8224 - Laserfiche WebLink

Home Browse Search

In some cases, it may not be possible to locate a well in the area of interest that meets all of the well-selection criteria for a study component and installation of a new well is not an option. In these cases, one option is to use an existing well that does not meet all the selection criteria, with the knowledge that the results might be biased in some way. Another option is to reduce the scope of the Study-Unit Investigation. In this case, discussion with appropriate NAWQA Regional, Headquarters, and National Synthesis staff is required. The field verification includes checking the construction integrity of the wells being con- sidered for selection. Checking construction integrity helps evaluate if there is a good hydrau- lic connection between the well screen and the aquifer and whether or not the well screen and casing are damaged. At a minimum, this includes a depth-to-bottom measurement in the well, if possible. USGS standard procedure is to check the depth to well bottom annually or the next time the well is visited for data collection if that occurs less than annually (USGS,1980). Other means of testing well integrity include the use of borehole-geophysical logs, comparison of water-level fluctuations over time, and periodic determination of barometric efficiency, and short-term slug, injection, pressure or partial vacuum tests (W. Lapham, U.S. Geological Survey, written commun., 1995--see footnote 1; Bedinger and Reed, 1988; Driscoll, 1986; U.S. Geological Survey, 1980; Lohman, 1972; and Stallman, 1971). Well-integrity tests should not be limited to a well-inventory site visit. If run periodically, well-integrity tests can indicate changes in well response that might be attributable to changes in well-construction integrity. Monitoring wells that are selected by the NAWQA Study Unit must be constructed to meet the specific objectives (table 1), water-quality-data requirements (table 2), and well- design criteria (table 3) of that study component. Well Installation Requirements for selection of wells for NAWQA Land-Use Studies and Flowpath Stud- ies are sufficiently restrictive that wells meeting those requirements generally will not be avail- able. Consequently, some wells will need to be installed. Before wells are installed it is important to take the following steps: (1) make site visits to assess conditions; (2) acquire the necessary well-drilling permits and approvals from site owners and Federal, State, and local regulatory authorities (figs. 2 and 3); and (3) obtain utility right-of-ways. Well installation requires: (1) preparation, (2) well drilling, (3) well completion, and (4) well development. Overall, it is important to recognize that the intended use of the well largely dictates the possible choices of methods and materials used during each of these steps. Preparation Factors considered for well installation include the nature of materials that make up and overlie the aquifer (for example, unconsolidated or consolidated materials; if consolidated ma- terials are fractured or have openings caused by dissolution); the depth to water, to the top of the aquifer of interest, and to the zone in the aquifer to be monitored; the type of drilling equip- ment available; access to the site; well casing and screen materials, length, and diameter, and 21