Chemical effects of highway runoff on the surficial aquifer, Broward County, Florida

In many areas of Broward County, swales are commonly designed to accept stormwater runoff from highways. Two sites adjacent to heavily traveled highways were studied to determine if stormwater percolating through unsaturated sand underlying the swales may affect the quality of water in the Biscayne aquifer. Concentrations of selected chemicals common in highway runoff were measured in swale stormwater, in the unsaturated-zone percolate, and in the surficial aquifer during 12 storms, May through November 1983. Analyses of the unsaturated lithologic material at the two sites and one control site were also made to indicate the extent of vertical attenuation of selected chemical constituents. Results of trace metal nutrient analyses indicated that there is no obvious water-quality effect on the surficial aquifer caused by highway runoff. In general, the data collected for dissolved trace metals indicated slight concentrations in stormwater samples with subsurface water samples usually indicating decreases in concentration below about 0.5 to 1 ft of unsaturated material. For most trace metals, the apparent decreases with depth were statistically significant based on nonparametric analysis of variance. Concentrations of dissolved nitrogen and phosphorus, however, were somewhat homogeneous and were not inferred to be significantly different with depth. Water hardness as calcium carbonate at both sites indicated a significant difference with depth because of calcium carbonate solution, but the effect of percolate hardness on the shallow groundwater was negligible because of regional mixing. The analyses of lithologic material at the two test sites indicated significant near-surface accumulation of some trace elements. For example, in the first foot below land surface, concentrations of iron, lead, and zinc were detected in the 1 to 7 mg/kg range, with concentrations decreasing sharply at lower depths. This near-surface accumulation was not indicated in the lithologic material at the control site. (Author 's abstract)