Revegetation of Taconite Tailings in Minnesota

The current taconite mining industry in Minnesota produces about 36Mt./a of taconite and employs about 4,000 employees. At the peak of the operation in the 1980s, eight taconite mines were in operation on the Mesabi range. Mined land reclamation rules adopted in Minnesota in 1980 require that all mined land be left in a stable and safe condition. The vegetation standards require that 90% vegetative cover be present after three growing seasons and require that the vegetation be self- sustaining after 10 years. Establishing vegetation on the soil removed during mining and glacial till overburden portion of the pit walls was not difficult. However, establishing 90% vegetation cover on the tailings required special methods through research.12

The typical standard reclamation approach uses 448 kg/hm (400 lbs/acre) of diammonium phosphate, 56 kg/hm (50 lbs/acre) of a cool-season grass mix and 4.6 t/hm(2 st/acre) of hay mulch.

This method was successful on the medium- to fine-grained tailings, it did not prove to be effective on the coarse tailing material used to build the retaining dams. After 5 years, the vegetative cover rarely exceeded 60%. Considerable research demonstrated that applying organic amendments improved vegetative success on coarse tailings.13

Site Description

EVTAC taconite processing plant and its tailing basin cover about 300 hm (750 acres) and is made from coarse tailings materials from the processing of the ore and truck hauled to construct the dams. The tailings dams are about 51-m (167 ft) high and contain three 17-m (56ft) high lifts, a final slope of 3:1, and an 8-m bench (26 ft) bench located at the top of each lift. Coarse tailings are low in nitrogen, phosphorous, w'ater holding capacity and organic matter. The coarse tailings are alkaline with a pH value of 8 and contained about 33% coarse sand, 40% medium sand, 22% fine sand and 5% silt and clay.

The fine tailings were also low in nutrients and organic matter, and range in size from fine sand to clay. Fifty per cent of the material is silt and clay.

Approach

Coarse Tailings

In the early 1990s, pilot studies were conducted at EVTAC to examine the feasibility of using organic amendments for growing vegetation on coarse tailings. Tests indicated that the use of peat, yard waste compost, municipal solid waste component, and paper mill residue increased the percent cover than in plots where standard mined land reclamation practices were adopted. The per cent cover generally increased with increasing amounts of organic material, 44.8 t/hm (20 st/acre) being selected as the cost-effective rate.

In 1996, tests were conducted to investigate the use of inorganic materials to improve vegetation on coarse tailings. The other four plots contained 50 mm (2 in) of fine tailings from two different sources, 50 mm (2 inch) of silty material, and 50 mm (2 inch) of peat. The materials were applied to the surface and mixed into underlying tailings.

In 1997, six 2-hm (5 acre) plots were established on the tailing’s dams. These were treated with municipal solid waste (MSW) compost at 44.8 t/hm (20 st/acre), paper mill residue at 44.8 t/hm (6 st/acre), Class В material from the nearby city of Virginia, MN and 44.8 t/hm (10 st/acre) biosolids. Standard mined land reclamation was used on one plot as a control. Producer of each material determined the application rate and by the results of previous research. The biosolids were applied at a rate that would provide the same amount of nitrogen as the inorganic fertilizer in the standard reclamation practice—100 kg/hm (89 lbs/acre). Because the carbon to nitrogen ratio in the paper residue far exceeded the desirable ratio of 30:1, extra nitrogen was added to these plots.

In the spring of 2000, EVTAC covered about of 18 hm (45 acres) of the coarse tailings dam with 100 to 150 mm (4 to 6 in.) of peat. EVTAC then applied seed, fertilizer, and mulch. In order to improve vegetation, biosolids from the Western Lake Superior Sanitary District were applied in the fall of 2000 as a top dressing on the peat areas. Application rates ranged from 100 kg/hm (89 lb/ acre) of nitrogen to 448 kg/hm (400 lb/acre) nitrogen.

Fine Tailings

EVTAC was interested in developing lands with a potentially higher economic value the grass meadows. Timber production was a major industry in northern Minnesota, with most of the harvest being used for paper and wood products. EVTAC found that it would be desirable to create tree plantations on reclaimed mined lands for economic benefits. The hybrid poplar has been used extensively in throughout northern Minnesota. Hybrid poplar can grow rapidly and, on natural forest soils, can be harvested in about 12 years, producing about 89 to 109 cords/hm (36 to 44 cords/ acre). EVTAC investigated the potential of creating a hybrid poplar plantation on the fine tailings. Test plots were established in 2001 to examine the survival and growth of hybrid poplars in the fine tailings with and without organic amendments (paper mill residue and biosolids).

Wetland Creation

EVTAC was originally built on wetlands. The US Army Corps of Engineers required that EVTAC conduct a study on the feasibility of creating wetlands on the tailings basins when they were permanently closed. In 1992, the State of Minnesota passed the Wetland Conservation Act, the idea of reclaiming tailings basin as wetlands received attention. The EVTAC task force-initiated studies to:

  • • Examine wetlands that had developed naturally, “incidental wetlands”
  • • Examine vegetative growth with various soil covers and seed mixes
  • • Study hydrology in the tailings basin and its effect on wetland establishment

In 1998, the State of Minnesota initiated a study to examine the use of dredged material from Lake Superior as a substrate for creation of wetlands. Two small areas of about 0.1 hm (0.25 acre) were established to examine the suitability of dredged material as a substrate for wetland vegetation and to investigate the impact on water quality. After 2 years, the per cent cover on the dredged material ranged from 80% to more than 90%. No impact on the water was measured.

The success of this study led to a large-scale demonstration project with the participation of the US Environmental Protection Agency. In the project 2 hm (5 acres) of land would be covered with a 100 mm (4 in) of dredge material and a wetland seed mix would be applied. In addition, EVTAC would cover about 400 hm (100 acres) with 50 mm (2 in) of the wetland soil obtained from road building projects.

Coarse Tailings

In 1996 plot tests, the addition of fine-grained inorganic material was not successful in producing vegetation that met mine land reclamation standards. After 3 years, the per cent cover ranged from 29.8% to 47.1%. The application of fine-grained material changed the infiltration characteristics of the coarse tailing slopes, and major erosion gullies formed during the first year following applications.

The peat plot had the highest per cent cover (71.9%) which did not meet the reclamation standard, but the slopes were stable, and erosion was minimal.

In 1997 test plots, after 3 years, the per cent cover on all amended plots exceeded the cover produced by the standard mine land reclamation practice. The cover ranged from 34% on the standard mine land reclamation plots to 86% on the MSW Table 14.12.

Although none of the covers met the legal requirements of 90% coverage, the cover was judged to be acceptable. Detailed cost information collected during the study indicated that since no inorganic fertilizer was required on the biosolids plot, the company saved $125/hm ($50/acre).

2000 reclamation: Reclamation was performed on the slopes and on the coarse tailings’ road. In the first year, the slopes which were covered with peat and top-dressed with biosolids were covered with a dense cover of grasses and legumes.

The first-year cover on the coarse tailings’ road grew a thick cover annual rye cover crop and the annual weed, Russian Thistle. The cover ranged from 14% on the standard mine land reclamation

TABLE 14.12

Per cent Cover, 1997 Coarse Tailings Demonstration Plots

Treatment Third Year

% cover

without Biosolids

% cover with Biosolids

Fourth Year

Standard mine land

34

42.1

51.0

Reclamation Blandin paper mill Residue

57

63.9

79.8

Consolidated paper Mill residue

53

74.8

58.1

Biosolids

68

71.5

90.8

Blandin paper mill Residue and biosolids

83

89.9

97.4

Municipal solid waste Compost

86

92.8

95.1

control to 45% on the plot with 448 kg N/hm (400 lb N/acre). No nitrogen was added to the inorganic biosolids plots, but some plots received phosphorous.

Fine Tailings

Tree plots: Visual observation indicated that the survival was good, and the first-year growth ranged from 0.3 to 1.2 m (1 to 4 feet). Wetland creation: Dredge and peat material would be spread in the fall of 2001. About of 100 mm (4 in) of dredge material will be placed on 2 hm (5 acres), while about 50 mm (2 in) of wetland soil will be placed on 40 hm (100 acres)

Bibliography

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