Water quality trends and dissolved gas dynamics in the Hudson River estuary.

摘要

Distributions of nutrients, dissolved non-reactive gases (chlorofluorocarbons, CFCs, and sulfur hexafluoride, SF{dollar}sb6{dollar}), and dissolved oxygen (DO) in the lower Hudson estuary show many similar features. Their distributions reflect large waste water sources in the water near Manhattan. Maxima in nutrient, CFC, and SF{dollar}sb6{dollar} concentrations and minima in DO concentrations vary systematically with freshwater discharge rates. These systematic changes result from variations in the freshwater replacement times which during periods of high flow are on the order of a week and during periods of low flow are on the order of months.; A second feature common in nutrient, CFC, SF{dollar}sb6{dollar} and DO distributions is that concentrations in surface and deep water with similar salinities are about the same. Typically, surface and deep water with similar salinities are found more than 6 km apart. This suggests that mixing along salinity (density) surfaces is fast.; A dual tracer experiment (SF{dollar}sb6{dollar} and {dollar}sp3{dollar}He) was completed to quantify gas transfer velocities. The results of the dual tracer experiment show that gas transfer velocities increase linearly with wind speed. This relationship is very similar to results of lake experiments suggesting that gas exchange rates are primarily forced by wind. Hence, current speed has relatively little effect.; Water quality trends were developed accounting for the general features mentioned above. The loading rate of soluble reactive phosphorus (SRP) decreased by 30-40% from the early 1970s to the mid-1970s as a result of removal of phosphorus based detergents. By the early 1990s, SRP loading rates were about 50% of the early 1970s load. Although the loading rate of NH{dollar}sb4{dollar} was relatively constant between the late 1970s and the early 1980s, the NH{dollar}sb4{dollar} concentration in the estuarine water varied. These variations were caused by changes in nitrification rates as well as changes in freshwater discharge rates. Minimum DO concentrations in the water adjacent to Manhattan increased significantly between the early 1980s and the late 1980s. During periods of similar freshwater flows, the minimum DO concentration had increased by 50-85 {dollar}mu{dollar}mol/1. During the later period, hypoxic waters were never found. The timing of the DO improvement agrees well with changes at the Passaic Valley and North River Treatment Facilities.