Ground Water in Alluvial Channel Deposits: Nobles County, Minnesota

The alluvial channel deposits described in this report are in Nobles County, Minn., about 150 miles southwest of Minneapolis and St. Paul. Although four municipalities and many farms obtain part or all of their water needs from the alluvium, it has not yet been fully developed for ground water. The extent of the alluvial channel deposits was mapped on high-altitude aerial photographs, and a power auger was used to bore 43 test holes to determine the thickness of alluvium and the water level at each of the test sites. Thicknesses of the alluvium differ in the several stream channels and across the same channel. The thickness ranges from zero to more than 61 feet and is expected to be greater in the trunk stream valleys than in most of the tributaries. Maximum thicknesses are expected to be similar along local segments in the same stream channel. The thickest section of alluvium is not necessarily coincident with the position of the present-day stream. A line of test holes across the stream valley is a positive means of determining the position of the thickest alluvial section. In grain size the deposits range from clay to boulders, with all possible admixtures. Textural changes occur both laterally and vertically in very short distances. The coarser sections are overlain by clayey silt which ranges in thickness from a few inches to more than 11 feet. The alluvial aquifers are under water-table conditions except where the coarser material is overlain by a relatively thick section of impermeable silt or clay and artesian conditions exist. The average water level in the 43 auger holes was about 7 feet below the land surface. If the average long-term storage coefficient of the alluvium is 0.2, about 1.5 gallons of water is available from storage in each cubic foot of saturated material. A pumping test on a well at Worthington showed the coefficient of transmissibility of the alluvium to be about 24,000 gpd per foot and the field coefficient of storage to be about 0.1. The surficial expression of the alluvial deposits facilitates the selection of good well sites. Factors to be considered in selecting well sites are (1) depth and width of deposits; (2) permeability; (3) saturated thickness; (4) location near surface water bodies; (5) location at confluence of two channels; and (6) proximity of valley walls. The water in the alluvium is of fairly good quality except that it is very hard. Five chemical analyses show a range in dissolved solids from 425 ppm (parts per million) to 870 ppm, and in hardness from 332 to 568 ppm. Some other ground waters in the county have dissolved solids and hardness of more than 1,000 ppm. The temperature of the water ranges from 46.5° to 51°F.