Tóm tắt
Hypotheses to explain the source of the 1011 tons of
salt in groundwaters of the Murray Basin, south-eastern Australia, are
evaluated; these are (a) mixing with original sea water,
(b) dissolution of salt deposits,
(c) weathering of aquifer minerals and
(d) acquisition of solutes via rainfall. The total
salinity and chemistry of many groundwater samples are similar to sea-water
composition. However, their stable isotopic compositions
(δ18O= –6.5 ‰;
δ2H = –35) are typical of mean winter
rainfall, indicating that all the original sea water has been flushed out of
the aquifer. Br/Cl mass ratios are approximately the same as sea water
(3.57 x 10-3) indicating that NaCl evaporites (which
have Br/Cl<10-4) are not a significant
contributor to Cl in the groundwater. Similarly, very low abundances of Cl in
aquifer minerals preclude rock weathering as a significant source of Cl. About
1.5 million tons of new salt is deposited in the Murray–Darling Basin
each year by rainfall.The groundwater chemistry has evolved by a combination
of atmospheric fallout of marine and continentally derived solutes and removal
of water by evapo-transpiration over tens of thousands of years of relative
aridity. Carbonate dissolution/precipitation, cation exchange and
reconstitution of secondary clay minerals in the aquifers results in a
groundwater chemistry that retains a ‘sea-water-like’ character.
