Monitoring nutrient dynamics in runoff following rainfall events in an agricultural catchment

Abstract

The Maltese Islands are critically vulnerable to freshwater nutrient contamination from agricultural runoff. High nutrient concentrations in surface runoff can adversely affect the environment, groundwater quality, agricultural yields and human health. Since the entirety of Malta is designated as a Nitrate Vulnerable Zone with limited natural freshwater sources, monitoring and managing surface water and soil quality is critical to prevent the deterioration of existing freshwater resources. This study assesses the temporal and spatial variation in surface water nutrient concentrations within one of Malta’s key agricultural catchments; Wied il-Għasel. Runoff water samples were collected during Malta’s early wet season, between September and February. The nutrient concentration of runoff was measured in the laboratory using photometric determination, while other water quality parameters - including pH, electrical conductivity and temperature - were measured in-situ. Soil samples from four agricultural plots near the runoff monitoring stations were also analysed to determine how land cover and local contamination sources contribute to runoff nutrient levels. The findings indicate severe nitrate contamination within the catchment’s surface waters, with concentrations reaching as high as 331 mg/L NO3 - . This is primarily driven by excessive fertiliser application on the surrounding fields, which is reflected in the high nitrate levels of the sampled agricultural soils. Nutrient concentrations shifted between upstream and downstream locations, varying based on land cover contributions and rainfall-runoff events. Ultimately, this study highlights the need for ongoing freshwater monitoring and stricter regulation of agricultural practices to minimise the contamination of Malta’s vulnerable freshwater resources.