Changes in iron (Fe) and phosphorus (P) cycling in Vadnais Lake, Minnesota, resulting from ferric chloride addition and hypolimnetic aeration are evaluated by repeat sampling of bottom sediments over a 13-year period: in 1985 (pre-treatment), and in 1990 and 1998 (post-treatment). Lead-210 derived accumulation rates for Fe, Mn, total-P, and P-fractions are combined with input/output monitoring to construct chemical mass-balances for each of the three time periods. Iron injections/aeration caused large and sustained reductions in water-column total-P by increasing phosphorus removal to the sediments and preventing its recycling during stratification. Annual whole-lake phosphorus sedimentation rose from 1.26 to 1.52 t between 1985 and 1990, equivalent to a doubling of in-lake retention of external P loads (from 19% to 38%). Most of the increase is represented in the labile Fe-bound fraction. The measured sedimentary fluxes for 1985 and 1990 are similar to those calculated by difference from inflow/outflow data, whereas measured P sedimentation in 1998 (2.43 t·yr−1) is three times higher than that estimated from input/output calculations. These results suggest surface enrichment by upward P diffusion within the sediment column. Present-day Fe accumulation rates (24 t·yr−1) are 33% greater than those immediately preceding treatment and 14% greater than pre-settlement fluxes (21 t·yr−1). The 6 t·yr−1 increase in Fe accumulation between 1985 and 1998 is roughly equal to the rate of iron injection to Vadnais Lake. Fifty-four tons of iron addition to the Lambert Creek tributary between 1990 and 1998 have also enriched littoral sediments in Fe, P, and Mn near the creek's outfall. These engineering solutions have substantially improved water quality in Vadnais Lake, but continued hypolimnetic aeration will be required to prevent internal phosphorus loading from the large reservoir of labile sedimentary P that has accumulated since treatment began.
Bibliographical noteFunding Information:
Major credit for the design, implementation, and long-term evaluation of water quality improvements for the SPWU belongs to William Walker, Dave Schuler, and Jim Bode. Sediment sampling in Vadnais Lake was done with the assistance of Schuler (1985), Bode (1990), and Kelly Thommes and Berit Engstrom (1998). Sediment extractions, phosphorus analyses, and 210Pb measurements for the 1998 collections were done by Kelly Thommes and Tara Bromenshenkel. Water-quality data from the SPWU were provided by Jim Bode. This research was funded by the St. Paul Board of Water Commissioners.
- Hypolimnetic aeration
- Internal loading
- Iron addition
- Phosphorus inactivation