DEVELOPMENT OF A GROUNDWATER CONTOUR MAP FOR THE WATER TABLE AQUIFER IN THE ATLANTIC COASTAL PLAIN DEPOSITS OF WASHINGTON, DC

Abstract

The District of Columbia is an urbanized region that has many human-influenced features that have significantly impacted both the surface and subsurface hydrology. The ability to evaluate the quantitative impacts of discharge sources requires an understanding of the groundwater flow system in the area. The construction of buildings with basement sump pumps and paved surface areas have significantly impacted surface hydrology, groundwater recharge, and general subdrainage basin hydrology throughout the area. In addition, the geology of the area is variable and further complicates the evaluation of the groundwater flow system. The District of Columbia straddles the boundary between the Atlantic Coastal Plain physiographic province and the Piedmont physiographic province. This research focused on the groundwater hydrology in the Atlantic Coastal Plain physiographic province. This province is characterized by alternating layers of unconsolidated sedimentary sands, silts, and clays. The contribution of this research was the development of a groundwater contour map of the Atlantic Coastal Plain aquifer that could be used for assessing urbanization impacts on the groundwater flow and provide insights into the interaction between surface water contamination and groundwater contamination. Several types of groundwater data were used to estimate the groundwater elevation contours. These data included surface water bodies, groundwater monitoring wells, and soil test borings. The location of each groundwater elevation data point was identified on a base map and assigned a coordinate based on the Universal Transverse Mercator coordinate system. A total of 466 data points were collected for this research. The Geostatistical Environmental Assessment computer program GEO-EAS was used to evaluate the data and examine the reliability of the elevation contours generated. Semi-variogram models were developed that plot the variance of paired sample measurements as a function of the distance between samples. Semi-variograms were used to characterize the similarity of the groundwater elevations within a homogeneous geological area. The semi-variogram model was then used to develop elevation contours using an ordinary block kriging method. Kriging is a weighted-moving-average interpolation method where the set weights assigned to the samples minimizes the estimation variance. In addition to the development of a groundwater elevation contour map, a contour map of the kriging standard deviations was developed and used to evaluate areas that may require additional research. Kriging standard deviation is the standard error of estimation computed for a kriged estimate and is dependent of the semi-variogram model. The groundwater elevation contour map suggests that four recharge areas are present in the Atlantic Coastal Plain region of Washington, DC. A perched water table was also identified in the downtown region near Logan Circle. Based on the groundwater contours, flow directions could be predicted and appeared to follow the natural topographic gradient of the region. Historical stream channels, that are now buried, also control the groundwater flow in the region. The geostatistical analysis indicated the greatest uncertainty in the data in the four recharge areas identified on the groundwater elevation contour map. In those areas, the groundwater elevations may be significantly effected by seasonal fluctuations. Additional research into the impacts of seasonal fluctuations of groundwater levels is recommended.