Historical pond systems as long-term composite archives of anthropogenic contamination in the Vrchlice River, Czechia

Abstract River damming has numerous benefits and adverse consequences. Unlike large dam reservoirs whose construction peaked in the twentieth century, historical ponds and their accumulated legacy sediments span much longer time periods. Small ponds in many areas worldwide are sinks for anthropogenic pollutants and affect sediment delivery to downstream river reaches. Thousands of ponds cover an area of >0.2% of the territory of Czechia, equal to the area of its ten largest dam reservoirs. This paper reports a sequence of constructed and removed pond dams along the Vrchlice River, Czechia, and their use as sedimentary archives, in order to reconstruct the long-term (>200 yr) history of contamination. Historical maps, geophysical profiles, activities of caesium-137 isotope, grain size, and sediment geochemistry in 16 sediment cores enabled the reconstruction of contamination in the pond, dam reservoir, and floodplain sediments since ∼1780. Contamination by lead, copper, zinc, arsenic, and cadmium from atmospheric, geogenic and agricultural sources increased since ∼1850 peaked in the 1970s and 1980s, and then decreased slightly. Polycyclic aromatic hydrocarbons and pesticides polluted post-1970 sediments, reflecting their use in agriculture. The removal of dams resulted in the rapid incision of newly formed river channels into the old pond deposits. Following the reinstatement of dams, channels filled rapidly (up to 5.2 cm/yr) with new lacustrine sediment, whereas sediment accumulated slowly (