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ad k a µ nr 3 a a"F 4 r v .t;. a 's wggr y 6 v�.,�.. �Sr.�v. �� , nutrients required to sustain microbial growlll. Environmental factors that affect microbial activity sort! population size need to be monitored and maintained within close toleran4el. !'ln•tllCl•Site InVestlgation and feasibility stnt(lies would be required to determine the applicability of this treaunCn[technique. 2.2 GROUNIMNIT'R CONTIZOI.()I2 I""C:OV"Ry'I"" II1�II(�EJ1:5 Below is a list of groundwater control or recovery techniques employed at remediation sites: liliZatir,n/scllidil'ien inn.is a process that isol;ues contamiu:uus from the ground- water by chemically fixing the soil ut prevent further leacllirtg of the contamination. This process does nothing to treat the groundwater, but caul prevent it from getting; worse. Stabilization is aast•d mostly for large amounts of soil that have been impact- ed by metals or nonvolatile organic compounds. I Iydrocarbons such as those found in gasoline are relatively easily removed from soil either above the ground or in rablc to remove tilt: source innpacting groundwater rather than leave 111ace. it is prcle it in place. 2. Recovery wel]_tiY—s-1.�n1s arc one 01 the most conitllon methods of controlling groundwater movement. Wells :are used to manipulate tile subsurface Ilydraulie gradient by either injecting or withdrawilig water. Recovery wells can also be used to depress the groundwater level, as will be required at former Exxon RS 7-3942. The groundwater recovery wells will be pumped at a rale sufficient to denvater tine area beneath and downgr;client of the former tank flet(], allowing a vapor extraction system to effect cleanup below the present water level. 3. Catania, refers to installing;a nearly impermeable barrier on the surface of the site. The cap is designed to prevent rainfall from penetrating the vadose zone and carry- ing the hydrocarbons into the aquifer or groundwater. Again, this process does nothing to trealt tile groundwater, but can prevent it from getting worse. As with stabilization, it is preferable to remove the source of hydrocarbons that lllay be impacting groundwater rather than leave it in place. 4. Infercc for trenches are another way to recover groundwater. A trench is excavated 3 to 5 feet below the water table,and a perforated pipe is installed in the trench with a sand or gravel pack around it to promote water flow into the pipe. Groundwater is recovered from the trench in the Sa1n1C way it is front a well. Installation of the trenches is restricted by the deptll of grourtciwaler. Groundwater-,it former Exxon RS 7-3912 is presently 30 feet bclrny the ground surface, making die cast of con- struction unfavorable compared to the construction of wells. 5. Physical barriers can be placed dowrigradient of the petroleum hydrocarbon plume to temporarily arrest groundwater movement. A recovery well system is also re- quired to reduce the hydraulic gradient near file barrier; otherwise,in the absence of natural barriers the g;rotinilwater will flow around or tinder the barrier, The three types of barriers fllat (mll he eolastructed are slurry walls, grout walls, and sheet piling walls. The 101al extent of !lie petroleum hydrocarbon !Hunte has not been fully determined so the bencfits of a barrier cannot be determined at this time. It is likely though that the Cost of consti'ajuting these wallls far outweighs the benefits that could be gained by them at this sits•. .w'nw:v 5