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2.3   Interpreting Well Construction Reports
Well construction reports are submitted by well drillers after a well is completed.
Most drillers lack formal geologic training, some logs are written up after well completion
rather than onsite, and often subtle differences in sediment are not reflected in cuttings.
Therefore, the quality of these data varies considerably. For example, terms such as
"hardpan" usually refer to glacial till, but so can stoney clay or clayey gravel, among other
designations. Considerable effort was made to be consistent and as accurate as possible in
transforming the driller's descriptions into geologic categories.
2.3.1 Three verses Four Units
Initially, the driller's descriptions were separated into three hydrofacies: 1) fine
grained till and lacustrine deposits such as clay and silt, 2) mainly silty and sandy till and
deposits of intermediate composition, and 3) sand and gravel deposits. However, because of
the wide range of hydraulic conductivity of sand and gravel deposits (10-1 to 10-5 cm/s
according to Stephenson et al., 1988), this hydrofacies was subdivided into a sand unit and a
gravel unit. These hydrofacies and the driller's descriptions included in each are shown in
Table 2.2. The few areas that had peat in the driller's logs were added to hydrofacies two,
since the average hydraulic conductivity for peat (Boelter, 1965; Holden and Burt, 2003) is
about the same as for glacial till.
2.3.2 Hydraulic Conductivity Determination
Literature values of hydraulic conductivity were assigned to each of the four
hydrofacies. Measurements of glacial deposits from North America (Stephenson et at.,
1988) were compared with local studies in southern Wisconsin (Simpkins et at., 1990;
Anderson et at., 1999) to determine the most likely values for each type of deposit. Field