ESM 2231ESM 223Response Actions:ContainmentESM 2232Overview of AlternativesContainmentIn-situ processesEx-situ processesExcavation and off-site treatmentESM 2233ContainmentHydraulic containmentPump groundwater at high rates to pull all of the dissolved contaminant plume into the extraction well(s)Slurry and Steel WallsPhysical barriers to slow down or stop groundwater flowMonitoringObservation wells for sample collection inside or outside the plume boundaryESM 2234Hydraulic containment Used to isolate contaminant plume from normal groundwater flow Prevents plume from reaching another aquifer, a water supply well or a surface water body (stream, river, lake) Requires above-ground treatment of extracted water (i.e. commonly known as pump-and-treat)ESM 2235Hydraulic containmentESM 2236Hydraulic containmentGroundwater normally flows along certain “streamlines’, driven by the slope of the water tableFunction of topographyrecharge ratesoil permeabilitiesdistance to the surface water discharge pointBy adding a pumping well, the streamlines around the well are diverted from the normal flow direction towards the wellESM 2237Hydraulic containmentESM 2238Hydraulic containmentESM 2239Hydraulic containment A well has a measurable “radius of influence”: Radius of influence indicates maximum distance at which a streamline can be captured by pumping at well)/2exp(QHrRwi∆=πESM 22310Hydraulic containment Radius of influence can be affected by other pumping wells nearby, or by a barrier (e.g. a building, bedrock, etc.) Proper design of the radius of influence (i.e. the “capture zone”) requires numerical modeling (e.g. using MODFLOW from USGS, a computer model for groundwater movement) Rules of thumb are commonly used by practitionersESM 22311Hydraulic containmentQ= flow (m3/s), B = aquifer thickness (m), U = GW velocity (m/s)ESM 22312Hydraulic containmentESM 22313Hydraulic containmentESM 22314Hydraulic containment Well-point system Several closely spaced wells connected to a main pipe Used in shallow aquifers where suction needed to pump water out of ground is relatively small Water table may be drawn down so far that shallow aquifer is completely dried around well: useful to extract any residual NAPLs using vapor extractionESM 22315Hydraulic containmentESM 22316Hydraulic containmentESM 22317Hydraulic containmentESM 22318Hydraulic containment Pressure Ridge System Injecting clean or treated water into subsurface along a line up- or down-gradient from plume Upgradient injection is used to force the contaminants into capture well(s), increasing flow into recovery wells, washing aquifer Downgradient injection is used to alter streamlinesProduces mound of water that acts as a barrierContains plumeKeeps clean water awayESM 22319Hydraulic containment Pumping or injection rates have to be balancedAchieve capture of plumeAvoid undesirable effects of excess drawdown of water table (e.g. settling, sink holes) Very high pumping rates are typically not effective, since do not allow for contaminants to desorb from soilPulsed pumping has been shown to be equivalent to slower pumpingESM 22320Hydraulic containmentObjective is to allow time for sorbed pollutant, or residual NAPL, to come to equilibrium with surrounding water between pumping intervalsESM 22321Hydraulic containmentPump-and-treat is generally ineffective in mobilizing residual NAPLPumping rates needed to mobilize residual NAPL are orders of magnitude greater than can be physically achieved in fieldESM 22322Hydraulic containment Volume of water in the polluted aquifer region to be treated is called a “pore volume” (PV) PV = L x H x W x θ Example: 100m x 50 m x 10m x 0.35 = 17,500 m3of PV Experimental work in bench scale studies indicates that at least 2 and sometimes more than 10 PV have to be pumped past the contaminated regionESM 22323Hydraulic containmentFor a quick, order of magnitude estimate on the time needed to dissolve NAPL from the source zone while pumping water through the zone:QCSPVQCSALtsatwnnsatwnndρρθ==ρn= density of NAPL = [kg/m3]Sn= saturation of NAPL = [-]Cwsat= water solubility (sat) = [kg/m3]Q = pumping rate = [m3/s]td= time for dissolution= [s]θ = porosity = [-]A = cross-section of polluted zone = [m2]L = length of polluted zone = [m]ESM 22324Hydraulic containment Can rarely achieve dissolution up to the water solubility; typically down to 1 - 10% Pollutant (NAPL) saturation in the soil is from a few samples (if lucky), and is probably not constant A is hard to define precisely=> estimating time to clean-up is an artESM 22325Hydraulic containment Cost Factors Design of pumping system (number and location of wells, pumping rates, estimate of pumping time, above-ground treatment) Permitting Well installation Above ground equipment installation Operation of wells and above-ground treatment equipment Monitoring (equipment, samples, analysis, reporting)ESM 22326Hydraulic containment Applicability of pump-and-treat: Very soluble organic contaminants (e.g. MTBE, Benzene) with low retardation factors (R < 3) Soluble inorganic pollutants with low background concentrations Sites where the source has already dissolved to a large extent Pumping Rates and removal typically 1 - 10 gpm per well quick calculation of pollutant removal should consider avg. conc.ESM 22327Physical barriers Slurry walls Place a low-permeability, subsurface cut-off wall that either directs flow of the contaminated plume through a small “gate”, orhydraulically isolates the contaminated zone Due to continuous recharge, even an isolated zone needs to be pumped to maintain a constant water tableESM 22328Physical barriersESM 22329Physical barriersESM 22330Physical barriersSlurry Walls“Funneling” the flow through a gate implies that pumping at the gate must be very high to capture all the plume“Walls” consist of vertical trenches, which can be excavated down to ~200ft (60m)Trench is filled with a very low permeability material (e.g. bentonite mixed with soil or cement)Physical barriersESM 22332Physical barriersESM 22333Physical barriersESM 22334Physical barriersESM 22335Physical barriersESM 22336Physical barriersESM 22337Physical barriersBentonite-soil walls are cheaper but last less timeCement-soil walls