Salinization of a fresh palaeo-ground water resource by enhanced recharge.

Deterioration of fresh ground water resources caused by salinization is a growing issue in many arid and semi-arid parts of the world. We discuss here the incipient salinization of a 10(4) km2 area of fresh ground water (<3,000 mg/L) in the semiarid Murray Basin of Australia caused by widespread changes in land use. Ground water 14C concentrations and unsaturated zone Cl soil water inventories indicate that the low salinity ground water originated mainly from palaeo-recharge during wet climatic periods more than 20,000 years ago. However, much of the soil water in the 20 to 60 m thick unsaturated zone throughout the area is generally saline (>15,000 mg/L) because of relatively high evapotranspiration during the predominantly semiarid climate of the last 20,000 years. Widespread clearing of native vegetation over the last 100 years and replacement with crops and pastures leads to enhancement of recharge rates that progressively displace the saline soil-water from the unsaturated zone into the ground water. To quantify the impact of this new hydrologic regime, a one-dimensional model that simulates projected ground water salinities as a function of depth to ground water, recharge rates, and soil water salt inventory was developed. Results from the model suggest that, in some areas, the ground water salinity within the top 10 m of the water table is likely to increase by a factor of 2 to 6 during the next 100 years. Ground water quality will therefore potentially degrade beyond the point of usefulness well before extraction of the ground water exhausts the resource.

Hydrogen-isotope composition of leaf water in C3 and C 4 plants: its relationship to the hydrogen-isotope composition of dry matter.

The natural abundance hydrogen-isotope composition of leaf water ([Formula: see text]) and leaf organic matter (δ D (org) ) was measured in leaves of C3 and C4 dicotyledons and monocotyledons. The [Formula: see text] value of leaf water showed a marked diurnal variation, greatest enrichment being observed about midday. However, this variation was greater in the more slowly transpiring C4 plants than in C3 plants under comparable environmental conditions. A model based on analogies with a constant feed pan of evaporating water was developed and the difference between C3 and C4 plants expressed in terms of either differences in kinetic enrichment or different leaf morphology. Microclimatic and morphological features of the leaves which may be associated with this factor are discussed. There was no daily excursion in the δ D (org) value in leaves of either C3 or C4 plants. When δ D (org) values were referenced to the mean [Formula: see text] values during the period of active photosynthesis, the discrimination against deuterium during photosynthetic metabolism (ΔD) was greater in C3 plants (-117 to -121‰) than in C4 plants (-86 to -109‰).These results show that the different water use "strategies" of C3 and C4 plants are responsible for the measured difference in deuterium-isotope composition of leaf water. However, it is unlikely that these physical processes account fully for the differences in hydrogen-isotope composition of the products of C3 and C4 photosynthetic metabolism.