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Contamination of groundwater resources by nitrate leaching under\udagricultural land is probably the most troublesome agriculture-related water\udcontamination worldwide. Contaminated areas often show large spatial\udvariability of nitrate concentration in wells. In this study, we tried to\udassess whether this spatial variability can be characterized on the basis of\udland use and standard agricultural practices. Deep soil sampling (10 m) was\udused to calibrate vertical flow and nitrogen-transport numerical models of\udthe unsaturated zone under different agricultural land uses. Vegetable fields\ud(potato and strawberry) and deciduous orchards (persimmon) in the Sharon area\udoverlying the coastal aquifer of Israel were examined. Average\udnitrate–nitrogen fluxes below vegetable fields were\ud210–290 kg ha−1 yr−1 and under deciduous orchards were\ud110–140 kg ha−1 yr−1. The output water and nitrate–nitrogen\udfluxes of the unsaturated-zone models were used as input data for a\udthree-dimensional flow and nitrate-transport model in the aquifer under an\udarea of 13.3 km2 of agricultural land. The area was subdivided into\udfour agricultural land uses: vegetables, deciduous orchards, citrus orchards,\udand non-cultivated. Fluxes of water and nitrate–nitrogen below citrus\udorchards were taken from a previous study in the area. The groundwater flow\udmodel was calibrated to well heads by changing the hydraulic conductivity.\udThe nitrate-transport model, which was fed by the above-mentioned models of\udthe unsaturated zone, succeeded in reconstructing the average nitrate\udconcentration in the wells. However, this transport model failed in\udcalculating the high concentrations in the most contaminated wells and the\udlarge spatial variability of nitrate concentrations in the aquifer. To\udreconstruct the spatial variability and enable predictions, nitrate fluxes\udfrom the unsaturated zone were multiplied by local multipliers. This action\udwas rationalized by the fact that the high concentrations in some wells\udcannot be explained by regular agricultural activity and are probably due to\udmalfunctions in the well area. Prediction of the nitrate concentration\ud40 years in the future with three nitrogen-fertilization scenarios showed that\ud(i) under the <q>business as usual</q> fertilization scenario, the nitrate\udconcentration (as NO3−) will increase on average by 19 mg L−1;\ud(ii) under a scenario of 25 % reduction of nitrogen fertilization, the\udnitrate concentration in the aquifer will stabilize; (iii) with a 50 %\udreduction of nitrogen fertilization, the nitrate concentration will decrease\udon average by 18 mg L−1

hydrology and earth system sciences

Modeling nitrate from land surface to wells' perforations under agricultural land: success, failure, and future scenarios in a Mediterranean case study