Measurement and analysis of aquifer-system compaction have been used to characterize aquifer and confining unit properties when other techniques such as flow modeling have been ineffective at adequately quantifying storage properties or matching historical water levels in environments experiencing land subsidence. In the southeastern Coastal Plain of Virginia, high-sensitivity borehole pipe extensometers were used to measure 24.2 mm of total compaction at Franklin from 1979 to 1995 (an average of 1.5 mm/yr) and 50.2 mm of total compaction at Suffolk from 1982 to 1995 (an average of 3.7 mm/yr). Analysis of the extensometer data reveals that the small rates of aquifer-system compaction appear to be correlated with withdrawals of water from confined aquifers. One-dimensional vertical compaction modeling indicates that the measured compaction is the result of nonrecoverable hydrodynamic consolidation of the fine-grained confining units and interbeds as well as recoverable compaction and expansion of coarse-grained aquifer units. The modeling results also provide useful information about specific storage and vertical hydraulic conductivity of individual hydrogeologic units. The results of this study enhance the understanding of the complex Coastal Plain aquifer system and will be useful in future modeling and management of ground water in this region. / Master of Science

Watersheds within northwestern Mississippi, a productive agricultural region referred to as the Delta, were recently identified as contributors of total nitrogen and phosphorus fluxes to the Gulf of Mexico. Water withdrawals for irrigation in the Delta have altered flow paths between surface-water and groundwater systems, allowing for more surface-water losses to the underlying alluvial aquifer. In order to understand how to manage nitrogen in a watershed, it is necessary to identify and quantify hydrologic flow paths and biogeochemical conditions along these flow paths, which ultimately combine to determine transport and fate. In order to evaluate the extent and role of surface-water losses to the alluvial aquifer on the transport of nitrate, a two-dimensional groundwater/surface-water exchange model was developed for a site within the Delta. Results from this model determined that groundwater/surface-water exchange at the site occurred regularly and recharge was laterally extensive into the alluvial aquifer. Nitrate was consistently reported in surface-water samples (n= 52, median concentration = 39.8 micromol/L), although never detected in samples collected from instream or near stream piezometers (n=46). Coupled model and water-quality results support the case for denitrification/ nitrate loss from surface water moving through an anoxic streambed. At larger scale, recent results from two Spatially Referenced Regressions on Watershed attributes (SPARROW) models imply that nitrogen is transported relatively conservatively once it enters the main channel of the Big Sunflower River Basin, which contributes much of the water discharging from the Yazoo River Basin to the Mississippi River. Net loss of nitrogen was assessed by comparing total nitrogen data from Lagrangian sampling events to chloride, drainage area, and predicted total nitrogen flux results from the SPARROW models. Results indicated relatively conservative instream transport of nitrogen at the scale of the Big Sunflower River Basin; however, two potential nitrogen loss mechanisms were identified: (1) transport and transformation of nitrogen through the streambed, and (2) sequestration and transformation of nitrogen above the drainage control structures downstream of Anguilla.