Elevated nitrate levels in the groundwater of the Gaza Strip: Distribution and sources
Introduction
Groundwater is one of the most precious natural resources in the Gaza Strip as it is the only source of drinking water for the majority of the population. The groundwater aquifer of Gaza is extremely susceptible to surface-derived contamination because of the high permeability of sands and gravels that compose the soil profile of Gaza (Shomar et al., 2005a). Several studies in Gaza reported high nitrate (NO3−) levels in groundwater as one of the major concerns among the public and governmental decision makers (Abu Maila et al., 2005, Shomar, 2006), but these studies did not include a scientific evaluation of source of NO3− nor their implications on the health of the population in Gaza.
Nitrate in contaminated water is known to cause methemoglobinemia in infants (AAP, 1970, Avery, 1999, Cynthia et al., 2002). The association of diarrhea and acidosis with methemoglobinemia is more common than previously thought and can produce dangerously high methemoglobin levels (Lebby et al., 1993, Gebara and Goetting, 1994, Kim et al., 1997, Avery, 2001, Freishtat et al., 2005, Venkateswari et al., 2007). Recent observations revealed a high positive correlation between the concentrations of NO3− (> 80 mg/l) in groundwater of the Gaza Strip and the occurrence of methemoglobinemia in babies younger than six months of age. Among 640 babies tested in Gaza, 50% showed signs of methemoglobinemia in their blood samples (Absi and Shomar, unpublished data). Additionally, one study suggested a potential link between high NO3− in drinking water and gastrointestinal cancer (Morales-Suarez-Varela et al., 1996). Nitrates could combine with amines in the body to form N-nitroso compounds that are known cancer causing agents. However, this association is controversial (Weyer et al., 2001).
Contamination of the groundwater can occur if input of NO3− into soil exceeds the consumption of plants and denitrification (Mcclain et al., 1994). Shomar (2006) proposed that the excess NO3− in the groundwater of the Gaza Strip occurred as a result of NO3− leaching from irrigation, wastewater septic tanks, sewage sludge, animal manure and synthetic fertilizers.
NO3− and nitrite (NO2−) are naturally occurring inorganic ions, which are part of the nitrogen (N) cycle. Microbial action in soil or water decomposes wastes containing organic nitrogen first into ammonia, which is then oxidized to NO2− and NO3− (Mcclain et al., 1994). Because NO2− is easily oxidized to NO3−, NO3− is the compound predominantly found in groundwater and surface waters under oxidizing conditions. Contamination with N-containing fertilizers, including anhydrous ammonia, as well as animal or human natural organic wastes, can raise the concentration of NO3− in groundwater (Thorburn et al., 2003). NO3−-containing compounds in the soil are generally soluble and readily migrate into groundwater (Rzan et al., 2001, Wakida and Lerner, 2002).
Nitrogen isotopes have been used to identify sources of NO3− contamination in groundwater (Brandes et al., 1996, Kendall, 1998, Ogawa et al., 2001). Different sources of NO3− and organic matter often have distinctive isotope “fingerprints” that can provide a better understanding of the sources of NO3− than more commonly used mass balance “black box models” (Kendall, 1998).
The main objectives of this study were: (1) to determine the average levels and distribution of NO3− in the groundwater of the Gaza Strip, and (2) to assess N and O isotopes to identify the major sources of NO3− that may contaminate groundwater.
Section snippets
Study area
The Gaza Strip is one of the most densely populated areas in the world (2638 people per km2; PCBS, 2000). For administrative purposes, the government divides the Gaza Strip into five regions (Fig. 1): North, Gaza, Middle, Khan Younis and Rafah. Approximately 85% of the population of the Gaza Strip drink from municipal groundwater wells and 15%, mostly in agricultural areas, use private wells to supply their drinking water (Shomar, 2006).
The study area is a part of the coastal zone in the
Nitrate in groundwater
The detailed results of general groundwater geochemical characteristics are given in Shomar (2006), whereas NO3− concentrations and related parameters from samples collected from 2006 to 2007 are presented in this study. NO3− concentrations in the Gaza aquifer ranged from 31 to 452 mg NO3/L. Almost 90% of the sampled wells had levels of NO3− above the World Health Organization's (WHO) standard of 50 mg/L and only 10 of the 115 sampled wells were at or below the WHO standard. These nitrate
Discussion
The findings of this study are consistent with earlier surveys conducted by the Palestinian water authority (PWA, 2001) where 90% of the wells had NO3− concentrations exceeding the WHO's maximum permissible drinking water limit of 50 mg/L NO3−. Since 2001, four of the groundwater sampling campaigns spanned the summer months of August–September and three in winter (January–March). The average concentration of nitrate in domestic wells is 128 mg/L in summer and 118 mg/L in winter, and for the
Conclusions
- 1.
Almost 90% of the groundwater wells of the Gaza Strip sampled between 2001 and 2007 showed NO3− concentrations two to eight times higher than the WHO standards.
- 2.
The δ18O and δ15N isotopic composition of NO3− revealed that manure and septic effluents are the main sources of NO3− in the groundwater of Gaza followed by sludge and synthetic fertilizers. Provided N compounds were well mixed in human waste before separation of sludge and septic effluents, the low δ15N values of sludge indicate that
Acknowledgments
The authors thank the staff of the “Environment and Information Center at Gaza Governorate” for collecting the needed samples and the field measurements. We thank Prof. Dr. R. Schuiling, Prof. Dr. Tim H.E. Heaton, Prof. Dr. Len Wassenaar, Dr. Ulrike Schulte, Dr. Arnold van Dijk, Mr. Sami Abu Fakher and Dr. Juliet VanEenwyk for their important and continuous efforts. This research was funded by the Federal Ministry of Education and Research, BMBF, Germany (Förderkennzeichen: 02WT0534).
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