Mercury in soils of the Long Valley, California, geothermal system

An evaluation of the Hg distribution in soils of the Long Valley, California, geothermal area, was made. A₁-horizon soil samples were collected utilizing a grid system from the resurgent dome area and the Long Valley area. In addition, samples were collected in five traverses across three fault systems and four traverses across east-west-oriented gullies to measure the importance of aspect. Additional samples were collected in an analysis of variance design to evaluate natural variability in soil composition with sampling interval distance.The primary objectives of this study were to further evaluate the applicability of anomalously high Hg concentration in soils to exploration for geothermal systems and the importance of secondary controls on Hg concentration in soils above geothermal systems.Statistical analysis indicates that there are two populations of Hg concentrations in soils; one affected by geothermal activity and the other unaffected. Samples from the resurgent dome are shown to be statistically different from the samples collected in Long Valley proper with respect to Hg, organic carbon, and pH. This suggests that secondary influences may be important in controlling Hg distribution in soils.Organic carbon in soils is shown by stepwise multiple regression to be the most important secondary parameter controlling Hg concentration. For the most part, the secondary controls of Hg are overwhelmed in an area of prominent geothermal activity. Some faults exhibit prominent anomalies in total Hg concentration and others do not, indicating the possibility of low levels of hydrothermal activity or effective sealing of faults to gas leakage.The analysis of variance results indicate that there is a regional trend in Hg concentration across the resurgent dome. Soils can be sampled for Hg by utilizing a grid of about 0.4 km spacing. However, some local irregularities appear in this pattern and anomalous areas should be prospected at intervals of 100 m or less.