Transport of Nitrate and Its Fate in an Unconfined Sandy Aquifer of Gareh-Bygon Plain, Islamic Republic of Iran

In response to the growing awareness of groundwater and surface water degradation, there has been increased concern about NO3ˉ mobility and retention in soils. Artificial recharge of groundwater (ARG) through floodwater spreading systems (FWSS) is conducted in the Gareh-Bygon Plain (GBP), south east of Iran, to facilitate both drinking and irrigating water for desert dwellers. However, floodwater used for ARG, contains 60.30 mg L-1 NO3ˉ on average and may contaminate groundwater. The main objective of this study was to identify the most suitable place for extracting safe drinking water supply in the GBP. Identifying natural sources and main flow pathways of NO3ˉ, impacts of different landuse on the groundwater NO3ˉ concentration, removal processes of NO3ˉ, and predicting NO3ˉ transport using suitable computer model, were specific objectives of this study. In the first study, 30 random geologic materials from the BZB were sampled and analyzed for NO3- concentration. This was carried out for atmospheric deposition during 27 rainfall events. Moreover, surfacial floodwater and incoming groundwater as flow pathways into the aquifer, were monitored monthly and analyzed for NO3ˉ, NH4+, dissolved O2 (DO), total organic carbon (TOC), Na+, K+, Ca+2, Mg+2, Fet, Mn+2, Cl-, SO4-2, CO3-2, HCO3-, alkalinity, EC and pH. Results showed NO3- concentration in the geologic materials ranged from 0.94 to 123.31 mg kg-1, however, that of the atmospheric deposition ranged from 0.88 to 19.12 mg L-1. The concentrations of NO3- in the incoming groundwater ranged from 1.50 to 39.94 mg L-1. Predominance of the oxidizing condition supports NO3ˉ stability in the flow pathways and in the aquifer as well. As a consequence, geologic materials and atmospheric deposition were not responsible for the serious NO3ˉ concentrations in the BZ groundwater. The impact of different landuse on the groundwater NO3ˉ concentration was investigated during the 12 months monitoring for the 2nd study (from 2003 to April 2004). Samples were analyzed for NO3ˉ, NH4+, DO , TOC, Na+, K+, Ca+2, Mg+2, Fet, Mn+2, Cl-, SO4-2, CO3-2, HCO3-, alkalinity, EC and pH. Groundwater NO3ˉ concentration ranged from 0.02 mg L-1 in the recharge area to 94.45 mg L-1 in the farming area which was higher than the permitted level of that ion in drinking water by United States Environmental Protection Agency (USEPA). Both farming areas, upstream and downstream the FWSS, increased NO3ˉ concentration in the groundwater. However, recharge flow decreased groundwater NO3ˉ through FWSS. Consequently, recharge area was found to be the most suitable and supply safe drinking water to inhabitants. In the 3rd study, the possible NO3ˉ removal processes in the GBP was investigated through; i) absorption, ii) adsorption, and iii) dilution. Planted and non-planted packed leaching columns in saturated and semi-saturated conditions were examined in the laboratory and in the open field, respectively. Results for semi-saturated condition showed the average NO3ˉ concentration in the planted leachate fractions (4.15 mg/L) was 6.54 times less than that of the control (27.15 mg/L). Breakthrough curve (BTC) obtained for saturated columns was asymmetric. Small average retardation factors, R = 2.38 and R ≈ 0.00, for planted and non-planted columns, respectively, suggested non-significant NO3- absorption by seedlings in saturated condition. This implies the potential of the Eucalyptus forested area to take NO3- up from the recharge water flow in the real FWSS. Batch adsorption isotherms and envelope tests were carried out for soil, sediment, and pure palygorskite as a vertical translocated clay species into the forested Eucalyptus rhizosphere. Results of all adsorption isotherms were best fitted to the Langmuir equation. Maximum NO3- adsorption occurred at pH < (PZC = 6.53) of the soil. Dilution was documented as the most effective NO3- removal process in the BZ aquifer using the average NO3-/ Cl- concentrations. The HYDRUS-1D was found to be a useful software for predicting vertical nitrate transport through saturated soil column. Nitrate BTC obtained from leaching columns and simulated data were compatible in general. Nitrate distribution pattern throughout the BZ aquifer was also simulated successfully using a MODFLOW_PMPATH computer code. As the main result of this study, it was found that the most suitable drinking water for the rural people in the BZB is the water extracted from wells located in the forested recharge area. However, the most degraded water was found in the vicinity of the farming areas. Keywords: Nitrate pollution, Drinking water, Denitrification, Artificial recharge, Floodwater spreading system, HYDRUS model, MODFLOW model.

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