Assessment of nitrate contamination of groundwater using lumped-parameter models
Introduction
Many regions all over the world depend entirely on groundwater resources for various uses (Babiker et al., 2003, Thirumalaivasan et al., 2003). However, the population growth and the increase in demand for water and food supplies place an increasing stress on the groundwater quantity and quality (Joosten et al., 1998, Lewis and Bardon, 1998, Thirumalaivasan et al., 2003, De Santa Olalla et al., 2007, Tait et al., 2008) where over-abstraction depletes the available quantity of groundwater (Ataie-Ashtiani, 2007). In addition, the increase in demand for food supplies may lead to groundwater contamination by nitrate since the major contributor to nitrate contamination in groundwater is the use of nitrogen-based fertilizers associated with cropping activities (Konikow and Person, 1985, Shamrukh et al., 2001, Wolf et al., 2003, Almasri and Kaluarachchi, 2005, Mao et al., 2006, Tait et al., 2008). Elevated nitrate concentrations in drinking water can cause methemoglobinemia in infants and stomach cancer in adults (Lee et al., 1991, Wolfe and Patz, 2002). Because of that the US Environmental Protection Agency (US EPA) has established a maximum contaminant level (MCL) of 10 mg/L NO3-N (US EPA, 2000).
Sources of groundwater contamination by nitrate can be classified into point and non-point sources. Non-point sources of nitrogen include fertilizers, manure application, leguminous crops, dissolved nitrogen in precipitation, irrigation return-flows, and dry deposition. Point sources such as septic systems and cesspits can also be major sources of nitrate pollution (Joosten et al., 1998, Stournaras, 1998, Mitchell et al., 2003, Babiker et al., 2003, Almasri and Kaluarachchi, 2005; Wolf et al., 2003; Santhi et al., 2006, Tait et al., 2008).
Nitrogen applied through fertilizers or manure is converted to plant-available-nitrate by bacteria living in the soil. The growing plants uptake part of this nitrate. The nitrate that is not taken up by crops, immobilized by bacteria into soil organic matter or converted to atmospheric gases by denitrification can leach from the root zone and possibly end up in groundwater (Bhumbla, 1999).
Nitrogen-based fertilizers used on a sandy soil have a high potential to cause nitrate to leach to groundwater when compared to a clay soil. Water moves rapidly through sandy or other coarse-textured soils (Kraft and Stites, 2003, Babiker et al., 2003). The negative charge on the clay particles retains ammonium ions, which prevents ammonia from leaching. Nitrate ions are negatively charged and are not retained by the clay particles.
Overall, groundwater contamination has become a major concern in the recent years (Kalivarapu and Winer, 2008). The Gaza Coastal Aquifer (GCA) is characterized by both quantity and quality problems due to the over-abstraction, excessive fertilization and untreated/poorly treated wastewater disposal (Assaf, 2001, Shomar et al., 2006). GCA is an important source of water to almost 1.5 million residents in Gaza Strip and is utilized extensively to satisfy agricultural, domestic, and industrial water demands (Metcalf and Eddy, 2000, UNEP, 2003). The GCA and the overlying soils are composed mainly of sands, which promote the vulnerability of the GCA to contamination through the high potential of nitrate leaching to groundwater. The groundwater that underlies Gaza City and Jabalia Camp (GCJC) is part of GCA and serves about half a million residents (see Fig. 1 for GCJC area). The groundwater of GCJC area represents a typical coastal aquifer where both over-pumping and the high-density population represent major water quantity and quality problems (see for instance Ataie-Ashtiani, 2007).
In order to simulate nitrate contamination in the groundwater of the GCJC area, two lumped-parameter models (LPMs) were developed. LPMs offer the opportunity to simulate a given system with fewer data requirements for parameterization and calibration compared with their distributed counterparts (Ling and El-Kadi, 1998). The literature is packed with studies that utilized LPMs for the analysis of groundwater systems as in Gelhar and Wilson, 1974, Mercado, 1976, Barrett and Charbeneau, 1997, Ling and El-Kadi, 1998 and Desbarats (2002). For instance, Mercado (1976) developed a single-cell model to study the regional chloride and nitrate pollution patterns in coastal aquifers. Barrett and Charbeneau (1997) developed an LPM for reproducing general historical trends for groundwater levels. Ling and El-Kadi (1998) developed an analytical LPM for the simulation of nitrate leaching from the unsaturated zone in agricultural areas.
Many of the abovementioned studies did utilize LPMs in assessing the efficacy of management options in remedying a situation. For instance, Mercado (1976) utilized a LPM in examining thirteen alternative protection measures to conserve groundwater quality from nitrate contamination. Such measures include advanced treatment of sewage water prior to their recharge to the aquifer, reduction of fertilizer dosage to crops, and exchange of nitrate-contaminated groundwater by low-nitrate surface waters.
The main objective of this paper is to develop LPMs for the simulation of water table elevation and nitrate concentration for the groundwater of GCJC area. The LPM development is depicted conceptually and mathematically. The developed LPMs consider all sources and sinks of water and nitrate and provide the simulated average nitrate concentration for the GCJC area. The LPMs were utilized for the assessment of the effectiveness of potential management options to mitigate the nitrate contamination problem in the GCJC area.
Section snippets
Model development
The mass balance approach was used for both water and nitrate to develop the LPMs. This concept of mass balance implies that the difference between inputs and outputs must equal the change in the storage for the system boundary or model domain (Freeze and Cherry, 1979).
The LPMs are comprised of two key components (models): the quantity (water) and quality (nitrate). Although the development of the nitrate model is the key target, the nitrate model requires the development of the water model and
Model application
The LPMs were implemented in the GCJC area located in the north of Gaza Strip, Palestine (see Fig. 1). The following subsections provide a description of the study area along with an illustration of the development of the mathematical models.
Model calibration
Model calibration was carried out in two stages. In the first stage, the quantity LPM was calibrated. The calibration process was carried out by forcing the model to produce a decline rate in water table elevation similar to the reported rates in the literature (Qahman, 2004). In doing so, the “Goal Seek” option of MS Excel was used to determine the appropriate hydraulic conductivity value after trying different values of wastewater leakage percentage. The calibrated average hydraulic
Management of nitrate contamination of the groundwater of GCJC area
In order to demonstrate the usefulness and applicability of the developed LPMs, the effectiveness of the different management options that aim at reducing nitrate concentration in the groundwater of GCJC area was assessed. Since the objective is to reduce nitrate concentration to the MCL, a management period was proposed such that the MCL would be met by the year 2015.
Summary and conclusions
This work focuses on the utilization of lumped-parameter models for the assessment of nitrate contamination of the groundwater of GCJC located in Gaza Strip, Palestine. Groundwater contamination by nitrate is an on-going problem in GCA and the GCJC area due to the disposal of untreated/poorly treated wastewater, leakage of wastewater from the sewerage system, the existence of heavy agriculture in the surrounding areas, and due to the cesspits. There is an emerging need to manage the nitrate
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