10As the world's largest distributed store of freshwater, groundwater plays a central role in 11 sustaining ecosystems and enabling human adaptation to climate variability and change. 12The strategic importance of groundwater to global water and food security will intensify 13 under climate change as more frequent and intense climate extremes (droughts, floods) 14 increase variability in soil moisture and surface water. Here we critically review recent 15 research assessing climate impacts on groundwater through natural and human-induced 16 processes as well as groundwater-driven feedbacks on the climate system.

The great spatial and temporal variability of nitrogen (N) processing introduces large uncertainties for quantifying N cycles in large scales, e.g. a watershed scale, and hence challenges the present techniques in measuring ecosystem N mass balance. The dual isotopes of nitrate (d 18 O and d 15 N) integrate signals for both nitrate sources and N processing, making them promising for studies on large scale N cycling. Here, the dual isotopes, as well as some ion tracers, from a subtropical river in south China were reported to identify the main nitrate sources and to assess the possible occurrence and degree of denitrification in the context of monsoon climate. Our results indicated that nitrification of reduced fertilizer N in soil zones was the main nitrate source, with sewage and manure as another important source in dry winter. Seasonal changes of denitrification was apparent by the *1:2 enrichment of 18 O and 15 N from April to August, and suggested to occur over the watershed rather than in the river. The lowest denitrification (10%) occurred in April, when the fertilizer application was strongest and the monsoon rainfall abruptly increased, causing enhancement of leaching. The highest denitrification (48%) took place in August due to the high soil temperature and moisture. In December, denitrification was significant (26%) perhaps due to the high enough temperature for microbial activities, whereas the low soil moisture appeared to limit the degree of denitrification. This study suggests that the seasonal variations in denitrification should be taken into account when estimating regional N mass balance.

[1] Naturally occurring chemical tracers were used to assess the magnitude of submarine groundwater discharge (SGD) during two different sampling periods at a coastal site south of the Yellow River delta, China. We used salinity and pH as indicators of the terrestrial and recirculated seawater components of discharging groundwater and radium isotopes to quantify offshore transport rates. We then used an hourly time series of multiple radium isotopes ( 224 Ra, 223 Ra, and 226 Ra) to quantify SGD rates and also used 222 Rn and seepage meters to independently quantify SGD rates as a comparison to the radium results. Offshore transport rates were found to range from 3.3 to 4.7 cm s À1 . Modeled time series radium activities indicated average SGD rates ranging from 4.5 to 13.9 cm d À1 in September 2006 and from 5.2 to 11.8 cm d À1 in July 2007. Temporal trends associated with the radium approach agree with SGD patterns revealed by automated seepage meters deployed nearby, but the absolute fluxes are about 70% lower than those determined by the seepage meters. Modeled SGD rates based on 222 Rn (mean = 13.8 cm d À1 in 2006 and 8.4 cm d À1 in 2007) agree with those determined by the radium analysis. Differences in derived SGD rates between the different radium isotopes ( 226 Ra highest; 224 Ra lowest) are likely results of uncertainties in the background activities and our limited selection of appropriate groundwater/pore water end-member values. Scaling our results to the entire Yellow River delta, we find SGD fluxes (and corresponding nitrate fluxes) 2-3 times that of the Yellow River.

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A survey of the quality of groundwater across a broad area of the North China Plain, undertaken in 1998 to 2000, indicates that nitrate pollution is a serious problem affecting the drinking water for a vast population. The use of nitrogen (N)-fertilizer in agriculture has greatly increased over the past 20 years to meet the food needs of the rapidly expanding population. During the study, 295 water samples were collected from wells and springs to determine the water chemistry and the extent of nitrate pollution. High concentrations of nitrate, especially in a recharge area along the western side, but also in the vicinity of Beijing and locally in other parts of the plain, pose a serious problem for the drinking water supply. In places, the nitrate concentration exceeds the maximum for safe drinking water of 45 mg/L. The intense use of N-fertilizer and the widespread use of untreated groundwater for crop irrigation contribute greatly to the problem, but no doubt the disposal of industrial and municipal waste into streams and infiltrating the aquifer also contribute to the problem; however, the lack of data prevents evaluation of those sources. In the recharge area, nitrate is found at depths of as much as 50 m. Near Beijing, relatively high concentrations of nitrate occur at depths of as much as 80 m. In the discharge area, in the vicinity of the Yellow River, high concentrations of nitrate occur at depths of <8 m.ResumØ Une campagne d'Øvaluation de la qualitØ de l'eau souterraine entreprise entre 1998 et 2000 à travers une vaste Øtendue de la plaine du nord en Chine indique que la pollution par les nitrates est un probl me sØrieux qui affecte la qualitØ de l'eau souterraine pour une abondante population. L'utilisation de fertilisants azotØs pour l'agriculture a augmentØ considØrablement durant les 20 derni res annØes afin de rencontrer les besoins alimentaires d'une population à forte croissance. Lors de cette Øtude, 295 Øchantillons d'eau ont ØtØ prØlevØs dans des puits et sources naturelles afin de dØterminer la composition chimique de l'eau souterraine ainsi que l'Øtendue de la pollution par les nitrates. Des concentrations ØlevØes de nitrates, en particulier dans une zone de recharge le long de la limite ouest de la plaine, mais aussi dans la rØgion de Beijing et sporadiquement dans la plaine, posent de sØrieux probl mes à la consommation de l'eau souterraine. A certains endroits, la concentration en nitrates exc de la valeur maximale acceptØe pour la consommation qui est de 45 mg NO 3 /L. L'utilisation intensive de fertilisants azotØs ainsi que l'utilisation rØ-pandue d'eau souterraine non traitØe pour l'irrigation contribuent pour beaucoup au probl me, mais il ne fait pas de doute que le rejet des dØchets municipaux et industriels dans les rivi res et aquif res y est aussi pour quelque chose. Par contre, le manque de donnØs ne permet pas d'Øvaluer la contribution de chacune de ces sources. Dans la zone de recharge, les nitrates se retrouvent à des profondeurs allant jusqu'à 50 m. Pr s de Beijing, ...