Operation of a coal-tar distillation and wood-preserving plant from 1918 to 1972 in St. Louis Park, a suburb of Minneapolis, Minn., resulted in ground-water contamination. This preliminary evaluation presents an overview of the problem based on the results of the first year (1979) of an ongoing study.

By 1932, water in the Prairie du Chien-Jordan aquifer, the region's major source of ground water, was contaminated 3,500 feet from the plant. It seems that this early contamination of the aquifer resulted in part from the introduction of coal tar directly into a multiaquifer well on the plant site. The Prairie du Chien-Jordan aquifer underlies the area at depths of 250 to 500 feet and is overlain by two bedrock aquifers (Platteville and St. Peter), two confining beds (Glenwood and the basal part of St. Peter), and 70 to 100 feet of drift.

The upper part of the aquifer (the Prairie du Chien Group) is carbonate rock having fracture and solution-channel permeability and low effective porosity. Contaminants in the Prairie du Chien Group can move more rapidly than those in drift and sandstone aquifers having intergranular permeability. The aquifer characteristics, the long contamination history, and seasonal potentiometric-surface fluctuations owing to heavy municipal and industrial withdrawals combine to create a complex distribution of coal-tar derivatives in the Prairie du Chien-Jordan aquifer.

In addition, at least 25 ungrouted or partly cased wells in the area may permit contaminated water from near-surface aquifers to flow downward into deeper bedrock aquifers along or through the well bores. Where possible, such wells have been geophysically logged and inspected by downhole television. Flow rates of 20 to 150 gallons per minute from the Platteville and St. Peter aquifers to the Prairie du Chien-Jordan aquifer were observed in five of nine wells. The water was contaminated in four of the five wells.

Drift materials on and south of the site have been contaminated by surface spills and by infiltration of contaminated process water. Near the contamination source, a hydrocarbon fluid phase is moving vertically downward relative to movement of the aqueous phase. Fluid pumped from an observation well in this area contained 6,000 milligrams per liter total organic carbon. Dissolved coal-tar constituents in the drift and the uppermost bedrock unit over most of the area, the Platteville aquifer, have moved at least 4,000 feet downgradient to a drift-filled bedrock valley. At the valley, it seems that the Platteville aquifer and the Glenwood confining bed have been removed by erosion and that contaminants with a concentration of approximately 2 milligrams per liter dissolved organic carbon are entering the underlying St. Peter aquifer. Chemical analyses of fluid pumped from observation wells suggest that soluble, low-molecular-weight compounds are moving preferentially through the drift and the Platteville aquifer.