J.P. Pope | USGS Science

USGS2007

A simulation of groundwater discharge and nitrate delivery to chesapeake bay from the lowermost delmarva peninsula, USA

A groundwater model has been developed for the lowermost Delmarva Peninsula, USA, that simulates saltwater intrusion into local confined aquifers and nitrate delivery to the Chesapeake Bay from the surficial aquifer. A flow path and groundwater-age analysis was performed using the model to estimate the timing of nitrate delivery to the bay over the next several decades. The simulated mean and median residence times of groundwater in the lowermost peninsula are 30 and 15 years, respectively. Current and future nitrate concentrations in coastal groundwater discharge were simulated based on local well data that include nitrate concentrations and groundwater age. A simulated future-trends analysis indicates that nitrate that has been applied to agricultural regions over the last few decades will continue to discharge into the bay for several decades to come. This study highlights the importance of considering the groundwater lag time that affects the mean transport time from diffuse contamination sources.

Conference Paper↗

USGS2005

Multiple-Aquifer Characterization from Single Borehole Extensometer Records

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 through 1995 (1.5 mm/year) and 50.2 mm of total compaction at Suffolk from 1982 through 1995 (3.7 mm/year). 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 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 calibrated modeling results indicate that nonrecoverable specific storage values decrease with depth and range from 1.5 x 10-5/m for aquifer units to 1.5 x 10-4/m for confining units and interbeds. The aquifer and Potomac system recoverable specific storage values were all estimated to be 4.5 x 10-6/m, while the confining units and interbeds had values of 6.0 x 10-6/m. The calibrated vertical hydraulic conductivity values of the confining units and interbeds ranged from 6.6 x 10-4 m/year to 2.0 x 10-3 m/year. These parameter values will be useful in future management and modeling of ground water in the Virginia Coastal Plain.

Ground Water↗