Confined glacial and bedrock aquifers are present within Quaternary and Cretaceous deposits that fill buried valleys incised in the Sioux Quartzite surface in Rock County, in southwestern Minnesota and Minnehaha County, South Dakota. This report describes the areal extent, thickness, water-bearing characteristics, water-supply potential, and water-quality characteristics of confined aquifers within buriedvalley deposits in Rock County.

Hydrogeologic units present within buried-valley deposits in Rock County include unconfined and confined drift aquifers, undifferentiated Cretaceous aquifers, the Split Rock Creek aquifer, and interbedded confining units. The undifferentiated Cretaceous aquifers consist of sandstone layers within interbedded claystone and siltstone overlying the Split Rock Creek Formation or Sioux Quartzite. The Split Rock Creek Formation consisting of sand units (comprising the Split Rock Creek aquifer) and interbedded layers of siltstone and claystone, is present in buried valleys incised in the Sioux Quartzite surface in southern and possibly northeastern Rock County, Minnesota.

Confined drift aquifers with thicknesses greater than 5 feet were penetrated in 6 of 10 test holes. Thicknesses of the confined drift aquifers in Rock County range from at least 2 to greater than 32 feet. Estimated horizontal hydraulic conductivity for a confined drift aquifer derived from specific-capacity information from one domestic well log was 73 feet per day.

No major (thickness greater than 5 feet) undifferentiated Cretaceous aquifers were penetrated in 10 test holes. Thicknesses of the undifferentiated Cretaceous aquifers compiled from the geologic logs for four domestic wells ranged from at least 7 feet to greater than 46 feet. Estimated horizontal hydraulic conductivity for an undifferentiated Cretaceous aquifer derived from specific-capacity information from one domestic well log was 55 feet per day.

Cumulative sand thicknesses for the Split Rock Creek aquifer in 10 test holes ranged from zero to 128.5 feet in 2 to 6 layers. The largest cumulative sand thicknesses were penetrated near the southern margin of the Sioux Quartzite high in northern Rock County and in an east-west trending buried valley (Brandon Embayment) entering Rock County from Minnehaha County, South Dakota. These comparatively large cumulative sand thicknesses are probably due to a high-energy depositional environment.

Estimated horizontal hydraulic conductivities for the Split Rock Creek aquifer in Rock County derived from analysis of three slug tests were 0.1, 0.2, and 1 foot per day. The corresponding aquifer transmissivities, calculated as the horizontal hydraulic conductivity multiplied by the cumulative sand thickness, were 3, 16, and 130 feet squared per day. The greatest horizontal hydraulic conductivity and transmissivity estimates were for a site near the southern margin of the Sioux Quartzite high. The watersupply potential of the Split Rock Creek aquifer in Rock County is generally limited by the low transmissivity of the aquifer due to the fineness of the aquifer material (generally very fine- to fine-grained sand).

Recharge to the Split Rock Creek aquifer is thought to be derived primarily from hydraulic connection to the Sioux Quartzite aquifer as infiltration of precipitation moves through the fractures and joints of the Sioux Quartzite to the Split Rock Creek aquifer. The regional directions of flow in the aquifer are to the south away from the Sioux Quartzite high and to the west in the Brandon Embayment in Minnehaha County and its east-west trending extension into Rock County.

The predominant ions in water from two wells screened in confined drift aquifers in Rock County were calcium and bicarbonate and in water from a third well were calcium and sulfate. The predominant ions in water from one well screened in an undifferentiated Cretaceous aquifer in Rock County were calcium and bicarbonate and in water from a second well were calcium and sulfate. The predominant ions in water from two wells screened in the Split Rock Creek aquifer in Rock County were calcium and bicarbonate and in water from a third well were calcium and sulfate.