Effects of Flow Regime on Metal Concentrations ... - ACS Publications

Nov 22, 2016 - constituents are often at their highest levels.9,10. The emphasis on low-flow conditions is .... as As, Cd, Cu, and Pb, decreasing the ...
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Effects of Flow Regime on Metal Concentrations and the Attainment of Water Quality Standards in a Remediated Stream Reach, Butte, Montana Robert L. Runkel* U.S. Geological Survey, 3215 Marine Street, Suite E127, Boulder, Colorado 80305, United States

Briant A. Kimball U.S. Geological Survey, 2329 W Orton Circle, West Valley City, Utah 84119, United States

David A. Nimick U.S. Geological Survey, 3162 Bozeman Ave., Helena, Montana 59601, United States

Katherine Walton-Day U.S. Geological Survey, Mail Stop 415, Denver Federal Center, Denver, Colorado 80225, United States S Supporting Information *

ABSTRACT: Low-flow synoptic sampling campaigns are often used as the primary tool to characterize watersheds affected by mining. Although such campaigns are an invaluable part of site characterization, investigations which focus solely on low-flow conditions may yield misleading results. The objective of this paper is to demonstrate this point and elucidate the mechanisms responsible for the release of metals during rainfall runoff. This objective is addressed using data from diel and synoptic sampling campaigns conducted over a two-day period. Low-flow synoptic sampling results indicate that concentrations of most constituents meet aquatic standards. This finding is in contrast to findings from a diel sampling campaign that captured dramatic increases in concentrations during rainfall runoff. Concentrations during the rising limb of the hydrograph were 2−23 times concentrations observed during synoptic sampling (most increases were >10-fold), remaining elevated during the receding limb of the hydrograph to produce a clockwise hysteresis loop. Hydrologic mechanisms responsible for the release of metals include increased transport due to resuspension of streambed solids, erosion of alluvial tailings, and overland flow. Rainfall also elevated the alluvial groundwater table and increased infiltration through the vadose zone, likely resulting in dissolution from alluvial tailings that were dry prior to the event.

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INTRODUCTION Watersheds affected by mining activities are inherently complex, with a variety of surface and subsurface inflows introducing acidity and metals to the receiving stream. These inflows range from natural sources emanating from mineralized bedrock, to mining-affected sources that come from draining adits, waste rock, and tailings piles. Clean up of contaminated watersheds is further complicated by a dearth of funding for remedial actions. As such, there is a need to accurately characterize mined watersheds, so that limited resources are targeted at the most deleterious sources. To this end, synoptic sampling techniques have been developed to quantify and rank sources of contamination at the watershed scale.1−4 Synoptic sampling provides a spatial snapshot of constituent concenThis article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

trations and streamflow that may be used to develop spatial profiles of constituent load (concentration times streamflow). Spatial profiles of mining-related constituents are in turn used to identify sources that account for the majority of mass loading to the watershed. Synoptic sampling campaigns are typically conducted under low-flow conditions, such as the August-October time period in the western United States. The reason for this focus on lowflow conditions is 2-fold. First, identification of constituent Received: July 1, 2016 Revised: October 13, 2016 Accepted: November 9, 2016

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DOI: 10.1021/acs.est.6b03190 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 1. Map of Blacktail/Silver Bow Creek study reach including stream and inflow sampling locations.

runoff. This objective is addressed using data from diel and synoptic sampling campaigns conducted over a two-day period on a remediated stream reach in Butte, Montana. Remedial actions have removed large quantities of tailings from the floodplain surrounding the study reach, resulting in considerable improvement in streamwater quality. Despite these actions, significant quantities of residual streamside tailings are present, as well as tailings and mine waste that have been left in place. These diffuse sources of contamination may be cause for concern, as demonstrated below, and additional techniques may be needed to supplement the synoptic approach. Synoptic and temporal (diel) data from Butte are used to stress the importance of sampling over a range of hydrologic conditions, including low and high flow regimes.

sources based on mass loading is predicated on the assumption that conditions within the watershed are temporally invariant during the sampling period. Numerous stream sites (20−50) are typically sampled during a single day, and a steady-state picture of contaminant loading may be obtained, provided constituent concentrations and streamflow are temporally invariant. Although these conditions are rarely met, and temporal variation caused by diel cycling5−7 and/or source variation8 is possible, steady-state conditions are often approximated at low flow. Second, the low-flow period is thought to represent a critical time period in terms of aquatic toxicity, when sources of contamination are undiluted by snowmelt and/or rainfall, and concentrations of mining-related constituents are often at their highest levels.9,10 The emphasis on low-flow conditions is most appropriate for watersheds in which draining mine adits and/or diffuse groundwaters are the primary sources of contamination.1,4,11−13 Mine adits and contaminated groundwater typically flow on a year around basis, and their effects on streamwater quality are most pronounced at low flow when dilution is negligible. Although low flow synoptic sampling represents a powerful tool for site characterization under these circumstances, additional techniques may be needed in other settings. Emphasis on lowflow conditions may be misleading, for example, when the watershed under study is affected by streamside tailings. Early mining practices often paid little regard to environmental considerations, and tailings were often disposed of in a haphazard manner. High-flow events subsequently moved these materials downstream, depositing them on the streambank above the water table. Harmful effects of these deposits on streamwater quality may be insignificant during dry, low-flow conditions. Wetter conditions, in contrast, lead to considerable leaching and erosion that adversely affects the stream. Streamside tailings thus represent a “perpetual nonpoint source,”5 that may be difficult to detect during low-flow synoptic sampling. The objective of this paper is to demonstrate the potential for misleading results when low-flow synoptic sampling is used exclusively for site characterization, and to elucidate the mechanisms responsible for the release of metals during rainfall

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MATERIALS AND METHODS

Field Setting. The study reach is located within the city of Butte, Montana, home to the Butte Mining District. The predominant rock type in the District is the late Cretaceous Butte Quartz Monzonite, that has been hydrothermally altered to include high grade Cu−Ag−Zn−Pb-Mn veins and lower grade porphyry style deposits.14 Underground mining began on Butte Hill in the 1860s and continued into the mid 1900s when open pit operations began. Large, economic quantities of Cu, Zn, Mn, Pb, Mo, Ag, and Au were produced, leading to the District being known as the “Richest Hill on Earth”.14 Mining, milling, and smelting activities within the District over a 150year period generated large quantities of mine waste and tailings that were historically released to nearby surface waters.15,16 Large flood events in the early 1900s washed these materials downstream, contaminating groundwater,17 soils,18,19 alluvial sediments,14 and the hyporheic zone.20,21 The extent of this contamination includes Blacktail and Silver Bow Creeks within the City of Butte, 35 km of Silver Bow Creek downstream of Butte, and 380 km of the Clark Fork River.22−25 An estimated 2−4 million m3 of mine waste has been transported downstream from Butte along this corridor,17,22 adversely affecting riparian vegetation and biota26,27 along the way. The contaminated floodplain, B

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transported to a central processing area where 125 mL aliquots were prepared for cation and anion analyses. Onsite processing included filtration, measurement of pH, and measurement of specific conductance. Filtration was completed using 0.45 μm capsule filters, and aliquots for cation analysis were acidified to pH < 2.0 with ultrapure HNO3. Dissolved anion concentrations were determined from filtered, unacidified samples by ion chromatography; alkalinity was determined from filtered, unacidified samples (SI Table S2). Total recoverable and dissolved cation concentrations were determined from unfiltered and filtered samples, respectively, using inductively coupled plasma-mass spectrometry (ICP-MS) (SI Tables S3−S6). Nine cations, denoted as “mining-related constituents”, are the focus of this paper (Al, As, Cd, Cu, Fe, Mn, Ni, Pb, and Zn). Estimating Streamflow by Tracer Dilution. Bromide serves as a conservative tracer at the circumneutral pH conditions observed within the study reach.4 As such, bromide concentrations from synoptic samples may be used to estimate streamflow using the tracer-dilution method.4,34 Given the negligible background bromide concentrations along the study reach, additions of water to the main stem should result in a continuously decreasing tracer profile, with any increases in bromide concentration being the result of laboratory and/or sampling error. A continuously decreasing bromide profile for use in the tracer dilution equation was therefore developed using the observed data and expert judgment (see “Br used”, SI Table S2). Loading Analysis. Estimates of constituent load provide an objective means of determining the sources that have the greatest effect on instream water quality. To this end, the study reach is divided into 25 stream segments that are demarcated by the 26 stream sampling sites (Figure 1). Constituent loads were calculated as the simple product of streamflow and totalrecoverable constituent concentrations (SI Tables S7−S8). Cumulative instream load is equal to the sum of all increases in constituent load.1 Stream segments in which the cumulative instream load increased are considered sources of constituent mass. The percent contribution of each source is equal to the within-segment increase divided by the cumulative instream load at the end of the study reach.4 Diel Sampling. Diel sampling was conducted at 3083 m from 13:15 on August 22 to 21:00 on August 23 (SI Tables S9−S13). Samples were collected approximately every 1.5 h and analyzed for total recoverable and dissolved cations using ICP-MS.7 As noted by Balistrieri et al.,7 temporal variation in constituent concentration was primarily attributable to the rain event on August 22, with constituent concentrations being nominally invariant during synoptic Sweep 2 (11:00−18:15, August 23).

extending from Butte to Missoula, Montana, constitutes the largest Superfund site in terms of areal extent.22,28,29 The head of the 5.5 km study reach is located on Blacktail Creek, approximately 1.6 km above its confluence with the Metro Storm Drain (MSD 1641 m, Figure 1). MSD is a realigned section of upper Silver Bow Creek that was historically used to convey mine waste, sewage, and stormwater from Butte Hill.17 The study reach continues past this confluence into Lower Area One (LAO), a 2.7 km section of Silver Bow Creek that was home to extensive milling and smelting activities that left behind large quantities of tailings and slag. Although Superfund removal actions have removed much of the contamination within the floodplain of Silver Bow Creek, considerable residual contamination remains.17,30 In addition, the Blacktail Creek floodplain has not been subject to remediation, and the MSD watershed has numerous locations where contaminated soils were “left in place”.31 Acidic, metalrich groundwater is also present in the vicinity of MSD and LAO, some of which is intercepted and sent to a lime treatment plant farther downstream.17 Sample locations along the study reach include 26 sites on the Blacktail/Silver Bow Creek main stem and 11 inflows that enter along the main stem (Figure 1, Supporting Information (SI) Table S1). Water quality within the study reach is characterized by circumneutral pH conditions and metal concentrations that generally exceed those found in unmineralized watersheds. Dissolved concentrations of Al and Fe are typically low, however, given the low solubility of these metals at circumneutral pH. Precipitated Al and Fe solids sorb other metals such as As, Cd, Cu, and Pb, decreasing the dissolved concentrations of these elements as well.32 Precipitation and sorption reactions result in the formation of colloidal size solids for all of the foregoing metals, and these colloids are transported in the water column prior to aggregation and settling. Tracer Injection and Synoptic Sampling. A continuous, constant rate injection of a concentrated sodium bromide (NaBr) solution was initiated at the head of the study reach at 16:30 on August 21, 2010. The injection was terminated at 19:00 on August 23, following completion of synoptic sampling. Bromide concentrations were subsequently used to estimate streamflow at all main stem sampling locations using the tracer-dilution method. Streamflow measurements were made at a subset of main stem sites using Acoustic Doppler Velocimetry. Stage was recorded every 1 min by pressure transducers located at 0, 1758, and 4309 m. The synoptic sampling campaign conducted on August 23 consisted of three synoptic “sweeps”, with Sweep 1 (8:15− 10:10) and Sweep 3 (18:50−19:35) including only a subset of sites and Sweep 2 (11:00−18:15) including all of the sample locations. These sweeps were designed to assess the effect of an August 22 rainfall event (13 mm of rainfall33) on streamflow and constituent concentration. An analysis of sweep data, stage data, and diel data indicate that the effects of rainfall had dissipated by 11:00 on August 23. As such, the longitudinal constituent concentration profiles obtained from Sweep 2 provide a steady-state picture of the study reach that may be used to quantify constituent loads and sources. Collection of stream samples during each sweep proceeded in an upstream-to-downstream direction, with the sampling team traveling along the stream bank to avoid contaminating downstream samples with resuspended streambed materials. Width and depth integrated samples were collected at all main stem sites using a DH-81 sampling device. Samples were

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RESULTS AND DISCUSSION Synoptic SamplingSpatial Concentration Profiles. Spatial pH profiles (Figure 2a) exhibit a gradual increase that may be attributable to CO2 degassing.12 Instream pH also changes in response to inflow waters from the lime and wastewater treatment plants (4175 and 4340 m, Figure 2a). The circumneutral pH results in the formation of colloidal phases within the water column (total recoverable > dissolved) for Cu and other constituents due to precipitation and/or sorption reactions (Figure 2b, SI Tables S3−S6). Concentrations of most constituents (Al, As, Cd, Fe, Ni, Pb, and Zn) fall below chronic aquatic life standards35 over the entire length C

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Figure 2. Spatial profiles from synoptic sampling: (a) pH (inflow at 2128 m with pH of 3.23 not shown); (b) copper concentration, log scale (inflow at 2128 m with dissolved concentration of 14 100 μg/L not shown); (c) copper load; and (d) percent contribution to copper load. Red circles denote potentially erroneous data.

of the study reach (n.b. with the exception of Al, all standards are in terms of total recoverable concentrations). Copper concentrations, in contrast, exceed the chronic standard over the lower 3.5 km of the study reach (Figure 2b). Synoptic SamplingConstituent Sources at Low Flow. Total recoverable loads of copper and other constituents begin to increase downstream of Grove Gulch (735 m), with further increases observed downstream of the MSD (vertical lines in Figure 2c, SI Tables S7 and S8). Numerous stream segments along this upper portion of the study reach contribute to load (1151−2900 m, Figure 2d), suggesting diffuse sources of copper and other constituents. Stream segments representing

the eight largest sources of copper are presented in SI Table S14. Observed loading within two of these eight segments is of questionable validity, with large increases in load immediately preceding (2900 m) or following (4161 m) large decreases in load (red circles, Figure 2c,d). These abrupt changes are likely attributable to the sampling error associated with total recoverable metal concentrations,4 when an observed segment concentration is erroneously high or low, relative to the neighboring segments (red circles, Figure 2b). Contributions from consecutive segments are aggregated to correspond to specific geographical areas that are potential sources of diffuse loading (Subreaches 1−7, Table 1). The D

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Table 1. Aggregate Contribution to Loading from Segments within Various Subreaches (Subreaches Representing Largest and Second Largest Contributions to Load Are Shaded in Red and Yellow, Respectively)

Contribution from stream segment ending at 2900 m is questionable due to noisy data  i.e., the gain in load is immediately followed by a loss in load. Percentage shown is in parentheses is the aggregate contribution with the contribution from 2900 m excluded. bContribution from stream segment ending at 4161 m is questionable due to noisy data  i.e., the gain in load is immediately preceded by a loss in load. Percentage shown is in parentheses is the aggregate contribution with the contribution from 4161 m excluded. a

the three synoptic sweeps. Data from Sweep 1 indicates that the effects of rainfall were still making their way through the study reach in the early morning hours of August 23. Values of pH from Sweep 1 are lower than those for Sweep 2 at all 5 sampling locations (Figure 2a). In addition, total recoverable concentrations for 7 of 9 mining-related constituents are higher for Sweep 1 at all 5 locations (Al, Cd, Cu, Mn, Ni, Pb, Zn; Figure 2b, SI Tables S3−S4). This comparison of Sweep 1 and Sweep 2 suggests the continued addition of acidic metal-rich waters during the final stages of flow recession (Figure 3a). Given these observations, detailed synoptic sampling (as in Sweep 2) during the early morning hours would have resulted in a biased loading analysis, due to the effects of temporal variation. Temporal variation appears to be minimal during Sweep 2 given observed stage (Figure 3a) and the close correspondence between Sweep 2 and 3 pH (Figure 2a). Further, total recoverable concentrations for most miningrelated constituents from Sweep 3 are closer to those of Sweep 2 than Sweep 1 (SI Tables S3−S4). Diel SamplingEffects of Rainfall Runoff. The effects of rainfall discussed above may be more fully investigated using the diel data set that documents metal concentrations every 1.5 h over a two-day period. The diel sampling campaign captured dramatic increases in mining-related constituents during the rain event, with peak concentrations 2−23 times the concentrations observed during synoptic sampling (most increases were more than 10-fold; Table 2). Concentrations of copper and other mining-related constituents peaked immediately after the increase in streamflow, and remained elevated during the receding limb of the hydrograph before returning to pre-event levels (Figure 3a,b, SI Tables S10, S11). This response produces a clockwise hysteresis loop36 in which the highest concentrations are observed on the rising limb of the hydrograph, with concentrations declining thereafter (Figure 3c). The potential effect of the rainfall-induced concentration increases is exacerbated by concomitant decreases in water hardness, as Ca and Mg concentrations are diluted by rainfall (Figure 3d, SI Tables S12, S13). The decrease in hardness results in a lower threshold for toxicity, as reflected by hardness-based standards for aquatic life. This increase in

majority of Cd, Cu, and Pb loading is within Subreach 4 (1470−3083 m; n.b., this conclusion is unaffected by the potentially erroneous concentration data discussed above, see footnote a, Table 1). Potential sources within this subreach include groundwater inflow from the MSD area, leakage from a sandbagged seep (2128 m), residual tailings, and slag. Sources above the study reach account for the majority of Fe load (Subreach 1, 45%), whereas the majority of the Zn load (37%) comes from Subreach 7 that receives inflow from the wastewater treatment plant. Zinc concentrations in the wastewater inflow exceed stream concentrations (SI Tables S4 and S6), and the wastewater treatment plant is known to represent a large percentage of the load during low-flow conditions (Curt Coover and Kent Whiting, written communication). Synoptic SamplingDiffuse Sources. One of the advantages of combining the accuracy of the tracer dilution method with the spatial detail of synoptic sampling is the ability to quantify diffuse sources. Increases in load are attributed to diffuse sources when an increase in load occurs within a stream segment without surface inflow. Diffuse sources include groundwater, bed sediments, and pore waters. Substantial loading in several subreaches can be attributed to diffuse sources. Subreach 3, for example, is the second largest source of Pb, and the bulk of this loading enters in a stream segment without observed surface inflow (segment ending at 1470 m, SI Table S8). This loading is attributable to floodplain tailings and contaminated sediments at the upstream end of the Blacktail Creek berm.30 Lead containing minerals representing potential sources include galena, anglesite, lead jarosite.19 Similarly, loads of Cd, Fe, Pb, and Zn in Subreach 4 are largely contributed by segments without surface inflow, while Cu load is predominately contributed in the segment that brackets the sandbagged seep (SI Tables S7 and S8). Although the Subreach 4 segment bracketing the MSD was not a large source of constituent load, Cd, Pb, and Zn concentrations in the MSD were much higher than those in the stream, and groundwater with similar composition may be responsible for the load increases observed in two segments immediately downstream (SI Tables S3−S8). Synoptic SamplingMulti-Sweep Approach. Effects of the August 22 rain event are evaluated here in light of data from E

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Figure 3. Temporal profiles and stage-concentration relationships: (a) Approximate stage at 3083 m, based on data from 1758 and 4309 m; (b) temporal profile of Cu concentration from diel sampling; (c) diel Cu concentration as a function of stage; (d) diel hardness concentration as a function of stage.

Mechanisms of Release During Rainfall Runoff. The rainfall event led to a decrease in pH (Sweep 1 versus Sweep 2, Figure 2a) and increases in both total recoverable (all miningrelated constituents) and dissolved (Cu, Cd, Mn, and Zn) concentrations, as discussed above. These changes in water chemistry are attributable to a suite of hydrologic and geochemical processes that release metals and acidity from areas with residual contamination and unremediated areas where contaminants have been left in place.

concentration of mining-related constituents, coupled with a decrease in hardness, has been observed in the Clark Fork River downstream of the study reach, and in other mining-impacted watersheds.37,38 For the case considered here, Cd, Cu, Fe, Pb, and Zn concentrations exceed the chronic aquatic life standards for extended periods of time (6−32 h), and acute standards are exceeded for Cu and Zn (Table 2). These results are in stark contrast to the results from synoptic sampling conducted the day after the rain event, when all mining-related constituents, with the exception of Cu, met the applicable standards. F

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underlying the Parrot Smelter site.31 Tailing deposits were up to 10% sulfide and generated dissolved leachate concentrations that were 2−3 orders of magnitude greater than those observed during the diel sampling discussed herein (e.g., leachate Cu concentration of 27 600 μg/L). Tucci also reported increases in metal concentrations within alluvial groundwater that corresponded to increases in water table elevation, a fact that further supports the foregoing analysis. Additional processes that potentially affect dissolved concentrations and pH include the dissolution of evaporative salts36 and increased contributions from contaminated groundwater. The August 22 rain event was preceded by relatively hot, dry weather ( dissolved concentrations, Figure 2b, SI Tables S3−S6). Resuspended solids from previous rainfall runoff events (e.g., flooding in lower Grove Gulch30) may also contribute to the increase. The foregoing hydrologic processes alone do not explain the observed decrease in pH and increase in dissolved constituent concentrations. These changes are attributable to geochemical processes that become more dominant as previously dry areas are wetted by infiltration and a rising water table. Floodplain tailings along Silver Bow Creek are known to overly natural floodplain soils, sometimes being perched above the water table.41 Although many of these floodplain deposits have been addressed during removal actions, contamination may remain below the vertical excavation limit.42 Considerable deposits of tailings and mine waste have also been left in place, including contaminated soils and slag within LAO,42 dispersed mine waste underlying clean fill, buildings, and parking lots within the City of Butte,17,28 and acid-generating lenses of tailings underlying the former Parrot Smelter site.31 Increases in water table elevation in response to rainfall could result in considerable leaching of metals and acidity from these previously dry deposits,20 and rainfall infiltration through the vadose zone would have a similar effect. Dissolution of metals is thus a likely mechanism of release that is supported by the work of Tucci, who conducted leach tests on alluvial tailings G

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year. Low flow synoptic sampling therefore provides the means to quantify the primary sources of contamination when they are undiluted by snowmelt and/or rainfall. Rainfall runoff events in these watersheds often result in dilution of metals and acidity.48 Synoptic sampling efforts in watersheds with extensive streamside tailings, in contrast, may be misleading as the primary source areas lie above the water table at low flow. For the case considered here, results of synoptic sampling indicate that concentrations of most mining-related constituents meet aquatic life standards at low flow. Results of diel sampling provide a contrasting picture, however, as metal concentrations spike following a summer rain event. These contrasting results, obtained from the same stream over a two-day period, demonstrate the limitations of low-flow synoptic sampling in watersheds with extensive tailing deposits. Additional sampling to document the effects of snowmelt and rainfall is needed in these watersheds, to adequately document source areas over a range of flow regimes.16,37,44 Consideration should be given to both the early snowmelt period where a first flush of contaminants is known to occur, and rainfall runoff events such as the one considered here. Sampling of these events can be facilitated by the use of automated sampling devices equipped with stage sensors that initiate sampling at the onset of high flow events.

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contributions to zinc loads in mining impacted catchments. J. Environ. Monit. 2010, 12, 1684−1698. (4) Runkel, R. L.; Walton-Day, K.; Kimball, B. A.; Verplanck, P. L.; Nimick, D. A. Estimating instream constituent loads using replicate synoptic sampling, Peru Creek, Colorado. J. Hydrol. 2013, 489, 26−41; DOI 10.1016/j.jhydrol.2013.02.031. (5) Brick, C. M.; Moore, J. N. Diel variation of trace metals in the Upper Clark Fork River, Montana. Environ. Sci. Technol. 1996, 30, 1953−1960; DOI 10.1021/es9506465. (6) Nimick, D. A.; Gammons, C. H.; Parker, S. R. Diel biogeochemical processes and their effect on the aqueous chemistry of streams: A review. Chem. Geol. 2011, 283, 3−17; DOI 10.1016/ j.chemgeo.2010.08.017. (7) Balistrieri, L. S.; Nimick, D. A.; Mebane, C. A. Assessing timeintegrated dissolved concentrations and predicting toxicity of metals during diel cycling in streams. Sci. Total Environ. 2012, 425, 155−168; DOI 10.1016/j.scitotenv.2012.03.008. (8) Runkel, R. L.; Bencala, K. E.; Kimball, B. A.; Walton-Day, K.; Verplanck, P. L. A comparison of pre- and post-remediation water quality, Mineral Creek, Colorado. Hydrol. Processes 2009, 23, 3319− 3333; DOI 10.1002/hyp.7427. (9) Grayson, R. B.; Gippel, C. J.; Finlayson, B. L.; Hart, B. T. Catchment-wide impacts on water quality: the use of “snapshot” sampling during stable flow. J. Hydrol. 1997, 199, 121−134; DOI 10.1016/S0022-1694(96)03275-1. (10) Besser, J. M.; Leib, K. J. Modeling frequency of occurrence of toxic concentrations of zinc and copper in the Upper Animas River. In USGS Water-Resources Investigations Report 99 4018A; Morganwalp, D. W., Buxton, H. T., Eds.; United States Geological Survey 1999; pp 75−81. (11) Runkel, R. L.; Kimball, B. A.; Walton-Day, K.; Verplanck, P. L. A simulation-based approach for estimating premining water quality: Red Mountain Creek, Colorado. Appl. Geochem. 2007, 22, 1899−1918; DOI 10.1016/j.apgeochem.2007.03.054. (12) Runkel, R. L.; Kimball, B. A.; Walton-Day, K.; Verplanck, P. L.; Broshears, R. E. Evaluating Remedial Alternatives for an Acid Mine Drainage Stream: A Model Post Audit. Environ. Sci. Technol. 2012, 46, 340−347; DOI 10.1021/es2038504. (13) Sullivan, A. B.; Drever, J. I. Spatiotemporal variability in stream chemistry in a high-elevation catchment affected by mine drainage. J. Hydrol. 2001, 252, 237−250; DOI 10.1016/S0022-1694(01)00458-9. (14) Gammons, C. H.; Metesh, J. J.; Duaime, T. E. An overview of the mining history and geology of Butte, Montana. Mine Water Environ. 2006, 25, 70−75. (15) Nimick, D. A.; Moore, J. N. Stratigraphy and chemistry of sulfidic flood-plain sediments in the Upper Clark Fork Valley, Montana. In Environmental Geochemistry of Sulfide Oxidation; Alpers, C. N., Blowes, D. W., Eds.; American Chemical Society: Washington, DC, 1994; pp 276−288. (16) Pascoe, G. A.; DalSoglio, J. A. Planning and implementation of a comprehensive ecological risk assessment at the Milltown ReservoirClark Fork River Superfund site, Montana. Environ. Toxicol. Chem. 1994, 13, 1943−1955. (17) Gammons, C. H.; Madison, J. P. Contaminated alluvial ground water in the Butte Summit Valley. Mine Water Environ. 2006, 25, 124− 129. (18) Davis, A.; Ruby, M. V.; Bergstrom, P. D. Bioavailability of arsenic and lead in soils from the Butte, Montana, mining district. Environ. Sci. Technol. 1992, 26, 461 468; DOI 10.1021/es00027a002. (19) Davis, A.; Drexler, J. W.; Ruby, M. V.; Nicholson, A. Micromineralogy of mine wastes in relation to lead bioavailability, Butte, Montana. Environ. Sci. Technol. 1993, 27, 1415 1425; DOI 10.1021/es00044a018. (20) Benner, S. G.; Smart, E. W.; Moore, J. N. Metal behavior during surface-groundwater interaction, Silver Bow Creek, Montana. Environ. Sci. Technol. 1995, 29, 1789−1795. (21) Nagorski, S. A.; Moore, J. N. Arsenic mobilization in the hyporheic zone of a contaminated stream. Water Resour. Res. 1999, 35, 3441−3450.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b03190. Additional information as noted in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 303/541-3013; fax: 303/541-3084; e-mail: runkel@ usgs.gov. ORCID

Robert L. Runkel: 0000-0003-3220-481X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was funded by the U.S. Geological Survey’s Toxic Substances Hydrology Program. Logistical support and field/ laboratory assistance was provided by Laurie Balistrieri, Chris Gammons, Joe Griffin, Joe Mills, Joe Naughton, Steve Parker, Kim Schierenbeck, Alan Shiller, Sara Sparks, Judy Steiger, and Nick Tucci. Helpful review comments were provided by Curt Coover, Kent Whiting, and two anonymous reviewers.

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REFERENCES

(1) Kimball, B. A.; Runkel, R. L.; Walton-Day, Katherine; Bencala, K. E. Assessment of metal loads in watersheds affected by acid mine drainage by using tracer injection synoptic sampling: Cement Creek, Colorado, USA. Appl. Geochem. 2002, 17, 1183−1207. (2) Kimball, B. A.; Walton-Day, K.; Runkel, R. L. Quantification of metal loading by tracer injection and synoptic sampling, 1996−2000. In Integrated Investigations of Environmental Effects of Historical Mining in the Animas River Watershed, San Juan County, Colorado; Church, S. E.; von Guerard, P.; Finger, S. E.; Eds.; USGS Professional Paper 1651, 2007; pp 417−495. (3) Banks, V. J.; Palumbo-Roe, B. Synoptic monitoring as an approach to discriminating between point and diffuse source H

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DOI: 10.1021/acs.est.6b03190 Environ. Sci. Technol. XXXX, XXX, XXX−XXX