Envlron. Sci. Technol. 1994, 28,2065-2073
Coupling of Hydrologic Transport and Chemical Reactions in a Stream Affected by Acid Mine Drainage Brlant A. Kimbali' U S . Geological Survey, 1745 W 1700 S,Room 1016, Salt Lake City, Utah 84104
Robert E. Broshears U S . Geological Survey, Denver, Colorado 80225
Kenneth E. Bencala U S . Geological Survey, Menlo Park, California 94025
Diane M. McKnlght U S . Geological Survey, Boulder, Colorado 80303 ~~~~~
~
Experiments in St. Kevin Gulch, an acid mine drainage stream, examined the coupling of hydrologic transport to chemical reactions affecting metal concentrations. Injection of LiCl as a conservative tracer was used to determine discharge and residence time along a 1497-m reach. Transport of metals downstream from inflows of acidic, metal-rich water was evaluated based on synoptic samples of metal concentrations and the hydrologic characteristics of the stream. Transport of SO4 and Mn was generally conservative, but in the subreaches most affected by acidic inflows, transport was reactive. Both 0.1-pm filtered and particulate Fe were reactive over most of the stream reach. Filtered A1 partitioned to the particulate phase in response to high instream concentrations. Simulations that accounted for the removal of Sod, Mn, Fe, and A1 with firstorder reactions reproduced the steady-state profiles. The calculated rate constants for net removal used in the simulations embody several processes that occur on a stream-reach scale. The comparison between rates of hydrologictransport and chemical reactions indicates that reactions are only important over short distances in the stream near the acidic inflows, where reactions occur on a comparable time scale with hydrologic transport and thus affect metal concentrations. Introduction
Instream biogeochemical reactions can affect concentrations in a stream reach if reaction rates are rapid with respect to transport rates. However, if hydrologic transport moves solutes much faster than chemical reactions can occur, solute transport will appear conservative rather than reactive. This dynamic coupling between the rate of hydrologictransport and rates of biogeochemical reactions influences the transport and transformation of solute concentrations in streams and rivers. Biogeochemical reactions in streams affected by acid mine drainage have received much attention (1-6)but have rarely been studied in the context of hydrologic transport. As described by Dzombak and Ali (7) in their review of hydrochemical modeling, few studies have evaluated the performance of reactive solute transport models for simulating field data. In this study, we put known biogeochemical reactions into the context of hydrologic transport by intensive sampling during the experimental injection of a conservative tracer. This helps
to understand the dynamic coupling in natural systems because the stream setting provides information on factors that cannot be duplicated in laboratory studies. However, this setting only permits observation of net biogeochemical reactions because all contributions to instream change cannot be individually quantified. Especially in mountain streams, the spatial distribution of water velocity and the control of chemical reaction by the local degree of watersediment interaction, light intensity, or patterns in other environmental variables can be complex (8, 4). By conducting an instream tracer-dilution experiment and using a transient-storage hydrologic model, the rates of key hydrologic processes can be determined (8). In this study, we determine instream rate constants for net biogeochemical reactions involving Sod, Mn, Fe, and A1 by matching steady-state solute transport simulations with the measured concentration profiles of these chemical species downstream from acidic, metal-rich inflows from an abandoned mine. Site and Methods
St. Kevin Gulch, near Leadville, CO, drains a basin of about 10km2 mostly underlain by a quartz-biotite-feldspar schist bedrock (9). Streamflow at the time of the experiment was less than 20 L/s. Pools, riffles, and cascades in the shallow stream caused a rapid mixing of inflows. The stream receives water that flows from a collapsed adit of an abandoned silver and zinc mine contributing metalrich, acidic inflow in a series of springs that discharge at the base of a mine dump (Figure 1). A 1497-m reach of St. Kevin Gulch was divided into 10subreaches, in terms of chemical and hydrologic characteristics (Figure 2a). Samples were collected at many stream sites, both upstream and downstream from the nine sampled inflows. Upstream from the acidic inflows (0-363 m), streamwater was slightly affected by mine drainage, but mostly reflected the natural weathering in the basin. Between 363 and 455 m, the stream became much more acidic, and SO4 increased from the four inflows of acid mine drainage in that subreach (Figure 2b). The effects of the acidic inflows were most pronounced between 455 and 484 m. Downstream from the acidic inflows, between 484 and 526 m, Shingle Mill Gulch entered St. Kevin Gulch (at 501 m). This nearly doubled the discharge (Figure 2c) and made the pH of St. Kevin Gulch higher. In the subreaches between 526 and 1497 m, there were other inflows, but
This article not subject to U S . Copyright. Published 1994 by the American Chemical Society
Environ. Scl. Technol., Vol. 28, No. 12, 1994 2065
EXPLANATION DRAINAGE BASIN BOUNDARY
v
525 SAMPLESITE4NONUMBER.. N"mherrrpnm~rd,,snr~ dllwnlfErm h l r n lithium M s n d r I",d,l'l"
,,,e.
I"
meter,
lNiPCTlON SITE AND NUMBER-. Nvmhrr rrpmnt,d,,ence di!wn,lwam lnlm lilhl""?
