Assessing traffic-derived soil contamination: Insights from earthworm bioaccumulation and histomorphological...

This study aims to evaluate traffic-derived soil contamination using Amynthas gracilis (Kinberg, 1867) earthworms as bioindicators, with insights from bioaccumulation and histomorphological responses. Road pollution arises from multiple sources, including vehicle emissions, tire and brake wear, and stormwater runoff, with extensive environmental impacts. The negative impacts of heavy traffic pollution are well-recognized, but less information is available on that of moderate traffic road pollution. Besides serving as proxies for soil contamination, earthworms provide information on contaminant bioavailability and biological responses. Soil and A. gracilis earthworm samples were collected on either side of three roads in São Miguel Island, Azores, Portugal, with varying average annual daily traffic: Lagoa-Ponta Delgada (44.300 annual daily traffic), Lagoa-Ribeira Grande (9092 annual daily traffic), and Coroa da Mata-Santa Iria (<400 annual daily traffic), labelled MHT, MT, and LT sites. Soil and earthworm elemental concentrations were measured; soil elemental contamination and earthworm bioaccumulation factors were calculated. Earthworm histomorphometrical and histomorphological data were gathered to assess the impact of chronic exposure to moderate traffic road pollution on bioindicator tissues. Enrichment factors were elevated for Bi in MHT and MT sites, and Co in MT site; earthworms bioaccumulated As, Cd, Hg, Sb, Zn. Histological analyses revealed alterations in earthworm tissues indicative of physiological stress, namely thicker integument and peritrophic matrix (both of which reflect enhanced barrier function), thinner intestinal epithelium (reflecting impaired nutrient absorption and higher cellular turnover), and a higher proportion of mucous cells (reflecting enhanced barrier function) in the integument of earthworms from MHT and MT sites. The impact of moderate levels of traffic pollution is demonstrated by its contribution to soil contamination. The potential of using A. gracilis earthworms as sensitive bioindicators to moderate levels of road pollution is highlighted.