We thought that by closing sinkholes, sediment to the spring could be controlled. Big Spring is Iowa's largest, and it has been used as a water source for trout-rearing ever since Mary and Otto Bankes put it to work in 1940. Iowa's DNR has owned the spring since 1961. These rocks have been slowly dissolved by circulating groundwaters, forming features such as caverns and sinkholes. Sinkholes are present in about one tenth of the basin, and when intense rains generate surface runoff, they capture and direct it into the aquifer.
Shallow aquifers are vulnerable to contamination from activities on the land surface.
Sinkholes funnel surface runoff directly into the underground aquifer. Photo by Bob Rowden. The Geological Survey Bureau began investigating the relationships between agriculture and groundwater quality in the late 1970s. Anecdotal evidence, often from dairy farmers and water-well drillers, suggested that widespread increases in nitrate concentrations were occurring in the extensive, vulnerable bedrock aquifers of northeast Iowa. The Big Spring basin, which is almost entirely agricultural, allowed for a direct study of agriculture's environmental effects.