Tides in confined well-aquifer systems

Abstract

Tidal water-level fluctuations were recorded in ten observation wells in Virginia. Six wells are located on the Atlantic coastal plain, and four are in the Appalachian mountain region. A quantitative explanation of the observed well tides is developed from consideration of the tidal dilatation of an idealized well-aquifer system. The response of a confined well-aquifer system to tidal dilatations requires consideration of the earth tidal dilatation ΔE, the barometric tidal dilatation ΔB, and the ocean tide dilatation Δo according to the equation: ΔA=-ρg dh(1-nEM+nEW)(1) where ρ is fluid density, g is gravitational acceleration, dh is the height of the well tide, n is porosity, EM is the bulk modulus of the aquifer matrix, andEW is the fluid bulk modulus. The aquifer dilatation ΔA is given by the expression ΔA=(ΔE+ΔBcosϕB+Δocosϕo)/cosϕA(2) where ϕB, ϕo, and ϕA are phase differences between the tide-generating potential and the barometric tide, ocean tide, and well tide, respectively. For wells located far inland Δo can be neglected. For wells located near the ocean the explanation of tides requires analysis of at least two tidal harmonic constituents and the ratio of the heights of the corresponding ocean tide constituents. In this situation the above equation can be written simultaneously for two tidal harmonic constituents. The magnitudes of the ocean tide dilatations can be eliminated in solving the simultaneous equations, but their ratio must be known. Calculation of aquifer porosity from well tide analysis is restricted by uncertainty about bulk modulus and ocean loading effects. If a well-aquifer system is located far enough inland so that ocean tide effects can be neglected, porosity values can be obtained to an accuracy subjectively estimated at ±5 percentage points. Specific storage capacity of a well-aquifer system can be more accurately calculated than porosity because independent estimates of aquifer bulk modulus are not required.