Nitrate and Fluoride Contamination in Soil and Drinking Water in District Mardan, Pakistan

Abstract

Contamination of drinking water with nitrate and fluoride is an issue of global concern. These contaminants are considered a priority in drinking water policy due to their detrimental impacts on health and environment. Long term exposure to high levels of fluoride and nitrate concentration in drinking water produce various ill health effects, affecting millions of people around the globe. These anions are prone to leaching from soil to groundwater and through runoff to surface water bodies. Like many growing economies of the world, Pakistan is an agricultural country with huge dependence on fertilizers (3,184.5, 880.9 and 23.7 thousand tonnes of Nitrogen, Phosphate and Potash respectively) to fulfil the ever-increasing demands for food. In addition, access to clean drinking water, proper sanitation and management of livestock waste are some of the common issues in the rural areas of the country. Keeping in view the potential of nitrate and fluoride contamination of groundwater from natural as well as anthropogenic sources, this study was conducted in an agriculturally intensive district of Khyber Pakhtunkhwa province. The first part of this work explores drinking water quality by analyzing various parameters, and the second part employs soil-related analyses. A total of 100 drinking water (mainly groundwater) and 96 composite soil samples were collected from 25 villages of District Mardan, encompassing four zones through random sampling techniques. Water samples were analysed for pH, electrical conductivity (EC), total dissolved solids, microbial contamination, selected anions (F-, NO3-, NO2-, Cl-, Br-, PO43- and SO42-) and cations (Na+, K+, Ca2+ and Mg2+). Soil samples were analyzed for pH, EC, moisture content, texture, bulk density, particle density, porosity, organic matter, phosphorus, potassium, mineral nitrogen, selected anions (F-, NO3-, NO2-, Cl-, Br-, PO43- and SO42-), and cations (Na+, K+, Ca2+, Mg2+ and Al3+). Groundwater samples of Alo, Dehri, Kunj and Katti Garhi villages in Katlang zone, Fazal Abad, Mayar, Garhi Kapura and Butseri villages in Mardan zone, Cheena, Palu Dehri and Surkhabi villages in Rustam zone, Hussai and Hatyan villages in Takhtbahi zone were positive for E. coli. Concentration of F- was in the range of 0.05 to 10.8 mg/L and a mean value of 0.79 mg/L. Zone-wise different mean values were determined for Katlang (0.37 mg/L), Mardan (1.57 mg/L), Rustam (0.68 mg/L) and Takhtbahi (0.55 mg/L). ANOVA showed significant values for F- and SO42- (p<0.05) in overall samples of the groundwater. Mean concentration of F- was above the permissible limit of 1.5 mg/L in three villages namely Fazal Abad and Garhi Kapura in Mardan zone while Surkhabi village in Rustam zone. In case of NO3- groundwater samples showed a mean value of 22.71 mg/L and a range 0-194.7 mg/L for district Mardan. Takhtbahi zone showed a xi relatively higher mean value of NO3- (29.28 mg/L) followed by Katlang zone (26.12 mg/Kg), Rustam zone (20.60 mg/Kg) and Mardan zone (15.20 mg/L). Two villages Dehri in Katlang zone and Palu Dehri in Rustam zone showed mean concentrations of NO3- beyond the WHO safe value of 50 mg/L. Soil of the district was generally alkaline, Mardan zone showed a high pH of 8.37 followed by Rustam (8.05), Takhtbahi (7.64) and Katlang zone (7.60) respectively. Soil Texture revealed a high content of silt followed by sand and clay. ANOVA showed most significant value (p<0.05) for silt and sand in all zones. Like the rest of the country soil of the study area was organic matter deficient i.e. Katlang zone (1.52%), Mardan zone (1.38%), Rustam zone (1.28%) and Takhtbahi zone (0.91%). Concentration of F- increased with the depth of the soil samples. Pandehri Bala village in Mardan zone has a minimum F- concentration (0.33 mg/Kg) and Butseri village of the same zone has a higher mean value of 4.24 mg/Kg in their soil samples. ANOVA showed significant value (p<0.05) for F- in soil samples. Mean concentration of NO3- was 12.06 for Katlang, 23.32 mg/Kg for Mardan, 22.98 mg/Kg for Rustam and 18.31 mg/Kg for Takhtbahi zone. Unlike F-, the concentration of NO3- decreased with the increasing depth of the soil samples. Soil of Alo village in Katlang zone showed a minimum mean concentration for NO3- (3.83 mg/Kg) and village of Palu Dehri in Rustam zone showed a maximum mean concentration (35.93 mg/Kg). ANOVA showed significant value (p<0.05) for NO3- in all soil samples both zone-wise and depth-wise. Depth-wise analysis of soil indicated that F- was a natural constituent of the parent rocks and NO3- was present due to excessive application of fertilizers especially nitrogenous. Organic matter and clay contents of soil samples revealed no significant correlation with the groundwater concentration of NO3- and F-. Irrespective of the fertilizers application rates, soil profiles of Mardan and Katlang zones showed evidence for greater NO3- retention and hence pose less threat to groundwater sources. On the other hand, soil profile of Mardan and Rustam zones showed more water-soluble F-. It is concluded that groundwater of the district is contaminated with various pollutants from natural and anthropogenic sources. Generally, mean values of the studied anions and cations for each zone were below the permissible levels of WHO and NEQS of Pakistan except F- in Mardan zone and E. coli contamination of groundwater sources in some villages. Regular monitoring of the groundwater sources of the study area is a way forward to protect public health from the associated risks.