Plant uptake, time trends, and natural attenuation of selected metals in an abandoned mining district

Abstract

Soils, vegetation, and water in the abandoned Tri-State mining district are contaminated by lead, zinc, and other metals. To investigate trends in lead and zinc concentrations in the environment for past periods when little environmental monitoring occurred, 32 tree cores were sampled in the mining district and 6 reference tree cores were sampled at a reference site in Norman, Oklahoma. The hypotheses of this work were that lead and zinc contents in tree growth rings would have decreased after mining ceased ca. 1970, increased with rising water tables and seepage of groundwater from the mine workings starting in late 1979, again increased with short-term disturbance of tailings caused by reclamation activities in the late 1990s and early 2000s, and that tree growth rings sampled from trees in the reference area would have different periods of significant changes of lead and zinc contents than tree growth rings sampled in the mining district. Except for lack of significant decreases in lead and zinc contents of tree growth rings after the mines closed ca. 1970 to 1980 and lack of significant decreases of zinc in the 2000s, content of lead and zinc in tree rings corroborated those hypotheses.

Tissues from 32 upland and 32 wetland trees in the abandoned mining district, and 6 reference site trees were sampled to test the hypotheses that wetland tree tissues have greater lead and zinc concentrations than upland trees and that mining district trees have elevated concentrations of those metals compared to trees at a reference site. Those hypotheses were only partially supported by lead and zinc concentrations of tree tissues in the three sample groups. Bole wood of upland trees had greater lead concentrations than bole wood of wetland trees and similar lead concentrations as those in bole wood of reference trees. Similar lead concentrations in samples of bole wood and other tissues in trees from both the mining district and the reference site, despite much smaller lead concentrations in soils at the reference site, may indicate natural limits to lead uptake by trees. Commercial nut samples had similar lead and zinc concentrations as nuts sampled from reference trees and wetland and upland trees in the mining district, refuting the hypothesis that nut tissues of trees in the mining district would have greater lead concentrations. Concentrations of zinc in bole wood, twigs, and leaves of wetland trees were 5-10 times greater than in upland trees in the mining district, supporting the hypothesis of greater zinc concentrations in wetland trees. However, zinc concentrations in nut and hull tissues were similar in upland and wetland trees. Zinc concentrations in reference tree bole wood samples were similar to those in bole wood samples from upland trees, but zinc concentrations of other types of reference tree tissues ranged from one-tenth about half of those in upland tree tissues.

Comparison to lead and zinc concentrations of tree tissues at other mining and commercial areas and estimates of lead and zinc concentrations in biomass of selected trees in the mining district indicate that trees may be useful for phytostabilization in this mining district, refuting the hypothesis that very large lead and zinc contents of trees in the mining district would preclude planting of trees for phytoremediation. Reference soil samples supported the hypothesis of having less total lead and zinc concentrations than mining district soils. Unlike soils in the mining district, zinc concentrations were not substantially greater than lead concentrations in the reference soils. Nearly half of the upland soil samples collected in the mining district had lead concentrations exceeding a residential clean-up standard of 500 mg/kg, with none of the reference soil samples having lead concentrations exceeding that standard.

At a stream site receiving drainage from most of the Oklahoma part of the mining district, total concentrations of iron, manganese, and zinc significantly decreased from the mid-1980s to the mid-2000s. Natural attenuation and long-term effects of reclamation activities probably are causing gradual decreases in metals concentrations in the aquatic environment of this abandoned mining district, similar to gradual decreases with time of lead and zinc contents in many, but not all of the tree cores.

Data collected from terrestrial and aquatic environments of the mining district in this dissertation indicate that metals concentrations have decreased gradually since cessation of mining ca. 1970. Natural events, such as filling of the mine workings with groundwater and subsequent seepage to local streams starting in 1979, and short-term disturbance and dissemination effects of reclamation activities in the late 1990s through early 2000s temporarily increased metals concentrations in the local environment.

Several different types of reclamation methods can be used to reclaim large metals-contaminated sites such as this mining district. Treatment of groundwater seepage from the mine workings, whether by passive or active systems, is being done at one site in the district to reduce quantities of metals discharging to a local stream. Phytoremediation, particularly with high-value hardwood trees, is likely to reduce erosion of fine tailings particles by wind and water, minimize bioavailability of metals to local wildlife, and improve the local economy by producing high-value agricultural products and forming a potential basis for industries using those materials. Better management of metals-rich fine tailings particles during excavation and transport of the millions of tons of tailings remaining in the mining district would minimize dissemination of fine tailings to the local environment and improve the quality of terrestrial and aquatic environments of the mining district during reclamation activities.

Monitoring of the media described in this dissertation and other environmental media such as growth rings in freshwater mussel shells from local rivers, age-dated cores from Grand Lake (the downstream recipient of most drainage from the mining district) metals in terrestrial wildlife and livestock, regular periodic water-quality sampling of local streams, and collecting air-quality and atmospheric deposition samples in the mining district would augment knowledge of trends of metals and the short- and long-term effects of natural attenuation and reclamation projects on the recovery of metals-contaminated terrestrial and aquatic systems in the past and the future.