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

Past and Present Research on Metal Transport In

St. Kevin Gulch, Colorado

Contents

• ABSTRACT
• INTRODUCTION
• PAST STUDIES
  • Methods for Assessing Iron-Rich Systems
  • Instream Processes
• PRESENT STUDIES
• REFERENCES

ABSTRACT

To prepare for mitigation of the effects of acidic mine drainage on upland watersheds, process-oriented research at St. Kevin Gulch has focused on chemical reactions that affect metal transport and partitioning among phases. We operationally defined dissolved and colloidal transport phases and the speciation of iron because the cycle of iron can affect the cycling of other metals. With these phases defined, our approach was to study chemical reactions in the context of hydrologic transport. By establishing the hydrologic setting with tracer-dilution injections, we studied temporal and spatial variability in metal concentrations resulting from combined hydrologic and chemical processes. Temporal and spatial aspects of variation were combined in an instream pH- modification experiment to evaluate the kinetics of reactions involving metals. These data are the focus of a simulation model that combines transport and reactive chemistry of metals. Ongoing studies of the ecological system are in anticipation of higher pH and lower metal concentrations that should folow remediation by a passive treatement system.

INTRODUCTION

Many streams in upland watersheds are affected by acidic mine drainage (Moore and Luoma, 1990). Efforts are underway to mitigate the effects of this acidic drainage. However, successful mitigation requires an understanding of the processes that affect metals in these streams. Since 1986, the Toxic Substances Hydrology Program of the U.S. Geological Survey has supported field-based research on processes that affect metals in streams. Objectives of this research include: (1) The study of instream chemical and biological processes in the context of transport processes that affect them, (2) determination of the role of sediment and colloids in the processes affecting metals, (3) quantification of the temporal and spatial variability of processes, and (4) simulation of the reactive transport of metals to evaluate field observations. The purpose of this report is to present the major findings from past studies of St. Kevin Gulch and to describe the focus of present studies.

St. Kevin Gulch is an upland watershed in the Rocky Mountains; its elevation is 2,500 to 2,800 m (meters) above sea level and it has an area of about 10²km (square kilometers, fig. 1). Most annual precipitation falls as snow; snowmelt runoff that occurs in late May or early June is the dominant hydrologic event each year. Bedrock is a quartz-biotite-feldspar schist. Mining of silver and zinc sulfides in vein deposits mostly occurred around 1920. St. Kevin Gulch has been a valuable research site because of its combination of physical and chemical characteristics: (1) Metal concentrations are sufficiently high to be quantified by routine methods and yet not so high as to be serious health and safety concerns, (2) a 100-m reach is a point source of acidic inflow, (3) physical and chemical processes that affect metal concentrations occur downstream from the principal acidic inflows, (4) a single algal species, Ulothrix sp., predominates (McKnight, 198S), and (5) a natural wetland is present just upstream from the confluence of St. Kevin Gulch and Tennessee Creek (fig. 1).

Figure 1. Location of (a) St. Kevin Gulch, near Leadville, Colorado, and (b) location of pH-modification experiment in 1988.

Downstream changes in the chemical characteristics and discharge in St. Kevin Gulch result from acidic inflows (fig. 2). Upstream from the principal acidic inflows (0-363 m, fig. 1) the chemical composition mostly results from natural weathering in the basin. Between 363 m and 501 m, discharge of the acidic inflows is only about 0.8 L/s (liters per second) compared to 6 L/s in the stream, but the inflow chemistry strongly affects instream pH and metal concentrations (figs. 2a and 2b). The confluence with the nonacidic Shingle Mill Gulch is a short distance downstream from the acidic inflows, at 501 m. At the confluence, discharge approximately doubles (fig. 2c) and pH increases from the nonacidic inflow. Downstream from 526 m there only are a few acidic inflows from small mines; these inflows do not substantially change instream metal concentrations upstream from the wetland. The mass loading of manganese (Mn) (fig. 2d) shows that the acidic inflows between 363 m and 501 m are virtually a point source to the stream.

Figure 2. Downstream profiles of (a) pH, (b) manganese, (c) discharge, and (d) manganese mass flow in St. Kevin Gulch, near Leadville, Colorado, August 1987.

PAST STUDIES

We have taken advantage of the characteristics of St. Kevin Gulch to conduct process-oriented research, studying well-known chemical reactions in the context of hydrologic transport. Using tracer-dilution injections to establish the hydrologic background, we have carried out experiments with intensive spatial and temporal sampling (table 1).

Table 1. Summary of experiments in St. Keven Gulch, 1986 through 1993.

Methods for Assessing Iron-Rich Systems

Particular methods have been used to define processes that affect metals in acidic mine drainage. We will review some of these methods and then discuss how these methods aid the study of instream processes.

Setting the hydrologic framework--Use of experimentally injected chemical tracers helps to define relevant physical characteristics of a stream With the physical characteristics defined, it is possible to distinguish between physical and chemical effects on instream metal concentrations (Stream Solute Workshop, 1990; Bencala and others, 1990; Broshears and others, 1993). However, a seasonal study has a greater temporal scale, and it is not always practical to inject tracers to set the hydrologic framework. In such cases, natural conservative solutes can substitute for injected tracers (Bencala and others, 1987). Although this procedure does not establish absolute values of discharge as does the tracer-dilution method, it accounts for the hydrologic effects by a discharge ratio so that chemical effects can be studied.

Defining trasport phases--In addition to defining the hydrologic framework, it is necessary to define transport phases for the metals. The cycle of iron (Fe) affects many metals and is strongly affected by precipitation (Kimball and others, 1991) and by photoreduction (McKnight and others, 1988; Kimball and others, 1992b). At pH values greater than about 2.2, Fe oxyhydroxides commonly precipitate to form colloids, which affect the cycling of other metals by sorption (Smith and others, 1991). Colloids in St. Kevin Gulch can contain as much as 130 ppm (parts per million) arsenic (As), 230 ppm copper (Cu), 600 ppm lead (Pb), and 1,200 ppm zinc (Zn). However, Ranville (1992) determined that most of the suspended sediments in St. Kevin Gulch are primarily aggregates of colloidal (40-nanometer diameter) Fe oxyhydroxides and Fe oxyhydroxysulfates. In opposition to precipitation, photoreduction of Fe III can dissolve colloids and return ferrous iron (Fe II) to the stream (McKnight and others, 1988). Other metals that may be sorbed to the colloidal Fe also can be released. This dynamic cycling of Fe on a daily time scale affects the transport and transformation of other metals and can be studied only by adequate sampling for Fe phases and species.

Operational definitions of phases can be defined by using multiple filtrations. The definitions that are most meaningful in terms of the Fe colloids include the following: (1) filtered concentration (representing "dissolved" solutes) determined by filtration through a 0.001-µm (micrometer) pore-size membrane, (2) colloidal concentration, determined by filtration through a 0.45-µm pore-size membrane and then subtracting the dissolved concentration, and (3) suspended particulate concentration determined by an unfiltered sample and then subtracting the colloidal and dissolved concentrations. These definitions are time consuming to obtain when we could not take the time for the sequence of filtrations. For intensive temporal and spatial sampling, we generally obtained only an unfiltered and a 0.1-µm filtered sample (McKnight and others, 1988; Kimball and others, 1992a).

Instream Processes

With these definitions of transport phases, we have described instream processes affecting metals in the context of transport.

Photoreduction of iron--Intensive temporal sampling showed the importance of the photoreduction process on the cycling of Fe and other metals. Quantification of discharge allowed the calculation of mass flow and flux of Fe II from the streambed to the water column in response to light intensity (see fig. 3 in Kimball and others, 1992b).

Figure 3. Variation of sampled and simulated (a) filtered iron and (b) colloidal iron with downstream distance in St. Kevin Gulch, near Leadville, Colorado.

Coupling of rates for hydrologic and chemical processes--Intensive spatial sampling along a 1,800-m reach allowed us to compare rates of transport to rates of chemical reaction by using steady-state solute transport simulation (fig. 3). If chemical reactions are relatively fast compared to rates of transport, the reactions can affect instream concentrations. The solid line in figure 3a illustrates conservative transport of Fe. Adding first-order rate constants to simulate Fe removal indicates the relative importance of chemical reaction (dashed line, fig. 3a). The increase of colloidal Fe (fig. 3b) corresponds to the decrease of filtered Fe. Loss of colloidal Fe (dashed line simulation, fig. 3b) is from sedimentation of colloidal aggregates.

Response to pH modification--Combining intensive temporal and spatial sampling, we modified the chemistry of St. Kevin Gulch to doeument the kinetics of meta1 reactions as pH was increased in two steps (fig. 4a). With an increase of pH to near 6.0, aluminum (Al) was completely partitioned from the filtered to the co11oida1 phases in the water column (fig. 4b). This is likely from the rapid formation of an Al oxyhydroxysulfate complex (Kimball and others, 1994). Sorption onto colloids affected the concentrations of Cu. However, Mn and Zn were little affected at this level of pH (Kimball and others, 1994).

Figure 4. Variation of (a) pH and (b) filtered and colloidal aluminum with time during pH-modification experiment in St. Kevin Gulch, near Leadville, Colorado, August 1988.

Exchange of streamwater and subsurface water--By injecting tracers both in the stream and in the hillslope, the exchange of water between the stream and the alluvium was documented (Harvey and Bencala, 1993). This changes the traditional concept of the stream as a pipe of water leaving a watershed. Instead, the stream continues to interact with the watershed, which can affect instream metal concentrations because metals are present in substantial concentrations in the alluvium (J.W. Harvey and B.A. Kimball, U.S. Geological Survey, 1991, unpublished data, Salt Lake Oty, Utah). The scale of this interaction can be on the order of 1-m stream segments.

Treatment of metals by natural wetlands--Before entering Tennessee Creek, the metal-rich water from St. Kevin Gulch passes through a natural wetland. Initial sampling of inflow and outflow water from this wetland indicated that metals were being treated by interaction with organic matter and minerals of the wetland. Metal flux and geochemical processes in this wetland have been studied by Walton-Day (1992) to determine whether the wetland is removing Fe, Mn, cadmium (Cd), Cu, Pb, and Zn from surface water flowing through the wetland. Careful measurement of metal fluxes indicates that only Fe was removed from surface water. Most metals were untreated by the wetland, and essentially passed through despite prolonged physical contact with the wetland environment.

PRESENT STUDIES

In our current work, we have used mathematical models to provide a frame of reference for evaluating data from field studies. We developed computer simulations for conservative and reactive solute transport in St. Kevin Gulch. Reactive transport simulation takes the study of biogeochemical processes beyond batch experiments and places it in the context of hydrology.

Simulation of conservative transport--Physical effects on the transport of solute mass must be quantified to distinguish the chemical and biological effects. Simulation of transport in mountain streams presents certain unique physical aspects, the most prominent being transient storage (Bencala and Walters, 1983). The transport model developed by Bencala and Walters (1983) was improved by using a new algorithm and by execution on new generations of computers (Runkel and Broshears, 1992; Runkel and Chapra, 1993). These improvements were the basis for building the simulation model for reactive solute transport.

Simulation of reactive solute transport--Data from the pH-modification experiment in 1988 (as in fig. 4) provide a unique opportunity to evaluate our understanding of rates for metal reactions. The complexity of reactive chemistry is simulated by coupling MINTEQA2 (Allison and others, 1991) with the transport code (Runkel and others, 1996; Broshears, 1996).

Our understanding of geochemical reactions in St. Kevin Gulch has provided the background to study remediation of acidic mine drainage. The State of Colorado plans to install a passive treatment system near the mine dump in St. Kevin Gulch, creating the opportunity to study the changes that occur in the recovering stream. Chemical changes in the water column, in the bed sediment, and in the alluvium will combine to affect the stream ecosystem. The current dominance of the blue-green algae, Ulothrix sp. (McKnight, 1988) will be affected by chemical changes, indicating the ecosystem response. Combining this remediation study with our past and present studies will complete a comprehensive process-oriented study at the St. Kevin Gulch site.

REFERENCES

Allison, J.D., Brown, D.S., and Novo-Gradac, K.J., 1991,

MINTEQ2/PRODEFA2, a geochemical assessment model for environmental systems-Version 3.0 User's Manual: EPA/600/3-91/021, U.S. Environmental Protection Agency, 106 p.

Bencala, K.E., McKnight, D.M., and Zellweger, G.W., 1987,

Evaluation of natural tracers in an acidic and metal-rich stream: Water Resources Research, v. 23, p. 827-836.

Bencala, K.E., McKnight, D.M., and Zellweger, G.W., 1990,

Characterization of transport in an acidic and metal-rich mountain stream based on a lithium tracer injection and simulations of transient storage: Water Resources Research, v. 26, p. 989-1000.

Bencala, K.E., and Walters, R.A., 1983,

Simulation of solute transport in a mountain pool-and-riffle stream-A transient storage model: Water Resources Research, v. 19, p. 718-724.

Broshears, R.E., Bencala, K.E., Kimball, B.A., and McKnight, D.M., 1993,

Tracer-dilution experiments and solute-transport simulations for a mountain stream, Saint Kevin Gulch, Colorado: U.S. Geological Survey Water-Resources Investigations Report 92-4081, 18 p.

Broshears, R.E., Kimball, B.A., and Runkel, R.L., 1996,

Simulation of reactive transport during a pH modification experiment in a mountain stream affected by acid mine drainag, in Morganwalp, D.W., and Aronson, D.A., ed., U.S. Geological Survey Toxic Substances Hydrology Program-Proceedings of the technical meeting, Colorado Springs, Colorado, September 20-24, 1993: U.S. Geological Survey Water-Resources Investigations Report 94-4015.

Harvey, J.W. and Bencala, K.E., 1993,

The effect of streambed topography on surface-subsurface water exchange in mountain catchments: Water Resources Research, v. 29, no. 1., p. 89-98.

Kimball, B.A., Broshears, R.E., Bencala, K.E., and McKnight, D.M., 1991,

Comparison of rates of hydrologic and chemical processes in a stream affected by acid mine drainage, in Mallard, G.E., and Aronson, D.A., eds., U.S. Geological Survey Toxics Substances Hydrology Program-Proceedings of the technical meeting, Monterey, California, March 11-15, 1991: U.S. Geological Survey Water-Resources Investigations Report 91-4034, p. 407-412.

Kimball, B.A., Broshears, R.E., McKnight, D.M., and Bencala, K.E., 1994,

Preliminary evaluation of the effects of an instream pH modification on transport of sulfide-oxidation products, in Alpers, C.L, and Bowles, D., eds., The Environmental Geochemistry of Sulfide Oxidation: Washington, D.C., American Chemical Society Books, p. 224-243.

Kimball, B.A., McKnight, D.M., Broshears, R.E., and Bencala, K.E., 1992a,

Effect of instream pH modification on aluminum, in Kharaka, Y. K. and Maest, A. S., eds., Water-Rock Interaction, Volume 1, Low Temperature Environments: Rotterdam, A.A. Balkema, p. 393-396.

Kimball, B.A., McKnight, D.M., Wetherbee, G.A., and Harnish, R.A., 1992b,

Mechanisms of iron photo reduction in a metal-rich, acidic stream (St. Kevin, Gulch, Colorado, U.S.A.): Chemical Geology, v. 96, p. 227-239.

McKnight, D.M., 1988,

Metal-tolerant algae in St. Kevin Gulch, Colorado, in, Mallard, G.E., ed., U.S. Geological Survey Toxic Substances Hydrology Program surface-water contamination-Proceedings of the technical meeting, Denver, Colorado, February 2-4, 1987: U.S. Geological Survey Open-File Report 87-764, p. 113-117.

McKnight, D.M., Kimball, B.A., and Bencala, K.E., 1988,

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Ranville, J.F., 1992,

Factors influencing the electrophoretic mobility of suspended sediments in acid mine drainage: Golden, Colorado School of Mines, unpublished Ph.D. dissertation T-4213, 270 p.

Runkel, R.L., and Broshears, R.E., 1992,

One-dimensional transport with inflow and storage (OTIS)-A solute transport model for small streams: Boulder, Colo., Center for Advanced Decision Support for Water and Environmental Systems, 85 p.

Runkel, R.L. and Chapra, S.C., 1993,

An efficient numerical solution of the transient storage equations for solute transport in small streams: Water Resources Research v. 29, p. 211-215.

Runkel, R.L., Bencala, K.E., and Broshears, R.E., 1996,

An equilibrium-based simulation model for reactive solute transport in small streams, in Morganwalp, D.W., and Aronson, D.A., ed., U.S. Geological Survey Toxic Substances Hydrology Program-Proceedings of the technical meeting, Colorado Springs, Colorado, September 20-24, 1993: U.S. Geological Survey Water-Resources Investigations Report 94-4015.

Smith, K.S., Ranville, J.F., and Macalady, D.L., 1991,

Predictive modeling of copper, cadmium, and zinc partitioning between streamwater and bed sediment from a stream receiving acid mine drainage, St. Kevin Gulch, Colorado, in Mallard, G.E., and Aronson, D.A., eds., U.S. Geological Survey Toxic Substances Hydrology Program-Proceedings of the technical meeting, Monterey, California, March 15, 1991: U.S. Geological Survey Water-Resources Investigations Report 91-4034, p. 380-386.

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Walton-Day, K., 1992,

Hydrology and geochemistry of a natural wetland affected by acid mine drainage affected by acid mine drainage St. Kevin Gulch, Lake County, Colorado: Golden, Colorado School of Mines, unpublished Ph.D. dissertation T-4033, 299 p.

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Calculation of conservative-tracer and flume-discharge measurements on a small mountain stream, in Mallard, G.E., and Aronson, D.A., eds., U.S. Geological Survey Toxic Substances Hydrology Program-Proceedings of the technical meeting, Monterey, California, March 11-15, 1991: U.S. Geological Survey Water-Resources Investigations Report 91-4034, p. 434-437.