Streamwater Particulate Mercury and Suspended Sediment Dynamics in a Forested Headwater Catchment

Due to the inherent differences in bioavailability and transport properties of particulate and dissolved mercury (Hg(P) and Hg(D)), it is important to understand the processes by which each is mobilized from soil to stream. Currently, there is a paucity of Hg(P) data in the literature despite the fact that it can be the dominant fraction in some systems. We analyzed Hg(P) in conjunction with volatile solids (VS, an estimate of organic content) and total suspended solids (TSS) and investigated the viability of using turbidity as a surrogate measure of Hg(P). Samples were collected for flow conditions ranging from 72 to 8,223 L s(-1) during October 2009 through March 2010 in a 10.5-km(2) forested headwater catchment. Total Hg concentrations ranged from 0.28 to 49.60 ng L(-1), with the relative amount of Hg(P) increasing with discharge from approximately 40% to 97%. Storm dynamics of Hg(P) and Hg(D) were not consistent, indicating unique controls on the export of each fraction. During high-flow events, Hg(P) was consistently higher on the rising limb of the hydrograph compared with the receding limb for a range of discharge events, with this hysteresis contributing to a degraded relationship between Hg(P) and streamflow. Overall, Hg(P) was strongly positively correlated with VS (r (2) = 0.97), confirming the known association with organic carbon. Due to a consistent organic fraction of the suspended solids (34 +/- 6%), Hg(P) was also well correlated with TSS (r (2) = 0.95), with an average of 0.10 ng of Hg(P) per milligram of TSS in this system. Stream turbidity measured with an in situ sonde also had a strong correlation with TSS (r (2) = 0.91), enabling commutative association with VS (r (2) = 0.86) and Hg(P) (r (2) = 0.76). Turbidity can explain more than twice the temporal variance in Hg(P) concentrations (n = 50) compared with discharge (r (2) = 0.76 versus 0.36), which leads to improved monitoring of Hg(P) dynamics and quantification of mass fluxes.