Since Horton in 1965, many authors have sought to aggregate different variables characterizing the state of water into a single value called Water Quality Index ( $WQI$ ). This index is intended to facilitate the operational management of water resources and their allocation for different uses. Detailed and operational description of the main $WQI$ calculations are here reviewed. The review contains: (1) an historical analysis of the evolution of $WQI$ calculation methods by looking both at the choice of variables, the methods of weighting and aggregating these variables into a final single value; (2) an illustration of the contradictions observed in the final result when, on the same database, the $WQI$ is calculated by different methods; (3) the significant progress possible via fuzzy logic to define a $WQI$ adapted to specific water use.

By combining a crop model (STICS) and a geochemical model (PHREEQC), a new approach to assess the sustainability of agrosystems is proposed. It is based upon aqueous geochemistry and the stepwise modifications of soil solution during its transfer from the surface till aquifer. Meadows of Crau (SE France), irrigated since the 16th century, were field monitored (2012-2015) and modelled. Except for N, the mineral requirements of hay are largely covered by dissolved elements brought by irrigation water with only slight deficits in K and P, which are compensated by P-K fertilizers and the winter pasture by sheep. N cycle results in a very small nitrate leakage. The main determinants of the chemical composition changes of water are: concentration by evaporation, equilibration with soil pCO2, mineral nutrition of plants, input of fertilizers, sheep grazing, mineral-solution interactions in superficial formations till the aquifer, including ion exchange. Inverse modelling with PHREEQC allows for quantifying these processes. For groundwater, measured composition fit statistically very well with those computed, validating thus this approach. This long-term established agrosystem protects both soil and water resources: soil nutritional status remains constant with even some P and (minor) K fixation in soils; long-term decarbonatation occurs but it is greatly slowed by saturation of irrigation water by carbonate; P fixation in soil protects groundwater from eutrophication.

Fougerite (IMA 203-057), from green rust (GR) group, is difficult to quantify due to its reactivity and its small concentration in soils and sediments. Chemical extractions with citrate-bicarbonate (CB) reagent, in kinetic mode, can be used for a pre-diagnosis. Performed by steps (0, 1, 6, 48, 168 and 504 h), the proposed protocol was applied on samples from Gleysol of Fougère’s forest with mineralogical controls by Mössbauer and XRD (X-ray diffraction) after each step of extraction. In less than 6 h, the first fraction extracted is composed of 70% Si, 80% Al, 23% Fe and 80% Mg of total element extractable by CB and is ascribed to the “indefinable mineral mixture Si-Al-Fe” named by Tamm. Between 6 and 168 h, the second fraction extracted is composed of Fe and Mg with a constant mole ratio Fe/Mg equal to 10 and is ascribed to the fougerite-GR phase. Analysis of XRD pattern and of Mössbauer spectra confirms: (i) all the other mineral phases containing Al, Mg, Si were not dissolved by CB after 6 h; (ii) the CB treatment extracts fougerite-GR completely. The residual fraction is composed of components not dissolved by CB extraction. Thus, the selectivity of CB can be used to quantitatively estimate the presence of fougerite-GRs in soils and sediments.

Rice is cultivated as staple for over half of the World’s population. In Camargue (South of France) rice fields have been established on very young soils developed from historic fluvial deposits of the Rhône River. The comparison of clay mineralogy in a paddy field cultivated for 60 years and in a control shows a significant increase of the clay crystallinity in the paddy field soil, which implies a decrease of their solubility. In the paddy soils, phytoliths, poorly crystallized clays, such as smectite and to a lesser extent kaolinite, are progressively dissolved to supply Si for rice requirements. The sustainability of the crop system requires the clearing of silica exportations.

Facing global changes and the challenge of food security, scientists are being questioned by decision-makers and stakeholders on the sustainability of agro-systems. The main difficulty in dealing with this question is to obtain enough data over long periods of time. Monitoring slow drifts and weak noises is needed to forecast tipping points that can jeopardize the present steady state. High-resolution datations by radiocarbon coupled with detailed palynological determinations in sediments, historical archives on yields and crop quality, and high-frequency field in situ measurements give information on climatic changes from multi-secular to seasonal and hourly time scales. In the long term, climatic forcing dominates agriculture performance, at that time only organic agriculture, with oscillations between prosperity and misery driven by climate and intermediated by civilization flourishing and collapsing; in the medium term, in modern agriculture, irrigation provides a provisional buffering effect on yield and crop quality despite present warming; in the short term, either under non-fertilized forested ecosystem or intensive rice cropping, the same patterns are evidenced and point to the importance of soil microflora shifting from aerobiosis to anaerobiosis. In all cases, geochemistry offers appropriate tools to decipher the climate-soil-agriculture complex interplay.