DRILLER SPECIFIC CAPACITY AS A MEASURE OF AQUIFER TRANSMISSIVITY AND A TEST OF THE HYDROGEOLOGIC UNITS IN THE GALLATIN LOCAL WATER QUALITY DISTRICT, GALLATIN COUNTY, MONTANA

Hydraulic properties of geologic units are needed to address groundwater quality and quantity problems.  This study uses abundant driller specific capacity data from Gallatin County, Montana to test whether eleven geologic units commonly thought to be different have statistically different hydraulic character.  Aquifer transmissivity was estimated from driller specific capacity using an empirical equation developed from fifty data pairs of transmissivity and specific capacity calculated from single well tests at unscreened or non-perforated wells pumped for one hour.  Recovery transmissivity and specific capacity are log-linearly related.  The best-fit regression is Log T = 1.3562(Log ^Q/_S) + 3.5209, where T = transmissivity expressed in m²/day and ^Q/_S (specific capacity) is in m²/min with a coefficient of determination, R², of 0.749.  A test of pumped, airlift, and bailed driller tests showed that bailed tests could not be used.  Although there are 7253 wells in the Gallatin Local Water Quality District only 1136 could be located, were unscreened, were tested for one hour, had specific capacity data, and were not bail tested.  The data only allowed statistical analysis of the Quaternary – Tertiary hydrogeologic units within the Gallatin Valley.  Older units had few located wells, missing, or inappropriate data such as short pumping times.  A Mann-Whitney statistical analysis of the estimated transmissivity values among identified hydrogeologic units indicates significant difference between neighboring units.  The eleven initial Quaternary – Tertiary hydrogeologic units were recombined into three distinct hydrogeologic units to reflect the results of the statistical analysis.  The calculated transmissivity values for the distinct hydrogeologic units utilizing driller reported specific capacity data agree well with published values from aquifer tests in the region, except for the high transmissivity West Gallatin alluvium aquifer.  This study demonstrates that driller reported specific capacity data can be used to estimate hydraulic parameters and that the values obtained are in good agreement with hydraulic parameters determined by traditional aquifer pump tests.