1.1 Location and Geological Overview
The study area, called Poplar Point, is located on the southern bank of the Anacostia River in Washington, DC, a little over a mile east of the confluence with the Potomac river (Figure-1). This is the southern most region of the larger Anacostia River Basin and is influenced by a daily tidal fluctuation of about three feet (House Document 104-257, 1996). Important features to note in Figure 1 are the topographic highs to the north and south (elevations south of the study area reach 250 ft above sea level) that have historically served as local sources of sediment transported to the Anacostia and the tidal flats (shown by a stippled pattern) which are characteristic of the current estuarine setting.
The Anacostia river is formed by the confluence of the Northwest and Northeast Branches at Bladensburg, Maryland. The watershed area is divided into three major portions: the northeastern, northwestern and the tidal area, respectively (Figure-2). The tidal watershed is drained by the Hickey Run, Lower Beaverdam Creek and Watts Branch (ICPRB, 1988). Tidal influence in the Anacostia extends to approximately 1000 feet above the confluence of the Northwest and Northeast Branches.
As shown in Figure-3, the Washington DC area is located along the boundary of two significant physiographic provinces: the Atlantic Coastal Plain and Piedmont Plateau. The boundary line that separates these two provinces is called the "Fall Line". The significance of the study area's proximity to the fall line is that in areas such as this there is typically a thin sedimentary section (consisting of modern and ancient sediment of the Coastal Plain) that directly overlies and is affected by the structure of the crystalline basement rock.
The Piedmont Plateau (Figure-3) consists of crystalline metamorphic rocks with occasional intrusions of igneous rocks. The granite, gneiss, schist and other crystalline rocks range in age from Precambrian to late Paleozoic. The structural geology of the Piedmont is complex, including tilted strata, folds, and normal as well as reverse faults. The Piedmont units are overlain to the east by the sediments of the Coastal Plain
The sedimentary strata of the Coastal Plain (Figure-3), consisting of a succession of Cretaceous to Quaternary age units, form a wedge that thins out onto the crystalline Piedmont to the west and becomes progressively thicker eastward. The depositional origins of the sediments that make up the Coastal Plain Stratigraphy range from fluvial, to deltaic, estuarine, and marine.
According to the surficial geology map shown in Figure-3 the study area is located within Quaternary alluvium deposited by the Anacostia river and its tributaries. Underlying the alluvial cover of the Anacostia and outcropping in the hills just south of the study area is the Cretaceous Bedrock of the Potomac group. The maximum thickness of the Potomac Group in Maryland is about 5,000 feet (Trapp and Horn, 1977). According to Maryland's surface geology map, the Potomac Group has three members: Patapsco Formation, which contains silt, clay, minor sand and gravel; Arundel clay with lignite contents; Patuxent formation (sands and gravels).
Over time human use and engineering in the region have significantly altered the morphology and hydrology of the tidal Anacostia. In its natural state the lower Anacostia region (including the study area) was covered with wetlands associated with the Anacostia river estuary, delta and floodplain. Over 90% of tidal wetlands, including those in the Poplar Point area, have been lost. The Army Corps of Engineers estimates that, between Bladensburg and the river's mouth, approximately 2,500 acres of tidal emergent wetlands have been destroyed, leaving less than 100 acres of emergent wetlands (http://response.restoration.noaa.gov/cpr/watershed/anacostia/guide/aboutar/ wetlands.htm).
Multiple tributaries to the Anacostia existed as well but have been filled along with the wetlands to facilitate the growth of the metropolitan DC area. As human land use in the area grew the rivers became narrower and shallower due to increased sediment supply caused by the denudation of forests for the creation of farmlands and urban construction sites (Yorke, 1975). Compiling historic, geologic and engineering data, Williams (1977) reconstructed the stream network and morphology that might have existed before colonial times. In his reconstruction (Figure-4) the approximate extent of the landfill around the Poplar Point area is shown as well as the number of tributaries to the Anacostia that may have indeed disappeared. Of note is a creek just at the southern edge of the study area of this report.
The Poplar Point area as it looks today is shown in the aerial photograph is presented in Figure-5.
The data that served as the core of our investigation was collected in 1997 by Thomas L. Brown and Associates. The data set consisted mainly of descriptive stratigraphic logs and the coordinates of 46 soil borings and 13 monitoring wells. The logs included brief characterization of the lithological units and their elevation above/below sea level. No age data were given in these borings. Water levels in the monitoring wells for 3 days were provided as well, as was the tidal stage at the time the elevation of the water table was measured. Lastly, engineering reports on subsurface investigations made for the Washington Metropolitan Area Transit Authority in addition to METRO report number 8 were received. 21 borings from the METRO reports were located in the study area (Poplar Point). Two additional borings north of the Anacostia river were used in the construction of our cross sections. The inFormation in these reports contained boring coordinates, main lithologic units, brief description of samples, and ages of the units.
It is important to note that the age data in the engineering reports were not based on absolute age measurements. Rather, these ages were based on the lithologic character of the rocks and a general knowledge of the geologic Formations. Therefore, some discrepancies in the given relative ages of lithologic units may occur. Despite this fact we have incorporated the revised age data available in these reports into our own study.
In addition to the stratigraphic data, basic inFormation such as aerial photography, watershed maps, river networks, roads, soil types, bathymetry, and surface elevation was provided in a digital format on the CD produced in 2001 by the Anacostia River Watershed Database and Mapping Project organized by NOAA's coastal Protection and Restoration Division.
2. Interpretation of the Stratigraphic Record
In the interpretation process, the lithologic descriptions were reviewed, correlations were made between borings, cross sections were constructed, and maps were made depicting the extent of the sand and gravel units on Poplar Point. Finally, a depositional history for the area was reconstructed. The locations of the borings and monitoring wells used in this study are shown together with the cross section locations in Figure-6.
2.1 Cross sections
The cross section A-A'(Figure-7) is the largest of the study, crossing the modern river channel and linking the study area to two borings on the north shore of the Anacostia river. The current position of the Anacostia river channel is shown on the section. The channel has a depth of 18 feet along this profile. The cross section shows that the river valley formed under the influence of extensional tectonic activity. The Cretaceous basement is bounded by normal faults on both the northern and southern sides and these faults are thought to have controlled the location of the original Anacostia river valley. Williams (1942) in the geological history of the region interprets uplift and tilting of the Atlantic Coastal Plain. The graben formation beneath the Anacostia river might have been associated with this tectonic activity in the Tertiary or lower Pleistocene. The age of the faulting and valley formation is post-Cretaceous, but the exact timing cannot be determined. The lack of Tertiary deposits in the area means that denudation may have occurred in association with valley formation at that time, or the Tertiary sediments might simply have been eroded in the Quaternary.
In the Pleistocene, as the initial form of the Anacostia valley took shape, coarse grained alluvial sediments were deposited on top of the eroded surface of the Cretaceous units near the valley edge. Borings HS-3, and SB-23 encountered this unit in the southern part of the study area and a similar sedimentary unit is found in boring F-226 on the northern edge of the valley. In our interpretation, this unit represents fan-delta type sediments deposited by creeks transporting mostly coarse grained sediments from the northern and southern hills. The formation (Formation code in this study: P-1-FD) is regarded as Pleistocene in age and was likely deposited prior to the last glaciation. This formation corresponds to T2-T3 unit in the engineering reports.
The fall in sea level associated with the last glaciation in the Pleistocene produced a deep incision into the tectonically preformed Anacostia valley and removed part of the earlier deposited sediments. Due to the incision, the river was transporting on high gradient slope and was carrying large amounts of sediments; the river type was most likely braided river. Gravels and sands were deposited in the study area as braided river channel bars. Borings F-227, F-231U and F-233 in Figure 9 encounter these deposits (Formation code in this study: P-2-BR). This formation corresponds with T5 in the engineering reports. It is significant to note that in our interpretation the braided river deposits are technically younger than the fan delta deposits. Previous work in the METRO reports interpreted T5 terrace units as being older than T2 and T3 deposits.
The Holocene sea level rise brought about significant fluvial Aggradation in the river valley represented by the thick clay, fine sand valley fill. The valley fill sediments eventually Onlapped the Pleistocene fan-deltas on the flanks of the valley. This configuration can be observed in this section in borings SB-23, SB-35 and HS-3 where clay deposited over the Pleistocene gravels. On the northern side of the valley artificial fill obscures this relationship somewhat. The fine grained, clay-rich sediments are interpreted as undifferentiated flood plain and tidal marsh facies. These deposits correspond to intervals A1-A2 in the engineering reports. Interspersed throughout the clay rich Holocene section are 5 to 10 ft thick lenses of coarse to fine sands. These sands and localized gravels (labeled H-TB1, 2, and 3 on all cross sections) are interpreted as preserved tidal bars much like those seen in the estuary today (see tidal flats on Figure 1). This depositional origin explains the discontinuous and relatively thin nature of the sands. This interpretation is also supported by the observation and analysis of Williams (1942) who stated that feeble currents produced by tides formed sand bars in the Anacostia river.
Figure-8 shows cross section B-B' close to the southern part of section A-A'. It is essentially a close up view of the southern portion of the cross section A-A'. The graben in the basement is evidenced by borings SB-35, SB-23 and MW-10 that reached the Cretaceous series. A drop of 40-45 feet in the basement along normal faults can be estimated based on the drilling inFormation. The Pleistocene fan-deltas lie on the southern flank of the valley, and the Pleistocene braided channel sediments are drilled by boring MW-10. A Holocene sand-gravel unit is detected in borings SB-22, SB-30 and SB-31 (H-TB2).
The cross section C-C' (Figure-9) is oriented perpendicularly to the previous sections and runs along the southern limit of the study area (Figure-6). The correlation between the deeper borings in this section shows that the basal Cretaceous units dip to the west-northwest, further evidence of the graben forming tectonic activity descriBed above. More importantly this section reveals that a smaller tributary valley was formed in addition to the main Anacostia valley, joining it from the south at an acute angle. The incision of this smaller valley is inferred by the absence of the P-1-FD unit (fan delta gravels) in borings SB-25, HS-32 and SB-24. The incision of this narrow tributary valley probably occurred during the last glaciation, coeval with the development of the larger ancestral Anacostia braided river. Further, the lowermost portion of boring HS-32, in the center of the interpreted tributary, contains a gravel rich layer which correlates well with the sediments interpreted as braided river deposits in borings F-227, F-231, F-232, and F-233 of the cross section A-A'. Another, smaller scale incision may have occurred at boring MW-4 where silt is deposited above the fan-delta unit.
The cross section D-D' (Figure-10) is another north-south oriented section and is located in the west-central part of the study area. The section, when compared to Figure 8 shows the overall thickening of the fan delta (unit P-1-FD) to the west as well as more of the isolated tidal bar sands within the Holocene alluvial clays. Cross section E-E' (Figure-11) shows that in the center of the study area the stratigraphic section is dominated by clays, except for the isolated braided river deposits at depth and thin lenses of tidal bar sands.
2.2 Mapped Extent of Permeable Units
Figure-12 shows the aerial extent of all sand and gravel deposits mapped in this study. It is important to note that while the tidal bar sands overlie the braided river deposits they are isolated from each other by more than 40 ft. of clay in most cases.
By far the most continuous unit mapped is the braided river deposit (P-2-BR). This unit is present from the southern edge of the study area north, extending beneath the river. While the data set used in this study is somewhat limited in its aerial extent, the braided river deposits are believed to be present at depth everywhere in the modern Anacositia Valley.
The fan delta deposits are mapped only on the southern edge of the study area, in association with the historic tributary drainage. These deposits are isolated from all others by the thicker clays filling the valley. Deposits similar to the fan delta mapped on Poplar Point are believed to be present along both the north and south sides of the Anacostia river valley.
The tidal bar deposits are distributed throughout the study site, but are laterally isolated from one another by the tidal marsh/floodplain clays. Lenses of sand such as these are most likely present throughout all of the Holocene sediments of the tidal Anacostia Basin.
2.3 Summary of Depositional Model
The depositional scenario discussed in section 2.1 and resulting in the geometry of the units shown on the map of section 2.2 is summarized in Figures 13-15. In the first stage, designated as Pleistocene 1 (Figure-13), the incipient Anacostia valley has already formed tectonically and the coarse grained fan deltas were deposited at the southern edge of the study area. These sediments were transported into the area by short, high energy creeks that originated in the hills south of the study area. Williams (1977)' reconstruction of the original drainage network supports this idea. In his map (Figure-4) a tributary is located at the site of these gravel sediments.
The next stage of evolution (Figure-14), Pleistocene 2, represents the period of valley incision caused by Relative Sea Level Fall associated with the last glacial period. During this time, the ancestral Anacostia valley was incised deeper into the incipient fault controlled valley and the braided river deposits were laid down on the valley floor. Due to a steep surface gradient at that time the Anacostia carried large loads of sediments giving the river a braided morphology, much different than the river appears today. Also at this time tributaries such as the one pictured in Figure 14 continued feeding from the north and south into the now deepened main valley. The steeper gradients resulted in incision into and removal of portions of the fan deltas they had deposited previously.
The final stage (Figure-15) was developed in the Holocene as sea level rose due to the end of glaciation. The rise of Relative Sea Level led to lower river gradients and the gradual Aggradational infilling of the Anacostia valley as the estuarine setting visible today developed. The lower energy depositional environments associated with the infill of the valley (aggrading river floodplains and tidal marshes) resulted in deposition of a clay rich section with isolated sandy lenses present wherever point bars and/or tidal bars and flats were preserved.
3. Ground Water Flow
The groundwater data that were reviewed for the Poplar Point site were elevations measured in the monitoring wells (MW) 1-11 in 1997 and 1999, and a series of pump tests conducted in 1981 (METRO Report No. 8).
Figure-16 shows the elevations of the water table in each well as measured on at 12:30 pm on 22 February 1999. The wells have been classified according to the units in which they are screened. Note that the "deep" wells are screened at depths of 27-57 feet below the surface and in the Pleistocene gravel units (P-1 FD and P-2 BR). The water levels in these wells (MW 1, 2, 7, and 10) group together and are the lowest of all wells, ranging from 0.45-2.51 feet above MSL. The water levels in the wells (MW 2A, 5, and 8) screened only in the Holocene sand and gravel "tidal bar" unit (HTPB 1-3) also group together, more tightly so, ranging from 5.6-5.8 feet above MSL. The water levels in wells that are screened in the fill material are the highest of all the wells, ranging from 4.65-6.75. Two wells are screened only in the Holocene clay unit, and they have identical water table elevations at 3.9 feet above MSL. Water table elevations reported for other dates show a similar pattern for the different stratigraphic units.
These trends in water table elevation indicate that the upper permeable units, that is, the fill and Holocene sand/gravel, do not have direct hydrologic connections to the deeper Pleistocene gravel/sand units. They each have distinct water table elevations. The Holocene clay appears to act as an effective aquitard. The fill unit has the highest water table elevations, and thus groundwater from the fill could migrate downward to the lower units. The Holocene "tidal bars" unit may be hydrologically connected to the fill material because its water table elevations are bracketed by those in the fill.
Elevations were measured hourly for different groups of wells on 22, 23, and 27 February 1999, for up to 7 hours a day. The time-series plots indicate that there is some influence of the tides in the Anacostia River on the water table in both the shallow and deep wells. However, pump tests done in 1981 on the Pleistocene unit (called T-5 and A-2/T-5 in the METRO reports) showed that there was no hydrological connection between the aquifer and the river. Our interpretations of the Stratigraphy of the Holocene sand and gravel units also indicate that they are isolated from the river. The changes in water level in the wells reflect the effects of aquifer deFormation resulting from the 3+ feet tidal fluctuations in the river.
The horizontal groundwater flow patterns can only be generally descriBed because there are only a few wells in each unit and the well distribution is poor. Figure-17 shows the water table elevation and surface elevation for the wells with known locations. Wells 1, 2, and 10 are in the same Pleistocene gravel and sand unit. For 9 out of 11 water table measurements, well MW-2 was higher than MW-1. For 6 out of 9 water table measurements, MW-10 had the highest water table. For this unit, there is no recognizable pattern in groundwater flow direction. The Holocene sand and gravel unit has three wells that are screened only in it (MW 2A, 5, and 8). Well-2A generally has the lowest water table elevation, by as much as 2 feet below the other wells in this unit. If these measurements are accurate, this pattern suggests horizontal flow in the Holocene unit is generally but weakly to the north.
4. Feasibility of 3D studies
In order to assess the feasibility of a 3D visualization of the Stratigraphy at Poplar Point, preliminary 3D plots of the interpretation presented above were developed. This was accomplished using the 3D Analyst and Spatial Analyst extensions of ESRIs Arcview GIS software. As shown in Figure-18, Figure-19, Figure-20 adequate data exist for an effective presentation of results in three dimensions. Such a study could be expanded to a three dimensional correlation of inFormation from all borings available, though a study extending to the northern Shoreline would be somewhat affected by the linearity of the dataset.
The analysis and correlation of over 50 stratigraphic logs from monitoring wells and soil borings on Poplar Point and the northern banks of the Anacostia revealed that the sand and gravel deposits present in the study area could be categorized in three groups. Each of these sandy units was isolated from the others by thick floodplain and tidal marsh clays. The deepest and most continuous permeable unit consisted of braided river sands of the ancestral Anacostia river. Other stratigraphically higher and less continuous units were interpreted as fan delta deposits and tidal bars. This stratigraphic interpretation was in agreement for the most part with existing METRO interpretations, the only significant difference being the identification of isolated sand lenses within the Holocene clays. Analysis of the groundwater data supported our stratigraphic interpretation in that wells screened in the deeper braided river deposits behaved similarly and had deeper ground water tables than those screened in shallow Holocene sands. The data available provides adequate coverage of the eastern half of Poplar Point. Data available within the river channel and along the northern shore is less extensive, occurring in dense linear trends associated with the subway installation. All data can easily be incorporated into a 3D site characterization.
Cleaves, E. T., Edwards, J., and Glaser, J. D., (eds.), 1968, Geologic Map of Maryland, Maryland Geological Survey.
House Document, 1996, Anacostia river and tributaries, District of Columbia and Maryland, Communication from The Assistant Secretary of the Army (Civil Works), House Document 104-257, 591p.
ICPRB (Interstate Commission on the Potomac River Basin), 1988, Anacostia: The other river, Rockville, Maryland, 16p.
Trapp, H., and Horn, M. A., 1997, Ground Water Atlas of the United States, Segment 11.
Williams, M. T., 1942, A history of erosion in the Anacostia drainage Basin, The Catholic University of America Press, Washington, D.C., 59p.
Williams, G. P., 1977, Washington, D.C.'s vanishing springs and waterways, Geological Survey Circular 752, 19p.
Yorke, T. H., 1975, Effects of sediment control on sediment transport in the Northwest Branch Anacostia river Basin, Montgomery County, Maryland, Journal of Research of U.S. Geological Survey, vol. 3, No. 4, p. 487-494.