Author: Idit Sofer

Abstract

Israel's coastal aquifer is a major water resource and there are many agricultural areas on the land overlying it. During the last decades many wells were disqualified for drinking water due to high nitrate (NO3-) concentration, above Israel's standard for nitrate in drinking water – 70 mg/l. Many of these wells are located in the Sharon region at the center of the coastal plain of Israel. In this area there are agricultural activities for many years on Hamra soils (Mediteranien red loamy-sand soils). High spatial variability in nitrate concentration is observed in groundwater wells in this area over small length scales, (i.e. hundreds of meter between wells with high nitrate-concentrations and wells with low concentrations). Inefficient nitrogen fertilization cause low intake to the roots and nitrogen leachate towards the deep unsaturated zone and groundwater, mainly as nitrate -nitrogen (NO3-N). This research examined the relationship between well's nitrate-concentration and the agricultural land-use in agricultural land in the Sharon area. Nitrate fluxes towards groundwater from three main crops in this area were used for assessment of nitrate in groundwater: 1) Citrus orchards (from Shapira 2012); 2) Persimmon orchard representing plantations of deciduous trees; 3) Potatoes and strawberry fields representing vegetables crops. The fields that were sampled have had a continuous agricultural activity of the same crop for decades. For each field, data on irrigation and fertilization regimes and irrigation water quality was collected. In each field three "Direct Push Drilling" were drilled and the top 10 meters of the unsaturated zone was sampled and standard soil analyses were carried out (bulk density, particle size distribution, water content, chloride in soil water extraction, NH4 and NO3 in KCl-solution extraction). Groundwater recharge and nitrate-nitrogen fluxes to the aquifer were calculated assuming steady-state by Chloride Mass Balance (CMB). For one representative sampling-hole in each field a 1-dimensional transient numerical model of water flow and nitrogen transport (HYDRUS-1D) was constructed and calibrated.
The average recharge flux toward the aquifer under vegetables and deciduous was found to be 300-400 mm/year from both CMB and multi-year averages of the numerical models.

Nitrate-Nitrogen flux was found on 100-200 kg/hectare/year in deciduous and 200-300 kg/hectare/year in vegetables.
In the second part of the work water flow and nitrate transport models in the aquifer (MODFLOW and MT3DMS, respectively) were constructed and calibrated below a 13.5 KM2 agricultural area in the Sharon region. The top boundary conditions of the groundwater models were the fluxes of water and nitrate which were outputs of the unsaturated zone flow and transport models. These were extended to the actual depth of the unsaturated zone (1 m resolution) for each grid's cell of the groundwater models. Hydraulic head and nitrate concentration data of the wells in this area were obtained from the Israeli Hydrological Service (IHS) and used for constructing the side boundary-conditions and for the calibration of the models. Using 1 characteristic model for the unsaturated zone for each land-use enabled to predict the average concentration of nitrate in the wells but it could not explain the high concentrations in the most contaminated wells. Therefore the nitrate fluxes from the unsaturated zone were multiplied by the factors of:10 (1% of the area), 5 (3% of the area), 2.8 (4% of the area) 1 (55% of the area), 0.6 (19% of the area) and 0.1 (18% of the area) keeping the same total nitrate flux for the model area . Only after this second stage of calibration, the model was able to produce the high concentration and the spatial variability of nitrate in groundwater with good fit to data.
The calibrated model was used for simulations of 40 years to the future (2012-2052) in a three fertilization scenarios: 1) 100% (regular fertilization, similar to the present); 2) 75% of the present fertilization applied; and 3) 50% of the present. In the first “business as usual” scenario the nitrate concentration in the aquifer will keep increasing and more wells will be disqualified as drinking water source (only 1 well in the modeled area will remain below standard by 2052). In the second scenario, the current nitrate conditions in the aquifer are expected in 40 years. In the third scenario (50% reduction in nitrogen fertilization), the nitrate concentration in the wells will decrease, the aquifer will rehabilitate and more well's than now days will have nitrate concentrations below the standard of 70 mg/l.