WATER RECHARGE AND SOLUTE TRANSPORT THROUGH THE VADOSE ZONE OF FRACTURED CHALK UNDER DESERT CONDITIONS

被引:122
作者
NATIV, R
ADAR, E
DAHAN, O
GEYH, M
机构
[1] BEN GURION UNIV NEGEV,JACOB BLAUSTEIN INST DESERT RES,SEDE BOQER,ISRAEL
[2] BEN GURION UNIV NEGEV,DEPT GEOL & MINERAL,SEDE BOQER,ISRAEL
[3] STATE GEOL SURVEY LOWER SAXONY,HANNOVER,GERMANY
关键词
D O I
10.1029/94WR02536
中图分类号
X [环境科学、安全科学];
学科分类号
08 ; 0830 ;
摘要
This study focuses on water flow and solute migration through unsaturated fractured chalk in an arid area. The chalk underlies a major industrial complex in the northern Negev desert, where groundwater contamination has been observed. Four dry-drilling holes were bored through the vadose zone. Core and auger samples, collected at 30- to 50-cm intervals, were used for chemical and isotopic analyses, enabling the construction of the following profiles: (1) a tritium profile, to estimate the rate of water flow through the unsaturated zone; (2) oxygen 18 and deuterium profiles, to assess the evaporation of water at land surface before percolation, and in the upper part of the vadose zone after infiltration; and (3) chloride and bromide profiles, as tracers for inert solutes and pollutants. The tritium and bromide profiles showed the rate of infiltration through the unsaturated matrix to be very slow (1.6-11 cm/yr). The chemical and isotopic data from the core holes suggested that the pore water changes characteristics with depth. Close to land surface, the pore water is strongly evaporated (delta(18)O = +5.94 parts per thousand) and highly concentrated (similar to 29 meg Cl/100 g rock), but changes gradually with depth to a more dilute concentration (similar to 4 meg Cl/100 g rock) and isotopically depleted composition (delta(18)O = -4.4 parts per thousand), closer to the isotopic composition of precipitation and groundwater. Nearby monitoring wells have shown anthropogenic contribution of heavy metals, organic compounds, and tritium (Nativ and Nissim, 1992). A conceptual model is proposed in which a small portion of the rainwater percolates downward through the matrix, while a larger percentage of the percolating water moves through preferential pathways in fractures. The water flowing through the fractures penetrates the matrix across the fracture walls, where it increases the tritium concentrations, depletes the stable isotopic composition, and dilutes the salt concentrations. The observed rapid downward migration of tritium and heavy metals through the profuse fractures makes the chalk inefficient as a hydrologic barrier.
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页码:253 / 261
页数:9
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