Groundwater assessment and modeling utilizing refraction seismic surveys

A series of refraction seismic surveys has been conducted over al­ tial deposits in Rapid City, South Dakota, to determine the thickness of saturated aquifer. The results indicate the presence of three rock strata consist of: (1) Unsaturated alluvium at the ver top, which yielded average seismic velocity of 215 m/s and thickness ranging from 1.3 m to 2.7 m; [2) Saturated alluvium, which had seismic velocity of 1200 m/s to 1765 m/s and thickness ranging from 7.2 m to 11.0 m; and (3) Impermeable red shale of Spearfish Formation, which yielded seismic velocity of 2300 m/s. The contact between the alluvium and the Spearfish Formation is undulator, representing the old Rapid Creel erosional surface, along which several major paleochannels were recognized adjacent to the current Rapid Creek channelData from seismic surveys were also used to asses groundwater resources and to model groundwater flow for best possible withdrawal methods. The results showed that a pumping well penetrated to the bottom of the aquifer was able to discharge more than 54,000 It/ hr or 3,200,000 tt/ day, and recharge from adjacent Rapid Creek could re­ turn the water table to nearl its original position within a few hours. Assuming that water from the Rapid Creek can sufficiently recharge the aquifer throughout the area of stud, Rapid Creek alluvium aquifer can provide all local water demands.