Assessing the accuracy of monitoring wells in tidal wetlands

Abstract

Groundwater movement in marshes is driven by tidal fluctuations, evapotranspiration, and the infiltration of rainwater. These forcings act on time scales of several hours. Monitoring wells in marshes must, therefore, be able to react rapidly and accurately to changes in these forcings. To assess the response characteristics of a given well design, I define a well's time constant as the square of its radius divided by the product of the aquifer conductivity and the length of its screen. Using numerical simulations, I show that the logarithm of the mean deviation over a tidal cycle between the head in the well and the head in the adjoining undisturbed aquifer is linearly related to the logarithm of the well time constant. Similar logarithmic relationships occur for the maximum deviations on flood and ebb tides and the well time constant. Thus, wells in soils with low hydraulic conductivity must have smaller radii and longer screens than those in more conductive soils in order to have similar accuracy. Alternatively, the time constant can be lowered by incorporating the pressure sensor in a slug that displaces most of the water above the sensor and thereby greatly reduces the volume of water that must move between the well and the aquifer. Copyright 2009 by the American Geophysical Union.