Effects of bucket dredging on water quality in the Delaware River and the potential for effects on fisheries resources

Abstract

The potential effects of bucket dredging on the fisheries resources in the Delaware River was assessed by evaluating the results of nine water quality monitoring studies conducted ·summer of 1992. The assessment was accomplished first by conducting analysis of variance (ANOVA) to determine if water quality conditions downstream of the dredge site were significantly different from upstream reference locations during the weeks of dredging. A second analysis included a literature review that identified critical levels of dissolved oxygen and turbidity from both a biological and regulatory perspective. Based on the critical values of turbidity and dissolved oxygen, the percentage of measurements that exceeded critical levels during dredging was evaluated. Analysis of variance tests indicated that only two of the nine studies resulted in significantly higher turbidity downstream of the dredging operations and the increase was about 15 nephelometric units (NTUs) over control turbidity (15 NTUs). Significant differences in dissolved oxygen were detected by the ANOVA, but mean downstream DO wa_s actually higher than the upstream reference location by about 0. 1 mg/I. A literature review suggested that concentrations of suspended solids can reach · thousands of milligrams per liter before an acute toxic reaction is expected in fish and that dissolved oxygen concentrations above 4 mg/I is required by striped bass. Based on the literature review and Delaware River Basin Commission's water quality standards, protective critical levels of 150 NTUs for turbidity and 4 mg/I for DO were ~elected. In more than 10,500 measurements, turbidity values greater than the critical level of 150 NTUs occurred only 13 times, and 7 of these observations occurred downstream of the Hog Island dredging operation. Over ninety-nine percent of dissolved oxygen measurements taken in downstream areas during dredging were above 4 mg/I and the percentage of observations below this critical level was not statistically different between the upstream reference and the downstream monitoring locations. Analysis of these data suggest that the bucket dredging carried out in 1992 did not result in adverse, biologically meaningful changes in DO or turbidity in the Delaware River