Abstract
Abstract. Saharan paleo-groundwater from the Hasouna area of Libya contains up to 1.8 mM of nitrate, which exceeds the World Health Organization limit for drinking water, but the origin is still disputed. Herein we show that a positive 17O excess in NO3− (Δ17ONO3 = Δ17ONO3 − 0.52 δ18ONO3) is preserved in the paleo-groundwater. The 17O excess provides an excellent tracer of atmospheric NO3−, which is caused by the interaction of ozone with NOx via photochemical reactions, coupled with a non-mass-dependent isotope fractionation. Our Δ17ONO3 data from 0.4 to 5.0 ‰ (n = 28) indicate that up to 20 mol % of total dissolved NO3- originated from the Earth's atmosphere (x[NO3−]atm), where the remaining NO3− refers to microbially induced nitrification in soils. High Δ17ONO3 values correspond to soils that are barren in dry periods, while low Δ17ONO3 values correspond to more fertile soils. Coupled high Δ17ONO3 and high x[NO3−]atm values are caused by a sudden wash-out of accumulated disposition of atmospheric NO3− on plants, soil surfaces and in vadose zones within humid–wet cycles. The individual isotope and chemical composition of the Hasouna groundwater can be followed by a binary mixing approach using the lowest and highest mineralised groundwater as end members without considering evaporation. Using the δ34SSO4 and δ18OSO4 isotope signature of dissolved SO42−, no indication is found for a superimposition by denitrification, e.g. involving pyrite minerals within the aquifers. It is suggested that dissolved SO42− originates from the dissolution of CaSO4 minerals during groundwater evolution.
Highlights
The accumulation of nitrate (NO−3 ) in groundwater is a wellknown phenomenon occurring worldwide (Clark and Fritz, 1997; Kendall, 1998)
In the present study we used an integrated hydrogeochemical and isotope approach to trace the origin of NO−3 and to reconstruct the paleoclimatic conditions during recharge of Saharan groundwater in the area of Hasouna, Libya
We applied the outstanding feature of 17ONO3 values to trace atmospheric NO−3, as the 17O excess of dissolved NO−3 is not affected by terrestrial fractionation processes (MDF) (Kendall and Doctor, 2004)
Summary
The accumulation of nitrate (NO−3 ) in groundwater is a wellknown phenomenon occurring worldwide (Clark and Fritz, 1997; Kendall, 1998). Individual NO−3 sources and mechanisms for its accumulation depend strongly on the environmental conditions during recharge, infiltration, and aquifer storage. High NO−3 concentrations of paleo-groundwater from the Hasouna area (Libya) have been measured for decades, but the NO−3 origin is still hotly debated (El-Baruni et al, 1985; Milne-Home and Sahli, 2007). Deciphering the source of NO−3 for Saharan groundwater in Libya is highly challenging, as (i) the present arid conditions preclude appreciable recharge and (ii) groundwater is about 90 % of the source of the water supply of Libya Several million m3 of fresh water per day are transferred to the Mediterranean for agricultural and domestic use including drinking water supply
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