New Mexico Geological Society Annual Spring Meeting — Abstracts

A 25-day aquifer test was conducted at a constant discharge rate of 1,249 gpm during the winter of 2003 at a municipal water supply well located in Los Alamos County. Continuous water level responses from numerous observation wells demonstrate that the regional aquifer displays pronounced resistance to vertical propagation of drawdown at shallower depths (i.e., hydraulic conductivity anisotropy). The aquifer also behaves like a semi-confined aquifer at depth with leaky units located above a highly conductive layer that averages about 850 ft thick. Drawdown in the highly permeable unit was recorded more than 8,800 ft away from the pumping well, while drawdown only 1,225 ft away in a multiple-screened observation well was directly related to individual screen depths. This test has demonstrated a remarkably complex aquifer response that is not easily interpreted without a combination of fully penetrating and multiple-screened observations wells, along with a dynamic spinner log from a second supply well. These data also suggest that an alternative conceptual model may present that regional aquifer in the central plateau area. This second leaky-confined model consists of alternating high and low hydraulic conductivity layer within the regional aquifer. Here the low-conductive layers within the alternating aquifer sequence are the leaky source beds that are adjacent to high-conductive layers, and the overlying units at the top of the regional aquifer are not a significant source of water to the municipal supply wells. Differentiation between alternative conceptual models has obvious implications for monitoring potential contaminant migration in the regional system.