EFFECTS OF IMPERVIOUS SURFACES ON RECHARGE IN THE TWIN CITIES METROPOLITAN AREA, 1986 – 2000

The purpose of this investigation was to assess the effects of impervious surfaces on recharge to water-table aquifers in the seven-county Twin Cities Metropolitan Area (TCMA). Annual recharge rates to water-table aquifers were estimated using stream-flow data from 21 stream gages within the TCMA and compared to percent impervious data compiled by Dr. Marvin Bauer, Department of Forest Resources, University of Minnesota, for 1986, 1991, 1998, and 2000. Annual recharge was estimated from the stream-flow records using recession-curve displacement, also known as the Rorabaugh Method. There was no significant correlation between percent impervious area and estimated recharge. This may be because other factors such as leakage from storm sewers and water mains offset the reduction of recharge as urban areas expanded. It also may be because there were not enough data from 1986 – 2000 for an accurate correlation. The 21 stream gages had from 2 to 28 years of continuous stream-flow record. Recharge was estimated at 2 stream gages for 1986, at 6 stream gages for 1991, at 12 stream gages for 1998, and at 21 stream gages for 2000. The results of this study demonstrate the importance of long-term monitoring of stream flow within the TCMA. This study was initiated in cooperation with the Metropolitan Council.