An investigation of the erosion characteristics of uncovered waste rock, Questa, New Mexico
Fluvial erosion, erosion by water, is a complex hydrologic process involving the interaction of a soil surface with the surrounding environment. A research program was undertaken at the Molycorp Inc. molybdenum mine in Questa, New Mexico to study the erosion characteristics of uncovered waste rock. The mine is situated on mountainous terrain, the waste rock piles were constructed by end-dumping material into the mountain valleys creating angle of repose waste rock slopes. The research study consisted of a field based program with laboratory and computer modeling programs. The field program spanned a period of one year from May 2000 to June 2001 and included a geomorphological study of the mine site, monitoring of test plot slope profiles, measurement of eroded material collected III sediment traps, and simulated rainfall erosion tests. The purpose of the geomorphological study was to evaluate the present condition of erosion on the waste rock piles and to investigate the mechanisms creating erosion. The test plot slope profiles monitored the change in the surface profile of four test plots to estimate the erosion rate and to observe where, within the slope profile, the material is lost. The sediment traps directly measured the erosion rate by collecting the runoff and eroded material emanating from a catchment area for individual natural storm events. Finally, additional rainfall events were produced with simulated rainfall using irrigation equipment. The waste rock piles are constructed of two materials, mixed volcanics and aplite rock. The surfaces of the mixed volcanics waste rock piles, approximately 60% of the total waste rock pile surface area, are covered with many gullies of moderate and large size. The aplite waste rock surfaces have fewer gullies and in many areas show no signs of erosion. The field research program was established on the mixed volcanics slopes in order to measure the erosion rate. The majority of the eroded material is transported to the base of the piles in debris flows or mudflows, mixtures of water and fine and coarse particles that flow en masse down the waste rock slope, typical of gully erosion. Sediments fall from the gully sidewalls or headcut into the gully channel where they are removed when sufficient runoff is present to liquify and transport the material down the slope. The measurements of the annual erosion rate in the field study program produced results of similar magnitude. The average erosion rate measured at the three sediment traps during the one year field period was 49.4 Mg/ha. Artificial erosion events produced by the simulated rainfall tests showed a similar erosion rate to the sediment traps. The erosion rate measured at the test plot slope profiles was higher with three plots averaging 130 Mg/ha while the erosion rate of the fourth test plot was over 13,000 Mg/ha. The 51.9 Mg/ha average annual erosion rate predicted by the RUSLE v1.06 computer code was slightly greater than the sediment traps, while measurement of the erosion test plots estimated a long-term erosion rate of 57.9 Mg/ha. Correlation of precipitation records to the erosion experienced at the sediment traps show rainstorms with maximum 30 minute intensities of less than 10 mmIhr and rainfall erosivity indexes (EI ^30) less than 10 MJ-mm-ha^-1-h^-1 produced low erosion «1 Mg/ha). Storms with 30 minute intensities around 10 to 20 mmlhr generate erosivity indexes between 10 and 40 MJ.mm·ha-l·h-1 and moderate erosion rates of 1.0 Mg/ha to 4.0 Mglha. Large events with erosion rates greater than 4 Mg/ha are caused by intense rain events (EI ^3o > 40 MJ·mm.ha-1·h-1) produced by 30 minute intensities greater than 20 mmIhr. The erosion rate of the mixed volcanics slope material, measured in this study, will be used in the design of the present and long-term pile maintenance procedures at the Questa mine. Knowledge of the relationship between storm intensity and duration and the erosion rate will allow installment of suitable containment systems to reduce and control erosion from average and high return period storm events.