Mine drainage from the SLT was fed continually at ≈ 113 L/min (30 gpm) to provide the entire range of seasonal water quality over a two-year period (2015–2016). During that time, water quality parameters and metal concentrations were measured at locations throughout the treatment system (Fig. 1).
During the freshet season of 2015, it was determined that the monitoring well point for evaluating the HFW was faulty and not representative of the treated water from the HFW. The monitoring point was switched to a new location at the discharge of the HFW-treated water manifold to the aeration channel (subsequently termed AC1INF). An analysis of the sampling location change, included in the supplemental information, shows that metal concentrations in the HFW effluent are best represented by the AC1 effluent data (AC1EFF) up until 9/10/2015 combined with AC1INF data from 10/12/2015 forward to provide a continuous record over the study (more detail in supplemental information). Water parameters heavily affected by the air sparging in the aeration channel include sulfide, oxygen, and CO2 concentrations, pH, and ORP measurements. The data from the original sampling location (well HSSWMP11) and the replacement sampling point (AC1INF) are the continuous data best suited to HFW evaluation and will be referred to as HFWEFF.
Seasonal Changes in Water Chemistry
The spring freshet at the Rico site brought several changes to the SLT mine water chemistry, including increased metal concentrations and decreased pH and alkalinity concentrations (Dean et al. 2022; Lewis-Russ et al. 2022), which affected the performance of the HWTT. Time-series data for treatment water pH throughout the HWTT are contained in Fig. 2. Values of pH in the influent were depressed during both (2015 and 2016) freshets and those effects were noted throughout all components of the HWTT. The 2015 freshet was different in that the pH values at the HFWEFF were lower than the influent pH, which is attributable to oxidation of elemental sulfur (prills) that was added to the HFW media to assist in low temperature operations (Marsland et al. 2010). Figure 3 contains time-series pH and sulfide concentrations for HFW effluent (HFWEFF) and Fig. 4 contains sulfide at HFWEFF and dissolved Zn flowing into the HFW (HFWINF). Sampling for the freshet events occurred when the SLT flow initially increased and the pH (based on in-place sondes) decreased, according to the conceptual site model (Lewis-Russ et al. 2022). The 2016 freshet had anomalously low flow; a sampling event was conducted at the influent/effluent locations on May 26, 2016, out of concern that the 2016 freshet had begun. A full sampling of the system was performed on June 6, 2016; however, peak metals concentrations for some system components were missed. This illustrates the temporal variability and importance of understanding the conceptual site model and when to collect samples to characterize freshet seasons. The HFWEFF sulfide concentrations were very high during system startup, from 1 to 8 mg/L during non-freshet periods, and decreased abruptly to < 0.05 mg/L during both freshet periods. The effects of the freshet on sulfide concentrations lasted one month following the 2015 freshet. The pH and/or Zn concentrations affected sulfide production in the HFW (Figs. 3, 4). The EC:50 (Zn concentration reported to cause a 50% decrease in sulfate reduction in mixed SRB cultures, Utgikar et al. 2001) and lower pH range for optimal SRB activity are both plotted for reference.
Changes in pH and ORP distribution during the spring freshet were apparent spatially within the HFW, as measured at 11 locations across transects at the inlet, middle, and outlet positions of the HFW (Fig. 5). The 2015 results of pH and ORP measurements (Figs. 6 and 7, respectively) illustrate the seasonal dynamics of these parameters within the HFW. Figure 6 shows that ambient pH during the 2015 freshet decreased by 0.2–0.3 units as water travelled across the entire HFW, reaching a minimum of 5.9 at one individual location during the 2015 spring freshet. Water pH values drop consistently along the flow path, suggesting a form of acidity in the wetland media in addition to acidity from the influent water (sulfur prills). The narrow transition zone (inlet end) of the HFW stayed aerobic the entire year, but ORP decreased rapidly as water travelled across the wetland during the non-freshet periods (Fig. 7). During the spring freshet, however, the aerobic portion of this transition zone extended across the entire length of the HFW. The increased ORP and decreased pH in the middle and outlet end of the HFW during the 2015 freshet were sub-optimal for SRB activity as reflected in decreased levels of sulfide during and after freshet (Fig. 3). This is consistent with the optimal pH and ORP requirements of most known freshwater SRBs (Hartzell and Reed 2006; Widdel 1988; Widdel and Bak 1992).
Metal Removal in the Horizontal Wetland System
Time series plots of As, Cd, Cu, Fe, Mn, Ni, Pb, and Zn concentrations for each component of the HWTT (influent, SBEFF, SFWEFF, HFWEFF, ACEFF, and RDEFF) are contained in supplemental Figs. S-1–S-8. Site-specific effluent criteria for the Rico site are not yet available for performance evaluation; therefore, a 2008 water quality assessment (WQA, Colorado Dept. of Public Health and Environment (CDPHE 2008)) for a potential lime treatment plant was used to develop project treatment goals (PTGs) for this research project (Table 1). The WQA contains effluent criteria that are protective of the most limiting surface water beneficial use, including toxicity to aquatic species, without regard to treatment technology or process. Although the WQA values are for dissolved metals concentrations (0.45 µm filtered), CDPHE generally requires potentially dissolved metals analyses (acid digestible metals, unfiltered) until it can be demonstrated that dissolved analyses correlate with potentially dissolved analyses (CDPHE 2017). For this study, we had very few potentially dissolvable metal data, so total metals (unfiltered) analyses were used as a conservative comparison to system performance (PTGs). The PTGs are included in Figs. S-1–S-8 to assist in performance evaluation. The frequency of PTG exceedances and removal efficiency for metals and As are reported in Table 2.
The fate and transport of metals in the HWTT were initially affected by whether they were in dissolved or particulate form. Iron oxyhydroxide is the dominant suspended solid in the mine drainage influent to the HWTT and was largely removed by settling (in the SB and, to a lesser extent, the SFW). Pb, As, and to some extent Cu were adsorbed/coprecipitated with Fe oxyhydroxides and removed primarily in the SB and SFW. Table 2 illustrates that As, Cu, Fe, Ni, and Pb generally met PTGs in the HWTT effluent (following system startup period) and so are not discussed further. Cd, Mn, and Zn were predominantly dissolved in the influent, were not attenuated appreciably in the SB and SFW, and so are the key metals of interest in this paper.
Zinc was present in highest concentrations during both freshet and non-freshet conditions, with freshet Zn concentrations almost 100 times those of Cd. For the post-startup period, total Zn concentrations in HWTT effluent (RDEFF) were below the PTGs 75% of the time (Figs. 8, S-8, Table 2), while dissolved Zn met PTGs 93.8% of the time. The average monthly removal efficiency for total Zn was 91.8% (Table 2) with highest removal efficiencies in the non-freshet periods and removal efficiencies as low as 60% during freshet periods when total concentrations were highest. Most of the total Zn removal occurred in the HFW (Figs. 8, S-8) and most of the total Zn in the HFW effluent was in the particulate form (Fig. 9). Analysis of the particulate Zn in a companion study (this issue) indicated that the suspended particles were ZnS (Dean et al. 2022). Nanophase ZnS particles can have significant mobility in saturated media and are common in natural systems (Deonarine et al. 2011) and other wetland and passive treatment studies (Gammons et al. 2000; Jarvis et al. 2015). Total Zn concentrations exceeded PTGs at times in the HFWEFF; however, the RD was effective at capturing most of the suspended material before system discharge (RDEFF). Post startup, total Zn only exceeded PTGs during the freshet periods and two samples during late 2016 (Fig. 9). In the more oxic environment of the RD, the dissolved Zn concentrations in the RDEFF were higher than upstream in the HFWEFF and dissolved Zn for the RDEFF (HWTT effluent) tended to increase over time.
Cadmium behaved similarly to Zn; however, much lower concentrations in the HWTT influent resulted in more frequent achievement of Cd PTGs, despite lower PTGs for Cd. Total Cd concentrations in the HWTT effluent (RDEFF) were below PTGs except for once during the system startup period and once during the 2015 freshet (Figs. 10, S-2, Table 2). Frequency of meeting PTGs after the startup period was 96.9% for total Cd and 100% for dissolved Cd (Table 2). The average total Cd concentration in the HWTT effluent after the startup period was 0.53 µg/L compared to the reporting limit of 0.5 µg/L (value of ½ the reporting limit used for data < RL). Most Cd mass removal was in the HFW (Figs. 10 and S-2) and most of the Cd in the effluent of the HFW was in the particulate form as CdS or Cd substituted into the ZnS structure (Fig. 11). The molar ratio of Cd:Zn in the HWTT influent was 0.3% averaged over the project and substitution of Cd in ZnS is the most probable Cd removal mechanism. Some of the total Cd was also removed in the RD, with most being settled as particulate matter (Figs. 10, S-2).
Total Mn concentrations were attenuated by the HWTT at an unexpectedly high efficiency (Figs. 12, S-5). Total and dissolved Mn met PTGs 100% of the time following the system startup period (Table 2). Post startup, attenuation of Mn was high in both the HFW and the RD components of the HWTT system (Fig. 12). The upgradient HFW removed most of the Mn, except during startup and the 2015 freshet. The Mn attenuation was negatively affected one month into the 2015 freshet and dissolved Mn from the HFWEFF increased above the lower PTG. The RD compensated for the HFW’s reduced attenuation during the 2015 freshet and prevented exceedance of PTGs at system effluent (RDEFF). As the study progressed, the ability of the HFW and RD to attenuate Mn improved. After Jan. 6, 2016, the HWTT effluent (RDEFF) averaged 14.3 µg/L for total Mn (calculated using ½ the reporting limit of 10 µg/L for < RL data) and an average removal efficiency of 99.4%. Unlike Zn and Cd concentrations that were dominated by particulate metals in the HFWEFF, Mn was almost completely dissolved, illustrating a different attenuation mechanism (Fig. 13).
Analysis of Precipitates in the RD and HFW
A chemical and mineralogical analysis of the precipitates and black deposits on surfaces of limestone rock media showed that greater attenuation of both Mn and Zn occurred in the upgradient position of the RD (Table 3). Visual observations of staining indicated that most Mn precipitation occurred in the upper portion of rock media (sample from 45 cm depth), with limited staining from 80 cm to the full depth (1 m) of the limestone media (Fig. 14).
Results from sequential extractions based on Tessier et al (1979) indicate that Mn was recovered primarily in the hydroxylamine extraction step, which is indicative of Mn oxides (Table 4). Analysis of RD particles and aggregates by SEM–EDX confirmed that the Mn oxides were primarily biogenic forms that occurred as tubular sheaths that formed as pseudomorphs around microbial colonies (Fig. 15). Microbial formation of Mn oxides have been reported in wetlands and passive treatment systems and natural systems (Hallberg and Johnson 2005; Luan et al. 2012). Particles of ZnS were also noted in the RD, resulting from sedimentation of particles transported from the HFW (Fig. 16).
Precipitates of Mn in the HFW were more difficult to identify. Manganese carbonates were common in the effluent end of the HFW (Fig. 17). The SEM–EDS analysis of the ≈ 2 mm red box in the micrograph had an average composition of Mn3Zn1Fe0.75Ca0.25(CO3)5. It cannot be determined, without additional mineralogic information, if this was a single phase, solid solution of carbonates, or contributions from several small particles excited by the electron beam.
Seasonal Water Temperatures in the HWTT
Cold winter ambient air temperatures and the ability to maintain active SRB populations in the HWTT was a concern prior to the study. Water temperature was continually recorded by thermistors located throughout the HWTT and are expressed as average monthly temperatures in Fig. 18. Influent mine drainage was consistently warm over the course of the study (18.3–19.3 °C) and reflects the hydrothermal source of mine water emerging from underground. Water temperature measurements ranged from 5 to 19 °C across the system (Fig. 18), despite winter ambient air temperatures decreasing below −20 °C. Temperatures in the HWTT effluent (RDEFF) ranged from 5 to 17 °C.
Managing the HWTT with Water Data
The sondes installed within the HWTT enabled continuous monitoring of water quality parameters (temperature, conductivity, pH, ORP, turbidity, and DO) at their locations (Fig. 1). Information from the sensors was remotely accessible and was used to manage the HWTT. For instance, water levels in the HFW were controlled in response to effluent ORP and sulfide concentrations. Water levels were manually raised when ORP was elevated and effluent sulfide concentrations were low, to increase HRT. Alternatively, water levels were lowered when effluent sulfide concentrations were excessive. Most changes were made during system startup and non-freshet sulfide concentrations in the HFW effluent ranged between 1 and 8 mg/L with very few additional adjustments required. Timing of performance monitoring was eventually based on minor changes in influent pH instead of minor changes in water flow.