Estimates of recharge to unconfined aquifers and leakage to confined aquifers in the seven-county metropolitan area of Minneapolis-St. Paul, Minnesota

Abstract

Recharge to unconfined aquifers in the seven-county Twin Cities metropolitan area of Minnesota was estimated by five methods. Mean recharge estimated as a percentage (27 percent) of precipitation ranged from 7.7 to 8.3 in./yr across the study area. The median recharge estimated from automated analyses of streamflow-recession displacements for seven basins ranged from 1.2 to 12.2 in./yr. Median recharge estimated from graphical analyses of groundwater level fluctuations for 11 wells ranged from 4.5 to 13.6 in./yr. Recharge estimated by age dating of shallow ground water at a nested well site was 8.8 in./yr. Minimal groundwater recharge estimated by statistical analyses of watershed characteristics (geologic data and streamflow records) ranged from less than 0.1 to 5.6 in./yr. Leakage, considered recharge to confined aquifers, in the seven-county Twin Cities metropolitan area was estimated by two methods. Leakage estimated by analyses of groundwater level fluctuations for 11 wells ranged from 3.2x10-3 to 1.1x10-2 in./yr. Leakage estimated by analyses of vertical-hydraulic gradients based on application of the Darcy flow equation for seven nested well sites ranged from 4.6x10-5 to 1.1x10-1 in./yr. Recharge estimates, which generally varied within 10 in./yr for each of the methods, generally were largest based on the precipitation, groundwater level fluctuation, and age dating of shallow ground water methods, slightly smaller based on the streamflow-recession displacement method, and smallest based on the watershed characteristics method. Leakage, which was less than 1 in./yr, varied within 1 order of magnitude based on the groundwater level fluctuation method and as much as 4 orders of magnitude based on analyses of vertical-hydraulic gradients.

Identification and quantification of leakage into the carbonate aquifer by using non- isotopic environmental tracers and hydro-chemical modeling

“…Leakage area delineation in multi-aquifer systems is one of the most challenging topics of regional hydrogeological studies. Existing methods for this purpose include the use of natural environmental tracers, mostly isotopes and chloride (Anker, 2003 Kleinig et al, 2015); and balance calculations (Walton, 1960;Ruhl et al, 2002;Anker, 2007;Messerschmid et al, 2018; Kunchao Lei et al, 2020). The best review of methods for aquifer connectivity determination is given in the report of CIESC (Harrington et al, 2014).…”