Heavy metals in Maltese agricultural soil

Abstract

Soil pollution has increased over the last few decades due to anthropogenic sources. Heavy metal pollution has been of great concern since it has been noted that these contaminants have entered our food chain. High concentration of heavy metals can adversely affect the health of the public wellbeing. The aim of the study was to evaluate whether i) heavy metals found in soil are present in high amounts; ii) heavy metals found in soil are present differently across Malta and Gozo; iii) there are soil limits regulating heavy metals. The soil samples were analysed with an X-Ray Fluorescence Spectrometer using the sample cup method. The method of analyses was validated by comparing it to the results obtained in an external laboratory. The samples were analysed to determine the concentration of heavy metals, namely aluminium, vanadium, chromium, cobalt, manganese, nickel, copper, zinc, molybdenum, arsenic, silver, cadmium, selenium, mercury and lead in soil. Samples were collected from five districts in Malta and from Gozo. A total of 103 samples were collected and analysed, out of which two samples were collected from organic farms and used as controls. Samples were analysed in triplicate and an average was calculated. Most of the soil samples tested showed that the heavy metals found in the soil were observed to be present at higher concentrations in the South-Eastern District and the Southern Harbour. Concentrations of the heavy metals tested were mapped out according to district and localities. The highest concentration of aluminium was found in the Western District at 38106.00 mg/kg. The highest concentration of lead was observed in the Southern Harbour and was statistically significantly (ρ-value<0.05) different from the rest of the districts with a concentration of 249.51 mg/kg. Other metals that were found at their highest concentrations in the Southern Harbour, were copper at 95.15 mg/kg and zinc at 251.33 mg/kg. The heavy metals vanadium at 64.25 mg/kg, chromium at 45.93 mg/kg, manganese at 586.67 mg/kg, nickel at 21.90 mg/kg and selenium at 15.51 mg/kg, to be observed at their highest concentrations in the South-Eastern District when compared to the other districts. The highest concentrations of molybdenum at 12.09 mg/kg, arsenic at 7.93 mg/kg and cadmium at 30.10 mg/kg were observed in the Northern District. Cobalt was seen with the highest concentration in the Gozo and Comino District, at 15.83 mg/kg. The Finland and Dutch standards were used to compare the results observed. Lead was found to exceed the threshold limits of the Finland standard which is 60 mg/kg and the Dutch standard which is 85 mg/kg. Cadmium was observed to exceed the threshold value of the Finland standard which is 1 mg/kg and the Dutch standard which is 0.8 mg/kg. The Finland standard concentration of arsenic is 1 mg/kg, which was much lower than the concentration observed in the Northern District. Zinc was found to exceed the threshold limits of the Finland standard which is 200 mg/kg and the Dutch standard value which is 50 mg/kg. The lower concentration of heavy metals observed in organic farms may be attributed to the lack of chemical use whereby instead biodiversity is applied to aid in pest and weed reduction. There is a need for the implementation of methods to address the high concentrations of heavy metals. Such methods which can be applied include phytoremediation, intercropping and organic farming.