Submarine groundwater discharge in the Southeast and Gulf Coast regions of the United States.

摘要

Submarine groundwater discharge (SGD) potentially transports significant volumes of water and dissolved geochemical constituents to continental shelf environments. SGD is variable from region to region, and is a function of local hydrogeological characteristics and anthropogenic influences. This work examines SGD at two locations in the Gulf Coast and Southeast regions (Louisiana continental shelf near the Mississippi Delta, and Onslow Bay, NC), and suggests a regional management plan for SGD.; Quantification of SGD on the continental shelf of Louisiana and in Onslow Bay was accomplished using 222Rn, a natural geochemical tracer. A box model approach was employed to quantify sources and sinks of 222Rn, which include: in situ production and decay, horizontal transport into and out of the box model, vertical evasion (across pycnocline or air-sea interface), and benthic advective-diffusive exchange. All sources and sinks were quantified except the benthic advective-diffusive exchange, which was determined by difference. SGD estimates on the Louisiana continental shelf were on the order of 0.01--0.14 cm d-1, which is equivalent to 1% the average discharge of the Mississippi River over the entire study area. Sources of SGD in the study area were primarily from production water, a product of the oil and gas extraction process, and seawater recirculation cells.; SGD estimates in Onslow Bay averaged approximately 0.1 cm d-1 at the regional scale, and estimates decreased from nearshore to offshore in three shore parallel models. Fresh groundwater discharge dominates in the nearshore region while tidal pumping provides most SGD farther offshore. Volumetrically, SGD over the entire study area is approximately 105% the average discharge of the Cape Fear River, the largest river near the study area that discharges directly to the continental shelf. The hydrogeology of Onslow Bay is similar to that of coastal South Carolina and Georgia. Assuming SGD rates are similar throughout in the region, SGD contributes 6--12% that of riverine discharge in the first 15 km offshore between Cape Lookout, NC and the FL/GA border.; Considering the importance of groundwater as a drinking water source and the potential influences of SGD on continental shelf environments, a regional plan is suggested, that is designed to manage groundwater resources in coastal zones. The management plan consists of increased assessment and monitoring of coastal aquifers, and a regional coastal groundwater resources oversight committee. The oversight committee would act as a central organization to collect and disperse groundwater information throughout the region, and mitigate and resolve local and state groundwater issues.