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I- S1* U 0 esi ns f 'or 4 P I - � A"uifer Cleanu In sitar treatment of aquifer contamination is gaining acceptance as a restoration technology. j Biostimulation in particular appears to hold the most promise. By Mchael R. Piotrowski, PhD ne quently,groundwater flow 0 approach rates tend to be restricted that holds n in fine-grained aquifers. great prom- ff1'd�a � Although slow ground- afflicted for restoring aquifers =��<•...; � �•� .,..r.r water flow rates tend to afflicted with long-term " KM~%~� slaw the spread of the sorbed and residual dissolved organic contami- • organic•phase contami- 7 nant plume, the slow Hants is to situ (in the groundwater flow rates ground) bioremediation. translate into a slow rate In this -approach, micro- of aquifer recovery. Bial agents are used either Fine-grained aquifers to destroy the organic con- also have a high potential taminants or transform for clays to be major geo- them into harmless ran logic components. Certain organic compounds clays (termed 2:1 layer directly within the Treating contaminated aquifers with hydrogen peroxide allows a high degree of silicate clays) allow a affected aquifer. flexibility and quick restoration. Photo courtesy Woodward-Clyde Consultants. degree of penetration of Because the microbial agents can act tion appears to have greater potential for organic contaminants into their lattice on the three primary forms of organic aquifer restoration because the added structures. contamination in groundwater concur- chemicals tend to be much more disper- Once the organics penetrate clay struc- rently, once these forms of contamination sive within the aquifer than augmented tures, microorganisms tend to have diffi- have been destroyed or rendered harm- microorganisms.As such,they will spread culty attacking the compounds. There- less,the aquifer will be restored, greater distances and clog flow paths less. fore, if the fine-grained aquifer contains Currently, two primary schools of Thus, it is possible to influence a larger 2:1 clays (for example, montmorillonite, thought regarding the means to imple- aquifer region using biostimulation. The smectite,illite),.not only will it be difficult ment bioremediation exist.One approach, remainder of this article will focus on this to adjust in situ aquifer conditions rap- bioaugmentation, involves the addition of approach. idly, it also may be difficult to reduce elevated densities of contaminant- Biostimulation is achieved by effec- sorbed organic contaminant..concentra- degrading microorganisms to the affected lively delivering biologically important tions to levels protective of the aquifer. material. The second involves the stimu- chemicals to the contaminated source Despite the potential difficulties lation of the existing microbial commu- areas at appreciable rates. The physical involved with remediatibn of finer- nity within the affected material to aspects of the aquifer will influence grained aquifers, in some cases;-the only degrade the contamination at an elevated strongly the groundwater flow charac- option is in situ bioremediation.For these rate by injecting or infiltrating biologi- teristics within the aquifer and therefore cases, use of infiltration trenches typi- cally stimulating chemicals. This the means used to apply in situ rally can introduce biologicallyiniportant approach has been termed biostimulation. bioremediation. If the aquifer consists of chemicals at greater rates than i0jection Although both approaches have been fine-grained geologic materials,it tends to wells to larger areas of the contaminated reported to be useful for treating organ- exhibit low hydraulic conductivity and aquifer. rally contaminated materials,biostimula- therefore low transmissivity- Conse- continued on page 36 34 ENViRONMENFAL PROTECTION