The Bilbao parish, located on the slopes of the Tungurahua volcano (Ecuador), was heavily impacted by ashfall during eruptions between 1999 and 2016. Volcanic ash may contain toxic metals such as Pb, Cd, Hg, As, and Se, which are linked to neurological, renal, skeletal, pulmonary, and dermatological disorders. This study evaluated metal concentrations in soil (40–50 cm depth, corresponding to the rooting zone of forage grasses), forage (English ryegrass and Kikuyu grass), and raw milk to assess potential risks to livestock and human health. Sixteen georeferenced sites were selected using a simple random probabilistic sampling method considering geological variability, vegetation cover, accessibility, and cattle presence. Samples were digested and analyzed with a SpectrAA 220 atomic absorption spectrophotometer (Varian Inc., Victoria, Australia). Soils (Andisols) contained Hg (1.82 mg/kg), Cd (0.36 mg/kg), As (1.36 mg/kg), Pb (1.62 mg/kg), and Se (1.39 mg/kg); all were below the Ecuadorian limits, except for Hg and Se. Forage exceeded FAO thresholds for Pb, Cd, As, Hg, and Se. Milk contained Pb, Cd, and Hg below detection limits, while Se averaged 0.047 mg/kg, exceeding water safety guidelines. Findings suggest soils act as sources with significant bioaccumulation in forage but limited transfer to milk. Although immediate consumer risk is low, forage contamination highlights long-term hazards, emphasizing the need for monitoring, soil management, and farmer guidance.

Heavy metal contamination in agricultural soils threatens food security and public health, especially in volcanic regions where ash alters soil properties. This study evaluates the effects of soil amendments on physicochemical properties, nutrient availability, and heavy metal accumulation in ash-affected (Mocha) and non-affected (Puyo) soils in Ecuador. A field experiment tested compost, poultry manure, inorganic fertilizer, and a control on onion (Allium fistulosum) and parsley (Petroselinum crispum). Soil analyses assessed the bulk density, texture, pH, electrical conductivity, organic matter, nutrients, metals, and metalloid concentrations of the soils and crops. Mocha soils exhibited volcanic Andisol characteristics, while Puyo soils resembled eastern Ecuadorian soils, both showing high nitrogen but deficiencies in phosphorus, potassium, and calcium. Arsenic (As), lead (Pb), and chromium (Cr) levels in soils varied between regions but not among treatments. In Mocha, As bioavailability decreased with poultry manure and compost, while other metals remained stable except in fertilized soils. In Puyo, organic amendments reduced Hg, Pb, Ni, and Cr but increased them in fertilized soils. All treatments met Ecuadorian limits for As, Cd, Pb, and Ni but exceeded those for Hg and Cr. Organic amendments improved soil quality, reduced metal mobility, and supported sustainable agriculture, with Mocha soils appearing more suitable for cultivation.