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<br />that may breach the long string casing could potentially
<br />move uphole and corrode through other parts of the cas-
<br />ing where salinization of the Navajo aquifer might take
<br />place.
<br />Annuli between the long string casing and the
<br />borehole walls (outside the casing) of active producing
<br />and injection wells, and plugged and abandoned wells,
<br />W are an additional avenue for potential movement of
<br />~. saline water between the upper Paleozoic aquifer and
<br />CO the Navajo aquifer in some areas, partIcularly In older
<br />oil wells and dry holes that were not plugged and aban-
<br />doned according to current standards and procedures.
<br />Investigation by the Navajo Nation Environmental Pro-
<br />tection Agency during this study indicates that almost
<br />200 active producing oil wells and injection wells were
<br />completed in a manner that did not completely isolate
<br />the upper Paleozoic aquifer from the Navajo aquifer
<br />and therefore, could allow crosstlow between these
<br />units (Melvin Capitan and James Walker, Navajo
<br />Nation Environmental Protection Agency, written com-
<br />mun., 1994). In these wells, the depth of surface casing
<br />is above the base of the Navajo aquifer, which averages
<br />about 1,500 ft, and the top of the cemenred interval is
<br />typically at a depth of 3,500 ft or greater. Because the
<br />depth of the top of the Cutler Formation generally is
<br />above 3,500 ft, an avenue or conduit could exist for
<br />water to move from the Cutler Formation to the Navajo
<br />aquifer where relative head differences are conducive
<br />to upward tlow. Upward movement of water between
<br />these units, however, could be inhibited by collapse of
<br />the borehole walls in less-competent intervals in the
<br />confining units (Gilbert Hunt, Utah Division of Oil,
<br />Gas, and Mining, oral commun., 1995) and by the
<br />weighted mud that would be present in the uncemented
<br />interval. Nonetheless, water that discharges from the
<br />ground around plugged and abandoned wells N32,
<br />N41, and N46 indicates that upward movement of
<br />water outside of well casings or from boreholes that
<br />have been plugged is taking place.
<br />
<br />SAMPLING, ANALYTICAL METHODS, AND
<br />QUALITY ASSURANCE
<br />
<br />Water samples collected during this study were
<br />analyzed for major and minor cations and anions,
<br />selected trace elemenrs, total organic carbon, dis-
<br />solved-solids concentration, density, and the isotopic
<br />ratios of oxygen-18/oxygen- I 6, hydrogen-2 (deute-
<br />rium)/hydrogen-I, sulfur-34/sulfur-32, and strontium-
<br />87/stronrium-86. All water samples except those col-
<br />lected for isotopic analysis were analyzed at the U.S.
<br />
<br />30
<br />
<br />Geological Survey National Water Quality Laboratory
<br />in Arvada, Colorado. Analytical methodology used at
<br />the National Water Quality Laboratory is described in
<br />Techniques of Water-Resources Investigations of the
<br />U.S. Geological Survey (Fishman and Friedman,
<br />1989). The stable isotope values for oxygen, hydrogen,
<br />and sulfur (sulfate) in water samples were determined
<br />at the U.S. Geological Survey Stable Isotope Labora-
<br />tory in Reston, Virginia. Isotope values for strontium in
<br />water samples were determined at the U.S. Geological
<br />Survey Srrontium Isotope Laboratory in Lakewood,
<br />Colorado.
<br />Isotopic values for oxygen, hydrogen, sulfur, and
<br />strontium are expressed in the del (I)) notation as permil
<br />differences between the sample and a standard. With
<br />oxygen, for example, ot80 is defined by
<br />
<br />~180 =
<br />
<br />
<br />where:
<br />
<br />(180;160 )sample is the isotope ratio of the sample,
<br />and
<br />{180;160)srandard is the isotope ratio of seawater.
<br />The comparative standard for oxygen, hydrogen,
<br />and strontium i50topes is standard mean ocean water
<br />(SMOW). The comparative standard for sulfur is an
<br />iron sulfide in the Canyon Diablo meteorite and is
<br />referred to as the Canyon Diablo troilite (Drever, 1988,
<br />p.368). Del notation in permil for the other isotopes is
<br />00 (hydrofen-21 hydrogen-I), 034S (sulfur-34/sulfur-
<br />32), and 0 7Sr (strontium-87/strontium-86). Specific
<br />analytical methods used in the analysis of each chemi-
<br />cal constituent, reporting units, and minimum reporting
<br />levels are summarized in table 2.
<br />Water samples collected for analysis of major
<br />and minor ions, trace elements, dissolved-solids con-
<br />centration, and 034S were filtered on site through a
<br />OA5-micron filter using a peristaltic pump and col-
<br />lected in field-rinsed polyethylene bottles. Water sam-
<br />ples collected for 1)180,00, 1)87Sr, and total organic
<br />carbon analysis were unfiltered and collected directly
<br />from the wellhead. Water sampled from tlowing wells
<br />was collected and processed immediately. Water sam-
<br />pled from wells powered by windmills was collected
<br />from the discharge line after the windmills had been
<br />operating for at least I day. Water from wells that had
<br />to be pumped and those that had valves to control arte-
<br />sian flow were sampled after 2 to 3 hours or until about
<br />three casing volumes were removed to obtain samples
<br />that were represenrative of the aquifer. Temperature,
<br />
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