Nearshore Seashell and Sand Mining: Environmental Impact, Gulf of Mexico Examples

ABSTRACT The recent precipitous decline of inshore-nearshore mudshell dredging on the Gulf is directly and indirectly related to a whole range of environmental concerns. Man-made hydrological and sedimentological changes in the last decades led to marked changes in certain estuaries. Due to increasing rates of shore erosion, offshore sand mining for beach restoration is expanding. However, its impact on nearhore blots of higher energy sandy seafloor appears to be much less pronounced than the influence of mudshell dredging in inshore environments. INTRODUCTION Coastal zone dredging for buried seashell and aggregate products involves risks of damaging not only live oyster reefs, but the entire adjacent inshore environment as a whole. In numerous estuarine areas, dredging for buried shell resources is prevented or restricted by the presence of oil and gas fields, pipelines, wildlife refuges and navigational channels. Various and often conflicting demands on the estuarine environment by different industries, conservationists, fishermen and others, created continuous, sometimes bitter conflicts with the shell dredging industry, often involving the state permiting process. A drastic reduction in the harvesting of commercial "mudshells" along the Gulf has been caused primarily by environmental constraints, partly by associated economic factors. The following account, based on published and unpublished reports and our own findings, deals with a variety of risks at a number of Gulf coastal localities. Most examples involve oyster and clam shell dredging. Examples of sand mining with the direct or indirect purpose of countering shore erosion are also noted. SHELL DREDGING Buried shell resources. Rangia clam (Rangia cuneata) and the American (or eastern) oyster (Crassostrea virginica) are among the most abundant bivalves found in brackish coastal environments and provide locally available industrial source materials. The bays, lagoons and sounds where these seashells thrive, receive abundant and regular fresh water inflow from mainland streams. Rangia prefers very low salinities (optimum for larvae: 6-10 ppt; LaSalle and de la Cruz1 and often is found densely scattered on muddy Gulf estuary bottoms. Oysters, usually much larger in size, prefer slightly higher salinity settings (c.10-20 ppt), that occasionally are subjected to sea water influx. They often form cemented Ienticular accumulations ("reefs") on sandy or muddy bottoms that can support their weight. These species provide much needed local sources of construction aggregate and a variety of calcium carbonate products. Industrial applications The bulk of dredged oyster shell and clam shell is used as aggregate for roadway base, road, parking lots petroleum drilling pads, and levee construction2. These raw materials had competed well against crushed stone, imported from distant, usually upstate or out-of-state locations. Smaller amounts of shell are required for road embankment construction in marshy areas, than the volume that would be needed of the heavier sand, Additional uses of shell include lime manufacturing, acid neutralization, water purification, filter media, poultry feed, glass manufacturing, and substrate cultch material for establishing new oyster beds. For clutch, limestone, imported from significant distances, has an advantage.