Understanding pesticide presence in upper coralline limestone aquifers beneath agricultural lands in Malta : a case study using chlorpyrifos

Abstract

Pesticides are commonly used in agriculture to protect crops from pests and ensure higher yields. These pesticides are formulated from various chemicals to protect crops from harmful pests and help improve yield. In recent years, more focus has been placed on the use of pesticides due to their negative impacts on health and the environment. One of these pesticides known to be harmful to humans is called chlorpyrifos, which was banned by the EU in 2020. After a pesticide is sprayed onto the soil and vegetation, rainfall events can cause these chemicals to run off and accumulate in groundwater sources. In this study, chlorpyrifos, its primary degradation product 3,5,6-trichloro-2-pyridinol (TCP), and supporting water quality parameters were investigated in the Upper Coralline Limestone (UCL) aquifers underlying agricultural land across the northern parts of Malta. Twenty-five water samples were taken from five UCL aquifers, which were three coastal aquifers, and two perched. Samples were taken from springs, boreholes and shafts, locally known as spieri. Parameters measured onsite included pH, temperature, electrical conductivity and redox, while nitrates, nitrites and phosphates were analysed on the same day back in the laboratory. To identify the presence of chlorpyrifos and TCP, a technique known as solid-phase extraction was used, using methanol to obtain 1 mL of sample. This sample was then analysed using UPLC-MSMS. Standards were purchased from Supelco for TCP and chlorpyrifos to prepare standard solutions in the concentration range of 0.05 - 1.00 μg/L. After analysing all standards and samples using UPLC-MS/MS, chlorpyrifos levels in all samples were found to be below the limit of detection (LOD), with a few samples showing no detectable levels. Detection was determined by analysing the areas of multiple reaction monitoring (MRM) peaks from the chromatograms and comparing them to standard reference samples. Although below the LOD, samples from Pwales and Marfa showed the highest presence of the pesticide. Samples from the Rabat-Dingli perched aquifer had MRM signal areas lower than those from Pwales and Marfa. No signal was detected from samples in Buskett (also part of Rabat-Dingli Perched) or Mellieħa Perched aquifer. Further analysis indicated that the coastal aquifers in Pwales had the highest levels of nitrate (NO3N−) and electrical conductivity, exceeding EU threshold limits. Perched aquifers also exhibited elevated nitrate levels, though with lower electrical conductivity. Phosphate and nitrite concentrations were notably low across all 25 samples analysed. The results show that four years after its ban in 2020, chlorpyrifos was not detected at significant concentrations, and none of its degradation products have been detected. This may mean that farmers have not been using the product, or the product is degraded in the unsaturated zone before reaching the saturated zone from where the sampling took place. From the nitrate concentration results, it can be noted that this is a persistent issue in UCL aquifers underlying agricultural lands where monitoring is needed and measures introduced to improve the status of these groundwater bodies.