The St. Peter-Jordan aquifer includes the Cambrian Jordan Sandstone and the overlying Ordovician Prairie du Chien Group and St. Peter Sandstone. The aquifer is present throughout Iowa and is confined beneath other aquifers in much of the State. Information on the aquifer available from drillers and contractors, provided estimates of aquifer transmissivity values ranging from about 500 to about 3,000 square feet per day. The largest transmissivity values are for dolomite and dolomite-cemented sandstone, indicating that permeability in much of the aquifer is due to secondary fractures. The aquifer is vertically bounded by an upper leaky confining unit with a vertical hydraulic conductivity of 10^-10 feet per second.

The aquifer was simulated using a two-layer finite-difference ground-water flow model. The upper layer simulated a source bed in aquifers composed of Silurian and Devonian rocks overlying the St. Peter-Jordan aquifer. The lower layer simulated flow in the St. Peter-Jordan aquifer. Lateral boundaries assigned in the model include constant heads in northeastern Iowa, where the aquifer is in contact with the Mississippi River or is unconfined, and no-flow boundaries in western and northwestern Iowa, where the rocks are insufficiently permeable to form an aquifer. The aquifer boundaries to the north, east, and south of Iowa were determined by geohydrologic conditions and the relation of the St. Peter-Jordan aquifer with the lateral extent of adjacent aquifers.

An assumption that the largest part of recharge to the aquifer is from outcrop areas in northeastern Iowa and from Minnesota is not supported by the results of this study. Vertical leakage from overlying rocks accounted for most of the recharge to the aquifer in northwestern Iowa. Discharge is mostly through lateral boundaries and to rivers.

Pumping has caused changes in the flow system that include regional declines in the potentiometric surface of the aquifer. Simulation indicates that pumping through 1980 increased net vertical leakage into the aquifer to about double the predevelopment rate. Discharge across lateral boundaries has been substantially reduced or reversed by pumping. Aquifer storage provided about one-third of the water required to supply pumping in the 1970's. Simulation of future conditions, assuming no increase in pumping rates, indicates that the rate of decline in water levels will decrease by the year 2020. As equilibrium with pumping is approached in 2020, 75 percent of the pumpage will be balanced by vertical leakage, eight percent by water released from aquifer storage, and 17 percent by increases in boundary recharge or decreases in boundary discharge. Future pumping at an increasing rate of about 10 percent per decade of the average pumping rate in 1975 will require about one and one-half times the vertical leakage of the 1971-1980 period and about fivetimes the net inflow from lateral boundaries; however, the rate of water released from aquifer storage will be about half the 1970's rate. Under these conditions, the head in the aquifer will continue to decline at an almost constant rate until 2020.