AN ESTIMATE OF LOW-PERMEABILITY CONFINING LAYER LEAKAGE INTO A BURIED VALLEY AQUIFER

The West Fargo Aquifer System (WFAS) is a complex set of buried channel aquifers consisting of unconsolidated glacial sands and gravels. The WFAS is carved into older tills, and occasionally into Cretaceous sediments down to depths of 150 to 350 feet below land surface. On top of the WFAS are more recent tills and about 70 to 80 feet of surficial lake clay (former Lake Agassiz). Ground-water withdrawals from the WFAS have resulted in lowered water levels, which in places have dropped below the top of the aquifer. Relatively large water-level decline suggests limited recharge to the WFAS. Deuterium and O-18 in ground-water samples collected from piezometers completed in the WFAS show a very cold ‘signature’ in relation to a meteoric water line, and thus indicate a lack of modern-day recharge. A water budget was estimated for the time of use. Population data was combined with estimated per capita water use to generate estimates of ground-water withdrawals for time periods without ground-water withdrawal data. Total withdrawal from 1870 to 1995 is estimated at about 33 billion gallons. Using numerous 1995 water-level measurements, the volume of water appearing to be ‘missing’ (depletion) from the WFAS since 1870 is only about 7 billion gallons. This depletion estimate attempts to account for the change in storativity in areas where the WFAS converts from confined to unconfined conditions. Based on the above, another ground-water source other than modern-day recharge is indicated. A plausible ‘other’ source could be derived from leakage in the overlying till/lake clay aquitards. Only two sites in the study area have piezometers completed in these aquitards. Limited data from these two sites show relatively large water-level declines indicative of aquitard leakage. Efforts are under way to install additional piezometers into the aquitards surrounding the WFAS to better quantify aquitard leakage.