THE ARSENIC SPECIAL CASING AREA IN THE FOX RIVER VALLEY OF EAST-CENTRAL WISCONSIN: AN EXAMPLE OF DATA INTEGRATION AND INTERAGENCY COOPERATION FROM INITIAL RESEARCH TO RULE DEVELOPMENT

High concentrations of arsenic and other heavy metals in private water wells present a potentially serious public health problem for much of the Fox River Valley region. The metals are released from naturally occurring sulfide minerals by a complex variety of natural chemical processes. For example, rapid residential development has resulted in heavy groundwater use for domestic and public water supply, which has caused a significant decline in the regional water table. The resulting oxidation of sulfide minerals, particularly iron-sulfide cement in the St. Peter sandstone aquifer, is the single most important release mechanism in this area.

The mineralization is typical of Mississippi Valley Type deposits, and occurs throughout the region, but is most commonly developed in the Ordovician St. Peter sandstone and in sandstones within the underlying Prairie du Chien Group. The Wisconsin Department of Natural Resources (DNR) determined that scientifically based guidelines were needed to assist well drillers in constructing wells that had the highest probability of avoiding the mineralized formations and thus the contaminated water. The Wisconsin Geological and Natural History Survey (WGNHS) and DNR worked together to assemble and integrate existing well construction data, match wells with address grids and digital orthophotography provided by county governments, interpret the geology and geometry of the aquifer system, and prepare a series of digital primary and derivative maps under tight time constraints. The resulting casing-depth requirements are illustrated on township-scale maps that specify the casing depth required to complete a well above or below the mineralized interval within a specific quarter section. These maps are easy for drillers to interpret and use in the field.

Much work remains to fully define the nature and extent of the mineralization, but this project is an example of what can be accomplished when intergovernmental cooperation works to provide a timely interim solution for an urgent public health concern. The cooperation and sharing of digital data by the involved counties with the DNR and WGNHS were essential to the success of this project and the timely publication of the guidelines.