Wellfield Development Along the Big Sioux Aquifer
Alluvial wellfields consisting of several dozen wells – both collector and vertical types – along the Big Sioux River have historically supported Sioux Falls’ water supply. HR Green, in partnership with Leonard Rice Engineers (LRE), was retained by the City to provide research and preliminary testing, hydrogeologic testing, well siting, and design and construction phase services for wellfield improvements.
Recent emphasis on maximizing yield from collectors made the siting of an additional collector critical. The objectives of the preliminary design were (1) provide a current and future wellfield development plan for a newly acquired property adjacent to the City’s existing well field, (2) coordinate with previous USGS work and conduct detailed geologic and hydrogeologic testing at the most favorable area to further characterize the aquifer, and (3) site and design a new high-capacity collector well.
Investigating the Site
An approximate 150-acre parcel was evaluated through geologic and hydrogeologic research. This included using existing information obtained from the USGS as part of their study entitled “Groundwater-Flow Model and Analysis of Groundwater and Surface-Water Interactions for the Big Sioux Aquifer, Sioux Falls, SD”. Two favorable areas were identified for collector wells and one was recommended for the final design.
Eleven exploratory borings were drilled to delineate depth to bedrock and the variability of the sediment grain size. The design team worked with USGS personnel to conduct an 8-hour step-rate and 72-hour constant-rate pumping test (350 GPM) were then conducted at a nearby irrigation well, with concurrent water level monitoring at nine observation wells, at the oxbow lake, and the nearby Silver Creek. The Big Sioux aquifer’s hydraulic conductivity and transmissivity were calculated and compared to other existing collector wells. The recharge contribution from the Big Sioux River, Silver Creek, oxbow lake, and groundwater storage were also estimated.
The Final Well Design
Well 72 was subsequently designed having a 16-foot inside diameter collector with four 200-foot long laterals which were oriented to capture maximum recharge. The new well will provide the City of Sioux Falls with additional capacity to meet the growing needs of the community while also achieving the City’s goal of having a redundant and reliable source of water for the Water Purification Plant.