Effects of ground-water withdrawals on flow in the Sauk River Valley Aquifer and on streamflow in the Cold Spring area, Minnesota

The U. S. Geological Survey, in cooperation with the city of Cold Spring, Minnesota conducted a study during 1998-99 to: (1) determine the contributing areas of groundwater flow to high-capacity wells, (2) delineate the 10-, 20-, and 30-year time-of-travel zones to high-capacity wells, and (3) determine changes in streamflow in the Sauk River due to ground-water withdrawals. Surficial aquifers underlie a portion of the uplands in the north-central part of the study area and the Sauk River Valley. The upland surficial aquifer is unconfined, with saturated thicknesses ranging from 6 to 37 feet. The Sauk River Valley aquifer consists of hydraulically connected sand and gravel units: (1) the generally continuous surficial unconfined unit underlying the Sauk River Valley and (2) buried confined units that are overlain by till or lake clays in the upland areas. Maximum saturated thickness of the Sauk River Valley aquifer is about 50 feet. Thicknesses of the buried units are generally 10 to 20 feet.

Ground-water flow in the upland surficial aquifer is (1) to the south and southeast toward the margins of the aquifer and (2) toward an unnamed creek valley in the northwestern part of the study area. Ground-water flow in the Sauk River Valley aquifer is from upland areas toward the Sauk River and the connected chain of lakes. Based on hydrograph analysis, ground-water recharge rates to the aquifers during 1999 ranged from 5.3 to 8.6 inches per year, with an average value of 7.0 inches per year.

Streamflow measurements indicated net gains of 44.8 and 25.8 cubic feet per second for the Sauk River from Cold Spring to Rockville during October 1998 and August 1999, respectively. In general, the Sauk River probably is a gaining stream in all reaches, except for the reach adjacent to the Gold'n Plump Poultry Processing Plant well field. Ground-water withdrawals from the well field induce infiltration of water from the Sauk River to the underlying aquifer. The measured gains in streamflow for Brewery Creek were 2.75 and 2.25 cubic feet per second during October 1998 and August 1999, respectively.

A numerical ground-water-flow model was constructed and calibrated for steady-state conditions. Based on the calibrated model, areal recharge accounts for 51.5 percent of the sources of water to the aquifers in the Cold Spring area and inflow through constant-head boundaries contributes 45.8 percent. The largest discharges from the aquifers are leakage from the Sauk River Valley aquifer to the Sauk River and Brewery Creek (53.7 percent) and outflow through constant-head boundaries (33.1 percent).

The simulated contributing areas for selected watersupply wells in the Cold Spring area generally extend to and possibly beyond the model boundaries to the north and to the southeast. The contributing areas for the Gold'n Plump Poultry Processing Plant supply wells extend: (1) to the Sauk River, (2) to the north to and possibly beyond to the northern model boundary, and (3) to the southeast to and possibly beyond the southeastern model boundary. The primary effects of projected increased ground-water withdrawals of 0.23 cubic feet per second (7.5 percent increase) were to: (1) decrease outflow from the Sauk River Valley aquifer through constant-head boundaries and (2) decrease leakage from the valley unit of the Sauk River Valley aquifer to the streams. No appreciable differences were discernible between the simulated steady-state contributing areas to wells with 1998 pumpage and those with the projected pumpage.