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atural attenuation proce se uch a eli p rsion, advection, and biodegradation serv to decrease concentrations of eli so lved contam inants a they are transported in all ground-water system . However, the efficiency of these natural attenuation processe. and the degree to which they help attain remediation goal , varies consid rabl y from site to ite. Thi . repo1t pro id a methodology for quantifying various natural attenuation mechanism . Thi . methodology incorporates information on ( I ) concentrations of contaminant in pace and/or time ; (2) ambient reduction/oxidation (redox) conditi n. ; (3) rates and direction of ground-water fl ow; (4) rate of contaminant biodegradation; and (5) demographic con ideration , such a the presence of nearby receptor expo ·ure points or property boundaries. Thi document outline the hydrologic, geochemical , and biologic data needed to asse s the efficien y of natural attenuation, prov ides a . creening tool for making preliminary assessments, and provid s examples of how to determine when natural att nuation can b a u eful component of . ite rem diation at leaking underground storage tank sites. At a site in the Piedmont Phys iographic Prov ince (Laurens, South Carolina), hyd rologic and water-chemi try data indicate that the natural attenuati on capac ity for benzene is approx imately 5 percent per foot of flowpath. As are. ult, b nzen concentration wou ld decrea e from about 28,000 microgram per lit r in ground war r at th ource area tole s than 5 microgram. per lit r 200 f t downgradient and prior to eli charg ing to a tream. Becau e of thi rapid attenuation, ontaminants do not pre ently impa t the tream downgradient or the it . In contra t, at a sit in the oasta l Plain Phys iographic Province (Charle ton , South Carolina) hydrologic and wat r-chemistry data indicate that, even though the ite ha a ubstantial natural attenuation capacity, it may not be ufficient to fully protect a nearby point of ground-water el i charg . The e two ite illustrate how the efficiency of natural attenuation proce . e acting on petroleum hydrocarbons can be y tematically evaluat d u ing hydrologic. geochemical. and microbiologic method . These methods, in turn, can be u d to a . es the role that the natural attenuation of petroleum hydrocarbon. can play in a hiev ing overa ll ite remediati on.

Methodology for applying monitored natural attenuation to petroleum hydrocarbon-contaminated ground-water systems with examples from South Carolina