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Project Title: Characterization of the Hydrogeologic Properties and Groundwater-Flow Conditions in the St. Lawrence Formation
Project Number: NQ00EAM - MN265
Project Chief: Jones, Perry
Project Start Date: 07/01/2010
Project End Date: 09/30/2013
Cooperator: Minnesota Geological Survey

Objective: Characterize hydrogeologic properties and groundwater-flow conditions of the St. Lawrence Formation.

Project Description
The St. Lawrence Formation is a Cambrian-age formation consisting of interbeds of the fine classic and carbonate rocks. The formation ranges between 110 to 120 feet in thickness and covers about 12,800 square miles of southeastern Minnesota. It lies between two Cambrian-age formations that are used for water supplies: the Jordan Sandstone and the Franconian Formation. The Jordan Sandstone is composed of coarse to fine clastic sediments and is part of a major aquifer used by many communities in southeastern Minnesota for a water supply. The Franconian Formation consists of fine clastic bedrocks and the upper part of the formation is widely used as an aquifer.

Hydrogeologic investigations have identified the St. Lawrence Formation as a confining unit, due mainly to the low vertical matrix permeability of the formation. Young (1992) and Delin and Woodward (1984) grouped the St. Lawrence and Franconia Formations as a regional confining unit, restricting vertical groundwater movement across southeastern Minnesota. However, recent hydrologic studies of the St. Lawrence formation have found that groundwater can travel at relatively high flow rates through the formation, putting into question the ability of the formation to be a confining unit. The vertical flow properties of the St. Lawrence Formation have not been well characterized, particularly at shallow depth and where the formation is uppermost bedrock. Understanding groundwater-flow conditions in the St. Lawrence formation is critical in protecting water supplies of the Jordan Sandstone where vertical groundwater flow from the St. Lawrence Formation is upward, and water supplies of the lower Franconia formation where vertical groundwater flow from the St. Lawrence Formation is downward. The objective of this study is to characterize the vertical and horizontal hydraulic conductivities and groundwater-flow conditions of the St. Lawrence Formation using borehole geophysical methods.

Progress During FY11:
A project proposal was prepared in cooperation with staff from the Minnesota Geological Survey (MGS) and with staff from the USGS Branch of Geophysics and the USGS Wisconsin Water Science Center. An agreement between the USGS and MGS was signed. The project chief met with a MGS geologist to discuss potential site locations for the borehole, well completion of the borehole following the borehole geophysical surveys, and a time schedule for the study. Potential sites in Scandia and Lakeville were discussed. The project chief, MGS hydrogeologists, and Minnesota Department of Health (MDH) hydrogeologists met with representatives from the City of Lakeville to discuss potential installing the proposed borehole near the city's wells. The city of Lakeville agreed to allow the borehole to be installed. MGS hydrologists worked with the MDH and Dakota County Environmental Management Department to obtain a variance needed to install the borehole and the multi-level groundwater monitoring system.

Statement of Work FY12:
When the variance is obtained from MDH and Dakota County Environmental Management Department, the borehole will be installed, borehole geophysical surveys will be conducted, and the multi-level groundwater monitoring system will be installed. Following installation of the multi-level groundwater monitoring system, a pressure transducer and data logger system will be installed to continuously monitor water levels in one of the ports of the monitoring system. Monthly water level measurements will be made in each of the ports of the monitoring system. Geophysicists from the Branch of Geophysics will conduct borehole geophysical surveys of the Jordan Sandstone, the St. Lawrence Formation, and Franconia Formation. Al Shapiro will conduct packer tests in the borehole. The project chief will work with USGS and MGS researchers on analyzing the collected geophysical and hydrogeologic data and summarize the results in a draft report.