Investigation of salinity and nutrient characteristics of two groundwater based flow systems on Virginia's Eastern Shore

The freshwater-saltwater transition zone was investigated in an unconfined aquifer on Virginia’s Eastern Shore. The Steelman’s Landing study site consisted of a well transect which began in an 800 meter wide upland agricultural field, and proceeded seaward through a 300 meter wide mesic forest, 300 meter wide saltmarsh, and 550 meters offshore into Magothy Bay. Wells in the surficial, Columbia aquifer were screened over 30 centimeters at depths of 3.05, 9.14, and 15.25 meters below the surface. Most monitoring sites consisted of clusters of multiple wells which were periodically sampled for inorganic nutrients and salinity. In the saltmarsh portion of the study site, salinity of monitoring well samples indicated the presence of two horizontal flow systems. The deeper flow system contained freshwater flowing seaward from upland areas. The shallower system contained saline water recharged by the tides and concentrated by evaporation. Salinity measurements and positive vertical (0.019-0.046 meters/meter) and horizontal (0.001-0.005 m/m) hydraulic gradients of wells located across the marsh suggested movement of freshwater offshore which was confirmed by the direct measurement of submarine groundwater discharge (SGWD) using seepage meters. Upland wells contained high nitrate freshwater (>600μmol / 1 NO₃⁻, <1.0% salinity), while shallow (3.05m) wells located from the marsh-forest interface across the saltmarsh to Magothy Bay contained increasingly saline water and reduced nitrate levels (<1μmol / 1 NO₃⁻, 4-21% salinity). Deeper (9.14m) wells across the study site from the agricultural field seaward 420 meters offshore, contained fresh water (<1.0%). A deep (9.14m) well located 550 meters offshore contained water of nearly equal salinity to ambient water in Magothy Bay (30-32%). This represented a point on the saline side of the transition zone. In order to more completely identify and account for movement of nutrients in groundwater across the study site from upland agricultural fields to Magothy Bay, the second portion of the study consisted of an investigation of nutrient movement through a small tidal creek located adjacent to the Steelman’s Landing study site. Creek water had a higher velocity and shorter residence time in comparison to groundwater. Exercises describing nutrient movement were conducted in February and May, 1993 in Wall’s Landing Creek. A seasonal component of nutrient reduction was investigated as indicated by ambient creek samples, and bulk flux of nutrients across the sediment-water interface using light and dark benthic dome chambers. Nitrate flux measurements for the February and May sampling exercises were approximately 14,500 μmol/sec (17.6kg/day as N) and 5400 μmol/sec (6.5kg/day as N), respectively. The primary mechanism of nitrate reduction on reaching the creek channel was physical dilution by seawater.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Biology

Department dc:contributor.department

Biology

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

1993

Author dc:creator

• Nippert, Howard Christian

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Dc Identifier Other

etd-11242009-020234

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/46014