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Published online 7 November 2005
Published in J Environ Qual 34:2234-2242 (2005)
DOI: 10.2134/jeq2004.0476
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Ecosystem Restoration

Volunteer Revegetation of Waste Rock Surfaces at the Bingham Canyon Mine, Utah

Richard K. Bordena,* and Rick Blackb

a Rio Tinto Technical Services, 1 Research Avenue, Bundoora, Victoria 3083, Australia
b HDR Engineering, Inc., 3995 South 700 East, Suite 100, Salt Lake City, UT 84107

* Corresponding author (rich.borden{at}riotinto.com)

Received for publication December 15, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 STUDY SITE
 METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Voluntary recolonization of sulfide-bearing waste rock dumps by native vegetation is inhibited by the harsh chemical and physical conditions. The success of volunteer vegetation on the waste rock surfaces at the Bingham Canyon (Utah) porphyry copper deposit is most strongly dependent on the soil pH and salinity, and to a lesser extent on physical characteristics such as compaction and distance from seed source. Vegetation cover and richness both decline below a paste pH of about 6 and above a paste conductivity of about 0.7 dS/m (for a 1:1 soil to water mixture). No significant vegetation establishment occurs below a soil pH of about 4.5. Young sulfide-bearing waste rock surfaces at Bingham Canyon have high salinity, but as reactive pyrite is depleted and salts are flushed from the soil, the salinity eventually declines, allowing volunteer native vegetation to become established on surfaces with a circumneutral pH. Under natural conditions, the pH of older acidic weathered surfaces will recover very slowly, but it can be rapidly raised by adding relatively small amounts of limestone because there are few intact reactive sulfides. For uncompacted waste rock surfaces with favorable chemical conditions, less than 90% gravel content, and that are located near a native seed source, the arithmetic mean volunteer vegetation cover was 56 ± 24% and the mean species richness was 17 ± 5. These data indicate that with adequate surface preparation and limestone addition, direct planting of older, acidic, but low salinity waste rock surfaces can greatly accelerate natural revegetation.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
STUDY SITE
 METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE HARSH CHEMICAL and physical environment typical of sulfide-bearing waste rock surfaces can severely inhibit vegetation establishment. Pyrite and other sulfide minerals in the waste rock oxidize when exposed to surface weathering conditions and release sulfate, acidity, and metals. Soils forming on sulfide-bearing waste rock surfaces commonly have very high salinity with low pH. Waste rock is devoid of organic matter, important nutrients, and beneficial microorganisms. It may also be very coarse textured, with low moisture retention characteristics. Flat surfaces can be heavily compacted by vehicle traffic, while angle of repose dump slopes may be subject to continuous surface creep and erosion. The establishment of volunteer vegetation may also be inhibited by the long distances to seed sources. To design effective revegetation programs and to understand the controls on volunteer revegetation, it is important to understand the influence and relative importance of these physical and chemical conditions.

Many investigators have examined the influence of one or more of these variables on volunteer revegetation on waste rock surfaces, but almost all of these studies have focused on coal spoils (Glenn-Lewin, 1979; Jonescu, 1979; Schafer and Nielsen, 1979; Hedin, 1988; Skousen et al., 1988, 1994; Pietsch, 1996; Kost et al., 1998; Wali, 1999). Many of these previous studies were also located in the eastern or midwestern United States and so may not be directly applicable to the semiarid climates of mining sites in the western United States.


   STUDY SITE
TOP
 ABSTRACT
 INTRODUCTION
 STUDY SITE
METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The present study was conducted at the Bingham Canyon porphyry copper deposit near Salt Lake City, Utah. Over three billion metric tons of waste rock have been produced at Bingham Canyon since open pit mining operations began in 1906. The existing waste rock surfaces cover more than 2000 ha and vary in age from less than 1 to more than 50 yr. The waste rock dumps were placed in valleys and on side slopes in the Oquirrh Mountain Range. The waste rock dumps investigated at Bingham Canyon are located in a semiarid to subhumid mountain climate between an elevation of 2000 and 2400 m. Annual precipitation within the study area varies from about 500 to 800 mm. The surrounding dry and rocky native hillsides are dominated by curl-leaf mountain mahogany (Cercocarpus ledifolius Nutt. ex Torr. & A. Gray) and Gambel oak (Quercus gambelii Nutt.) communities. Wetter, north facing or higher altitude slopes also support Douglas fir [Pseudotsuga menziesii (Mirb.) Franco], quaking aspen (Populus tremuloides Michx.), and bigtooth maple (Acer grandidentatum Nutt.)–chokecherry (Prunus virginiana L.) communities. Big sagebrush (Artemisia tridentata Nutt.) and rubber rabbitbrush [Chrysothamnus nauseosus (Pall.) Britton] are dominant on exposed ridges and on lower slopes.

Copper mineralization at Bingham Canyon is centered on a monzonite stock intruded into a Pennsylvanian sedimentary sequence, and the waste rock is composed of a highly variable mixture of monzonite, quartzite, and limestone. The waste rock is also variably mineralized and may contain from 0 to greater than 5% sulfide sulfur when it is first placed on the dumps (Borden, 2003). Pyrite is the dominant sulfide mineral in the waste rock with lesser chalcopyrite, sphalerite, and galena. Gravel content of the soils forming on the waste rock surfaces varies from between 20 and 100% and averages about 65%. The mining operation at Bingham Canyon has created waste rock surfaces with a great diversity of age, chemical, and physical characteristics. As a result, the establishment of volunteer vegetation on the waste rock surfaces is also highly variable. The waste rock surfaces at the Bingham Canyon mine provide a unique area to study volunteer revegetation of waste rock surfaces in the intermountain west.

The objectives of this study were to (i) evaluate the influence and relative importance of waste rock pH, salinity, surface age, compaction, and distance to seed source on the success of volunteer vegetation; and (ii) identify the key pioneer species that have successfully colonized the waste rock surfaces. Fieldwork was conducted between 1998 and 2004.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 STUDY SITE
 METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Selection
A total of 75 study sites were surveyed on the waste rock dump surfaces. Approximate sample locations were selected to ensure that dump surfaces with different vegetation covers and surface characteristics were surveyed. The study sites were restricted to dump surfaces between 2000 and 2400 m above mean sea level to limit the variation in annual precipitation and mean temperature between the sites. The small percentage of the waste rock dump surfaces composed of greater than 90% gravel (defined as clasts greater than 2 mm in diameter) was excluded from the study because these areas support little vegetation. Surfaces that had received reclamation treatments were also excluded from the study. In the field, the exact sample location was chosen to be representative of the immediately surrounding area. For example, on generally nonvegetated surfaces with small, scattered areas of volunteer growth, the study site boundaries were limited to the nonvegetated areas.

Soil Sampling and Analysis
Soil samples were collected at each site and analyzed for paste pH, paste conductivity, and percent gravel. Each sample was composited from three to five subsample locations spaced 3 to 9 m apart on the waste rock surface. At each subsample location, a hole was dug to a depth of 10 cm and the subsample was collected by removing the entire 0- to 10-cm vertical interval from one side of the hole.

A representative split from the composite sample was then submitted to the Kennecott Utah Copper Environmental Laboratory for pH and conductivity analysis. Paste conductivity is a measure of the total salinity of the soil and is expressed in decisiemens per meter (dS/m). For all samples, the paste pH and paste conductivity analyses were performed on a mixture composed of 5 g of soil mixed with 5 g of distilled water (1:1 soil to water mixture). The pH and conductivity measurements were performed on the water sample after it was decanted from the solids with an Orion (Beverly, MA) Model 230A pH meter and an Amber Science Incorporated (Eugene, OR) EC Model 2052. Twenty-two samples with a pH above 5 and with paste conductivity values between 0.05 and 1 dS/m were also analyzed by the saturation extract method (Richards, 1954). In this method, distilled water is added to the sample until it becomes a saturated paste, then the water is extracted from the soil with a vacuum filter or centrifuge.

Data on elevation, surface age, distance to the closest undisturbed native hillside, and surface compaction were also compiled for each study site. Compaction and resistance to penetration was estimated in the field by counting the number of blows required to drive a 1.3-cm-diameter rebar 20 cm into the waste rock surface with a 1.8-kg hammer. The rebar was driven into the ground at three locations at each site and the results were averaged. The same person performed the compaction measurements at all sites.

Vegetation Community Analysis
Vegetation community analyses were performed at 33 study sites that supported vegetation following the Releve or "simple stand" method (Barbour et al., 1987). Plant identification and nomenclature generally follows Welsh et al. (1993), while exotic species were identified from Whitson et al. (1992). Using the Releve method, variable-sized quadrats were sampled at representative locations at each study site. The number of individual quadrats sampled at each site varied from one to five, depending on the size of each study site and the variability in the vegetation cover. Within each quadrat, the absolute percent aerial cover of each species present was occularly estimated. Percent cover estimates were assigned within the general cover classes defined by the Braun-Blanquet cover scale (Braun-Blanquet, 1964). The cover for each observed species was assigned to one of the following categories: rare, 0.5 to 1%, 1 to 5%, 6 to 25%, 26 to 50%, 51 to 75%, and 76 to 100%. At a few small sites that were thinly vegetated, aerial cover for each species was calculated by direct measurement of canopy diameter for the few individuals of that species. The percent cover of the species for the site was calculated by dividing the measured species cover by the total area of the site. The aerial cover for each species present was then assigned to the appropriate Braun-Blanquet class.

The total cover in each quadrat was calculated by adding the median point of the Braun-Blanquet cover class for each species present. For survey sites with multiple quadrats, the total cover was calculated by averaging the total cover in each quadrat. Species richness was approximated by the total number of species observed within each quadrat. For survey sites that included more than one quadrat, the number of species observed in each quadrat was averaged to yield the number of species observed for the entire survey site. This prevented overstating the richness at larger sites with numerous quadrats compared to small sites that were characterized with a single quadrat.

Statistical Interpretation
Arithmetic mean values for vegetation cover and species richness are reported along with the 95% confidence interval ({alpha} = 0.05) for each sample population. Sample means were compared for statistical significance using a t test, and p values (two-tailed) are reported whenever means were determined to be significantly different at the {alpha} = 0.05 level. Pearson product moment correlation coefficients and the coefficient of determination (R2) values were calculated to investigate the relationships between physical and chemical variables and measures of vegetation success. All physical and chemical data except pH were log10 transformed because this yielded a better correlation with log10 transformed cover and unmodified richness data. The coefficient of determination can be considered a measure of the proportion of variance in one variable that is attributable to variation in the other variable. The statistical significance of the correlations was also assessed at the {alpha} = 0.05 level.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 STUDY SITE
 METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Of the variables investigated during this study, soil paste pH and soil paste conductivity were the most highly correlated with vegetation cover and species richness (Table 1). Soil pH had a positive correlation with vegetation cover and species richness, and soil paste conductivity had an inverse correlation. The distance to the closest seed source exhibited a weaker inverse correlation with cover and species richness, while surface age exhibited a weaker positive correlation. Compaction did not have a statistically significant correlation with vegetation cover or richness.


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  		Table 1. Correlation coefficient (CC) and coefficient of determination (R2) between chemical and physical variables and volunteer revegetation success for the entire 75-sample data set.{dagger}

 
Soil pH and Salinity
The paste pH of the waste rock dump surfaces varied between about 2 and 8, and the paste conductivity varied between 0.05 and 10 dS/m (Fig. 1) . Comparisons between the conductivity and the total dissolved solids content of the 1:1 soil to water leachate indicate that the two values are highly correlated (R2 = 0.94) and that almost all of the salinity is provided by sulfate, calcium, and magnesium in solution (Borden, 2001).



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  		Fig. 1. Soil paste conductivity (dS/m) versus soil paste pH for the 75 vegetation survey sites. Open diamonds are sites with less than 1% vegetation cover, open triangles are sites with 1 to 5% vegetation cover, solid triangles are sites with 5 to 25% cover, and solid squares are sites with more than 25% cover.

 
The pH and salinity of the surfaces were primarily controlled by the concentration and distribution of sulfide minerals in the original waste rock, the amount of limestone present, and the age of the waste rock surface (Borden, 2001). In general, the greater the sulfide concentration, the lower the pH was and the greater the salinity of the soil. Conversely, the greater the limestone content of the waste rock surface, the higher the pH. On most surfaces there was a sharp drop in waste rock pH and a peak in salinity within 6 yr of placement on the dumps. This is caused by the rapid release of acidity and sulfate during the oxidation of freshly exposed pyrite. The salinity of the soils begins to decline as pyrite becomes depleted, and sulfate is flushed from the soil by infiltration and runoff more rapidly than it is replenished by sulfide oxidation. It is unclear to what depth the pyrite oxidation and the flushing of soluble salts has occurred in the waste rock column because only the upper 10 cm were sampled. However, the depth of weathering was sufficient to allow deep rooting woody shrub and tree species to become established on many of the sample sites.

Volunteer vegetation was observed on 44% of the waste rock surfaces that were surveyed. No vegetation was observed on any of the 22 sites that had high salinity (>0.7 dS/m) in conjunction with low pH (<5). Conversely, the mean vegetation cover for the 16 sites with a pH of greater than 6 and a conductivity of 0.7 dS/m or less was 36 ± 14%. The mean number of species observed on these sites with favorable chemistry was 15 ± 3. None of the sites with circumneutral pH and low salinity supported less than 9% cover or fewer than six species. Much lower and less consistent vegetation cover was observed on low salinity sites with marginally acidic soils, and on circumneutral pH sites with elevated salinity. Many of the sites with marginally favorable chemistry supported less than 5% cover (Fig. 1). Vegetation on sites with less than 5% cover generally did not exhibit a uniform distribution, instead it was clustered in relatively small pockets within the survey area. These pockets were probably related to microhabitats where the soil chemistry was more favorable, and which may not be representative of the composite sample chemistry reported for the entire site.

At pH values below 6, nitrogen and phosphorus availability typically begins to decline (Buckman and Brady, 1969; Donahue et al., 1976). At pH values below 5 to 5.5, Al, Mn, and Cu solubility and plant toxicity may also begin to inhibit the growth of most agronomic species (Tucker et al., 1987). As shown on Fig. 2 and 3 , both vegetation cover and species abundance declined below a pH of around 5.5 to 6. The 12 surfaces with low salinity (<0.7 dS/m) and marginally acidic pH (4.2 to 6) have a mean vegetation cover of only 11 ± 10% and a species richness of 6 ± 3. Both vegetation cover (36 ± 14%) and species richness (15 ± 3) were significantly higher on low salinity sites with pH above 6 compared to low salinity sites with marginally acidic soils (p = 0.01 and p = 0.00002, respectively). Leach tests conducted on 24 Bingham waste rock soil samples indicate that Al, Mn, and Cu solubility increases by one to two orders of magnitude below a pH of 4 to 4.5 (Borden, 2001). The increase in soluble metals concentrations at low paste pH coincides with the observed lower growth limits for volunteer vegetation.



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  		Fig. 2. Volunteer vegetation cover versus soil paste pH. Only sites with soil paste conductivity values ≤ 0.7 dS/m are included on the plot. Cover values of less than 1% were assigned a value of 1% to allow the cover data to be plotted on a log10 scale.

 


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  		Fig. 3. Number of species present at each site versus soil paste pH. Only sites with soil paste conductivity values ≤ 0.7 dS/m are included on the plot.

 
Elevated soil salinity limits the availability of soil water to plants by increasing the osmotic potential. Elevated soil salinity also reduces the availability of nutrients to plants (Jurinak et al., 1987). Salt tolerance among plants is highly variable. Halophytes may be tolerant of soil salt concentrations that are more than an order of magnitude higher than for salt-sensitive species. In the field, the salt tolerance of a given species is related to a number of interrelated factors including climate, nutrient availability, and the physical characteristics of the soil. The lack of nutrients and the coarse texture of the soils forming on the waste rock surfaces likely make the volunteer vegetation more susceptible to salinity-related problems. On the Bingham Canyon waste rock dumps, both vegetation cover and species richness decrease at a soil paste conductivity of between 0.5 and 0.8 dS/m (Fig. 4 and 5) . It should be noted that the paste conductivity values that are presented in the figures and discussed in the text are based on analysis of a 1:1 soil to water mixture. However, most agricultural assessments of plant salt tolerance that are available in the literature are based on the saturation extract procedure. For Bingham Canyon waste rock soils, the 22 paired saturation extract and 1:1 soil to water conductivity results were highly correlated (R2 = 0.97). The conductivity derived by the saturation extract method was on average about 1.9 times greater than the results derived from a 1:1 soil to water mixture. According to this relationship, plant growth on the Bingham waste rock dumps appears to become salinity-limited in the range of 1 to 1.5 dS/m as measured by the saturation extract method. This is consistent with the salinity tolerance cited by Maas (1990) for salt sensitive crops, who noted that salt-sensitive species begin to exhibit decreased yield at saturation extract soil conductivity values of about 1.3 dS/m.



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  		Fig. 4. Volunteer vegetation cover versus soil paste conductivity (dS/m). Only sites with soil paste pH ≥ 6.0 are included on the plot. Cover values of less than 1% were assigned a value of 1% to allow the cover data to be plotted on a log10 scale.

 


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  		Fig. 5. Number of species present at each site versus soil paste conductivity (dS/m). Only sites with soil paste pH ≥ 6.0 are included on the plot.

 
For the seven sites with a pH of greater than 6 and a conductivity of 0.8 to 2.5 dS/m, the mean cover was only 3 ± 2%, and the mean number of species observed was 5 ± 3. The maximum total cover that was observed on any of the circumneutral but high salinity sites was only 7%. The vegetation cover (36 ± 14%) and species richness (15 ± 3) for circumneutral pH sites with conductivity of less than 0.7 dS/m was significantly higher than for circumneutral sites with conductivity of between 0.8 and 2.5 dS/m (p = 0.0003 and p = 0.00004, respectively).

Young waste rock surfaces at Bingham Canyon were characterized by variable pH but almost uniformly high salinity. The 10 waste rock surfaces that were 6 yr or less in age had paste conductivity values of between 1.7 and 10 dS/m with a mean conductivity of 4.1 ± 1.7 dS/m. As has been noted in other studies, the high salinity inhibits vegetation establishment on most young waste rock surfaces (Skousen et al., 1988; Nawrot et al., 1988; Wali, 1999). The youngest site that supported 10% or more cover is 20 yr old, and none of the surfaces that were 6 yr or younger support more than 1% cover or more than three species. The 16 surveyed waste rock surfaces that were 30 yr or older had paste conductivity values of between 0.6 and 1.9 dS/m with a mean conductivity of 0.4 ± 0.2 dS/m. These oldest surfaces have a significantly higher mean cover (22 ± 15%) and species richness (8 ± 4) than the surfaces that are 6 yr or younger (p = 0.01 and 0.0007, respectively).

Physical Characteristics
Most of the circumneutral pH and low salinity sites on Fig. 2 through 5 that have low vegetation cover or species richness also have some physical barrier to plant establishment. Within the restricted population of sites with favorable chemistry (pH > 6 and conductivity < 0.7 dS/m), the degree of compaction had a significant impact on volunteer vegetation cover, and the distance to seed source had a significant impact on species richness. No other comparisons were found to be statistically significant.

Compaction increases bulk density and resistance to mechanical penetration and can create physical barriers to plant root growth. Many previous studies have demonstrated that compaction can severely limit revegetation success (Barnhisel, 1988; Burger and Torbert, 1992; Andrews et al., 1998; Conrad et al., 2002). The degree of compaction on the Bingham Canyon waste rock surfaces was highly variable depending on the amount of past vehicle traffic. Studies conducted on waste rock dumps at the Chino Mine using haul trucks of a comparable size to those used at Bingham Canyon indicate that truck compaction can increase the waste rock bulk density by an average of 20% and up to 60% in some locations (Uhrie and Koons, 2001). At Bingham Canyon the number of blows required to drive a 1.3-cm-diameter rebar 20 cm into the waste rock surface varied from 2 to 14 for uncompacted surfaces such as end dumped piles, angle of repose slopes, and ripped surfaces (mean 6 ± 1). For haul roads the number of blows required varied from 51 to greater than 100 (mean 89 ± 8). For the survey sites with circumneutral pH and low salinity, the degree of compaction of the waste rock surfaces is inversely proportional to vegetation cover (Fig. 6) . These compacted surfaces (>14 blows/20 cm) had a mean cover (14 ± 5%) that was significantly lower than the mean cover for uncompacted surfaces (45 ± 18%) (p = 0.006). The difference in mean species richness between compacted and uncompacted surfaces was not statistically significant.



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  		Fig. 6. Volunteer vegetation cover versus compaction as measured by the number of blows required to drive a 1.3-cm-diameter rebar 20 cm into the waste rock surface. Only sites with soil paste pH ≥ 6.0 and paste conductivity values ≤ 0.7 dS/m are included on the plot.

 
The waste rock survey sites ranged from 30 m to more than 560 m from the closest undisturbed native hillsides. Numerous studies have noted that the success of volunteer species establishment decreases with increasing distance to a seed source (Wagner et al., 1978; Skousen et al., 1994; Kost et al., 1998). At more distant sites, colonization is often limited to those species with effective seed dispersal mechanisms and high seed production rates. There was an inverse correlation between distance to seed source and species richness for sites with circumneutral pH and low salinity (Fig. 7) . Surfaces located within 50 m of an undisturbed native hillside had a significantly higher mean species richness (23 ± 3) compared to sites located more than 100 m from a potential seed source (14 ± 4) (p = 0.02). The difference in mean cover between sites located less than 50 and more than 100 m from a seed source is not statistically significant.



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  		Fig. 7. Number of species present at each vegetated site versus distance to the closest seed source in meters. Only sites with soil paste pH ≥ 6.0 and paste conductivity values ≤ 0.7 dS/m are included on the plot.

 
Pioneer Species
A total of 87 different species were identified at the 33 sites that supported volunteer vegetation. Many of the plants were relatively rare, and only 41 of these species were observed at three or more sites. Eight of the species that were identified are on official state noxious weed lists for Utah or one of the immediately surrounding states (Arizona, Colorado, Idaho, Nevada, New Mexico, and Wyoming).

Waste rock surfaces with different soil chemistry tend to have very distinct species compositions. On surfaces with favorable soil chemistry (pH > 6.0, conductivity < 0.7 dS/m), woody shrubs and trees provided on average about 62% of the total cover, forbs about 32%, and grasses only 6% (Table 2). Native legumes were only observed on sites with circumneutral pH and low salinity where they were present on about 50% of the surfaces. Weed species only comprised 4% of the total cover on sites with favorable chemistry. On marginally low pH and low salinity sites, woody shrubs and trees provided on average about 56% of the total cover, forbs about 40%, and grasses about 4% (Table 3). Weedy species comprised about 12% of the minimal total cover that was present. On waste rock surfaces with circumneutral pH but elevated salinity, woody shrubs and trees on average only provided about 9% of the total cover, forbs about 79%, and grasses about 12% (Table 4). Weedy species comprised about 54% of the minimal total cover that was present on these high salinity sites.


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  		Table 2. Common species observed on neutral pH and low salinity sites (pH from 6.0 to 7.9 and conductivity from 0.06 to 0.5 dS/m). Data compiled from 16 sites, all of which had cover. Surface age varies between 15 and 43 yr and averages approximately 30 yr.

 

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  		Table 3. Common species observed on low pH and low salinity sites (pH from 4.2 to 6.0 and conductivity from 0.06 to 0.44 dS/m). Data compiled from 12 sites, 4 of which had no cover. Surface age varies between 8 and 41 yr and averages approximately 28 yr.

 

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  		Table 4. Common species observed on neutral pH and high salinity sites (pH from 6 to 7.2 and conductivity from 0.8 to 2.5 dS/m). Data compiled from seven sites, two of which had no cover. Surface age varies between 4 and 41 yr and averages approximately 19 yr.

 
Several species have successfully colonized a broad variety of waste rock surfaces at Bingham Canyon. The most successful tree and shrub species include rubber rabbitbrush [Chrysothamnus nauseosus (Pall.) Britton], Douglas fir [Pseudotsuga menziesii (Mirb.) Franco], curlleaf mountain mahogany (Cercocarpus ledifolius Nutt. ex Torr. & A. Gray), and big sagebrush (Artemisia tridentata Nutt.). The most successful native forbs included silverleaf phacelia (Phacelia hastata Dougl. ex Lehm. var. hastata), milfoil yarrow (Achillea millefolium L.), Douglas' dusty-maiden [Chaenactis douglasii (Hook.) Hook. & Arn.], Wasatch penstemon (Penstemon cyananthus Hook.), and sulfur buckwheat (Eriogonum umbellatum Torr.). The most common legumes were Rydberg's sweetpea (Lathyrus brachycalyx Rydb.) and various lupine species. Although native grass species are rarely dominant on any of the revegetated surfaces, Kentucky bluegrass (Poa pratensis L.), sheep fescue (Festuca ovina L.), and bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Á. Löve] are relatively widespread. The waste rock surfaces have also been successfully invaded by several noxious weed species including Dalmatian toadflax [Linaria dalmatica (L.) Mill.], cheatgrass (Bromus tectorum L.), various thistle species, and woolly mullein (Verbascum thapsus L.). The weedy species only appear to attain dominance on those sparsely vegetated surfaces with low pH or high salinity that prevented the establishment of a dense, healthy native plant community.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 STUDY SITE
 METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The success of volunteer vegetation on the Bingham Canyon waste rock dumps is most strongly dependent on the soil pH and salinity, and to a lesser extent on physical characteristics such as compaction and distance from seed source. Vegetation cover and richness both declined below a pH of about 6 and above a conductivity of around 0.7 dS/m for a 1:1 soil to water mixture. This equates to a saturation extract conductivity of between 1 and 1.5 dS/m. The native vegetation colonizing the waste rock dumps would be considered salt sensitive according to the classification scheme of Maas (1990).

When waste rock is first placed on the dumps, the weathering of freshly exposed sulfides causes oxidation products to accumulate in the material faster than they are removed by infiltration and runoff. Young waste rock surfaces have uniformly high conductivity, and even neutral pH soils support little vegetation. The little vegetation that is present is dominated by weedy forbs. As sulfide minerals are depleted and salts are flushed from the soil, the salinity eventually declines so that native vegetation can become successfully established on surfaces with a circumneutral pH. Neutral pH surfaces that are more than 30 yr old generally have low salinity and are generally able to support a diverse community of native woody shrubs, trees, forbs, and grasses. Surfaces that have acidified will not support vegetation until there are very few intact sulfides and the pH rises above about 4.5. Natural neutralization processes in waste rock soils that have acidified are slow. They must rely on the relatively slow reaction kinetics of silicate mineral neutralization and on flushing with rainwater at a pH of 5 to 5.5. Neutralization of waste rock surfaces can be greatly accelerated by human intervention. As has been noted by previous authors working with coal spoils, older weathered surfaces with low pH and low salinity can be successfully treated with relatively small amounts of a limestone because there is very little potential acidity in the form of sulfide minerals remaining in the material (Nawrot et al., 1988).

For waste rock surfaces without any obvious physical or chemical barriers to vegetation establishment (uncompacted surfaces with pH ≥ 6, conductivity ≤ 0.7 dS/m, <90% gravel, and located within 100 m of a seed source) the mean volunteer vegetation cover was 56 ± 24% and the mean species richness was 17 ± 5. These data indicate that with adequate lime or limestone addition, and with surface preparation including deep ripping to break up compacted layers and to potentially move finer material to the surface, direct planting into older, low salinity waste rock soils at Bingham Canyon and similar sites can be expected to greatly accelerate natural revegetation.

Several of the most successful volunteer species on the Bingham Canyon Waste rock dumps such as milfoil yarrow, sulfur buckwheat, big sagebrush, rubber rabbitbrush, bluebunch wheatgrass, curl-leaf mountain mahogany, penstemon species, and lupine species have been previously identified as good colonizers and favorable species for reclamation programs (Thornburg, 1982; Richardson, 1985; Monsen, 1989; Hansen et al., 1991). However, other very successful volunteer species at Bingham Canyon such as Douglas fir, silverleaf phacilia, Rydberg's sweetpea, and Douglas dusty maiden are not commonly identified in the reclamation literature and may also prove useful as reclamation species in the semiarid to subhumid intermountain West.


   ACKNOWLEDGMENTS
 
The authors wish to thank the staff of the Bingham Canyon Mine and the Kennecott Utah Copper Environmental Laboratory for their help in completing this study. Thanks also to Mark Logsdon for his helpful reviews.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 STUDY SITE
 METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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The SCI Journals Agronomy Journal Crop Science
Vadose Zone Journal Journal of Plant Registrations
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal