Temporal and spatial variability of Fe and Mn in perched groundwater flowing through weathered argillite underlying a steep forested hillslope

Groundwater flowing through weathered bedrock dictates the runoff chemistry to streams in many catchments yet; its chemical evolution has been rarely documented. In particular, observations of Fe and Mn dynamics in groundwater are extremely challenging due to their high reactivity. To preserve the sample integrity for these elements we have developed a new sampling scheme that is applicable to autosamplers; a gravitational filtration system (GFS). GFS is capable of filtering samples by gravity within 30 minutes after the sampling. The GFS samples showed a good agreement with reference samples, which were collected following the standard sampling method for trace metals (i.e. immediate filtration and acidification). Since October 2011, GFS has been employed to monitor Fe and Mn in perched groundwater that moves through weathered argillite in an intensively instrumented hillslope (Rivendell), in the Angelo Coast Range Reserve. The study site is located at the headwaters of the Eel River, northern California, characterized by a typical coastal Californian Mediterranean climate. We collected groundwater samples at 3 wells along the hillslope (upslope (W10), mid-slope (W3) and near the creek (W1)) with 1-3 day intervals. Additionally, rainwater and throughfall samples were collected at a meadow near the hillslope and at the middle of the hillslope, respectively. The results from our observations indicate that Fe and Mn exhibit distinct spatial and temporal behavior under variable hydrologic conditions. The concentrations of Fe in throughfall vs. rainwater were similar (0.45μM vs. 0.49μM), but Mn in throughfall was 10-fold higher than that in rainwater (1.2 μM vs. 0.1 μM). In the early rainy season, W10's water table was deep (-18m) and Fe and Mn in W10 were 30-150 nM and 1-2 μM, respectively. As the rainy season proceeds, W10's water table rose by 4-6m, indicating the arrival of new water. At this time, Mn in W10 decreased to ~0.1 μM, synchronizing with the water table rise, and remained unchanged throughout the season. In contrast, Fe slowly declined to <10nM for this high water table regime. During the summer recession limb, Fe and Mn concentrations in W10 began to increase. During the dry summer, the concentrations of Fe and Mn at W3 were 2-3μM and 15-20 μM, respectively. At the beginning of the rainy seasons, the W3 water table slowly rose (<1 m) and both Fe and Mn decreased by 10-fold. The concentrations of Fe and Mn decreased to 20-70nM and 0.1 μM, respectively, when W3's water table became highly dynamic and fluctuated about 4 m. At W1, Fe and Mn remained in the 50-100nM and 5-10 μM ranges, respectively; however, the water table was extremely responsive to rainfall inputs. Mn in W1 was briefly diluted to <0.1 μM during large rainstorms and rebounded within several days. In the late summer of 2012, Fe and Mn in W1 increased up to 2-6 μM and 80 μM, respectively. These high-frequency observations of Fe and Mn will provide insight into the biogeochemical cycles of redox sensitive elements in upland terrains, allowing for better quantitative estimation of these elemental fluxes.