Robert E. Trouwborst

Soluble Mn(III) in Suboxic Zones

Soluble manganese(III) [Mn(III)] has been thought to disproportionate to soluble Mn(II) and particulate MnIVO2 in natural waters, although it persists as complexes in laboratory solutions. We report that, in the Black Sea, soluble Mn(III) concentrations were as high as 5 micromolar and constituted up to 100% of the total dissolved Mn pool. Depth profiles indicated that soluble Mn(III) was produced at the top of the suboxic zone by Mn(II) oxidation and at the bottom of the suboxic zone by MnIVO2 reduction, then stabilized in each case by unknown natural ligands. We also found micromolar concentrations of dissolved Mn(III) in the Chesapeake Bay. Dissolved Mn(III) can maintain the existence of suboxic zones because it can act as either an electron acceptor or donor. Our data indicate that Mn(III) should be ubiquitous at all water column and sediment oxic/anoxic interfaces in the environment.

Iron and Sulfur Chemistry in a Stratified Lake: Evidence for Iron-Rich Sulfide Complexes

A four month study of a man-made lake used for hydroelectric power generation in northeastern Pennsylvania USA was conducted to investigate seasonal anoxia and the effects of sulfide species being transported downstream of the power generation equipment. Water column analyses show that the system is iron-rich compared to sulfide. Total Fe(II) concentrations in the hypolimnion are typically at least twice the total sulfide levels. In situ voltammetric analyses show that free Fe(II) as [Fe(H2O)6]2+ or free H2S as H2S/HS- are either not present or at trace levels and that iron-rich sulfide complexes are present. From the in situ data and total Fe(II) and H2S measurements, we infer that these iron-rich sulfide complexes may have stoichiometries such as Fe2SH3+ (or polymeric forms of this and other stoichiometries). These iron-rich sulfide complexes appear related to dissolution of the iron-rich FeS mineral, mackinawite, because IAP calculations on data from discrete bottle samples obtained from bottom waters are similar to the pKsp of mackinawite. Soluble iron-sulfide species are stable in the absence of O2 (both in lake waters and the pipeline) and transported several miles during power generation. However, iron-sulfide complexes can react with O2 to oxidize sulfide and can also dissociate releasing volatile H2S when the waters containing them are exposed to the atmosphere downstream of the powerplant. Sediment analyses show that the lake is rich in oxidized iron solids (both crystalline and amorphous). Fe concentrations in FeS solids are low (<5 μmole/grdry wt) and the pyrite concentration ranges from about equal to the solid FeS to 30 times the solid FeS concentration. The degree of pyritization is below 0.12 indicating that pyrite formation is limited by free sulfide, which can react with the iron-rich sulfide complexes.

The roles of anoxia, H2S, and storm events in fish kills of dead-end canals of Delaware inland bays

In 2001, the development of seasonal anoxia was studied in two waterways located at the head of Delaware’s northern inland bay, Rehoboth Bay. Bald Eagle Creek is a northern tributary of the bay, which has tidal exchange with Torquay Canal (a dead-end canal) via a short channel with a 1.4 m sill. Mean low water depth in Torquay Canal is about 2 m, but dredging produced over a dozen depressions with a total water depth of 5.5 m. During the summer of 2000, four major fish kills were reported in Torquay Canal and Bald Eagle Creek with more than 2.5 million juvenile menhaden (Brevoortia tyrannus) killed. Low O2 concentration was assumed to be the problem but production of toxic H2S is more likely. From late spring 2001, we conducted in situ determination of temperature, salinity, pH, dissolved O2, and H2S in Torquay Canal and Bald Eagle Creek. During spring, water column stratification began in the depressions with warmer and less salty water observed in the upper layer, and cooler, saltier water below 2 m. O2 was at saturation levels in the surface waters but was not detectable below 2 m by the end of May. The depressions were anoxic with H2S accumulating to mM concentrations in June. A storm event prior to July 12 mixed these two layers with a subsequent loss of H2S. The H2S levels again increased in the deep water due to stratification and reached another maximum in late August. Another storm event occurred at this time resulting in no detectable O2 and up to 400 μM H2S in surface waters. H2S appears to be the primary reason for fish kills in these tributaries. Aerators installed in Torquay Canal on June 21 had no significant effect on abating stratification and anoxic conditions beyond their immediate area.