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• Dissolved iron(EPA method 200 7) . Total iron(EPA Method 236 1 or 60 10) • Methane(field measurement) • Alkalinity(EPA method 310 1) • Dissolved carbon dioxide(with alkalinity or method SM406C) Certain parameters, notably DO and Eh, may be measured in the field using downhole meters Most of the other parameters require laboratory analysis of a groundwater sample for accurate quantification Trends in methane concentrations may be identified using an organic vapor meter fitted with an appropriate filter at the wellhead The combination of parameters that Clearwater will monitor at a particular site will depend on site-specific conditions and previous site investigation The minimum set of parameters as defined by Rifai will always be included for at least one cleanack and well and at least one well rc resenlative of turd- lame conditions The following sections provide a detailed description of monitoring methods and anticipated results for Indicator parameters outlined above Microbial Populations Hydrocarbon degrading bacteria are generally ubiquitous, however, the total population of microbes(measured in counts per liter) is dependent on the available energy source (ie, hydrocarbons) To evaluate natural attenuation, microbial counts should Include separate enumerations for hydrocarbon degrading bacteria and for total heterotrophic bacteria, both normally obtained from cultured plate counts The ratio of hydrocarbon degraders to total heterotrophs is the most useful in assessing natural attenuation This ratio should be relatively large in samples from contaminated wells, compared to the ratio in samples from clean wells, indicating a proliferation of the indicator species in contaminated areas, independent of overall microbial population variations Such a distribution of bacteria may require 1 to 2 years to become established once hydrocarbon contanunation is present As further confirmation, it may be useful to establish that sufficient concentrations of microbial nutrients such as • nitrogen and phosphorous are present in the subsurface Groundwater typically contains total microbial counts of 103 to 108 counts per liter Lower counts in contaminated areas may indicate toxic conditions In sites with organic rich soils, microbial populations may be high but hydrocarbon degradation may be inhibited because the microbes preferentially degrade the naturally occurring carbon compounds found in the soils (Cookson, 1995) pH pH is best measured with a meter or by collecting a sample for laboratory analysis The probe portion of pH meters must be regularly cleaned and periodically soaked in solutions designed to remove oil and protein build up Lowered pH corresponding spatially to the plume may be indicative of the production of organic acid metabolic end-products of aerobic hydrocarbon degradation Uncontaminated groundwater is commonly slightly alkaline,but pH vanes widely depending on many natural and human influenced factors pH between 6 and 8 is optimal for BTEX degradation Redox Potential(Eh) Eh is a measure of electron activity within a solution Each pathway of degradation is generally restricted to a prescribed range of Eh values Hydrocarbon degradation reduces the Eh of the system in which it occurs, unless the groundwater recharge rate exceeds the utilization rate of the electron acceptor (this is normally not the case since mixing is limited) Once an electron acceptor has been utilized and thereby depleted in the system, Eh conditions determine which next electron acceptor in the sequence will become predominant The utility of Eh measurement is as an adjunct to electron acceptor concentration measurements(discussed below) Eh must be measured in situ to avoid atmospheric influence as described in the section on DO sampling Eh units are millivolts (mV) Decreased Eh should coincide with elevated contaminant concentrations, and depleted DO Very low Eh (reducing conditions <0 mV) should coincide with depleted anaerobic electron acceptors Table 2 • presents Eh values typical of each biodegradation pathway CLEARWATER GROUP(NATURAL ATTENUATION) 3 revised October 3,2002