Ethanol Plant Hydrogeologic Assessment: Cottonwood County, Minnesota
Leggette, Brashears & Graham, Inc. (LBG) recently completed a water supply assessment at a proposed ethanol plant site in northern Cottonwood County, Minnesota. The proposed plant will require a groundwater supply that will produce over 800 gallons per minute (gpm) or approximately 1.2 Million Gallons per Day (MGD). This assessment involved conducting a test to evaluate the available water supply, and the collection of technical data necessary to support regulatory requirements of the Minnesota DNR. The purpose of the testing program was to assess the potential impact of the site’s planned production well on the source water aquifer and surrounding water supply wells prior to the issuance of a water use permit for the planned site.
DOMESTIC WELL SEARCH AND AQUIFER PUMPING TEST
LBG conducted the aquifer test in close cooperation with a local drilling contractor who installed the test production well and two observation wells, provided the generator, pump and meter, and collected manual water level measurements. The aquifer test work plan was developed to satisfy the requirements and procedures provided of the DNR, Division of Waters. LBG also assisted the client with a domestic well survey to identify nearby water users who could be affected by the proposed well.
The aquifer selected for the site water supply is a confined sand and gravel aquifer. During the test, water levels were monitored in the aquifer and in the underlying sandstone bedrock aquifer system to assess potential regional affects associated with the pumping. A constant-rate test was conducted by pumping the test well at a rate of 860 gpm for a period of 4.5 days. A recovery test was conducted immediately following the completion of the constant-rate test.
AQUIFER PARAMETER DETERMINATION
In order for the long-term effects of pumping to be reliably projected and modeled, hydrogeologic characteristics of the aquifer at the test site needed to be determined. About 30 feet of drawdown occurred in the test well after 4.5 days of pumping. Conditions in the sand and gravel aquifer were found to conform to the assumptions of analytical methods for a confined aquifer. These methods were used to analyze the data and establish aquifer parameters necessary to assess the interference, long-term aquifer sustainability, and satisfy DNR requirements related to the potential cumulative impact on the groundwater resources in the area.
LONG-TERM GROUND-WATER SUSTAINABILITY
Based on the results of the test, LBG concluded that the single production well completed at the site could sustain a pumping rate of 850 gpm without adversely affecting the ground-water supply for the existing wells identified and monitored in this study.