ASTM D5785/D5785M-20 PDF Download

The ASTM D5785/D5785M-20 standard describes a procedure for determining transmissivity using the measurement of the damped oscillation about the equilibrium water level of a well-aquifer system after a sudden change in water level. This practice is applicable for wells tapping highly transmissive confined aquifers or deep wells with long water columns where underdamped response occurs.

The standard makes several assumptions about the physical system, including that the aquifer is of uniform thickness and confined by impermeable beds, it has constant homogeneous porosity and matrix compressibility, and its hydraulic conductivity is homogeneous and isotropic. The cylindrical coordinate system is used, with the origin at the top of the aquifer. The aquifer is assumed to be fully screened, and the flow through the well screen is assumed to be uniformly distributed over the entire aquifer thickness.

The assumptions made in defining the momentum balance include the approximation of average water velocity in the well being constant, laminar and negligible frictional head losses from flow across the well screen, and negligible change in momentum from the water velocity changing from radial flow through the screen to vertical flow in the well. The system response is assumed to be an exponentially decaying sinusoidal function.

The standard also mentions the limitations of the slug test method, such as the requirement of knowing the storage coefficient and the assumption of a fully penetrating well, although the method can be used with partially penetrating wells under certain conditions. It also specifies that the test is applicable for slug tests where the initial water-level displacement is less than 0.1 or 0.2 of the length of the static water column.