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Oil, grease, and other foreign materials on drilling and associated equipment can be in- troduced to water-bearing units during drilling, well completion, and well development if not removed from equipment prior to its use. This potential for contamination needs to be consid- ered when gathering and interpreting information about candidate wells. Selection of low-capacity rather than high-capacity wells is recommended for Study-Unit Surveys, and generally is required for wells selected for Land-Use and Flowpath Studies. Evaluating if a well has a low or high capacity involves consideration of the effect of the pump- ing rate on the aquifer in addition to consideration of the pumping rate. For example, pumping a few tens of gallons per minute from a well screened in a poorly transmissive aquifer might induce significant leakage from or through confining beds, whereas pumping a few thousands of gallons per minute from a well screened in a highly transmissive aquifer might not. Pumping rates of domestic wells generally are low, whereas pumping rates of municipal, commercial/ industrial, and irrigation wells generally are high. Each has several advantages and disadvan- tages (table 4) to consider when selecting wells for study components. Well Inventory The inventory process is used to collect and document all relevant information needed to select wells for use in data collection and begin creating site files for the wells selected. This information is gathered from records and site visits. The basic information compiled for a well inventory is that which is needed to create a Ground-Water Site Inventory file (see "Documen- tation") and that which is needed to complete a Well Inventory Form (fig. 1). The inventory also includes gathering any information identified as necessary to evaluate the well with re- spect to well-selection criteria (table 3). Written permission must be obtained to gain site access and to collect and publish data from currently used or abandoned (fig. 2) wells. Information from records is used to start the well inventory. In addition to the GWSI and well inventory forms, any available records of well installation and development, well mainte- nance, geophysical logs and surveys, aquifer tests, geological and geochemical data, and land use are compiled, reviewed, and incorporated into the well site file. Site visits to candidate wells are useful to obtain permission to measure or sample the candidate well, and field verify information about the well. On-site evaluation will help to en- sure that data from that well will meet study-component objectives. Well-inventory visits prior to sampling might seem impractical, particularly on a large scale for the Study-Unit Surveys, but field verification prior to sampling can save time, money, and effort in the long run. During the inventory visit, well identification and location are verified, along with access to the site, to the well, and to the apertures needed for making measurements and collecting samples. A site sketch indicating well location, surface-water bodies, and major landmarks is drawn. Well depth and depth to water are measured. Potential point and nonpoint sources of ground-water contamination are identified. During the inventory visit(s), it is recommended that Study Units schedule collection of preliminary data to help plan for sample collection, such as purge vol- ume and purge time, routine field measurements (for example, pH, conductivity, dissolved oxygen), and any appropriate analyte screening (for example, for VOCs). 13