Geochemkal Transport Through the Miramichi Estuary

Abstract

A 2-yr seasonal study of the physical oceanography, geochemistry, and sedimentology of the Miramichi Estuary, New Brunswick, demonstrated that the overall system consists of two compartments: a drowned river, variable salt wedge estuary which is normally highly stratified, and the connected Inner Bay which is slightly stratified to well mixed. Freshwater inputs at the head of the estuary vary from 50 to 250 m3∙s−1 during summer and winter to high flows of 300 to 1700 m3∙s−1 during spring. Suspended particulate matter (SPM) and several labile trace metals including Fe, Mn, Zn, Cu, Pb, and Cd were analyzed to determine the total transport of potential contaminants within the system and into the Gulf of St. Lawrence. Wide variations in concentrations of SPM and labile metal by more than an order of magnitude are a function of freshwater input rates, tidal prism flushing characteristics, and geochemical interactions within the estuary. The transports for SPM, particulate metals, and labile metals were calculated using a tidal prism flushing model. Although 70% of the annual input of SPM (95 000 t) is exported from the estuarine system, about 60% of the total Fe (2000 t) is trapped in the estuarine bottom sediments. By contrast, 50% more Mn (200 t) is exported in combined particulate and labile forms than is brought into the estuary by the river. The source of this additional Mn is remobilization from fine-grained reduced bottom mud. The most important sink for Fe, Zn, Pb, and Cd appears to be the organic-rich bottom muds of the upper estuary. During spring flood periods, which last for an average of 39 d, about 85% of the annual supply of SPM enters the river estuary from the Miramichi River. Also during this time, 30–55% of the annual supply of labile metals Fe, Mn, Zn, and Cu enter the system. Much of this spring influx of SPM and metal is remobilized in the estuary over the rest of the year.