ABSTRACT: Results of a study of the effects of highway runoff on the chemical quality of water and bed sediments of a cypress wetland and a freshwater marsh in central Florida indicate that detention of the runoff prior to release into the wetland reduces concentrations of automobile-related chemicals in the water and bed sediments in the wetland. Detention of highway runoff for the cypress wetland occurs in a 68- by 139-foot detention pond and in a 12- by 25-foot trash retainer for the fresh water marsh. The analysis of the chemical data for water and bed sediments indicates that many of the observed differences in chemistry are due to the difference in detention facilities.
Water quality generally improved from the inlet to the outlet of both wetlands. Only inlet and outlet data were collected at the cypress wetland and these showed a reduction in concentrations through the wetland. Spatial data collected at the freshwater marsh indicated that constituent concentrations in water generally decreased with distance from the inlet. Results of analysis of variance of grouped data for 40 water-quality variables at the freshwater marsh inferred that 26 of 40 variables tested were significantly different among five general locations within the wetland: inlet outlet, near, intermediate, and far sites (with respect to the inlet). Further statistical tests inferred significant differences between the inlet location and all other locations except the near sites, in color and total organic carbon (increased with distance), pH, nutrients, aluminum, lead, and zinc (decreased with distance). Comparisons between the near-sites and other locations were analagous to those of results for the inlet location.
The use of a primary sedimentation facility reduces the impact of highway runoff on the chemical composition of wetland bed sediments. For example, median concentrations of trace metals in bed sediments were much lower in the cypress wetland than in the detention pond preceding it. The median lead concentration in pond sediments was 620 ug/g (micrograms per gram), and 20 ug/g in the wetland; median zinc concentrations were 250 ug/g (pond) and 14 ug/g (wetland), and chromium concentrations were 20 ug/g (pond) and 2 ug/g (wetland).
Median concentrations of selected constituents in bed sediments at the freshwater marsh were much higher than in the cypress wetland but were similar in magnitude to those detected in the detention pond at the cypress wetland. Median concentrations for 10 sites in the freshwater marsh were: lead 390 ug/g; zinc, 175 ug/g; and chromium, 40 ug/g.
Results from this study indicate that detention structures, larger than the trash retainer at the freshwater marsh, may cause sufficient sorption and settling of substances contained in highway runoff to minimize the transport and deposition of some undesirable chemicals into wetlands.