Aquatic invertebrate colonization as a river restoration success criterion: a case study of the upper Blackfoot mining complex superfund site

Abstract

In 1975 the Mike Horse Dam partially collapsed, releasing 200,000 tons of cadmium, copper, iron, lead, manganese, and zinc into the streams and floodplains on the Upper Blackfoot Mining Complex (UBMC) in Montana, USA. The magnitude of the material that was toxic to humans from this event triggered the federal Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), which currently governs 1,329 sites across the USA. Portions of the $39 million lawsuit in 2008 with the American Smelting and Refining Company (ASARCO), funded the remediation and restoration of 37 hectares of floodplains, wetlands, and stream channels. Although CERCLA's success criteria focus on reducing risk to human health from hazardous substances, the Montana Natural Resource Damage Program was interested in aquatic invertebrate colonization of the restored river ecosystems, since they are monitoring progress of restoration. To answer this, I explored whether observations of invertebrate colonization could gauge restoration success and identify aquatic invertebrate-based tools for future restoration projects. Over three years, I compared invertebrate communities at five impacted "restored" sites on the UBMC with ten unimpacted "reference" sites. I then quantified colonization using seven indices: four statistical taxonomic diversity and similarity indices, the River Invertebrate Prediction and Classification System (RIVPACS), the Benthic Index of Biotic Integrity (B-IBI), and a new Stable Isotopic Colonization Index (SICI) which estimated isotopic complexity using metrics derived from delta 15N and delta 13C stable isotopes. Statistical diversity and similarity indices showed the restored sites were diversifying quickly. For example, from 2020 to 2023, the average (+ or - 1SD) Shannon Diversity of restored sites increased from 1.1 + or - 0.5 to 1.8 + or - 0.43 while reference was 2.1 + or - 0.3. The average B-IBI of restored sites increased from 11.1 + or - 4.8 in 2020 to 31.7 + or - 7.7 in 2023 while reference B-IBI was 65.7 + or - 4.5, indicating ongoing ecosystem recovery, but this index required taxonomic identification to the genus level. The average SICI for restored sites was 23.3 + or - 6.1 and reference was 54 + or - 9.2, and SICI required identification to the family level. Restoration efforts on the UBMC have resulted in a promising trajectory, but continuous monitoring is imperative to ascertain if restored streams have reached reference conditions.