The coordinates for this Earthcache will take you in front of the city of Watertown Water Treatment Plant. The city’s source of water for the plant is the Big Sioux aquifer. The Big Sioux aquifer is the major source of water within the basin, serving municipal, industrial, rural, and irrigation needs. Overall, the groundwater quality is excellent and ranks among the highest quality water in the State. Because the surface water supplies within the basin are insufficient for existing needs and the water quality of the bedrock aquifers is generally poor, the Big Sioux aquifer is the only major source of good quality water within the basin. Predominant chemical constituents in water from the Big Sioux River aquifer are calcium and bicarbonate (hardness).
The Big Sioux aquifer is a shallow glaciofluvial aquifer associated with the Big Sioux River system. This system drains the easternmost tier of counties in the State with the aquifer underlying several thousand square miles of this area. The Big Sioux aquifer consists of poorly to well-sorted surficial outwash that ranges from medium sand to medium gravel; it generally is less than 10 ft below land surface. The aquifer is limited to the flood plain of the Big Sioux River and its tributaries, and is underlaid by till. In most locations, the aquifer becomes coarser and more sorted with depth. The aquifer generally is under water-table conditions, except in the Lake Pelican area where the aquifer is confined by 3 to 5 feet of till.
Recharge to the aquifer is by direct infiltration and subsequent percolation of rainfall and snowmelt through the overlying 1 to 2 feet of topsoil. Recharge to the aquifer ranges from 4 inches to as much as 10 inches per year. The general direction of water movement in the aquifer is to the south and toward the Big Sioux River, which flows from north to south. The gradient of the water-table surface generally is about 6 to 10 feet per mile. Lake Kampeska and Pelican are connected hydraulically to the Big Sioux aquifer. During spring and early summer, lake levels commonly exceed the water level in the aquifer and, thus, the lakes recharge the aquifer. During summer and fall, when evaporation exceeds precipitation, the lake levels are less than the water levels in the surrounding Big Sioux aquifer and water from the aquifer discharges to the lakes.
The Big Sioux aquifer is very productive because:
1. It is shallow (typically at ground surface to 10 feet below ground surface) and receives recharge directly from infiltrating precipitation;
2. It consists of coarse-grained outwash deposits, which are very permeable; and
3. It is in direct hydraulic connection with the Big Sioux River and its tributaries, which provide recharge to pumping wells and limit drawdown.
LOGGING REQUIREMENTS:
In order to count this Earthcache as a find, you must complete the following tasks and email the answers to me.
- Post a picture of yourself/team with the Water Treatment Plant in the background (optional).
- What is the height of the water storage tank near ground zero?
- What is the elevation at ground zero?
- Approximately how far below the ground’s surface (in feet) is the aquifer?
- Based on the map below, can the Big Sioux aquifer be found in McCook County?
Resources:
Water Resources of Codington and Grand Counties, South Dakota by Donald S. Hansen.
Open File Report No. 1-BAS, Task I - Completion Report Big Sioux Aquifer Study, Eastern South Dakota by Robert Stach, James Mika, Dennis Tomhave, and Diane Wittenhagen.