of an HMZO crust from High Ore Creek; (b) a closeup image, showing diatom fustules imbedded in HMZO matrix.

Fig. 4. Results of the Series 1 mesocosm experiments: (a) complete mesocosm, including water, boulders, and biofilms from High Ore Creek; (b) HOC water with filamentous green algae; (c) control tank with HOC water only. Shaded regions denote times when the artificial lights in the laboratory were off.

Fig. 5. Results of the Series 2 mesocosm experiments: (a) complete mesocosm, including water, boulders, and biofilms from High Ore Creek; (b) HOC water with filamentous green algae. The shaded regions denote times when the artificial lights in the laboratory were off.

Fig. 6. Comparison of pH and dissolved Zn concentrations from the Series 2a experiment of this study vs. field data from High Ore Creek. The latter were collected by the US Geological Survey in July, 2000, and were reported in Nimick et al. (2003).

Fig. 7. Results of the Series 3 mesocosm experiments with artificial pH control. Arrows show the sequence in which the samples were collected.

Fig. 8. Results of the Series 4 and 5 experiments with HMZO crusts and pH control.

Fig. 9. The dependence of dissolved Zn and Mn concentration on pH from the Series 4 and Series 5 experiments with HMZO crusts.

Fig. 10. Results of the Series 6 experiments with HMZO crusts and temperature control.

Fig. 11. The dependence of dissolved Zn and Mn concentration on temperature (1000 × reciprocal T, in Kelvin) from the Series 6 and Series 7 experiments with HMZO crusts.

Fig. 12. Results of the Series 8 experiments with HMZO crusts and artificial Eh control. Dashed lines separate periods of time in which either O₂ gas or H₂ gas was bubbled through the reaction vessel.

Fig. 13. The dependence of dissolved Zn and Mn concentration on temperature (1000 × reciprocal T, in Kelvin) from all experiments with HMZO crusts. The data were adjusted for minor differences in pH between samples. The slopes of the linear regressions were used to derive enthalpies for the adsorption of Zn²⁺ and Mn²⁺ onto HMZO.

Fig. 14. A comparison of observed and predicted Zn concentrations for six different diel investigations conducted at High Ore Creek by the US Geological Survey (Nimick et al., 2003, and unpublished data). Streamflow (Q, ft³ s⁻¹ = 0.0283 m³ s⁻¹) is shown on the right y-axis for Fig. 14e and f. The predictions are based on a model that takes into account pH changes only, or a combination of pH and temperature changes (see text).

Composition (acid-available metals) of HMZO crusts from High Ore creek

wt%, weight%; at%, atomic%, normalized to 100.