Defining interface parameters between ground and surface water in a fault-controlled alluvial valley

A perennial spring forming a tributary to Battle Creek in the Black Hills was studied to determine controls on both shallow and deep water sources.  Characteristics defining the interface between these sources are poorly understood but are integral to development of a tenable water supply.  Seismic reflection has indicated a large fault extending to the basement, potentially cutting the Mississippian Madison Group aquifer.  Alluvial deposits consist of interbedded gravel and silt and overlie the Triassic Spearfish formation.   Gypsum beds in the upper Spearfish formation dissolve forming conduits for surface runoff to infiltrate into coarse-grained alluvial beds that are in hydraulic connection with the stream channel.  Thus, a mixed spring source seems likely.  Six piezometers were installed up- and downstream of the spring and monthly water table measurements have been made.  Observed water levels have remained steady at each location and are unaffected by precipitation.  Shallowing of the water table proceeds southward from five to zero meters at the spring representing the decrease in surface elevation in the down valley direction.  The subsurface water table gradient is low, much less than the land surface, and the intersection of these 2 features results in the spring.  Analysis of these data have suggested that deep-sourced water rises through the brecciated rock along the fault to the surface due to artesian pressure within the underlying aquifers.  This deep source provides a continuous supply that is enhanced by flow through the dissolution features and shallow gravels in wet precipitation years.


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