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2.4 Regional Ground Water Quality <br />Shell has been measuring ground water quality in Piceance Basin wells for about 10 years. <br />Several wells were drilled on a single well pad into any or all of the water bearing intervals of the <br />Uinta, L7, L6, L5, L4, and L3 zones. Ground water wells were sampled for a broad range of <br />analytes, including at least one sample each of the full suite of regulated organics, numerous <br />analyses of all regulated inorganics and major elements, and a single set of isotopic <br />measurements for both stable and radiogenic isotopes including C, O, H, S, B, Sr, and radiogenic <br />Cl. As a result of these studies, Shell has an excellent foundation to interpret the ground water <br />chemistry. <br />Mineral dissolution has released constituents to ground water in quantities that in some cases <br />cause ambient ground water to exceed standards. Nahcolite apparently contains trace quantities <br />of arsenic (As), barium (Ba), boron (B), and fluoride (F); halite releases chloride (CI); and the <br />oxidation of iron sulfides increases sulfate concentrations. The dissolution of nahcolite and halite <br />also has led to TDS concentrations that commonly exceed 10,000 mg/L near the Dissolution <br />Surface. <br />Ground waters in the oil shale sequence contain dissolved gases carbon dioxide and biogenic <br />methane, derived perhaps from the dissolution of nahcolite (sodium bicarbonate) and from the <br />aging of kerogen under very shallow burial conditions. In all waters from the Parachute Creek oil <br />shale sequence (1_3 through 1-7), carbon dioxide is likely to exsolve from water as it is lifted to <br />surface, causing samples to effervesce like a soda bottle when the cap is removed and pressure <br />released. This causes sampling for obtaining gas-free volatile samples for volatile organic <br />compound measurements, a subject addressed in the Standard Operating Procedures. <br />In deeper intervals (particularly L3 and 1-4), the abundance of solid nahcolite can produce water <br />fully saturated with carbon dioxide, which releases so much gas during pumping that discharge <br />becomes a turbulent mix of C02 and water, and gas separation in the pump can cause the pump <br />to gas-lock (pumps generate gas faster than they can expel it). Pumps with higher horsepower <br />rating are set deeper in L4 wells to counteract this tendency to gas-lock; still, it may be <br />necessary, as detailed in sampling SOPS (Appendices B and C), to shut a pump down to allow <br />gas to escape and restart it several times to purge a well. <br />Gases in L3 and L4 intervals comprise mostly carbon dioxide (-85%) and methane (-15%) with <br />approximately 1 % each of H2 and N2. The total gas concentration in the formation is not known. <br />The headspace of a tank into which these gassy waters discharge is likely to be devoid of oxygen <br />and exhibit flammable gas content up to or above the explosive limit. As with the hydrogen <br />sulfide, these gases pose a potential hazard about which samplers must be fully aware. <br />Sampling persistently is preceded by metering for the lower explosive limits of CH4 and H2S, the <br />exceedance of which is found to be rare. The monitoring and control measures for these gases <br />in the sampling program are discussed in sampling SOPS (Appendices B and C). <br />EAST RDD ENVIRONMENTAL MONITORING, SAMPLING AND ANALYSIS PLAN <br />5 <br />