Influence of forest management on water quality and Hg dynamics : a synoptic research

Abstract: Forest management is hypothesized to increase the mercury (Hg) load towards aquatic systems. In this study a synoptic research was carried out assessing 55 sites across Sweden, whereby the sampling scheme was designed to compare reference (N = 18), stump harvested (N = 16) and site prepared (N = 21) areas. The influence of forest treatment irrespective the separation into stump harvest and site preparation on Hg export to surface water streams was evaluated with regard to water quality (TOC, suspended solids, TSS) and with special focus on Hg and MeHg levels. Furthermore, the parameters affecting THg and MeHg concentrations were determined. Total phosphor (Ptot), total nitrogen (Ntot) and total organic carbon (TOC) concentrations were significantly higher in the treated areas compared to the reference sites (p = 0.0465, p = 0.0251 and p = 0.0487, respectively). THg revealed nearly significant higher concentration in the treated areas than in the reference site (p = 0.077). MeHg concentrations were not significantly differing between treated and reference areas, although a high standard deviation in the treated areas indicate that site specific catchment characteristics play an important role in the overall MeHg and THg mobilization. The individual comparison of stump harvested and site prepared areas showed no statistical significant difference. This might indicate that in general the treatment methods did not differ regarding their effects of Hg mobilization as well as nutrient and ion leaching on surface water quality. THg and MeHg correlated positive with TOC (R² = 0.70, p < 0.0001 and R² = 0.48, p < 0.0001). Furthermore, the quite strong significant correlation of THg and MeHg with absorption (unfiltered, 420 nm) that was R² = 0.66, p < 0.0001 and R² = 0.60, p < 0.0001, respectively, could favor the implementation of absorption measurement as a first estimate of Hg concentration in water. This study could not confirm the indications in literature that forest harvesting is severalfold increasing THg and MeHg load to aquatic systems.