Evapotranspiration (store and release) soil covers have been proposed as a means to limit acid mine drainage. Soil water balance models, like UNSAT-H are commonly used to assess the effectiveness of store and release covers. Plant-related attributes are required as inputs to these models. In particular, UNSAT-H requires leaf area index (LAI) and root length density (RLD) inputs. Published LAI and RLD data are generally lacking for semi-arid plant communities. To resolve this data gap, we collected leaf area and root density measurements in native and reclaimed shrub-grassland communities in southwestern New Mexico. Leaf area indices were determined using digital image analysis of harvested leaves at the end of the growing season. These data were used to estimate peak LAI and develop an annual LAI distribution. The average LAI ranged from 0.29 in reclaimed plant communities to 0.42 in native shrub-grasslands. LAI values for the reclaimed site did not correspond to soil cover thickness, which ranged from 23 to 62 cm. However, higher LAI values were typically associated with plots with higher amounts of shrub cover. Preceding drought and heavy grazing probably affected the LAI data in both native and reclaimed areas. Root density was measured in soil excavations using a grid-count method. Root density measurements indicated that nearly two-thirds of the roots occurred in the upper 20 cm of the soil in both the reclaimed and native areas. Very few roots occurred below 1.0 m. RLD was described by the quartile function 69-20-7-4 in the upper meter of soil. Preliminary water balance simulations using a 100-year climate record indicate that average drainage was less than 1 percent of mean annual precipitation when the measured LAI and RLD functions were applied to a 60 cm thick cover.Additional Key Words: leaf area index, root length density, soil cover, soil water balance model, UNSAT-H
Upward migration of either acidity or sodium is often cited as a concern when soil covers are placed over mine wastes. Regulatory concerns associated with upward migration are commonly used to justify capillary barriers or increased cover thickness. Upward migration of soluble salts is known to occur in soils with permanent or seasonal water tables near the soil surface and is a common problem in agricultural contexts. Short-term studies conducted at mine reclamation sites where changes in pH or salinity occurred at the contact between the soil cover and waste have been used to infer that upward migration would ultimately degrade cover quality. Thus, we investigated the potential for upward migration in soil covers overlying acid mill tailing and coal spoils 20 to 25 years after cover placement and revegetation. The soils were sampled and analyzed at discreet intervals above the soil-waste contact to assess upward migration. Changes in soil pH and/or extractable constituents were inconsistent among sites and varied from about 2 to 10 cm above the soil cover-waste contact. Field observations suggest that physical mixing of the cover and tailing during construction, rather than advection, accounted for acidification of the basal soil cover. Based on evidence from our investigations and an understanding of soil developmental processes in arid-regions, we conclude that upward migration of constituents from wastes is not a time-transgressive process that will result in cover degradation in arid and semi-arid regions where permanent or seasonal water tables occur well below the soil surface. Finally, we consider the changes in chemistry at the soil cover above the waste interface to be biologically insignificant.
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