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water table was dry and fractured from the long period of drought, but after continuous infection is started, a decrease in soil permeability is expected from the swelling of the clay particles in the soil underlying the site, which consist mostly of silt and clay with lenses of more-sandy sedi- ments Fractures will decrease or disappear when the soil becomes saturated In addition, ' hydrocompaction might occur during water infection, which will further decrease the permeabili- ty of the underlying soil. At least 10-15 gpm of groundwater is expected to be extracted at the site, and even more if any additional extraction wells must be installed offsite to capture the ' plume To secure sufficient capacity of the injection wells, more than one was necessary Exist- ing monitoring wells MW4 and MW7 were connected to the injection system (Figure 4) To spread injected water more evenly and provide an efficient barrier separating the site from an ' upgradient plume, one more injection well (IW I) midway between MW4 and MW7 was in- stalled (see Figure 4). To monitor the groundwater level outside of the injection wells, two ' piezometers (P1, P2, Figure 4) were installed Assuming that each well is injecting about 5 gpm of water, the rise in groundwater level upgradi- ent near the middle injection well was estimated using the same equations as for the system of three pumping wells in line (Muskat 1937) Although these equations are used to estimate drawdown in the pumping wells, they are also used to estimate the mound or a rate of injection ' with good approximation (Driscoll 1986) Mound in the middle well (IW1) can be derived from the equation SPH - sd = PQ log(R/[rd2l 033))/1 36k ' for outer wells (MW4, MW7) so(2H - so) = (3Q log(R/12rd2]0 33))/i 36k ' where ' Q = single well pumping rate 26.5 m3/day sc = mound in the central well (m) s = mound in the outer well (m) 0 = hydraulic conductivity, 3 04 m/day ' H = aquifer thickness, 6 1 m d = distance between wells, 12 m R = radius of influence (assumed 48 m as a distance to a downgradient stagnation ' point for pumping wells RW1-RW3) r = well diameter, 0 15 m ' Mound in well IW 1 is estimated to be about 2 43 m (8 feet) and in wells MW4 and MW7, 2 18 m (7.15 feet) This would create a hydraulic barrier sufficient to isolate former Exxon RS 7-3942 and prevent an upgradient plume from moving toward the site The height of this bamer can be monitored using the proposed piezometers (P1 and P2) (Figure 4) ' �9 10