Project Title: Multiple Time-Scale Investigations of the Chemistry and Biological Activity of a Stream Receiving Acid Mine Drainage
Mendenhall Fellow: Thomas Chapin, (303) 236-5795, email@example.com
Duty Station: Denver
Start Date: January 27, 2002
Education: Ph.D. (Oceanography), University of Washington, 1997
Research Advisor: Richard B. Wanty, (303) 236-1819, firstname.lastname@example.org
Dr. Thomas Chapin is currently a research scientist with the Crustal Imaging and Characterization Team of the USGS in Denver, Colo.
Abstract and Goals
Historically, most investigations of biogeochemical cycling in acid rock drainage systems involved synoptic sampling over longer time periods (one sample per week or month) or intensive time series studies over a few hours or days. However, many important episodic loading events, such as seasonal meltwater and storm events occur on timescales that are rarely observed by these traditional sampling approaches. Monitoring these transient processes, which may contribute the bulk of the annual metal loading, is essential for accurate assessment in acid rock drainage areas. Recent work also has demonstrated dramatic diel (24 hour) variations in trace metal concentrations in response to diel changes in temperature, pH, oxygen, and photochemical cycling (Brick and Moore, 1996, Nimick et al. 2003). To characterize a system that may respond to diel, seasonal, and episodic events, a monitoring program must sample at a frequency that captures the details of these major forcing events.
The overall goal of this Mendenhall project was to develop new chemical monitoring instrumentation to examine the effects of temporal variability in acid mine drainage systems. We have developed two novel instruments for this task, the Zn-DigiScan (Zn-Digital Submersible Chemical Analyzer) and the Bubble Sampler. The Zn-DigiScan is a battery powered near real-time in situ chemical analyzer designed for long-term deployments with hourly analyses of Zn and the Bubble Sampler is a long-term water sampling instrument. Additional instruments were deployed with the Zn-DigiScan and Bubble Sampler to provide concurrent water quality data for temperature, conductivity, dissolved oxygen, pH, turbidity, and depth.
The use of brand names in this summary is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey. The primary instrument used in this research was the DigiScan, a submersible in-situ chemical analyzer developed during my previous work at the Monterey Bay Aquarium Research Institute. The DigiScan was originally designed to measure nitrate in oceanic environments and an improved version of this instrument, the YSI 9600 nitrate analyzer, is currently available from YSI (Yellow Springs International). We adapted the DigiScan instrument to monitor Zn in streams affected by acid mine drainage. The Zn-DigiScan uses micro solenoid pumps to preconcentrate and separate Zn from interfering metals with a strong anion exchange column, add reagents, and propel the reaction product to a photometric detector (Figure 1). A sensitive colorimetric reagent, PAN (pyridyl-azo napthol), was used to determine Zn concentrations. A battery powered data logger controls the instrument, records the data and the Zn-DigiScan can operate autonomously for over 3 months. The linear dynamic range for the Zn-DigiScan was 20-3000 g/L Zn with a detection limit of 20 g/L (3 blank). The analytical precision below 100 g/L was ±10 g/L. Onboard Zn standards and blanks were analyzed every 10 hours to provide in-situ calibration. The Zn-DigiScan was deployed with an YSI Sonde 6600 water quality sonde that provided concurrent temperature, pH, conductivity, dissolved oxygen, turbidity, and depth information.
Figure 1. Schematic of Zn-DigiScan and Bubble Sampler.
The second instrument, the Bubble Sampler, was developed for long-term high resolution water sampling. The Bubble Sampler injects a small sample volume (1-10 mL) into an acid cleaned, 3.2 mm I.D. Teflon coil and a bubble of nitrogen gas is inserted between successive samples to prevent mixing (Figure 1). The Bubble Sampler has the capacity to take over fifty samples and can be programmed to take either discrete samples every hour or one sample integrated over 24 hours. Laboratory tests indicated that carryover between successive samples was <3%. Upon recovery, samples were diluted and analyzed by inductively coupled plasma mass spectrometry (ICP-MS) for a suite of elements. Bubble Sampler results were compared with samples collected by an Isco 3700 automated sampler and analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). Online sample acidification is also possible with the Bubble Sampler but was not done during this project.
Near real time in-situ Zn
The following discussion highlights results from an August 12-15, 2002 deployment at Fisher Creek, Montana (Figure 2). In-situ Zn concentrations at Fisher Creek obtained by the Zn-DigiScan ranged from 25-65 g/L and showed a clear diel cycle (Fig. 3G). Higher Zn concentrations were observed during the late evening-early morning and lower Zn concentrations were observed during the late afternoon. This diel Zn cycle was observed over 3 successive days at Fisher Creek. Temperature and pH were out of phase with Zn, with higher temperature and higher pH values during periods when Zn concentrations were low (Fig. 3). The turbidity signal displayed a modest diel cycle in phase with the Zn cycle suggesting that the increase in Zn concentration may be linked to desorption of Zn from suspended particulates (Fig. 3).
Figure 2. Map of Fisher Creek and location of sampling site.
Figure 3. Three day record of flow (A), conductivity (B), DO (C), turbidity (D), pH (E), temperature (F), and in-situ Zn (G) at Fisher Creek, MT.
Nimick et al. (2003) identified a number of processes that could result in diel metal cycling including:
- variations in metal input;
- diel variations in stream flow;
- redox and photochemical reactions;
- precipitation-dissolution reactions;
- biological uptake-regeneration; and
- adsorption reactions.
They observed diel Zn cycling in a number of near neutral pH streams in Montana and noted strong inverse Zn correlations with temperature and pH (Nimick et al., 2003). Their results suggest that the diel metal cycling is driven primarily by temperature and pH dependent adsorption. Since the diel pH change in Fisher Creek was relatively small, it appears that temperature was the dominant forcing mechanisms for diel Zn cycling at this location.
Intercomparison of Zn-DigiScan with Isco Sampler
Figure 4 compares near real-time in-situ Zn-DigiScan data to results from samples collected by the Isco sampler and analyzed by ICP-OES. The same diel Zn cycle was observed in both data sets but the Zn-DigiScan results showed a greater range between the high and low Zn concentrations. The Zn-DigiScan was operating near its detection limit which may account for the discrepancy between the Zn-DigiScan results and the ICP-OES results. In addition, high concentrations of Cu relative to Zn observed in Fisher Creek can produce a 10% positive interference in the Zn analysis. Further intercalibration efforts and refinement of the analytical technique will address these discrepancies. In spite of these difficulties, the near real-time in-situ Zn-DigiScan results clearly showed the diel Zn cycle and were in general agreement with the Zn data collected by standard methods.
Figure 4. Comparison of in-situ Zn concentrations from the Zn-DigiScan with ICP-AES results from the ISCO sampler.
Bubble Sampler vs. Isco Sampler
Diel cycles in Mn, Cu, and Zn were also observed in the data from both the Bubble Sampler and the Isco sampler (Fig. 5). These cationic metals followed the predicted pattern of higher metal concentrations during the coldest, lowest-pH part of the diel cycle and lower metal concentrations during the warmest, highest-pH part of the diel cycle. We found very good agreement between the Bubble Sampler and the Isco Sampler throughout the diel Zn cycle. Cu concentrations agreed well at the beginning of the deployment but diverged after 6 hours. Comparison of the Mn data collected by the different samplers show the same diel pattern but with a 25 g/L offset. More intercalibration efforts are needed but these initial results are very promising and indicate that the Bubble Sampler can collect samples cleanly and preserve the subtle concentration changes found in diel metal cycling.
Figure 5. Comparison of Mn (A), Cu (B), and Zn (C) from the Bubble Sampler and the Isco sampler.
Conclusions and future work
We successfully developed a new in-situ chemical analyzer to provide hourly monitoring of Zn concentrations and a new water sampler to capture the details of diel metal cycling in streams affected by acid mine drainage. However, the longer-term reliability of the Zn-DigiScan has been problematic, especially in shallow deployments in winter conditions or in cold high altitude streams. These cold streams are often super saturated with air and micro-bubbles degassing in the instrument flow path would usually lodge in the detector within 1-7 days. Once a bubble lodged in the detector, the small solenoid pumps were not strong enough to consistently flush the bubbles out of the detector and subsequent data was not reliable. A number of modifications were made to address the bubble issue but we were not able to reliably solve the Zn-DigiScan bubble problem.
To address some of the limitations of the Zn-DigiScan, I spent the last portion of my Mendenhall appointment developing a completely new in-situ analyzer, the Field Sequential Injection Analyzer (Field-SIA, Figure 6). The Field-SIA is more reliable in the field, more flexible analytically, and costs much less than the Zn-DigiScan. The Field-SIA is designed to integrate with existing USGS gauge station telemetry systems to provide real-time chemical and real-time flow information. Integration of the Field-SIA with existing USGS telemetry stations will allow USGS scientists to monitor real-time chemical fluctuations and relate them to seasonal or episodic processes (e.g. seasonal meltwater runoff, volcanic and tectonic events, storms, etc.), as well as processes that respond to daily cycling (e.g. diel temperature and photochemical cycles, photosynthesis, diel metal cycling, etc.). Real-time chemical information combined with real-time flow data will greatly improve estimates of total maximum daily loads (TMDLs). Real-time chemical monitoring is critical to the USGS goal of providing time-relevant data for hazard mitigation and we expect that the Field-SIA will make a significant contribution to this goal.
Figure 6. The new Field Sequential Injection Analyzer (Field-SIA) under development.
This research was funded by the USGS Mendenhall Postdoctoral Research Fellowship Program and the Crustal Imaging and Characterization Team. David Nimick (USGS) and Chris Gammons (Montana Tech) provided Isco trace metal data and assisted in the interpretation of the data.
Brick, C.M. and Moore, J.N. 1996. Diel variation of trace metals in the upper Clark Fork River, Montana. Environ. Sci.Technol. 30:1953-1960.
Nimick, D.A., Gammons, C.H., Cleasby, T.E., Madison, J.P., Skaar, D., and Brick, C.M. 2003. Diel cycles in dissolved metal concentrations in streams - Occurrence and possible causes, Water Resour. Res., 39: 1247-1264.
Completed Publications From This Project
Chapin, T., Wanty, R., Development of a Real-Time In Situ Zinc Analyzer and Water Sampler - Results From High Resolution Monitoring of a Stream Impacted by Acid Rock Drainage. Sixth International Conference on Acid Rock Drainage, July 2003, Cairns, Australia, conference proceedings, pp 987-999.
Chapin, T., Wanty, R., Diel cycling of Zn and other metals in a stream impacted by acid rock drainage: results from an in-situ Zn analyzer and water sampler: Water Rock Interaction Conference, June 2004, Sarasota Springs, NY, conference proceedings pp 1463-1467.
Chapin, T., Wanty, R., Development of a solenoid pumped in-situ zinc analyzer for environmental monitoring: submitted to Analytica Chimica Acta.
Chapin, T., Nimick, D., Gammons, C., and Wanty, R., Diel cycling of metals in a stream impacted by acid rock drainage; initial results from a new in-situ Zn analyzer and water sampler, in internal USGS review, to be submitted to Environmental Science and Technology.
Original Project Description:
Background.--Historically, most investigations of biogeochemical cycling in acid mine drainage systems have involved synoptic sampling (one sample per week or month) over longer time periods or intensive time series studies over short time periods (hours to days). However, many important processes, such as diurnal and seasonal meltwater events, and storm events, which may contribute the bulk of the metal loading, are rarely recorded by these traditional sampling approaches. To characterize an ecosystem that may respond to diurnal, seasonal, and episodic events, a monitoring program must be able to sample at a frequency that captures the details of these major forcing events.
Research Objectives.--In order to examine low and high frequency temporal changes in acid mine drainage areas, we are deploying an instrument package for the high-resolution long term in-situ monitoring of biogeochemically active elements in the Snake River basin near Montezuma, CO. Two different types of monitoring instrumentation will be used: 1) The DigiScanner-DigiSampler, a novel in-situ analyzer and in-situ sampler, and 2) a YSI Sonde 6600 water quality monitor for basic physical properties.
The Zn-DigiScanner will be set up to provide hourly in-situ monitoring of Zn to study the temporal nature of Zn cycling. The DigiSampler will collect daily-integrated samples for multi-element analysis of metals (Al, Cd, Cu, Mn, Fe, Pb, Zn, etc.), nutrients (NO2, NO3, PO4), and anions (F, Cl, SO4). The DigiScanner-DigiSampler can operate unattended for over 3 months performing in-situ analyses and collecting in-situ samples. Basic physical and biological parameters will be monitored by a YSI 6600 Sonde multi-parameter water quality logger. Physical parameters (temperature, pH, flow, conductivity, turbidity) and biological parameters (dissolved oxygen, fluorescence) will greatly aid in the interpretation of results.
The data generated from this study will provide insights to the relative importance of the physical, chemical, and biological processes that affect short and long term changes in metal and nutrient concentrations in acid mine drainage areas. We expect the dominant hydrologic forcing to be meltwater events, the dominant biological forcing to be photosynthesis and respiration, and the dominant geochemical forcing to be pH changes and colloid formation and precipitation (Fe and possibly Al and Mn colloids). Due to the inherent complexity of even the simplest stream system, a multi-parameter approach will provide the best understanding of the biogeochemistry of the Snake River.
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