Particle-size distribution analysis and sediment deposition on the pit floor at Malakoff Diggins State Historic Park

Information about the patterns of particle-size distributions at localities within the pit of Malakoff Diggins can be useful in determining effective mitigation strategies for sediment retention. Particle-size variation was analyzed as lateral variation from east to west within the pit; as vertical variation; and as temporal variation by comparison to data collected by David Peterson for his 1980 Master’s thesis. Sediment samples were collected at 6 localities within the pit of Malakoff Diggins, with 3 replicates per locality, and particle sizes were all analyzed by laser diffraction analysis and some by sieve analysis. Mean D50 values decreased from east to west in the pit but large error bars complicated interpretation. Mann-Whitney tests using median D50 values of surface samples showed that at a 91.9% confidence level, locality 1 had a statistically distinct median D50 value (N = 3; p = 0.0809); localities 2 and 3 (east to west) had similar D50 values (N = 3; p = 0.6625); localities 4 and 5 had similar D50 values (N = 3; p = 0.3827); and locality 6 was unlike any of the others (N = 3; p = 0.0809). Mann-Whitney tests using median D50 values of depth and surface samples demonstrated that at a 91.9% confidence level, the median D50 value of a deeper sample at locality 6 was significantly coarser than the median D50 value of the surface sample (N = 3; p = 0.0809). This indicates a finingupward sequence at locality 6, which shows that over time, progressively finer sediments may be retained within the pit. Wilcoxon signed-rank tests that compared D50 values of surface samples for the present study (840, 6800, and 800 μm at locality 1; 130, 440, and 130 μm at locality 2; 300, 160, and 190 μm at locality 3; 80, 30, and 69 μm at locality 4; 66, 32, and 26 μm at locality 5; and 20, 17, and 12 μm at locality 6) with D50 values of the study by Peterson (1980) (610 μm at locality 1; 840 μm at locality 2; 570 μm at locality 3; 620 μm at locality 4; 350 μm at locality 5; and 2 m at locality 6) found no significant differences at the 95% confidence level (N = 3; p = 0.181); but at the 81.9% confidence level found that the median D50 values for locality1 and locality 6 were coarser in samples of this study than in Peterson’s study, and that the median D50 values for localities 2 – 5 were finer in samples of this study than in Peterson’s study.