By Mary Ann Thomas
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Ground-water quality was assessed in the northeastern part of the Corn Belt, where tile-drainedrow crops are underlain by fractured glacial till. Data were collected from 30 shallow monitor wells and 18 co-located domestic wells as part of the U.S. Geological Survey’s National Water-Quality Assessment in the Lake Erie-Lake St. Clair Basin.
Pesticides or pesticide degradates were detected in 41 percent of the monitor wells and 6percent of the domestic wells. The pesticides detected closely correspond to those most heavily applied—herbicides used on corn and soybeans. Pesticide degradates were detected three times more frequently, and at higher concentrations, than were parent compounds. No pesticide concentration exceeded a USEPA Maximum Contaminant Level (MCL), but MCL’s have not been established for 9 of the 11 compounds detected.
Thirty-seven percent of monitor-well samples had nitrate concentrations indicative ofhuman influences such as fertilizer, manure or septic systems. Nitrate was the only chemical constituent detected at a concentration greater than an MCL. The MCL was exceeded in 7 percent of samples from monitor wells which were too shallow to be used as a source of drinking water.
Pesticide and nitrate concentrations in the study area are low relative to other agriculturalareas of the Nation. Several authors have suggested that ground water in parts of the Upper Mid-west is minimally contaminated because it is protected by the surficial glacial till or tile drains. These ideas are examined in light of the relations between concentration, well depth, and ground-water age in the study area.
Most of the shallow ground water is hydraulically connected to the land surface, based onthe observations that 83 percent of waters from monitor wells were recharged after 1953, and 57 percent contained a pesticide or an elevated nitrate concentration. Fractures or sand-and-gravel stringers within the till are the probable pathways.
In some areas, deeper parts of the ground-water-flow system are also hydraulically connectedto the land surface. Almost half the waters from wells 50 to 100 feet deep were recharged after 1953. Anthropogenic constituents were detected in samples from three domestic wells 60 to 121 feet deep, in areas where the till is relatively coarse-grained.
The hydrogeologic system has several geochemical characteristics conducive to transformationsor sorption of nitrate or pesticides: (1) the till is clay-rich, has a high organic-carbon content, and contains an abundance of pyrite-rich shale fragments, (2) the ground water has low dissolved-oxygen concentrations, and (3) iron and manganese oxides and oxyhyroxides line the faces of fractures in the unsaturated zone.
Although the aquifer system appears be protected from contamination in some areas, thefact that the surficial till is heterogeneous and of variable thickness suggests that the protection is not uniform. The protection can be breached by fractures or sand-and-gravel stringers, which are apparent in core samples but not noted on domestic-well logs.
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