1 Investigation and Remediation of Groundwater Contamination at a Pesticide Facility • Carter et al., 1995. Water Quality Research Journal of Canada, pp. 469- 491. 2 Site History • Since 1972 Ciba-Geigy Canada has operated an agrichemical formulating and warehousing facility in Cambridge, Ont. • In 1991 soil impacted with Dinoseb was discovered. 3 Preliminary Investigation • Wells installed just below the water table confirmed that dinoseb was present in groundwater at about 400 mg/L (40 times the ODWO). • Other organic compounds were also found. 4 Environmentally Sensitive Site • About 30% of Cambridge drinking water is pumped from 8 wells located between 600 and 2200 m from the site in the underlying bedrock. 5 Location of Site and Municipal Supply Wells Hiway 401 Hiway 24 Franklin Boulevard 6 Further Investigation • Two pesticides (metolachlor and dinoseb) have been measured above ODWO in deep groundwater in the bedrock about 30 m below the site. 7 Pesticide Characteristics • Significant impacts to groundwater are not expected under typical agricultural use. • Recent well surveys indicate that deep migration of pesticides is uncommon. 8 Ciba initiated a four-phase soil and groundwater remediation program. • Phase 1: Background information review. • Phase 2: Remedial investigation. • Phase 3: Remedial alternative analysis. • Phase 4: Remedial action plan implementation. 9 All 4 Phases have been initiated. • Localized excavation of impacted soils and implementation of interim measures to control and treat contaminated groundwater. • Site remediation for 10-20 years. 10 Investigation Methodology 1 Process and Waste Management Audit 2 Private Well Survey 3 Soil Vapour Survey 4 Drilling Program 11 Methodology... 5 Geophysics 6 Groundwater Flow Modeling 7 Regional Well Testing 8 Monitoring Program 12 Methodology... 9 Treatability Studies/Interim Groundwater Control 1 0 Regulatory/Public Liaison 13 Process and Waste Management Audit • Current and historical site operational practices. • Interviews with current and former employees. • Records of all chemical compounds handled at the site. 14 Private Well Survey • Review water well records on file with MOE. • Door-to-door survey within a 1 km. radius. • Only two private wells discovered, neither were registered with MOE. 15 Soil Vapour Survey • Volatile organic solvents were used at the plant. • Shallow soil vapour survey conducted and information used to identify areas for drilling and soil sampling. 16 Drilling Program • 87 monitoring wells installed as of Aug. 1994. • Using mainly 10.8 cm hollow stem augers with continuous coring. • Wells constructed of 5 cm PVC pipe and slotted well screens set in silica sand filter pack. 17 Drilling Program... • Shallow wells near water table are constructed with 3m screens. • Deeper wells have 1.5 m screens sealed with bentonite grout. • Measure K in situ. 18 Groundwater Flow Modeling • Used a 2-D model (FLOWPATH) to estimate groundwater flow velocities, regional groundwater flow conditions in the bedrock, and the potential effectiveness of a groundwater pump and treat system. 19 Regional Well Testing Program • How to use nearby municipal wells? • Shut-down municipal wells for 48 hours and measure water level response in on-site wells. • Measurable drawdown of 0.1 to 1 m at a distance of 1 km. 20 Monitoring Program • Wells sampled up to 4 times per year until 1994. • Use Waterra foot valve samplers. • Sample protocol according to MOE standards. 21 Treatability Studies • Interim groundwater pump and treat facility with two granular activated carbon units in series. • Alternatives are ultraviolet-peroxide destruction and in situ /ex situ bioremediation . 22 Regulatory/Public Liaison • Regular meetings with municipal, regional, and provincial regulators and the general public. • Major technical reports in the local library. 23 Results: Hydrogeologic Setting Unit Description Thickness (m) K (cm/s) 1 Silty sand/sandy silt 10 10-2 - 10-5 2 Silty clay/clayey silt 3 10-5 - 10-7 3 Sand 12 10-3 - 10-4 4 Silt till 4 10-4 5 Sand, gravel 1 not tested 6 Dolomite 100 10-2 - 10-5 24 Summary Surficial Sand Aquifer Clay and Silt Aquitard Lower Sand/Till Aquifer* Bedrock Aquifer * Unit 3 is confined, semi-confined, and unconfined on site. 25 Pesticide Distribution on Site • Metolachlor has the greatest exceedance of ODWO (50 mg/L). • Up to 10,000 mg/L in surficial sand aquifer. 26 Pesticide Distribution... • Maximum concentration in bedrock is 14 mg/L. • Mixing/dispersion is suspected as the principal process of reduction. • Some metabolites are present. • Ciba-derived compounds have not been detected in the municipal wells. 27 On-Site Groundwater Flow Patterns • Shallow groundwater flow from east to west. • Velocities from FLOWPATH 0.4 to 75 m/yr. • Maximum migration distance of 250m. 28 Water Table Configuration 306 305 302 303 304 29 Groundwater Flow Directions... • Lateral flow direction in lower aquifer and bedrock is south and southwesterly. • Dilute concentrations of pesticides in bedrock south and southwest of the site. 30 Equipotential Lines: Bedrock 299 298 297 296 31 Results from Shut-Down Tests • Water levels in bedrock and lower aquifer are influenced by municipal pumping. • Vertical gradients in lower aquifer increased up to a factor of about 2. 32 Municipal Wells that Influence Water Levels in Bedrock Wells on Site Hiway 401 Hiway 24 Franklin Boulevard 33 Interim Groundwater Control/Treatment • Install pumping wells in upper and lower aquifers to control lateral migration off site. • Capture zone analysis with FLOWPATH indicates that 2 wells at 49 and 65 m3/day are enough. 34 On-Site Water Treatment • Two 454 Kg. granular activated carbon units in series. • Reduce pesticide concentrations to below detection limits of 1 mg/L. 35 Conclusions • Unplanned release of pesticides and other organic compounds. • Pesticide levels in shallow aquifers exceed ODWO. 36 Conclusions... • Lateral and vertical transport of pesticides a function of source area, physical/chemical properties, and groundwater flow conditions. 37 Conclusions • Six pesticides have migrated 30 m downward through five layers and laterally approximately 200 m. • Concentrations of five pesticides in bedrock aquifer are below ODWO. 38 Conclusions... • Downward movement of pesticides largely influenced by nearby municipal wells. • Variations in vertical gradients caused by irregularities in hydraulic connection between overburden and bedrock fracture system. 39 Conclusions... • Groundwater pump and treat system to control further migration of pesticides. • Activated carbon units efficiently remove contamination. • System operates at 114 m3/day.
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