The surf icial sands in the Pomme de Terre and Chippewa River valleys in Grant, Pope, Stevens, and Swift Counties have been studied to determine the occurrence, availability, and quality of ground water in these aquifers.

In the northern part of the Pomme de Terre and Chippewa River valleys, the aquifers consist of coarse sand and gravel ranging from 0 to 100 feet in thickness; transmissivities range from 0 to 35,000 feet squared per day in narrow, steep-sided erosional valleys. In the north, well yields commonly exceed 1,000 gallons per minute and may be as much as 4,000 gallons per minute locally. Farther south, the deposits are medium to fine grained, range from 0 to 90 feet thick, and reach a maximum width of 10 miles near Benson, Minnesota. Transmissivities range from 0 to 25,000 feet squared per day. Wells may yield as much as 1,500 gallons per minute locally. Southeast of Clontarf, well yields generally do not exceed 500 gallons per minute because the deposits are thinly saturated and fine grained.

Ground water in the surf icial aquifer is a mixed calcium magnesium-sulf ate bicarbonate type that is chemically suitable for most uses. Concentrations of most constituents analyzed were below limits recommended by the Minnesota Pollution Control Agency for drinking water, but concentrations of manganese, iron, nitrite plus nitrate, and dissolved solids exceed recommended limits locally. Salinity, as indicated by the specific conductance (values ranged from 580 to 1,000 micromhos per centimeter) was in the medium to high range at several locations.

An analytical model was used to estimate the effect on streamflow of pumpage from the surficial aquifer in the narrow, 50-mile reach of the Pomme de Terre River valley in Stevens and Grant Counties. The model indicates that the 43 existing wells pumping at maximum potential yields could reduce streamflow by 55 cubic feet per second. Addition of 23 wells also pumping at maximum potential yields could reduce streamflow by 77 cubic feet per second; this rate exceeds low base flow of the Pomme de Terre River.

Finite-difference models were used to simulate flow in the surficial aquifer along the Pomme de Terre River near Appleton in Swift County and along the Chippewa River between Cyrus in Pope County and Danvers in Swift County. In the Appleton area/ model analyses indicate that pumping lowered water levels as much as 3 feet from 1973-80 and reduced streamflow by about 14 cubic feet per second. Additional regional water-level declines of 1 to 2 feet/ and up to 4 feet locally near aquifer-till boundaries, can be expected after about 4 years if pumping continues at the 1980 rate and area! recharge from precipitation is near normal. However/ simulation of increased pumping rates and decreased area! recharge during a 3-year drought indicates that water levels may decline as much as 9 feet near aquifer-till boundaries and streamflow may be reduced by about 41 cubic feet per second, which is about 95 percent of the available flow in the Pomme de Terre River at the 55-percent flow duration. Model results also suggest that/ during the first year of a drought/ the combined pumpage from wells operated during 1980 along the Pomme de Terre River in Stevens and Grant Counties and in the Appleton area could reduce streamflow to zero during base flow. Model-computed streamflow deficiencies are 48 and 60 cubic feet per second at the 55- and 70-percent flow duration/ respectively. Under such conditions/ pumping could not be sustained at the rates simulated unless there was sufficient water stored in the stream channel or streamflow was augmented.

In the Cyrus-Benson area/ model results indicate that tinder 1980 development and average area! recharge/ dynamic equilibrium would be reached in less than 4 years and additional drawdown would be less than 2 feet. A 3-year drought coupled with increased pumping from irrigation wells operated during 1980 would lower water levels as much as 6 feet and reduce flow in the Chippewa River by about 26 cubic feet per second. At maximum hypothetical development in terms of the number of wells and normal area! recharge/ water levels would be lowered as much as 9 feet and streamflow would be reduced about 12 cubic feet per second. At maximum hypothetical development/ drought conditions and increased pumping would lower water levels as much as 12 feet and reduce flow in the Chippewa River by about 30 cubic feet per second/ which equals about 75 percent of available streamflow at the 70-percent flow duration.