Application of low doses of alum to treatment wetlands to reduce elevated outflow winter phosphorus
concentrations were tested in mesocosms vegetated with either Typha domingensis, Schoenoplectus californicus,
or submerged aquatic vegetation (SAV) (Najas guadalupensis-dominated). Alum was pumped
to experimental units at a rate of 0.91 g Al m2 d1 and water quality monitored for 3 months. The alum
application significantly improved the outflow water quality and overall the growth of the plants was
unaffected by the alum application. Biomass and growth varied between species and through time, but
no significant effects of alum application were detected. The concentrations of nutrients and mineral elements
in the aboveground tissues differed between species and over time, but only the concentration of
Al in plant tissue was increased by alum additions. The concentration of Al was 50-fold higher in alumtreated
SAV as compared to the control, and in Typha and Schoenoplectus the concentrations were 4- and
2-fold, higher, respectively. The N/P ratios in the plant tissues were low (
Application of low doses of alum to treatment wetlands to reduce elevated outflow winter phosphorus
concentrations were tested in mesocosms vegetated with either Typha domingensis, Schoenoplectus californicus,
or submerged aquatic vegetation (SAV) (Najas guadalupensis-dominated). Alum was pumped
to experimental units at a rate of 0.91 g Al m2 d1 and water quality monitored for 3 months. The alum
application significantly improved the outflow water quality and overall the growth of the plants was
unaffected by the alum application. Biomass and growth varied between species and through time, but
no significant effects of alum application were detected. The concentrations of nutrients and mineral elements
in the aboveground tissues differed between species and over time, but only the concentration of
Al in plant tissue was increased by alum additions. The concentration of Al was 50-fold higher in alumtreated
SAV as compared to the control, and in Typha and Schoenoplectus the concentrations were 4- and
2-fold, higher, respectively. The N/P ratios in the plant tissues were low (<10) suggesting that their
growth and biomass was limited by nitrogen. The research suggests that a continuous or seasonal
low-dosage alum application to treatment wetlands provides an effective tool to maintain discharge concentrations
within permitted values during the inefficient winter treatment times. We suggest that the
use of alum should be restricted to treatment wetland areas dominated by emergent vegetation as the
effects of the elevated Al concentrations in SAV needs further study.
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