Presentation on theme: "Upper Fall Brook Watershed Acid Mine Drainage and Acid Rain Impacts Conceptual Treatment Approaches Terry A. Rightnour Water’s Edge Hydrology, Inc. January."— Presentation transcript:
Upper Fall Brook Watershed Acid Mine Drainage and Acid Rain Impacts Conceptual Treatment Approaches Terry A. Rightnour Water’s Edge Hydrology, Inc. January 15, 2007 firstname.lastname@example.org, http://wehydro.com
Upper Fall Brook Watershed Problems: –The upper Fall Brook watershed is impacted by acidification from natural and man-made sources. –The majority of acidification and metals contamination comes from acid mine drainage (AMD) sources DFB001, DFB002, & DFB003. –Additional acidification comes from acid rain and natural wetland (bog) acidity in the headwaters. Goals: –Treat the three AMD sources above Fall Brook Road. –Reduce non-AMD impacts in the Fall Brook headwaters. –Establish good water quality in upper Fall Brook as justification for treating other downstream AMD sources in lower Fall Brook.
AMD Source Analyses Sampled by TCCCC from June to November 2006 – 8 rounds to date on DFB001, DFB002, and DFB003. Analyzed for average conditions and 95% confidence interval (CI) flows. Plotted concentrations versus flow to predict conditions at 95% CI flows. 95% CI values are usually design maximums for stream restoration systems. Conditions were predicted for combined flows to represent the system influent. Previous SRBC sampling was not included due to possible climatic and analysis method differences.
Basic Treatment Alternatives Passive Treatment –Uses natural contaminant removal processes with non- continuous addition of power or reagents. –Most applicable to low to moderate contaminant loadings. –Suited to sites with a low availability of access or O&M labor. –Best technologies for this site are oxidation/precipitation basins (OPBs), vertical flow wetlands (VFWs), and surface flow wetlands (SFWs). Chemical Treatment –Involves continuous controlled application of a neutralizing reagent, with or without need for power. –Most applicable to moderate to high contaminant loadings. –Suited to accessible sites with readily available O&M labor. –Best technology for this site is pebble quicklime addition.
Oxidation/Precipitation Basins Remove iron and aluminum by forming precipitate sludge through oxidation. Usually have little effect on acidity or manganese. Work best with 24 hours or more detention time. Additional volume needed to store accumulating sludge – typically 40% of total volume. 24 hour removal rates in alkaline water are approx. 35% for aluminum and 65% for iron; rates decline in acidic waters. Sludge generated at approx. 1 liter per 5 grams of aluminum or 10 grams of iron removed. Require periodic cleaning to remove sludge as detention capacity is approached.
Vertical Flow Wetlands Deep basins filled with a bottom layer of limestone aggregate and an upper layer of spent mushroom compost. Influent migrates down through both layers, neutralizing acidity and generating alkalinity. Acidity capacity based on loading over surface area of compost (25 g/day-m 2 average, 50 g/day-m 2 maximum) Alkalinity generation based on detention time in limestone (18 hours average, 12 hours minimum). Remove metals based on percentage of influent concentration (90% of aluminum, 80% of iron, 10% of manganese). Metals precipitates eventually clog substrates, requiring flushing or substrate replacement. Usually applied in pairs, with one cell able to maintain treatment while the other is off-line for maintenance.
Surface Flow Wetlands Vegetated basins with shallow surface flow (1 – 6 inches). Work best for polishing metals from alkaline waters at the discharge end of treatment systems. Metals removal rates are directly related to influent concentration: –Aluminum rate approx. 0.21 x (Inf. Conc.) g/day-m 2 –Iron rate approx. 0.17 x (Inf. Conc.) g/day-m 2 Cells may require periodic water level adjustment and vegetative management, but seldom major substrate maintenance if influent metals are low.
Conceptual Passive Treatment Plan Collect and combine AMD sources in an OPB for initial aluminum and iron removal. Split flow to two parallel VFWs for acidity removal and alkalinity generation. Pass VFW discharges through an SFW for final polishing, with target aluminum < 0.1 mg/L on average. A site-specific model was run for basic sizing considerations.
Passive System Model Initial OPB – 40,000 CF Paired VFWs Each at – 62,000 SF Surface Area 43,500 CF Detention Polishing SFW – 27,000 SF
Passive System Model Results Overall system size estimated at ≈ 6 acres with earthwork OPB cleaning cycle predicted at ≈ 10 years VFW cleaning cycle predicted at ≈ 4 years Construction cost estimated at ≈ $810,000 Annualized O&M estimated at ≈ $64,000 15-Year total cost estimated at ≈ $1.8 million Depending on effluent metals goals, the SFW size could be reduced.
Pebble Quicklime Treatment Systems Based on a waterwheel-driven applicator. Pebble quicklime has about twice the neutralization capacity and reactivity of limestone. Easily scaleable to flow increases. Provide a consistent neutralization delivery rate. Aquafix systems available in scalable sizes between small hoppers (1/2 – 1 ton) to silos (up to 100 ton). Bulk delivery approx. $120/ton for pebble quicklime. Metals are removed as sludge in an OPB similar to passive systems. Sludge generation tends to be greater than passive sludge, estimated at 1 liter for 2.5 grams of aluminum or 5 grams of iron removed.
Aquafix Pebble Quicklime Systems 1 Ton Hopper System35 Ton Silo System
Conceptual Chemical Treatment Plan Collect and combine AMD sources in a single channel. Apply pebble quicklime using a flow split to drive an Aquafix system. Precipitate sludge in paired downstream OPBs, with one OPB capable of maintaining 24 hour detention while the other is offline for cleaning. Site-specific model was run for basic sizing requirements using specifications of pebble quicklime and Aquafix systems.
Chemical System Model Aquafix 35 ton silo system for 1 year storage capacity. Approx. 175 lbs/day addition yields discharge alkalinity of 45 mg/L on average and 15 mg/L at 95% CI flow. Paired OPBs each at 70,000 CF total volume.
Chemical System Model Results Overall system size estimated at ≈ 1.5 acre Silo refilling cycle estimated at ≈ 1 year OPB cleaning cycle predicted at ≈ 2.5 years Construction cost estimated at ≈ $300,000 Annualized O&M estimated at ≈ $35,000 15-Year total cost estimated at ≈ $820,000
Conclusions Chemical treatment with pebble quicklime appears to be the more cost-effective alternative. Both alternatives will require establishment of long-term O&M funds. Chemical treatment will require more frequent supervision by skilled mechanical labor, potentially adding to long-term costs. There may not be sufficient area to construct an adequately sized passive treatment system capable of receiving gravity flow from all AMD sources. Reducing the size of the passive system would result in shorter longevity, a lesser degree of treatment, or both. Chemical treatment is recommended as the most viable option for this site.
Fall Brook Headwaters Impacted by non-AMD acidity from acid rain and bog tannin. TCCCC has sampled five tributaries between June and October, 2006, with 5 rounds to date. Acid Neutralization Capacity (ANC) was used instead of acidity and alkalinity as a better measure of non-AMD acidification. Results were analyzed for average and 95% CI conditions, and alkaline deficiency. Objective is to locate and design an alkalinity-generating VFW to improve headwaters conditions above AMD impacts.
Preliminary Conclusions A standard VFW for non-AMD application (approx. 1 acre size) will produce about 50 lbs/day alkalinity. The combined alkaline deficiency in the five sample points is 32 lbs/day on average and 145 lbs/day under 95% CI conditions. One VFW would be adequate to correct average deficiencies, but up to three VFWs may be needed to correct high flow conditions. FBH1, FBH3, and FBH4 appear to be the best candidates for VFWs based on deficiencies and flow volumes. FBH5 may also be considered due to its location near the top of the watershed to maximize main stem restoration length.