Abstract:

An analysis is presented for the hydrodynamic behavior of the Puelche aquifer in the Matanza River basin, Buenos Aires, Argentina, relative to the section overlying it or Epipuelche aquifer which contains the water table. By comparison between the isophreatic and isopiestic contours, a general similarity exists between the potentiometric levels, the hydraulic gradients, and the distribution of local recharge and discharge areas of both aquifers. Also, a coincidence occurs between the surface‐ and ground‐water divides, the latter acting as a hydrological barrier which prevents ground‐water inflows to the Matanza basin from neighboring basins. The volume of water flowing away from the Puelche aquifer by underground lateral movement in the northeastern section of the basin is 20,000 cu m/d (5.3 Mgal/d). This yield is supplied by the Epipuelche aquifer through vertical downward leakage, because the Puelche: (1) does not crop out within the study area, (2) does not receive water recharge from underground flows from neighboring basins, and (3) is a semiconfined aquifer. From comparison between piezometric and phreatimetric maps, curves of equal difference of hydraulic head between both aquifers can be constructed. From this analysis it is determined that the Puelche potentiometric surface is lower than the Epipuelche (water table) in 75% of the ground‐water basin area. Therefore, of the total of the ground‐water basin area (1,600 km2= 620 mi2), the Puelche is recharged by the Epipuelche over 1,200 km2 (465 mi2). In 25% of the area (400 km2= 155 mi2), the Puelche exhibits positive levels relative to the Epipuelche, hence discharging into the latter. Finally, from the analysis of the relation between the vertical transmissivity of an aquitard (T′) and the storage coefficient of a semiconfined aquifer (S) related to the former, it may be concluded that when a considerable difference of ground‐water levels occurs, T’is less or equal in equivalence than S. The mean storage coefficient of the Puelche obtained by pumping tests is 5 × 10−4. If a vertical regional transmissivity equivalent to 5 × 10−4 day−1 is assumed, it would be necessary to have a mean hydraulic head difference of only 5 cm (2 in.) along the remaining 800 km2 (310 mi2), for the Puelche to receive from the Epipuelche 20,000 m3/d, which constitutes its effective recharge. The remaining area (800 km2) appears as a difference between the recharge and discharge areas of the Puelche aquifer. If the mean vertical transmissivity of the aquitard were 5 × 10−5 day−1, the effective recharge of the Puelche would be supplied by the Epipuelche with 50 cm (1.6 ft) of hydraulic head differential. The latter value seems to be more accurate than the former, taking into account the difference of hydraulic head measured in both aquifers. Copyright © 1986, Wiley Blackwell. All rights reserved