New Mexico Geological Society Annual Spring Meeting — Abstracts

Two separate aquifer tests were performed using an extensive network of observation wells. Initially, a 25-day aquifer test was conducted at well PM-2 at a constant discharge rate of 1,249 gpm, while supply wells PM-4 and PM-5 were used as observation wells. Then a 21-day aquifer test was conducted at well PM-4 at a constant discharge rate of 1,494 gpm while supply wells PM-2 and PM-5 were used as observation wells. These data reveal horizontal propagation of drawdown in the regional aquifer beyond 8,700 ft from each production well, and a pronounced resistance to vertical drawdown propagation at shallower depths. Hydraulically, the regional aquifer seems to behave like a semi-confined aquifer with leaky units located above a highly conductive layer that averages about 850 ft in thickness. While these analyses yield excellent type-curve matches, they fail to explain differences in measured vertical pressure responses from multi-screened observation wells. In addition, multidimensional numerical flow models mimic both horizontal and vertical hydraulic responses very well, and suggest that the aquifer is actually phreatic. These tests demonstrate that the regional aquifer below Los Alamos is strongly heterogeneous, and exhibit pronounced horizontal and vertical anisotropy in hydraulic transmitting properties. These results also illustrate that model dimensionality can influence inferred aquifer behavior, and reinforce theoretical predictions (Neuman, 1975) that say unconfined aquifers may exhibit confined or leaky-aquifer behavior at early times. Finally, the transition from leaky-confined to phreatic conditions is estimated to occur after about 200 days of continuous pumping.