Water Protection Prize

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KACTS logo King Abdulaziz City for Science and Technology (KACST)

Contamination of groundwater by nitrate is considered an environmental problem all over the world.  Nitrates are introduced in the ground and surface waters from a variety of sources which include agricultural activities, wastewaters, leaching from solid wastes disposal locations, and dissolution of nitrogen from the atmosphere.  The majority of the population in Saudi Arabia depends on groundwater as the primary source of their drinking water requirements.  However, the massive expansion in agricultural production in the Kingdom leading to the extensive use of nitrogen and organic fertilizers and continued reliance on septic tanks as a means of wastewater disposal has aggravated the nitrate contamination of the groundwater.  Therefore, the problem of nitrate contamination deserves an in depth study to identify the extent of the problem and the measures that are need to be taken to confront it.

The objectives of KACST's project included carrying out a survey of well waters used for drinking purposes in different regions of the Kingdom and their analysis for nitrate content and other physical and chemical parameters, evaluation of seasonal nitrate variations, assessment of drinking water treatment plants in terms of nitrate removal, identification of water quality in water distribution network, correlating the nitrate levels with land use and aquifers, and conducting laboratory and field experiments using ion exchange resins for the purpose of nitrate removal from water.

The study plan included the following activities:

  1. Collection of available information on wells used for drinking purposes in different regions of the Kingdom.
  2. Collection of 1060 water samples from wells used for drinking purposes in different regions of the Kingdom.
  3. Collection of 111 water samples from different locations within 18 water treatment plants in Riyadh, Gasseem, and Hail.
  4. Collection of 197 water samples from water distribution networks in Riyadh, Hail, and Unaizah..
  5. Collection of 153 water samples during one year from nine wells in Riyadh and Gasseem for evaluating seasonal variation in nitrate concentrations in the selected wells.
  6. Conducting laboratory experiments on six different ion exchange resins types and designing and constructing a mini pilot plant in which a total of 21 experiments were conducted to evaluate the effect of regenerant concentration, and duration on the performance of resins.  In addition a pilot plant was designed and constructed in which 90 experiments were conducted using three different types of resin and of waters from three sources with varying concentrations of nitrates.

 

Standard methods were followed in the collection and analysis of water samples.  In addition to nitrate total dissolved solids, total alkalinity, total hardness, calcium, magnesium, potassium, sodium, chlorides, sulphates, iron and manganese were analyzed in the laboratory.  Temperature, pH, conductivity, and turbidity of the water samples were measured at the time of sample collection in the field.  In addition, the locations of wells were recorded using GPS.

The results indicated variation in nitrate levels from 1.1 to 884 mg/L, TDS in the range of 132 to 10082  mg/L and  total hardness in the range of 16 to 4530 mg/L as CaCO3.  The average nitrate levels (mg/L) with respect to the regions were as follows: 65.7 (Jazan), 60.3 (Assir), 60.0 (Gasseem), 51.3 (Hail), 42.0 (Makkah Al Mukarramah), 41.3 (Al Madinah Al Munawarrah), 39.0 (Al Baha), 38.0 (Najran), 30.7 (Tabuk), 25.2 (Al Sharqiah), 18.5 (Riyadh), 15.8 (Al Jouf) and 9.1 (Hudood Shamalyah). 

The results indicated that nitrate levels exceeded the maximum allowable limit in drinking water in a number of wells in all 13 regions.  The maximum percentage of wells exceeding the limits were in Jazan (52%) where as the least were in Hudood Shamalyah (4.9%).  In general, the nitrate levels decreased with an increase in well depth with no clear relationship between TDS, hardness and well depth.  It was noticed that the high nitrate concentrations were found in the shallow wells and in the unconfined aquifers whereas the lowest concentrations were found in Al Wajid Aquifer.

Most of the wells that exceeded the nitrate MCL were located in agriculture-housing areas.  Forty percent of the wells in those areas exceeded the MCL.  In general, the average nitrate levels in wells with respect to the location of wells in descending order were:  agriculture-housing, agriculture, housing-desert, housing, desert, and industrial.  In terms of seasonal variation in nitrate concentrations, relatively small fluctuation was observed in Riyadh wells during one year of samples collection.

With regard to the relationship between nitrate levels and other parameters, a maximum positive correlation was observed between nitrate and calcium in shallow wells whereas a low degree liner correlation was found between NO3 and other major parameters.

The results indicated that all water treatment plants (with the exception of Al Rass WTP) produce water with nitrate levels below the MCL in the range of 1.2 to 23 mg/L with a removal rate of 0.0 to 69.2%.  The WTP with no salt reduction processes gave lower nitrate removal in the range of 0.0 to 2.9% whereas the WTP with RO gave removal of 66.3% on an average.

The nitrate levels in the water distribution network (WDN) were in the range of 1.7 to 15.0 mg/L in Riyadh city, whereas in Hail there were 35 mg/L on average.  In Unaizah, the areas with a direct water supply from the WTP, the average nitrate level was 23.7 mg/L whereas in the areas supplied by a mix of WTP and local wells, the average nitrate level was 37.2 mg/L.  For the areas supplied only by the wells, nitrate level was 28.7 mg/L on an average.

The ion exchange experiments revealed that the capacity of the resins was higher with moderate level of nitrate in the raw water (107 mg/L).  The average quantity of nitrate adsorbed by resin was the highest for the nitrate selective resin, which was 36.1 gmNO3/Kg resin.  Results also indicate that regeneration doesn't provide complete removal of nitrate adsorbed by the resin.

Based on the results of this study, KACST recommended the establishment of a database which would include detailed information on wells and their water quality.  It is also important to control the possible sources of groundwater contamination by enforcing drinking water standards and monitoring agriculture activities, and speed up completion of the sewage system.  It is also recommended to treat the water in areas with nitrate levels exceeding the MCL.  In addition, measures should be taken to close down wells that contain high nitrate concentrations and efforts be made for searching alternate sources of water with acceptable nitrate concentration of those locations.

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