METHODS FOR PROCESSING OF WATER-WELL RECORDS TO ESTIMATE HYDRAULIC CONDUCTIVITY AND WATER LEVELS FOR A REGIONAL GROUND-WATER MODEL

The U.S. Geological Survey is analyzing the availability of water in the Great Lakes Basin, which includes constructing a ground-water model for the Lake Michigan part of the Basin. Most ground-water flow is in the unconsolidated sediments. The horizontal and vertical hydraulic conductivity (Kh and KZ, respectively) of these sediments is important to the accuracy of model simulations because of the heterogeneity in the sediments. Programs and procedures were developed to efficiently process records for about 1,800,000 well sites from three state well data bases to obtain data useful to determine initial estimates of unconsolidated lithology, hydraulic conductivity, and ground-water levels.

The processing of well records into hydraulic conductivities involved four steps. First, an automated interpolation program classified well-record text descriptions of lithology into aquifer and non-aquifer deposits. A second program calculated the equivalent Kh and KZ from the thickness of aquifer and non-aquifer deposits in three predefined layers. The equivalent Kh was based on 100 ft/day for aquifer deposits and 1 ft/day for non-aquifer deposits. The equivalent KZ was based on 10 ft/day for aquifer deposits and 0.001 ft/day for non-aquifer deposits. These values are assumed for convenience to determine a relative contrast between aquifer and non-aquifer material. The point values of Kh and KZ for wells were interpolated into a continuous distribution of values using inverse distance weighting. Lastly, the hydraulic conductivities of aquifer deposits were calculated using the specific-capacity data from the driller well records as input to a modified version of the Theis equation. These calculated hydraulic conductivities were used to adjust the continuous distribution of Kh for aquifer deposits to initial values used in the modeling.

Well records were also used for steady-state and point-in-time water levels. Many water levels in the well-record data set are erroneous and should be eliminated from the set of calibration water levels. To obtain a potentially more accurate data set of calibration water levels, the water-level data were kriged to determine a water-level surface, then the difference between the kriged surface and individual water levels was used to remove outliers.