Restoration of the Kissimmee River, Florida: Water quality impacts from canal backfilling

The planned restoration of the Kissimmee River ecocystem will backfill appr oximately 35 km of flood control canal (C-38) that cuts through the meander ing river channel, re-establish natural flow patterns, and restore the rive r/floodplain ecosystem. Water quality monitoring, including nutrients, tota l suspended solids (TSS), turbidity, dissolved oxygen (DO), and mercury, wa s conducted during a pilot 'test fill' project to determine if soil disturb ance during canal backfilling would negatively impact these water quality c onstituents. Surface water nutrient concentrations varied little between si tes. Generally, highest concentrations occurred prior to construction, with lowest concentrations occurring during and after construction. During cons truction, TSS concentrations increased at sites immediately upstream, downs tream, and adjacent to the construction area. Increased turbidity was gener ally restricted to areas immediately upstream and downstream of the test pl ug, with maximum levels occurring during the initial construction phase. So me downstream increases in turbidity were observed; however, impacts were s hort-term, lasting less than 24 h. Depresssed DO levels (<2 mg/l) were obse rved upstream of the test plug following completion of the initial plug acr oss C-38. Dissolved oxygen levels remained low for approximately 6 weeks, w ith no apparent ecological impacts. Total mercury (HgT) within canal sedime nt ranged from 9.2-180 ng/g and methylmercury concentrations ranged from 0. 037-0.708 ng/g. Concentration of total mercury and total methylmercury (MeH gT) in the backfill material were much lower than concentrations in the can al sediment. No significant change in aqueous HgT concentrations occurred o ver the sampling period, although construction-induced turbidity could have temporarily caused a slightly elevated concentration immediately downstrea m of the construction site. Methylmercury concentrations in the water colum n ranged from 0.033-0.518 ng/l. No significant differences in mean MeHgT co ncentrations occured between sites or between sampling dates, except at one downstream site where MeHgT declined significantly over the sampling perio d.