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For installation of wells for sampling ground-water quality, preferred drilling methods are <br />those that minimize (1) the possibility of contamination of aquifers and aquifer pore water by <br />foreign drilling fluids, and (2) cross-contamination between aquifers by drilling fluid, pore <br />water, and drill cuttings. Given that the primary objective of installing wells for NAWQA stud- <br />ies is for water-quality sampling, the selection of a drilling method that minimizes the potential <br />for subsurface contamination by the drilling process should be of primary concern, and well- <br />installation needs should be budgeted accordingly. Use of other drilling methods must be done <br />with the knowledge that increased time and cost might have to be committed to adequately <br />develop and purge the well prior to sampling. <br />In some cases, a method of drilling that minimizes the potential for subsurface contamina- <br />tion by the drilling process might severely limit collection of other data at the well that are also <br />important to meeting the study-component objectives. For example, many of the most useful <br />borehole-geophysical logs must be run in uncased, fluid-filled boreholes. This requirement is at <br />odds with the most preferred methods of drilling for installing wells for water-quality sampling. <br />Study Units must weigh the cost benefit of data desired against the practical constraints of the <br />drilling methods being considered and the primary objective of collecting ground-water-quality <br />samples that accurately represent ground-water chemistry. <br />Well-construction information must be documented at the time of well installation, as <br />discussed in the section "Documentation." <br />Table 8. Factors to consider when selecting a drilling method (modified from W. E. Teasdale, <br />U.S. Geological Survey, written commun., 1992) <br />Logistical considerations <br />Accessibility of the drilling site <br />Ability to obtain permits and approval to drill at the site <br />Availability of necessary equipment <br />Time available to complete drilling program <br />Ease of equipment decontamination at and between sites <br />Experience of the driller <br />Drilling considerations <br />Types and competency of water-bearing units to be drilled and sampled <br />Types and quality of lithologic and other borehole logs required <br />Types and quality of aquifer samples required <br />The importance of minimizing contamination of aquifers by a drilling fluid <br />The importance of minimizing cross contamination between aquifers <br />The importance of minimizing disturbance of aquifers during drilling <br />Total depth of drilling anticipated <br />Casing diameter and casing material selected for the monitoring well <br />Ease of completing the monitoring well as designed, for example ease of <br />installation of filter pack, grouting, and instrumentation <br />Economic considerations <br />Cost of drilling and sampling to meet data needs. <br />34