STAGNANT AQUIFER CONCEPT .3. STAGNANT MINIAQUIFERS IN THE STAGE OF FORMATION, MAKHTESH-RAMON, ISRAEL

Observation wells in the arid Ramon National Geological Park were moni tored at three sites (C, T, and E), each with wells spaced tens of met ers apart, the distances between the sites being 860-1060 m. The follo wing characteristics were observed: (1) water level differences of up to a few meters occur between wells that are 20-60 m apart at each sit e, and level differences of over 10 m occur between wells at the three sites, (2) the water levels were steady over 2 years of monitoring, ( 3) Cl concentrations in the wells vary over three orders of magnitude: 0.4-1.8 g l(-1) in site C, 1.8-3.7 g l(-1) in site T, and 2.5-13 g l( -1) in site E, (4) the relative abundance patterns of the dissolved io ns vary remarkably along with the concentration variations, from water dominated by Ca and SO4 to water dominated by Na and Cl, (5) the chem ical composition is steady in each well, as indicated by chemical anal yses repeated over 6 years, and conductivity monitoring over 2 years. The observed significant and steady compositional differences are inte rpreted as indicating the existence of small-scale isolated aquifers, with no lateral flow between them in spite of the pronounced water lev el differences. Flow in fractures is ruled out because of the steady w ater levels and very slow water level recovery in pumped wells. Dating strongly confirmed the lack of flow in the investigated shallow syste m. A well situated in the flood plain of Nahal Ramon revealed a tritiu m concentration of 8 TU and C-14 concentration of 69 p.m.c. - indicati ng post-1954 water, but two wells outside the river bed contained no m easurable tritium and C-14 values of 14 and 22 p.m.c. - indicating wat er ages in the order of 10 000 years. In 15 additional wells no measur able tritium was determined. Thus, a case of ancient groundwater trapp ed in shallow aquifers, and bounded by efficient hydraulic barriers, i s demonstrated. The geological set up seemed rather homogeneous, with no clue to small-scale efficient hydrological barriers. Yet, a detaile d geological study, conveyed in the light of the hydrochemical finding s, revealed the existence of dykes and small-scale structural and lith ological controls, coinciding with some of the hydraulic discontinuiti es defined by the water properties.