Baltimore WATERS Test Bed

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Dr. Nigel Crook of Stanford University and CUAHSI HMF Geophyics was in residence from July 2 – 18, 2007 to carry out the following work in collaboration with the Baltimore Waters Testbed project:

(1) Riparian zone geophysics – Ten transects across and around the stream channels in Dead Run watershed were characterized using resistivity, ground penetrating radar and seismic refraction. The aim was to delineate depth to bedrock and the water table topography as a function of distance from the river channel. Required ancillary data included a topographic survey using a total station and installation of piezometers for ground-truthing of water levels. The interpreted data will be incorporated into the groundwater model of Dead Run.

(2) Microgravity survey - A semi-regional scale survey was conducted over the Gwynns Falls watershed and a smaller, higher-resolution scale survey was conducted around the Dead Run watershed. The survey consisted of placing a microgravity meter at selected locations and determining earth’s gravity at these points. The idea is that there is a greater gravitational pull under a point when pores are filled with water. Repeat measurements were taken at the same points in February 2008, which enabled calculation of the change in groundwater storage. This method has been previously applied to the Tuscon area by USGS. A map showing the point coverage of the region with the first gravity map is shown below. 90 data points were collected. A poster summarizing this work was presented at Fall 2008 AGU. The abstract can be viewed here.

Interpretrions of data from both efforts have been summarized in a manuscript for submission to the journal Ground Water.

Phil Larson assisting
Nigel Crook with
resistivity survey.

Locations in Dead Run chosen for geophysical transects. From downstream to upstream: locations 1, 4, 8, 9, 10, 11, 15, 16, 18, 20.

Locations in the Gwynns Falls watershed chosen for the microgravity survey.

Example Resistivity Results, Transect 9

Top: Wenner electrode configuration (provides greater sensitivity to horizontal variations in resistance, greater depth pentration), 2m electrode spacing. MIDDLE: With interpretation of water table depth and extent of sapprolite.
Bottom:Dipole-dipole electrode configuration (provides greater sensitivity to vertical variations in resistance, higher resolution), 2m electrode spacing.

Electrical resistivity inversion model output (topography included)

• The two models depict a 3 layer structure, with an upper resistive layer interpreted as a thin soil cover, this varies in thickness across the transect (<2m) with some regions of higher resistivity around 20m and between 40-50m possibly reflecting areas of rocky ground. The area associated with the stream channel displays higher resistivity values as would be expected due to the coarse grained sediment of the stream bed.
• Below this is a more conductive layer which is interpreted as the saprolite layer which varies between ~3-5m thick, thinning towards the start of the transect. The resistivity of this layer displays some variation with both depth and horizontal distance. The upper more conductive region of this layer (the blues and purples in the figures) could be reflecting the position of the water table – this correlates well with water levels taken from a piezometer close to the transect. Additionally it might reflect a zone of enhanced weathering in the saprolite associated with water table fluctuations, hence the thickening of this layer away from the stream channel where such fluctuations would be greater. The discontinuous nature of this layer illustrated in the dipole-dipole configuration could reflect variations in composition of the saprolite. The increase in resistivity with depth of this layer is probably related to a decrease in the degree of weathering.
• A high resistivity layer underlies the saprolite, interpreted as bedrock.

July 2007 microgravity map.

Nigel Crook taking a microgravity reading.